Comprehensive Model of the Degradation of Organic Light-Emitting Diodes and Application for Efficient, Stable Blue Phosphorescent Devices with Reduced Influence of Polarons
Bomi Sim, Jong Soo Kim, Hyejin Bae, Sungho Nam, Eunsuk Kwon, Ji Whan Kim, Hwa-Young Cho, Sunghan Kim, Jang-Joo Kim
Abstract
We present a comprehensive model to analyze quantitatively and predict the process of degradation of organic light-emitting diodes (OLEDs) considering all possible degradation mechanisms, i.e., polarons, excitons, exciton-polaron interactions, exciton-exciton interactions, and impurity effects. The loss of efficiency during degradation is presented as a function of quencher density. The density and generation mechanisms of quenchers are extracted using a voltage-rise model. The comprehensive model is applied to stable blue phosphorescent OLEDs, and the results show that the model describes the voltage rise and external-quantum-efficiency loss very well, and that the quenchers in the emitting layer (EML) are generated mainly by polaron-induced degradation of dopants. Quencher formation is confirmed by mass spectrometry. The polaron density per dopant molecule in the EML is reduced by controlling the emitter doping ratio, resulting in the highest reported lifetime ${\mathrm{LT}}_{50}$ of 431 h for an initial brightness of $500\phantom{\rule{0.1em}{0ex}}\mathrm{cd}/{\mathrm{m}}^{2}$ with a commission internationale de l'\'eclairage y coordinate less than 0.25, and a high external quantum efficiency greater than 18%.