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.