In Situ IR Spectroscopy Studies of Atomic Layer-Deposited SnO₂ on Fullerenes for Perovskite Photovoltaics

In recent years, atomic layer deposition (ALD) has established itself as the state-of-the-art technique for the deposition of SnO₂ buffer layers grown between the fullerene electron transport layer (ETL) and the ITO top electrode in metal halide perovskite-based photovoltaics. The SnO₂ layer shields the underlying layers, i.e., the fullerene-derivative materials such as C60 and PCBM, as well as the perovskite absorber, from water ingress and damage induced by the sputtering of the transparent front contact. Our study undertakes a comprehensive investigation of the impact of SnO₂ ALD processing on fullerenes by means of <i>in situ</i> spectroscopic ellipsometry (SE) and transmission infrared spectroscopy (FTIR). While no difference in SnO₂ bulk properties is observed and the perovskite absorber degradation is nearly entirely avoided during exposure to heat and vacuum, when the absorber is introduced beneath the organic ETLs, a SnO₂ growth delay of about 50 ALD cycles is measured on PCBM, whereas the delay is limited to 10 cycles in the case of growth on C60. Notably, FTIR measurements show that while C60 remains chemically unaffected during SnO₂ ALD growth, PCBM undergoes chemical modification, specifically of its ester groups. The onset of these modifications corresponds with the detection of the onset, after the initial delay, of ALD SnO₂ growth. It is expected that the modification that the PCBM layer undergoes upon ALD SnO₂ processing is responsible for the systematic lower photovoltaic device performance in the case of PCBM-based devices, with respect to C60-based devices.