New fullerene design enables efficient passivation of surface traps in high performance p-i-n heterojunction perovskite solar cells

Abstract

Defect states within perovskite crystals are thought to induce undesired charge recombination and photocurrent hysteresis in perovskite solar cells. Although the processing of perovskite films with electron-rich molecules that can efficiently passivate the surface traps, the exact mechanism remains unclear. As the electron-rich units are key components for such a function, a rigorous analysis using controlled electron density in passivators can provide the means to understand these underlying mechanisms and thereby improve future improvements. In the study reported here, we combined electron-rich functional groups with fullerenes to design a new series of hydrophilic fullerene derivatives, in which each phenyl group of the diphenylmethanofullerene (DPM) moiety was decorated with an oligoether (OE) side group. These new materials were introduced as alternative electron transport layers (ETLs) to replace the commonly used PCBM in p-i-n planar-heterojunction perovskite solar cells. Our tests indicate that electron-rich OE chains can both passivate perovskite trap states and reduce the work function of the metal cathode. By adjusting the numbers of OE chains, relevant properties such as the energy levels, charge carrier mobilities, surface energy and dipole layer features could be tuned at the interfaces. Furthermore, devices with these fullerene ETLs showed significant improvements in power conversion efficiency (PCE) compared to devices with PCBM ETLs. A high PCE of 16% was achieved by applying the monoadduct fullerene derivative C70-DPM-OE as the ETL of the device.