Research Towards Composite Transparent Electrode Employed In Organic Light-emitting Diodes

In the modern society with advanced information technologies,the acquisition,storage,processing,transmission and even expression of information are important factors that determine the speed of information exchange,that is,the speed of social development.However,unlike the further development of other events of information,the display of information is limited by the senses of human beings,which has been maintained in terms of sound and light.Among all kinds of information obtained by human senses,visual information accounts for80%of its total.This also makes display the most important technical means to express information.Nowadays,market competition for display technology is particularly fierce,and display technologies such as liquid crystal,organic electroluminescence,quantum dot display,and laser projection are all unanimous.In development of flat display,the size of display products grows.however,it is limited by space showing an upper limit.Thus,to further increase the density and functionality of information display has become a critical requirement to current light-emitting displays.Emergence of augmented reality?AR?technology provides a direction for the development of display.similar to a breakthrough for the display limit,AR provides a chance to simultaneously receive information from different media,such as see-through displays and backgrounds.The invention of AR provides a means to effectively increase the density of information displays,but needs the realization of a novel display device,that is,transparent display.Transparent display can emit light without blocking the optical transmission of information.Small-molecule organic materials in organic light-emitting devices?OLEDs?have high transparency and relatively high luminous efficiency.Moreover,they are also ready for mass productions,so OLED is the ideal choice for transparent display.However,the main obstruct that currently limits transparent OLED is the selection and preparation of transparent cathodes.Small-molecule materials are fragile under high-temperature and high-energy particle bombardment,while easily soluble in most solvents,which limits the process in fabricating electrodes on organic layers.In cases of materials such as indium tin oxide,graphene,and nanowires,it is difficult to avoid damage to the organic layer during the fabricating process,even causing device failures.Metal thin films do not damage the device while the fabrications,but causing low transmittance of the OLED,which is failed to meet the requirements of transparent display.Based on the above problems,this thesis takes composite transparent conductive electrodes as research objects and explores the possibility of using them as the top cathode of transparent display devices.Apart from the previous discussions on electrode focusing on conductivity and optical transmittance.By analyzing the carrier distribution and energy level change during the metal-semiconductor contact process in composite electrode,the electrode design strategy to maintain high transmittance and metal-like properties of the composite electrode have been developed.the above mechanism has been verified in different metal materials.Finally,using the electrode designed with the above mechanism as the cathode,a transparent OLED with high transmittance and high efficiency was prepared.Specific content is laid below.1.Based on the theory of energy band bending,differences of energy band bending in metal-semiconductor contact under different conditions were discussed.By employing ZnO and Ag,it was found that with 7.5 nm thick ZnO in contact with Ag.The entire ZnO layer was in a degenerate state,which caused changes in the carrier concentration,mobility,and work function.Based on the above phenomena,a composite transparent electrode ZnO/Ag/ZnO?ZAgZ?with self-encapsulation ability and surface degenerate property was designed.The ZnO coating on Ag made the electrode more stable than the Ag thin film.Moreover,the presence of the ZnO layer improved the film-forming ability of the Ag layer,thereby improving the conductivity and transparency of the ZAgZ electrode.ZAgZ electrode with 10 nm thick Ag as the metal layer had a transmittance of 80%at 550 nm,and the sheet resistance is14.3?/?.By using Hall effect and UV photoelectron spectroscopy characterizations,ZnO surface of ZAgZ electrode was confirmed as degenerate state.The degenerate ZnO layer made the entire ZAgZ electrode have a uniform metallicity,and the overall carrier concentration can reach 10²² cm^-2.Moreover,the mobility also showed a linear relationship with the overall thickness of the ZAgZ layer.Work function on the surface of the ZAgZ was also affected by this contact degeneracy and decreased to 4.0 eV.Using ZAgZ electrode,bottom-emitting OLEDs and perovskite light-emitting diodes exhibited excellent photoelectric performance.Compared with ITO-based devices,the efficiency of the two ZAgZ devices increased by 80%and 30%,respectively.2.Based on the above works,ZnO/Ca/Al₂O₃?ZCA?transparent electrode was designed.ZnO layer was applied to protect alkaline earth metal Ca from reacting with organics in OLED and Al₂O₃ layer was deposited to protect Ca from reacting with water and oxygen in the air.Based on the extended Maxwell-Garnett equivalent medium and transfer matrix simulation,the surface microstructures formed on the Ca layer in ZCA were simulated,and the high transmittance of the ZCA electrode was predicted.The ZCA electrode was further experimentally verified with a transmittance of 81.0%when the square resistance is 40.6?/?.By further adjusting the structure,average transmittance can reach 95%,with sheet resistance remaining 161.4?/?.In addition to the electrical and optical performance,the surface work function of the ZCA electrode was maintained at 3.31 eV,which was efficient to the injection of electrons.All above results were also supported by first-principles.3.Using the ZCA electrode designed as the transparent cathode and ITO as the transparent anode,green,yellow and blue transparent OLEDs were prepared.Transparent OLEDs show excellent luminous efficiency and transparency.The maximum current efficiency of the three-color transparent OLEDs were 79.2 cd/A,62.4 cd/A,and 33.5 cd/A,respectively,with external quantum efficiency of 22.7%,19.3%,and17.9%.While maintaining high efficiency,the transparent OLEDs still held extremely high transparency.Its average transmittance in the visible light range was 86%,which meets the requirements of the most transparent display device.Furthermore,the transparent OLEDs exhibited a balanced light emission in both directions of top emission and bottom emission with an asymmetric electrode structure,which proved the excellent light extracting ability of ZCA.Multi-functional specialized display devices have been explored base on the transparent OLEDs.In summary,based on the theoretical analysis and first-principles calculations,a method to achieve surface degeneration and work function adjustment in oxide by contacting with metal is proposed.By applying the above method to two kinds of transparent electrodes with different structures,range of applications and electrical characteristics of this theory were fully characterized.Moreover,ZCA electrode prepared based on the above theory enables Ca to stably exist as the conducive part of the transparent conductive electrode for the first time,enabling high performance transparent OLEDs.This work function adjustment method is widely applicable to a variety of metals and semiconductors and provides a new method and related theoretical basis for the development of not only transparent electrode,but various semiconductor devices.