Structural defects at the surface and the bulk of perovskite films hinder efficient energy conversion in solar cells due to the loss of charge carriers through non-radiative recombination. Post-passivation approaches...

Metal oxide semiconductors have been highlighted in recent decades owing to their versatility in a wide range of electronic and optoelectronic applications, such as photovoltaics, light-emitting diodes, thin-film transistors, and electrochromic and resistive switching memory devices. [1][2][3][4][5][6] While n-type conductivity can be achieved in various types of metal oxides, the candidate transition metals for p-type oxides are relatively limited for the use of transparent conducting layers toward efficient optoelectronic devices. Cuprous oxide (Cu 2 O) and tin mono-oxide (SnO) exhibit p-type characteristics with a delocalized electronic wavefunction at the valence band maximum (VBM), resulting from the hybridization of Cu 3d and Sn 5s with O 2p orbitals, respectively. [7][8][9][10] However, their phases can readily transform into low-bandgap (1.6 eV) cupric oxide (CuO) and n-type tin dioxide (SnO 2 ) at room temperature. [11] In contrast, nickel oxide (NiO x ) shows excellent stability under ambient conditions with respect to bandgap and electrical conductivity, allowing its utilization for hole transport layers in optoelectronic device applications including photovoltaics and light-emitting diodes. [12][13][14][15][16] The wide bandgap and energy levels of NiO x make it suitable as a hole transport layer, particularly in perovskite solar cells, enabling good hole collection and electron blocking. [17,18] However, strong interfacial recombination and shrinkage of quasi-Fermi-level splitting by a large mismatch in energy levels limit the open-circuit voltage (V oc ). [19][20][21]