Fig. 6: Mechanism of the operation-induced efficiency elevation.
a Flat-band energy-level diagrams of the pristine QLEDs. The current flows (arrows) and space-charge accumulations (symbols) of electrons (red) and holes (blue) in the QLED under steady-state operation are illustrated. The high space-charge concentration of electrons in the QD layer results in considerable consumptions of Je via the efficiency-loss channels of non-radiative Auger recombination (Je-Auger) and electron leakage (Je-leakage). Consequently, the pristine devices possess low exciton-generation efficiency (ηX), i.e., the conversion ratio of electron current into exciton-recombination current (Je-X), b Schematic diagram summarizing the EL process in the QDs before (red shaded) and after the efficiency elevation (blue shaded). As the electron-injection capability degrades, the EL process is gradually dominated by the electrical generation and recombination of the highly emissive single exciton state (QDX, top right). c Schematics illustrating the distributions of charge carriers and current flows in the QLED after the efficiency-elevation process. More electrons are accumulated at the degradation site (dashed square). The contributions of Je-Auger and Je-leakage channels are suppressed due to the decreased electron concentration in the QD layer (compared with a), which leads to a higher conversion ratio of the electrons into exciton recombination.
