In Situ IR Spectroscopy\nStudies of Atomic Layer-Deposited\nSnO<sub>2</sub> on Fullerenes for Perovskite Photovoltaics

In recent years, atomic layer deposition (ALD) has established\nitself as the state-of-the-art technique for the deposition of SnO<sub>2</sub> buffer layers grown between the fullerene electron transport\nlayer (ETL) and the ITO top electrode in metal halide perovskite-based\nphotovoltaics. The SnO<sub>2</sub> layer shields the underlying layers,\ni.e., the fullerene-derivative materials such as C60 and PCBM, as\nwell as the perovskite absorber, from water ingress and damage induced\nby the sputtering of the transparent front contact. Our study undertakes\na comprehensive investigation of the impact of SnO<sub>2</sub> ALD\nprocessing on fullerenes by means of <i>in situ</i> spectroscopic\nellipsometry (SE) and transmission infrared spectroscopy (FTIR). While\nno difference in SnO<sub>2</sub> bulk properties is observed and the\nperovskite absorber degradation is nearly entirely avoided during\nexposure to heat and vacuum, when the absorber is introduced beneath\nthe organic ETLs, a SnO<sub>2</sub> growth delay of about 50 ALD cycles\nis measured on PCBM, whereas the delay is limited to 10 cycles in\nthe case of growth on C60. Notably, FTIR measurements show that while\nC60 remains chemically unaffected during SnO<sub>2</sub> ALD growth,\nPCBM undergoes chemical modification, specifically of its ester groups.\nThe onset of these modifications corresponds with the detection of\nthe onset, after the initial delay, of ALD SnO<sub>2</sub> growth.\nIt is expected that the modification that the PCBM layer undergoes\nupon ALD SnO<sub>2</sub> processing is responsible for the systematic\nlower photovoltaic device performance in the case of PCBM-based devices,\nwith respect to C60-based devices.