Light Outcoupling in Organic Light-Emitting Devices
Since the first reports of efficient and practical organic light-emitting devices (OLEDs) in 1987 by Tang and VanSlyke , OLEDs have been the subjects of intensive studies owing to their advantageous flat panel displays and lighting applications, such as high efficiency, wide viewing angle, fast response, and potentially low cost. In addition, their low processing temperatures and thus versatility in substrates render them suitable for some novel applications, such as flexible displays and lighting. Over the past two decades, the OLED technology has made rapid progresses and various types of OLED displays have been demonstrated and commercialized. With the continuous improvements of OLED efficiencies and white-emitting OLEDs, the application of OLEDs in solid-state lighting is also getting realistic [1–3].
Currently, the optical outcoupling technique for OLEDs has become an important issue for achieving high efficiencies [3–8] compatible with other solid-state lighting techniques, like fluorescent tubes and LEDs. Indeed, by adopting the phosphorescence mechanisms, the internal quantum efficiencies of OLEDs can reach nearly 100% [9, 10]. However, due to the significant refractive index mismatches at air/substrate and substrate/ITO interfaces, OLED internal emission usually suffers total internal reflection and hence most of internal radiation is trapped and guided inside ...