Improving performance of Cu2ZnSnS4 solar cell via back contact interface engineering

Although kesterite Cu2ZnSnS4 (CZTS) has emerged as a potential absorber material for development of low cost thin film solar cells, the performance of the CZTS devices has been limited by a large deficit in open circuit voltage caused by recombinations of charge carriers primarily at the back contact interface. Recently, field passivation through engineering of the back contact interface has been shown to control the interface recombination and thereby minimise the open circuit voltage deficit. In this work, we have numerically established the limiting role of defect densities in the bulk of CZTS and at interfaces (CZTS-CdS and CZTS-Mo) on performance metrics of the champion CZTS cell (similar to 11%). Furthermore, we have shown that field passivation by using back surface field (BSF) layers in the form of very thin p(+) type Cu2O and SnS layers, sandwiched between the bottom electrode Mo and CZTS, greatly improved the device efficiency (to about 15%) despite significant high bulk and interface defect densities. Numerical calculations have revealed critical dependence of back contact interface recombination on charge carrier concentration gradient and band offset at the interface, and thickness of the BSF layer. Based on the results, a mechanism for suppressing the recombinations at back contact interface has been illustrated.