Pyridine-functionalized fullerene additive enabling coordination interactions with CH3NH3PbI3 perovskite towards highly efficient bulk heterojunction solar cells

Incorporating n-type fullerene derivatives into the perovskite layer affording bulk heterojunction (BHJ) perovskite solar cells (PSCs) is an effective strategy to passivate trap states, consequently reducing the current-voltage hysteresis and improving the device performance. Herein, we report the synthesis of a novel pyridine-functionalized fullerene derivative (C-60-PyP) via a 1,3-dipolar cycloaddition reaction, and the unambiguous determination of its molecular structure by X-ray single crystal diffraction. Upon incorporating C-60-PyP as an additive with an optimized doping ratio of 0.13 wt% into the CH(3)NH(3)Pbl(3) (MAPbI(3)) perovskite precursor, the regular-structure bulk heterojunction PSC devices exhibit a best power conversion efficiency (PCE) of 19.82%, which is dramatically enhanced relative to that of the control devices (17.61%). The C-60-PyP additive provides heterogeneous nucleation sites, leading to the decrease of the nucleation Gibbs free energy and consequently enlarged grain size and improved crystallization of the MAPbI(3) perovskite film. Besides, incorporation of C-60-PyP results in enhanced crystalline orientation as confirmed by grazing-incidence X-ray diffraction (GIXRD) measurements. Meanwhile, the coordination interaction between the N atom of the pyridine moiety within C-60-PyP and the Pb2+ ion within MAPbl(3) leads to the passivation of the trap states of the perovskite layer, jointly contributing to the improved device performance and markedly suppressed current-voltage hysteresis. Moreover, the ambient stability of the devices is improved upon C-60-PyP incorporation due to the hydrophobic nature of the C-60-PyP molecule, which presumably resides at the grain boundaries.