Investigation of External Quantum Efficiency Roll-Off in OLEDs Using the Mean-Field Steady-State Kinetic Model

Organic light-emitting diodes (OLEDs) are promising candidates for solid-state lighting but suffer from decreased efficiency at the high current density required for lighting applications. This decreased efficiency is called roll-off, and its cause remains the subject of much debate. Here, a combination of targeted experiments and novel modeling is used to quantify the importance of different microscopic processes in OLED roll-off. This study utilizes a new formalismmean-field steady-state kineticsthat can rapidly simulate macroscopic device characteristics based on molecular-scale rates. Specifically, the model can disentangle exciton–charge annihilation (ECA) from exciton–exciton annihilation (EEA)two processes that dominate roll-off. Next, two OLEDs are fabricated that primarily differ in the radiative lifetime of the emitters. This change is found to have a critical effect on the roll-off mechanism; the faster-emitting device is dominated by ECA and the slower device by EEA. One set of molecular rate constants is able to reproduce both the external quantum efficiency and photoluminescence quantum yield roll-off in each device, suggesting that the resulting mechanistic insights are robust. These results suggest that tuning the emitting layer can change the roll-off behavior in an OLED, and the speed and simplicity of the kinetic model opens the door for rapid design of more efficient devices.