**2. Deposition of CdTe thin films by close‐spaced sublimation (CSS) technique**

The CSS technique is particularly attractive for the deposition of CdTe films since it offers high deposition rates and can be simply scaled up for manufacturing purposes. CSS equipment can be constructed with a wide variety of options such as vacuum, temperature, spacing between source and substrate. One can design first based on required material to be coated on substrates. As for example, the source and substrate in case of CdTe are kept at a close distance of 2 mm in our case, which can also be changed by some simple techniques like increasing spacer of certain thickness. Now, top and bottom heaters can be chosen according to the temperature needed to evaporate either in atmospheric pressure or evacuating to the range of millitorr, filling with certain gas like Ar or N2. There is a need for good temperature controller which can be programmable in steps such as increment or hold time. Finally, natural cooling is followed to let the temperature falls back to room temperature. Interestingly, the whole CSS deposition takes place within only 10 min, excluding the evacuation or purging time. We have successfully grown a required thickness of 1 micron for CdTe to be used in solar cells, which is the most challenging part in CSS to learn temperature profiles for various thickness with the best coating or film properties needed in each application.

The main design prerequisite for the close-spaced sublimation (CSS) equipment is to minimize the influence of extrinsic impurities that could be incorporated from ambient to the deposited films. This requirement minimizes the number of impurity sources that would be heated to high temperatures together with the number of wires required to feed through the chamber. Sublimation illustrates the direct phase transition between a solid and gas state. A close-spaced sublimation is dependent on the following factors:

**•** Sublimation process at the surface of the source

**1. Introduction**

362 Modern Technologies for Creating the Thin-film Systems and Coatings

technologies.

temperature.

Close-spaced sublimation (CSS) is one of the simplest methods in physical vapor deposition. Materials especially semiconductors that evaporate below 800°C can be coated on substrates like glass in both vacuum and atmospheric pressure. The target materials have to be in the form of solid in either chunk or powder form [1–3]. As for example, compound semiconductor like cadmium telluride (CdTe) can be deposited at around 600*°*C with a thickness of 1–10 μm within 10 min of deposition time, which is one of the fastest deposition times among other physical vapor deposition (PVD) methods. Needless to mention, CSS and binary compound material cadmium telluride (CdTe) are densely interrelated due to extensive usage of CSS in the growth of CdTe thin film [4, 5]. The binary compound CdTe has been recognized as one of the promising thin film photovoltaic materials owing to its near

absorbs over 90% of available photons (hν > 1.44 eV) in 1 μm thickness, and hence, films of only 1–3 μm are sufficient for solar cells [6, 7]. Several types of CdTe solar cells such as Schottky barrier, homojunction, heterojunction, and p-i-n have been investigated to date [8– 10]. Among all, the most successful configurations are the heterojunctions where a wide bandgap semiconductor can be used as the heterojunction partner or "window." However, cadmium sulfide (CdS) has been the most widely studied and most appropriate window material for CdTe solar cells to date. Most recent development in CdTe thin film solar cells has found noteworthy improvements in small area conversion efficiencies. A number of techniques such as atomic layer epitaxy, spraying, electrodeposition (ED), and close-spaced sublimation (CSS) have been employed for the fabrication of CdTe thin film solar cells with significant efficiencies of over 20% with various configurations. It is quite notable that even though important dissimilarities exist, the performances achieved are independent to processing demonstrating the versatility of CdTe and its superior status in the photovoltaic

The deposition method for CdTe thin films differs widely and can considerably affect the material properties and device performance. Since CdTe has high absorption coefficient hence thicknesses for CdTe thin films are limited within 2–10 μm [11, 12]. There are various methods to deposit CdTe, which includes close-spaced sublimation (CSS), vapor transport deposition (VTD), electrodeposition (ED), physical vapor deposition (PVD), sputtering, etc. [13–15]. Among all the deposition methods, the highest efficient CdTe thin film solar cell was obtained by close-spaced sublimation (CSS). The substrate temperature is one of the crucial parameters for CdTe deposition as it could be observed that most of the deposition techniques demonstrated has substrate heating. Higher growth temperature not only enhances the deposition rate, but it also determines the quality of junction formation. Moreover, some research studies illustrated that CdTe deposited at higher temperature exhibits better performance. Therefore, the resistivity of the CdTe films decreases with an increase in substrate

/cm. CdTe, therefore,

optimum bandgap of 1.44 eV and high absorption coefficient over 105


Phase diagrams of CdTe demonstrate a very narrow composition where the CdTe binary compound exists as a solid without any other phases, with an atomic percentage of tellurium equal to approximately 50%. The melting point of the material is also a maximum at this point and equal to 1099*°*C [16]. Greenberg calculated the range of tellurium concentration over which the material is in a purely solid phase to be 49.996 to 50.012 atomic percent [17]. A pressure phase temperature diagram of CdTe has also been published, which indicates the boundary of a solid to vapor transition [18]. The sublimation of CdTe occurs in the region where the CdTe is in a pure solid phase as calculated by Greenberg. This region is defined by the solid-liquidvapor transition as the upper limit, the sides are defined by the limits of Cd and Te saturation in CdTe, and the bottom is defined by the line of stoichiometric congruent sublimation where the vapor has the same composition as the solid [19]. The sublimation reaction happens in the following equation [20]:

$$2\text{ CdTe}\ (\text{s}) \leftrightharpoons 2\text{ Cd}\ (\text{g})\ +\ \text{Te}\_2\ (\text{g})\tag{1}$$

The consequence of manipulating certain deposition parameters on the growth rate has also been reported earlier. Alamri demonstrated two distinct growth rate regions when increasing the source and substrate distance for a series of depositions [21]. This would specify a change in the growth mechanism, apparently from a sublimation limited to diffusion limited case. The growth rate was found to be independent of the substrate temperature (over a 100°C region), for temperatures significantly lower than source temperature and with a low deposition chamber pressure (10-5 mbar). This result does not emerge to agree with the theory for any sublimation limited cases suggested by others groups. No reason for this disagreement can be found from the experimental detail provided. As the distance between the source and substrate increased at low pressures, the growth rate was reported to decrease due to the divergence of the gas. Research works by Nagayoshi and Suzuki observed that the deposition rate increased as the source temperature increased for a deposition under vacuum, in agreement with a sublimation limited case [22]. They also varied the substrate temperature showing for a source temperature of 650°C and a substrate temperature of 520°C, the growth rate reached maximum in that case. The incidence of a maximum deposition rate for a series of experiments where only the substrate temperature was varied would imply varying this parameter changed the deposition mechanism. Work by Anthony et al. showed the composition of the environment gas also has an effect on the diffusion limited process. They recommend that the faster growth rate in a helium environment compared to argon was due to the small molecular weight and diameter of the helium [23].

#### **2.1. Close‐spaced sublimation (CSS) system**

CSS technique has been mainly emphasized for the deposition of CdTe films on foreign substrates. This technique has been extensively investigated because of the relatively high efficiency of solar cells prepared from CSS grown CdTe films. Deposition by reaction condensation from vapor generated from direct sublimation of the compounds has produced the highest efficiency devices. A schematic diagram of the CSS apparatus is shown in **Figure 1**.

**Figure 1.** Schematic view of close-spaced sublimation (CSS) apparatus.

The substrate and source, separated by a small distance (1 mm for instance) and placed on appropriate holders, are enclosed in a fused silica tube with gas inlet and outlet tubes to keep controlled environment inside. The system is maintained at desired temperatures by using infrared radiation, and thermocouples inserted into the sample holders are used to monitor their temperatures. The important process parameters are the temperatures of the source and the substrate, the nature of the atmosphere, the pressure in the reaction tube, and the composition of the source material. These parameters are inter-related. For example, the partial pressure of Cd and Te2 in the reaction tube is important in determining the rate of deposition, and these pressures, or the dissociation pressure of CdTe, depend exponentially on temperature. At a given source temperature, the sublimation rate increases rapidly as the pressure in the chamber is reduced from the atmospheric pressure. At low pressures, such as 1 Torr, the mean free path of the gaseous species in the reaction tube increases, and the condensation process is no longer limited to the space between the substrate and the source material. The effect of the nature of the ambient gas is related mainly to its thermal conductivity. In case of fixed source-substrate spacing, high thermal conductivity of the ambient gas tends to increase the substrate temperature, thus reducing the growth rate. High deposition rate of CdTe films (up to 10 μm/min) is therefore a special feature of the CSS process that benefits the thin film growth in a short span of time.

Deposition pressures between 1 and 30 Torr, substrate temperature from 500 to 600°C, and source temperatures between 700 and 800°C are commonly used for CSS. Sublimation of the compounds produces monatomic Cd and diatomic group VI (Te2 or S2) vapor, which recombines by the reverse reaction on the relatively cool substrate. Essentially, the coating process is a chemical vapor deposition with locally generated vapor (sublimation) and a reverse reaction (deposition) evolving no by-products. Due to high vapor pressures of the elements relative to the compounds, depositions of CdTe at temperatures above 500°C result in single phase films with stoichiometry to better than 100 ppm. **Table 1** provides a summary in the range of conditions used for the deposition of the CdTe films.


**Table 1.** Deposition conditions used in the CSS growth of CdTe films.

() ( ) ( ) <sup>2</sup> 2 CdTe s 2 Cd g Te g Æ + (1)

The consequence of manipulating certain deposition parameters on the growth rate has also been reported earlier. Alamri demonstrated two distinct growth rate regions when increasing the source and substrate distance for a series of depositions [21]. This would specify a change in the growth mechanism, apparently from a sublimation limited to diffusion limited case. The growth rate was found to be independent of the substrate temperature (over a 100°C region), for temperatures significantly lower than source temperature and with a low deposition chamber pressure (10-5 mbar). This result does not emerge to agree with the theory for any sublimation limited cases suggested by others groups. No reason for this disagreement can be found from the experimental detail provided. As the distance between the source and substrate increased at low pressures, the growth rate was reported to decrease due to the divergence of the gas. Research works by Nagayoshi and Suzuki observed that the deposition rate increased as the source temperature increased for a deposition under vacuum, in agreement with a sublimation limited case [22]. They also varied the substrate temperature showing for a source temperature of 650°C and a substrate temperature of 520°C, the growth rate reached maximum in that case. The incidence of a maximum deposition rate for a series of experiments where only the substrate temperature was varied would imply varying this parameter changed the deposition mechanism. Work by Anthony et al. showed the composition of the environment gas also has an effect on the diffusion limited process. They recommend that the faster growth rate in a helium environment compared to argon was due to the small molecular weight and

CSS technique has been mainly emphasized for the deposition of CdTe films on foreign substrates. This technique has been extensively investigated because of the relatively high efficiency of solar cells prepared from CSS grown CdTe films. Deposition by reaction condensation from vapor generated from direct sublimation of the compounds has produced the highest efficiency devices. A schematic diagram of the CSS apparatus is shown in **Figure 1**.

diameter of the helium [23].

**2.1. Close‐spaced sublimation (CSS) system**

364 Modern Technologies for Creating the Thin-film Systems and Coatings

**Figure 1.** Schematic view of close-spaced sublimation (CSS) apparatus.

In this study, the substrate temperature is varied in the range of 550–620°C. The source temperature, the total pressure, and spacing are adjusted to result in deposition rates of about 1.0–5.0 μm/min. However, for some films, the pressure during the deposition is kept at 2 Torr. One of the important efforts of this study is to control the film thickness toward stable pinhole free CdTe films of thickness <5 μm. By keeping other deposition parameters constant, the temperature profile is changed according to the desired film thickness. The chamber is evacuated several times and then is kept at 1.8 Torr of Ar gas to create the appropriate atmosphere for deposition.

### **2.2. Sintering of source material**

Sintering is the procedure of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction. In CSS process, sintering is usually executed to prepare the source material, while fine‐powdered CdTe or other materials are used. In cases of chunks, one can skip the sintering process. In sintering, CdTe powder is first put on the empty well on the bottom holder. Then, the powder is heated at specific pressure and temperature to make the powder compact as well as to form solid as shown in **Figure 2**.

**Figure 2.** CdTe source material as sintered for CSS process.

The source material on the graphite box is exhausted for each experiment, so the volume of the material decreases after each sublimation process. CdTe is also deposited on the inside wall of the chamber in regions where the temperature is cooler than the graphite blocks. This means that more than just the amount of CdTe deposited on the sample is released from the source material during each experiment. The parameters used during the sintering are given in **Table 2**.


**Table 2.** CdTe sintering parameters used in CSS.

The chamber has a rotary vacuum pump attached to one flange with the choice to allow gas to flow into the other flange. To control the pumping rate, a valve is positioned between the pump and deposition chamber, a pipe which bypassed the valve has a smaller diameter to limit the flow of gas. The pressure in the chamber is normally measured by pressure gauges. Argon and nitrogen gases are attached to the system. The argon gas is used as a deposition gas. Nitrogen is generally used for purging and venting the chamber to open the quartz tube (**Figure 3**).

evacuated several times and then is kept at 1.8 Torr of Ar gas to create the appropriate

Sintering is the procedure of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction. In CSS process, sintering is usually executed to prepare the source material, while fine‐powdered CdTe or other materials are used. In cases of chunks, one can skip the sintering process. In sintering, CdTe powder is first put on the empty well on the bottom holder. Then, the powder is heated at specific pressure and temperature to make the powder compact as well as to form solid as shown in **Figure 2**.

The source material on the graphite box is exhausted for each experiment, so the volume of the material decreases after each sublimation process. CdTe is also deposited on the inside wall of the chamber in regions where the temperature is cooler than the graphite blocks. This means that more than just the amount of CdTe deposited on the sample is released from the source material during each experiment. The parameters used during the sintering are given in

The chamber has a rotary vacuum pump attached to one flange with the choice to allow gas to flow into the other flange. To control the pumping rate, a valve is positioned between the

atmosphere for deposition.

**2.2. Sintering of source material**

366 Modern Technologies for Creating the Thin-film Systems and Coatings

**Figure 2.** CdTe source material as sintered for CSS process.

**CSS sintering parameters Value** Source temperature 700°C Spacing 1 mm

Heating time 30 min

**Table 2.** CdTe sintering parameters used in CSS.

Pressure 10 Torr (Ar gas atmosphere)

**Table 2**.

**Figure 3.** (a) CSS deposition system and (b) CSS grown CdTe thin film on glass.

In general, the CSS chamber is left under vacuum between experiments. When it is needed to open the chamber, gas is fed inside to bring the pressure up to atmosphere. The end flange with the gas inlet can be removed, and the graphite blocks can also be removed from the chamber. The sample is placed in the substrate block, and two glass samples are kept on top working as heat spreaders. The graphite blocks are then placed at the end of the tube, and the thermocouples are positioned within the blocks. The blocks are then pushed into the centre of the quartz tube, and the chamber is pumped to a vacuum. After a vacuum has been reached, the gas pressure gauge could then be adjusted to acquire the desired pressure within the chamber after a few purging of ambient inside. When the environment is stabilized, the heating sequence starts, and the temperature monitoring begins. The superstrate configuration is basically used to deposit CdTe on top of CdS deposited films. The system is maintained at the set temperatures by means of radiative heating using a total of 2 kW halogen lamps on top and bottom. The thermocouples are inserted into the graphite holders to monitor the growth temperatures (**Figure 4**).

**Figure 4.** Inside view of the CSS chamber with spacers placed on top of bottom holder.

#### **2.3. Significance of temperature profiles**

The substrate temperature of CdTe thin films during the growth is an essential parameter to obtain high-quality films ultimately resulting in high-efficiency CdTe/CdS thin film solar cells. During the fabrication of CdTe/CdS full cell, high temperature is required for CdTe thin films for inter-diffusion of CdS and CdTe to form CdS1-xTex layer. The formation of a CdS1–xTex mixed crystal layer modifies the electrical junction into the CdTe improving the electrical and photovoltaic characteristics of the junction [24]. The extent of inter-diffusion depends on the rate of deposition and the substrate temperature [25].

**Figure 5.** A typical temperature profile for CdTe growth of 5 μm and above.

In many literatures, it is found that the high conversion efficiencies obtained from CSS CdTe films have been attributed to the high substrate temperature. The use of high substrate temperatures is considered to uphold the interface reaction between the CdS and CdTe. Considering all the above facts, the temperature profile (described as heat profile in some literatures) is highlighted strongly. In order to achieve high efficiency, it is more significant to attain high-quality CSS CdTe films. Several temperature profiles have been utilized to differentiate the cell performance. The temperature profile of the growth process is illustrated in **Figure 5**.

### **2.4. High‐efficiency CdTe solar cells (>5 μm) from CSS technique**

sequence starts, and the temperature monitoring begins. The superstrate configuration is basically used to deposit CdTe on top of CdS deposited films. The system is maintained at the set temperatures by means of radiative heating using a total of 2 kW halogen lamps on top and bottom. The thermocouples are inserted into the graphite holders to monitor the growth

The substrate temperature of CdTe thin films during the growth is an essential parameter to obtain high-quality films ultimately resulting in high-efficiency CdTe/CdS thin film solar cells. During the fabrication of CdTe/CdS full cell, high temperature is required for CdTe thin films for inter-diffusion of CdS and CdTe to form CdS1-xTex layer. The formation of a CdS1–xTex mixed crystal layer modifies the electrical junction into the CdTe improving the electrical and photovoltaic characteristics of the junction [24]. The extent of inter-diffusion depends on the

**Figure 4.** Inside view of the CSS chamber with spacers placed on top of bottom holder.

**2.3. Significance of temperature profiles**

rate of deposition and the substrate temperature [25].

**Figure 5.** A typical temperature profile for CdTe growth of 5 μm and above.

temperatures (**Figure 4**).

368 Modern Technologies for Creating the Thin-film Systems and Coatings

Although a number of important factors in CSS growth, which are supposed to bear significant roles in overall performance, are demonstrated earlier, the most important issue is to grow high-quality CdTe film regardless of its growth technique. To study as well as to make an effort in achieving such high-quality CdTe films, several temperature profiles were adopted during the growth by CSS here. All these temperature profiles were evaluated from their related results such as solar cell characteristics. In general, it has been noticed that the temperature profiles have important effect on the crystallinity as well as the thickness of the CdTe layer (**Figure 6**).

