Temperature dependent impedance spectroscopy on polyaniline based devices

Polyaniline (PANI) sandwich structures with Al and/or Au electrodes have been investigated using temperature dependent I-V measurements and impedance spectroscopy in the 50 mHz -1 MHz frequency range. The I-V measurements on the Au/PANI/Au structure have shown a practically Ohmic behavior, while the Au/PANI/Al structure has presented a clear non-linear behavior in the voltage range studied. From the complex impedance data analysis we have observed that while in the Au/PANI/Au structure the conductive process is best described with a single distribution of relaxation times, associated only to the bulk region, in the Au/PANI/Al structure the active layer is divided in two regions with distinct resistances: one associated to the bulk region, and the other, much higher, associated to a thin region close to the PANI/Al interface. We have also analyzed the real capacitance data of the Au/PANI/Al structure using two models based on an abrupt cut-off frequency concept. The application of these models has enabled the determination of some important parameters, such as: Debye length (ap20 nm), energetic position (ap320 meV) and associated density of states (ap2times10
¹⁸
eV
^-1
.cm
^-3
) of the bulk Fermi level, width of the space charge region (ap70 nm), built-in potential (ap780 meV), and the gap states distribution. Correlating the results obtained from the I-V measurements, impedance and capacitance spectra, we have concluded that a highly resistive layer is formed in the vicinity of the polymer/Al interface in the Au/PANI/Al structure, which is responsible for the presence of a high density of localized states. In the Au/PANI/Al structure, such localized density of gap states dominates the electronic transport, in contrast to the Au/PANI/Au structure, where this resistive layer can be neglected and the bulk effects dominate the electronic transport