**5.1.1 Discussion on the Cu(In,Ga)Se2 thin film based solar cells results**

Two Cu(In,Ga)Se2 samples were used to be processed into solar cell devices: sample JS17 had a CdS layer prepared with a recipe based on CdCl2 and sample JS18 with a recipe based on CdSO4 as the Cd source. The *J*-*V* parameters for devices JS17 are: area = 0.47 cm2, *V*oc = 536 mV, *J*sc = 31.70 mA/cm2, fill factor = 64.0 %, and = 10.9 % (see figure 13) and for JS18 are: area

Chemical Bath Deposited CdS for CdTe and Cu(In,Ga)Se2 Thin Film Solar Cells Processing 249

two layers of Cu and Au (2nm and 350 nm, respectively) were evaporated, with an area of 0.08 cm2, onto the CdTe and annealed at 180 °C in Ar. The front contact was taken from the

> Au, 350 nm Cu, 2 nm

> > CdTe

CdS SnO2:F

Soda-lime glass substrate

**5.2.1 Variation of the S/Cd ratio in the solution for deposition of CdS by chemical bath** 

The variation of the S*/*Cd ratio in the solution used in the preparation of the CdS films modifies the morphology, the deposition rate, the crystal grain size, the resistivity and the optical transmittance of these films and have an influence upon the structural and electrical properties of the CdTe layer itself, in addition to modifications of the CdS–CdTe interface. Hence, our study shows the influence of the S*/*Cd ratio in the solution for CdS thin films prepared by chemical bath upon the characteristics of CdS*/*CdTe solar cells with a

The concentrations of NH3, NH4Cl and CdCl2 were kept constant in every experiment, but the thiourea [CS(NH2)2] concentration was varied in order to obtain different S*/*Cd relations *(R*tc*)* in the solution. All the films were grown on SnO2:F conducting glasses (10 ohm-cm) at 75 °C. Deposition times were also varied, according to our previous knowledge of the growth kinetics (Vigil O. et al., 2001), with the purpose of obtaining films with similar thickness in all cases. The selected thiourea concentrations and deposition times for each

> Thiourea concentration in the bath (mol/l)

Table 2. Thiourea concentration and deposition time for each S*/*Cd relation

1 2.4 x 10-3 120 2.5 6 x 10-2 100 5 1.2 x 10-2 120 10 2.4 x 10-2 120

Solar cells were prepared by depositing CdTe thin films on the SnO2:F*/*CBD-CdS substrates by CSVT-HW. The atmosphere used during the CdTe was a mixture of Ar and O2, with an O2 partial pressure of 50%. In all cases, the total pressure was 0.1 Torr. Prior to deposition the system was pumped to 8 × 10−6 Torr as the base pressure. CSVT-HW deposition of CdTe

Deposition time (min)

conducting glass substrate (0.5 μm thick SnO2:F/ glass with sheet resistivity of 10 Ω/).

Fig. 15. Schematic configuration of a typical CdTe based solar cell

Sunlight

**and its effect on the efficiency of CdS/CdTe solar cells** 

superstrate structure (Vigil-Galán, et al., 2005).

A

S*/*Cd relation are shown in table 2.

S/Cd ratio Rtc

= 0.47 cm2, *V*oc = 558 mV, *J*sc = 29.90 mA/cm2, fill factor = 63.1 %, and = 10.5 % (see figure 14). From these results, we can see that sample JS17 shows a conversion efficiency a little bit higher than JS18, this is due to the different recipe used to prepare the CdS layer as it was mentioned before. This was the only difference between the two devices, everything else was the same. From these figures, a low Voc is observed, but we should expect to have a higher Voc value, compared to the Ga content. The roll-over in forward bias could be indicative of a low sodium content in the Cu(In,Ga)Se2 films. Also, the low current collection, observed for the Cu(In,Ga)Se2 thin film devices, may be due to incomplete processing of the absorber layer. Improvements in device performance are expected with optimization of absorber processing.

