Correlation between the Size of Microlenses and the Outcoupling Efficiency of Organic Light Emitting Diodes Studied by Finite-Difference Time-Domain Simulation

Outcoupling efficiency (OCE) of organic light emitting diodes (OLEDs) with four different microlens arrays was studied by two-dimensional finite-difference time-domain simulation to find out correlation between the size of microlenses and OCE. The OCE decreased as the size of microlenses was reduced irrespective of the shape of microlenses. Especially, the OCE became similar to that of basic OLED without any microlenses when the size of microlenses was lower than the wavelength of visible light. This result was attributed to the backscattering caused by microlenses, which was enhanced significantly at lower sizes and confirmed by additional Mie-scattering simulation. Insertion of high-index layer with embedded micropores between the glass substrate and the transparent electrode was effective in extracting the light trapped in organic layers. The combination of optimized microlenses and high-index layer resulted in the increase of OCE by 196%.