Out-coupling Efficiency Improvement In Organic Light-emitting Diodes

OLEDs（Organic light-emitting devices） have attracted wide interest due to theirapplicationsfor illumination application. Because of the good properties such as highefficiency lighting, wide viewing angle, wide choice of materials, low driving voltage,being large-area having flexible, transparent substrates and all-solid self-luminous, andactually the planar light source, the OLEDs are considered as one of the candidates ofthe next generation planar light source.The internal quantum efficiency of OLEDscan reach values near to100%.However, due to the difference among the refractive indices of the substrate, anode,organic thin films, the fraction of light emitted from emission layer is suppressed by thewave guide mode in the glass substrate and organic layers. How to improve the lightout-coupling efficiency of OLEDs is important and popularin the field of OLEDs.Out-coupling efficiency has been studied for many years, and many both theoretical andexperimental conclusions are found to improve the out-coupling efficiency.In this paper, we demonstrate wavelength-independent methods to increase thelight extraction by modifying the glass substrate. A novel mechanism is developed tosimulate the optical luminous field for OLEDs based on inverted pyramid substratestructure. And the microlens was developed to simulate the optical luminous field forOLEDs. Monte Carlo method and FDTD（Finite-difference time-domain） mode wereused to investigate enhancement of the external extraction efficiency by changing theslide angle of the substrate and microlens arraysfabrication.The modified glass substratewas simulated by using the TracePro software package to find thebest performance.Byusing the inverted pyramid substrate and the microlenssubstrate, the lightingout-coupling efficiency of OLEDs can reach45%and38.7%.The microlens substratewas simulated by using the OptiFDTD software package to analyze the Poyntingvector.It is enhancement by a factor of0.24.According to the simulation results, the modified glass substrate is produced. Weuse the substrate cutting process and metal evaporation process to produce invertedpyramid substrate. The lithography process was used to produce microlens arrays. The optimization production process was developed in this paper. According to a series ofexperiment results, the considerable enhancement in the extraction efficiency of theOLED is expected, and over38%increasing in the light out-coupling of device wasachieved in experiment.