Elucidation of carrier injection and recombination characteristics with impedance and capacitance in organic light-emitting diodes and the frequency effects

Impedance ( Z ), phase ( φ ) and capacitance ( C ) versus bias voltage ( V ) characteristics are studied to clarify carrier injection and recombination characteristics in organic light-emitting diodes (OLEDs). The Z – V transition starts at a characteristic voltage ( V c ), which is strongly frequency dependent, i.e. V c shifts to a high voltage with increasing measuring frequency. The electron–hole recombination starts at a voltage above V c revealed by the φ – V and C – V transitions, which correspond to a phase approaching 0, a sharp rise in current density and a decrease in capacitance. Hole injection starts at a low V c and corresponds to charge carrier accumulation and a slight rise in capacitance. Cole–Cole impedance plots illustrate that the interfacial resistance corresponds to the impedance at ultrahigh frequencies and shows bias independence, while the impedance at low frequencies represents the sum of interfacial resistance and organic stacks, and exhibits considerable bias dependence.