Frequency-Dependent Electrical Transport Properties of 4,4',4

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 0.246 eV with no phase change in the temperature range 220–320 K. These results indicate that the IS method is applicable for organic semiconductors having a wide band gap.