NiO as Hole Transporting Layer for Inverted Perovskite Solar Cells: A Study of X-Ray Photoelectron Spectroscopy

Abstract

Hygroscopic and acidic nature of organic hole transport layers (HTLs) insisted to replace it with metal oxide semiconductors due to their favorable charge carrier transport with long chemical stability. Apart from large direct bandgap and high optical transmittance, ionization energy in the range of -5.0 to -5.4 eV leads to use NiO as HTL due to good energetic matching with lead halide perovskites. Analyzing X-ray photoelectron spectroscopic (XPS) data of NiO, it is speculated that p-type conductivity is related to the NiOOH or Ni2O3 states in the structure and the electrical conductivity can be modified by altering the concentration of nickel or oxygen vacancies. However, it is difficult to separate the contribution from nonlocal screening, surface effect and the presence of vacancy induced Ni3+ ion due to very strong satellite structure in the Ni 2p XPS spectrum of NiO. Thus, an effective approach to analyze the NiO XPS spectrum is presented and the way to correlate the presence of Ni3+ with the conductivity results which will help to avoid overestimation in finding the oxygen-rich/deficient conditions in NiO. The shoulder peak of Ni2p XPS spectrum is important to understand p-type semiconducting behavior. Both the Ni 2p and O 1s XPS spectra shall be carefully recorded with fixed take-off angle (and/or depending on take-off angle) and compare results with transport measurements.