Investigation On The Degradation Mechanism Of Inverted Quantum Dot Light Emitting Diodes

Colloidal quantum dot?QD?is a kind of semiconductor material with a size of several nanometers to several tens of nanometers,which have attracted wide attention due to their excellent optical characteristics such as superior color purity,adjustable light emission wavelength and outstanding optical stability.Many photoelectric devices based on QD have emerged,for example,solar cells,detectors,and light emitting diodes.Especially the QD-based light emitting diode?QLED?,which have opened a new world for the display industry.It is considered to be the most promising next-generation display technology.Since the first demonstration by V.L.Colvin et al in1994,great progress has been made in the QLED.To date,the device operation lifetime of T₅₀0 has been over 2,200,000 h at 100 cd/m² for the red QLED.And the green QLED has achieved a peak external quantum efficiency?EQE?of 22.9%and the T₅₀ is similar to that of red ones.In fact,some electronic products based on QD as the emissive units,such as TVs,monitors and tablets,have already appeared in the consumer market.In order to meet people's increasing requirements for display and solid-state lighting,novel inverted structure QLED devices has been demonstrated to further promoted the commercialization of quantum dots in LED with the development of technology,due to the simple solution process,and good compatibility with driving circuits of n-channel field effect transistors.However,to date,most of the investigations on the physics mechanism are focused on the QLED with a conventional structure.There are rare reports on the degradation mechanism,especially for the QLED based on inverted structure.This is still a big obstacle to the development of QLED technologies in the applications.Therefore,in this paper,the degradation mechanism is studied with inverted red QLED as the platform due to their mature device design and performance.In this paper,we introduce the degradation mechanism of invented QLED based on CdSe that its basic structure is ITO/ZnO/QDs/4,4',4''-Tris?carbazol-9-yl?triphenylamine?TCTA?/MoO₃/Al.The degradation mechanism of QLED is studied by means of impedance spectroscopy,transient and steady-state photoluminescence?PL?spectroscopy and electrical characteristics.We use scanning electron microscope to characterize the cross-sectional image of QLED,and further verify its mechanism.Based on the device structure,adding modified layers of polyethylenimine ethoxylated?PEIE?and poly?methylmethacrylate??PMMA?further improves device stability.The main contents of this paper are as follows:1.To described in detail the current research progress and existing problems of QLED.Considering the practical applications of the QLED,a low constant current density is used to drive the inverted device based on CdSe QD.By transient and steady-state PL spectroscopy,we find the exciton emission efficiency in the QD is not affected and changed during the aging processes,but the PL intensity gradually decreases.Which means that during the charge injection process in device,QD emissive centers are decreased but no defects are generated for the QD.Therefore,we speculate that some of the QD are lying in the permanent dark states or a static quenching occurs to the QD.2.To further study the root cause of device degradation.By electrical characteristics,there are unbalanced carrier charges in devices,which electron concentration is greater than hole concentration.By impedance spectroscopy,during the degradation process,the internal resistance of the device induced by the degraded hole transport layer increases.The hole transport layer degradation is caused by the Joule heating effect caused by excessive electron injection.Then,the cross-sectional scanning electron microscope images of the device provide a visually evidence to prove our conclusion.3.To propose method to improve device stability.Adding modified layers PEIE or/and PMMA between QD layer and charge transport layers can suppress device degradation.We add a PEIE layer between electron transport and the QD layers to block the injection of excess electrons,and add a PMMA layer between the hole transport layer and QD layer to suppress the damage of the hole transport layer induced by electrons,improving the stability of the device.