Fig. 13. J-V curves for the best Cu(In,Ga)Se2 thin film device prepared with a CdS bath solution based on CdCl2

Fig. 14. J-V curves for the best Cu(In,Ga)Se2 thin film device prepared with a CdS bath solution based on CdSO4

#### **5.2 CdTe/CdS thin film solar cells**

The typical superstrate structure of a hetero-junction CdTe/CdS solar cell is composed of a soda lime glass substrate, coated with a sputtered transparent conducting oxide (TCO) to the visible radiation, which acts as the front contact, then a CdS layer with a thickness 120 nm is chemically bath deposited, followed by the deposition of the absorber CdTe layer by close spaced vapor transport technique and finally the CdS/CdTe device is completed by depositing the ohmic back contact on top of the CdTe layer, see figure 15. For the back contact,

= 0.47 cm2, *V*oc = 558 mV, *J*sc = 29.90 mA/cm2, fill factor = 63.1 %, and = 10.5 % (see figure 14). From these results, we can see that sample JS17 shows a conversion efficiency a little bit higher than JS18, this is due to the different recipe used to prepare the CdS layer as it was mentioned before. This was the only difference between the two devices, everything else was the same. From these figures, a low Voc is observed, but we should expect to have a higher Voc value, compared to the Ga content. The roll-over in forward bias could be indicative of a low sodium content in the Cu(In,Ga)Se2 films. Also, the low current collection, observed for the Cu(In,Ga)Se2 thin film devices, may be due to incomplete processing of the absorber layer. Improvements in device performance are expected with optimization of absorber processing.

Fig. 13. J-V curves for the best Cu(In,Ga)Se2 thin film device prepared with a CdS bath

**-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0**

**V(volts)**

**V(volts)**

**JS17**

**-50 -40 -30 -20 -10 0 10 20 30 40 50**

**10.2% 10.5% (b) JS18**

**J (mA/cm2)**

**JS17-002**

**JS18-004**

**10.5% 10.9% (a)**

**J (mA/cm2)**

Fig. 14. J-V curves for the best Cu(In,Ga)Se2 thin film device prepared with a CdS bath

**-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0**

**-50 -40 -30 -20 -10 0 10 20 30 40 50**

The typical superstrate structure of a hetero-junction CdTe/CdS solar cell is composed of a soda lime glass substrate, coated with a sputtered transparent conducting oxide (TCO) to the visible radiation, which acts as the front contact, then a CdS layer with a thickness 120 nm is chemically bath deposited, followed by the deposition of the absorber CdTe layer by close spaced vapor transport technique and finally the CdS/CdTe device is completed by depositing the ohmic back contact on top of the CdTe layer, see figure 15. For the back contact,

solution based on CdCl2

solution based on CdSO4

**5.2 CdTe/CdS thin film solar cells** 

two layers of Cu and Au (2nm and 350 nm, respectively) were evaporated, with an area of 0.08 cm2, onto the CdTe and annealed at 180 °C in Ar. The front contact was taken from the conducting glass substrate (0.5 μm thick SnO2:F/ glass with sheet resistivity of 10 Ω/).

Fig. 15. Schematic configuration of a typical CdTe based solar cell

### **5.2.1 Variation of the S/Cd ratio in the solution for deposition of CdS by chemical bath and its effect on the efficiency of CdS/CdTe solar cells**

The variation of the S*/*Cd ratio in the solution used in the preparation of the CdS films modifies the morphology, the deposition rate, the crystal grain size, the resistivity and the optical transmittance of these films and have an influence upon the structural and electrical properties of the CdTe layer itself, in addition to modifications of the CdS–CdTe interface. Hence, our study shows the influence of the S*/*Cd ratio in the solution for CdS thin films prepared by chemical bath upon the characteristics of CdS*/*CdTe solar cells with a superstrate structure (Vigil-Galán, et al., 2005).

The concentrations of NH3, NH4Cl and CdCl2 were kept constant in every experiment, but the thiourea [CS(NH2)2] concentration was varied in order to obtain different S*/*Cd relations *(R*tc*)* in the solution. All the films were grown on SnO2:F conducting glasses (10 ohm-cm) at 75 °C. Deposition times were also varied, according to our previous knowledge of the growth kinetics (Vigil O. et al., 2001), with the purpose of obtaining films with similar thickness in all cases. The selected thiourea concentrations and deposition times for each S*/*Cd relation are shown in table 2.


