Electrical conductivity and oxygen doping of vapour-deposited oligothiophene films

We have measured current-voltage (I-V) characteristics of vapour-deposited films of various oligothiophenes in the direction parallel to the substrate using a two-point probe technique with symmetric contacts. For applied field strengths up to 2 × 104 V cm-1, the 1-V characteristics are linear. The conductivity (σ) of freshly prepared films is very low (below 10-1 S cm-1 for EC6T, an oligothiophene with end-substituted 4,5,6,7-tetrahydrobenzo groups ('end caps') and with six thiophene units), but can be increased by four orders of magnitude on doping with oxygen (σ = 4 × 10-7 S cm-1), the doping process being strongly promoted by light and/or applied current. Using capacitance-voltage (C-V) spectroscopy a charge carrier density of 1017 cm-3 was measured. The temperature dependence of σ (140-320 K) can best be fitted by an exponential law (exp(αT) ).