Simplified formation process for Cu₂ZnSnS₄-based solar cells

Abstract

Cu2ZnSn(S,Se)4 absorber layers for thin film solar cells contain up to five separate elements making it challenging to synthesize due to the necessity of achieving chemical uniformity on a nanometer scale. In a two-step synthesis procedure a metallic Cu-Sn-Zn precursor can be converted uniformly to the semiconductor by the reaction with gaseous chalcogens. Here, it is demonstrated that only a simplified precursor containing Cu and Zn is required to form the Cu2ZnSnS4-semiconductor when the annealing step is carried out in the presence of gaseous tin sulfide and sulfur. It is shown that the formation of Cu2ZnSnS4 progresses in a self-limiting manner, independently of the amount of sulfur and tin-sulfide provided. Using ex-situ investigations, the reaction pathway is resolved and a model of the reaction process is established. The model furthermore explains the formation of the undesired ZnS phase at the back interface of the absorber, limiting the device performance. The influence of various annealing conditions on the formation of ZnS and hence the device performance with efficiencies of up to 2.3 % is shown. Lowering of the background forming gas pressure to 100 Pa was revealed to be beneficial as it influences the distribution of ZnS at the back interface.