Innovative Design and Synthesis of Fullerene-Terpyridine Derivatives for Enhanced Electron Transport in Inverted Perovskite Solar Cells

In this study, three fullerene derivatives─C60tBu, C60TPY, and C60TPY-Cl─were synthesized and investigated as additives in PC₆₁BM-based electron-transporting layers (ETLs) for inverted perovskite solar cells (PVSCs). The incorporation of C60tBu and C60TPY into the ETLs led to improved ETL morphology and passivation of crystal defects on the surface of the methylammonium lead iodide (MAPbI₃) layer. This defect passivation enhanced crystal quality, increased UV-vis absorption, reduced charge recombination, and improved electron mobility in the C60tBu- and C60TPY-based PVSCs. The passivation effect of C60TPY, which contains a 2,2':6',2″-terpyridine (TPY) unit, was found to be superior to that of C60tBu, which features a t-butyl ester group. As a result, PVSCs utilizing C60TPY exhibited enhanced photovoltaic performance compared to those incorporating C60tBu. To further investigate the contribution of the TPY moiety to the passivation effect, C60TPY was neutralized with HCl to afford C60TPY-Cl. As anticipated, the protonation of the TPY group in C60TPY-Cl resulted in poorer ETL morphology and diminished defect passivation within the MAPbI₃ layer. Consequently, no improvement in photovoltaic properties was observed for PVSCs treated with C60TPY-Cl. The architecture of the inverted PVSCs doped with fullerene derivatives consisted of indium tin oxide/NiO_x/MAPbI₃/fullerene derivative: PC₆₁BM/bathocuproine/Ag. Among the fullerene-based additives, C60TPY demonstrated the highest photovoltaic performance, achieving a power conversion efficiency (PCE) of 20.10%, an open-circuit voltage of 1.07 V, a short-circuit current density of 24.85 mA cm^-2, and a fill factor of 75.6%. Furthermore, the C60TPY-based PVSC retained 80% of its initial PCE after 450 h of storage under ambient conditions (30 °C, 40% relative humidity).