**Figure 6.** FESEM cross-sectional view of CdTe solar cell grown by CSS.

The most common temperature profile, which is recommended by the leading comrades in this field of CdTe solar cell fabrication, consisted of three steps with all high temperatures is shown in **Figure 5**. After the initial ramp-up step, the second step is supposed to have a great role in the crystallization of CdTe. The third step is important for stabilizing the microcrystals and the gaining the growth of CdTe layer. A part of the study also found unintentional growth of CdTe during the rise of temperature at the first step of the temperature profile. Even at low temperatures, small seed grains of CdTe were observed. Therefore, investigations have been carried out for all possible temperature profiles in high temperature region and finally proposed the optimum one, as shown earlier in **Figure 5**, for better quality CdTe films (5–7  μm thick). The key point of this temperature profiles lies in the reverse mode during temperature rise as substrate temperature gets higher than the source temperature at any time dimension until the set temperature. Once the substrate temperature reaches its set point (e.g., 595°C), the source temperature increases until its set point (e.g., 625°C) and then both of them keep the set temperature constant, thus create a temperature difference of 30°C viable for CdTe growth in this case. This type of reverse mode in temperature rise could control the unintentional growth of CdTe at lower temperatures, which (CdTe) is suspected to be lower in quality with pinholes etc. In the modified temperature profile, the unexpected CdTe growth was suppressed to as low as 0.15 μm when the temperature rise of both source and substrate was stopped at the set point of substrate temperature. These CdTe films are then used to make full solar cell devices with the configuration of glass/ITO/CdS/CdTe/C:Cu/Ag. The conversion efficiency of 15.31% with open-circuit voltage (Voc) of 0.811 V, short-circuit current (Jsc) of 26.3 mA/cm2 , fill factor (F.F.) of 0.718 was achieved for the best cell as the highest to date found in the current configuration and process. The thickness of CdTe layer was about 7 μm, and the effective area of the cell was 1 cm2 . The current-voltage (J-V) characteristics are shown in **Figure 7**.

**Figure 7.** J-V characteristics of the CdTe thin film solar cell.

#### **2.5. Thickness reduction in CdTe by low‐temperature growth**

The key concern is to obtain high-quality CdTe films regardless of the growth technique. Typically, the temperature corresponds to the source temperature of 625°C and substrate temperature of 595°C for high-efficiency cells, where 5-μm-thick CdTe is grown in this profile.

Close‐Spaced Sublimation (CSS): A Customisable, Low‐Cost, High‐Yield Deposition System for Cadmium... http://dx.doi.org/10.5772/66040 371

**Figure 8.** Temperature profile for low-temperature growth of CdTe.

carried out for all possible temperature profiles in high temperature region and finally proposed the optimum one, as shown earlier in **Figure 5**, for better quality CdTe films (5–7  μm thick). The key point of this temperature profiles lies in the reverse mode during temperature rise as substrate temperature gets higher than the source temperature at any time dimension until the set temperature. Once the substrate temperature reaches its set point (e.g., 595°C), the source temperature increases until its set point (e.g., 625°C) and then both of them keep the set temperature constant, thus create a temperature difference of 30°C viable for CdTe growth in this case. This type of reverse mode in temperature rise could control the unintentional growth of CdTe at lower temperatures, which (CdTe) is suspected to be lower in quality with pinholes etc. In the modified temperature profile, the unexpected CdTe growth was suppressed to as low as 0.15 μm when the temperature rise of both source and substrate was stopped at the set point of substrate temperature. These CdTe films are then used to make full solar cell devices with the configuration of glass/ITO/CdS/CdTe/C:Cu/Ag. The conversion efficiency of 15.31% with open-circuit voltage (Voc) of 0.811 V, short-circuit current (Jsc) of

, fill factor (F.F.) of 0.718 was achieved for the best cell as the highest to date found

. The current-voltage (J-V) characteristics are shown in

in the current configuration and process. The thickness of CdTe layer was about 7 μm, and the

The key concern is to obtain high-quality CdTe films regardless of the growth technique. Typically, the temperature corresponds to the source temperature of 625°C and substrate temperature of 595°C for high-efficiency cells, where 5-μm-thick CdTe is grown in this profile.

26.3 mA/cm2

**Figure 7**.

effective area of the cell was 1 cm2

370 Modern Technologies for Creating the Thin-film Systems and Coatings

**Figure 7.** J-V characteristics of the CdTe thin film solar cell.

**2.5. Thickness reduction in CdTe by low‐temperature growth**

**Figure 9.** Effect of growth time on the (a) solar cell performance and (b) thickness of CdTe films, grown at low temperature.

Therefore, the temperature reduction in both the source and the substrate was carried out with the aim of obtaining a high-quality, as well as thinner, CdTe layer. No CdTe growth was observed until the source temperature of 530°C and substrate temperature of 500°C were maintained. In the profile shown in **Figure 8**, both the source and the substrate were first heated to a high temperature of 595°C for surface cleaning and then lowered to 575 or 565°C in the case of the source and 550°C in the case of the substrate. Controlling the time in step 4, thickness control (reduction) of the CdTe layer to 1.5 and 2 μm was achieved for source temperatures of 565 and 575°C, respectively.

**Figure 9** illustrates the effect of growth time in step 4 on the thickness of the CdTe layer, as well as on the overall performance of solar cells. Thickness could be controlled down to 1.5  μm with a conversion efficiency of 10.4% (Voc: 0.76 V, Jsc: 22 mA/cm2 , F.F.: 0.62, area: 1 cm2 ) at a source temperature of 565°C. Meanwhile, if the source temperature is increased to 575°C while keeping the substrate temperature unchanged at 550°C, the growth rate remarkably increases. However, the thickness can be controlled down to 2 μm with this profile, with an efficiency of 10.8% (Voc: 0.76 V, Jsc: 22.7 mA/cm2 , F.F.: 0.63, area: 1 cm2 ). Therefore, a low-growth temperature demonstrates lower possibility of achieving thin CdTe films (1 μm) with high efficiency.

### **2.6. Control of CdTe thickness by reducing temperature difference**

With the aim of achieving high-quality thin films and considering the results presented in previous section, the temperatures of the source and the substrate were increased to 600 and 595°C, respectively. The temperature profile is shown in **Figure 10**.

Moreover, the temperature difference between the two was set to 5°C to reduce the thickness. Thickness control down to 2 μm was possible with an efficiency of 11% (Voc: 0.77 V, Jsc: 23.7  mA/cm2 , F.F.: 0.60, area:1 cm2 ). The overall performance deteriorated with the decrease in thickness but showed an improvement compared to the cells grown at lower temperatures as can be found in **Figure 11**. Regardless of the temperature profiles, all the cell performances were affected with the decrease in CdTe film thickness.

**Figure 10.** Temperature profile for CdTe growth in CSS with minimal temperature difference between source and substrate.

Close‐Spaced Sublimation (CSS): A Customisable, Low‐Cost, High‐Yield Deposition System for Cadmium... http://dx.doi.org/10.5772/66040 373

**Figure 11.** Characteristics of CdTe solar cells (a) and thickness of CdTe films (b), deposited using minimal temperature difference between source and substrate in CSS.

#### **2.7. Growth of 1 μm‐CdTe thin films by CSS**

Therefore, the temperature reduction in both the source and the substrate was carried out with the aim of obtaining a high-quality, as well as thinner, CdTe layer. No CdTe growth was observed until the source temperature of 530°C and substrate temperature of 500°C were maintained. In the profile shown in **Figure 8**, both the source and the substrate were first heated to a high temperature of 595°C for surface cleaning and then lowered to 575 or 565°C in the case of the source and 550°C in the case of the substrate. Controlling the time in step 4, thickness control (reduction) of the CdTe layer to 1.5 and 2 μm was achieved for source temperatures of

**Figure 9** illustrates the effect of growth time in step 4 on the thickness of the CdTe layer, as well as on the overall performance of solar cells. Thickness could be controlled down to 1.5 

a source temperature of 565°C. Meanwhile, if the source temperature is increased to 575°C while keeping the substrate temperature unchanged at 550°C, the growth rate remarkably increases. However, the thickness can be controlled down to 2 μm with this profile, with an

temperature demonstrates lower possibility of achieving thin CdTe films (1 μm) with high

With the aim of achieving high-quality thin films and considering the results presented in previous section, the temperatures of the source and the substrate were increased to 600 and

Moreover, the temperature difference between the two was set to 5°C to reduce the thickness. Thickness control down to 2 μm was possible with an efficiency of 11% (Voc: 0.77 V, Jsc: 23.7 

thickness but showed an improvement compared to the cells grown at lower temperatures as can be found in **Figure 11**. Regardless of the temperature profiles, all the cell performances

**Figure 10.** Temperature profile for CdTe growth in CSS with minimal temperature difference between source and sub-

, F.F.: 0.63, area: 1 cm2

). The overall performance deteriorated with the decrease in

, F.F.: 0.62, area: 1 cm2

). Therefore, a low-growth

) at

μm with a conversion efficiency of 10.4% (Voc: 0.76 V, Jsc: 22 mA/cm2

**2.6. Control of CdTe thickness by reducing temperature difference**

595°C, respectively. The temperature profile is shown in **Figure 10**.

were affected with the decrease in CdTe film thickness.

efficiency of 10.8% (Voc: 0.76 V, Jsc: 22.7 mA/cm2

372 Modern Technologies for Creating the Thin-film Systems and Coatings

, F.F.: 0.60, area:1 cm2

565 and 575°C, respectively.

efficiency.

mA/cm2

strate.

As described in earlier sections, several temperature profiles have been used during CSS growth of CdTe to determine the effect of temperature. It has become apparent that temperature plays a significant role in thickness control. Following the temperature profile shown in **Figure 12**, the temperatures of both the substrate and the source were raised together to several peak temperatures, and it was possible to grow thin CdTe layers with thickness from 0.5 to 1.5 μm for peak temperatures from 595 to 620°C.

**Figure 12.** Temperature profile of 1-μm-thick CdTe thin film growth by CSS.

**Figure 13.** XRD patterns of CdTe films of different thicknesses.

To determine the performances of solar cells with various CdTe layer thicknesses, the I-V characteristics, spectral response, and reflectance were measured, and scanning electron microscope (SEM) and AFM images were taken to evaluate the quality of the films. From the XRD measurement, significant difference among the CdTe films from 5 to 1 μm thick can be observed, as shown in **Figure 13**. In the case of 1 μm films, some other peaks, probably from ITO or CdS, were found. Compared to thicker films, all the 1-μm-thick films showed strong preferential orientation in the (111) direction. SEM images of the 1-μm-thick CdTe surface shown in **Figure 14(a)** exhibit the larger grain size of the films. Differently shaped grains have been found in the case of thicker CdTe films.

Close‐Spaced Sublimation (CSS): A Customisable, Low‐Cost, High‐Yield Deposition System for Cadmium... http://dx.doi.org/10.5772/66040 375

**Figure 12.** Temperature profile of 1-μm-thick CdTe thin film growth by CSS.

374 Modern Technologies for Creating the Thin-film Systems and Coatings

**Figure 13.** XRD patterns of CdTe films of different thicknesses.

been found in the case of thicker CdTe films.

To determine the performances of solar cells with various CdTe layer thicknesses, the I-V characteristics, spectral response, and reflectance were measured, and scanning electron microscope (SEM) and AFM images were taken to evaluate the quality of the films. From the XRD measurement, significant difference among the CdTe films from 5 to 1 μm thick can be observed, as shown in **Figure 13**. In the case of 1 μm films, some other peaks, probably from ITO or CdS, were found. Compared to thicker films, all the 1-μm-thick films showed strong preferential orientation in the (111) direction. SEM images of the 1-μm-thick CdTe surface shown in **Figure 14(a)** exhibit the larger grain size of the films. Differently shaped grains have

**Figure 14.** SEM micrographs of the surfaces of (a) 1 μm, (b) 2 μm, (c) 4 μm and (d) 6 μm-thick CdTe films [24].

Films deposited at substrate temperatures of 550–620°C exhibited preferential orientation along the (111) direction as indicated by X-ray diffraction studies. Scanning electron microscope (SEM) images also revealed the high quality of the deposited films of CdTe. The microstructure of CdTe films depends on the substrate temperature, source-substrate temperature gradient, and the crystallinity of the substrate. In general, the grain size increases with the increasing substrate temperature and film thickness.

An efficiency of 8.3% (Voc: 0.73 V, Jsc: 20.2 mA/cm2 , F.F.: 0.57, area: 1 cm2 ) was achieved for cells with 0.5-μm-thick CdTe, whereas 9.9% (Voc: 0.75 V, Jsc: 22 mA/cm2 , F.F.: 0.6, area: 1 cm2 ) and 11.4% (Voc: 0.77 V, Jsc: 23.7 mA/cm2 , F.F.: 0.63, area: 1 cm2 ) were achieved for solar cells with 1 and 1.5-μm-thick CdTe, respectively. The most significant achievement of this effort was the establishment of the growth technique of such thin, high-quality CdTe films, along with reproducibility.

#### **2.8. Overall optimization of CSS grown CdTe solar cells**

To be used as a working device, optimization is needed. In order to form an ohmic contact to CdTe thin films, Cu-doped graphite carbon paste was screen-printed, and then, the resulting stacks were subjected to annealing in controlled atmospheres. One related study showed that Cu is distributed as effective acceptors in the CdTe layer, rendering it p-type after annealing. Despite having optimum data for 5-μm-thick CdTe layers, optimization of the annealing temperature was carried out. Significant improvement was found in comparatively lowtemperature regions. An excellent improvement in efficiency to 11.2% (Voc: 0.77 V, Jsc: 23.1  mA/cm2 , F.F.: 0.63, area: 1 cm2 ) was achieved at the annealing temperature of 345°C. The current-voltage (I-V) characteristic is shown in **Figure 15**. Copper, which is believed to have diffused into the CdTe layer from the carbon layer, functioned as an effective acceptor after annealing at the optimum temperature. Supported by the spectral response data where the cells treated at high temperature exhibit a significant shift near the CdTe absorption edge suggests a possible inter-diffusion of CdS into the CdTe and causes the bandgap of this material to decrease slightly. Since the junction is exposed to such high temperatures, excessive interdiffusion (which forms CdSxTe1-x) is believed to occur, which leads to almost complete consumption of the 1-μm-thick CdTe layer grown during CSS. Therefore, it can be concluded that the uncontrolled formation of such CdSxTe1-x alloy in 1-μm-thick CdTe solar cells can adversely affect the cell performance, which is not possible to overcome by merely treating the stack of layers in any optimum conditions. Therefore, growth during CSS has to be carefully controlled by temperature profiles to achieve high-quality films in terms of grain size, defects, uniformity or homogeneity.

#### **2.9. Conclusion**

Close-spaced sublimation (CSS) is one of the most cost-efficient, high-throughput semiconductor coating techniques that offers industrial scalability as well. We developed our custom-

**Figure 15.** J-V characteristics of 1 -μm-thick CdTe solar cell (top record to date) [24].

ized close-spaced sublimation (CSS) apparatus and achieved growth of our material of interest, such as group II-VI compound semiconductor, that is, CdTe for solar cell application. By controlling the temperature profile in steps, thickness of the CdTe film is controlled over 7 to 1 μm without any pinholes in order to realize material conservation as well as to improve the performance through controlling the carrier recombination loss in relatively thicker CdTe absorption layers. The films are investigated by all possible means of thin film characterization like XRD, SEM, UV-Vis, and so on to find its optimized usage in thin film solar cell. The growth of 1-μm-thick CdTe films was achieved by controlling the temperature profile during CSS growth, with reproducibility. Gradual improvements were found in the glass/ITO/CdS/CdTe solar cells as conversion efficiencies of 15.3% for 7-μm-thick, 14.3% for 5-μm-thick, 11.4% for 1.5-μm-thick, and 9.9% for 1-μm-thick CdTe films grown by CSS. Moreover, after a rigorous optimization in post-deposition and back electrode formation annealing profiles, conversion efficiency of 11.2% in the case of 1-μm-thick CdTe was achieved, in air mass 1.5 without antireflection coating as the best value to date. All the results have shown successful deposition of CdTe by this customized close-spaced sublimation (CSS) technique and therefore verify its implication to similar kind of semiconductors for solar cells or other purposes.

### **Acknowledgements**

**2.8. Overall optimization of CSS grown CdTe solar cells**

376 Modern Technologies for Creating the Thin-film Systems and Coatings

, F.F.: 0.63, area: 1 cm2

uniformity or homogeneity.

**2.9. Conclusion**

mA/cm2

To be used as a working device, optimization is needed. In order to form an ohmic contact to CdTe thin films, Cu-doped graphite carbon paste was screen-printed, and then, the resulting stacks were subjected to annealing in controlled atmospheres. One related study showed that Cu is distributed as effective acceptors in the CdTe layer, rendering it p-type after annealing. Despite having optimum data for 5-μm-thick CdTe layers, optimization of the annealing temperature was carried out. Significant improvement was found in comparatively lowtemperature regions. An excellent improvement in efficiency to 11.2% (Voc: 0.77 V, Jsc: 23.1 

current-voltage (I-V) characteristic is shown in **Figure 15**. Copper, which is believed to have diffused into the CdTe layer from the carbon layer, functioned as an effective acceptor after annealing at the optimum temperature. Supported by the spectral response data where the cells treated at high temperature exhibit a significant shift near the CdTe absorption edge suggests a possible inter-diffusion of CdS into the CdTe and causes the bandgap of this material to decrease slightly. Since the junction is exposed to such high temperatures, excessive interdiffusion (which forms CdSxTe1-x) is believed to occur, which leads to almost complete consumption of the 1-μm-thick CdTe layer grown during CSS. Therefore, it can be concluded that the uncontrolled formation of such CdSxTe1-x alloy in 1-μm-thick CdTe solar cells can adversely affect the cell performance, which is not possible to overcome by merely treating the stack of layers in any optimum conditions. Therefore, growth during CSS has to be carefully controlled by temperature profiles to achieve high-quality films in terms of grain size, defects,

Close-spaced sublimation (CSS) is one of the most cost-efficient, high-throughput semiconductor coating techniques that offers industrial scalability as well. We developed our custom-

**Figure 15.** J-V characteristics of 1 -μm-thick CdTe solar cell (top record to date) [24].

) was achieved at the annealing temperature of 345°C. The

The authors would like to acknowledge and appreciate the contribution of The National University of Malaysia (Universiti Kebangsaan Malaysia) through the research grants with code DIP-2015-021 and GUP-2016-042.

