Engineering of Volatile Cations in Tetrafluoroborate-Based Spontaneous Heterointerface Modulators for Perovskite Solar Cells

The recent emergence of spontaneous heterointerface modulators (SHMs) has led to a potent advancement in perovskite solar cells (PSCs). Practical PSCs comprise multiple layers; therefore, modulating their defect-prone heterointerfaces is crucial for achieving highly efficient PSCs. SHMs are introduced as additives in precursor solutions of PSC components and spontaneously modulate these heterointerfaces during the deposition process. Consequently, SHMs enhance the process efficiency of the PSC fabrication. Despite the advantage of SMHs in terms of engineering aspects, their intrinsic concentration sensitivities limit their applicability. Thus, a general strategy is necessary to mitigate these sensitivities and exploit their advantages in device engineering. Herein, we propose a novel strategy: utilizing the volatile components in SHMs to address their concentration sensitivity issue. This strategy is demonstrated by engineering volatile cations in tetrafluoroborate (BF₄)-based SHMs. BF₄ salts of ammonium (NH₄⁺), methylammonium (MA⁺), and formamidinium (FA⁺), which are cations of different volatilities, were used as additives for formamidinium lead halide (FAPbI₃) perovskite photoabsorbers, and their effects are compared. The optimal amount of each BF₄-based SHM was effective for enhancing the photovoltaic performance of PSCs via their SHM functions, with similar outcomes achieved for all cation types. This indicates that BF₄^- anions play a pivotal role in optimal SHM functionality. Meanwhile, cation composites affect their concentration sensitivity; NH₄BF₄, comprising the most volatile cation led to the most moderate concentration sensitivity, successfully demonstrating the proposed strategy. The material design of leveraging the volatility of SHM components to remove unused species can be applied in the development of general SHMs and as such will be helpful in addressing the intrinsic issue of SHMs. These findings support broader implementation of spontaneous heterointerface modulation technics and allow innovation in SHM design, ultimately promoting the development of high-performance PSCs.