Fig. 1. Structure, optical bandgap, and energy level of NiOx and Mg-NiOx NC films. (a) XRD patterns; (b)–(d) XPS spectra of (b) Mg 1s, (c) O 1s, and (d) Ni 2p; (e) Tauc plot; (f) UPS spectra.

Fig. 2. (a)–(c) AFM images of the (a) ITO/NiO, (b) ITO/Mg-NiOx, and (c) ITO/NiOx/Mg-NiOx NC films; (d) transmittances of PEDOT:PSS and NiOx/Mg-NiOx HIL films deposited on the ITO substrate.

Fig. 3. (a) Schematic of the layers in the device structure; (b) energy level diagram of the devices; (c) normalized EL spectra of devices. The inset shows a photograph of a device. (d) Luminance–voltage–current density (L–V–J) characteristics of QLEDs; (e), (f) EQE and CE as a function of luminance; (g) lifetime measurements at the initial luminance of 1000  cd m−2 of three typical QLEDs; (h) histogram of peak EQEs of NiOx/NiMgOx QLEDs obtained from 50 devices; (i) histogram of accelerated T95 lifetimes with an initial luminance of 1000  cd m−2 estimated from 1/2 of the total number of QLEDs based on a bilayered HIL of NiOx/Mg-NiOx.

Fig. 4. (a)–(c) Charged QD− states, charge transport mechanisms, and hole injection barriers of the (a) PEDOT:PSS, (b) Mg-NiOx, and (c) NiOx/Mg-NiOx HIL devices; (d) J-V characteristic curves of the hole-only devices and electron-only devices for the three kinds of HILs of PEDOT:PSS, NiOx, and NiOx/Mg-NiOx; (e) C-V characteristics and (f) impedance spectra of InP-based QLED with HILs of PEDOT:PSS, NiOx, and NiOx/Mg-NiOx.