Impedance of space-charge-limited currents in organic light-emitting diodes with double injection and strong recombination

The impedance model for a one-carrier space-charge-limited (SCL) current has been applied to explain some experimental features of double carrier organic light-emitting diodes. We report the analytical model of impedance of bipolar drift transport in SCL regime in the limit of infinite recombination. In this limit the ac impedance function is identical to that of a single carrier device, with a transit time modified by the sum of mobilities for electrons and holes, mu(n)+mu(p). The static capacitance C(omega -> 0) is a factor of 3/4 lower than the geometric capacitance, as observed for single carrier devices, but it is shifted to higher frequencies. It follows that impedance measurements in the dual-carrier organic diodes with strong recombination provide the combination of mu(n)+mu(p). For the mobilities of the different carriers to be determined separately, additional information is required.