High Charge Transfer from C₆₀Pyrrolidine Tris-Acid to SnO₂Electron Transport Layer Directly Observed by ESR Spectroscopy

Charge transfer characteristics of perovskite solar cells (PSCs) are effectively modified using fullerene-passivated SnO2 electron transport layers (ETLs), which are used as electron injection components from the perovskite layer in PSCs. Characterizing the charge accumulation states of decisive scientific evidence is highly expected. Therefore, to investigate the charge accumulation states over time from a microscopic viewpoint, this study presents electron spin resonance (ESR) spectroscopy of two fullerenes layered with SnO2. The advantages of a C60 pyrrolidine tris-acid (CPTA) over [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) were demonstrated using their high performance in terms of the number of accumulated charges, location of charge accumulation, and comparison of the charge trap depth. In terms of the number of accumulated charges, a decrease in electrons on CPTA and an increase in the electrons on PC61BM were directly observed in their layered films with SnO2 under light irradiation, which indicates that the CPTA can be sufficient to transfer photocarriers under light irradiation, while PC61BM remains saturated during the transfer of photocarriers leading to the accumulation of charges. In terms of the location of charge accumulation, the electrons on CPTA are accumulated at bulk areas, whereas those on PC61BM are accumulated at the interface between PC61BM and SnO2. The interfacial charge accumulation will demonstrate a negative effect on the device's performance. In terms of the comparison of the charge trap depth, the number of accumulated electrons on CPTA decreased rapidly, whereas those on PC61BM showed almost no change under dark conditions after light irradiation, indicating that the electrons on CPTA were trapped at a shallower depth than those on PC61BM. The direct observation of charge accumulation states on fullerenes of the fullerene-passivated SnO2 ETLs under light irradiation over time will be essential for optimizing the cell structures and enhancing cell power conversion efficiency and stability.