Fig. 1. (a) PL and absorbance spectra of type-I ZnCdSe/ZnSe/ZnS (C-QDs) and anti-type-I ZnCdSe/ZnS/ZnSe (A-QDs), respectively. Inset: nanostructure profiles and emission images of the C-QDs and A-QDs, respectively. Schematic band alignment diagram and the heterojunction charge injection process of (b) C-QDs and (c) A-QDs. Notes: in QLEDs, a typical sandwich structure formed by a QD and adjacent film is as follows: hole transport layer/QD/electron transport layer (abbreviated as HTL/QD/ETL). Spatial probability distribution of the hole (green region) and electron (red region) as a function of (d) C-QDs and (e) A-QDs of different shell structure alignments. Gray dashed lines are boundaries between regions of core and shell space that correspond to different localization regimes.

Fig. 2. (a) AC-HAADF-STEM image and (b) corresponding magnified image and the electron energy loss spectroscopy analysis of the A-QDs. (c) Temperature-dependent PL intensity, fitting curves, and emission peaks evolution, and (d) time-resolved PL spectra for A-QDs and C-QDs, respectively. (e) PLQYs of the A-QDs and C-QDs in solution, on glasses, on Cl-treated ETL, and on ETL, respectively. (f) Comparison of time-resolved PL decay kinetics of the QDs.

Fig. 3. High-resolution XPS spectra of (a) Zn 2p and Cd 3d regions and (b) Se 3d and S 2p regions of A-QDs and C-QDs, respectively. (c) UPS spectra of A-QDs film (red) and C-QDs film (green). Inset: schematic band position of A-QDs and C-QDs, respectively. (d) Current−voltage characteristics of A-QDs and C-QDs films. Current density−voltage characteristics of (e) HODs and (f) EODs implementing A-QDs versus C-QDs. Inset: schematic structure of HODs and EODs, respectively.

Fig. 4. (a) Schematic diagram and (b) flat band energy level of each layer in flexible QLEDs. (c) Current density and luminance versus voltage characteristics for the device with A-QDs and C-QDs, respectively. (d) Luminance dependent EQE of the devices. (e) EL spectra of the QLEDs. Inset: photograph of flexible QLEDs and the corresponding CIE coordinates. (f) Lifetime (T50) of QLEDs at different initial luminance.

Fig. 5. Degradation mechanism of the flexible QLEDs. Capacitance–voltage characteristics of (a) A-QDs and (b) C-QDs QLEDs. (c), (d) Mott–Schottky measurements of the pristine and aged devices; both the extracted carrier concentration (NCV) and built-in potential (Vbi) are calculated from these profiles. (e), (f) Corresponding trap density of states (tDOS) distributions of the pristine and aged devices calculated from thermal admittance spectroscopy (TAS).

Fig. 6. TEM images of ZnCdSe/ZnSe/ZnS QDs (C-QDs).

Fig. 7. AC-HAADF-STEM images and simulated EDS elemental mapping of A-QDs.

Fig. 8. Device performance of A-QDs on glass substrates. (a) Schematic diagram of QLEDs. (b) Current density and luminance versus driving voltage characteristics of A-QDs devices with ZnS intermediate shell thicknesses of 0.9, 2.1, and 3.2 nm, respectively. Corresponding (c) EQEs of QLEDs in the luminance range for display and lighting applications. (d) Lifetime (T50) of QLEDs.

Fig. 9. Photographs of red flexible QLEDs with different voltages.

Fig. 10. Bending cycles test of flexible QLEDs. The device is driven by a constant current 10  mA cm−2.