Degradation And Performance Optimization Of Quantum-dot Light-emitting Diodes

In recent years,quantum dot light-emitting diodes(QLEDs)have shown great promise in display and lighting applications due to the unique properties of quantum dots(QDs),such as wide excitation spectra,narrow emission spectra,size-tunable emission wavelength,inherent photo-and thermal-stability and high electron mobility The performance of QLEDs has been improved substantially in the past several decades and the lifetime attracts attention gradually.In this paper,we research the effects of external air and carrier dynamics on lifetime of QLEDs and optimize the device performance based on the resultFirstly,the degradation mechanisms of QLEDs were studied in the external environment and internal carrier characteristics.In the external environment,the devices prepared under the same conditions were placed in the air for a certain period(20/40/60min)before the aging test.The result show that the initial luminance and subsequent increase magnitude of QLEDs was increased as the placement time is extended.In terms of internal carrier dynamics,the effects of carrier injection intensity and bound charge or electron spontaneous transfer on the lifetime of the device were investigated.We tested the lifetime of QLEDs at different current densities and applied reverse voltages beforehand and the results indicate that excessive carrier injection and the carriers trapped by the defect state or charges transferred from QDs to ZnO spontaneously have a large negative effect on QLEDs'lifetimeThen we optimized the performance of QLEDs based on the results of the aging mechanism.The effect of air on the brightness of the device is presumed to be due to the effect of moisture on the passivation of quantum dot defect states.We fabricated a series of QLEDs with the same structure and the quantum dot layer was subjected to moisture treatment during the preparation of the QLED device.The results show that the performance of the QLEDs can be improved by moisture treatment.Especially,the lifetime of the treated QLEDs was enhanced by 70%when the processing time was lmin.The effect of moisture on the luminance of QLEDs is proved and the moisture treatment in QDs layer under the appropriate time is effective for the enhancement of the lifetime of the QLEDs.In order to reduce the influence of carrier injection and charge spontaneous transfer on device stability,the alkali halide,lithium fluoride(LiF),is inserted between the green QD EML and ZnO ETL.The result show that The LiF interlayer acts as a gate for electron injection,which hampers the electron injection at low voltages and enhances the electron injection at high voltages,leading to the much balanced carrier distribution inside EML.Moreover,the LiF interlayers also reduce the exciton quenching induced by ZnO at EML/ZnO interface.Consequently,an optimal current efficiency of 17.5 cd/A is achieved in the QLED with a 6 nm-thick LiF interlayer,which is obviously higher compared with 14.3 cd/A of the standard device.Finally,the role of the LiF interface layer was demonstrated by using TFB as the hole transport layer and the lifetime of the QLEDs was also greatly improved.