Charge-trapping-associated threshold-voltage shift during electrical aging in OLEDs with electron-transport layers having a giant surface potential

Organic light-emitting diodes (OLEDs) employing electron-transport layers (ETLs) with substantial spontaneous orientation polarization (SOP) can exhibit a giant surface potential (GSP), which produces polarization-induced interfacial charge (IC) and influences charge injection. In this study, we investigate the threshold-voltage (Vth) increase in aged OLEDs employing an 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) ETL with a GSP. Electrical aging was accompanied by an increase in Vth delayed hole injection, and a decrease in the estimated polarization-induced interfacial charge obtained from capacitance–voltage (C–V) measurements. To examine the role of localized charge trapping, a hole-trap device was fabricated by introducing TAPC into a 5-nm-thick region of the OXD-7 ETL adjacent to the emitting layer. The hole-trap device reproduced electrical characteristics similar to those observed in the aged devices, including delayed hole injection and increased Vth. C–V and displacement-current measurements, together with bias-dependent photoluminescence analysis, support an association between localized charge trapping and an apparent weakening of the polarization-related electrostatic contribution to hole injection. These findings suggest a degradation pathway that is particularly relevant to OLEDs employing ETLs with a GSP and highlight the importance of controlling localized charge trapping in such layers to maintain stable charge-injection characteristics during operation. Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give... [797 chars]