### **Author details**

Nowshad Amin\* and Kazi Sajedur Rahman

\*Address all correspondence to: nowshad@ukm.edu.my

The National University of Malaysia, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia

### **References**


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[14] Ringel, S. A., Smith, A. W., MacDougal, M. H., Rohatgi, A. The effects of CdCl2 on the electronic properties of molecular‐beam epitaxially grown CdTe/CdS heterojunction

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#### **Effects of CdCl2 Treatment on Physical Properties of CdTe/CdS Thin Film Solar Cell Effects of CdCl2 Treatment on Physical Properties of CdTe/CdS Thin Film Solar Cell**

Nazar Abbas Shah, Zamran Rabeel, Murrawat Abbas and Waqar Adil Syed Nazar Abbas Shah, Zamran Rabeel Murrawat Abbas and Waqar Adil Syed

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67191

#### **Abstract**

We report CdTe, CdS, and ITO thin films on glass substrates for solar cell fabrication by closed space sublimation and chemical bath deposition. CdTe and CdS thin films were sublimated to chemical treatment at 25°C in a saturated CdCl2 solution (1.04 g/100 ml methanol) and heat treated at 400°C for 30 minutes. Indium tin oxide and tellurium films were analyzed by spectrophotometer and scanning electron microscopy. It has been observed that solar cell performance can be improved by depositing a CdCl2 layer on the CdTe/CdS layers. The optical, structural, and morphological changes of CdTe and CdS surfaces on CdTe/CdS/ITO/glass solar cells due to CdCl2 solution treatment followed by annealing for 400°C were studied. Optical analysis showed about 15% decrease in transmittance after CdCl2 heat treatment in case of CdTe thin film, whereas CdS thin film demonstrated an increase of about 10–15% transmittance after CdCl2 heat treatment. Similarly, a decrease in band gap values was found for both CdTe and CdS thin films after CdCl2 heat treatment. XRD and SEM results of CdCl2 heat‐treated CdTe and CdS samples showed recrystallization, reorientation, and progressive increase in grain size. The grain sizes of CdTe and CdS samples demonstrated an increase of about 0.2 µm.

**Keywords:** CdCl2 , cadmium sulfide, cadmium telluride, heat treatment, morphological, optical, structural

### **1. Introduction**

Considering the high absorption coefficient, near optimum band gap, and manufacturability of cadmium telluride (CdTe), it can quite easily be regarded as one of the most favorable photovol‐ taic materials realizable for use as high‐efficiency and low‐cost thin film solar cell. Typically, a

and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CdTe solar cell structure consists of Au/CdTe/CdS/ITO/glass. It has a direct band gap of 1.45 eV and only 1 µm CdTe can absorb more than 90% of the photons having energy greater than 1.45 eV that is why it is well suited for solar cell applications. CdTe is stable up to 500°C. CdTe has lattice constant of 0.68 nm at 300 K. The thermal conductivity of CdTe is 6.2 W m/m2 K at 293 K. The specific heat capacity is about 210 J/kg K at 293 K. In solar cell fabrication CdTe is used as a p‐type semiconductor which has a junction with cadmium sulfide (CdS) as an n‐type semi‐ conductor. Infrared detector material (HgCdTe) is manufactured when CdTe is alloyed with mercury. An important application of CdTe is as a radiation detector that is used to detect X‐ray and alpha, beta, and gamma rays, and for this purpose, CdTe is doped with chlorine. CdTe has electron affinity of 4.3 eV. The work function is 5.5 eV for p‐type CdTe. It can be doped both as p‐type as well as n‐type semiconductors [1–3].

A II‐VI semiconductor compound cadmium sulfide (CdS) has much importance because of its many applications in several heterojunction photovoltaic thin film devices of CdTe, copper indium gallium diselenide (Cu(In*<sup>x</sup>* Ga1‐*<sup>x</sup>* )Se2 ), or copper indium gallium sulfide (CIGS) and solar cells. Many other devices in the fields of electronics, optics, and infrared are also fabri‐ cated by using CdS [1–3]. The CdS is a suitable n‐type material, which can be fabricated by a variety of fabrication techniques like sol‐gel technique [4], close spaced sublimation (CSS), chemical bath deposition (CBD), thermal evaporation, chemical vapor deposition, molecu‐ lar beam epitaxy, and spray pyrolysis. Each and every deposition process provides different optical, structural, electrical, and morphological properties [5–10].

First solar cell has reported CdS deposited by high rate vapor transport deposition (HRVTD). CdS is called window layer due to its higher rate of light transmission [11–14]. CdS thin films are very suitable for so many other semiconductor devices and radiation detectors. CdS thin films with wide band gap are highly used for photovoltaic devices. Since CdS is used as window layer in solar cell, it should be fabricated very thin and high amount of light should pass through CdS and be absorbed in CdTe. The CdS thin film must be continuous to reduce the effect of short circuit in the cell. The CdS thin films deposited by CBD can fulfill these requirements. So the thickness of CdS has much importance for the high‐efficiency of solar cell [15–17]. CdS (CBD) thin films are grown by cadmium chloride, ammonium nitrate, and potassium hydroxide. After heating to a specific temperature, thiourea is added to start the fabrication process [18–20].

CBD is a very easy process for the fabrication of CdS thin films on ITO glass. For the fabri‐ cation of thin film solar cell, one needs a very thin film up to 60–80 nm. It is a very suitable process especially for solar cell point of view. It is a process in which substrate is placed in a hot chemical solution stirring vigorously for a specific time, positive and negative ions will reach and meet on the substrate and thin film is grown. The advantage of this technique is that neither vacuum and nor very high temperature is required for CBD [5, 21].

CdS thin films fabricated by the CSS are also another moderate temperature fabrication technique in a vacuum chamber. More than 20% efficiency has been achieved by using CdTe/CdS heterojunction thin film solar cell by this technique. CSS is a moderate tempera‐ ture procedure so it provides better results in some cases [22, 23]. The CSS technique is one of the techniques that have produced encouraging results [15] mainly because it is a simple deposition apparatus and high transport efficiency and deposition can be done in low vacuum at moderate temperatures. There is minimum use of material in the CSS sys‐ tem as compared to other methods. As the substrate is close to the source materials in the CSS technique, roughness increases in the thin films and has high absorption, which makes it a suitable material for solar cell applications. The CSS has a disadvantage as one cannot introduce a thickness monitor.

CdTe solar cell structure consists of Au/CdTe/CdS/ITO/glass. It has a direct band gap of 1.45 eV and only 1 µm CdTe can absorb more than 90% of the photons having energy greater than 1.45 eV that is why it is well suited for solar cell applications. CdTe is stable up to 500°C. CdTe has

The specific heat capacity is about 210 J/kg K at 293 K. In solar cell fabrication CdTe is used as a p‐type semiconductor which has a junction with cadmium sulfide (CdS) as an n‐type semi‐ conductor. Infrared detector material (HgCdTe) is manufactured when CdTe is alloyed with mercury. An important application of CdTe is as a radiation detector that is used to detect X‐ray and alpha, beta, and gamma rays, and for this purpose, CdTe is doped with chlorine. CdTe has electron affinity of 4.3 eV. The work function is 5.5 eV for p‐type CdTe. It can be doped both as

A II‐VI semiconductor compound cadmium sulfide (CdS) has much importance because of its many applications in several heterojunction photovoltaic thin film devices of CdTe, copper

solar cells. Many other devices in the fields of electronics, optics, and infrared are also fabri‐ cated by using CdS [1–3]. The CdS is a suitable n‐type material, which can be fabricated by a variety of fabrication techniques like sol‐gel technique [4], close spaced sublimation (CSS), chemical bath deposition (CBD), thermal evaporation, chemical vapor deposition, molecu‐ lar beam epitaxy, and spray pyrolysis. Each and every deposition process provides different

First solar cell has reported CdS deposited by high rate vapor transport deposition (HRVTD). CdS is called window layer due to its higher rate of light transmission [11–14]. CdS thin films are very suitable for so many other semiconductor devices and radiation detectors. CdS thin films with wide band gap are highly used for photovoltaic devices. Since CdS is used as window layer in solar cell, it should be fabricated very thin and high amount of light should pass through CdS and be absorbed in CdTe. The CdS thin film must be continuous to reduce the effect of short circuit in the cell. The CdS thin films deposited by CBD can fulfill these requirements. So the thickness of CdS has much importance for the high‐efficiency of solar cell [15–17]. CdS (CBD) thin films are grown by cadmium chloride, ammonium nitrate, and potassium hydroxide. After heating to a specific temperature, thiourea is added to start the

CBD is a very easy process for the fabrication of CdS thin films on ITO glass. For the fabri‐ cation of thin film solar cell, one needs a very thin film up to 60–80 nm. It is a very suitable process especially for solar cell point of view. It is a process in which substrate is placed in a hot chemical solution stirring vigorously for a specific time, positive and negative ions will reach and meet on the substrate and thin film is grown. The advantage of this technique is that

CdS thin films fabricated by the CSS are also another moderate temperature fabrication technique in a vacuum chamber. More than 20% efficiency has been achieved by using CdTe/CdS heterojunction thin film solar cell by this technique. CSS is a moderate tempera‐ ture procedure so it provides better results in some cases [22, 23]. The CSS technique is one of the techniques that have produced encouraging results [15] mainly because it is a

neither vacuum and nor very high temperature is required for CBD [5, 21].

), or copper indium gallium sulfide (CIGS) and

K at 293 K.

lattice constant of 0.68 nm at 300 K. The thermal conductivity of CdTe is 6.2 W m/m2

Ga1‐*<sup>x</sup>* )Se2

optical, structural, electrical, and morphological properties [5–10].

p‐type as well as n‐type semiconductors [1–3].

382 Modern Technologies for Creating the Thin-film Systems and Coatings

indium gallium diselenide (Cu(In*<sup>x</sup>*

fabrication process [18–20].

The general working properties of solar cells can be best described by three parameters: short circuit current (ISC), open circuit voltage (VOC), and fill factor (FF). Postdeposition process‐ ing of polycrystalline CdTe/CdS heterojunction thin film solar cells with cadmium chloride (CdCl2 ) heat treatment has been demonstrated to improve the short circuit current and open circuit voltage of CdTe/CdS thin film solar cell, by recrystallization, reorientation, and grain enhancement of films for photovoltaic operation [24–26]. Heat treatment with CdCl2 has been known to be a key step in high quality CdTe/CdS solar cells preparation [27–31]. Irrespective of the method being used for deposition of CdTe and CdS layer, CdCl2 treatment has become a customary and vital process in fabrication of high‐efficiency CdTe/CdS‐based photovoltaic devices. Three different methods of CdCl2 treatment are widely known to be used; solution CdCl2 treatment, evaporated CdCl2 treatment, and vapor CdCl2 treatment [12]. CdCl2 treat‐ ment is basically known to activate a chemical reaction between CdTe and CdS, which is the driving force for the bulk and grain‐boundary inter‐diffusion of CdTe and CdS [13]. However, regardless of the CdCl2 treatment method being used, the basic mechanism by which CdCl2 effects CdTe and CdS can be expected as a similar process [12–15].

The optical, structural, and morphological properties of CdTe and CdS thin films are mainly subjective to the preparation route. Hence, a variety of methods have been applied for the synthesis of such materials, e.g., thermal evaporation [16], chemical pyrolysis deposition (CPD) [17], metal organic chemical‐vapor deposition (MOCVD) [18–20], closed space subli‐ mation (CSS) [21] and chemical bath deposition (CBD) [22–25]. CSS and CBD are known to produce optimal and encouraging results for CdTe/CdS based solar cells. Both the techniques have many advantages for production of photovoltaic devices under controlled conditions, such as exceptional uniformity and reproducibility of film thickness even for a large‐scale module [26, 27].

There are different materials being used for the manufacturing of thin film solar cells. The silicon photovoltaic cells are covering more than 70% of the world market. Silicon solar cells containing amorphous, polycrystalline, crystalline, and now a days silicon thin film solar cells are being used. Thin film solar cells (TFSC) containing (II–VI) and (III–V) semiconductor materials have high efficiencies. These include copper indium gallium sulfide (CIGS), GaAs, Cu (InGa)Se2 , and CdTe/CdS. The selection of material depends upon the band gap, absorbing ability of material, and cost of fabrication process.

CdTe is very suitable for thin film solar cells because it has direct band gap at room tempera‐ ture. In the world PV market, the CdTe based solar cells have attained 16.5% efficiency. It has very good match with CdS on ITO glass substrate. CdS is used as a window layer; it means more than 70% of light will pass through the first n‐type layer of solar cells. Different tech‐ niques can be used for the manufacturing of thin film CdTe based solar cells [19–26].

It has some native defect cation vacancies (Vcd) that can behave as double accepters and anion vacancies (VTe) can behave as double donors. These vacancies can be formed with other extrinsic impurities [5, 6]. A problem of CdTe is that it is toxic if ingested and if its powder is inhaled. If properly processed, manufactured, and encapsulated, then it may be harmless. It is observed that elemental cadmium is more toxic than CdTe. It is also studied that CdTe quantum dots causes extensive reactive oxygen damage to cell membrane, mitochondria, and cell nucleus.

If it is to be used at large scale commercialization of solar panels then exposure and long term safety of CdTe will be serious issue. So attempts are being made to overcome all these issues. The BNL has given the research that CdTe large‐scale PV modules have neither any health risks nor any threats for environment. These modules can be recycled. These modules do not produce any pollutants during their working for long term [7, 8]. Major issue for the CdTe PV modules is the availability of tellurium. Actually, tellurium is an extremely rare element (1–5) parts per billion in the earth's crust. Manufacturing of CdTe PV solar panels at large‐commer‐ cial scale will cause depletion of tellurium [9].

In the present work, effects of CdCl2 thermal treatment on physical properties of CdTe/ CdS heterojunction solar cells, fabricated using CSS and CBD, have been investigated. Correspondingly, the results have demonstrated that performance of solar cell can be improved significantly after this treatment. In this regard, optical properties such as film thickness, refractive index, absorption, and optical band gap, crystallographic properties such as crystallite size and plane orientation, and morphological properties such as grain size, have been investigated using UV‐VIS‐IR spectrophotometer, X‐ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, at room temperature (25°C) [28–31].

### **2. Why thin films**

Thin film technology is well known technology for physics and engineering applications. It is projected to be one of the major processing techniques to fabricate electronic, optical, and magnetic data storage devices, solar cells, light emitting diodes (LED), etc.

Thin film science and technology play a crucial role in the high‐tech industries that will bear the main burden of future, while the major exploitation of thin films are in microelectronics, communications, optical electronics, coatings of all kinds, and energy generation. Thin films of various materials are currently in use as protective and optical coatings, electronics, antire‐ flection films, polarizers, radiation detectors, and solar energy converters.

It is not only thickness that is defining a film but also the way of fabrication which is respon‐ sible for the uniformity, structure, and other properties of the film. The optical, structural, and electrical properties of thin film are different from the bulk materials; it is the reason these are being used. Characterization and fabrication of thin films is very easy. The structural, chemical and physical properties depend on the deposition parameters and thickness of the thin film. The electrical, optical, and mechanical behavior of thin film also depends on microstructure, surface morphology, purity, and homogeneity. These things are strongly dependent on the fabrication methods and selected parameters and post depositions treatments also.

It has some native defect cation vacancies (Vcd) that can behave as double accepters and anion vacancies (VTe) can behave as double donors. These vacancies can be formed with other extrinsic impurities [5, 6]. A problem of CdTe is that it is toxic if ingested and if its powder is inhaled. If properly processed, manufactured, and encapsulated, then it may be harmless. It is observed that elemental cadmium is more toxic than CdTe. It is also studied that CdTe quantum dots causes extensive reactive oxygen damage to cell membrane, mitochondria, and

If it is to be used at large scale commercialization of solar panels then exposure and long term safety of CdTe will be serious issue. So attempts are being made to overcome all these issues. The BNL has given the research that CdTe large‐scale PV modules have neither any health risks nor any threats for environment. These modules can be recycled. These modules do not produce any pollutants during their working for long term [7, 8]. Major issue for the CdTe PV modules is the availability of tellurium. Actually, tellurium is an extremely rare element (1–5) parts per billion in the earth's crust. Manufacturing of CdTe PV solar panels at large‐commer‐

CdS heterojunction solar cells, fabricated using CSS and CBD, have been investigated. Correspondingly, the results have demonstrated that performance of solar cell can be improved significantly after this treatment. In this regard, optical properties such as film thickness, refractive index, absorption, and optical band gap, crystallographic properties such as crystallite size and plane orientation, and morphological properties such as grain size, have been investigated using UV‐VIS‐IR spectrophotometer, X‐ray diffraction (XRD) and scanning

Thin film technology is well known technology for physics and engineering applications. It is projected to be one of the major processing techniques to fabricate electronic, optical, and

Thin film science and technology play a crucial role in the high‐tech industries that will bear the main burden of future, while the major exploitation of thin films are in microelectronics, communications, optical electronics, coatings of all kinds, and energy generation. Thin films of various materials are currently in use as protective and optical coatings, electronics, antire‐

It is not only thickness that is defining a film but also the way of fabrication which is respon‐ sible for the uniformity, structure, and other properties of the film. The optical, structural, and electrical properties of thin film are different from the bulk materials; it is the reason these are being used. Characterization and fabrication of thin films is very easy. The structural, chemical and physical properties depend on the deposition parameters and thickness of the thin film. The electrical, optical, and mechanical behavior of thin film also depends on microstructure,

electron microscopy (SEM), respectively, at room temperature (25°C) [28–31].

magnetic data storage devices, solar cells, light emitting diodes (LED), etc.

flection films, polarizers, radiation detectors, and solar energy converters.

thermal treatment on physical properties of CdTe/

cell nucleus.

cial scale will cause depletion of tellurium [9].

384 Modern Technologies for Creating the Thin-film Systems and Coatings

In the present work, effects of CdCl2

**2. Why thin films**

Stable, accurate, efficient, reliable low‐cost electronic, insulating, sensors, and many other industrial devices with minimum use of material are possible due to thin film technology. Thin film devices are easy to manufacture, especially suitable for flexible solar cells.

Thin films are layers of ferromagnetic, semiconductor or ceramic materials. It ranges from fractions of a nanometer to several micrometers in thickness (usually 100 A‐1 µm). It is a microscopically thin layer of material that is deposited onto a metal, ceramic, semiconductor or plastic base. Thin Films can be conductive or dielectric (non‐conductive) and are used in myriad applications. Thin films have almost a two dimensional structure, so they give a bet‐ ter insight into the structural properties of the material. The earliest of what might be called latest thin film optics was discovered by Robert Boyle and Robert Hooke, independently of the phenomenon now known of older material even without any visible signs of tarnish, was too low. One possible explanation which he suggested was the formation on the surface of the thin layer from the underlying material.

