Thermal assisted up-conversion electroluminescence in quantum dot light emitting diodes

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%.

Fig. 1. Temperature-dependent EL characteristics.

The luminance-voltage (L–V) characteristics of a red-, b green-, and c blue-QLEDs under different temperatures (RT= room temperature): the V_T (turn-on voltage) is remarkably reduced as the temperature is increased. d EL spectra of red-, green- and blue-QLEDs at different temperatures (the orange/blue arrows represent an increase/decrease in temperature). e The V_T (solid circle), photon voltage V_ph ($V_{ph}=h\nu /e$, open circle) and up-conversion efficiency (star) of red-, green- and blue-QLEDs at different temperatures: the up-conversion efficiency is gradually increased as the temperature is increased, indicating that the up-conversion EL is triggered by a thermal-assisted process. f EQE-V characteristics of red-QLEDs at different temperatures: at sub-bandgap bias of 1.7~2.0 V, the devices exhibit an EQE of 12.5~15%, which is higher than the upper limit (~10%) of the Auger-assisted process, thus disapproving the Auger-assisted mechanism. g Photographs of up-conversion EL at 1.6 V under different temperatures: the luminance is gradually increased as the temperature is elevated (also demonstrated in Supplementary Movie 1).

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 (${\mathrm{\varphi }}_{\mathrm{TFB}}$ and ${\mathrm{\varphi }}_{\mathrm{ZnMgO}}$), 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 ${\varphi }_h$. 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 ${\varphi }_h=△E_V-{PE}_{Coulomb}$. 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.

Fig. 3. Thermal-assisted hole-injection.

a The probability of finding the holes at different energy: when the temperature is elevated, the probability that the holes can be injected into QDs is remarkably increased. b Up-conversion EL at 1.6 V under different temperatures: the calculated results agree fairly well with the measured one when the temperature is lower than 80 °C. At high temperature, the discrepancy is caused by the thermal excitation that increases the hole concentration of TFB. c The current density (J)-V characteristics of hole-only devices and capacitance-V characteristics of the red QLEDs at RT and 100 °C: at elevated temperature, the hole current is substantially enhanced, which thus reduces the peak capacitance of the devices. d The transient EL of red-QLEDs: at elevated temperature, the devices are turned-on more rapidly due to enhanced hole injection.

Fig. 4. The up-conversion EL in different structured QLEDs.

The energy levels diagrams of a regular red-QLEDs with PVK HTL and d inverted red-QLEDs with CBP HTL. (black/white circle represents electron/hole). The L–V characteristics of b PVK based- and e CBP based-QLEDs under different temperatures: the $V_T$ is remarkably reduced as the temperature is increased. Turn-on voltage $V_T$ (solid triangle), photon voltage $V_{ph}$ (open triangle) and the up-conversion efficiencies (star) of c PVK based- and f CBP based-QLEDs at different temperatures: the up-conversion efficiency is gradually increased as the temperature is increased. When the temperature is higher than 100 °C, the up-conversion efficiency of both QLEDs can exceed 100%.