Frequency-dependent electrical transport properties of 4,4′,4″-tri(N-carbazolyl)-triphenylamine(TCTA)are analyzed by impedance spectroscopy(IS)as functions of bias and temperature.The Cole-Cole plot shows a single semicircle which indicates that the equivalent circuit can be designed as a single parallel resistor R_(p)and capacitor C_(p)network with a series resistance R_(s).The bulk capacitance C_(p)remains unchanged while the resistance R_(p)decreases along with bias voltage.Conduction mechanism matches well with the space-charge-limited current(SCLC)model with exponential trap charge distributions.The temperature-dependent impedance studies reveal the activation energy of 0.246eV with no phase change in the temperature range 220-320K.These results indicate that the IS method is applicable for organic semiconductors having a wide band gap.
