Tuning the band structure of nickel oxide for efficient hole extraction in perovskite solar cells

Inorganic materials for the charge transport layer in solar cells are increasingly being investigated. Non-stoichiometric nickel oxide (NiO_X), owing to its larger band gap and stability is a promising hole transport material. In this work, we critically examine the electronic structure and hole-selectivity of NiO_X films at various oxygen partial pressure (p_O2) values during radio frequency (RF) sputtering. At p_O2 of 0.25, a maximum in the Ni³⁺/Ni²⁺ ratio (7.09) is obtained. At this ratio, the band alignment of NiO_X with that of methyl-ammonium lead iodide (MAPI) is observed to be most suitable. There is no misalignment of the valence band edges while conduction band edges separated by about 1 eV. Initial results of the MAPI perovskite solar cell with this optimized NiO_X as hole transport layer showed 7.7% efficiency. This result is promising to take the work further for enhancement of efficiency as well as for in-depth engineering of the energy bands of NiO_X.