Hole injection enhancement of a single-walled carbon nanotube anode using an organic charge-generation layer

초록

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. (C) 2014 Elsevier Ltd. All rights reserved.

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