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 x 10(4) V cm(-1), the I-V characteristics are linear. The conductivity (a) of freshly prepared films is very low (below 10(-11) 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 (sigma = 4 x 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 10(17) cm(-3) was measured. The temperature dependence of sigma (140-320 K) can best be fitted by an exponential law (exp(alpha T)).