**3. Chemical surface deposition of CdS thin films from CdSO4, CdCl2, CdI2 aqueus solutions**

#### **3.1 Introduction**

One of the methods to increase SC efficiency based on CdS/CdTe, CdS/CuIn1-xGaxSe2, with the CdS film as the window is increasing the current density value (Stevenson, 2008). This can be achieved by reducing losses in the photons optical absorption from λ> 500 nm by reducing CdS film thickness. To provide a spatially homogeneous work of the device the CdS films should not only be thin, but solid, durable and resistant to further technology of SC production. To produce ultra-thin (from 30 to 100 nm) and homogeneous CdS films the technology of bath chemical deposition is widely used (Estela Calixto at al., 2008, Mugdur at al., 2007).

Chemical deposition technology is quite simple, inexpensive and suitable for the deposition of polycrystalline CdS films on large areas. Deposition of thin CdS films from aqueous solutions is a reaction between cadmium salt and thiocarbamid (thiourea) in alkaline medium. Mostly are used simple cadmium salts: CdSO4 (Chaisitsak at al., 2002, Contreras at al., 2002, Tiwari & Tiwari, 2006, Chen at al., 2008), CdI2 (Nakada & Kunioka, 1999, Hashimoto at al., 1998), Cd(CH3COO)2 (Granath at al., 2000, Rau & Scmidt, 2001) and CdCl2 (Qiu at al., 1997, Aguilar-Hernández at al., 2006). Thiourea (TM) is used as sulfide agent in the reactions of sulfide deposition, as has a high affinity to metal cations and decomposes at low temperatures. Deposition process can be described by two mechanisms (Oladeji, 1997, Soubane, 2007). Homogeneous mechanism involves formation of layer with the CdS colloidal particles, which are formed in solution and consists of several stages.

1. Ammonium dissociation:

$$\mathrm{NH}\_4^+ + \mathrm{OH}^- \rightleftharpoons \mathrm{NH}\_3 + \mathrm{H}\_2\mathrm{O} \tag{1}$$

In alkaline medium due to interaction Cd2+ ions with the OH- environment ions is possible formation of undesirable product - Cd(OH)2:

$$\text{Cd}^{2+} + \text{OH}^- \rightarrow \text{Cd(OH)}\_2\downarrow \tag{2}$$

2. Thiourea hydrolysis (NH2)2CS with the the formation of sulfide ions

$$\text{(NH}\_2\text{)}\_2\text{CS} + \text{H}\_2\text{O} \rightleftharpoons \text{HS}^- + \text{H}^+ + \text{(NH}\_2\text{)}\_2\text{CO} \tag{3}$$

$$\text{HS}^- + \text{OH}^- \rightleftharpoons \text{S}^{2-} + \text{H}\_2\text{O} \tag{4}$$

3. Final product formation

$$\text{Cd}^{2+} + \text{S}^{2-} \rightleftharpoons \text{CdS} \downarrow \tag{5}$$

Deposition of thin CdS films from the aqueous solutions through the stage of cadmium tetramin [*Cd*(*NH*3)4] 2+ complex ion formation, which reduces the overall speed of reaction and prevents Cd(OH)2 formation by the heterogeneous mechanism.

$$\text{Cd}^{2+} + 4\text{NH}\_4\text{OH} \rightarrow \text{[Cd(NH}\_3\text{)}\_4\text{]}^{2+} + 4\text{H}\_2\text{O} \tag{6}$$

$$\left[\text{Cd(NH}\_3\text{)}\_4\right]^{2+} + S^{2-} \rightarrow \text{CdS} \downarrow + \text{4NH}\_3 \tag{7}$$

In general form:

386 Solar Cells – Thin-Film Technologies

This paper describes CSD technology of CdS thin films from aqueous solutions of cadmium salts CdSO4, CdCl2, CdI2. The properties of CdS films deposited on glass and ITO/glass from the nature of the initial salt and solar cells based on CdTe/CdS with CSD CdS films as

