Delayed recombination of detrapped space-charge carriers in poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]-based light-emitting diode -: art. no. 114505

We report the observation of a spectroscopically resolved delayed electrofluorescence (DEF) in the time domain of nanosecond to microsecond (depending on temperature, in the range of 30-290 K, as well as bias) from light-emitting diodes based on poly[2-methoxy-5-(2(')-ethyl-hexyloxy)-1,4-phenylene vinylene]. The decay kinetics of this DEF are always found to be biexponential in nature. The fast decaying component with a lifetime of similar to 40 ns is attributed to the back transfer of nonemissive (or very weakly emissive) interchain excited singlets (partially charge-transfer states) to emissive intrachain excited singlets (this component is called DEFCT). The relatively slower decaying component with a lifetime of similar to 0.2-6.2 mu s (depending on temperature as well as bias) is attributed to the recombination of detrapped space-charge carriers at the polymer-electrode interfaces (this component is called DEFSC). The intensity of DEFSC increases as the temperature is increased from 30 to 290 K, although it is weak at low temperature (< 100 K). The temperature dependence of the recombination rate of the detrapped space-charge carriers yields two activation energies of 2.2 and 40 meV below and above similar to 130 K, respectively. The existence of these two activation energies is explained on the assumption of electrons being in shallow traps and holes in deep traps. Also, our data indicate that the space-charge carriers generally act as major quenching sites (especially at 290 K) for triplet excitons in polymer light-emitting diodes. (C) 2005 American Institute of Physics.