Fluorinated interfaces for efficient and stable low-temperature carbon-based CsPbI${}_2$Br perovskite solar cells

Carbon-based inorganic perovskite solar cells (C-PSCs) have attracted intensive attention owing to their low cost and superior thermal stability. However, the bulk defects in perovskites and interfacial energy level mismatch seriously undermine their performance. To overcome these issues, a multifunctional dual-interface engineering is proposed with a focus on low-temperature CsPbI${}_2$Br C-PSCs, where the potassium trifluoroacetate (KTFA) and the 4-trifluorophenyl methylammonium bromide (CF${}_3$PMABr) are introduced beneath and on top of the perovskite layer, respectively. It is found that TFA${}^{-}$ ions locate at the SnO${}_2$/CsPbI${}_2$Br interface, whereas a small amount of K${}^{+}$ ions diffuse into perovskite lattice to participate in nucleation and crystallization, resulting in more favored interfacial energy level alignment, improved film quality, passivated interfacial defects, released interfacial strain, as well as suppressed charge recombination and ion migration. Meanwhile, the CF${}_3$PMABr passivates I/Br vacancies and forms 2D perovskite capping layer to facilitate hole extraction at the CsPbI${}_2$Br/carbon interface. As a result, a remarkable power conversion efficiency (PCE) of 14.05% with an open-circuit voltage of 1.273 V is achieved. To the best of the authors' knowledge, it is currently the highest PCE reported for low-temperature CsPbI${}_2$Br C-PSCs. Furthermore, the nonencapsulated device exhibits improved moisture, thermal, and illumination stability in ambient air. (© 2022 Wiley‐VCH GmbH)