▎ 摘　　要

In Sn-Pb based low bandgap perovskite solar cells (LB-PSCs), the large energy loss of -0.5 eV lags its development, originating from unfavorable band alignment at the interface and serious charge recombination in the device. Here, judiciously designed graphene quantum dots by functional N/Cl doping are applied at PEDOT:PSS/ perovskite interface to modulate the energy level and charge transfer simultaneously. Among three designed graphene quantum dots, the N, Cl codoped one (N,Cl-GQDs) induces the best band alignment, because of the enhanced p-doping. Besides, N,Cl-GQDs best regulate the charge distribution and passivate the defect states at the interface by pi-conjugated effect and electronegative Cl, paving the way for efficient interface charge transport. Upon N,Cl-GQDs optimization, the FA0.6MA0.4Sn0.6Pb0.4I3 based PSCs achieves impressive efficiency of 21.5% (1 sun, AM1.5) with a high photovoltage of -0.89 V (energy loss of -0.36 eV) and fill factor over 80%, which are among the best parameters in reported LB-PSCs. The N,Cl-GQDs modified device exhibits the best shelf stability after 1000 h, due to the least trap states and reformative interface. This work highlights designed graphene quantum dots for photovoltaic applications.