Investigation on the escaped and trapped emission in organic light-emitting devices

The escaped and trapped emission of small molecule based bottom emission organic light-emitting diodes was investigated by using an integrating sphere and a fiber spectrometer and a glass hemisphere. It was found that the maximum external coupling ratio r _ext, namely the ratio of escaped emission to the emission of escaped and trapped in the substrate, is 56%. We also extended the "half-space" dipole model by taking dipole radiation pattern into account, which agreed well with our experiments. Our experimental and theoretical results will benefit the optimization of device structures for the higher out-coupling efficiency.