Dennis Taylor, in 1891, published the first edition of his famous book on the adjustment and testing of telescopic objectives. In fact Taylor developed method of artificially producing the tarnish by chemical etching. Kollmorgen followed this work and developed the chemical process for different types of glass. In Nineteenth century at the same time a great deal of progress was made in the field of interferometry. The most significant development from thin film point of view was the Fabry‐Perot inferometer described in 1899 which became one of the basic structures of thin film filters. Development became more rapid in the 1930s, an indeed it is in this period that we can recognize the beginning of the modern thin film optical coating. In 1932, Rouard observed that very thin metallic film reduced the internal reflectance of the glass plate, although the external reflectance was increased. In 1934, Bauer in the course of fundamental investigations of optical properties of halides produced reflec‐ tion‐reducing coatings and pfund evaporated zinc sulfide layers to make low loss beam split‐ ters for Michelson interferometers. Noting incidentally that titanium dioxide could be a better material. In 1936, John Strong produced anti‐reflection coatings by evaporation of fluorite to give inhomogeneous films which reduced the reflectance of glass to visible light to as much as 89% a most impressive figure. Then in 1939, Geffcken constructed the first thin film metal‐ dielectric interference filters. The most important factor in this sudden expansion of thin film coatings was the manufacturing process. Although sputtering was discovered by the middle of nineteenth century and vacuum evaporation in the beginning of twentieth century, these were not considered as useful manufacturing proccesses. The main difficulty was the lack of suitable pumps and it was not until the early 1930s that the works of CR Burch on diffusion pump oil made it possible for this process to be used satisfactorily. Since the tremendous trades have been made particularly in last few years, filters with greater than 100 layers are not uncommon and uses have been found for them in almost every branch of science and technology.

Much research has been done on the mechanism of thin film growth with evaporated films. It has been found by observation of films evaporated directly in the viewing field of an electron microscope that film growth may be divided into certain stages. Pashley et al. distinguished four stages of growth process: nucleation and island structure, coalescence of islands, channel filling, and formation of continuous film. Nucleation is the initial stage of a film. The particles which have been evaporated from the evaporation source and have reached the substrate, on which a thin film is to deposit, generally lose part of their energy on impingement. Therefore, the mobility of the atoms or molecules on the surface decreases as the atoms give up their energy to the substrate. The effect of elevated substrate temperature is to permit the atoms to retain sufficient energy to make the movements necessary for the accommodation on the sub‐ strate and among them. Most of the flux arrives at the substrate in atomic form. These ad‐atoms diffuse around the substrate and their diffusivities are dependent on the interaction between the ad‐atoms (adsorbed atoms) and the substrate and the temperature of the substrate. Any defects or crystallographic variations on the substrate surface acts as a potential well and the atoms have to overcome this potential barrier to keep moving around. Occasionally, they suc‐ ceed so well that they are re‐evaporated. In diffusing randomly they come across other atoms and join them to form doublets, which have lower diffusivities. Beyond a critical nucleus size (on the order of 10–100 Å), the larger nuclei grow at the expense of the smaller ones, and so the number of nuclei on the substrate is continually decreasing during growth. The initial nucleation is enhanced by the presence of defects on the substrate. At this stage the individual crystallites (about 100 Å in diameter) are quite perfect structurally. As more flux arrives at the surface, the nuclei sizes grow and eventually islands are formed. During this stage of film growth some islands come into mutual contact and coalescence ensues. The coalescence phase is critical for the formation of grain boundaries and dislocations. As the larger nuclei (perhaps several thousand atoms in size) combine, the amount of disorder at the merging boundary depends on the orientation of each nucleus before contact. If the nuclei are aligned (as in epitaxial growth on a well‐matched substrate lattice), either a twin boundary or none at all is formed. If the islands are not aligned before contact (as in growth on an amorphous substrate or growth at low temperature), a grain boundary is formed. As the nuclei become large, the energy of the aggregate becomes smaller so the larger nuclei have less ability to hold to each other as they combine.

It is believed that certain energy is liberated by coalescence, which is sufficient to affect a temporary melting of the crystallites in contact. After coalescence, the temperature drops and newborn island occurs again. It has been established that when two islands which are of different sizes and crystallographic orientation coalescence, the resultant crystallite assumes the orientation of larger one. As the islands grow, there is a decreasing tendency for them to become completely rounded after coalescence. Large shape changes still occur, but these are confined mainly to the regions in the immediate vicinity of junction of the islands. Consequently, the islands become elongated and join to form a continuous network structure in which the deposit material is separated by long, irregular, and narrow channels of width 50–200 Å. As deposition continues, secondary nucleation occurs in these channels, and the nuclei are incorporated into the bulk of the film as they grow and touch the sides of the chan‐ nel. At the same time, channels are bridged at some points and fill in rapidly in liquid like manner. Eventually, most of the channels are eliminated and the film is continuous but con‐ tains many small irregular holes. Secondary nucleation takes place on substrate with in holes, which are produced during channel filling stage. The hole contains many secondary nuclei which coalesce with each other to form secondary islands which then touch edge of hole and coalesce with the main film to leave a clean hole. Further, secondary nuclei then form, and the process is repeated until the hole finally fills. The liquid like behavior of the deposit persists until a complete film is obtained. These processes are substantially complete before appre‐ ciable growth in thickness occurs.

### **3. Deposition techniques for thin films**

microscope that film growth may be divided into certain stages. Pashley et al. distinguished four stages of growth process: nucleation and island structure, coalescence of islands, channel filling, and formation of continuous film. Nucleation is the initial stage of a film. The particles which have been evaporated from the evaporation source and have reached the substrate, on which a thin film is to deposit, generally lose part of their energy on impingement. Therefore, the mobility of the atoms or molecules on the surface decreases as the atoms give up their energy to the substrate. The effect of elevated substrate temperature is to permit the atoms to retain sufficient energy to make the movements necessary for the accommodation on the sub‐ strate and among them. Most of the flux arrives at the substrate in atomic form. These ad‐atoms diffuse around the substrate and their diffusivities are dependent on the interaction between the ad‐atoms (adsorbed atoms) and the substrate and the temperature of the substrate. Any defects or crystallographic variations on the substrate surface acts as a potential well and the atoms have to overcome this potential barrier to keep moving around. Occasionally, they suc‐ ceed so well that they are re‐evaporated. In diffusing randomly they come across other atoms and join them to form doublets, which have lower diffusivities. Beyond a critical nucleus size (on the order of 10–100 Å), the larger nuclei grow at the expense of the smaller ones, and so the number of nuclei on the substrate is continually decreasing during growth. The initial nucleation is enhanced by the presence of defects on the substrate. At this stage the individual crystallites (about 100 Å in diameter) are quite perfect structurally. As more flux arrives at the surface, the nuclei sizes grow and eventually islands are formed. During this stage of film growth some islands come into mutual contact and coalescence ensues. The coalescence phase is critical for the formation of grain boundaries and dislocations. As the larger nuclei (perhaps several thousand atoms in size) combine, the amount of disorder at the merging boundary depends on the orientation of each nucleus before contact. If the nuclei are aligned (as in epitaxial growth on a well‐matched substrate lattice), either a twin boundary or none at all is formed. If the islands are not aligned before contact (as in growth on an amorphous substrate or growth at low temperature), a grain boundary is formed. As the nuclei become large, the energy of the aggregate becomes smaller so the larger nuclei have less ability to hold

386 Modern Technologies for Creating the Thin-film Systems and Coatings

It is believed that certain energy is liberated by coalescence, which is sufficient to affect a temporary melting of the crystallites in contact. After coalescence, the temperature drops and newborn island occurs again. It has been established that when two islands which are of different sizes and crystallographic orientation coalescence, the resultant crystallite assumes the orientation of larger one. As the islands grow, there is a decreasing tendency for them to become completely rounded after coalescence. Large shape changes still occur, but these are confined mainly to the regions in the immediate vicinity of junction of the islands. Consequently, the islands become elongated and join to form a continuous network structure in which the deposit material is separated by long, irregular, and narrow channels of width 50–200 Å. As deposition continues, secondary nucleation occurs in these channels, and the nuclei are incorporated into the bulk of the film as they grow and touch the sides of the chan‐ nel. At the same time, channels are bridged at some points and fill in rapidly in liquid like manner. Eventually, most of the channels are eliminated and the film is continuous but con‐ tains many small irregular holes. Secondary nucleation takes place on substrate with in holes,

to each other as they combine.

The process by which thin film is deposited onto a substrate or onto a previously deposited layer is called thin film deposition. The process will be followed according to the require‐ ments and economic conditions. There are some major techniques used for thin film fabri‐ cation such as physical vapor deposition (PVD), closed space sublimation (CSS), chemical vapor deposition (CVD), chemical bath deposition (CBD), pulsed laser deposition (PLD), elec‐ tro‐deposition (ED), sputtering technique, and atomic layer epitaxy (ALE) [16–18]. So many techniques can be used for the CdTe. It is the versatility of the CdTe because it provides many ways for its deposition. The method of deposition should be economical, easily scalable, and easy to handle and can give good conversion efficiency of the device. The method should be easily applicable at large industrial level. The deposition technique will be preferred which have maximum conversion efficiency, low cost, and high deposition rate. Many techniques have efficiency up to 12% in laboratory but not at the industrial level. There should be such kind of deposition techniques that will give good conversion efficiency in laboratory as well as at industrial level [10].

### **3.1. Physical vapor deposition (PVD)**

It is the process in which material is sublimated from solid or liquid source and condensed upon the substrate; mostly the whole process is done in a vacuum. The deposition rate for physical vapor deposition is approximately from 1 to 10 nm per second. This deposition tech‐ nique is used for the deposition of alloys, elements, and compounds by the reactive deposi‐ tion. In reactive deposition, a gas environment is used which has a reaction with the source material (depositing material) to form a compound, the gas environment may be a nitrogen gas or some other gas.

The physical vapor deposition can be categorized as:

(i) vacuum evaporation, (ii) sputter deposition, and (iii) molecular beam epitaxy.

Physical vapor deposition is a technique whereby physical processes, such as evaporation, sublimation or ionic impingement on a target, facilitate the transfer of atoms from a solid or molten source onto a substrate. Evaporation and sputtering are the two most widely used processes and PVD method is used for depositing films. Important factors in controlling the structure of a growing film are growth flux or deposition rate, substrate temperature, source temperature, and evaporation time.

The ratio of the substrate temperature to the melting temperature of the film material is an important factor in determining the structure of a polycrystalline film. These factors deter‐ mine the degree to which ad‐atoms are able to seek out minimum energy positions and grain boundaries are able to adopt morphologies of minimum energy. The driving force for grain growth is the reduction in the total grain boundary surface area and the attendant reduc‐ tion in the total energy associated with grain boundary surfaces. Another mechanism for the development of structure in polycrystalline films involves recrystallization, the driving force for which is the minimization of energy associated with defects, such as pre‐existing disloca‐ tions, in addition to grain boundary energy. Minimization of stored elastic energy arising from intrinsic and mismatch strains in the film can also serve as an additional factor contribut‐ ing to grain growth and recrystallization.

The factors that control the very early stages of growth of a thin film on a substrate are described in atomistic terms. The process begins with a clean surface of the substrate mate‐ rial, which is at substrate temperature Ts, exposed to a vapor of a chemically compatible film material, which is at the vapor temperature TV.

To form a single crystal film, atoms of the film material in the vapor must arrive at the sub‐ strate surface, adhere to it, and settle into possible equilibrium positions before structural defects are left behind the growth front.

To form an amorphous film, on the other hand, atoms must be prevented from seeking stable equilibrium positions once they arrive at the growth surface. In either case, this must happen in more or less the same way over a very large area of the substrate surface for the structure to develop. At first sight, this outcome might seem unlikely, but such films are produced routinely.

Atoms in the vapor come into contact with the substrate surface where they form chemical bonds with atoms in the substrate. The temperature of the substrate must be low enough so that the vapor phase is supersaturated in some sense with respect to the substrate, an idea that will be made more concrete below. There is a reduction in energy due to formation of the bonds during attachment. Some fraction of the attached atoms, which are called ad‐atoms, may return to the vapor by evaporation if their energies due to thermal fluctuations are suf‐ ficient to occasionally overcome the energy of attachment. High energy ad‐atoms stick on the growth surface where they arrive, and the film tends to grow with an amorphous or very fine‐ grained polycrystalline structure. The growth surface invariably has some distribution of sur‐ face defects crystallographic steps, grain boundary traces, and dislocation line terminations, for example which provide sites of relatively easy attachment for ad‐atoms. For semiconduc‐ tor films, the epitaxial structure is essential to the electronic performance of the material. For metal films, on the other hand, the electrical conductivity of a polycrystalline film is nearly as large as that of a single crystal film.

In the vacuum evaporation process, source material is evaporated thermally inside the vac‐ uum and its vapors are condensed at the substrate. The vacuum has a benefit of controlling the contaminations and reducing the melting temperature of the source material. Vacuum also increases the mean free path for the motion of deposited species. The vacuum required for deposition is from 10−5 to 10−10 Torr. Thermal evaporation is obtained by thermal heating sources such as tungsten coils. The vacuum evaporation technique is used for fabrication of decorative coating, corrosion protective coatings, and electrically conducting films [13]. The sputter deposition is done by physical sputtering. It is the process in which material is not heated thermally so it is a non‐vaporizing process. Material from target is ejected and then deposits onto a substrate. When energetic particles like ions are bombarded on the sputter‐ ing target, a plume of material is released and deposits onto a substrate just like a shower of sand when a golf ball lands in the bunker. The bombarding particles are usually gaseous ions accelerated from plasma. The sputtering gas is often an inert gas like argon gas. The spacing between source and substrate is less than vacuum deposition.

The plasma pressure for sputter deposition is from 5 to 30 mTorr. The sputtered particles are thermalized by gas phase collision before they reach substrate surface. The ions are generated from local plasma (diode or planar magnetron sputtering) or a separate ion beam source (ion beam deposition) [13, 14], as shown in **Figure 1**.

In molecular beam epitaxial (MBE) growth, the fabrication of crystalline thin film can be grown epitaxial above the other crystalline substrate with the beam of molecules or atoms.

This method required an ultra‐high vacuum up to (10⁻8 ) pa for the film deposition. The rate of deposition is very slow which is 1 µm/h. The materials are then evaporated and reach the sub‐ strate individually on the wafer and reaction takes place between these vapors. This method can give high‐purity epitaxial layers of compound semiconductors. The word "beam" shows that sublimated atoms of the material do not interact with each other and with vacuum cham‐ ber until reach the wafer, due to long mean free path of atoms. The crystalline film is fabri‐ cated on the substrate of the same material which is called as homoepitaxy. So this epitaxy is done with only single material. Heteroepitaxy is performed with different materials. In heteroepitaxy, crystalline thin film is fabricated on the substrate of a different material. The disadvantage of this method is that it is very expensive. In MBE technique; vapor phase, liq‐ uid phase, and solid phase methods can be used [16].

**Figure 1.** Sputtering apparatus.

The ratio of the substrate temperature to the melting temperature of the film material is an important factor in determining the structure of a polycrystalline film. These factors deter‐ mine the degree to which ad‐atoms are able to seek out minimum energy positions and grain boundaries are able to adopt morphologies of minimum energy. The driving force for grain growth is the reduction in the total grain boundary surface area and the attendant reduc‐ tion in the total energy associated with grain boundary surfaces. Another mechanism for the development of structure in polycrystalline films involves recrystallization, the driving force for which is the minimization of energy associated with defects, such as pre‐existing disloca‐ tions, in addition to grain boundary energy. Minimization of stored elastic energy arising from intrinsic and mismatch strains in the film can also serve as an additional factor contribut‐

The factors that control the very early stages of growth of a thin film on a substrate are described in atomistic terms. The process begins with a clean surface of the substrate mate‐ rial, which is at substrate temperature Ts, exposed to a vapor of a chemically compatible film

To form a single crystal film, atoms of the film material in the vapor must arrive at the sub‐ strate surface, adhere to it, and settle into possible equilibrium positions before structural

To form an amorphous film, on the other hand, atoms must be prevented from seeking stable equilibrium positions once they arrive at the growth surface. In either case, this must happen in more or less the same way over a very large area of the substrate surface for the structure to develop. At first sight, this outcome might seem unlikely, but such films are produced routinely.

Atoms in the vapor come into contact with the substrate surface where they form chemical bonds with atoms in the substrate. The temperature of the substrate must be low enough so that the vapor phase is supersaturated in some sense with respect to the substrate, an idea that will be made more concrete below. There is a reduction in energy due to formation of the bonds during attachment. Some fraction of the attached atoms, which are called ad‐atoms, may return to the vapor by evaporation if their energies due to thermal fluctuations are suf‐ ficient to occasionally overcome the energy of attachment. High energy ad‐atoms stick on the growth surface where they arrive, and the film tends to grow with an amorphous or very fine‐ grained polycrystalline structure. The growth surface invariably has some distribution of sur‐ face defects crystallographic steps, grain boundary traces, and dislocation line terminations, for example which provide sites of relatively easy attachment for ad‐atoms. For semiconduc‐ tor films, the epitaxial structure is essential to the electronic performance of the material. For metal films, on the other hand, the electrical conductivity of a polycrystalline film is nearly as

In the vacuum evaporation process, source material is evaporated thermally inside the vac‐ uum and its vapors are condensed at the substrate. The vacuum has a benefit of controlling the contaminations and reducing the melting temperature of the source material. Vacuum also increases the mean free path for the motion of deposited species. The vacuum required for deposition is from 10−5 to 10−10 Torr. Thermal evaporation is obtained by thermal heating

ing to grain growth and recrystallization.

defects are left behind the growth front.

large as that of a single crystal film.

material, which is at the vapor temperature TV.

388 Modern Technologies for Creating the Thin-film Systems and Coatings

### **3.2. Close spaced sublimation (CSS) technique for thin film deposition**

Close space sublimation is a type of thermal evaporation technique. Advantage of CSS pro‐ cess is simplified deposition and high transport efficiency conducted under low vacuum conditions at moderate temperatures. In CSS technique, desired source material is placed in powder form in a graphite boat which is being heated by halogen lamps [5]. The substrate is placed in a mica sheet, which acts as a thermal gradient between the source and the substrate. The material starts to sublimate and deposit on substrate. The source is maintained at higher temperature than substrate. The deposited film presents a high crystallographic orientation and adequate opto‐electrical properties for photovoltaic applications.

The CSS is a process for a thin film deposition of materials in a vacuum as shown in **Figure 2**. The material is sublimated by heating and its vapors condensed onto a substrate which is placed above the source material. The basic phenomenon of thin film deposition based on dissociation at high temperature.

Before the fabrication of thin film by close spaced sublimation, the substrate needs a proper cleaning by these methods like acetone, isopropyl alcohol, a rinse with distilled water, and ultrasonic cleaning.

The quality of thin films, material transport and deposition rate depends upon the fabrication parameters [17, 18].

It is observed that high substrate temperature of CdTe provides good performance of solar cell devices. Resistivity of CdTe decreases by increasing the substrate temperature and grain size reduces by increasing substrate temperature. Deposition rate also improved at high substrate

**Figure 2.** Schematic diagram of CSS designed at CIIT.

temperature [19]. The pacing between source and substrate is inversely proportional to the rate of deposition. Vacuum level for CdS and CdTe deposition is from 10−3 to 10− 5 mbar.

