Investigation of the effect of selenium addition to window and absorption layers on cell performance in CdS/CdTe solar cells

In the first part of this thesis, the effect of heat treatment on the characteristics of CdS/CdSe window layers and the cell performance was investigated. It was observed that cubic CdS and CdSe phases were formed in CdS/CdSe bilayer structures as a result of heat treatment applied for up to 10 minutes. However, after 10 minutes of heat treatment in the presence of CdCl2, CdSSe alloy was formed and the initial cubic structure turned into hexagonal. Solar cells with CdSe/CdTe, CdS/CdTe and CdS/CdSe/CdTe stacks yielded 8.39%, 10.12% and 11.47% efficiency, respectively. Using a CdS/CdSe window layer in solar cell structure increased the short-circuit current density by improving the long wavelength response of the device. In the second part, the effects of CdS/CdSexTe1-x/CdTe device structure on cell parameters were investigated by examining the properties of CdSexTe1-x alloys. In CdSexTe1-x alloys, the crystal structure changed from cubic to mixed phase (cubic+hexagonal) with increasing Se-concentration. The optical band gap values of the alloys revealed the existence of bowing effect. It was showed that CdSexTe1-x thin films (CST-20, CST-25, CST-30) used in devices have a graded alloy structure and nearly optimum band gap. Finally, the efficiencies of solar cells with CST-20/CdTe, CST-25/CdTe and CST-30/CdTe structures were obtained as 9.59%, 11.69% and 10. 13%, respectively. These results shows that CdSexTe1-x alloy layer in front of the CdTe absorption layer in the device structure can be concluded the short circuit current density.