Emission enhancement of microlens on OLED with different layer structures

Abstract

Low out-coupling efficiency is one of the most critical problems in organic light-emitting device (OLED) application. Only 20~30% of the emitting light from OLED can propagate into air [1]. Therefore, several methods have been utilized to extract more light from device. Here, we use the microlens array attached on device to couple out wave-guiding mode in the glass substrate. We found that, the luminous enhancement behavior has great dependence on OLED structure. When light emitted in the layered structure of OLED, the wide angle interference and multi-beam interference occurred, and far-field emission profile change simultaneously. For different emission profile, microlens array film shows a different enhancement behavior. For a conventional OLED device, the most critical interference will occur at the electron transport layer (ETL). We fabricated a series of OLEDs with different ETL thicknesses to investigate the influence to the optical properties, such as spectrum, CIE coordinate change, and emission profile at different view angles. By controlling the emission dipole position, we investigate the relation between the emission profile and the efficiency enhancement by microlens array attachment. When increasing the ETL thicknesses from 30nm to 150nm, the weaker micro cavity effect results in broader spectrum and more light extracted. In these devices, the luminous enhancement varies from 25.1% to 51.3%.