Unusual Hole Transfer Dynamics of the NiO Layer in Methylammonium Lead Tri-iodide Absorber Solar Cells.

Nickel oxides (NiO) as hole transport layers (HTLs) in inverted-type perovskite solar cells (PSCs) have been widely studied mainly because of their high stability under illumination. Increases in the power conversion efficiency (PCE) with NiO HTLs have been presented in numerous reports, although the photoluminescence (PL) quenching behavior does not coincide with the PCE increase. The dynamics of the charge carrier transport between the NiO HTLs and the organic-inorganic halide perovskite absorbers is not clearly understood yet and quite unusual, in contrast to organic/polymerics HTLs. We deposited NiO HTLs with precisely controlled thicknesses by atomic layer deposition (ALD) and studied their photovoltaic performances and hole transfer characteristics. Ground state bleaching (GSB) recovery was observed by ultrafast transient absorption spectroscopy (TAS), which suggested that backward hole injection occurred between the perovskites and NiO HTLs, so that the uncommon PL behaviors can be clearly explained. Backward hole injection from the NiO HTL to the perovskite absorber originated from their similar valence band (VB) energy positions. The thickness increase of the NiO HTLs induced VB sharing, which caused a red-shift of the photoinduced hole absorption spectrum in near-infrared (NIR) femtosecond TAS and a decrease in the PL intensity. Our studies on inorganic metal oxide transport layers, NiO in this work, with a thickness dependence and the comparison with organic layers provide a better understanding of the interfacial carrier dynamics in PSCs.