A thiourea additive-based quadruple cation lead halide perovskite with an ultra-large grain size for efficient perovskite solar cells

Quadruple cation-based perovskite solar cells (PVSCs) have crossed the power conversion efficiency (PCE) of 25.2% because of their effective light harvesting ability. The perovskite materials and type of additives play a crucial role in improving the photovoltaic performance and stability. Therefore, here, we demonstrated a simple approach to reduce the grain boundaries and increase the grain size by adding thiourea (TU) as an additive in mixed halide (FAPbI₃)_0.85(MAPbBr₃)_0.15, triple cation Cs_0.05[(FAPbI₃)_0.85(MAPbBr₃)_0.15]_0.95 and quadruple Rb_0.05{Cs_0.05[(FAPbI₃)_0.85(MAPbBr₃)_0.15]_0.95}_0.95 cation perovskite absorbers. Our results indicate that the TU-added perovskite thin films have positive effects on the grain size, which improved up to 2.6 μm for the quadruple cation. Final optimization with the quadruple cation containing TU additive-based PVSC exhibited a 20.92% PCE, which is higher than additive-free PVSCs. Furthermore, the stability of the additive-modified PVSCs is much higher than that of bare films due to their ultra-large grain size with reduced grain boundaries. In addition, our thermal stress results exhibited that the additive-based PVSC devices display better thermal stability of more than ∼100 h at 60 °C without encapsulation.