Cu2ZnSn(S, Se)4 thin film absorbers based on ZnS, SnS and Cu3SnS4 nanoparticle inks: Enhanced solar cells performance by using a two-step annealing process

Abstract

In this paper, we present the fabrication of Cu2ZnSn(S, Se)4 (CZTSSe) thin film absorbers by a four-step solution process based on ZnS, SnS and Cu3SnS4 nanoparticle precursors and their application in thin film solar cells. The influence of ligand-exchange process on the morphologies of the resulting CZTSSe thin films was studied. Ligand exchange with each sequential spun coat layers leads to cracking films which can be avoided by combining ligand-exchanged and non-ligand-exchanged processes. Moreover, a two-step annealing process yields the most homogeneous films. CZTSSe thin films consisting a large grain and fine nanoparticle grain layered structure was formed. The formation of layered structure for the absorbers was found to be due to the existence of high content of carbon left near the back contact and the out diffusion of Cu and Zn from the bottom layer to the surface layer. As a result, solar cell conversion efficiency was improved from 1.2% to 3.0% upon adoption a two-step annealing process. Temperature dependent I–V characteristic analysis reveals the dominant loss mechanism of the solar cells is the strong CZTSSe and CdS buffer interface recombination.