**5.1 Cu(In,Ga)Se2/CdS thin film solar cells**

246 Solar Cells – Thin-Film Technologies

block and the other is the substrate block. Figure 11 shows the block diagram of the CSVT system used to prepare the CdTe thin films. Between the source graphite block "A" and the substrate graphite block "B" is located the graphite boat that contains the material to be sublimated, and on top of this boat the substrate is located, in a very close proximity or close spaced. The material growth is carried out under the presence of an inert atmosphere like argon, nitrogen, etc. The growth rate of the material to be deposited can be controlled by controlling the pressure and gas flow rate. Also this inert gas can be mixed with a reactive gas like oxygen, which benefits the growth of CdTe with the characteristic p-type conductivity. The deposition parameters for this technique are: a) Ts: temperature of the source, b) Tsub: substrate temperature, it has to be lower than the Ts in order to avoid the re-sublimation of the material, c) ds-sub: distance between the material to be deposited and the substrate and d) *Pg:*

For the processing of CdTe thin film solar cells, it is necessary to use a *superstrate* structure, so that the CdS is deposited on SnO2:F, in such a way that the growth process allows the film to be deposited over the whole surface, becoming a surface free of holes and caverns without empty spaces among the grains, and with a uniform grain size distribution. It is also required that the CdS layer matches well with the CdTe host, thus favoring a good growing kinetics for CdTe, as well as the formation of the CdSxTe1-x ternary compound in the interface due to the diffusion of S from CdS to CdTe. The high-efficiency CdTe solar cells to date have essentially the same *superstrate* structure. The superstrate structure is composed of a sodalime glass substrate, coated with a SnO2:F; a transparent conductor oxide as the front contact, then a CdS layer is chemically bath deposited, followed by the deposition of a CdTe layer and finally the deposition of two layers of Cu and Au to form the back contact to complete the CdS/CdTe device. In order to achieve solar cells with high conversion efficiencies, the physical and chemical properties of each layer must be optimized (Morales-Acevedo A., 2006). The deposition of CdTe was performed by using CdTe powder 99.999% purity. The deposition atmosphere was a mixture of Ar and O2, with equal partial pressures of O2 and Ar. In all cases the total pressure was 0.1 Torr. Prior to all depositions the system was pumped to 8×10−6 Torr as the base pressure. In the CSVT-HW (hot wall) deposition, the separation between source and substrate was about 1 mm. 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 2 – 4 μm were obtained. The CdTe thin films were also thermally treated with CdCl2. As discussed before, a very important treatment independently of the deposition technique for both CdS and CdTe layers is a thermal annealing after the deposition of CdCl2 on top of the CdTe layer. If the CdCl2

pressure of the inert gas inside the chamber.

Fig. 11. Schematic diagram of a CSVT system

The substrate structure of a Cu(In,Ga)Se2 thin film based solar cell is composed of a soda lime glass substrate, coated with a sputtered 0.7 – 1 m Mo layer as the back contact. After the thermal co-evaporation of Cu(InGa)Se2 deposition, the junction is formed by chemically bath depositing the CdS with thickness 30 - 50 nm. Then a high-resistance (HR) ZnO layer and a doped high-conductivity ZnO:Al layer are subsequently deposited, usually using the sputtering technique. Finally, the deposition of a current-collecting grid of Ni/Al completes the device as shown in figure 12. The total cell area is defined by removing the layers on top of the Mo outside the cell area by mechanical scribing.

Fig. 12. Schematic configuration of a typical Cu(In,Ga)Se2 thin film solar cell
