Using the phase separation of a binary immiscible polymer blend of poly(amic acid) and polyimide in combination with SiO2 nanoparticles, we have designed random nanopore and nanopillar patterns by a simple spin-coating technique. The spontaneously formed pattern structure was investigated by an atomic force microscope technique, which enhanced the external quantum efficiency of conventional green OLEDs up to a highest factor of 1.42. This enhancement is mainly attributed to the reduction of the total internal reflection at the interface of glass and air by the polymer hybrid layer on the backside of OLED devices. This work provides a new and practical approach to the development of efficient external outcoupling structures for OLEDs.