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

Abstract

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 (C₆₀-PyP) via a 1,3-dipolar cycloaddition reaction, and the unambiguous determination of its molecular structure by X-ray single crystal diffraction. Upon incorporating C₆₀-PyP as an additive with an optimized doping ratio of 0.13 wt% into the CH₃NH₃Pbl₃ (MAPbI₃) 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 C₆₀-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 MAPbI₃ perovskite film. Besides, incorporation of C₆₀-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 C₆₀-PyP and the Pb²⁺ ion within MAPbl₃ 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 C₆₀-PyP incorporation due to the hydrophobic nature of the C₆₀-PyP molecule, which presumably resides at the grain boundaries.