Annealing of the films provides improvement of surface morphology and it reduces roughness of surface of CdS and CdTe films. Recombination centers reduce by annealing. The crystallinity of film also improves by the annealing process. Open circuit voltage also increased by increas‐ ing annealing temperature. Spectral response especially in the range of 500–600 nm is also improved by annealing temperature. Hence efficiency of solar cell can be improved [20–22].

The particles movement from higher concentration to lower concentration is called diffusion. The distance particles can travel without any collision is called mean free path. Diffusion of one kind of particles into other kind of material can change its characteristics like a semiconductor material can be converted into an n‐type or a p‐type. The advantages of CSS are as follows: (1) the evaporation source and substrate are heated directly by halogen lambs and their tempera‐ ture is controlled using temperature controllers; (2) the source and substrate are separated by a mica sheet of about 1–3 mm. In this way, the source vapors are confined to closed space, leading to less wastage of evaporated material as compared to other methods. The mica sheet maintains the source and substrate at different temperatures, due to which the evaporating materials will always have better access to the substrate; (3) the films deposited by this method present a high crystallographic orientation and adequate opto‐electrical properties for photo voltaic applica‐ tions; (4) the system is very simple and easy to use; and (5) it has high transport efficiency conducted under low vacuum conditions at moderate temperature. There are also some limita‐ tions: (1) the main limitation of CSS deposition system is that there is no quartz crystal to moni‐ tor the growth rate and thickness of the film deposited; and (2) this method can only be used for limited number of materials, i.e., materials which can be sublimated at moderate temperatures.

Thermal evaporation method for preparing thin films is becoming very popular since 30 years or so. In this method the pressure of gas in chamber is reduced to value as low as possible, this is called creating vacuum. Generating vacuum properly is very important because when evaporation is performed in poor vacuum or close to the atmospheric pressure, the resulting deposition is generally non‐uniform. The purity of deposited films also depends on the qual‐ ity of vacuum and on the purity of source material. Vacuum is created by different pumps for example we have rotary vane pump and oil diffusion pump. In our vacuum coating unit, these pumps create different vacuum ranges, 10−3 to 10−6 mbar. In this vacuum chamber, source material is evaporated by heating at suitable temperature for particular time. In this method one must make sure that in order to deposit a material, the evaporation system must be able to sublimate it and also pressure of gas in chamber is low enough so that mean free path of the atoms of evaporated material is larger than source‐substrate distance.

### **3.3. Chemical vapor deposition (CVD)**

**3.2. Close spaced sublimation (CSS) technique for thin film deposition**

390 Modern Technologies for Creating the Thin-film Systems and Coatings

and adequate opto‐electrical properties for photovoltaic applications.

dissociation at high temperature.

**Figure 2.** Schematic diagram of CSS designed at CIIT.

ultrasonic cleaning.

parameters [17, 18].

Close space sublimation is a type of thermal evaporation technique. Advantage of CSS pro‐ cess is simplified deposition and high transport efficiency conducted under low vacuum conditions at moderate temperatures. In CSS technique, desired source material is placed in powder form in a graphite boat which is being heated by halogen lamps [5]. The substrate is placed in a mica sheet, which acts as a thermal gradient between the source and the substrate. The material starts to sublimate and deposit on substrate. The source is maintained at higher temperature than substrate. The deposited film presents a high crystallographic orientation

The CSS is a process for a thin film deposition of materials in a vacuum as shown in **Figure 2**. The material is sublimated by heating and its vapors condensed onto a substrate which is placed above the source material. The basic phenomenon of thin film deposition based on

Before the fabrication of thin film by close spaced sublimation, the substrate needs a proper cleaning by these methods like acetone, isopropyl alcohol, a rinse with distilled water, and

The quality of thin films, material transport and deposition rate depends upon the fabrication

It is observed that high substrate temperature of CdTe provides good performance of solar cell devices. Resistivity of CdTe decreases by increasing the substrate temperature and grain size reduces by increasing substrate temperature. Deposition rate also improved at high substrate

> CVD is a chemical process used to produce high‐purity and high‐performance solid materi‐ als. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by‐products are also produced, which are removed by gas flow through the reaction chamber.

Micro fabrication processes widely use CVD to deposit materials in various forms, including: nanocrystalline, polycrystalline, amorphous, and epitaxial. These materials include: silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2 , silicon‐germanium, tung‐ sten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, and various high‐k dielectrics. The CVD process is also used to produce synthetic diamonds. It is a chemical reac‐ tion which transforms gas molecules into the solid material and then into thin film or powder form, on the substrate surface. CVD is widely used to fabricate semiconductor devices.

Atomic layer epitaxy (ALE), or atomic layer chemical vapor deposition (ALCVD), now a days is known as atomic layer deposition (ALD) technique used for the production of high quality and thin solid films of specific crystal structures or orientations. This method provides very fine control of film thicknesses to one atomic layer. It has a wide range of applications in areas such as thin film ceramics, gas sensors, radiation detectors, optical/infrared filters, surface hardening, and fiber optical materials. The term *epitaxy* comes from the Greek roots *epi*, mean‐ ing "above", and *taxis*, meaning "in ordered manner". It can be translated "to arrange upon". It is a method that is based on sequential use of gas phase chemical process. Mostly in (ALE) two chemicals are used called precursors. Chemical reaction takes place between these pre‐ cursors with surface one at a time in sequential manner. Using ALE alternating monolayers of two different elements can be deposited onto a substrate. In this technique material amount deposited in each cycle is constant. This method provides crystalline and uniform films espe‐ cially if very thin film is required. In the laboratory of advanced energy systems, ALE used to grow both CdS and CdTe layers in a single process. ALE reactor with four individually con‐ trolled sources to control inside solid reactants, and has number of sources for external inert gas flow and liquid reactants. The spacing between two substrates is 1–3 mm.

The deposition of a substance on an electrode by the process of electrolyses is another CVD method. It is a technique in which electric current is passed through a chemical solution and ionization occurs then these ions deposit on the substrate.

Electrolytic deposition‐cathodic film is also a versatile method of depositing film on an elec‐ trode (cathode) of the cell in which electrode is placed in a solution and the ions in that solution are impelled to the electrodes by an electric anodization was used as electrolyte in the middle of nineteenth century. A few years later, sulfuric acid bath was used in same process. This tech‐ nique can be related to that of electrolytic deposition method. Same apparatus is used for both, but in this case, the thin film is formed at the anode or positive electrode rather than at cathode.

### **4. Fabrication of CdS thin film by chemical bath deposition (CBD) and (CSS) techniques**

A II–VI semiconductor compound cadmium sulfide (CdS) has much importance because of its so many applications in several heterojunction photovoltaic thin film devices of CdTe, cop‐ per indium gallium diselenide (Cu(In*<sup>x</sup>* Ga1‐*<sup>x</sup>* )Se2 ), or copper Indium Gallium sulfide (CIGS), and solar cells. Many other devices in the fields of electronics, optics, and infrared are also fabricated by using CdS [1–3]. The CdS is a suitable n‐type material, can be fabricated by a variety of fabrication techniques like sol‐gel technique [4], close spaced sublimation (CSS), chemical bath deposition (CBD), thermal evaporation, chemical vapor deposition, molecular beam epitaxy and spray pyrolysis. Each and every deposition process provides different opti‐ cal, structural, electrical, and morphological properties [5–10].

Micro fabrication processes widely use CVD to deposit materials in various forms, including: nanocrystalline, polycrystalline, amorphous, and epitaxial. These materials include: silicon,

sten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, and various high‐k dielectrics. The CVD process is also used to produce synthetic diamonds. It is a chemical reac‐ tion which transforms gas molecules into the solid material and then into thin film or powder form, on the substrate surface. CVD is widely used to fabricate semiconductor devices.

Atomic layer epitaxy (ALE), or atomic layer chemical vapor deposition (ALCVD), now a days is known as atomic layer deposition (ALD) technique used for the production of high quality and thin solid films of specific crystal structures or orientations. This method provides very fine control of film thicknesses to one atomic layer. It has a wide range of applications in areas such as thin film ceramics, gas sensors, radiation detectors, optical/infrared filters, surface hardening, and fiber optical materials. The term *epitaxy* comes from the Greek roots *epi*, mean‐ ing "above", and *taxis*, meaning "in ordered manner". It can be translated "to arrange upon". It is a method that is based on sequential use of gas phase chemical process. Mostly in (ALE) two chemicals are used called precursors. Chemical reaction takes place between these pre‐ cursors with surface one at a time in sequential manner. Using ALE alternating monolayers of two different elements can be deposited onto a substrate. In this technique material amount deposited in each cycle is constant. This method provides crystalline and uniform films espe‐ cially if very thin film is required. In the laboratory of advanced energy systems, ALE used to grow both CdS and CdTe layers in a single process. ALE reactor with four individually con‐ trolled sources to control inside solid reactants, and has number of sources for external inert

gas flow and liquid reactants. The spacing between two substrates is 1–3 mm.

ionization occurs then these ions deposit on the substrate.

**(CSS) techniques**

per indium gallium diselenide (Cu(In*<sup>x</sup>*

The deposition of a substance on an electrode by the process of electrolyses is another CVD method. It is a technique in which electric current is passed through a chemical solution and

Electrolytic deposition‐cathodic film is also a versatile method of depositing film on an elec‐ trode (cathode) of the cell in which electrode is placed in a solution and the ions in that solution are impelled to the electrodes by an electric anodization was used as electrolyte in the middle of nineteenth century. A few years later, sulfuric acid bath was used in same process. This tech‐ nique can be related to that of electrolytic deposition method. Same apparatus is used for both, but in this case, the thin film is formed at the anode or positive electrode rather than at cathode.

**4. Fabrication of CdS thin film by chemical bath deposition (CBD) and** 

A II–VI semiconductor compound cadmium sulfide (CdS) has much importance because of its so many applications in several heterojunction photovoltaic thin film devices of CdTe, cop‐

and solar cells. Many other devices in the fields of electronics, optics, and infrared are also fabricated by using CdS [1–3]. The CdS is a suitable n‐type material, can be fabricated by a

), or copper Indium Gallium sulfide (CIGS),

Ga1‐*<sup>x</sup>* )Se2 , silicon‐germanium, tung‐

carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2

392 Modern Technologies for Creating the Thin-film Systems and Coatings

First, solar cell has reported CdS deposited by high rate vapor transport deposition (HRVTD). CdS is called window layer due to its higher rate of light transmission [11–14]. CdS thin films are very suitable for so many other semiconductor devices and radiation detectors. CdS thin films with wide band gap are highly used for photovoltaic devices. Since CdS used as win‐ dow layer in solar cell so it should be fabricated very thin, high amount of light should pass through CdS and absorb in CdTe. The CdS thin film must be continuous to reduce the effect of short circuit in the cell. The CdS thin films deposited by CBD can fulfill these requirements. So the thickness of CdS has much importance for the high efficiency of solar cell [15–17]. CdS (CBD) thin films are grown by cadmium chloride, ammonium nitrate, and potassium hydrox‐ ide. After heating to a specific temperature, thiourea is added to start the fabrication process [18–20]. CBD is very easy process for the fabrication of CdS thin films on ITO glass. For the fabrication of thin film solar cell, one needs a very thin film up to (60–80) nm. It is very suitable process especially from solar cell point of view. It is a process in which substrate is placed in a hot chemical solution stirring vigorously for specific time, positive and negative ions will reach and meet on the substrate and thin film is grown. The advantage of this technique is that neither vacuum and nor very high temperature is required for CBD [5, 21].

Thin films of CdS were deposited on microscope glass sides and ITO glass sides. Two fab‐ rication procedures were used for CdS deposition for the first time in our lab. Firstly, CdS thin films were fabricated by CBD technique. For fabrication of thin film for solar cells, we need a very thin film up to (60–80) nm. In this process substrate was placed in a hot chemical solution stirring vigorously for 10–30 minutes with magnetic stirring at 3 Hz at 75°C by com‐ bining positive and negative ions on the substrate thin film was grown. The advantage was that neither vacuum nor very high temperature is required for CBD. The chemical solutions of (0.02 M) about 80 ml solution of cadmium chloride (CdCl2 ), (1.5 M) about 80 ml solution of ammonium nitrate (NH4 NO3 ), and (0.5 M) about 200 ml solution of potassium hydroxide (KOH) were made in a beaker. When temperature reached at 75°C, thiourea of (0.2 M) about 80 ml solution was added to start fabrication. The films were fabricated with magnetic stirring of 3 Hz for 10–30 min at 75<sup>o</sup> C as described in [5]. It is a process in which substrate is placed in a hot chemical solution stirring vigorously for specific time, positive and negative ions will reach and meet on the substrate and thin film is grown. The advantage of this technique is that neither vacuum and nor very high temperature is required for CBD [24]. Substrate was cleaned by using detergent then rinsed in distilled water then again clean with acetone and rinsed in distilled water then clean with isopropyl alcohol (IPA).

These three solutions are added to the beaker. The substrates are fixed inside the solution with the help of a substrate holder. The beaker is placed on the hot magnetic plate with mag‐ netic stirrer inside. A pH meter and thermometer is also dipped in the solution to measure pH and temperature, respectively. Provide heat to solution up to temperature of 75°C. The solution is provided by stirring continuously throughout the experiment.

When temperature reached at 75°C add the thiourea about 0.2 M (80 ml).

As thiourea is added to the solution, the reaction starts suddenly. So thiourea is the last com‐ ponent which is added to the solution. The CdS deposition starts when thiourea is added. Use stop watch now and substrates will be taken out after the 10, 20, and 30 min, respectively.

Films are retired from the solution according to the specific deposition time and rinsed imme‐ diately with distilled water into an ultrasonic cleaner. The CdS deposited films with pale yel‐ low color are obtained. The CdS film is deposited on both sides of the ITO glass. So film from glass side is cleaned by using 10% hydrochloric acid (HCl) solution. It needs much care when using HCl acid on glass side for cleaning because drops of HCl can remove film from ITO sides as well. After deposition, these CdS films will be annealed at 400°C for 30 min [25–27].

CdS thin films fabricated by the CSS are also another moderate temperature fabrication tech‐ nique in a vacuum chamber. More than 20% efficiency has been achieved by using CdTe/CdS heterojunction thin film solar cell by this technique. CSS is moderate temperature procedure so it provides in some cases better results [22, 23]. The CSS technique is one of the techniques that have produces encouraging results [15] mainly because it is simple deposition apparatus and high transport efficiency and deposition can be done in low vacuum at moderate temper‐ atures. There is minimum use of material in the CSS system as compared to other methods. As the substrate is close to the source materials in the CSS technique, roughness increases in the thin films and has high absorption, which makes it suitable material for solar cell applications. The CSS has a disadvantage that one cannot introduce thickness monitor.

CdS (CSS) fabrication was carried out in a vacuum chamber of approximately 4 × 10−1 mbar. The CdS sigma Aldrich powder 99.999% pure was used. Substrate temperature was maintained at 400°C and source temperature was in between (500–600)°C. Source material was heated by 1000 W halogen lamp and substrate was heated by a 500 W halogen lamp. Time of deposition was about (3–5) min as shown in **Figure 2**. The film was fabricated after collected after cooling. It is very easy process for the fabrication of CdS thin films on ITO glass. The disadvantage of closed spaced sublimation is that thickness cannot be controlled. For the fabrication of thin film solar cell we need a very thin film up to 60–80 nm, which is not easily possible by using close spaced sublimation process. It is very suitable process especially from solar cell point of view. CdS thin films were deposited on ITO glass substrate by CBD technique as shown in **Figure 3** using CdCl2 , ammonium nitrate (NH4 NO3 ), potassium hydroxide (KOH), and thiourea as starting materials. ITO coated glass was used as available from sigma‐Aldrich. Deposition was carried out with magnetic stirring at 3 Hz for 30 min at 75°C. The resulting CdS layer was annealed at 400°C for 30 min in a vacuum chamber at 4 × 10−2 mbar, and its thickness was found to be 800 nm. The CdTe layer was deposited on CdS by CSS technique under vacuum of 4 × 10−1 mbar. Deposition time was kept as 3–5 min. The substrate and source temperature were maintained at 400 and 530°C, respectively. A 500 W halogen lamp was used to heat the CdS/ITO/glass substrate, whereas a graphite boat was used to heat the source material with a 1000 W halogen lamp. The source and substrate spacing was measured as 4–5 mm. CdTe film thickness was measured as 5 µm. To enhance the p‐type properties of CdTe, tellurium (Te) layer (400 nm) was deposited on CdTe by CSS technique under vacuum of 4 × 10−2 mbar. Deposition time was kept as 5 min, whereas, source and substrate temperatures were kept as 350 and 200°C, respectively. Resulting Te layer was annealed at 200°C for 1 h.

**Figure 3.** Chemical bath deposition apparatus.

When temperature reached at 75°C add the thiourea about 0.2 M (80 ml).

394 Modern Technologies for Creating the Thin-film Systems and Coatings

The CSS has a disadvantage that one cannot introduce thickness monitor.

NO3

CdCl2

, ammonium nitrate (NH4

Resulting Te layer was annealed at 200°C for 1 h.

CdS (CSS) fabrication was carried out in a vacuum chamber of approximately 4 × 10−1 mbar. The CdS sigma Aldrich powder 99.999% pure was used. Substrate temperature was maintained at 400°C and source temperature was in between (500–600)°C. Source material was heated by 1000 W halogen lamp and substrate was heated by a 500 W halogen lamp. Time of deposition was about (3–5) min as shown in **Figure 2**. The film was fabricated after collected after cooling. It is very easy process for the fabrication of CdS thin films on ITO glass. The disadvantage of closed spaced sublimation is that thickness cannot be controlled. For the fabrication of thin film solar cell we need a very thin film up to 60–80 nm, which is not easily possible by using close spaced sublimation process. It is very suitable process especially from solar cell point of view. CdS thin films were deposited on ITO glass substrate by CBD technique as shown in **Figure 3** using

materials. ITO coated glass was used as available from sigma‐Aldrich. Deposition was carried out with magnetic stirring at 3 Hz for 30 min at 75°C. The resulting CdS layer was annealed at 400°C for 30 min in a vacuum chamber at 4 × 10−2 mbar, and its thickness was found to be 800 nm. The CdTe layer was deposited on CdS by CSS technique under vacuum of 4 × 10−1 mbar. Deposition time was kept as 3–5 min. The substrate and source temperature were maintained at 400 and 530°C, respectively. A 500 W halogen lamp was used to heat the CdS/ITO/glass substrate, whereas a graphite boat was used to heat the source material with a 1000 W halogen lamp. The source and substrate spacing was measured as 4–5 mm. CdTe film thickness was measured as 5 µm. To enhance the p‐type properties of CdTe, tellurium (Te) layer (400 nm) was deposited on CdTe by CSS technique under vacuum of 4 × 10−2 mbar. Deposition time was kept as 5 min, whereas, source and substrate temperatures were kept as 350 and 200°C, respectively.

