Efficient, Nonintrusive Outcoupling in Organic Light Emitting Devices Using Embedded Microlens Arrays

We demonstrate efficient light extraction from the active region of bottom-emitting organic light emitting devices (OLEDs) using a high refractive index, nondiffractive hemispherical microlens array located between the transparent anode and embedded in the low refractive index glass substrate (n = 1.5). The subelectrode microlens array (SEMLA) results in a maximum external quantum efficiency of 70 +/- 4% for green phosphorescent OLEDs (PHOLEDs). Furthermore, the wavelength- and viewing-angle-independent light extraction structure results in white PHOLED external efficiencies of 50 +/- 3%. The SEMLA light extraction structure is nonintrusive; that is, it lies completely outside of the OLED structure. Since this design has no effect on the image resolution, it is compatible with applications for both displays and white light illumination, with no dependence on molecular transition dipole orientation and the active organic layers (and hence diode electrical characteristics) used in the PHOLED. Finally, due to the micrometer-scale feature size of the SEMLA, it is achieved using conventional photolithography prior to the OLED array deposition.