Processing CdS and CdSe containing window layers for CdTe solar cells

Influence of heat treatment steps on the characteristics of (CdS,CdSe) junction partners and on solar cell performance were studied. CdS films were obtained by chemical bath deposition, CdSe layers were evaporated. Structural and compositional properties of CdS/CdSe bilayer stacks did not change upon heat treatment at 400 C up to 10 minutes, whereas heat treatment in presence of CdCl2 for 10 minutes caused formation of a CdSSe alloy with a bandgap value of about 2.05 eV. Originally cubic structure of the stack was also transformed into hexagonal during this treatment. CdSe-CdTe interdiffusion was also studied using CdS/CdSe/CdTe triple layer stacks. CdTe films were deposited by close spaced sublimation method. Limited CdSe-CdTe interdiffusion was seen when CdTe was deposited over the as-deposited CdSe layer at 580C. However, such interdiffusion was not detected for samples where CdTe deposition was carried out on CdS/CdSe stacks pre-annealed in presence of CdCl2. This suggests that partial crystallization of CdS/CdSe bilayer stack by CdCl2 reduced such interaction. Solar cells with CdSe/CdTe, CdS/CdTe and CdS/CdSe/CdTe structures with efficiencies of 8.39%, 10.12% and 11.47% were fabricated using 4.5-5 µm thick CdTe layers and a final CdCl2 treatment. Quantum efficiency measurements demonstrated the benefit of CdSe-CdTe alloying during the final CdCl2 treatment in improving the short circuit current values.