A review on the light extraction techniques in organic electroluminescent devices

Organic electroluminescent devices are becoming increasingly important because of their potential applications for large area flat-panel displays and general lighting. The internal quantum efficiency of these devices have been achieved near 100% using electro-phosphorescent materials with proper management of singlet and triplet excitons, however, the external quantum efficiency of conventional devices remains near 20% because of losses due to wave-guiding effect. Recently, there has been great progress to enhance the light out-coupling efficiency of organic electroluminescent devices by means of various internal and external device modification techniques. In this review we report recent advances in light out-coupling techniques, such as, substrate modification methods, use of scattering medium, micro-lens arrays, micro-cavity effect, photonic crystals and nano-cavity, nano-particles, nano-structures and surface plasmon-enhanced techniques that have been implemented to enhance the external extraction efficiency of organic electro-luminescent devices.