An ammonia-free chemical-bath-deposited ZnS(O,OH) buffer layer for flexible Cu(In,Ga)Se2 solar cell application: an eco-friendly approach to achieving improved stability

Abstract

A low-cost, environmentally friendly process of chemical-bath-deposited (CBD) ZnS(O,OH) buffer layers for flexible Cu(In,Ga)Se₂(CIGS)-based solar cells is reported. Unlike the conventional CdS buffer layers, in which toxic cadmium and ammonia are used, our CBD-ZnS(O,OH) buffer layer is fabricated without the addition of ammonia or other complexing agents. The ammonia-free ZnS(O,OH) buffer layer is robust under light illumination so the metastabilities in the device characteristics (the so-called light-soaking effect), typically observed in CIGS/CBD-ZnS(O,OH) solar cells, are eliminated. To further adjust the composition of the ammonia-free CBD ZnS(O,OH) buffer layer, we employed an oxygen plasma post-treatment so that the oxygen content of the CBD buffer layer could be controlled in a straightforward way and the conduction band offset at the CIGS/ZnS(O,OH) interface could be engineered. Significantly reduced series resistance was observed after optimizing the oxygen content, leading to 10.1% cell efficiency. Our ammonia-free process demonstrates a comparable efficiency to the conventional process but without any light soaking.