Impact\nof Irreversible\nAdsorption on Molecular Ordering\nand Charge Transport in Poly(3-hexylthiophene) Thin Films on Solid\nSubstrates

We\ninvestigate the irreversible adsorption of poly(3-hexylthiophene)\n(P3HT) polymer thin films on silicon dioxide/silicon (SiO<sub>2</sub>/Si) substrates during thermal annealing at a temperature below the\nmelting temperature (<i>T</i><sub>m</sub>) but far above\nthe glass transition temperature (<i>T</i><sub>g</sub>),\ni.e., <i>T</i><sub>g</sub> ≪ <i>T</i> =\n170 °C < <i>T</i><sub>m</sub>, and its effect on\ntheir crystalline ordering and charge transport properties. It was\nfound that short-time annealing enhances the molecular ordering of\nP3HT films, while prolonged thermal annealing gradually disrupts the\ncrystalline structures and reduces the overall crystallinity of the\nfilm. Concurrently, thermal annealing at this temperature facilitates\nthe slow irreversible adsorption of P3HT chains at the polymer-solid\ninterface, resulting in the formation of a 1.7 <i>R</i><sub>g</sub>-thick (∼18 nm thick) adsorbed layer on SiO<sub>2</sub>/Si substrates that is fully amorphous and contains a large fraction\nof loosely adsorbed chains. We postulate that such irreversible adsorption\nis responsible for the reduced crystalline packing of P3HT at the\npolymer-solid interface at <i>T</i><sub>g</sub> ≪ <i>T</i> < <i>T</i><sub>m</sub>, which further disrupts\nthe molecular ordering of the entire 46 nm thick P3HT film by a long-range\nperturbation effect. Electrical measurements using an organic field-effect\ntransistor (OFET) device reveal that the enhanced charge carrier mobility\nof P3HT films correlates with an optimized annealing time at <i>T</i><sub>g</sub> ≪ <i>T</i> < <i>T</i><sub>m</sub>, which achieves a balance between maximizing molecular\nordering and minimizing the impact of irreversible chain adsorption.\nThese findings provide new insights into the underlying mechanism\nof thermal annealing in tailoring the structure and property of conjugated\npolymer thin films prepared on solid substrates.