Crossover from space-charge-limited to recombination-limited transport in polymer light-emitting diodes

By performing admittance spectroscopy as a function of frequency on polymer light-emitting diodes, inductive and capacitive charge-relaxation processes with different characteristic time scales are separated. The inductive contributions arise from the finite transit time of injected carriers, while the capacitive contributions stem from dielectric redistribution of charge density in the device. The crossover from inductive charge relaxation at low bias to capacitive charge relaxation at high bias marks the transition from space-charge-limited to recombination-limited current flow. This unexpected result shows that, while the individual carrier mobilities are strongly enhanced by the applied field, the recombination mobility remains unaffected.