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Finite-Difference Time-Domain Simulation of Fractal-Like Microlens Arrays for High Outcoupling Efficiency of Organic Light-Emitting Diodes.

The outcoupling efficiencies (OCEs) of organic light-emitting diodes (OLEDs) were studied for a fractal-like two-dimensional structure consisting of three layers of semicircular microlens on a glass substrate using a finite-difference time-domain (FDTD) method. The OCE with only one semicircular microlens layer was 29.5%, 1.75 times larger than that of the basic OLED. Additional layers with smaller diameters on the first layer did not improve the OCE. The OCE remained constant or slightly decreased with the increase of the number of layers. Two possible origins of this result were suggested; first, the possibility that the escaped light enters the nearby microlens becomes higher with the introduction of an additional protruded layer; second, the Mie scattering effect becomes important with the decrease of the diameter of the semicircular microlens from 20 μm to 0.8 μm. An additional FDTD simulation was performed for the OLED with only one microlens array as a function of the diameter. The OCE decreased approximately monotonously with the decrease of the diameter from 20 μm to 0.2 μm. In particular, the OCE became lower than that of the basic OLED when the diameter decreased from 0.5 μm to 0.2 μm. This is consistent with the observation that smaller fractal-like structures on the large microlens array did not further enhance the OCE.