Spontaneous Heterointerface Modulation by a Methylammonium Tetrafluoroborate Additive for a Narrow-Bandgap FAPbI₃ Photoabsorber in 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 the bulk quality improvement of FAPbI₃ perovskite. 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^- over the heated FAPbI₃ surface, suggesting a defect-suppression mechanism that 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^- 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), a time-resolved microwave conductivity measurement revealed that MABF₄ addition eliminates carrier traps at the heterointerfaces. Our findings provide insights into promising FAPbI₃-based PSCs, offering a valuable tool for their further development.