Charge carrier dynamics and degradation phenomena in organic light-emitting diodes doped by a thermally activated delayed fluorescence emitter

We report on the charge carrier dynamics and their degradation phenomena in an organic light-emitting diode (OLED) doped by a thermally activated delayed fluorescence emitter; (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN). Displacement current measurement (DCM) data revealed the presence of negative interface charge originating from the spontaneous orientation polarization of the electron transport layer (ETL), 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene. The negative interface charge acts as a hole reservoir and thus confines the recombination zone near the emission layer (EML); 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP):4CzIPN (5 wt%)/ETL interface. By keeping the recombination zone far from the hole transport layer, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD), a better electroluminescence efficiency is expected because the Dexter energy transfer from the triplet state of 4CzIPN to that of α-NPD is suppressed. Moreover, we found an excellent linear relation between the accumulated hole amount and luminance losses owing to device aging. The results are consistent with hole traps originating at the degradation products of CBP as the main factor of device degradation. We suggest device architectures based on the charge carrier dynamics for better efficiency and lifetime.