Solution-Processed Zn_<i>x</i>Cd_1–<i>x</i>S Buffer Layers for Vapor Transport-Deposited SnS Thin-Film Solar Cells: Achieving High Open-Circuit Voltage

Abstract

As an alternative buffer material to CdS, Zn_<i>x</i>Cd_1-<i>x</i>S buffer layers for vapor transport-deposited SnS thin-film solar cells (TFSCs) were fabricated using the successive ionic layer adsorption and reaction (SILAR) method. Varying the Zn-to-Cd ratio resulted in a series of Zn_<i>x</i>Cd_1-<i>x</i>S thin films with controllable band gaps in the range of 2.40-3.65 eV. The influence of the Zn-to-Cd ratio on the cell performance was investigated in detail. The Zn_0.34Cd_0.66S buffer layer was found to be the optimal composition for SnS TFSCs, and a record open-circuit voltage (<i>V</i>_oc) of 0.405 V was achieved with an efficiency of 3.72%, whereas the SILAR-CdS buffer layer rendered a <i>V</i>_oc of 0.324 V. The improvement in <i>V</i>_oc when using the Zn_0.34Cd_0.66S buffer layer was corroborated by the spike-type conduction band offset of 0.35 eV with the SnS absorber, as revealed by the X-ray photoelectron spectroscopy analysis. In addition, minimized interfacial recombination at the SnS/Zn_0.34Cd_0.66S heterojunction was confirmed by the temperature-dependent <i>V</i>_oc analysis under illuminated conditions.