Correlating halide segregation of wide-bandgap perovskites with the methoxy group in organic hole-selective materials

Abstract

Mixed-halide wide-bandgap (WBG) perovskites are widely used in constructing tandem photovoltaics, but their practical application is challenged by a phenomenon known as photo-induced halide segregation (PIHS), which is detrimental to the stability of the devices. The origin of PIHS is not fully understood yet, restricting the further advancement of mixed-halide WBG perovskites. Here, we report the serendipitous discovery that the PIHS of WBG perovskites is highly related to the presence of the 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 which differed only in the substituent groups (MeO, methyl or hydrogen) on the triphenylamine. In situ photoluminescence (PL) measurements revealed that all HSMs with MeO groups exhibited severe PIHS, and this observation was further validated by commercial PACz-series HSMs. Temperature-dependent PL experiments and density functional theory calculations suggest that contact between the MeO group and perovskites reduces the diffusion energy barrier of the halide ion, thus accelerating the PIHS. Removing the MeO group from the HSMs not only improves the power conversion efficiency of 1.76 eV WBG perovskite solar cells from 19% to 21% but also enhances their operational stability, with T90 increasing from 180 h to 650 h. This work discloses PIHS caused by the molecular structure of HSMs and suggests that the MeO group should be avoided when designing interfacial materials for WBG-perovskite-related optoelectronic devices.