A strategy to improve the efficiency of hole transporting materials: Introduction of a highly symmetrical core

The electronic, optical and hole transport properties of three new hole transporting materials (HTMs) with a planar core have been investigated by using density functional theory and Marcus theory. A reliable semi-rational formula was adopted to calculate the highest occupied molecular orbital (HOMO) levels of new HTMs. The results showed that the HOMO levels of new HTMs were 0.07-0.30 eV lower than those of Spiro-OMeTAD, and their absorption peaks appeared outside or close to the visible region and overlapped slightly with the absorption band of perovskites. Moreover, the Stokes shifts of the designing molecules were calculated to lie in a range from 72 to 124 nm, meaning that they could undergo large geometrical changes on excitation. More importantly, the hole mobility of new HTMs (0.099-0.27 cm² v^-1 s^-1) was approximately two orders of magnitude higher than that of Spiro-OMeTAD (0.0056 cm² v^-1 s^-1) due to strong hole coupling from a face-to-face packing pattern. Our results indicated that planar core-based HTMs could become potential candidates to replace the widely established Spiro-OMeTAD.