Investigation of local compositional uniformity in Cu2ZnSn(S,Se)4 thin film solar cells prepared from nanoparticle inks

Abstract

Cu₂ZnSn(S,Se)₄ (CZTSSe) is a promising material for low-cost photovoltaics (PV), however despite rapid improvements in recent years, the performance of CZTSSe based solar cells has yet to reach the level required for commercial relevance. In order to understand the sources of efficiency losses in CZTSSe thin film solar cells, a detailed analysis of a solution-processed CZTSSe solar cell with a power conversion efficiency of 8.6% was performed using transmission electron microscopy (TEM). The composition of the CZTSSe active absorber layer was found to be highly non-uniform laterally which contributed to efficiency losses due to band gap and potential fluctuations. To further understand how compositional non-uniformity develops during the annealing process, the microstructure and local composition of a series of CZTSSe thin films annealed for differing lengths of time were investigated using TEM. It was found that longer annealing times improved both the microstructure of the CZTSSe layer and the compositional uniformity across the thickness of the layer. However, the lateral grain-to-grain variation in composition increased with increasing annealing time. As a result, despite improvements in the short circuit current resulting from larger CZTSSe grains with increased annealing time, no significant net gain in efficiency was obtained due to a counterbalancing decrease in open circuit voltage as a result of the increased lateral CZTSSe composition non-uniformity. The results show a challenging need to improve the control of CZTSSe thin film compositional uniformity at all length scales in order to improve the PV device performance.