Optical And Electrical Characteristics Of Electrodes In Organic Light-emitting Diodes

Organic light-emitting diodes(OLEDs)consist of organic layers sandwiched between anode and cathode.The multi-layered OLED structures form a Fabry-Perot microcavity,so the optical properties of each film have influence on the luminescence characteristics of the OLEDs.The optical characteristics of the electrodes differ from organic layers,which will greatly affect the luminescent properties of the OLEDs.The scope of this thesis focuses on studying how the optical and electrical properties of the electrodes influence the luminescence characteristics of the OLEDs.The studied electrodes include opaque electrodes,transparent electrodes,and connecting electrodes.The corresponding device structures are bottom-emission,top-emission and tandem devices,respectively.It is found that:(1)by engineering the optical characteristics of a phase-changing cathode,a black OLED device with an ultra-low reflectance in the full range of visible light(?5%,lower than the glass substrate)can be fabricated.This greatly improves the device's optical contrast ratio.Theoretical modeling studies show that,as long as the optical properties of the phase changing layer are consistent with the theoretical results,the high contrast OLEDs can achieve 0%reflection.This research finding provides a theoretical guidance for manufacturing flexible high-contrast OLEDs.(2)The mixing ratio effect of Mg:Ag alloy cathode on the performance of OLEDs has been systematically studied.In the bottom-emission OLEDs,the cathodes chosen from the AgMg3-Mg phase region(Mg content is greater than 50 wt.%)are found to yield good electron injection properties and to produce the highest out-coupling efficiency;in the top-emission OLEDs,the Mg:Ag/Ag double layer transparent cathodes are found to improve the electron injection performance while weakening the micro-cavity effect.This research works provide a guide for the selection of transparent and opaque cathodes for OLED manufacuring.(3)Application of Mg:Ag alloys in the connecting electrodes of tandem OLEDs is studied.In weak microcavity bottom-emission tandem OLEDs,Ag rich alloy is found to yield better electrical and optical properties.In a strong microcavity top-emission tandem OLEDs,Mg rich alloy is found to yield better optical characteristics.The results show that the optical design of the connecting electrode has to consider both optical transmittance and overall device microcavity effect.All optical calculations in this thesis are based on transfer matrix theory,and programmed by the Matlab softwere.The optical constants of the organic films and the metal films are measured by an ellipsometer.The simulation results are highly consistent with the experimental results,and thus both experimental findings and theoretical method reported in this thesis provide an important guide for future OLED development.