Table 2. Thiourea concentration and deposition time for each S*/*Cd relation

Solar cells were prepared by depositing CdTe thin films on the SnO2:F*/*CBD-CdS substrates by CSVT-HW. The atmosphere used during the CdTe was a mixture of Ar and O2, with an O2 partial pressure of 50%. In all cases, the total pressure was 0.1 Torr. Prior to deposition the system was pumped to 8 × 10−6 Torr as the base pressure. CSVT-HW deposition of CdTe

Chemical Bath Deposited CdS for CdTe and Cu(In,Ga)Se2 Thin Film Solar Cells Processing 251

Fig. 17. Typical *J* –*V* characteristics of CdS*/*CdTe solar cells under illumination at 100 mW

There are several factors directly or indirectly influencing the cell behaviour, in particular the amount of S in the CBD CdS layers may influence the formation of the CdS1-*x*Te*<sup>x</sup>* ternary compound at the CdS–CdTe interface. CdTe films grown at high temperatures, such as those produced by CSVT, produce a sulfur enriched region due to S diffusion. The amount of S penetrating the bulk of CdTe from the grain boundary must be dictated by the bulk diffusion coefficient of S in CdTe and of course by the amount of S available in the CdS films. The re-crystallization of CdTe could be affected by the morphological properties of the CdS layers grown with different S*/*Cd ratios. These facts have been studied by Lane (Lane D. W. et al., 2003) and Cousins (Cousins M. A. et al. 2003). From this point of view the formation of CdS1-xTe*<sup>x</sup>* may be favored when the *R*tc is increased in the bath solution. This ternary compound at the interface may cause a lower lattice mismatch between CdS and CdTe, and therefore a lower density of states at the CdTe interface region will be obtained, causing a lower value for the dark saturation current density *J*0. The resistivity of the CdS and CdTe layers and their variation under illumination also change the characteristics of the cell under dark and illumination conditions. In other words, a better photoconductivity implies smaller resistivity values under illumination, with the possible improvement of the solar cell properties. In addition, optical transmittance, thickness and morphological measurements of the CBD-CdS films showed the following characteristics when increasing *R*tc: i) band gap values are observed to increase (from 2.45 eV to 2.52 eV when changing *R*tc from 1 to 10), ii) grain sizes become smaller (from 55.4 nm to 47.2 nm when S*/*Cd = 1 and 10, respectively) and iii) the average optical transmission above threshold increases from 68% to 72% when *R*tc is increased from 1 to 10. Higher band-gap values of the window material improve the short circuit current density of the solar cells. Thin films with smaller grain sizes show fewer pinholes with a positive effect on the open circuit voltage and fill factor. In this regard, the properties of the CdS layers are correlated with the kinetic of the deposition process when the concentration of thiourea is changed. For instance, for high thiourea concentration, the reaction rate becomes large enough to promote a quick CdS precipitation which leads to the formation of agglomerates in the solution rather than nucleation on the substrate surface, while for low thiourea concentration a very slow growth process can be expected, leading to a thinner but more

cm−2, with *R*tc as a parameter

homogeneous layer.

was done by placing a CdTe graphite source block in close proximity (1 mm) to the substrate block. The deposition time was 3 min for all the samples deposited with substrate and source temperatures of 550 °C and 650 °C, respectively. Under these conditions, CdTe layers of approximately 3.5 *μ*m were obtained. The CdTe thin films were coated with a 200 nm CdCl2 layer and then annealed at 400 °C for 30 min in air. For the back contact, two layers of Cu and Au (2 nm and 350 nm, respectively) were evaporated, with an area of 0.08 cm2, on the CdTe film and annealed at 180 °C in Ar. The growth conditions of CdTe were maintained constant for all solar cells.

