Fig. 2: Charge injection processes in QLEDs.

a Energy levels of the functional layers of the typical red-QLEDs. b At thermal equilibrium, the surfaces of TFB and ZnMgO are depleted so that the Fermi levels are aligned through the system. Due to the presence of build-in surface potentials (\({\upphi }_{{{{{{\rm{TFB}}}}}}}\) and \({\upphi }_{{{{{{\rm{ZnMgO}}}}}}}\)), charge injection is impossible. c At \({V}_{{FB\_QD}}\), flat-band is achieved in QD layer and thus electron injection into QDs is possible, while hole injection is still unfavorable due to the large injection barrier \({\phi }_{h}\). However, at high temperature (HT), with sufficient thermal energy provided, holes could be injected into QDs via the thermal-assisted thermionic-emission mechanism. d At \({V}_{{FB\_TFB}}\), flat-band is achieved in TFB layer, and the hole injection barrier is reduced to a minimum value of \({\phi }_{h}=\triangle {E}_{V}-{{PE}}_{{Coulomb}}\). At RT, with the assistance of thermal energy, the holes can overcome a barrier of 0.4 eV and injected into QDs. e At \(h\upsilon /e\), all depletion regions are vanished and the electric field in all layers turn positive, and thus the holes can be accelerated towards the QDs. Hole injection is enabled by both thermal- and field-assisted thermionic-emission mechanisms. f At \(V \, > \, h\nu /e\), due to the presence of strong positive electric field in TFB, hole injection is mainly dominated by the field-assisted thermionic-emission mechanism.