Plasma-Assisted Atomic Layer Deposition of Nickel Oxide as Hole Transport Layer for Hybrid Perovskite Solar Cells

Low-temperature atomic layer deposition (ALD) offers significant merits in terms of processing uniform, conformal and pinhole-free thin films, with sub-nanometer thickness control. In this work, plasma-assisted atomic layer deposition (ALD) of nickel oxide (NiO) is carried out by adopting bis-methylcyclopentadienyl-nickel (Ni(MeCp)2) as precursor and O2 plasma as co-reactant, over a wide table temperature range of 50-300 °C. A growth rate of 0.32 A per cycle is obtained for films deposited at 150 °C with an excellent thickness uniformity on a 4 inch silicon wafer. Bulk characteristics of the NiO film together with its interfacial properties with a triple cation hybrid perovskite absorber layer are comprehensively investigated, with the aim of integrating NiO as hole transport layer (HTL) in a p-i-n perovskite solar cell (PSC) architecture. It is concluded that “key” to efficient solar cell performance is the post-annealing treatment of the ALD NiO films in air, prior to perovskite synthesis. Post-annealing leads to better wettability of the perovskite layer and increased conductivity and mobility of the NiO films, delivering an increase in short-circuit current density (Jsc) and fill factor (FF) in the fabricated devices. Overall, a superior 17.07% PCE is achieved in the post-annealed NiO-based PSC when compared to the 13.98% PCE derived from the one with pristine NiO.