Electromodulation of fluorescence in hole-transporting materials (TPD, TAPC) for organic light-emitting diodes Original Research Article

Electric-field-modulated fluorescence (EMF) was measured in films of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and 1,1′-bis(di-4-tolylaminophenyl) cyclohexane (TAPC), the diamine compounds commonly used as hole-transporting materials in organic light-emitting diodes. External electric fields of 2×106 V/cm reduce the integral emission intensity from about 0.3% down to 10% dependent on the excitation photon energy and spectral emission range. The analysis of the experimental data provides evidence that the EMF in TPD is due to the field imposed changes of the dissociation rate of localized molecular excited states. The observed difference in the EMF for the short-wavelength (monomolecular) and long-wavelength (excimer) emission bands of TAPC can be ascribed to the different field response of these two types of emitting species. All these results are consistent with a kinetic scheme that includes formation of molecular excited states, their localization and relaxation into charge pairs within extended trapping domains, and formation of excimer states at a field-dependent rate mediated by the field-assisted dissociation of the charge pairs. The 3D-Onsager theory of dissociation satisfactorily explains the electric field and excitation wavelength dependence of EMF in TPD and that for the excimer emission of TAPC. The quadratic Stark effect on fluorescence quantum yield must have been invoked to understand the EMF characteristics of monomolecular emission of TAPC.