Study On The Origin Of Subthreshold Turn-On In Quantum-Dot Light-Emitting Diodes

In recent years,Quantum-Dot Light-Emitting Diodes(QLEDs)have been favored by academia and industry because of their characteristics of cold light emission,high color purity,adjustable emitting-wavelength,and natural adaptation to printed displays.QLED is expected to become the next generation of quantum dots display technology.Although the luminous efficiency of QLEDs is currently comparable to that of organic LEDs,the study of device physics is insufficient,limiting the pace of industrialization of this technology.For example,the issue of subthreshold(sub-band gap)turn-on of electroluminescence,typically observed in solution-made QLEDs,has been generally misinterpreted.The focus of the controversy is whether or not energy up-conversion occurs when the QLED emits photons at a sub-threshold biasHere,we study the sub-threshold turn-on problem of QLED with various characterization represented by electro-absorption energy spectrum.The results show that the basic principle of carrier injection and transport is the source of this phenomenon,not the energy up-conversion mechanism.First,by measuring and fitting the QLED open-circuit voltage as a function of temperature in photovoltaic mode,we find that the bias voltage at subthreshold turn-on is not enough to excite charge transfer(CT)excitons at the interface.Therefore,the de-excitation of CT states is not the source of sub-bandgap turn-on.Next,we analyzed the relationship between the internal electric field and the bias voltage of the emitting layer(EML)and hole-transport layer(HTL)in QLED by monitoring the Stark effect in the electro-absorption phenomenon.The results show that the turn-on voltage of electroluminescence is consistent with the flat band voltage of EML,but it is significantly lower than that of HTL.When the bias voltage just exceeds the flat band voltage of the EML,the QLED is in a sub-threshold bias state.Although the hole injection at this time is still limited by the lower hole concentration in the HTL in the depleted state,the forward internal electric field in the EML has ensured that the holes provided by the HTL to the EML increase significantly with the bias,which in turn causes carrier recombination increases significantly.This work not only provides a reasonable explanation for the so-called sub-threshold turn-on phenomenon,but also suggests that QLED has a charge injection mechanism that helps to achieve charge balance.This provides an important reference for further understanding of QLED's other device mechanism issues.