Comparative study of organic light emitting diode efficiency enhancement by the use of optical films

An optical film with some microstructures can enhance external quantum efficiency of a bottom-emission type organic light emitting diode (OLED) by out-coupling the light trapped inside its substrate. We measure the luminous intensity of two OLEDs coupled to several optical films. The films we try are four types of prism films and three kinds of diffuser films. A CCD-based measurement system rapidly evaluates the angular dependency of luminous intensity for each film and OLED combination. Integration of these distributions over the azimuth and polar angles gives luminous flux. We define an enhancement factor as the ratio of the luminous flux of an OLED-film combination to that of an OLED alone. In all the cases, the largest factor is 1.60 and the second largest is 1.54. The OLED with a higher reflectance always has a larger enhancement factor. A highly reflective OLED recycles the light reflected or scattered by the microstructure of the film more efficiently. Ray tracing analysis, assuming a Lambertian source, more or less reproduces the enhancement factors and their dependency on the OLED reflectance. It also indicates that an OLED with 100% reflectance coupled to a film with some spherical surface would give the highest efficiency of 1.68.