Fig. 1. (Color online) (a) Radar chart illustrating the key property difference between NiO_x, SAMs, and PEDOT:PSS. (b) Comparison of the optoelectronic properties of NiO_x, SAMs, and PEDOT: PSS^[10−14].

Fig. 2. (Color online) The schematic diagram of the topics in this review.

Fig. 3. (Color online) (a) Crystal structure of NiO_x^[23]. (b) The conduction mechanism of NiO_x, where V_c represents Ni²⁺ vacancies^[25]. (c) Schematic diagram of the electronic structure of NiO_x^[26]. (d) Resistivity of Zn-doped NiO_x films with different doping ratios^[29]. (e) Top-view SEM images of NiO_x thin films deposited on FTO^[28]. (f) XRD patterns of NiO_x thin films^[33].

Fig. 4. (Color online) Schematic diagram of NiO_x thin film deposition by (a) magnetron sputtering deposition, (b) spin-coating deposition^[37], (c) spray pyrolysis droplet deposition^[42], (d) electrochemical deposition^[44], (e) pulsed laser deposition^[46], and (f) ALD techniques^[48].

Fig. 5. (Color online) (a) Schematic diagram of the sputtering under low and high sputtering pressure conditions^[50]. (b) AFM images of NiO_x films and bare glass deposited at different pressures. (c) Transmittance of NiO_x thin films deposited by magnetron sputtering under various pressures^[52]. (d) Conductivity variation with Ni³⁺/Ni²⁺ ratio, and XPS images of NiO_x films at 4 mTorr (inset)^[54]. (e) XRD patterns of Cu-doped NiO_x films with different substrate temperatures^[55]. (f) The Ni³⁺/Ni²⁺ ratio of NiO_x thin films sputtered under various oxygen content^[58]. (g) A chart of the energy levels of device functional layers^[60]. (h) Structure diagram of aluminum-doped NiO_x. (i) J−V curve of perovskite devices based on NiO_x and Al_yNi_1−yO_x^[64].

Fig. 6. (Color online) (a) The energy levels of common perovskites used in inverted PSCs with magnetron-sputtered NiO_x as the HTL^{[20, 52, 71, 72, 77]}. (b) Efficiency evolution of PSCs based on magnetron-sputtered NiO_x as HTL^{[20, 52, 54, 57, 65−74]}. (c) Long-term stability of devices based on NiO_x and MeO-4PADBC at different temperatures^[18]. (d) The J−V curve of the 100 cm² minimodules based on NiO_x/Me-4PACz^[72]. (e) Schematic diagram of the tandem PSCs with ITO/NiO_x as the connecting layer^[75]. (f) Flexible perovskite solar cell devices based on NiO_x^[76].

Fig. 7. (Color online) (a) The energy band diagram of NiO_x, interlayer, and perovskites with different bandgaps. (b) The J−V curves of 1.53 eV devices with NiO_x, MeO-4PADBC, and NiO_x/MeO-4PADBC as HTLs^[18]. (c) Schematic diagram of the redox degradation mechanism at the NiO_x/perovskite interface^[73]. (d) Dark and light J−V curves with excess A-site ion PSCs^[80].

Fig. 8. (Color online) (a) Schematic diagram of molecular anchoring group of SAMs^[81]. (b) Electrostatic potential of PC molecules and mechanism of co-SAMs modified interface^[82]. (c) The corresponding potential histograms of ITO and ITO/NiO_x films before and after the adsorption of MeO-2PACz^[70]. (d) Energy level diagram for pristine NiO_x and NiO_x/MeO-2PACz^[65]. (e) Schematic diagrams of the NaIO₄-modified NiO_x film experimental process and the binding of SAMs^[83].

Fig. 9. (Color online) (a) Schematic diagram of the structure with NaCl interface introduced between NiO_x and perovskite^[84]. (b) XRD patterns of the perovskite layer with and without CsBr interfacial layer between NiO_x and perovskite^[85]. (c) Top-view SEM of NiO_x/perovskite and NiO_x/CsBr/perovskite. (d) UPS spectra of the work function and valence band regions for NiO_x film and NiO_x/CsBr film^[86]. (e) Chemical structure of the N719 molecule, and the electrostatic potential diagram, and three-dimensional charge density difference diagram of the NiO_x(001)/N719/PbI₂-rich MAPbI₃(001) interface^[71]. (f) Schematic electronic structure and carrier distribution of AlO_x/perovskite and SiO_x/perovskite heterojunctions^[89].