Electrical conductivity and oxygen doping of vapour-deposited oligothiophene films

Abstract 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 2×10 4 V cm −1 , the I-V characteristics are linear. The conductivity (σ) 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 (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 10 17 cm −3 was measured. The temperature dependence of σ (140–320 K) can best be fitted by an exponential law (exp( αT )).