Halogen-Bonded Hole-Transport Material Enhances Open-Circuit Voltage of Inverted Perovskite Solar Cells

Interfacial properties of a hole-transport material (HTM) and a perovskite layer are of high importance, which can influence the interfacial charge transfer dynamics as well as the growth of perovskite bulk crystals particularly in inverted structure. The halogen bonding (XB) has been recognized as a powerful functional group to be integrated with new small molecule HTMs. Herein, a carbazole-based halo (iodine)-functional HTM (O1), is synthesized for the first time, demonstrating a high hole mobility and suitable energy levels that align well with those of perovskites. The strong interaction between O1 and perovskite, i.e., I···I^-, induces the formation of an ordered interlayer, which are verified by both theoretical and experimental studies. Compared to the reference HTM (O2) without any halo-function, the XB-induced interlayer effectively enhances the interfacial charge extraction efficiency, while significantly hindering the non-radiative charge recombination by reducing the surface traps upon the strong passivation effect. This is reflected as a big increase in the open-circuit voltage by up to 114 mV in the fabrication of inverted devices with the highest power conversion efficiency of 22.34%. Moreover, the ordered XB-driven interlayer at the interface of O1 and perovskite is mainly responsible for the extended lifespan under the operational conditions.