We measured the complex impedance and nonlinear conductivity for regioregular poly(3-hexylthiophene) (P3HT) by recording and analyzing AC waveforms at their fundamental frequency and at higher order harmonic frequencies. We used 50 µm thick films of P3HT with head-to-tail regioregularity of more than 99 %. Gold electrodes were prepared by evaporation. The complex impedance of the semiconducting P3HT decreases exponentially with increasing electric field strength. Furthermore, our broadband dielectric measurements indicate that the apparent semiconducting character of P3HT ceases above a critical frequency, above which the material becomes a dielectric. At room temperature the semiconducting to dielectric transition takes place at about 5 kHz. At frequencies below 5 kHz, P3HT shows a semiconducting character, and the overall response is dominated by conductivity. At low electric fields, the combined temperature and field dependent conductivity of P3HT obeys the bulk-limited Poole-Frankel (PF) model. At higher electric fields, above 104 V/cm, the electric field affects the rate of carrier generation or injection, and a bulk limited charge transport undergoes a transition to the electrode limited conduction, which can be expressed as a Richardson-Schottky effect. The non-linear charge transport in P3HT is dominated by the third-order conductivity, which originates from extended p-type electronic states in P3HT. The presented waveform technique is a novel method, which can be used to determine complex impedance and conductivity of semiconducting polymers at high AC electric fields. Our results demonstrate that the third order conductivity can be used to quantify the effect of electric field on conduction mechanism in organic semiconductors and to correlate the intrinsic charge carriers mobility with molecular structure.
Citation: Abstracts of Papers of the American Chemical Society, v231, pp168-IEC (2006)
Pub Type: Others
Organic semiconductors, Nonlinear conductivity, Polytiophene, Impedance, AC Waveforms, Harmonics