Capacitance-voltage characteristics of a 4,4′-bis[(N-carbazole)styryl]biphenyl based organic light-emitting diode: Implications for characteristic times and their distribution

The characteristic times and their distributions of a fluorescent 4,4′-bis[(N-carbazole)styryl]biphenyl-based organic light-emitting diode (OLED) were studied using impedance spectroscopy. The four defined characteristic times (transit time, recombination time, trapping time, and ac characteristic time) and their dc bias dependences were obtained from the frequency dependence of capacitance. The trapped carrier density distribution in the time domain was revealed from the voltage dependence of the OLED. The relative value of each characteristic time is suggested to play an important role in determining the profile of the frequency- and voltage-dependent capacitance of OLEDs.