Enhancing light extraction in organic light-emitting devices via hemispherical microlens arrays fabricated by soft lithography

We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 +/- 5 deg) and array fill factor of (85 +/- 3)% were obtained with 100-mu m PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 x 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 +/- 7)% in the light-extraction efficiency was achieved. (C) 2011 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3528267]