Enhancing Carrier Mobilities in Organic Thin-Film Transistors Through Morphological Changes at the Semiconductor/Dielectric Interface Using Supercritical Carbon Dioxide Processing

Charge-carrier mobilities in poly(3-hexylthiophene) (P3HT) organic thin-film transistors (OTFTs) increase 5-fold when OTFTs composed of P3HT films on trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (FTS) monolayers supported on SiO₂ dielectric substrates (P3HT/FTS/SiO₂/Si) are subjected to supercritical carbon dioxide (scCO₂) processing. In contrast, carrier mobilities in P3HT/octadecyltrichlorosilane (OTS)/SiO₂ OTFTs processed using scCO₂ are comparable to mobilities measured in as-cast P3HT/OTS/SiO₂/Si devices. Topographical images of the free and buried interfaces of P3HT films reveal that scCO₂ selectively alters the P3HT morphology near the buried P3HT/FTS-SiO₂ interface; identical processing has negligible effects at the P3HT/OTS-SiO₂ interface. A combination of spectroscopic ellipsometry and grazing-incidence X-ray diffraction experiments indicate insignificant change in the orientation distribution of the intermolecular π-π stacking direction of P3HT/FTS with scCO₂ processing. The improved mobilities are instead correlated with enhanced in-plane orientation of the conjugated chain backbone of P3HT after scCO₂ annealing. These findings suggest a strong dependence of polymer processing on the nature of polymer/substrate interface and the important role of backbone orientation toward dictating charge transport of OTFTs.