Dopant-free Hole-transporting Materials for CH₃NH₃PbI₃ Inverted Perovskite Solar Cells with an Approximate Efficiency of 20%

A D-π–A-π–A-π-D structure-based small organic molecule (YJ01) has been demonstrated as an efficient dopant-free hole-transporting material (HTM) for inverted perovskite solar cells (iPSCs). The conjugation and twisted angles are balanced by the 1,4-di((E)-styryl)benzene core that bears two electron-withdrawing cyan groups. The cyan positions on the central core have a significant impact on the HTM properties. The outstanding performance of the iPSC device employing indium-doped tin oxide/pristine HTM/CH3NH3PbI3/PCBM/BCP/Ag based on YJ01 yielded an efficiency of 19.85% with a Voc of 1.08 V, Jsc of 22.53 mA cm–2, and FF of 0.815. The device which was based on YJ02 yielded power conversion efficiency (PCE) of 17.5%. The devices retained 80% of their original PCE after 500 h in an ambient environment. The hole motilities of YJ01 and YJ02 were determined to be 5.68 × 10–3 cm2 V–1 s–1 and 1.55 × 10–3 cm2 V–1 s–1, respectively. The large efficiency difference between YJ01 and YJ02 is attributed to the enlargement of the substitution distance between the two cyan units given the much greater reduction of solid-state fluorescence emission for YJ01 compared with YJ02. Thus, it is hoped to provide a good strategy for the HTM design, facilitating highly efficient and stable iPSCs.