Solution-processed hexaazatriphenylene hexacarbonitrile as a universal hole-injection layer for organic light-emitting diodes

The use of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) thin layers, particularly the solution-processed type, as an efficient hole-injection layer (HIL) for organic optoelectronic devices is demonstrated herein. Among the solvents commonly used for solution processing, 2-propanone was found to selectively dissolve HAT-CN, allowing the fabrication of a rigid film. The alignment of the electronic energy levels of the solution-processed HAT-CN and thermally polymerized 2,7-disubstituted fluorene-based triaryldiamine (VB-FNPD) species was evaluated using ultraviolet photoelectron spectroscopy. The results revealed that the lowest unoccupied molecular orbital of HAT-CN and the highest occupied molecular orbital of VB-FNPD were very close to the Fermi level, which facilitated charge transfer at the interface and improved hole injection. The utilization of HAT-CN as HIL resulted in a dramatic enhancement of the performance of solution-processed red, green, and blue organic light-emitting diodes. The external quantum efficiency, current efficiency, and power efficiency of the HAT-CN-based devices were higher than or almost similar to those of optimized poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-based devices. Because of the efficient carrier-injection capability and the capacity to prevent interfacial mixing and erosion during fabrication, solution-processed HAT-CN is promising as a novel alternative to conventional PEDOT:PSS HILs. (C) 2013 Elsevier B.V. All rights reserved.