Engineering Efficiency Roll-Off in Organic Light-Emitting Devices

Previous studies have identified triplet-triplet annihilation and triplet-polaron quenching as the exciton density-dependent mechanisms which give rise to the efficiency roll-off observed in phosphorescent organic light-emitting devices (OLEDs). In this work, these quenching processes are independently probed, and the impact of the exciton recombination zone width on the severity of quenching in various OLED architectures is examined directly. It is found that in devices employing a graded-emissive layer (G-EML) architecture the efficiency roll-off is due to both triplet-triplet annihilation and triplet-polaron quenching, while in devices which employ a conventional double-emissive layer (D-EML) architecture, the roll-off is dominated by triplet-triplet annihilation. Overall, the efficiency roll-off in G-EML devices is found to be much less severe than in the D-EML device. This result is well accounted for by the larger exciton recombination zone measured in G-EML devices, which serves to reduce exciton density-driven loss pathways at high excitation levels. Indeed, a predictive model of the device efficiency based on the quantitatively measured quenching parameters shows the role a large exciton recombination zone plays in mitigating the roll-off.