Manifold benefits of vacuum annealed molybdenum back contact for enhanced CZTSSe thin film properties and device

Cu₂ZnSn(S_xSe_1−x)₄ (CZTSSe) thin-film solar cell (TFCS) are emerging as favourable materials for sustainable energy production, offering advantages such as low cost, abundance, and non-toxicity. Despite the significant progress in CZTSSe solar cells, optimizing their performance remains challenging due to factors like back contact material and interfacial layer formation. Molybdenum (Mo) is commonly used as a back contact material due to its robustness and compatibility with the absorber layer. However, the work function of Mo is highly sensitive to its deposition conditions, which can influence the device's open-circuit voltage (V_oc) and resistance. Our study investigates the impact of Mo work function optimization through post-deposition treatments, such as vacuum annealing, to enhance the electrical and morphological properties of Mo films. Additionally, the growth of Mo(S,Se)₂ layers at the Mo/CZTSSe interface and its influence on the device's performance is studied. We thereby demonstrate that post-deposition annealing of Mo can significantly improve the work function, reduce interfacial layer thickness, and enhance the overall photovoltaic performance of CZTSSe solar cells. Our findings reveal that the optimized Mo back contact results in an improved power conversion efficiency, with Mo_VA-treated films achieving 7.7 % efficiency compared to 0.71 % efficiency for as-sputtered Mo films, highlighting the critical role of back contact optimization in CZTSSe-based photovoltaics.