), potassium hydroxide (KOH), and thiourea as starting

As thiourea is added to the solution, the reaction starts suddenly. So thiourea is the last com‐ ponent which is added to the solution. The CdS deposition starts when thiourea is added. Use stop watch now and substrates will be taken out after the 10, 20, and 30 min, respectively. Films are retired from the solution according to the specific deposition time and rinsed imme‐ diately with distilled water into an ultrasonic cleaner. The CdS deposited films with pale yel‐ low color are obtained. The CdS film is deposited on both sides of the ITO glass. So film from glass side is cleaned by using 10% hydrochloric acid (HCl) solution. It needs much care when using HCl acid on glass side for cleaning because drops of HCl can remove film from ITO sides as well. After deposition, these CdS films will be annealed at 400°C for 30 min [25–27]. CdS thin films fabricated by the CSS are also another moderate temperature fabrication tech‐ nique in a vacuum chamber. More than 20% efficiency has been achieved by using CdTe/CdS heterojunction thin film solar cell by this technique. CSS is moderate temperature procedure so it provides in some cases better results [22, 23]. The CSS technique is one of the techniques that have produces encouraging results [15] mainly because it is simple deposition apparatus and high transport efficiency and deposition can be done in low vacuum at moderate temper‐ atures. There is minimum use of material in the CSS system as compared to other methods. As the substrate is close to the source materials in the CSS technique, roughness increases in the thin films and has high absorption, which makes it suitable material for solar cell applications.

1.04 g CdCl2 was dissolved in 50 ml methanol at 65°C with constant magnetic stirring at 3 Hz for 10 min CdCl2 /methanol solution was then allowed to cool down at room temperature (25°C). The CdS/ITO/glass and the CdTe/CdS/ITO/glass structures were soaked in the CdCl2 solution for 7 s. After drying at room temperature, the structures were annealed in a tube furnace for 30 min at 400°C with constant flow of argon gas (30 ml/min).

Transmission spectra, X‐ray diffraction (XRD), and scanning electron microscope (SEM) with energy dispersive X‐ray (EDX) investigations were carried out in order to understand the opti‐ cal, crystallographic, and morphological effects of CdCl2 treatment on CdTe and CdS films. Optical analysis using UV‐VIS‐NIR spectrophotometer was also used to study the thickness of films. Rutherford back scattering (RBS) analysis was used to identify the elemental composi‐ tion of CdS thin films and further confirm their thickness. Rutherford back scattering spec‐ trometry with an accuracy of 7% was used. High energy alpha particles were bombarded on the CdS film. Backscattered He2+ ion energy distribution and yield at a given angle were mea‐ sured. Backscattered ions were detected by using a surface barrier detector with 17 keV resolu‐ tion kept at a scattering angle of 160°. Since cross section of backscattering for each element is known, it is possible to get a quantitative compositional analysis from the RBS spectrum. Silver (Ag) paste was used for ohmic contact formation in the ITO/CdS/CdTe solar cells structure.

### **5. Comparative analysis of optical properties of thin films**

(UV‐VIS‐IR) spectrophotometer was used to study optical properties of CdS (CBD) and CdS (CSS) thin films. The transmission for (CBD) films starts after 300 nm wavelength but in case of (CSS) transmission starts after 500 nm. So in CdS (CSS), blue portion of light was absorbed. Transmission increased especially above 550 nm wavelength region. In the region of 600–800 nm the transmission was more than 70% for both (CBD) and (CSS), so light transmission through (CBD) films was higher than (CSS). Due to this high transmission characteristic which showed that CdS a good window layer for the thin film solar cells of many kinds, fabricated by both of the techniques. The Swanepoel model provided calculations about thickness and refractive index. Energy values were calculated by plotting a graph between energy and (*αhυ*) 2 . Formula for refrac‐ tive index (*n*) is given in Eqs. (1)–(3).

$$d = \frac{M\lambda\_{\text{max}}\lambda\_{\text{min}}}{4n(\lambda\_{\text{max}} - \lambda\_{\text{min}})} \tag{1}$$

where *M* is the number of oscillations between maximum and minimum transmission wave‐ lengths λmax and λmin, respectively. The obtained thickness of CdTe films was 5 µm. *n* can be calculated using the relation:

$$n = \frac{[N + (N^2 - 4s^2)^{\frac{1}{2}}]}{2} \tag{2}$$
 
$$\text{Where, } N = 1 + s^2 + 4s \left(\frac{T\_{\text{M}} - T\_{\text{m}}}{T\_{\text{M}} \cdot T\_{\text{m}}}\right) \tag{3}$$

$$\text{Where, } N = 1 + s^2 + 4s \left( \frac{T\_{\text{M}} - T\_{\text{m}}}{T\_{\text{M}} T\_{\text{m}}} \right) \tag{3}$$

Here, the refractive index of glass, s = 1.52, *T*max and *T*min are the maximum and minimum transmissions, respectively. The values of *d* and *n* are calculated from Eqs. (1) and (2). The optical properties demonstrate a slight increase in absorption in IR region. Consequently, the transmittance is seen to slightly decrease after CdCl2 heat‐treatment. The band gap can be determined using the following relation:

$$
abla \nu = A \left( h \nu - E\_g \right)^{\mathbb{N} \mathbb{Z}} \tag{4}$$

Here *A* is a constant, *hv* is the photon energy, *E*<sup>g</sup> is the optical energy band gap. *N* depends on the nature of the transition (*N* = 1 for direct band gap, while *N* = 4 for indirect band gap transition). *hv* can be calculated by

$$h\nu(\text{eV}) = 1.24/\lambda(\mu\text{m})\tag{5}$$

The band gap can be obtained by extrapolating (α*hv*) 2 versus the incident photon energy (*hv*) plot. CdCl2 heat‐treatment results showed a slight decrease in band gap values as well. The band gap values extracted from these plots are 1.50 eV for as‐deposited and 1.49 eV for CdCl2 heat‐treated sample.

#### **5.1. Optical properties of CdTe by closed space sublimation (CSS)**

The optical measurements using the spectrophotometer can provide the information about the transmittance, thickness, refractive index, absorption coefficient, and energy band gap. The as‐deposited CdTe films have a high absorption in the visible and near‐infrared regions, which did not change significantly after the CdCl treatment. The transmission in the as‐ deposited sample is about 80% at higher wavelength. The same is improved to above 85% in the heat‐treated sample while the CdCl2 treated sample has about 80% transmission in higher wavelength region as shown in **Figure 4**.

The crystallographic orientation of CdTe samples was investigated by X‐ray diffraction. The main reflections of the samples are the same and can be indexed according to fcc CdTe lattice.

Effects of CdCl2 Treatment on Physical Properties of CdTe/CdS Thin Film Solar Cell http://dx.doi.org/10.5772/67191 397

**Figure 4.** Transmittance vs. wavelength and energy band gap of CdTe samples.

Transmission increased especially above 550 nm wavelength region. In the region of 600–800 nm the transmission was more than 70% for both (CBD) and (CSS), so light transmission through (CBD) films was higher than (CSS). Due to this high transmission characteristic which showed that CdS a good window layer for the thin film solar cells of many kinds, fabricated by both of the techniques. The Swanepoel model provided calculations about thickness and refractive index.

where *M* is the number of oscillations between maximum and minimum transmission wave‐ lengths λmax and λmin, respectively. The obtained thickness of CdTe films was 5 µm. *n* can be

Here, the refractive index of glass, s = 1.52, *T*max and *T*min are the maximum and minimum transmissions, respectively. The values of *d* and *n* are calculated from Eqs. (1) and (2). The optical properties demonstrate a slight increase in absorption in IR region. Consequently, the

*αhν* = *A* (*hν* − *E*<sup>g</sup> )*<sup>N</sup>*/2 (4)

on the nature of the transition (*N* = 1 for direct band gap, while *N* = 4 for indirect band gap

*hυ*(eV) = 1.24/*λ*(µm) (5)

band gap values extracted from these plots are 1.50 eV for as‐deposited and 1.49 eV for CdCl2

The optical measurements using the spectrophotometer can provide the information about the transmittance, thickness, refractive index, absorption coefficient, and energy band gap. The as‐deposited CdTe films have a high absorption in the visible and near‐infrared regions, which did not change significantly after the CdCl treatment. The transmission in the as‐ deposited sample is about 80% at higher wavelength. The same is improved to above 85% in the heat‐treated sample while the CdCl2 treated sample has about 80% transmission in higher

The crystallographic orientation of CdTe samples was investigated by X‐ray diffraction. The main reflections of the samples are the same and can be indexed according to fcc CdTe lattice.

2

heat‐treatment results showed a slight decrease in band gap values as well. The

\_\_1

*T*M − *T* \_m

(

2

*<sup>T</sup>*<sup>M</sup> *<sup>T</sup>*<sup>m</sup> ) (3)

heat‐treatment. The band gap can be

is the optical energy band gap. *N* depends

versus the incident photon energy (*hv*)

<sup>4</sup>*n*(*λ*max <sup>−</sup> *<sup>λ</sup>*min ) (1)

<sup>2</sup> ] \_\_\_\_\_\_\_\_\_\_\_ <sup>2</sup> (2)

. Formula for refrac‐

Energy values were calculated by plotting a graph between energy and (*αhυ*)

*<sup>d</sup>* <sup>=</sup> *<sup>M</sup> <sup>λ</sup>*max *<sup>λ</sup>* \_\_\_\_\_\_\_\_\_\_\_ min

396 Modern Technologies for Creating the Thin-film Systems and Coatings

*<sup>n</sup>* <sup>=</sup> [ *<sup>N</sup>* <sup>+</sup> (*N*<sup>2</sup> <sup>−</sup> <sup>4</sup> *<sup>s</sup>* <sup>2</sup> )

Where, *N* = 1 + *s* <sup>2</sup> + 4*s*

transmittance is seen to slightly decrease after CdCl2

Here *A* is a constant, *hv* is the photon energy, *E*<sup>g</sup>

The band gap can be obtained by extrapolating (α*hv*)

**5.1. Optical properties of CdTe by closed space sublimation (CSS)**

determined using the following relation:

transition). *hv* can be calculated by

wavelength region as shown in **Figure 4**.

plot. CdCl2

heat‐treated sample.

tive index (*n*) is given in Eqs. (1)–(3).

calculated using the relation:

The data analysis gave the lattice constant as 6.395 Å, which agreed with the reported value of 6.410 Å for the as‐deposited and CdCl2 lattice constant (ASTM Cards 15‐0770, 75‐2086). Two values of the lattice constant are attributed to the recrystallized lattice. The strongest (1 1 1) reflection in the as‐deposited sample indicates that a preferential orientation of (1 1 1) matches well with the observation earlier reported. The loss in the texture of CdTe is exhibited in CdCl2 ‐treated sample. However, the intensity of the (1 1 1) peak is lower in CdCl2 ‐treated sample CdTe‐63, implying that the samples are losing (1 1 1) texture and at the same time reorienting themselves in the (2 2 0) direction. The structure of CdTe is Cubic. The nature of deposited film is polycrystalline as shown in **Figure 5**.

#### **5.2. Scan electron microscopy (SEM) of CdTe samples CdTe‐20 and CdTe‐63, CdTe‐20 and CdTe‐63**

The morphology of the as‐deposited heat‐treated and CdCl2 ‐treated samples CdTe‐20 and CdTe‐63 show the change in the shape and size of the CdTe grains as in **Figure 6**. The average grain size of the as‐deposited sample is under 01.7 µm and 0.86 µm in the heat‐treated sample respectively while some of the bigger grains divide into smaller grains and reorient themselves, which results into an entirely different microstructure. The SEM images support the XRD results.

**Figure 5.** X‐ray diffraction spectrum for CdTe samples.

**Figure 6.** SEM micrographs of CdTe‐20 and CdTe‐63 samples.

### **5.3. Optical analysis of CdS thin film by chemical bath deposition**

The CdS layer has good transmittance especially above 550 nm wavelength region as shown in **Figure 7**. Transmission starts after 300 nm by CBD process. In the region of 600–800 nm, the transmission is more than 70% in samples CdS 21 and CdS/CBD 23. This is the characteristic which shows that CdS is good window layer for the thin film solar cells of many kinds.

The band gap can be obtained by extrapolating (α*hv*)2 versus the incident photon energy (*hv*) plot. CdS 23 and CdS 24 samples fabricated by chemical bath deposition results showed a slight decrease in band gap values as well. The band gap values extracted from these plots are 2.36 eV and 2.33 eV.

### **5.4. Structural analysis of CdS by chemical bath deposition (CBD)**

20 30 40 50 60 70 80

**C[220]**

**C[311]**

**Figure 5.** X‐ray diffraction spectrum for CdTe samples.

**CdTe-20**

**CdTe-63**

**Figure 6.** SEM micrographs of CdTe‐20 and CdTe‐63 samples.

**2**θ

20 30 40 50 60

**H[103]**

**C[311]**

**C[220]**

d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o

**CdTe-63**

**2** θ

0

14

1000

2000

**Counts**

3000

4000

**C[111]**

**CdTe-20**

398 Modern Technologies for Creating the Thin-film Systems and Coatings

0

1000

2000

**Counts**

3000

4000

**C[111]**

Structural properties were studied by X‐ray diffraction using CuKα radiations of 1.5418 Å. The data analysis provided lattice constant as 6.50 Å for as‐deposited sample. The lattice parameter can be calculated by using the relation for only cubic structure.

**Figure 7.** Transmittance vs. wavelength and energy band gap of CdS samples.

$$a = d \left( h^2 + k^{2+} l^2 \right)^{1/2} \tag{6}$$

where *h*, *k*, and *l* are Miller indices, for hexagonal structure formula for "*d*'' is

$$1/d^2 = 4/\Im\{\left(h^2 + hk + k^2\right) + \frac{l^2}{c^2}}\tag{7}$$

By using the Scherer formula, crystalline size can be calculated

$$\text{Crystalline size} = \frac{0.9\lambda}{\beta \cos \theta} \tag{8}$$

*λ* is X‐rays wavelength, 0.9 is a constant shape factor, *β* is full width half maxima in radi‐ ans, *θ* is Bragg's angle [25]. Peaks relating to CdS can be identified by using standard card JCPDS‐00‐041‐1049. The *d*‐spacing values were compared with standard values of ASTM cards to find the structure. It was observed in this experimental work that CdS thin films had a mixed structure of H (0 0 2) and C (1 1 1) for both techniques. So CdS has a polycrystalline behavior [20–22, 30–39]. Preferred orientation is (0 0 2) and structure is hexagonal as shown in **Figure 8**.

#### **5.5. Optical analysis of CdS thin film by close sublimation technique (CSS)**

The CdS deposited by close spaced sublimation have transmission for light above 550 nm then it again decreases for infrared region as shown in **Figure 9**. So if we consider only visible spectrum as for the solar cell point of view then it can be observed that from 600 to 800 nm, the transmission is more than 60% which shows that CdS is a good and efficient window layer for the visible spectrum or solar spectrum exclusively. The energy band gap is also showing the 2.44 and 2.38 eV for samples CdS 13 and CdS 318, respectively.

**Figure 8.** X‐ray diffraction spectrum for CdTe samples.

Effects of CdCl2 Treatment on Physical Properties of CdTe/CdS Thin Film Solar Cell http://dx.doi.org/10.5772/67191 401

**Figure 9.** Transmittance vs. wavelength and energy band gap of CdS samples.

*a* = *d* (*h*<sup>2</sup> + *k*<sup>2</sup> <sup>+</sup> *l*

400 Modern Technologies for Creating the Thin-film Systems and Coatings

1/*d*<sup>2</sup> = 4/3((*h*<sup>2</sup> + *hk* + *k*<sup>2</sup> ) +*<sup>l</sup>*

By using the Scherer formula, crystalline size can be calculated Crystalline size = \_\_\_\_\_\_ 0.9*<sup>λ</sup>*

where *h*, *k*, and *l* are Miller indices, for hexagonal structure formula for "*d*'' is

**5.5. Optical analysis of CdS thin film by close sublimation technique (CSS)**

the 2.44 and 2.38 eV for samples CdS 13 and CdS 318, respectively.

H(002)


**Figure 8.** X‐ray diffraction spectrum for CdTe samples.

0

50

100

**Counts**

150

200

*λ* is X‐rays wavelength, 0.9 is a constant shape factor, *β* is full width half maxima in radi‐ ans, *θ* is Bragg's angle [25]. Peaks relating to CdS can be identified by using standard card JCPDS‐00‐041‐1049. The *d*‐spacing values were compared with standard values of ASTM cards to find the structure. It was observed in this experimental work that CdS thin films had a mixed structure of H (0 0 2) and C (1 1 1) for both techniques. So CdS has a polycrystalline behavior [20–22, 30–39]. Preferred orientation is (0 0 2) and structure is hexagonal as shown in **Figure 8**.

The CdS deposited by close spaced sublimation have transmission for light above 550 nm then it again decreases for infrared region as shown in **Figure 9**. So if we consider only visible spectrum as for the solar cell point of view then it can be observed that from 600 to 800 nm, the transmission is more than 60% which shows that CdS is a good and efficient window layer for the visible spectrum or solar spectrum exclusively. The energy band gap is also showing

20 30 40 50 60 70 80

C(311)

**CdS23 CBD**

**2**θ

H(110)

2)1/2 (6)

*<sup>β</sup>* cos*<sup>θ</sup>* (8)

*<sup>c</sup>* <sup>2</sup> (7)

2 \_\_

#### **5.6. Structural analysis of CdS by close sublimation technique (CSS)**

The XRD spectra were taken scanning the values of 2*θ* from 20 to 80° as shown in **Figure 10**. In CdS (CSS) high intensity planes were grown during the film growth due to high tempera‐ ture. In CdS (CSS) strong peaks C (1 1 1), and H(0 0 2) were for two samples 317 and 318 as observed, which proved the polycrystalline behavior of CdS thin films. The height of peak/ intensity for CdS (CBD) was only up to 200, 150, and 400, first strong peak was H (0 0 2) for two samples and C (1 1 1) for third sample at 2*θ* value of (26.49, 25.19, 26.74) degree of angle. In our research work for both CBD and CSS techniques, first hexagonal strong peak was dom‐ inant. The size of grains was different for different fabrication techniques this may be due to film thickness, temperature or different nucleation of CdS for different deposition methods.

#### **5.7. SEM analysis of CdS (CBD) and CdS (CSS)**

Surface morphology was studied by using scanning electron microscopy (SEM), and grain size for CdS (CSS) was measured to be 300–400 nm as shown in **Figure 11**.

