Phosphorescent organic light-emitting diodes (PhOLEDs) have great potentials for applications in displays and lightings. The use of phosphorescent emitter molecules in OLEDs is essential to realize internal electron-photon conversion efficiencies of 100%. However, the efficiencies of phosphorescent OLEDs decline at high current densities ? an effect known as efficiency roll-off that has been known to be mainly dominated by bimolecular interactions and charge carrier imbalance.
In this thesis, the origin of efficiency roll-off is analyzed through quantitative measuring the contribution of monomolecular and bimolecular quenching processes and charge carrier imbalance with newly modified modeling of EQE including microcavity effects and excitons profiles. By combining electrical and optical excitation in time-resolved spectroscopic experiments, the contributions of quenching and charge carrier imbalance to efficiency roll-off are analyzed. To analyze precisely, the exciton profiles and the width of recombination zone in EML are identified by the sensing layer method based on energy transfer from the emitter to the sensitizer.
In this work, two different PhOLEDs, one with a single host, 4,4’-Bis(carbazol-9-yl)biphenyl(CBP), and the other with an exciplex-forming co-host(TCTA:B3PYMPM), are studied. Both CBP single host system and the exciplex forming co-host system have low efficiency roll-off, however the tendency of efficiency roll-off is significantly different. In exciplex forming co-host system, the high charge carrier balance and less quenching rate with wide recombination zone is the origin of low efficiency roll-off. In CBP single host system, the reduced charge balance and increased quenching rate in narrow recombination zone is mainly contribute to efficiency roll-off.