Space-charge limited conduction with traps in poly(phenylene vinylene) light emitting diodes

Current-voltage, impedance, and transient conductance measurements have been carried out on indium-tin-oxide/poly(phenylene vinylene)/Al light emitting diodes. In these devices injection and transport is expected to be dominated by positive carriers. Fowler-Nordheim tunneling theory cannot account for the temperature dependence, the thickness dependence, or the current magnitude of the current-voltage characteristics. Space-charge limited current theory with an exponential distribution of traps is however in extremely good agreement with all of the recorded current-voltage results in the higher applied bias regime (approximately 0.7≤V/d≤1.6 × 106 V cm-1). This gives a trap density Ht of 5(±2) × 1017 cm-3 and the product of μNHOMO of between 1014 and 5 × 1012 cm-1 V-1 s-1. Assuming NHOMO is 1020 cm-3 gives an effective positive carrier mobility between 10-6 and 5 × 10-8 cm2 V-1 s-1. The characteristic energy Et of the exponential trap distribution is 0.15 eV at higher temperatures (190≤T≤290 K), but this decreases as the devices are cooled, indicating that the distribution is in fact a much steeper function of energy closer to the highest occupied molecular orbital (HOMO) levels. The current-voltage characteristics in the lower applied bias regime (approximately V/d≤0.7×106 V cm-1) can be fitted to pure space-charge limited current flow with a temperature and field dependent mobility of Arrhnenius form with a mobility at 290 K close to the above values. If NHOMO lies between 1021 and 1019 cm-3, then the trap filled limit bias gives a mobility independent value of Ht of 3(±1) × 1017 cm-3. Capacitance-voltage measurements show that at zero bias the devices are fully depleted, and that the acceptor dopant density NA must be less than about 1016 cm-3. The impedance results show that the devices can be modeled on a single, frequency independent, parallel resistor-capacitor circuit with a small series resistor The variation of the resistor and capacitor in the parallel circuit with applied bias and temperature are consistent with the space-charge limited current theory with the same exponential trap distribution used to model the current-voltage characteristics. Initial results for transient conductance measurements are reported. The transients have decay times greater than 300 s and exhibit a power-law dependence with time. This is shown to be exactly the behavior expected for the decay of an exponential trap distribution. Measurements at higher temperatures (290≥T ≥ 150 K) give an Et of 0.15 eV, in excellent agreement with that found from the current-voltage measurements. This value of Et is exactly that found by similar analysis of the current-voltage characteristics in negative carrier dominated dialkoxy poly(phenylene vinylene) and Mq3 devices. It is proposed that this bulk transport dominated behavior is purely a consequence of hopping conduction through an approximately Gaussian density of states in which the deep sites act as traps. © 1997 American Institute of Physics.