Rearrangement of the molecular orientation of Alq3 in organic light-emitting diodes under constant current aging investigated using sum frequency generation spectroscopy

Sum frequency generation (SFG) spectroscopy has been used to study the operational degradation of 4, 4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD)/tris(8-hydroxylquinoline) aluminum (Alq3) organic light-emitting diodes (OLEDs). A reduction in the SFG peak strengths of Alq3 was found in aged OLEDs, indicating that the permanent dipole orientation in the Alq3 layer was randomized as a result of long-term continuous operation. In contrast, the SFG signals derived from the α-NPD in aged samples were enhanced as compared to those of pristine OLEDs, and it can be attributed to an increase of the internal electric field near the interface by the long-term operation.