Sub-Band Gap Operation Mechanism of InP Quantum Dot-Based Light-Emitting Diodes
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Colloidal quantum dot-based light-emitting diodes (QLEDs) are being to the fore as next-generation technology for future information displays. Though the research for QLEDs have made much progress from perspective of efficiency and stability, the origin of sub-band gap (sub-Eg) operation is still undisclosed. Here, we investigate the sub-Eg phenomenon in QLEDs. In the course of assessment using hole transport materials with different energy levels, the diode and light turn-on voltages display a clear dependence on the difference between electron and hole carrier transport levels, not hole injection barrier by QDs’ shell material. Fermi level pinning between ZnO electron transport layer and QDs revealed by ultraviolet photoelectron spectroscopy implies that an involvement of surface states of QDs to the alignment of energy level landscape. We attribute the sub-Eg turn on to the vacuum shift originating from surface states of QDs which allows to minimizing the hole injection barrier.