The Effect of Work Function of Back Contact Metals on the Performance of CdTe Solar Cells

Abstract

The CdTe thin film solar cells suffer power losses at the back contact due to charge losses due to poor mismatch of the work function of  interfacing back contact materials. Thus, there is a need to find appropriate back contact materials that do not reduce cell performance  by creating large Schottky barriers. One of the established strategies employed to reduce the barrier height is depositing metal(s) with  high work function to the CdTe back surface. In this study, we used the average of the commonly reported values of the work function of  metals, to investigate the effects of various metals, when used as back contacts, on the performance of CdTe solar cells. Without any  other strategy employed to overcome the Schottky contacts, the solar cells' efficiency is gravely diminished when metals with work  functions lower than 5.0 eV are used. While metals like Pt, Pd, and Au with very high work functions produce cells with efficiencies of up to  18.6%, lower work functions metals like Al almost completely diminished the efficiency to less than 0.5%. our findings indicate that the  Schottky barrier affects mainly Voc and FF, as Jsc showed only a slight reduction. When subjected to extreme operating temperatures, the  efficiencies largely decreased with temperature. Temperature coefficients of -0.3 to -0.45/oC have been obtained, for the highest- performing metals.