#### **5.2.2 Discussion on CdTe thin film solar cells results**

Figure 16 shows the set of *I*–*V* characteristics for CdS*/*CdTe solar cells made with the same *R*tc (S*/*Cd ratio = 5). According to our experimental conditions, the solar cells made with the same technological process have similar characteristics.

Fig. 16. J –V characteristics of three CdS/CdTe solar cells made with CdS layers grown with Rtc = 5 during the CBD-CdS growth process

The *I*–*V* characteristics of CdS*/*CdTe solar cells under AM1.5 illumination (normalized to 100 mW cm-2) as a function of *R*tc are shown in figure 17. In table 3, the average shunt *(R*p*)* and series *(R*s*)* resistances, the short circuit current density *(J*sc,), the open circuit voltage *(V*oc*)*, the fill factor (FF) and the efficiency (*η*) of solar cells prepared with different *R*tc are reported. The averages were taken from four samples for each *R*tc. As can be seen in table 3, *η* increases with *R*tc up to *R*tc = 5 and drops for *R*tc = 10.


Table 3. Photovoltaic parameter results for CdS*/*CdTe solar cells with different S*/*Cd ratio *(R*tc*)* in the CdS bath

was done by placing a CdTe graphite source block in close proximity (1 mm) to the substrate block. The deposition time was 3 min for all the samples deposited with substrate and source temperatures of 550 °C and 650 °C, respectively. Under these conditions, CdTe layers of approximately 3.5 *μ*m were obtained. The CdTe thin films were coated with a 200 nm CdCl2 layer and then annealed at 400 °C for 30 min in air. For the back contact, two layers of Cu and Au (2 nm and 350 nm, respectively) were evaporated, with an area of 0.08 cm2, on the CdTe film and annealed at 180 °C in Ar. The growth conditions of CdTe were

Figure 16 shows the set of *I*–*V* characteristics for CdS*/*CdTe solar cells made with the same *R*tc (S*/*Cd ratio = 5). According to our experimental conditions, the solar cells made with the

Fig. 16. J –V characteristics of three CdS/CdTe solar cells made with CdS layers grown with

The *I*–*V* characteristics of CdS*/*CdTe solar cells under AM1.5 illumination (normalized to 100 mW cm-2) as a function of *R*tc are shown in figure 17. In table 3, the average shunt *(R*p*)* and series *(R*s*)* resistances, the short circuit current density *(J*sc,), the open circuit voltage *(V*oc*)*, the fill factor (FF) and the efficiency (*η*) of solar cells prepared with different *R*tc are reported. The averages were taken from four samples for each *R*tc. As can be seen in table 3,

> Jsc (mA/cm2)

1 6.8 318 20.8 617 55.2 7.1 2.5 5.4 800 21.8 690 55.5 8.3 5 2.9 787 23.8 740 70.5 12.3 10 5.9 135 22.7 435 52 5.4 Table 3. Photovoltaic parameter results for CdS*/*CdTe solar cells with different S*/*Cd ratio

Voc (mV)

FF (%)

 (%)

maintained constant for all solar cells.

**5.2.2 Discussion on CdTe thin film solar cells results** 

same technological process have similar characteristics.

Rtc = 5 during the CBD-CdS growth process

Rs (ohm-cm2)

S/Cd ratio Rtc

*(R*tc*)* in the CdS bath

*η* increases with *R*tc up to *R*tc = 5 and drops for *R*tc = 10.

Rp (ohm-cm2)

Fig. 17. Typical *J* –*V* characteristics of CdS*/*CdTe solar cells under illumination at 100 mW cm−2, with *R*tc as a parameter

