2D Versus 3D Crystalline Order in Thin Films of Regioregular Poly(3‐hexylthiophene) Oriented by Mechanical Rubbing and Epitaxy

Abstract

Highly oriented films of regioregular poly(3‐hexylthiophene) (P3HT) are prepared by two methods: mechanical rubbing and directional epitaxial crystallization. The structure, nanomorphology, and optical and charge‐transport properties of the oriented films are investigated by electron diffraction, high resolution transmission electron microscopy (HR‐TEM), absorption spectroscopy, and transistor field‐effect measurements. In rubbed films, P3HT chains align parallel to the rubbing direction and the crystalline domains orientation changes from preferential edge‐on to flat‐on orientation. The maximum in‐plane orientation probed by absorption spectroscopy is a function of the polymer molecular weight Mw; the lower the Mw, the higher the in‐plane orientation induced by rubbing. The anisotropy of field‐effect mobility measured parallel and perpendicular to the rubbing shows the same trend as the absorption. The Mw‐dependence of the orientation is explained in terms of chain folding and entanglement that prevent the reorientation and reorganization of the π‐stacked chains, especially when Mw ≥ 50 kDa. For comparison, P3HT films are oriented by directional epitaxial crystallization using a zone‐melting technique. Electron diffraction and HR‐TEM show that epitaxial and rubbed films differ in terms of intralamellar order within layers of π‐stacked chains. Comparison of UV‐vis absorption spectra for the different samples suggests that the vibronic structure is sensitive to intralamellar disorder.

Large Scale Alignment and Charge Transport Anisotropy of pBTTT Films Oriented by High Temperature Rubbing

“…Annealing at T ≥ 200 °C helps to recover some of the 600 nm feature in the spectra, indicating that local crystalline order has been partly restored. This behavior is similar to that reported for rubbed P3HT films …”

Impact of Regioregularity on Alignment and Thermoelectric Properties of Sequentially Doped and Oriented Films of Poly(3-hexylthiophene)

“…Although multiple orders of the lamellar periodicity are seen, the ED pattern of oriented R71-P3HT films remains poor without multiple index reflections. This is a consequence of the regiorandom branching of the side chains that is responsible for the absence of the 0 0 2 reflection in the ED pattern; it is similar to that of the smectic-like phase of rubbed P3HT, i.e., without registry of stacking of thiophene units inside individual π-stacks . This is fully consistent with the results of polarized UV–vis–NIR spectroscopy.…”

“…This is a consequence of the regiorandom branching of the side chains that is responsible for the absence of the 0 0 2 reflection in the ED pattern; it is similar to that of the smectic-like phase of rubbed P3HT, i.e., without registry of stacking of thiophene units inside individual π-stacks. 28 This is fully consistent with the results of polarized UV−vis−NIR spectroscopy. Accordingly, this study shows that high-temperature rubbing of R71-P3HT at 165 °C is able to enforce smectic-like order similar to that observed in RR-P3HT films rubbed at T R close to 100 °C.…”

Segregated versus Mixed Interchain Stacking in Highly Oriented Films of Naphthalene Diimide Bithiophene Copolymers

“…A similar mixed stacking was also observed in F8BT films after annealing of the thin films above the melting temperature . Interestingly, the observed stem length in the direction of the chain axis exceeds 50 nm which is much larger than the average stem length in crystalline P3HT lamellae observed by HR-TEM (it exceeds hardly 20 nm in epitaxied thin films). ,, This observation confirms the higher rigidity of P(NDI2OD-T2) chains and the extended-chain crystallization mode. The two different types of packing of form I and form II are schematically depicted in Figure .…”