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

In this study, three fullerene derivatives─<b>C60tBu</b>, <b>C60TPY</b>, and <b>C60TPY-Cl</b>─were synthesized and investigated as additives in PC₆₁BM-based electron-transporting layers (ETLs) for inverted perovskite solar cells (PVSCs). The incorporation of <b>C60tBu</b> and <b>C60TPY</b> 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 <b>C60tBu</b>- and <b>C60TPY</b>-based PVSCs. The passivation effect of <b>C60TPY</b>, which contains a 2,2':6',2″-terpyridine (TPY) unit, was found to be superior to that of <b>C60tBu</b>, which features a <i>t</i>-butyl ester group. As a result, PVSCs utilizing <b>C60TPY</b> exhibited enhanced photovoltaic performance compared to those incorporating <b>C60tBu</b>. To further investigate the contribution of the TPY moiety to the passivation effect, <b>C60TPY</b> was neutralized with HCl to afford <b>C60TPY-Cl</b>. As anticipated, the protonation of the TPY group in <b>C60TPY-Cl</b> 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 <b>C60TPY-Cl</b>. The architecture of the inverted PVSCs doped with fullerene derivatives consisted of indium tin oxide/NiO_<i>x</i>/MAPbI₃/fullerene derivative: PC₆₁BM/bathocuproine/Ag. Among the fullerene-based additives, <b>C60TPY</b> 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 <b>C60TPY</b>-based PVSC retained 80% of its initial PCE after 450 h of storage under ambient conditions (30 °C, 40% relative humidity).