**3. Chemical surface deposition of CdS thin films from CdSO4, CdCl2, CdI2**

One of the methods to increase SC efficiency based on CdS/CdTe, CdS/CuIn1-xGaxSe2, with the CdS film as the window is increasing the current density value (Stevenson, 2008). This can be achieved by reducing losses in the photons optical absorption from λ> 500 nm by reducing CdS film thickness. To provide a spatially homogeneous work of the device the CdS films should not only be thin, but solid, durable and resistant to further technology of SC production. To produce ultra-thin (from 30 to 100 nm) and homogeneous CdS films the technology of bath chemical deposition is widely used (Estela

Chemical deposition technology is quite simple, inexpensive and suitable for the deposition of polycrystalline CdS films on large areas. Deposition of thin CdS films from aqueous solutions is a reaction between cadmium salt and thiocarbamid (thiourea) in alkaline medium. Mostly are used simple cadmium salts: CdSO4 (Chaisitsak at al., 2002, Contreras at al., 2002, Tiwari & Tiwari, 2006, Chen at al., 2008), CdI2 (Nakada & Kunioka, 1999, Hashimoto at al., 1998), Cd(CH3COO)2 (Granath at al., 2000, Rau & Scmidt, 2001) and CdCl2 (Qiu at al., 1997, Aguilar-Hernández at al., 2006). Thiourea (TM) is used as sulfide agent in the reactions of sulfide deposition, as has a high affinity to metal cations and decomposes at low temperatures. Deposition process can be described by two mechanisms (Oladeji, 1997, Soubane, 2007). Homogeneous mechanism involves formation of layer with the CdS

*NH OH NH H O* <sup>4</sup> 3 2

In alkaline medium due to interaction Cd2+ ions with the OH- environment ions is possible

<sup>2</sup> *HS OH S H O*<sup>2</sup>

(1)

<sup>2</sup> *Cd OH Cd OH* ( ) (2)

(4)

2 2 *Cd S CdS* (5)

22 2 2 2 ( ) *NH CS H O HS H NH CO* ( ) (3)

colloidal particles, which are formed in solution and consists of several stages.

2

2. Thiourea hydrolysis (NH2)2CS with the the formation of sulfide ions

windows was investigated.

Calixto at al., 2008, Mugdur at al., 2007).

1. Ammonium dissociation:

3. Final product formation

formation of undesirable product - Cd(OH)2:

**aqueus solutions 3.1 Introduction** 

$$\text{C}\left[\text{Cd(NH}\_3\text{)}\_4\right]^{2+} + \text{(NH}\_2\text{)}\_2\text{CS} + \text{OH}^- \rightarrow \text{CdS}\downarrow + 4\text{NH}\_3 + \text{H}^+ + \text{(NH}\_2\text{)}\_2\text{CO} \tag{8}$$

The sulphides films deposition from thiocarbamid coordination compounds has some chemical peculiarities. Depending on the nature and the salt solution composition may be dominated different coordination forms, and with thiourea molecules in complex inner sphere may contain anions Cl-, Br-, J-, and SO4 2- under certain conditions. Thus, the cadmium atoms close environment are atoms of sulfur, halogens and oxygen, and at the thermal decomposition part of the Cd-Hal or Cd-O bonds are stored and in the sulfide lattice are formed HalS• and OS•defects. In conjunction with the substrate the thiocarbamid complexes orientation on active centers of its surface is observed. The complex particles that can interact with active centers on the substrate are the link that provides sulfide link with the substrate. The nature of this interaction determines the nature of film adhesion. In the case of cadmium sulfide deposition on quartz or glass substrates the active centers are sylanolane groups (≡SiOH) which interact with halide or mixed hydroxide complexes. In result of such interaction the CdOSi oxygen bridges are created. This explains the good adhesion of the cadmium sulfide films deposited from thiocarbamid coordination compounds to glass substrates (Palatnik & Sorokin, 1978).

