Effects of FeCl3 doping on polymer-based thin film transistors

Polymer-based thin film transistors (PTFTs) were fabricated on glass substrates with anodized Al2O3 as gate insulators. RR-P3HT (regioregular poly-3-hexylthiophene) and MEH-PPV [poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene)] were respectively used as semiconducting active layers for the transistors. A two orders of magnitude increase in field effect mobility (from 7.2x10(-4) cm(2)/V s to 7.4x10(-2) cm(2)/V s) deduced from electrical data of transistors fabricated using FeCl3 doped RR-P3HT was observed. This increase is believed to be mainly due to a large reduction in contact resistance (from 10(8) Omega to 10(3) Omega) to the source and drain Au contacts. The conductivity of RR-P3HT was found to increase only slightly with the doping. For MEH-PPV, doping with FeCl3 also decreased its contact resistance. However, it (4 GOmega) was still much larger than the channel (polymer) resistance (1 MOmega), leading to a slight improvement in its field effect mobility. Theoretically, contacts between Au and P3HT should have very small energy barrier heights (<0.2 eV) for hole injection. We believe that a negative vacuum level shift introduced by metal to organic interfacial dipoles might be the origin of this large energy barrier, as well as to large contact resistance. (C) 2004 American Institute of Physics.