Mixed-halide wide-bandgap (WBG) perovskites are widely used in constructing tandem photovoltaics, but their practical applications are challenged by a phenomenon called photo-induced halide segregation (PIHS), which is detrimental to stability of the devices. The origin of such PIHS is not fully understood yet, restricting their further advancement. Here, we report a serendipitous discovery that PIHS of WBG perovskites is highly related to the presence of methoxy group (MeO) in organic hole-selective materials (HSMs). Based on a model compound with triphenylamine as the hole-selecting group and cyanovinyl phosphonic acid as the anchoring group, we developed a series of HSMs with only difference in substituent groups (MeO, methyl or hydrogen) on the triphenylamine. In-situ photoluminescence (PL) measurements reveal that all MeO carrying HSMs results in severe PIHS and this observation is further validated by commercialized PACz-series HSMs. Temperature dependent PL experiments and density functional theory calculations suggest that contact between MeO group and perovskites reduces diffusion energy barrier of halide ion, thus accelerating the PIHS. Removing MeO group from HSMs not only improves power conversion efficiency of 1.76 eV WBG perovskite solar cells from 19% to 21%, but also enhances operational stability with T90 augmented from 180 h to 650 h. This work discloses molecular structure of HSMs caused PIHS, and suggests to avoid MeO group in designing interfacial materials for WBG perovskites related optoelectronic devices.
