# Solution-Processed NiO as a Hole Injection Layer for Stable Quantum Dot Light-Emitting Diodes
## 3 Results and Discussion
For the device with an NiO annealed at 500 \({}^{\circ}\)C, the peak  
Figure 3: (**a**) Device structure and (**b**) energy band diagram of a standard-structured QLED with NiO as the HIL. Figure 2: (**a**) XRD patterns of annealed NiO films and (**b**) optical transmittance spectra of PEDOT:PSS, as-deposited NiO, and annealed NiO films. luminance of 34,125 cd/m\({}^{2}\) was achieved at 15.5 V (corresponding to a current density of 164.5 mA/cm\({}^{2}\)), and a current efficiency of 25.1 cd/A was obtained at a current density of 60.4 mA/cm\({}^{2}\). The higher the annealing temperature applied to the NiO layer, the higher the conductivity obtained (Figure S2 in the Supplementary Materials), which solves the charge imbalance in the EML due to the slow hole transport. As shown in Figure 5d, there is no big difference in the shape and peak position of the EL peak and the PL peak. Further, QLEDs based on NiO layers annealed at 500 \({}^{\circ}\)C were fabricated either with UV-ozone treatment or without it, and their optoelectronic properties were investigated (Figure 6). In the absence of a UV-ozone treatment, the electrical properties were unstable, and a high leakage current was observed in the Ohmic region. By contrast, after 10 min UV-ozone treatment of the NiO layer, the current density, luminance, and current efficiency increased significantly. [PERSON] et al. reported that the UV-ozone treatment was an effective technique to improve the performance of QLEDs [PERSON] et al. (2019). The enhanced hole injection with the NiO layer after the UV-ozone treatment led to improved optoelectronic properties, as shown in Figure 6.  
Figure 4: AFM images of the (**a**) as-deposited NiO film and NiO films annealed at (**b**) 300 \({}^{\circ}\)C and (**c**) 500 \({}^{\circ}\)C. (**d**) RMS roughness vs. the annealing temperatures. The application of inorganic HILs can thus provide many advantages, especially improved device stability. Figure 7 shows that the half-life of QLEDs with NiO layers was 2.3 times longer than that of QLEDs with PEDOT:PSS. As shown in Figure S1, the electrical characteristics of two HODs (NiO or PEDOT:PSS) were very similar to each other. Therefore, the device stability mainly depends on the used material itself. Additionally, UV-ozone treated QLEDs with NiO layers annealed at 500 \({}^{\circ}\)C exhibited long shelf lives; moreover, they showed stable EL emission even after 2 weeks. This result suggests that our QLEDs with inorganic HILs have better resistance to oxygen and humidity than traditional QLEDs do. Figure 5: (**a**) Current density–voltage, (**b**) luminance–voltage, and (**c**) current efficiency–current density characteristics of QLEDs with the NiO at different annealing temperatures; (**d**) spectral comparison of PL of a dilute QD dispersion with EL of the device at 10 V.  
Figure 6: (**a**) Current density–voltage, (**b**) luminance–voltage, and (**c**) current efficiency–current density characteristics of QLEDs using NiO with and without UV-ozone treatment. Figure 7: The operational lifetime characteristics of QLEDs with NiO or PEDOT:PSS as the HIL. The luminance of QLEDs was measured at a constant current density (0.07 mA/cm\({}^{2}\)) corresponding to an initial luminance of 500 cd/m\({}^{2}\).