Spectral tuning of light emitting diodes with phenyl-thiophenes

We report electroluminescence data on single and double layer devices based on well-defined phenyl-thiophenes (PTs) (PPPPT, PPPTP, PPTPP), wherein 2,5-thiophene (T) is systematically translated through a p-quinquephenyl (PPPPP) framework. Single layer light emitting diodes (LEDs) were prepared by vacuum deposition of the PTs and an aluminum top electrode onto an ITO substrate; double layer devices were prepared using Alq(3) as the electron-transport and emitter layer. The single layer LEDs exhibited light emission that could be tuned across the spectral range from blue to green depending upon the location of the thiophene moiety. All of the double layer devices emit in the 510-nm range which suggests that this emission originates from Alq(3) and that the PTs function as hole-transport materials. For both types of devices we present spectroscopic data, the wavelength dependence of the electroluminescence, and the current-voltage characteristics. Cyclic voltammetry was used to determine the redox behavior and the pertinent electronic energy levels of the PTs. The experimental UV lambda(max) values and HOMO values of the PTs were contrasted with ab initio calculations and found to be in good agreement. (C) 1999 Elsevier Science S.A. All rights reserved.