# Solution-Processed NiO as a Hole Injection Layer for Stable Quantum Dot Light-Emitting Diodes
## 3 Results and Discussion  
Figure 2a presents the XRD pattern of the annealed NiO films that crystallized in the face-centered cubic phase without the formation of any new phase. In detail, the XRD pattern presents peaks at 20 = 37.20\({}^{\circ}\), 43.20\({}^{\circ}\), 62.87\({}^{\circ}\), 75.20\({}^{\circ}\) and 79.38\({}^{\circ}\) corresponding to the (111), (200), (220), (311), and (222) crystal planes of the NiO, respectively. The optical transmittance data for the NiO layer, which annealed at 500 \({}^{\circ}\)C (Figure 2b), show a high transmittance of more than 90% in the entire visible region and no difference from those of the PEDOT:PSS layer. This transmittance value is either similar to or higher than the alternative inorganic materials for HIL [11; 15]. As shown in Figure 2a, the transmittance of films annealed at elevated temperatures increases because of the improved crystalline microstructure [16]. Because the materials under the EML inevitably absorb light emitted from the QDs in the bottom emission structure, the transmittance must be ensured before the fabrication of the devices. Figure 3a illustrates a schematic of the device structure of a standard QLED with NiO as the HIL. It consists of ITO (anode)/NiO (HIL)/TFB (HTL)/green QDs (EML)/TPBi (ETL)/LiF/Al (cathode). According to the energy level diagram in Figure 3b [17; 18; 19], PEDOT:PSS and NiO have similar work functions (Table S1 in the Supplementary Materials). However, NiO not only has a great hole injection ability, but also an electron-blocking function because of its deep valence band and wide bandgap. Owing to the high energy level of its conduction band minimum, the NiO can block further transport of electrons to the anode, while its well-matched valence band maximum allows holes to move efficiently to the EML. Figure 4 shows the AFM images of solution-processed NiO thin films annealed at different temperatures. The charge transport layers formed by the solution process often show a rough surface or pinholes, which leads to poor performance or severe electrical leakage of devices. The AFM images confirm the formation of ultra-smooth surfaces of the NiO thin films with a root mean square (RMS) roughness of less than 0.3 nm, indicating  
that all the NiO films are smoother than the ITO film on glass [20]. These results indicate that the NiO layer effectively covers the ITO surface, and the annealing temperature hardly affects the morphology of the NiO film. Figure 5 presents the optoelectronic properties of the QLEDs with the NiO films annealed at different temperatures. Improvements in both the electrical properties and the luminance were achieved with an increase in the annealing temperature. It is well known that three different conduction regimes (ohmic, trap-limited, and space charge limited conduction) are apparent in the QLEDs, and different power-law relationships in each region are shown in Figure 5a [21]. The leakage current in the Ohmic region decreased significantly, and the current density above the turn-on voltage improved after the annealing process. The device with the NiO annealed at 500 \({}^{\circ}\)C clearly showed trap-limited behavior above the turn-on voltage, and then changed to space-charge limited conduction as the operating voltage was increased. Interestingly, the electrical properties of the devices annealed at 500 \({}^{\circ}\)C were found to be almost identical to those of the devices with PEDOT:PSS. To further investigate the hole injection into the EML, hole-only devices (HODs) were fabricated using NiO layers annealed at 500 \({}^{\circ}\)C, and their current densities were compared with those of HODs with PEDOT:PSS (Figure S1 in the Supplementary Materials). The HODs consisted of ITO/NiO or PEDOT:PSS/TFB/QDs/MoO\({}_{3}\)/Al layers. The current density of the HODs with an NiO layer annealed at 500 \({}^{\circ}\)C is comparable to that of the HODs with PEDOT:PSS.