Rational design of novel star-shaped organic molecules as hole-transporting materials in perovskite solar cells

Designing highly efficient hole-transporting materials (HTMs) has recently become one of the effective approaches to increasing the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs). Herein, twelve novel star-shaped organic small molecules (A1-F2) are simulated by using triphenylamine as the core group, introducing different electron-accepting pi-bridges and modulating terminal groups. The equilibrium geometries, electronic structures, optical properties, stabilities, solubilities, hole mobilities and adsorption features on the perovskite surface of the isolated molecules are calculated by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) in combination with the Marcus charge transfer theory. Our theoretical results demonstrate that the electron-withdrawing ability of pi-linkers and the molecular planarity have an important influence on the various properties of the studied molecules. Compared with the reference HTMs, the designed molecules with benzothiadiazole-based and benzoxadiazole-based electron-accepting bridges, especially E1, F1, C2, D2, E2 and F2, exhibit more suitable frontier molecular orbital character, good optical properties, larger Stokes shifts, similar or better solubility, good stability and higher hole mobilities, and are expected to be potential HTM candidates to help create more efficient solar cells.