Tailoring of the core structure towards promising small molecule hole-transporting materials for perovskite solar cells: a theoretical study

Design of new molecules with theoretical chemistry methods and further to obtain a fundamental understanding of the structure-property relationship is important for the development of high-efficient hole-transporting materials (HTMs). Herein, the effect of semi-locked and fully-locked cores was systematically investigated based on two conformation-tunable tetrathienylethene (TTE) and tetraphenylethylene (TPE) units. Our results show that the highest occupied molecular orbital (HOMO) levels of the locked TTE-2 and TTE-3 are clearly down-shifted compared with that of the unlocked TTE-1, which derives from the decreased electronic conjugation between the locked cores and triphenylamine (TPA) arms, whereas the same situation has not been found for the TPE-3 due to the twisted core configuration. Compared with the TTE-series, the TPE-series exhibit less optical absorption in the visible light region and enhanced stability. Meanwhile, the hole mobility of designed HTMs displays an increased trend from the unlocked core to the semi-locked and fully-locked cores due to the gradually increased hole transfer integral with enhanced molecular planarity. In addition, we also find that the reorganization energy of the locked TTE cores is obviously lowered than that of the unlocked one, which plays an important role for the increase of hole mobility. In summary, this work can provide some useful clues for designing of high- efficient two-dimensional HTMs, and two potential promising candidates (TTE-3 and TPE-3) are proposed.