High-performance Quantum Dot Light Emitting Diodes By Controlling Hole-injection

Quantum dots?QD?are paid much attention in quantum dot light emitting diodes?QLED?due to their unique properties.Since QLED was found by Alivisatos et al.in1994,people has been doing lots of work in developing the performance of QLED.There are two methods to improve the performance of devices.On the one hand,from the perspective of QDs,we could prepare core-shell QDs to limit emission of QDs in the core or choose suitable ligands to modify the surface of QDs.On the other hand,from the view of device architecture,we could select the appropriate on the basis of energy level arrangement and carrier mobility.And another way is modifying surface of the charge transport layer to ameliorate carrier injection of QD layer.Recently,it was reported that the luminance of red,green and blue achieved 35600 cd/m²,2614000cd/m² and 62600 cd/m² respectively.This experimental result made a breakthrough in improving device performance.However,in these years,although the performance of QLED has been increased rapidly,high-performance of QLED rely on the emission of cd-containing QDs.Cd is a heavy metal element that not only pollutes environment,but also damages human health.Therefore,when fabricated the devices,it is necessary to choose friendly non-toxic quantum dots such as?ZnS,CuInS₂,InP,AgInS₂ etc.?as emitting materials.But,according to the literature reported previously,the device performance of non-toxic QDs lags far behind that of cadmium-based QDs.In our report,we have improved the brightness and efficiency of Zn-Ag-In-S quantum dot electroluminescent diodes by designing the device structure elaborately.It provides an idea for the future research on the performance of non-toxic and friendly electroluminescent devices.The main work is as follows:We fabricated QLED with stepwise structure two-layer of hole transport layer?HTL?.We achieved the maximum luminance?over 2000 cd/m²?and high current efficiency?2.1 cd/A?of Zn-Ag-In-S quantum dot devices by measuring the electrical properties of the device.Besides,we measured the impedance spectrum to determine the accumulation of holes in the devices.The effect of different charge transport layerson the lifetime of quantum dots was tested by TCSPC.By analyzing the data,we found that reducing the accumulation of interface between holes and the quantum dot emitting layer could help improving performance of devices.In addition,another work is to insert a TPBi interlayer which modifies the surface of hole transport layer at the interface between the QDs layer and the HTL.Compared to the standard device without TPBi,the power efficiency of the device is improved by85%,and the current efficiency is improved by 57%,when the thickness of TPBi is10 nm.By measuring the electroluminescence?EL?spectrum of the device,we conclude that excitons are formed on the quantum dot luminescent layer.This work has shown that modifing the interface between QD and hole transport layer will greatly improve the performance of the device.