Anchoring Fullerene onto Perovskite Film via Grafting Pyridine toward Enhanced Electron Transport in High-Efficiency Solar Cells

Fullerene derivatives have been popularly applied as electron transport layers (ETLs) of inverted (p- i-n) planar heterojunction perovskite solar cells (iPSCs) due to their strong electron-accepting abilities, and so far, [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) has been the most commonly used ETL, which suffers, however, from high cost due to the complicated synthetic route. Herein, novel pyridine-functionalized fullerene derivatives (abbreviated as C-60-Py) were synthesized facilely via a one-step 1,3-dipolar cycloaddition reaction and applied as ETLs superior to PCBM in iPSC devices. Three pyridine-functionalized fullerene derivatives with different alkyl groups, including methyl, n-butyl, and n-hexyl, grafted onto the pyrrolidine moiety (abbreviated as C-60-MPy, C-60-BPy, and C-60-HPy, respectively) were synthesized. According to cyclic voltammogram study, the chain length of the N-alkyl group has negligible influence on the molecular energy level of C-60-Py. However, the ETL performance of C-60-Py is sensitively dependent on the chain length of the N-alkyl group, with C-60-BPy exhibiting the highest power conversion efficiency (PCE) of 16.83%, which surpasses that based on PCBM ETL (15.87%). The PCE enhancement of C-60-BPy device is attributed to the coordination interactions between the pyridine moiety with the Pb2+ ion of CH3NH3PbI3 perovskite, which anchor C-60-BPy onto perovskite film and reinforce the passivation of the trap state within the CH3NH3PbI3 perovskite film and suppress the non-radiative electron-hole recombinations, leading to enhanced electron transport reflected by the increase of short-circuit current density (J(sc)). The ambient stability of C-60-HPy-based device is much better than that based on PCBM ETL since its long N-alkyl group can function as a superior encapsulating layer protecting the CH3NH3PbI3 layer from contact with the ambient moisture.