Impedance spectroscopy study on transport properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine

The transport properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenhyl-4,4'-diamine (NPB) have been investigated by using impedance spectroscopy (IS) and J-V measurements based on a single-layer structure of ITO/NPB/Al. The J-V characteristic of the device obeys Ohmic law below 0.6 V, and is in the space-charge limited current (SCLC) regime above 0.6 V. The density of the thermally generated charge carrier is determined to be about 3.1x1016 cm-3. Impedance spectroscopy measurements showed that the device can be simulated by an equivalent parallel RC network with a contact resistance R S in series, in which the bulk capacitance C of NPB remains unchanged while the resistance R decreases along with bias voltage. The variation of the whole conductance and capacitance of the device with frequency and bias can be understood by the carrier response and the transit time dependence on the bias in the NPB film