Engineering the Active Layer of Lead-Free Perovskite (CH3NH3SnI3) Solar Cells Using Numerical Simulation

Abstract We conduct a thorough numerical simulation to examine the impact of the thickness, defect density, and doping density of the active layer on the photovoltaic performance of the lead-free CH 3 NH 3 SnI 3 perovskite solar cell (PSC). We observe that increasing the thickness of the active layer initially from 100 nm to 400 nm improved the power conversion efficiency (PCE) from 10.4% to 11.6%. However, further increasing the thickness to 800 nm resulted in a slight decline in PCE to 11.1%. This unexpected trend can be attributed to the high carrier mobility of charges in the CH 3 NH 3 SnI 3 perovskite, which enables fast extraction of charge carriers, offsetting losses due to charge recombination. Increasing active layer trap density substantially declines the PCE from 11.5% at 1014 cm -3 to 7.5% at 1018 cm -3 , as a result of the noticeable drop in open-circuit voltage (VOC) and fill factor (FF) with a growing defect density due to the enhancement in trap-assisted recombination. This is backed by a striking reduction in the shunt resistance upon increasing the defect density. Raising the active layer doping firstly enhances the PCE, reaching a peak value of 12.5% at the active layer doping density of 1017 cm-3, after which the PCE decreases as the doping density continues to increase. We explain these observations by energy level diagrams deduced at various doping levels.