#### **3.2 Chemical surfact deposition of CdS thin films**

In CSD, a solution at ambient temperature containing the desired reactants is applied to a pretreated surface. Glass or ITO/glass (16×20 mm) substrates, CdTe (10×10 mm) and Si (30×20 mm) wafers were used in the entire work. After that sample with working solution is heated and endured for a given temperature (Fig. 1). To ensure uniformity of heating plate

Fig. 1. Scheme of CdS films thin chemical surface deposition

Chemical Surface Deposition of CdS Ultra Thin Films from Aqueous Solutions 389

The film thickness was determined by ellipsometric measurement of light polarization change after light reflection from an air-film interface on the LEF-3M instrument, allowing precision from 5 to 10 nm, for film thickness less then 100 nm. Morphology of the film surface and the elemental composition were investigated using the scanning electron microscopes REMMA-102-02 with EDS and WEDS and JSM-6490LV. Crystallinity of the CdS film structure was investigated using the automated X-ray diffractometer HZG-4A (with CuKα radiation, λ=0,15406 nm). The optical transmission measurements have been done at room temperature with unpolarized light at normal incidence in the wavelength range from 300 to 1000 nm using Shimadzu UV-3600 double beam UV/VIS spectrophotometer. The

*II t <sup>t</sup>* <sup>0</sup> exp

where *t* is the film thickness, *It* and *Io* are the intensity of transmitted light and initial light, respectively. The absorption coefficient α is related to the incident photon energy *hν* as:

> 

where *А* is a constant dependent on electron and hole effective mass and interband transition, *Eg* is the optical band gap, and *n* is equal to 1 for direct band gap material such as CdS. The band gap *Eg* was determined for each film by plotting *(αhν)2* vs *hν* and then

The peculiarity of the CSD method is that after the first deposition the function of the substrate is performed not by glass, but by formed CdS film. All subsequent depositions are conducted on the same substrate. Through this growth rate of successive layers is approximately the same, and the total film thickness increases in equal size. The data of film thickness measurements and calculated average growth rate is shown on Fig. 2. The accuracy of ellipsometric measurements of thickness increased as the total thickness of the film growth, so that the absolute error varied from ± 10 nm to ± 5 nm. The highest thickness

Apparently, among all other Cd salts, CdI2 always results in a much thinner film. This observation was in agreement with what was reported earlier (Kitaev at al., 1965, Ortega-Borges & Lincot, 1993). This can be explained by different values of stability constant of Cd complexes complementary (Khallaf at al., 2008).While using for CSD the CdCl2 (Fig. 2, a) were obtain almost linear dependence increase of film thickness on the deposition time. For films deposited with CdSO4 and CdJ2 (Fig. 2, b and c, respectively), the dependence of film thickness on deposition time was more complicated, but also had a character close to linear. This fact can be used for CdS films thickness control with high precision in the CdS/SdTe HJ fabrication. The differences in the nature of layer growth of thin CdS films can be explained by the process stages. When solution is applied to the substrate and heated, thiocarbamid complexes start to orient on active centers of the substrate surface and form CdS growth centers. The maximum possible number of growth centers is determined by the number of active centers on the substrate surface, which is considerably less than reactive particles in solution. Under the influence of continuous solution flow the grow centers increases and turn into islands. After a surface filling the islands are merging and form netted

 <sup>2</sup> *n g*

(9)

*h Ah E* (10)

optical absorption coefficient α was calculated for each film using the equation

obtained was in the case of CdSO4, and the least thickness in the CdI2 case.

**3.3 Properties of CSD CdS thin films** 

extrapolating the straight line to the energy axis.

**3.3.1 Thickness and deposition rate** 

with working solution is previously placed on thermostated (343 K) surface. Surface tension of the solution provides a minimum volume of reaction mixture and its maintenance on the substrate. Film deposition occurs through the heterogeneous growth of compounds on the substrate surface by transfer of heat to the work solution. Heterogeneous growth is preferred over homogeneous loss due to thermal stimulation of chemical activity on warmer surface. At a result we receive a high proportion of cadmium from a solution in film and depending on the substrate, the heteroepitaxial film growth. The outflow of heat from the solution to environment helps to keep the favorable conditions for the film heterogeneous growth in time required for film deposition. After heating the plate was removed, the surface was rinsed with distilled water and dried in the air.

The combination of factors of the heat delivery to phase division surface (substrate-solution) and small volume of working solution in the CSD allows to receive coverage with satisfactory performance, increase the efficiency of the reagents, and therefore simplify their utilization. For deposition of CdS films were used freshlyprepared aqueous solutions of one of three cadmium salts: CdSO4, CdCl2, CdI2. Solution ingredients and the corresponding concentrations are presented in Table. 1.


