Abstract
Up-conversion electroluminescence, in which the energy of a emitted photon is higher than that of the excitation electron, is observed in quantum-dot light-emitting diodes. Here, we study its mechanism by investigating the effect of thermal energy on the charge injection dynamic. Based on the results of temperature-dependent electroluminescence and theoretical analysis, we reveal that at sub-bandgap voltage, holes can be successfully injected into quantum-dots via thermal-assisted thermionic-emission mechanism, thereby enabling the sub-bandgap turn-on and up-conversion electroluminescence of the devices. Further theoretical deduction and experimental results confirm that thermal-assisted hole-injection is the universal mechanism responsible for the up-conversion electroluminescence. This work uncovers the charge injection process and unlocks the sub-bandgap turn-on mechanism, which paves the road for the development of up-conversion devices with power conversion efficiency over 100%.
Highlights
Up-conversion electroluminescence, in which the energy of a emitted photon is higher than that of the excitation electron, is observed in quantum-dot light-emitting diodes
For the red QLEDs, when the temperature is increased, the VT is significantly reduced, while the Vph is mildly decreased, and as a result, the up-conversion efficiency is rapidly increased from 96% to 156%. These results indicate that: (1) the reduction of VT at elevated temperatures should not be attributed to the change of Vph, but instead, should be ascribed to the increase of the thermal energy; (2) the sub-bandgap turnon and up-conversion EL are enabled by the thermal-assisted charge injection process
Based on the experimental results of temperature-dependent EL, we reveal that thermal energy plays an essential role in the sub-bandgap charge injection processes
Summary
Up-conversion electroluminescence, in which the energy of a emitted photon is higher than that of the excitation electron, is observed in quantum-dot light-emitting diodes. Based on the results of temperature-dependent electroluminescence and theoretical analysis, we reveal that at sub-bandgap voltage, holes can be successfully injected into quantum-dots via thermal-assisted thermionic-emission mechanism, thereby enabling the sub-bandgap turn-on and up-conversion electroluminescence of the devices. The radiative recombination of excitons leads to the generation of photons with energy of hν, which is roughly equal to or slightly smaller than the bandgap energy (Eg) of the QDs. Because the photons are converted from the electrons, the energy of the injected electrons at a applied voltage of V should be equal to the energy of photons, i.e., eV 1⁄4 hν % Eg, which implies that the minimum applied voltage or the turn-on voltage VT to induce detectable electroluminescence (EL) should satisfy VT ≥ hν=e. For typical QLEDs driven by a sub-bandgap voltage, electron injection into QDs is relatively efficient, but hole injection into
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