The authors thank the National Science Foundation (NSF-STC program under DMR-0120967). A. K.-Y. J. thanks the Boeing-Johnson Foundation for its support for support of this research. They also thank Prof. David Ginger and Dr. Liam Pingree, U. of Washington, for Kelvin probe measurements.
Self-assembled Electroactive Phosphonic Acids on ITO: Maximizing Hole-Injection in Polymer Light-Emitting Diodes†
Article first published online: 10 NOV 2008
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Advanced Functional Materials
Volume 18, Issue 24, pages 3964–3971, December 22, 2008
How to Cite
Bardecker, J. A., Ma, H., Kim, T., Huang, F., Liu, M. S., Cheng, Y.-J., Ting, G. and Jen, A. K.-Y. (2008), Self-assembled Electroactive Phosphonic Acids on ITO: Maximizing Hole-Injection in Polymer Light-Emitting Diodes. Adv. Funct. Mater., 18: 3964–3971. doi: 10.1002/adfm.200800033
- Issue published online: 15 DEC 2008
- Article first published online: 10 NOV 2008
- Manuscript Revised: 9 JUL 2008
- Manuscript Received: 8 JAN 2008
In order to fulfill the promise of organic electronic devices, performance-limiting factors, such as the energetic discontinuity of the material interfaces, must be overcome. Here, improved performance of polymer light-emitting diodes (PLEDs) is demonstrated using self-assembled monolayers (SAMs) of triarylamine-based hole-transporting molecules with phosphonic acid-binding groups to modify the surface of the indium tin oxide (ITO) anode. The modified ITO surfaces are used in multilayer PLEDs, in which a green-emitting polymer, poly[2,7-(9,9-dihexylfluorene)-co-4,7-(2,1,3-benzothiadiazole)] (PFBT5), is sandwiched between a thermally crosslinked hole-transporting layer (HTL) and an electron-transporting layer (ETL). All tetraphenyl-diamine (TPD)-based SAMs show significantly improved hole-injection between ITO and the HTL compared to oxygen plasma-treated ITO and simple aromatic SAMs on ITO. The device performance is consistent with the hole-transporting properties of triarylamine groups (measured by electrochemical measurements) and improved surface energy matching with the HTL. The turn-on voltage of the devices using SAM-modified anodes can be lowered by up to 3 V compared to bare ITO, yielding up to 18-fold increases in current density and up to 17-fold increases in brightness at 10 V. Variations in hole-injection and turn-on voltage between the different TPD-based molecules are attributed to the position of alkyl-spacers within the molecules.