Enhancing High Humidity Stability of Quasi-2D Perovskite Thin Films through Mixed Cation Doping and Solvent Engineering

Perovskite materials show excellent photovoltaic performance along with simple processing and low-energy requirements. Despite their high power conversion efficiency (PCE), instability in the presence of moisture is still a major challenge. An effective method to enhance perovskite stability is by reducing dimensionality through incorporation of long organic cations into the perovskite crystal, which improves charge-carrier extraction efficiency of the perovskites compared to conventional 3D perovskites. Quasi-2D perovskites or 2D/3D perovskites strike a good balance between PCE and stability, having much improved stability compared to 3D structures while retaining excellent optoelectronic properties. Yielding better thermal stability and broader absorption into the near-infrared, formamidinium iodide (FAI) doping has positive influences yet tends to cause poor surface morphology. Here, we introduce highly stable MA/FA-based quasi-2D perovskite fabricated by mixed cation doping (MCD), which is repeated deposition of MA and FA cations onto a quasi-2D perovskite layer. MCD enables better morphology and surface passivation, leading to fewer defects. MA/FA-based quasi-2D perovskite with quasi-cubic structure has high humidity resistivity, remaining intact after 90 days under 60% relative humidity without encapsulation. The underlying mechanism is further explained by binding and formation energies of cation mixture in solution and perovskite structure through computational analysis.