**Figure 10.** X‐ray diffraction spectrum for CdS samples.

The surface of CBD films was slightly nonuniform but CdS (CSS) structure was fine with bet‐ ter crystallinity. SEM analysis showed that CBD‐CdS thin films grain size approximately was found to be (50–100) nm and CSS‐CdS films grain size was approximately (200–300) nm. It was also observed that at high source temperature, grain size was bigger in the CSS technique. It is reported in the literature that even less crystalline and non‐uniform films of CBD process gave high‐efficiency as compared to CSS [5, 40–42].

**Figure 11.** SEM micrographs of CBD‐CdS and CSS‐CdS samples.

The transmission spectra of ITO coated thin films and ITO/CdS films shows more than 90% light is passes in the visible as well as near infrared regions shown in **Figure 12**. Since a CdS thin film (100–200 nm) is used for solar cells, the CdS film is well suited as the TCS for CdTe solar cells.

**Figure 12.** Transmission as a function of wavelength of ITO substrates and ITO/CdS thin films.

### **6. Conclusions**

The surface of CBD films was slightly nonuniform but CdS (CSS) structure was fine with bet‐ ter crystallinity. SEM analysis showed that CBD‐CdS thin films grain size approximately was found to be (50–100) nm and CSS‐CdS films grain size was approximately (200–300) nm. It was also observed that at high source temperature, grain size was bigger in the CSS technique. It is reported in the literature that even less crystalline and non‐uniform films of CBD process

**0**

**500**

**1000**

**counts**

**1500**

**2000**

H(0 0 2)

**20 30 40 50 60**

O(7 2 2)

H(1 0 2)

**CdS 318**

**C(3 1 1)**

**2** θ

gave high‐efficiency as compared to CSS [5, 40–42].

**Figure 11.** SEM micrographs of CBD‐CdS and CSS‐CdS samples.

**20 30 40 50 60**

H(1 0 2)

d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o d e m o

402 Modern Technologies for Creating the Thin-film Systems and Coatings

H(1 0 1)

H(1 0 3)

C(3 1 1)

**CdS 317**

**2** θ

**Figure 10.** X‐ray diffraction spectrum for CdS samples.

**0**

**500**

**1000**

**1500**

**counts**

**2000**

**2500**

**3000**

C(1 1 1)

An ITO/CdTe/CdS/glass solar cell was successfully prepared using CSS and CBD techniques. Optical analysis of ITO demonstrated transmittance of over 85% for whole wavelength. The effects of CdCl2 immersion and heating on the optical, structural, and morphological prop‐ erties of CdTe and CdS surfaces were studied with an effort to promote recrystallization, reorientation, and progressive increase in grain size. Structural properties investigated by using XRD exhibited shift in 2*θ* diffraction angles of peaks, change in crystalline size, change in intensity of preferred orientation, and change in the total number of peaks, which indi‐ cates toward recrystallization and reorientation. The optical results after CdCl2 heat‐treat‐ ment showed a decrease in transmittance in the case of CdTe thin film and increase in the case of CdS thin film. Similarly, band gap values were also observed to decrease after the CdCl2 heat‐treatment. Surface morphology of CdTe thin films was affected by CdCl2 heat‐treatment as well. The SEM micrographs assisted in investigating the changes. A grain size of CdTe as‐deposited sample was found to improve after the CdCl2 heat‐treatment. Similarly, a grain size of CdS as‐deposited sample was found to improve after the CdCl2 heat‐treatment. In conclusion, the significant improvement to the CdTe/CdS films solar cell performance can be achieved when annealed at 400°C in the presence of CdCl2 on the free surfaces of CdTe and CdS. These results can be used in turn to improve the short circuit current and open circuit voltage of solar cells.

### **Author details**

Nazar Abbas Shah1 \*, Zamran Rabeel2 , Murrawat Abbas3 and Waqar Adil Syed4

\*Address all correspondence to: nabbasqureshi@yahoo.com

1 Thin Films Technology Research Laboratory, Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan

2 Higher Education Department, Punjab, Lahore, Pakistan

3 Federal Urdu University of Arts & Science, Islamabad, Pakistan

4 Islamic International University, Islamabad, Pakistan

### **References**


[10] Menezes, CA, ''Low Resistivity CdTe‐Te Films by a Combined Hot‐Wall‐Flash Evaporation Technique.'' ECS Journal of Solid‐State Science and Technology, 127, 155 (1980).

**Author details**

Nazar Abbas Shah1

**References**

1982.

cm2

Florence, 2001.

JJAP.19.703.

\*, Zamran Rabeel2

404 Modern Technologies for Creating the Thin-film Systems and Coatings

Information Technology, Islamabad, Pakistan

Conference, p. 1138, 1982.

\*Address all correspondence to: nabbasqureshi@yahoo.com

2 Higher Education Department, Punjab, Lahore, Pakistan

4 Islamic International University, Islamabad, Pakistan

3 Federal Urdu University of Arts & Science, Islamabad, Pakistan

, Murrawat Abbas3

1 Thin Films Technology Research Laboratory, Department of Physics, COMSATS Institute of

[1] Werthen, JG, Anthony, TC, Fahrenbruch, AL, Bube, RH, 16th IEEE Photovoltaic Specialists

[2] Tyan, YS, Perez‐Albuerne, EA, 16th IEEE Photovoltaic Specialists Conference, p. 794,

[3] Chakrabarti, R, Dutta, J, Maity, AB, Chaudhuri, S, Pal, AK, "Photoconductivity of CdTe

[4] Touskova, J, Kindl, D, Tousek, J, ''Preparation and Characterization of CdS/CdTe Thin Film Solar Cells.'' Thin Solid Films, 293, 272 (1997). doi:10.1016/S0040‐6090(96)09113‐4. [5] Matsumoto, H, Kuribayashi, K, Komatsu, Y, Nakano, A, Uda, H, Ikegami, S, ''30 × 30

[6] Wu, X, Keane, JC, Dhere, RG, Dehart, C, Albin, DS, Duda, A, Gessert, TA, Asher, SD, Levi, H, Sheldon, P, "Seventeenth European Photovoltaic Solar Energy Conference." Proceedings from the International Conference, Vol. 1, p. 995. WIP Munich and ETA‐

[7] Abbas Shah, N, Ali, A, Ali, Z, Maqsood, A, Aqili, AKS, ''Properties of Te‐Rich Cadmium Telluride Thin FilmsFabricated by Closed Space Sublimation Technique.'' Journal of

[8] Nakayama, N, Matsumoto, H, Nakano, A, Ikegami, S, Uda, H, Yamashita, T, ''Ceramic Thin Film CdTe Solar Cell.'' Journal of Applied Physics, 19, 703 (1980). doi:10.1143/

[9] Khan, MA, Shah, NA, Ali, A, Basharat, M, Hannan, MA, Maqsood, A, ''Fabrication and Characterization of Cd Enriched CdTe Thin Films by Close Spaced Sublimation.''

CdS/CdTe Single Substrate Module Prepared by Screen Printing Method.'' Japan.

Films." Thin Solid Films, 32, 288 (1996). doi:10.1016/S0040‐6090(96)08801‐3.

Journal of Applied Physics, 22, 891 (1983). doi:10.1143/JJAP.22.891.

Crystal. Growth, 284, 477 (2005). doi:10.1016/j.jcrysgro. 08.005 2005.

Journal of Coatings Technology and Research 6, 251–256 (2009).

and Waqar Adil Syed4


[41] Dluzewski, P, ``TEM Characterization of MBE Grown CdTe/ZnTe Axial Nanowires'', Journal of Microscopy, 237, 337–340, (2010).

[24] Bonnet, D, "Manufacturing of CSS CdTe solar cells." Thin Solid Films, 361, 547 (2000). [25] Birkmire, RW, "Recent progress and critical issues in thin film polycrystalline solar cells and modules." IEEE Photovoltaics: 26th Conference. Proceedings. pp. 295–300, 1997.

[27] Laks, DB, Van de Walle, CG, Neumark, GF, Pantelides, ST, "Role of native defects in wide‐band‐gap semiconductors." Physical Review Letters, 66, 648–651 (1991).

[28] Chegaar, M, Ouennoughi, Z, Guechi, F, Langueur, H, ``Determination of Solar Cells Parameters under Illuminated Conditions'', Journal of Electronic Devices, 2, 17–21 (2003).

[29] Salinger, J, ``Measurement of Solar Cell Parameters with Dark Forward I‐V Characteristics'',

[30] Green, MA, "Solar Cell Fill Factors: General Graph and Empirical Expressions", Journal

[32] Shah, NA, Adil, WA, Atta, MA, ``Characterization of II–VI Semiconductor Thin Films Fabricated by Close Spaced Sublimation'', Nanoscience and Nanoletters, 1, 62–65 (2009).

[33] Shah, NA, Ali, A, Maqsood, A, Characterization of CdTe Thin Films Fabricated by Close Spaced Sublimation Technique and a Study of Cu Doping by Ion Exchange Process''.

[34] Kaydanov, VI, Ohno, TR, Studies of Basic Electronic Properties of CdTe‐Based Solar Cells and Their Evolution during Processing and Stress, Final Technical Report 16

[35] Fthenakis, VM, ``Life Cycle Impact Analysis of Cadmium in CdTe PV Production''.

[36] Kumar, V, ``Characterization of Large Area Cadmium Telluride Films and Solar Cells Deposited on Moving Substrates by Close Spaced Sublimation'', MS thesis, University

[37] Ferekides, CS, "Thin Films and Solar Cells of Cadmium Telluride and Cadmium Zinc

[38] Khrypunova, G, Romeo, A, Kurdesauc, F, Bätzner, DL, Tiwarie, AN, ``Recent Developments in Evaporated CdTe Solar Cells'', Solar Energy Materials and Solar Cells,

[39] Alvin Compaan, D, Gupta, A, Lee, S, Wang, S, Drayton, J, ``High Efficiency, Magnetron

[40] Feuillet, M, Charleux, G, Mariette, M, Pub, H, ``Atomic Layer Epitaxy of CdTe and

October 2001—31 August 2005 Colorado School of Mines Golden, Colorado.

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Telluride", Ph.D. Dissertation, University of South Florida, USA (1991).

Sputtered CdS/CdTe solar Cells, Solar Energy, 77, 815–822 (2004).

MnTe Hartmann'', Journal of Applied Physics,79, 3035 (1996).

Method'', Journal of Coatings Technology and Research, 7, 105–110 (2010).

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[26] Chopra, KL, "Thin Film Phenomena." 1969 (New York: McGraw‐Hill).

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406 Modern Technologies for Creating the Thin-film Systems and Coatings

of Solid‐State Electronics, 24, 788–789 (1981).

[31] Shah, NA, Ali, A, Hussain, S, ``CdCl2

of south Florida, USA (2003).

90, 664–677 (2006).

[42] Romeo, N, Bosio, A, Canevari, V, Podesta, A, ``Recent Progress on CdTe/CdS Thin Film Solar Cells'', Solar Energy, 77, 665–994 (2004).

### **Silver-Based Low-Emissivity Coating Technology for Energy-Saving Window Applications Silver-Based Low-Emissivity Coating Technology for Energy-Saving Window Applications**

Guowen Ding and César Clavero Guowen Ding and César Clavero

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67085

#### **Abstract**

Low-emissivity (low-E) technology is a unique and cost-effective solution to save energy in buildings for different climates. Its development combines advances in materials science, vacuum deposition, and optical design. In this chapter, we will review the fundamentals of energy saving window coatings, the history of its application, and the materials used. The current low-E coating technologies are overviewed, especially silverbased low-E technologies, which comprise more than 90% of the overall low-E market today. Further, the advanced understanding of generating high-quality silver thin films is discussed, which is at the heart of silver-based low-E product technology development. How the silver thin film electrical, optical, and emissivity properties are influenced by their microstructure, thickness, and by the materials on neighboring layers will be discussed from a theoretical and an experimental perspective.

**Keywords:** low-E, emissivity, silver, coating, glass, window, optical, materials

### **1. Introduction to low-E applications**

The growing awareness of global warming has intensified efforts to make buildings and vehicles more energy efficient. "Building heating, ventilation, and air conditioning (HVAC) accounted for 14% of primary energy consumption in the United States in 2013" [1]. Windows are often considered the least energy-efficient component in a building. Efficiency upgrades that improve the energy efficiency of windows are among the most promising and costeffective energy technology options available now. A National Academy of Sciences study concluded that, "by an order of magnitude, the largest apparent benefits [of the technologies examined] were realized as avoided energy costs in the buildings sector in energy efficiency"[2].

and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Modern coating technologies provide architectural window coatings with adequate illumination levels in building interiors, while controlling energy transfer to save either cooling or heating energy. In 2003, 95% typical new windows in U.S. were double-glazed windows, and 50% have a low-E coating [3].Thus, energy saving windows are very popular in developed countries.

### **1.1. Why low-E coating windows are needed**

Typical commercial buildings waste 30% of the energy they consume, mostly by heat and cooling loss through the building envelope (windows, doors, roofs, etc.) [4]. Losses through windows alone are estimated to cost U.S. consumers roughly \$40 billion each year [2]. Radiation losses occur through the window glass and represent about 60% of the total heat loss in a standard window [4]. How can the heat transfer through windows be effectively controlled? One cost-effective solution today is through low-emissivity (low-E) coatings.

### **1.2. What is low-emissivity coating?**

To understand low-emissivity coatings, let us first define emissivity. Emissivity is the ratio of heat emitted from a given material compared to that from a blackbody, from zero to one. A blackbody would have an emissivity of 100% and a perfect reflector would have zero value. The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. The typical common materials emissivity is listed below in **Table 1**.

When the emissivity of a window coating is low, the window coating is called low-E coating. The standard varies for different countries. Pyrolytic low-E coating for single-pane glass normally can achieve around 20% emissivity, while silver-based sputter coatings can achieve 8–2% emissivity, which represents currently 90% of the low-E market. These two types of low-E coatings will be discussed in more detail in the later sections.


**Table 1.** Emissivity of common materials [39].

### **1.3. How can low-E coating windows save energy?**

Let us first review a few common terminologies in the low-E field according to Ref. [5]:

**Insulating glass unit (IGU)** commonly consists of double or triple panes of glass separated by a vacuum or gas-filled space and sealed together at the edge to reduce the heat transfer of the buildings. The common insulating filled gas are air, argon, or krypton. There are four surfaces for double-paned IGUs, commonly labeled as surface 1–4 from exterior to interior, as shown in **Figure 1** [5].

**Figure 1.** The structure of IGU.

Modern coating technologies provide architectural window coatings with adequate illumination levels in building interiors, while controlling energy transfer to save either cooling or heating energy. In 2003, 95% typical new windows in U.S. were double-glazed windows, and 50% have a low-E coating [3].Thus, energy saving windows are very popular in developed countries.

Typical commercial buildings waste 30% of the energy they consume, mostly by heat and cooling loss through the building envelope (windows, doors, roofs, etc.) [4]. Losses through windows alone are estimated to cost U.S. consumers roughly \$40 billion each year [2]. Radiation losses occur through the window glass and represent about 60% of the total heat loss in a standard window [4]. How can the heat transfer through windows be effectively controlled?

To understand low-emissivity coatings, let us first define emissivity. Emissivity is the ratio of heat emitted from a given material compared to that from a blackbody, from zero to one. A blackbody would have an emissivity of 100% and a perfect reflector would have zero value. The emissivity of the surface of a material is its effectiveness in emitting energy as thermal

When the emissivity of a window coating is low, the window coating is called low-E coating. The standard varies for different countries. Pyrolytic low-E coating for single-pane glass normally can achieve around 20% emissivity, while silver-based sputter coatings can achieve 8–2% emissivity, which represents currently 90% of the low-E market. These two types of

One cost-effective solution today is through low-emissivity (low-E) coatings.

radiation. The typical common materials emissivity is listed below in **Table 1**.

low-E coatings will be discussed in more detail in the later sections.

**Materials surface\*\* Thermal emissivity**

Aluminum foil 0.03 Asphalt 0.88 Brick 0.9 Concrete, rough 0.91 Glass, smooth (uncoated) 0.91 Limestone 0.92 Marble, Polished or white 0.89–0.92 Marble, Smooth 0.56 Paper, roofing or white 0.88–0.86 Plaster, rough 0.89 Silver, polished 0.02

**1.1. Why low-E coating windows are needed**

410 Modern Technologies for Creating the Thin-film Systems and Coatings

**1.2. What is low-emissivity coating?**

**Table 1.** Emissivity of common materials [39].

**Visible light transmittance (Tvis)** is the transmitted percentage of visible light (380–780 nm) through glass, (or insulating glass unit, IGU).

**Solar heat gain coefficient (SHGC)** is the percentage of the solar energy passing through the window over the incident solar energy (including direct solar transmittance and indirect reradiation).

**U-factor** is a measure of air-to-air heat transmission (loss or gain) in indoor and outdoor temperatures of a 1-m high glazing due to the thermal conductance and the difference. It is an overall coefficient of heat transfer, the lower the U-factor, the better the insulating properties of the windows.

Second, let us explain how low-E coating could save energy, which is discussed in two climate conditions:


concept is needed, and it is called LSG (light-to-Solar gain ratio). We desire LSG to be as high as possible and current highest LSG window product in the market is around 2.4.

In the spectra level, the ideal low-E coating spectra for cold and hot climates were illustrated from reference [6] as in **Figure 2**.

**Figure 2.** The idealized low-E coating spectra for hot or cold climates.

#### **1.4. Examples of low-E windows in saving energy cost**

The energy savings by installing low-E window are solid. Guardian Industries demonstrated two examples at the 2012 AIA National Convention and Design Exposition [7]: Two similar buildings were located in Chicago and Miami, respectively. The floor area of the six stories buildings were 120,000 square feet for both, with floor-to-floor height of 12 feet; slab on grade foundation, strip-type windows, window area 20,000 square feet, R-13 wall insulation, natural gas for heat, and electric power A/C. Several Guardian window products were installed as listed below (the IGU configuration were 6 mm glass/12 mm space/6 mm glass, and coating was at #2 surface) (**Table 2**).

Silver-Based Low-Emissivity Coating Technology for Energy-Saving Window Applications http://dx.doi.org/10.5772/67085 413


**Table 2.** Several typical glass product and their basic parameters [7] .

concept is needed, and it is called LSG (light-to-Solar gain ratio). We desire LSG to be as high as possible and current highest LSG window product in the market is around

In the spectra level, the ideal low-E coating spectra for cold and hot climates were illustrated

The energy savings by installing low-E window are solid. Guardian Industries demonstrated two examples at the 2012 AIA National Convention and Design Exposition [7]: Two similar buildings were located in Chicago and Miami, respectively. The floor area of the six stories buildings were 120,000 square feet for both, with floor-to-floor height of 12 feet; slab on grade foundation, strip-type windows, window area 20,000 square feet, R-13 wall insulation, natural gas for heat, and electric power A/C. Several Guardian window products were installed as listed below (the IGU configuration were 6 mm glass/12 mm space/6 mm glass, and coating

2.4.

from reference [6] as in **Figure 2**.

