π-conjugated poly(3-alkylthiophenes) [P3ATs] are soluble in a variety of common organic solvents which make them possible for the fabrication of devices using simple solution processing techniques. They are semi-crystalline in nature, having crystalline P3AT domains as well as amorphous regions. The chemical incompatibility between the alkyl side chains and the polythiophene backbone is responsible for the crystalline ordering of the polymer. P3AT molecules can adopt two types of orientation on a substrate-(i) edge-on orientation, in which lamellae are perpendicular to the substrate, and (ii) face-on, in which they are parallel to it. The orientation and ordering in P3AT films greatly influence their performance as semiconducting materials, as the field-effect mobilities of devices strongly depend on them. The orientation and ordering can be influenced by various factors such as regioregularity and molecular weight, length of alkyl side chain, the solvent from which the film is cast, nature of the substrate and deposition technique used, such as drop-casting, spin-coating, dip-coating and directional epitaxial crystallization, thermal annealing, solvent vapour treatment, etc. P3AT usually agglomerate on the water surface due to their low amphiphilic nature and strong intermolecular (π–π) interaction. Such agglomerated Langmuir film creates massive hindrance in the formation of well-ordered layer-by-layer structured P3AT film onto the solid substrate. Thus, controlling the structure of the Langmuir film of P3AT is of prime importance for the growth of well-ordered structured film onto the solid substrate through layer-by-layer transfer.
  Here the growth and structural evolution of stearic acid (SA) blended poly(3-hexylthiophene) [P3HT] Langmuir and Langmuir–Blodgett (LB) films were studied using complimentary surface and interface sensitive techniques to understand the possibility of ordering and layering of promising charge carrier mobility polymers, at the air–water interface and on the transferred solid substrate. SA-induced and subsequent compression-induced transitions in P3HT structure, from aggregated-3D to soft-2D and from in-plane mixed to unmixed layer, are evident at low and high pressures, respectively. The blending of SA molecules enhances the amphiphilic character of P3HT, which reduces the extent of the out-of-plane aggregation to form edge-on oriented (EO) bottom side-chain folded-bilayer (f-BL) islands (of size ∼60 nm) within SA monolayer (ML), of commensurate thickness (∼2.6 nm). Further compression, gradually rejects the less hydrophilic f-BL islands from the mixed layer to form EO P3HT BL islands (of coverage in-tune with starting composition) on top of SA ML. The formation of nearly covered P3HT(BL)/SA(ML) structured film on solid substrate is evident for the first time, which (even of limited P3HT thickness) has immense importance in the device properties, as the current in the bottom-gated organic thin-film transistors is known to travel only within few ML region near gate-dielectric [published in J. Colloid Interface Sci. 606, 1153 (2022)].