Research On Inverted Organic Light-emitting Devices Based On Inorganic Metal Oxide Transport Layer

In recent years,organic light emitting devices?OLEDs?have been widely applied in flexible displays and solid-state lighting due to various advantages including self-emission,fast response time and low driving voltage.However,conventional OLEDs suffer a poor environmental stability because of the high moisture/oxygen sensitivity and low glass transition temperature?T_g?of organic materials,which hinder the further development of OLEDs in practical applications.In this paper,inorganic metal oxides are used as carrier transport materials to improve the stability of the devices.As an electron transport material,n-type inorganic metal oxide would be directly coated on indium tin oxide?ITO?glass substrate to avoid organic small molecules being destroyed in the spin coasting process.For ITO as the cathode in this device,the inverted device structures are different from the conventional OLEDs.Moreover,thin film transistors?TFTs?based on intrinsic amorphous silicon and transparent amorphous oxide semiconductors operate in the n-channel mode,which indicates that inverted organic light-emitting devices?iOLEDs?should be advantageous over conventional OLEDs to integrate with the n-channel TFTs.Therefore,the research of iOLED based on inorganic metal oxide transport layer has practical significance.The main contents of this dissertation are as follows:Firstly,inorganic metal oxide ZnO was used to prepare inverted organic light-emitting device,which is called I-iOLED.Considering ZnO has a higher T_g,the Joule heat produced by devices will not cause the crystallization of ZnO even if after a long time operation,which will not cause damage to the upper organic layer.So that the stability of the device has been initially improved.The fluorescence microscopy images were shown to prove the stability of the I-iOLED.Besides,we also discussed the influence of polyethyleneimine?PEI?interlayer and the hole blocking layer?HBL?on the device photoelectric properties.Secondly,based on the structure of I-iOLED,p-type inorganic metal oxide MoO₃was used as hole transport layer to prepare II-iOLED.MoO₃ can effectively block water and oxygen in the air from entering the device,further improve the stability of the device.The influence of the thickness of MoO₃ on the performance of our device was discussed to determine the optimal thickness of MoO₃ layer.Furthermore,compared to organic molecules,the advantage of MoO₃ hole transport material was proved.Finally,the inverted devices were driven by DC power and AC power resources,respectively.When the iOLEDs were continuously operated under DC voltage,the luminance of the devices will decay rapidly within 30 minutes.While under AC voltage,they operated for 24 hours or more in the atmosphere without any encapsulation and still has high luminance.To explain the phenomena,we tested the C-V curves and transient electroluminescence characteristics of the devices and found these results caused by the trapped charges in ZnO layer.