412 Modern Technologies for Creating the Thin-film Systems and Coatings

**1.4. Examples of low-E windows in saving energy cost**

**Figure 2.** The idealized low-E coating spectra for hot or cold climates.

was at #2 surface) (**Table 2**).

The annual cost for cooling and heating was calculated in comparison to clear glass (without low-E coating) as shown below. The savings purely from heating and cooling could be \$8000 annually in the Chicago building, and double in the Miami building, as shown in **Figure 3**. The reason for the reduction of heating and cooling cost is not only due to the low-E window preventing the heat transfer to make the room warm at night and cool at day time, but also due to a significant reduction of the solar IR heat through the windows.

**Figure 3.** Comparison of annual heating and cooling saving cost for a building in Chicago and Miami [7].

In addition, the cooling and heating system capacity could be reduced too, so that one-time savings from reduction in HVAC system cooling capacity was calculated in comparison to a noncoating unit, as shown in **Figure 4**. That cost reduction could be \$30,000 for the buildings.

Considering a 10-year period, the cost saving is very attractive: \$100,000 in Chicago, and double that amount in Miami (\$200,000). The exact number with different low-E product is showed in **Figure 5**, in comparison with clear glass windows (without low-E coatings).

**Figure 4.** One-time savings from reduction in HVAC system cooling capacity (compared to a noncoating unit) [7].

**Figure 5.** Ten-year savings including heating/cooling cost saving and one-time saving from reduction in HVAC system cooling capacity (compared to a noncoating unit) [7].

### **2. Low-E coating technology**

#### **2.1. Brief history of low-E coating technology**

Low-E coating technology's roots can be traced back more than 100 years ago, from Drude, Hagen, and Rubens's theories. The first glass coating that was able to selectively reflect radiation can be traced back to 1958, when Holland and Siddall [8] demonstrated a gold coating on glass with high-transmittance and high-heat reflections. In the 1960s, a product called "Stop Ray" was first released in the market for solar control glass to reduce the cooling cost of the building. Later, a product called "Infrastop" was also introduced to the market. In the 1970s and 1980s, another type of low-E glass appeared, called "K-Glass," which demonstrated high environmental and chemical durability, and also reduced the solar IR heat transfer. In the 1980s, the silver coating breakthrough was demonstrated, with higher transmittance, much lower emissivity, and friendly color. Quickly, silver coating low-E became the dominant low-E product in the market. Today, silver coatings can be further developed with multiple silver blocks, known as double silver and triple silver products. A brief history of low-E is covered in **Table 3** [9, 10].

#### **2.2. What materials can be used for low-E window coating**

Around the 1900s, the German physicist Paul Drude explained the optical behavior of free electrons in a solid based on the kinetic theory of free electrons in a metal. This theory is still widely used in literature today. In the early twentieth century, physicists Hagen and Rubens found that the heat emission from bulk metals described by their emissivity, *ε*, correlates strongly with their conductivity, *σ*, i.e., with the concentration of free electrons [11]. Based on the Drude model, they derived a formula to connect conductivity *σ* and emissivity:

$$
\varepsilon = \sqrt{\frac{8\varepsilon\_0\omega}{\sigma}}\tag{1}
$$

Thus, the higher the conductivity, the lower the emissivity. In the low-E industry, there are a few practical estimations on the relationship of emissivity and sheet resistivity, such as *ε* = 0.0106 R*□*, where R*□* is the film sheet resistivity [11]. This is the reason why film-sheet resistivity is a very important parameter in qualifying low-E products, and it is easily measured by a four-point probe. In addition, Ref. [12] found a way to estimate the optical properties of silver (refractive index) at near IR by resistivity measurements [12].

There are two types transparent low-E coatings in today's market: (1) semiconductive coatings, e.g., ITO (indium tin oxide) and FTO (fluorine-doped tin oxide) and (2) metallic coating. Some common low-E materials are listed in **Table 4** for comparison.

### **2.3. Major industry Low-E window coating technologies**

**2. Low-E coating technology**

cooling capacity (compared to a noncoating unit) [7].

414 Modern Technologies for Creating the Thin-film Systems and Coatings

**2.1. Brief history of low-E coating technology**

Low-E coating technology's roots can be traced back more than 100 years ago, from Drude, Hagen, and Rubens's theories. The first glass coating that was able to selectively reflect radiation can be traced back to 1958, when Holland and Siddall [8] demonstrated a gold coating on glass with high-transmittance and high-heat reflections. In the 1960s, a product called "Stop Ray" was first released in the market for solar control glass to reduce the cooling cost of the

**Figure 5.** Ten-year savings including heating/cooling cost saving and one-time saving from reduction in HVAC system

**Figure 4.** One-time savings from reduction in HVAC system cooling capacity (compared to a noncoating unit) [7].

Although there are many thin film coating methods available for glass coatings, such as solgel, PECVD, ALD, and E-beam evaporation, there are only two major low-E window coating technologies in today's market: sputtered coating and pyrolytic coating, which provide costeffective, high durablility, excellent uniformity on jumbo glass (3 m or more wide glass). The two technologies are discussed in the following:

**1.** Chemical vapor deposition (CVD) coating, or called pyrolytic coating, is a low-E coating technology that appeared in the market in the 1970s. This method deposits films directly on the hot glass while it is still on the float line, so it is also called online low-E. The layout is shown in **Figure 6**. The CVD process is chosen right after the float/tin bath on the production line with around 600°C. Because the substrate glass moves about 1 ft/s as it travels down the float line, only 1–1.5 s are available for the coating to form. The precursor compounds (both gases and liquids) are vaporized in a reactor that spreads the resulting gas mixture uniformly over an advancing, newly formed glass ribbon. Chemical reactions occur in the gas above the glass and on the growing surface of the deposited film. Temperature


**Table 3.** Brief history of low-E technology, data obtained from [9, 10]. control is easier because the large thermal mass of the system also keeps the temperature relatively uniform across the ribbon. Strongly adhered coatings maintain their integrity when the product is bent and tempered [13].


**Table 4.** The common low-E materials comparison, data obtained from Ref. [11] .

**1950s**

> Notes

Market for

First solar

control glass

to reduce

the cooling

cost of the

building

Firm: Film stack

Trade

"Stop ray"

"Infrastop"

Thermoplus

Thermoplus

k-Glass

Iplus

neutral

name

New

High

High IR

High IR

High

Transmission

Highly

Aging

Higher

Better

Better

resistant

transmission,

solar gain

solar gain

better neutral

control,

control,

higher

higher

LSG, >1.7

LSG >2.2

durable

form

40==>60%, U:

chemical/

and color

mechanical

neutral

color

resistance

better than

greenish/

pinkish

color by

Au/Cu

coating

1.3W/m 2 k,

Jumbo glass

transmittance

low-e coating

reflection,

low thermal

reflection,

low thermal

emissivity,

emissivity,

good

good

transmittance

Deposition

Sputter/

Sputter

Evaporation

Pyrolysis /

Sputter

Pyrolysis/

Sputter

Sputter

Sputter

Sputter

APCVD

APCVD

evaporation

method

**Table 3.**

Brief history of low-E technology, data obtained from [9, 10].

transmittance

features

transmittance/

high heat

reflection

**BiOx/Au/BiOx**

**BiOx/Au**

**ZnS/Au/ZnS**

**Tin oxide**

**BiOx/Au,** 

**Tin oxide**

**BiOx/**

**Silver-based** 

**Oxide /**

**3 cycles of** 

416 Modern Technologies for Creating the Thin-film Systems and Coatings

**Ag/oxide** 

**Oxide/Ag/**

**/Ag/**

**oxide**

**oxide**

Solarban

glass

**PbOx/**

**coating**

**Ag/PbOx/**

**BiOx**

**IGU filled Ar**

BOC Edwards

Flaverbel

Heraeus /

Philips (NL)

Flashglass /

Pilkington

Interpane

Interpane

Cardinal,

PPG

etc.

DELOG

DETAG

heat reflection

glass

**1960s**

**1960s**

**1970s** Heat

1st

insulating

energy

glass units of

crisis

building

**1974**

**1975–1980s**

**1980s**

**1981**

**1988** Asymmetrical

Double

Tripple

silver layer

silver

silver

system

**1990s**

**2006**

**Figure 6.** Schematic of a float bath production line used to deposit fluorine-doped tin oxide. Simplified from Ref. [13].

The typical deposition materials by pyrolytic method is fluorine-doped tin oxide (FTO). Their extinction coefficient *k* is very small (0.01 at 550 nm), so that the typical thickness is of µm scale, whose transmittance of >80% is acceptable. The refractive index is typically around 2. This kind of coating shows extremely good environmental and chemical resistance. Thus, sometimes it is called hard-low-E coating.

**2.** Today, more than 90% of the low-E window coatings are manufactured by sputtered coaters. Worldwide, billions of square meters per year of glass is coated by sputtering method and this amount is increasing steadily. Metallic low-E coating is conveniently manufactured by a sputtering method, because it provides a cost-effective solution and excellent coating uniformity; in addition, it also provides rich products with variety of choices on color, transmittance, solar heat gain, etc. The sputtering coater provides improved emissivity (below 0.08), therefore better heat radiation control, better solar heat control, and better optical performance. In addition, the price for current low-E coating products is very affordable, below \$1/ft2 for single silver coating on 3 mm soda-lime glass.

Sputtering coater could process many different types of materials including:


The typical metal sputter deposition coater layout is shown in **Figure 7**.

**Figure 7.** Schematic of a continuous-batch sputter-coating reactor, simplified from Ref. [13].

The sputtering coater is typically independent of the glass production, so it is also called offline low-E coating. The coater line starts with a glass washer, and an entrance chamber to pump down to vacuum condition. The glass travels through each chamber for different layers of material deposition, finally passing through the exit chamber to the ambient condition. Being an offline process, sputtering process allows for a high level of flexibility, such as flexible layer system, the scale of production, etc. Also, it is generally regarded to be environmentally safe, without waste products. Thus, the sputter coating is the most used technology in the low-E coating industry today.

### **3. Silver-based low-E coating technology overview**

Among the sputtered low-E coating products, silver-based low-E is dominant. There are three major categories of silver-based low-E products: single-silver, double-silver, and triple-silver products [14]. The structures are similar, with single-silver structure being the simplest one. A typical structure is as follows (**Figure 8**).

**Figure 8.** Typical single-silver coating layer structure.

It has been reported that the electrical/optical properties of Ag thin film strongly depend on its microstructure, such as crystallite size [15], grain size [16], grain boundaries [17], and surface roughness [18, 19], and also on the microstructure of the dielectric under-layers [20–22]. Thus, the R&D direction for the low-E industry has been focused on how to improve silver-thin film microstructure for better optical and thermal performance. Generating high-quality silver thin films is at the center of the technological development of silver-based low-E products. This will be further discussed in Section 3. The base and top layers are typically transparent dielectric materials layers, which are critical to the visible optical performance. Seed and blocker layers are very important to the emissivity properties, and they will be discussed in the following sections.

#### **3.1. Seed layer**

**2.** Today, more than 90% of the low-E window coatings are manufactured by sputtered coaters. Worldwide, billions of square meters per year of glass is coated by sputtering method and this amount is increasing steadily. Metallic low-E coating is conveniently manufactured by a sputtering method, because it provides a cost-effective solution and excellent coating uniformity; in addition, it also provides rich products with variety of choices on color, transmittance, solar heat gain, etc. The sputtering coater provides improved emissivity (below 0.08), therefore better heat radiation control, better solar heat control, and better optical performance. In addition, the price for current low-E coating products is

• *Metal materials*: most metal could be deposited by sputtering method. The most widely used metallic low-E coatings are silver or gold. The extinction coefficient *k* is very high for such metals, such as 3.5 for sliver and 2.6 for gold at wavelength of 550 nm, so only thin

• *Semiconductor*: such as indium-doped tin oxide (ITO), aluminum-doped zinc oxide (AZO), etc. • *Dielectric materials*: such as bismuth oxide, tin oxide, zinc oxide, titanium oxide, silicon

The sputtering coater is typically independent of the glass production, so it is also called offline low-E coating. The coater line starts with a glass washer, and an entrance chamber to pump down to vacuum condition. The glass travels through each chamber for different layers of material deposition, finally passing through the exit chamber to the ambient condition. Being an offline process, sputtering process allows for a high level of flexibility, such as flexible layer system, the scale of production, etc. Also, it is generally regarded to be environmentally safe, without waste products. Thus, the sputter coating is the most used technology in the low-E coating industry today.

Among the sputtered low-E coating products, silver-based low-E is dominant. There are three major categories of silver-based low-E products: single-silver, double-silver, and triple-silver products [14]. The structures are similar, with single-silver structure being the simplest one.

Sputtering coater could process many different types of materials including:

films such as 20 nm are acceptable for good transmittance.

**3. Silver-based low-E coating technology overview**

A typical structure is as follows (**Figure 8**).

The typical metal sputter deposition coater layout is shown in **Figure 7**.

**Figure 7.** Schematic of a continuous-batch sputter-coating reactor, simplified from Ref. [13].

for single silver coating on 3 mm soda-lime glass.

very affordable, below \$1/ft2

418 Modern Technologies for Creating the Thin-film Systems and Coatings

oxide, silicon nitride, etc

Using a seed layer is a common deposition technique to promote thin-film microstructure, and to enhance the thin-film properties, such as optical and mechanical properties. It is often reported that the ZnO seed layer can enhance silver thin film crystallite size and grain size, so that its resistivity and absorption is greatly reduced [22, 23]. Arbab et al. [23] have demonstrated that ZnO seed layer is better than other oxides, such as zinc stannate, as shown in **Figures 9** and **10**. The well-crystallized Ag (111) atop of ZnO (002) basal plane induced lower resistivity than that of polycrystalline Ag atop of zinc stannate. ZnO is a material that crystallizes very easily at room temperature even at very thin thickness such as 5 nm. The ZnO lattice sites are at the corners and center of a hexagon. Three silver atoms at alternate fourfold hollow sites form the unit cell of a (111) plane of silver, with 2.6% lattice mismatch between the ZnO and Ag layers [23], thus, the crystallized ZnO lattice promote the silver growing at (111) direction.

**Figure 9.** Well-crystallized Ag (111) atop of ZnO (002) basal plane induced lower resistivity than that of polycrystalline Ag atop of zinc stannate.

**Figure 10.** Ag (111) growth atop of ZnO (002).

#### **3.2. Blocker layer**

The blocker layer is extremely important in silver-based low-E coating. Treichel et al. [24] gave a good description of blocker layer functions by noting that the typical top layer is comprised by oxide materials, such as SnO2 , TiO2 , or ZnO. Without a blocker layer, the deposition of the top layer takes place directly on top of the unprotected silver film. In such a case, the silver layer meets a highly reactive sputter process, with the presence of oxygen radicals. The silver crystal lattice will be damaged and the silver atoms will agglomerate, until the top layer forms close to the surface, preventing further reacting species from reaching the silver [24]. Since the quality of the low-E coating is mainly determined by the crystallized silver, an additional barrier, the blocker layer, becomes necessary for high-quality low-E coating. In the study shown below, the metal titanium was chosen. In low-E applications, emissivity and resistivity is in nearly linear relationship. When the blocker is too thin, no close protecting layer formed yet, the emissivity/ resistivity (from silver) is high, which is called region 1. As the blocker thickness is increased, the emissivity/resistivity reaches a minimum, called region 2. Further increasing the thickness, called region 3, leads to emissivity/resistivity flat or increasing slightly, as shown in **Figure 11**.

**Figure 11.** The resistivity of single-silver stack is dependent on the blocker thickness.

The optimized blocker layer consists of two portions: a metallic portion close to the silver layer and a region with higher oxidization ratio close to the top layer. The typical blocker layer thickness is below 10 nm [24].

### **3.3. Double silver and triple silver**

**3.2. Blocker layer**

by oxide materials, such as SnO2

**Figure 10.** Ag (111) growth atop of ZnO (002).

420 Modern Technologies for Creating the Thin-film Systems and Coatings

The blocker layer is extremely important in silver-based low-E coating. Treichel et al. [24] gave a good description of blocker layer functions by noting that the typical top layer is comprised

top layer takes place directly on top of the unprotected silver film. In such a case, the silver layer meets a highly reactive sputter process, with the presence of oxygen radicals. The silver crystal lattice will be damaged and the silver atoms will agglomerate, until the top layer forms close to the surface, preventing further reacting species from reaching the silver [24]. Since the quality of the low-E coating is mainly determined by the crystallized silver, an additional barrier, the blocker layer, becomes necessary for high-quality low-E coating. In the study shown below, the metal titanium was chosen. In low-E applications, emissivity and resistivity is in nearly linear relationship. When the blocker is too thin, no close protecting layer formed yet, the emissivity/ resistivity (from silver) is high, which is called region 1. As the blocker thickness is increased, the emissivity/resistivity reaches a minimum, called region 2. Further increasing the thickness, called region 3, leads to emissivity/resistivity flat or increasing slightly, as shown in **Figure 11**.

, or ZnO. Without a blocker layer, the deposition of the

, TiO2

**Figure 11.** The resistivity of single-silver stack is dependent on the blocker thickness.

The coating stack in **Figure 8** can now be used as a building block for multi-low-E stacks. Introducing a sequence of two blocks in **Figure 8** leads to double silver stacks, which will enhance low-E coating with higher selectivity between IR and visible. The typical structure is illustrated in **Figure 12**.

**Figure 12.** The double silver stack layout from PPG products with the data obtained from Ref. [25].

In this case, the dielectric material is zinc stannate, the seed layer is zinc oxide, and the blocker layer is titanium. The zinc stannate layer in the middle has a thickness roughly equal to the total thickness of the base and the top layer of the single silver. The typical double low-E coating glass spectra is shown in **Figure 13**, with transmittance around 70%, and reflectance from film and glass sides below 10% in the visible region. However, the transmittance is near zero at the IR region (*λ* > 1000 nm), and the reflectance for IR is very high >90% (for *λ* > 1000 nm).

**Figure 13.** The typical double silver spectra of transmittance and reflectance from film side and glass side [26].

Further, if the three blocks of single silver of **Figure 8** were put together [13], it would make a triple silver stack which is shown in **Figure 14**. The selectivity of IR and visiblity is the best in today's market, however, the cost is higher.

**Figure 14.** A typical triple silver stack.
