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

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.