Table 1. Ingredients and concentrations of solutions for CSD of CdS films, T=343 K, pH=12

Several modifications of films CSD were used. First modification (A) includes single applying of working solution and it different time exposure (5 to 12 min.) on the substrate. The second modification (B) provided repeated addition (3 min intervals.) of fresh working solution on the substrate surface. The difference of the third modification (C) consistent in applying (with 3 min. time exposure) and subsequent flushing of working solution on the substrate surface, ie in layer deposition. In such way we achieved increase and regulation of CdS film thickness.


Table 2. The CdS films maximum thickness and deposition rate depending on the CSD modification

Aplying of A modification results in the smallest CdS film thickness, as seen from Table. 2. This is because the main part of the film (80-90 % thickness) is deposited in 2-3 min. Further time exposure of the working solution-substrate system is not accompanied by visible changes in the appearance of the formed film, apparently due to exhaustion of working solution. Therefore, during the multistage (CSD modifications B and C) CdS films deposition the duration of elementary expositions deposition was 3 min. Based on the structural studies results for further work modification B was selected.

#### **3.3 Properties of CSD CdS thin films**

388 Solar Cells – Thin-Film Technologies

with working solution is previously placed on thermostated (343 K) surface. Surface tension of the solution provides a minimum volume of reaction mixture and its maintenance on the substrate. Film deposition occurs through the heterogeneous growth of compounds on the substrate surface by transfer of heat to the work solution. Heterogeneous growth is preferred over homogeneous loss due to thermal stimulation of chemical activity on warmer surface. At a result we receive a high proportion of cadmium from a solution in film and depending on the substrate, the heteroepitaxial film growth. The outflow of heat from the solution to environment helps to keep the favorable conditions for the film heterogeneous growth in time required for film deposition. After heating the plate was removed, the

The combination of factors of the heat delivery to phase division surface (substrate-solution) and small volume of working solution in the CSD allows to receive coverage with satisfactory performance, increase the efficiency of the reagents, and therefore simplify their utilization. For deposition of CdS films were used freshlyprepared aqueous solutions of one of three cadmium salts: CdSO4, CdCl2, CdI2. Solution ingredients and the corresponding

salt С(cadmium salt), mol/l С(CS(NH2)2), mol/l С(NH4OH), mol/l

Table 1. Ingredients and concentrations of solutions for CSD of CdS films, T=343 K, pH=12 Several modifications of films CSD were used. First modification (A) includes single applying of working solution and it different time exposure (5 to 12 min.) on the substrate. The second modification (B) provided repeated addition (3 min intervals.) of fresh working solution on the substrate surface. The difference of the third modification (C) consistent in applying (with 3 min. time exposure) and subsequent flushing of working solution on the substrate surface, ie in layer deposition. In such way we achieved increase and regulation of

maximum thickness, nm 62 65 105 deposition rate, nm/min. ≤6 4–6 ≥8 Table 2. The CdS films maximum thickness and deposition rate depending on the CSD

Aplying of A modification results in the smallest CdS film thickness, as seen from Table. 2. This is because the main part of the film (80-90 % thickness) is deposited in 2-3 min. Further time exposure of the working solution-substrate system is not accompanied by visible changes in the appearance of the formed film, apparently due to exhaustion of working solution. Therefore, during the multistage (CSD modifications B and C) CdS films deposition the duration of elementary expositions deposition was 3 min. Based on the

structural studies results for further work modification B was selected.

modifications A B C

CdCl2 0,001; 0,0001 0,1; 0,01 1,8; 1,2

surface was rinsed with distilled water and dried in the air.

concentrations are presented in Table. 1.

