Solution-processed oxide thin films are actively pursued as hole-injection layers (HILs) in quantum-dot light-emitting diodes (QLEDs), aiming to improve operational stability. However, device performance is largely limited by inefficient hole injection at the interfaces of the oxide HILs and highionization-potential organic hole-transporting layers. Solution-processed NiO x films with a high and stable work function of ≈5.7 eV achieved by a simple and facile surface-modification strategy are presented. QLEDs based on the surface-modified NiO x HILs show driving voltages of 2.1 and 3.3 V to reach 1000 and 10 000 cd m −2 , respectively, both of which are the lowest among all solution-processed LEDs and vacuum-deposited OLEDs. The device exhibits a T 95 operational lifetime of ≈2500 h at an initial brightness of 1000 cd m −2 , meeting the commercialization requirements for display applications. The results highlight the potential of solution-processed oxide HILs for achieving efficient-driven and long-lifetime QLEDs.

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To understand the electronic processes in quantum-dot light-emitting diodes (QLEDs), a comparative study was performed by time-resolved transient electroluminescence (TREL). We fabricated red, green, and blue (R-, G-, and B-) QLEDs with poly­(9,9-dioctylfluorene-co-N-(4-sec-butylphenyl)­diphenylamine) as the hole-transporting layer with conventional structures. The external quantum efficiency (EQE) and current efficiency were 19.2% and 22.7 cd A–1 for R-QLEDs, 21.1% and 93.3 cd A–1 for G-QLEDs, and 10.6% and 10.4 cd A–1 for B-QLEDs, respectively. The TREL results for B-QLEDs were remarkably different from those for R- and G-QLEDs because of the insufficient electron injection crossing the type II heterojunction between the emission layer and the electron-transporting layer. We further applied poly­(N-vinylcarbazole) as the hole-transporting layer and obtained much better performance for B-QLEDs, with EQE and current efficiency of 15.9% and 15.4 cd A–1, respectively. Concomitant with the increase in EQE are an increase in the turn-on voltage from 2.3 to 3.7 V and a transient electroluminescence spike after voltage turn-off.

Films based on colloidal ZnO nanocrystals are promising electron-transporting layers (ETLs) widely used in solution-processed optoelectronic devices. However, adsorption of gas molecules in the ambient atmosphere, such as O2 and H2O, onto the surfaces of ZnO nanocrystals may cause significant changes in the electrical conductance of the ZnO-nanocrystal films. Such ambient-dependent conductivities of the ZnO-nanocrystal films impose a challenge to overcome in practical applications. Here, we develop a thiol–ligand exchange strategy to passivate the surface-adsorption sites of the colloidal ZnO nanocrystals, resulting in thiolate-capped ZnO nanocrystals. Thin films consisting of thiolate-capped ZnO nanocrystals show unprecedented atmosphere-independent conductivities. We further demonstrate that the thiol–ligand exchange strategy is readily extended to ZnO nanocrystals with different sizes. In consequence, the atmosphere-stable conductivities of films based on the thiolate-capped ZnO nanocrystals can be tuned over 5 orders of magnitudes.

Thin films of ZnO nanocrystals are actively pursued as electron-transporting layers (ETLs) in quantum-dot light-emitting diodes (QLEDs). However, the developments of ZnO-based ETLs are highly engineering oriented and the design of ZnO-based ETLs remains empirical. Here, we identified a previously overlooked efficiency-loss channel associated with the ZnO-based ETLs: i.e., interfacial exciton quenching induced by surface-bound ethanol. Accordingly, we developed a general surface-treatment procedure to replace the redox-active surface-bound ethanol with electrochemically inert alkali carboxylates. Characterization results show that the surface treatment procedure does not change other key properties of the ETLs, such as the conductance and work function. Our single-variable experimental design unambiguously demonstrates that improving the electrochemical stabilities of the ZnO ETLs leads to QLEDs with a higher efficiency and longer operational lifetime. Our work provides a crucial guideline to design ZnO-based ETLs for optoelectronic devices.