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 ${\text{0.7⩽V/d⩽1.6×10}}^6\hspace{0.17em}{\mathrm{V\hspace{0.17em}cm}}^{\mathrm{-1}}$ ). This gives a trap density $H_t$ of ${\text{5(±2)×10}}^{17}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}$ and the product of ${\mathrm{\mu N}}_{\mathrm{HOMO}}$ of between ${10}^{14}$ and ${\text{5×10}}^{12}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-1}}{\mathrm{\hspace{0.17em}V}}^{\mathrm{-1}}{\mathrm{\hspace{0.17em}s}}^{\mathrm{-1}}.$ Assuming $N_{\mathrm{HOMO}}$ is ${10}^{20}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}$ gives an effective positive carrier mobility between ${10}^{\text{−6}}$ and ${\text{5×10}}^{\text{−8}}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{2}}{\mathrm{\hspace{0.17em}V}}^{\mathrm{-1}}{\mathrm{\hspace{0.17em}s}}^{\mathrm{-1}}.$ The characteristic energy $E_t$ of the exponential trap distribution is 0.15 eV at higher temperatures $\text{(190⩽T⩽290\hspace{0.17em}}\mathrm{K}\mathrm{),}$ 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 ${\text{V/d⩽0.7×10}}^6\hspace{0.17em}{\mathrm{V\hspace{0.17em}cm}}^{\mathrm{-1}}$ ) 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 $N_{\mathrm{HOMO}}$ lies between ${10}^{21}$ and ${10}^{19}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}},$ then the trap filled limit bias gives a mobility independent value of $H_t$ of ${\text{3(±1)×10}}^{17}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}.$ Capacitance–voltage measurements show that at zero bias the devices are fully depleted, and that the acceptor dopant density $N_A$ must be less than about ${10}^{16}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}.$ 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 $\text{(290⩾T⩾150\hspace{0.17em}}\mathrm{K})$ give an $E_t$ of 0.15 eV, in excellent agreement with that found from the current–voltage measurements. This value of $E_t$ is exactly that found by similar analysis of the current–voltage characteristics in negative carrier dominated dialkoxy poly(phenylene vinylene) and ${\mathrm{Mq}}_3$ 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.