Methylammonium Tetrafluoroborate Additive for Spontaneous Heterointerface Modulation in a Narrow-Bandgap FAPbI3 Photoabsorber for Perovskite Solar Cells

Over the past decade, the photovoltaic (PV) performance of perovskite solar cells (PSCs) has been considerably improved with the development of perovskite photoabsorbers. Among these, formamidinium-lead-iodide (FAPbI3) is a promising photoabsorber owing to its narrow bandgap and is mainly used in n–i–p-structured PSCs. The property modulation of FAPbI3 photoabsorbers while retaining their narrow bandgap is imperative for further development of PSCs. Molecular tetrafluoroborate anion (BF4−)-based materials can be used as additives in perovskite layers to prevent bandgap widening, while facilitating perovskite crystal growth; thus, they are suitable for FAPbI3 photoabsorbers in principle. However, BF4−-based additives for narrow-bandgap FAPbI3 photoabsorbers have not been developed. This is presumably because of the higher temperatures required for FAPbI3 formation than that for other wide-bandgap perovskites, which likely changes the effects of BF4-based additives from those for wide-bandgap perovskites. In this study, we verified the applicability of methylammonium tetrafluoroborate (MABF4) as an additive in narrow-bandgap FAPbI3 photoabsorbers for improving their PV performance primarily via the spontaneous modulation of the heterointerfaces between FAPbI3 and carrier-transport materials, rather than through crystal growth facilitation. At the interface of the hole-transport material and FAPbI3, MABF4 addition effectively eliminates the surface defects in all FAPbI3 components, even in the absence of BF4− anions over the heated FAPbI3 surface, suggesting a defect-suppression mechanism, which differs from that observed in conventional ones. Moreover, at the interface of FAPbI3 and the TiO2 electron-transport material, the BF4−-derived species, which likely includes decomposed BF4− anions owing to the high-temperature heating, spontaneously segregates upon deposition, thereby modulating the heterointerface. Furthermore, in addition to the carrier mobility ratio in FAPbI3 (e−:h+ ≈ 7:3), time-resolved microwave conductivity measurement revealed that BF4 addition eliminates carrier traps at the heterointerfaces. Our findings provide insights into the promising FAPbI3-based PSCs, offering a valuable tool for their further development.