Buried Interface Engineering in Perovskite Solar Cells: Selective vs Spontaneous Heterointerface Modulation Using Tetrafluoroborate-Based Materials

Heterointerface modulation plays a crucial role in obtaining efficient perovskite solar cells (PSCs) owing to their multilayered architecture, which forms defect-prone heterointerfaces. Recently, spontaneous heterointerface modulation (SPHM) materials have emerged as additives for PSC component solutions and spontaneously modulate heterointerface(s) during solution deposition. Thus, SPHMs are advantageous because they eliminate the additional processes that conventional heterointerface modulators require and avoid atmospheric exposure of the modulated interface. Specifically, tetrafluoroborate (BF 4 – )-based materials exhibit SPHM functionalities; the BF 4 – anions spontaneously modulate the buried perovskite/electron transport material (ETM) interface in n-i-p PSC architectures. In principle, BF 4 -based additives are well suited to a narrow-bandgap FAPbI 3 photoabsorber because the BF 4 – additives retain its narrow bandgap and can enhance photovoltaic performance. However, a limitation of FAPbI 3 in the BF 4 -based SPHM system was recently identified: the BF 4 – composite deteriorates FAPbI 3 bulk properties via its decomposition during the relatively high-temperature heating required for FAPbI 3 . As such issues may not be confined to BF 4 -based materials, the development of strategies for buried interface modulation using materials prone to decomposition is imperative. In this study, an ex situ treatment for the TiO 2 surface is defined as selective heterointerface modulation (SEHM) and demonstrated its effectiveness for FAPbI 3 /ETM interface using a BF 4 -based material. SEHM with the BF 4 -based material negligibly affected the FAPbI 3 bulk, unlike SPHM, yet effectively modulated the buried perovskite/ETM interface, demonstrating the unique benefit of SEHM relative to SPHM. Therefore, SEHM enhances photovoltaic performance via increasing open-circuit voltage, an improvement that SPHM did not achieve. Thus, SEHM proves to be a potent technology when employing such heterointerface modulators that are susceptible to decomposition, and it is generally applicable to materials beyond BF 4 -based systems. This work highlights the advantages and disadvantages of both SEHM and SPHM techniques and provides insights for the further development of nascent and potent SPHM, contributing to advancements in materials science.