Unraveling Its Intrinsic Role of CH3NH3Cl Doping for Efficient Enhancement of Perovskite Solar Cells from Fine Insight by Ultrafast Charge-Transfer Dynamics

The addition of CH3NH3Cl (MACl) in perovskite precursor has become one of the most effective strategies for enhancing the photovoltaic performance of perovskite solar cells (PSCs). To further determine its relevant intrinsic modification mechanism, a series of PSCs with variant MACl contents are prepared. Apart from the analysis of crystal morphology and defect states, molecular-level photophysical processes related closely to photovoltaic performance are systematically investigated by transient absorption (TA) and time-resolved photoluminescence spectroscopy. Promisingly, by a diffusion-coupled charge-transport model via global fitting of TA spectra, the kinetic of perovskite/SnO2 heterojunction films is resolved into four distinct photophysical processes. Among the processes, as the MACl concentrations increase, the charge carriers' bulk diffusion in perovskite and interfacial transfer in perovskite/SnO2 heterojunction accelerate simultaneously, while the back charge recombination from SnO2 to perovskite decelerates, which correlates closely with larger grains featuring fewer grain boundaries and defect sites of perovskite induced by MACl doping. The aforementioned modified charge dynamics constitute the origin of the excellent optoelectronic properties in the resultant device, which exhibits an optimal conversion efficiency of 23.6%.