Mechanism for enhancing the power conversion efficiency of Cu2ZnSn(S,Se)4 solar cells via MgF2 anti-reflection coating deposition

In this study, MgF2 anti-reflection coatings (ARCs) with different thicknesses were deposited on the surface of Indium Tin Oxide (ITO) located between Ag electrodes of a CZTSSe solar cell by vacuum evaporation deposition. The thickness of the MgF2 ARC was controlled by varying its weight in the range of 0.01–0.04 g of MgF2. By optimizing the weight, the power conversion efficiency (PCE) of the CZTSSe solar cell increased from 10.40% without MgF2 ARC to 12.21% with MgF2 ARC produced with 0.02 g of MgF2. It was found that the increased PCE stems not only from an increase in short-circuit current density (JSC) but also from fill factor (FF). This is different from the previously reported literature results, where the increased PCE was usually attributed to an enhancement in JSC. By calculating the percentage contribution of photogenerated current density (JL) and electrical parameters to JSC and FF, it was demonstrated that the increased JSC is mainly attributed to an increase in JL, and the increased FF is attributed to a decrease in series resistance (Rs), reverse saturation current density (J0), and ideality factor of diode (A). The increase in JL results from the reduction of incident light loss by the anti-reflection effect of MgF2, the decrease in Rs from a decrease in resistivity of ITO caused by diffusion of F from MgF2 into ITO, and the decrease in A and J0 from a slight increase in carrier recombination due to Mg diffusion into ITO and a significant reduction in interfacial recombination because the device was annealed during thermal evaporation of MgF2.