Revealing the Subtle Role of DC-Sputtered Mo Back Contact Work Function in Mitigating the Detrimental Effects of the MoS2/MoSe2 Interfacial Layer in Kesterite-Based Thin Film Solar Cells

Due to thermal robustness and chemical stability at high working temperatures during photo-absorber layer deposition, molybdenum (Mo) is favored as the back electrode material in chalcogenide-based thin film photovoltaic devices. However, the unintentional formation of an interfacial layer between Mo and the absorber layer adversely affects the deviceperformance. In addition, the lower work function of Mo compared to the semiconductor work function of other emerging p-type chalcogenide absorber layer materials results in low open-circuit voltage caused by the inhibition of the hole-transport process at the back contact interface. Therefore, efforts toward the enhancement of the work function of Mo thin film are imperative to improve the device performance. In this research, the impact of post-deposition heat treatment in vacuum condition at 580 °C on the microstructural, electronic, electrical, and morphological properties of DC-sputtered Mo thin films was studied. It was shown that vacuum annealing improves the microstructural properties, which in turn promotes better surface topology and electron conduction mechanism. More importantly, the work function of the Mo thin film was increased up to 5.05 eV after vacuum annealing. The obtained experimental work function of 5.05 eV was employed in numerical simulation study through SCAPS-1D to highlight the critical role of work function of Mo in reducing the negative impact of the MoS₂/MoSe₂ interfacial layer in terms of open-circuit voltage (V_oc), short-circuit current (J_sc), fill factor (FF), and efficiency (PCE) of CZTS/CZTSe devices. Based on the simulation results, Mo with a work function of 4.6 eV (average for a polycrystalline Mo) results in efficiency of around 15% for both CZTS and CZTSe devices, which was modeled together with low-quality interfacial layer (low carrier mobility and high defect density). However, Mo with a work function of 5.05 eV results in improved efficiency of 19% and 23% for CZTS and CZTSe devices, respectively. Hence, the approach of enhancing the Mo back contact work function is shown as a plausible pathway to enhance the overall performance of the photovoltaic device despite the formation of a detrimental interfacial layer.