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

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 V_oc, 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.