ELECTRICAL-CONDUCTIVITY AND OXYGEN DOPING OF VAPOR-DEPOSITED OLIGOTHIOPHENE FILMS

We have measured current-voltage (I-V) characteristics of vapour-depos ited 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 characte ristics 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 t hiophene units), but can be increased by four orders of magnitude on d oping with oxygen (sigma = 4 x 10(-7) S cm(-1)), the doping process be ing strongly promoted by light and/or applied current. Using capacitan ce-voltage (C-V) spectroscopy a charge carrier density of 10(17) cm(-3 ) was measured. The temperature dependence of sigma (140-320 K) can be st be fitted by an exponential law (exp(alpha T)).