Fabrication And Characterization Of All-inorganic Quantum Dot Light Emitting Diodes

In recent years, light emitting devices (LEDs) using colloidal quantum dots (QDs)as emitting layers have received numerous interests because of their highly saturatedemission color as well as the capability of tuning the emission color by engineering itssize. Although quantum dot LEDs based on organic semiconductor materials canobtain high injection efficiency, the short lifetime caused by the degradation of theorganic layers under moisture and oxygen and the instability of the metal contactslimits its advanced application in QLEDs. Inorganic charge-transport layers can avoidthese issues due to their high stability under oxygenic and humid environment andallowing higher operating currents. Recently, metal oxides as charge injectioninterfaces have been invested extensively, because metal oxides possess a largenumber of unique properties, such as high transparency, excellent electricalconductivity, tunable morphology and the possibility of deposition on large areas withlow costs. Among them, NiO and ZnO are widely-used p-type and n-type transitionmetal oxides, respectively. The transition metal oxide films are usually prepared byhigh cost methods, such as chemical vapor deposition, electrochemical deposition andsputtering.Herein, we report the preparation of an all-inorganic quantum dot light-emittingdiode using ZnO and NiO thin films as charge transport layers prepared through afacile sol-gel spin-coating method, which is a simple, low-cost process and it is easyto obtain products of high purity. The structure of the device is ITO/NiO/QDs/ZnO/Al.The devices show current-voltage rectification behavior and very pure QD lightemission. The turn-on voltage can be lower to be2V and the leakage current isinappreciable. The electroluminescence spectrum of the device is in good agreementwith the photoluminescence spectra of the CdSe/ZnS quantum dots. The electronsrecombine with holes predominantly in the QD layer and emit high pure.Aiming to improve the efficiency of the devices, we introduced molybdenumtrioxide (MoO3) to make the energy level match better. We explore the effects ofdifferent thickness of MoO3film on the strength of electroluminescent intensity andthe current density. MoO3layers with different thicknesses are introduced between theITO and NiO layer via a thermal evaporation process. The MoO3films exert noapparent effect on the transmission properties of the device, thanks to its excellenttransmission property over95%in the visible light region. Our results show that an ultrathin (~5nm) MoO3intermediate layer can significantly enhance theelectroluminescence intensity of the QD-LED. All the peaks of EL spectra have noobvious shift even when a high voltage was applied， which shows that therecombination of holes and electrons are limited in the region of the QD layer, Also,no apparent quantum confined Stark effect (QCSE) is observed. It is suggested thatthe electroluminescence enhancement originates from the effectively facilitatedinjection of holes into QDs through MoO3intermediate layer.