Solubility‐Induced Ordered Polythiophene Precursors for High‐Performance Organic Thin‐Film Transistors

Abstract

With the aim of enhancing the field‐effect mobility of self‐assembled regioregular poly(3‐hexylthiophene), P3HT, by promoting two‐dimensional molecular ordering, the organization of the P3HT in precursor solutions is transformed from random‐coil conformation to ordered aggregates by adding small amounts of the non‐solvent acetonitrile to the solutions prior to film formation. The ordering of the precursor in the solutions significantly increases the crystallinity of the P3HT thin films. It is found that with the appropriate acetonitrile concentration in the precursor solution, the resulting P3HT nanocrystals adopt a highly ordered molecular structure with a field‐effect mobility dramatically improved by a factor of approximately 20 depending on the P3HT concentration. This improvement is due to the change in the P3HT organization in the precursor solution from random‐coil conformation to an ordered aggregate structure as a result of the addition of acetonitrile. In the good solvent chloroform, the P3HT molecules are molecularly dissolved and adopt a random‐coil conformation, whereas upon the addition of acetonitrile, which is a non‐solvent for aromatic backbones and alkyl side chains, 1D or 2D aggregation of the P3HT molecules occurs depending on the P3HT concentration. This state minimizes the unfavorable interactions between the poorly soluble P3HT and the acetonitrile solvent, and maximizes the favorable π–π stacking interactions in the precursor solution, which improves the molecular ordering of the resulting P3HT thin film and enhances the field‐effect mobility without post‐treatment.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Meanwhile, the π-π stacking into ordered aggregates such as nanofibrils is also an entropy-declining process [156]. Therefore, the decreasing of solution temperature and adding bad or non-solvent mentioned above also promote the the π-π stacking, reflected not only in the enhanced absorbance of lower energy electronic transition in spectra but also by the increased precursor size in solution, fibril length and development of clearer fibrillar morphology in films [152,154]. Besides the driven force, there are two important factors affecting the inter-chain aggregation and final morphology: growth kinetics and the nucleation-growth scheme in solution.…”

“…According to the definition of free energy change, ΔG = ΔH − TΔS, there are two pathways to increase the thermodynamic driven force favored by a lower free energy (ΔG < 0) for the coil-to-rod transition with the entropy-declining nature (ΔS < 0): (1) ΔH nearly unchanged but decreasing T and (2) constant T but increasing |ΔH| with ΔH < 0. Reflected by the red-shift and enhanced absorbance at bigger wavelengths in spectra, in situation (1), the straight method is lowering the temperature of the solution to initiate the coil-to-rod transition [68,138,141,144,152], and in situation (2), the transition is induced by adding non-solvent or bad solvent into the solution in a good solvent because of both the minus ΔH arising from the unexposure of conjugated planes with high surface energy towards solvents to lower the total energy and the increased |ΔH| arising from the larger difference of solubility parameters between solvents and polymers according to |ΔH| ∞ (Δδ) 2 [140,[153][154][155]. With the further decrease of temperature or addition of bad or non-solvent, the tendency of coil-to-rod transition is more obvious and the rod-like conformation is more prevailing.…”

Optically switchable organic field-effect transistors based on photoresponsive gold nanoparticles blended with poly(3-hexylthiophene)

“…Spin-coated P3HT film shows a low absorption window between 300-370 nm, minimizing any possible interference with the AZO-coating transcis photoisomerizaton (λ ¼ 365 nm) (24).…”

The effects of P3HT crystallinity in bilayer structure organic solar cells

“…These fibril structures increased with increasing amounts of AN up to 7.5%. As the volume of added AN was increased above 7.5%, the fibril structures disappeared again because of the significant aggregation of P3HT, which may be associated with the secondary aggregation of P3HT [16,17]. Fig.…”

“…A self-assembled nano-fibril morphology of the P3HT thin films was obtained by adding a small amount of non-soluble AN solvent [16]. Fig.…”