Inorganic cesium halide perovskites (CsPbX3, X=I,Br) are promising candidates as the light-harvesting materials of new-generation photovoltaic devices owing 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 ability and phase stability has attracted ever-increasing attention in the field 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 necessity of developing facile but effective strategy to modify buried interface towards achieving superior cell performance. In this work, we report a facile additive strategy 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 can't incorporate into the crystal lattice of CsPbI2Br perovskite due to its large size. Therefore, 4-MPBA molecules are excluded from CsPbI2Br perovskite crystal and pushed downwards the buried interface of TiO2 electron-transport-layer and CsPbI2Br perovskite film during the perovskite crystallization process. Because of 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 demonstrate that the formation of 4-MPBA interlayer greatly enhance the interface contact, improve the interfacial energy level structure, and passivate 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 to the efficiency of the cell without 4-MPBA interlayer. Moreover, the cell without any encapsulation retains ~90% of the original efficiency after 960 h of aging in ambient air, suggesting a superior long-term stability. Therefore, this work highlights a facile strategy to in situ modify the buried interface for effectively enhancing the photovoltaic performance of inorganic perovskite solar cells.
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