Push-pull substituent design of fullerene dimer at the buried interface toward stable and efficient perovskite solar cells

Defective interface contact is detrimental to the performance of perovskite solar cells (PSCs), by inducing hysteresis, reducing power conversion efficiency (PCE) and deteriorating operation stability. Interlayer materials are commonly utilized to eliminate interfacial defects; however the design of efficient molecules with superior charge transport and defect passivation capabilities remains a big challenge. Herein, four novel fullerene dimers DC60-R1-R2 (R1 = H or Cl, R2 = H or MeO) are synthesized as the interlayers between metal oxides and perovskites. The polar substituents of these fullerene dimers are found to determine the inter-molecular interaction between these fullerene molecules, with DC60-Cl-MeO showing the greatest intermolecular charge transport and passivation ability due to the strongest push-pull effect of the electron-accepting chloride (-Cl) and electron-donating methoxy (-MeO). The non-encapsulated planar (FAPbI(3))(x)(MAPbBr(3))(1-x) PSC with DC60-Cl-MeO as the interlayer delivers a maximum PCE of 23.3% with no hysteresis, maintaining nearly 100% of the initial efficiency after being stored in a high humidity environment over 500 h. The extrapolated T-S80 lifetime (the time required to reach 80% of initial performance) of the PSC device could be extended to 1110 h at the maximum power point operation condition under one sun illumination without any encapsulation or inert gas protection, superior to the pure SnO2-based device (0.5 h). Fullerene derivatives with push-pull substituents are therefore promising candidates as the interfacial materials for PSCs to receive high performance.