CdSO4

CdI2

CdS film thickness.

modification

The film thickness was determined by ellipsometric measurement of light polarization change after light reflection from an air-film interface on the LEF-3M instrument, allowing precision from 5 to 10 nm, for film thickness less then 100 nm. Morphology of the film surface and the elemental composition were investigated using the scanning electron microscopes REMMA-102-02 with EDS and WEDS and JSM-6490LV. Crystallinity of the CdS film structure was investigated using the automated X-ray diffractometer HZG-4A (with CuKα radiation, λ=0,15406 nm). The optical transmission measurements have been done at room temperature with unpolarized light at normal incidence in the wavelength range from 300 to 1000 nm using Shimadzu UV-3600 double beam UV/VIS spectrophotometer. The optical absorption coefficient α was calculated for each film using the equation

$$I\_t = I\_0 \exp\left(-\alpha t\right) \tag{9}$$

where *t* is the film thickness, *It* and *Io* are the intensity of transmitted light and initial light, respectively. The absorption coefficient α is related to the incident photon energy *hν* as:

$$
\alpha \cdot \hbar \nu = A \left( \hbar \nu - E\_{\text{g}} \right)^{\eta \prime \prime} \tag{10}
$$

where *А* is a constant dependent on electron and hole effective mass and interband transition, *Eg* is the optical band gap, and *n* is equal to 1 for direct band gap material such as CdS. The band gap *Eg* was determined for each film by plotting *(αhν)2* vs *hν* and then extrapolating the straight line to the energy axis.

#### **3.3.1 Thickness and deposition rate**

The peculiarity of the CSD method is that after the first deposition the function of the substrate is performed not by glass, but by formed CdS film. All subsequent depositions are conducted on the same substrate. Through this growth rate of successive layers is approximately the same, and the total film thickness increases in equal size. The data of film thickness measurements and calculated average growth rate is shown on Fig. 2. The accuracy of ellipsometric measurements of thickness increased as the total thickness of the film growth, so that the absolute error varied from ± 10 nm to ± 5 nm. The highest thickness obtained was in the case of CdSO4, and the least thickness in the CdI2 case.

Apparently, among all other Cd salts, CdI2 always results in a much thinner film. This observation was in agreement with what was reported earlier (Kitaev at al., 1965, Ortega-Borges & Lincot, 1993). This can be explained by different values of stability constant of Cd complexes complementary (Khallaf at al., 2008).While using for CSD the CdCl2 (Fig. 2, a) were obtain almost linear dependence increase of film thickness on the deposition time. For films deposited with CdSO4 and CdJ2 (Fig. 2, b and c, respectively), the dependence of film thickness on deposition time was more complicated, but also had a character close to linear. This fact can be used for CdS films thickness control with high precision in the CdS/SdTe HJ fabrication. The differences in the nature of layer growth of thin CdS films can be explained by the process stages. When solution is applied to the substrate and heated, thiocarbamid complexes start to orient on active centers of the substrate surface and form CdS growth centers. The maximum possible number of growth centers is determined by the number of active centers on the substrate surface, which is considerably less than reactive particles in solution. Under the influence of continuous solution flow the grow centers increases and turn into islands. After a surface filling the islands are merging and form netted

Chemical Surface Deposition of CdS Ultra Thin Films from Aqueous Solutions 391

The results of the CdS films investigation by scanning electron microscopy, deposited from diferent aqueous salt solutions are shown in Fig. 3-7, in the reflected and secondary

Fig. 3. Surface morphology of CdS film deposited from CdSO4 aqueus solution, A modification

(a) and C modification (b). REMMA-102-02, accelerating voltage 20 kV, scale 1:2000

Fig. 4. Surface morphology of CdS film deposited from CdSO4 aqueus solution on ITO coated glass in the secondary-electron mode (a) and reflected-electron mode (b). REMMA-

102-02, accelerating voltage 30 kV, scale 1:8000

**3.3.2 Surface morphology** 

electrons mode.

Fig. 2. The CdS thin film thickness dependence on time and quantity of deposition from aqueus solution: CdCl2 (a); CdSO4 (b); CdJ2 (c). The mean deposition rate of CdS thin films on figure inset.

structures that consist of pores and channels. Further film growth is, in fact, filling the pores and channels. It slows the increase of film thickness, but does not alter the film weight gain. At later stages of the growth occurs reflection of the particles stream from the surface that leads to film growth rate decrease, and in the future - to its almost complete stop. Maximum growth rate of CSD at 343 K had films deposited from CdI2 solution. Big deposition rates cause to the significant film defections, which confirm the results of their structural studies.
