We demonstrate significant hole injection enhancement of single-walled carbon nanotube (SWCNT) anodes in flexible organic light-emitting devices (OLEDs) by the insertion of a strong electron-accepting organic charge-generation layer (CGL), hexaazatriphenylene hexacarbonitrile (HAT-CN). To clarify the origin of hole injection improvement, we investigated interfacial electronic structures using in situ ultraviolet photoelectron spectroscopy, inverse photoelectron spectroscopy, theoretical calculations, and electrical measurements. The HAT-CN layer significantly increased the work function of SWCNT anodes and acted as an efficient CGL due to its deep-lying lowest unoccupied molecular orbital level, which arises from the strong electron-accepting characteristics of the carbonitrile endgroups. We compared the energy level alignment at the interface of the N,N′-bis(1-naphthyl)-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) hole transport layer/HAT-CN/SWCNTs with that of NPB/SWCNTs, and found that the highest occupied molecular orbital level of the NPB changed from 1.20 to 0.40 eV with insertion of the HAT-CN layer. As a result, flexible OLEDs with the HAT-CN layer showed an order of magnitude larger current density and luminance than those without the HAT-CN layer.
Bibliographical noteFunding Information:
This work was supported by a research project of the National Research Foundation of Korea (Grant Nos. 2013R1A1A1004778 and 2012-0006893 ) and Samsung Display Company . We sincerely appreciate Dr. J.T. Han and J.S. Woo for providing SWCNT substrates and LG chemistry for providing the HAT-CN material.
All Science Journal Classification (ASJC) codes
- Materials Science(all)