There are several factors directly or indirectly influencing the cell behaviour, in particular the amount of S in the CBD CdS layers may influence the formation of the CdS1-*x*Te*<sup>x</sup>* ternary compound at the CdS–CdTe interface. CdTe films grown at high temperatures, such as those produced by CSVT, produce a sulfur enriched region due to S diffusion. The amount of S penetrating the bulk of CdTe from the grain boundary must be dictated by the bulk diffusion coefficient of S in CdTe and of course by the amount of S available in the CdS films. The re-crystallization of CdTe could be affected by the morphological properties of the CdS layers grown with different S*/*Cd ratios. These facts have been studied by Lane (Lane D. W. et al., 2003) and Cousins (Cousins M. A. et al. 2003). From this point of view the formation of CdS1-xTe*<sup>x</sup>* may be favored when the *R*tc is increased in the bath solution. This ternary compound at the interface may cause a lower lattice mismatch between CdS and CdTe, and therefore a lower density of states at the CdTe interface region will be obtained, causing a lower value for the dark saturation current density *J*0. The resistivity of the CdS and CdTe layers and their variation under illumination also change the characteristics of the cell under dark and illumination conditions. In other words, a better photoconductivity implies smaller resistivity values under illumination, with the possible improvement of the solar cell properties. In addition, optical transmittance, thickness and morphological measurements of the CBD-CdS films showed the following characteristics when increasing *R*tc: i) band gap values are observed to increase (from 2.45 eV to 2.52 eV when changing *R*tc from 1 to 10), ii) grain sizes become smaller (from 55.4 nm to 47.2 nm when S*/*Cd = 1 and 10, respectively) and iii) the average optical transmission above threshold increases from 68% to 72% when *R*tc is increased from 1 to 10. Higher band-gap values of the window material improve the short circuit current density of the solar cells. Thin films with smaller grain sizes show fewer pinholes with a positive effect on the open circuit voltage and fill factor. In this regard, the properties of the CdS layers are correlated with the kinetic of the deposition process when the concentration of thiourea is changed. For instance, for high thiourea concentration, the reaction rate becomes large enough to promote a quick CdS precipitation which leads to the formation of agglomerates in the solution rather than nucleation on the substrate surface, while for low thiourea concentration a very slow growth process can be expected, leading to a thinner but more homogeneous layer.

**1. Introduction** 

smart textiles as well [3].

**12** 

*Poland* 

**Innovative Elastic Thin-Film** 

The idea of thin films dates back to the inception of photovoltaics in the early sixties. It is an idea based on achieving truly low-cost photovoltaics appropriate for mass production, where usage of inexpensive active materials is essential. Since the photovoltaic (PV) modules deliver relatively little electric power in comparison with combustion-based energy sources, solar cells must be cheap to produce energy that can be competitive. Thin films are

Replacement of single crystalline silicon with poly and amorphous films, caused the decline of material requirements, which has led to lower final prices [2]. Furthermore, the thickness of cell layers was reduced several times throughout the usage of materials with higher optical absorption coefficients. Unique, thin film and lightweight, devices of low manufacturing costs and high flexibility, were obtained by applying special materials and production techniques, e.g. CIS, CIGS or CdTe/CdS technologies and organic elements. Taking advantage of those properties, there is a great potential of new, useful applications, such as building integrated photovoltaics (BIPV), portable elastic systems or clothing and

Low material utilization, mass production and integrated module fabrication are basic advantages of thin film solar cells over their monocrystalline counterparts [4]. Figure 1 (by

The development of cadmium telluride (CdTe) based thin film solar cells started in 1972 with 6% efficient CdS/CdTe [5] to reach the present peak efficiency of 16.5% obtained by NREL researchers in 2002 [6]. Chalcopyrite based laboratory cells (CIS, CIGS) have recently achieved a record efficiency of 20% [7], which is the highest among thin film PV cells (see Table 1). Solar modules based on chalcopyrites, uniquely combines advantages of thin film technology with the efficiency and stability of conventional crystalline silicon cells [4].

Thin film solar cell type **CIGS CdTe/CdS a-Si**  Cell area [cm2] 0.5 1.0 0.25 Highest efficiency [%] 20.0 16.5 13.3 Typical efficiency range [%] 12.0 – 20.0 10.0 – 16.5 8.0 – 13.3

NREL) shows the development of thin film photovoltaic cells since 1975.

Table 1. Efficiencies of CIGS, CdTe and a-Si thin film solar cells [8].

considered to be the answer to that low-cost requirement [1].

Maciej Sibiński and Katarzyna Znajdek *Technical University of Łódź, Department of Semiconductor and Optoelectronic Devices,* 

**Solar Cell Structures** 
