Abnormal temperature dependent behaviors of intersystem crossing and triplet-triplet annihilation in organic planar heterojunction devices

Abstract

Anomalous temperature dependent magneto-electroluminescence was observed at low and high magnetic field strength from organic planar heterojunction devices incorporated common phosphorescent host materials of N,N′-dicarbazolyl-3,5-benzene (mCP) or 4,4′-N,N′-dicarbazole-biphenyl (CBP) as an emissive layer. We found that intersystem crossing became stronger with decreasing temperature and that triplet-triplet annihilation (TTA) occurred at room temperature but ceased at low temperature. Analyses of the electroluminescence spectra of these devices and their temperature dependences indicated that the population of exciplex states increased at low temperature, which caused the abnormal behavior of intersystem crossing. Additionally, long lifetime of the excitons within mCP or CBP layer may allow TTA to occur at room temperature, while the reduced population of excitons at low temperature may account for the disappearance of TTA even though the excitons had increased lifetime.