Recent advances and comprehensive insights on nickel oxide in emerging optoelectronic devices

Metal oxide semiconductors, exhibiting excellent optical transparency, high carrier mobilities, good mechanical stress tolerance and environmental stability, have been widely used in emerging optoelectronic devices (OEDs), such as dye-sensitized solar cells (DSSCs), perovskite solar cells (PerSCs), organic solar cells (OSCs) and various light-emitting diodes (LEDs). Among them, nickel oxide (NiO) is a typical p-type metal oxide and plays important roles in these emerging OEDs, such as photocathodes in p-type DSSCs, and hole transport layers (HTL) or hole injection layers (HIL) or electron blocking layers (EBL) in PerSCs, OSCs and LEDs. This comprehensive review delivers a summary of recent progress in emerging OEDs with NiO, explains how NiO works in different devices, demonstrates the influence of the electronic structure and surface chemistry of NiO on the device performance, and emphasizes the effects of morphology, components and post-treatment on the optoelectronic properties of NiO. The extant problems and future directions for further development of NiO in OEDs are discussed in the end.