Modifying Low-Temperature-Annealed NiO_<i>x</i> Nanoparticle Films for Improved Performance of Perovskite Light-Emitting Diodes

As a potential hole transport material (HTM) for perovskite light-emitting diodes (PeLEDs), nickel oxide (NiO<i>_x</i>) presents advantages of high stability and low cost. However, the undesirable NiO<i>_x</i>–perovskite interface and the high annealing temperature of NiO<i>_x</i> limit its application. Here, we report the modification of low-temperature-annealed NiO<i>_x</i> nanoparticle films to improve the NiO<i>_x</i>–perovskite interface and the performance of PeLEDs. NiO<i>_x</i> nanoparticles are presynthesized and poly(sodium-4-styrene sulfonate) (PSSNa) is introduced into the dispersion of NiO<i>_x</i> nanoparticles for modification. The morphology of the NiO<i>_x</i> film is improved by optimizing the PSSNa concentration, showing higher uniformity and compactness with a reduced surface roughness. The modification also increases the Ni³⁺/Ni²⁺ ratio and shifts the valence band maximum (VBM) of the NiO<i>_x</i> film. Furthermore, the modification of NiO<i>_x</i> film results in a remarkable improvement of the perovskite film quality and suppresses photoluminescence (PL) quenching at the NiO<i>_x</i>–perovskite interface. As a result, the PeLEDs based on the modified NiO<i>_x</i> HTM exhibit a prominently increased external quantum efficiency (EQE) from 2.13 to 6.24%, accompanied by an enhanced operational stability. This work provides a feasible way toward realizing cost-effective PeLEDs based on low-temperature-processed HTM.