Contact dependence of α-sexithienyl thin film transistor characteristics

We have fabricated thin film transistors (TFTs) using α-sexithienyl (α-6T) as the active material and have investigated the dependence of transistor characteristics on the choice of source/drain contact metal. Using α-6T synthesized from terthiophene and purified by vacuum gradient sublimation, we have fabricated TFTs with material deposited by evaporation onto substrates held at both room and elevated temperature. We have studied devices fabricated by depositing the active material onto previously patterned source/drain contacts and also by depositing the source/drain contacts after the active layer deposition, in both cases with the gate contact and dielectric underneath the active material. For both device types we find a clear dependence on the choice of source/drain contact metal with correlation to the metal work function. For our devices the best performance is obtained using metals with largest work function (Pd, Pt, or Au), intermediate performance is obtained using a metal with somewhat smaller work function (Cr), and significantly degraded performance is obtained using a metal with relatively small work function (Al). Despite the apparent correlation to the metal work function, we cannot yet rule out oxidation or other chemical effects with the more reactive metals. We have also examined doping of the α-6T layer as a possible route to improved contacts and find that FeCl₃ can be used to shift the threshold voltage of α-6T TFTs by more than 50 V.