Inorganic cesium halide perovskites (CsPbX₃, X = I, Br) are promising candidates as the light-harvesting materials of new-generation photovoltaic devices due to their intrinsic advantages, such as the high thermal stability, excellent optoelectronic properties, and facile solution fabrication process. In particular, CsPbI₂Br perovskite which balances the light-harvesting capability and phase stability has attracted ever-increasing attention in the fields of the single junction, the tandem, and the semitransparent photovoltaic devices. In the past several years, inorganic CsPbI₂Br perovskite solar cells (PSCs) have achieved great progress in both the power conversion efficiency and the stability through versatile device engineering. Nevertheless, the inferior buried interface derived from the uncontrollable up-to-bottom perovskite crystallization process leads to the serious charge recombination and energy loss within CsPbI₂Br PSCs, which considerably hinders the further development and practical deployment of CsPbI₂Br PSCs. This highlights the need to develop a simple but effective strategy to modify the buried interface to achieve excellent cell performance.

In this work, we present a simple additive approach to in situ modify the buried interface of CsPbI₂Br PSCs through forming a dipolar interlayer. The polar 4-mercaptophenylboronic acid (4-MPBA) additive is directly added into CsPbI₂Br precursor solution. 4-MPBA molecules cannot be incorporated into the crystal lattice of CsPbI₂Br perovskite because of its large size. Therefore, in the crystallization process of perovskite, 4-MPBA molecules are excluded from CsPbI₂Br perovskite crystal and pushed down to the buried interface between the TiO₂ electron-transport-layer and the CsPbI₂Br perovskite film. Owing to the strong interaction between the —B(OH)₂ group of 4-MPBA molecule and TiO₂, 4-MPBA molecules tend to accumulate at the buried interface between CsPbI₂Br perovskite and TiO₂ layer and form a dipolar interlayer. Scanning electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy measurements clearly show that the formation of 4-MPBA interlayer greatly enhances the interface contact, improves the interfacial energy level structure, and passivates the interface defects, which effectively suppresses the charge recombination and promotes the charge collection within the cell. As a result, the assembled carbon-based CsPbI₂Br PSC without hole-transport layer delivers a power conversion efficiency of 14.83%, which is increased by 26% compared with the efficiency of the cell without a 4-MPBA interlayer. Moreover, the cell without any encapsulation retains ~90% of the original efficiency after 960-hour aging in ambient air, indicating its superior long-term stability. Therefore, this work highlights a simple strategy for in situ modifying the buried interface to effectively enhance the photovoltaic performance of inorganic perovskite solar cells.