Imidazole cores for the construction of dopant-free asymmetric hole-transporting materials for efficient inverted perovskite solar cells

As the sandwiched hole transporting materials (HTMs) layers are in direct contact with up-coated perovskite layer and bottom electrode in p-i-n structured perovskite (i-PSCs), the film quality of HTMs determines the morphological properties of the perovskite absorbent. To achieve excellent surface morphology of HTM films, two asymmetric structured HTMs, which featuring with methoxytriphenylamine electron donor and imidazole core, has been successfully developed through two simple reactions of cyclization and Buchwald-Hartwing coupling. The DI-TPA was designed by incorporating an extra methoxytriphenylamine unit in the C2 site of imidazole core, whereas an isolated benzene ring was introduced for DI-Ar. The asymmetrical conformation contributes to high glass transition temperatures and supports thermal-stable amorphous properties in both HTM films. Comparing with DI-Ar, DI-TPA featuring with extended backbone realizes more effective capability of hole extraction arising from the enhanced intermolecular - interaction in the solid state. Consequently, the i-PSCs with DI-TPA as dopant-free HTM generates an encouraging power conversion efficiency of 16.54%, significantly exceeding that with DI-Ar (13.77%). Noted that the efficiency is shown to outperform that of commercial dopant-free HTM PEDOT:PSS (13.41%) under the same condition. This work offers a fundamental strategy by reinforcing electron-donation to improve the hole mobility of asymmetric HTMs for dopant-free p-i-n structured inverted PSCs.