Study On Light Extraction Of Organic Light-Emitting Diodes

Organic light-emitting diode(OLED)is considered as a new star for illumination and display because of its high color contrast,wide gamut,low power consumption and so on.However,due to the low external quantum efficiency relative to inorganic light-emitting diode(LED),OLEDs have not been widely used in the field of illumination.In my thesis,different light extraction structures were fabricated to enhance the light output of OLED by reducing different optical loss modes:substrate mode,waveguide mode and Surface Plasmon Polaritons(SPP)mode.Theoretical simulation based on finite difference time domain(FDTD)was used to verify and analyze the experimental results.The representative findings are highlighted as follows:1.Ideal microlens array(IMLA)based on polystyrene(PS)microspheres with small diameters is introduced to the surface of indium tin-oxide(ITO)glass substrates to enhance the out-coupling efficiency of OLEDs.IMLA was fabricated by a replica molding process using annealed closely packed PS spheres as the template.IMLA is used to extract the substrate mode which is limited in glass substrate.Experiment results show that the best performance of OLED devices is achieved when the diameter of IMLA is 6?m.We also demonstrate that the IMLA with a diameter of 6 ?m can be applied for the extraction of green,red,blue and white light.Furthermore,the use of IMLA is not detrimental to the electrical and optical performance of the OLED devices.2.Nano-modified ITO glass substrates are fabricated for internal light-extraction structure to enhance the out-coupling efficiency of OLED devices.This nano-modified anode with low average haze(less than 4%)was prepared by wet chemical etching.Polystyrene hemispherical spheres,which were distributed randomly on the ITO anode surface,were used as the etching mold.An IMLA with a diameter of 6 ?m was used as an external light extraction structure.The combination of nanostructured ITO and an IMLA can greatly enhance the current efficiency,power efficiency,and external quantum efficiency of OLEDs.The use of nanostructured ITO anodes does not have any detrimental effect on the optical performance of OLED device.3.Two different periodic light extraction structures,PC1 and PC2.were prepared.PC1 is prepared by reactive ion etching(RIE)of PS microspheres,while PC2 is formed by spin-coating of SiO2 colloid on PS microspheres and then subject to annealing and washing.PC1 and PC2 are used as a glass substrate for the fabrication of FIrpic based OLEDs to extract waveguide modes confined in the devices.We investigated experimentally and theoretically how these two internal light out-copuling strutures improve the device efficiency and influence the electroluminescent(EL)spectra of OLEDs.4.PS microspheres with diameters of 350 nm and 300 nm were used as a template to prepare periodic nanostructured ITO substrates,which could extract waveguide modes and SPP modes confined within the OLED devices effectively.We investigate how these nanostructured ITO substrates determine the device performance theoretically and experimentally.To enhance the light extraction of white OLEDs,PS microspheres with diameters of 300 and 350 nm were mixed,and corrugated ITO substrates with different periodic nanostructures were obtained by adjusting the mass ratio of PS microspheres.With these periodic ITO structures,the current efficiency and power efficiency of white OLEDs are significantly improved.In addition,the EL spectra of white OLEDs can be controllably fine tuned by adjusting the mass ratio of PS spheres.When the viewing angle is changed from 0 to 80 degrees,the CIE color coordinates are located in the warm-white region.Therefore,this method provides a new idea for the preparation of efficient warm white emission for illumination,and it is not only simple but also low cost.It is suitable for large area fabrication as well,which has great potential in the fabrication of large-area OLED panels.