Ammonium cations with high pKa in perovskite solar cells for improved high-temperature photostability

Phenethylammonium (PEA+) and butylammonium (BA+) are widely used in three-dimensional (3D) perovskites to form two-dimensional perovskites at film surfaces and grain boundaries (GBs) for defect passivation and performance enhancement. Here we show that these cations are unstable with 3D formamidinium (FA+)-containing perovskites under high-temperature light soaking. PEA+ and BA+ are found to deprotonate to amines, which then react with FA+ to produce (phenethylamino)methaniminium (PEAMA+) and (butylamino)methaniminium (BAMA+), respectively, severely limiting device high-temperature photostability. Removing these cations greatly improves the photostability but compromises device efficiency by leaving non-fully passivated surfaces and GBs. Ammonium cations with a high acid dissociation constant (pKa), including PEAMA+ (pKa = 12.0) and BAMA+ (pKa = 12.0), can replace PEA+ or BA+ for passivation and are stable with FA-based perovskites due to their resistance to further deprotonation. P–i–n structure solar cells with PEAMA+ additive maintained over 90% of their initial efficiency after light soaking at open circuit and 90 °C for 1,500 hours. Ammonium cations can improve the power conversion efficiency of perovskite solar cells yet might pose an issue to the device stability. Wang et al. show that cations with a high acid dissociation afford improved operational stability at high temperatures owing to their resistance to deprotonation.