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 7.2×10−4 cm2/V s to 7.4×10−2 cm2/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 108 Ω to 103 Ω) 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 GΩ) was still much larger than the channel (polymer) resistance (1 MΩ), 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.