The effect of pH level and annealing temperature on NiO thin films as Hole Transport Material in Inverted Perovskite Solar Cells

Perovskite solar cells (PSCs) have the potential for high-power efficiency and drawn interest from both experimental and theoretical perspectives. The PSCs using inorganic materials for hole transport layers (HTL) such as Cu
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O, CuI, CuO, and NiO can afford high efficiency of about 20% which is comparable to the values achieved by the Spiro-OMeTAD and PEDOT:PSS, frequently used HTL. Nickel oxide (NiO) has considerable attention because of its better chemical stability, good transmittance, and suitable matching energy level with perovskite. The main objective of this paper is to present the experimental result on the influence of different pH levels for the top (supernatant) and bottom (precipitate) layer through the spin coating method that is annealed at 700°C which is then characterized using UV-Vis spectroscopy to investigate their optical properties. GPVDM (General-purpose Photovoltaic Device Model) simulator was used to run the results from the experimental data to determine the optimized parameters built on the framework of ITO/NiO/MAPbI
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/ZnO/Ag. The experimental result, from UV-Vis spectra, revealed that in the visible region, the bottom layer of as-deposited pH 11 NiO thin film showed the highest absorption. Meanwhile, treated at 700°C top layer pH 11 annealed NiO thin film displayed the highest absorption towards the shorter wavelength region. The reduction of energy bandgap from 3.80 to 3.50eV due to change in lattice symmetry of NiO thin film crystallites and the refractive index increased from 2.07 to 3.92 attributed to the enhanced scattering losses as temperature grew were noticed at top and bottom layer of pH 11 NiO thin film. From the simulation results, the highest efficiency was at 7.20%, 0.87a.u, 1.34V, and 61.57 Am
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achieved by top layer pH 11 annealed NiO thin film. The simulated results agree well with the experimental results based on optical studies that top layer pH 11 annealed NiO thin film exhibited the characteristics suitable for HTL in PSCs.