Photovoltaic effect in earth abundant solution processed Cu₂MnSnS₄ and Cu₂MnSn(S,Se)₄ thin films

Abstract

In this work, we present the first report on thin film solar cells that employ Cu2MnSnS4 (CMTS) and Cu2MnSn(S, Se)4 (CMTSSe) as the absorber. CMTS and CMTSSe thin films are fabricated using a low cost spray pyrolysis technique in ambient atmosphere using water as a solvent. The crystal structure of the materials are similar to the established kesterite type photovoltaic materials such as Cu2ZnSn(S, Se)4 (CZTSSe). The bandgap of these materials is between 1.4−1.7 eV, which is ideal for optimum solar absorption and can be tuned by varying the ratio of the elemental constituents. The photovoltaic device with a structure of Mo/CMTS/CdS/TCO/top electrode is fabricated as a proof-of-concept and yields a power conversion efficiency of ~0.73% for the best optimized CMTS device with Na doping. Through analysis of the device characteristics, we identify a key problem of very high carrier density in the CMTS/CMTSSe absorber that leads to short collection length, low Voc, low quantum efficiency at long wavelength and high shunt conductance that quench the fill factor. We also discuss several routes to improve the device performance.