Semiconducting organic molecules and polymers are becoming increasingly important in the electronics industry due to their flexibility, easier fabrication and low cost. Ordering and orientation of such molecules significantly influence the physical properties of organic materials and their corresponding device properties. Therefore ordering organic molecules, especially conjugated polymer molecules, in the active layer has been a hot topic in organic electronics due to their high performance. Poly(3-alkylthiophenes) (P3ATs) are one such kind of polymer having flexible alkyl side chains attached to stiff backbones consisting of thiophene units. They are soluble in a variety of common organic solvents which makes it 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.
  The influence of poor solvent and thermal annealing, and their specific roles, in the crystalline ordering of poly(3-dodecylthiophene) (P3DDT) films, which are of immense importance in their performance as semiconducting materials, were investigated using complementary techniques. Edge-on oriented (EO) crystallites (Form-II-like) are enhanced in the as-cast films prepared after addition of a poor solvent. However, the coil-to-rod-like conformational transition is more prevalent compared to the crystallites, suggesting that a poor solvent predominantly helps to overcome the unfavorable conformational transition. A very large enhancement in the amount of the crystallites (Form-I-like) is observed for the films annealed above the melting temperature of the alkyl side chains, suggesting thermal annealing essentially helps to promote the diffusion of rod-like chains and to overcome the hindrance of the ϖ-ϖ stacking. Perfectly EO crystallites, which are enhanced with increasing annealing temperature, start to deteriorate when the melting temperature of the polymer backbone is reached. The domain-like morphology of the as-cast film, however, remains almost unchanged upon thermal annealing suggesting the spontaneous organization of ϖ-stacked layers through alkyl side chains to form crystallites is essentially within the small domains. The best EO crystallites are found for the P3DDT films prepared from a solution containing a large amount of poor solvent and subsequently annealing the film at around 130^oC [published in RSC Adv. 5, 665 (2015)].
  The strong influence of solvents on the ordering of P3DDT due to EO stacking, in the spin-coated thin film on the Si substrate, both near the substrate and away from it, depending upon the substrate surface nature, is observed from the X-ray reflectivity (XR) study. The absence of any appreciable amount of coil-like P3DDT chains (i.e. charge localized states) and formation of ϖ-stacked aggregates (i.e. charge delocalized states) in the spin-coated thin films, with slightly better uniformity for the film prepared from toluene (TL) compared to that prepared from chloroform (CF) and chlorobenzene (CB), are well evident from the optical absorption study. No ordering near the weakly hydrophobic H–Si substrate is found in the films prepared from TL, probably due to less diffusion of P3DDT in TL and the appreciable pinning (film–substrate interaction) effect, while appreciable ordering near the film–air interface, overcoming the pinning effect, is likely to be related to the moderate values of the viscosity and the evaporation rate of the solvent. A better ordered Form-I-like relaxed structure near the film–substrate interface and a less ordered interpenetrating Form-II-like structure toward the film–air interface are found in the films prepared from CF, probably related to the low viscosity and high evaporation rate, respectively, of the solvent. Less ordered and mixed but more toward Form-II-like structures are formed throughout the film prepared from CB, probably due to the high viscosity of the solvent, even though its evaporation rate is low. The high evaporation rate of CF and high viscosity of CB probably create hindrance in the formation of continuous films on the weakly hydrophilic O–Si substrate at low speed, while the moderate values of both the parameters for TL, help to form continuous films on the O–Si substrate even at low speed. Such moderate values also help to form less variable (and more toward Form-I-like) structures and better ordering in the latter film. The relative fluctuation between aggregates along the film-thickness is, however, found slightly more in the film prepared from TL compared to that prepared from CF [published in Soft Matter 11, 3724 (2015)].
  The structures of the spin-coated P3DDT thin films along the out-of-plane and in-plane directions, and the influence of solvent vapor annealing (SVA) and thermal annealing (TA) on their structures, which are of immense importance for their performance as semiconducting materials, were investigated using complementary XR and AFM techniques. The P3DDT films of different thicknesses (~3.4 nm, i.e. ‘monolayer’ and ~11 nm i.e. ‘multilayer’) prepared using different polymer concentration and solvents (CF and TL) are found to compose of grain-like structures (of size ξ) of predominantly edge-on orientation (probably to maximize the coverage of the low energy methyl groups at both the interfaces) with some defects. EO grains are particularly prominent for the monolayer films (ξ ≈ 50 nm), where the interactions at both the interfaces are weak, but not in the case of the multilayer films (ξ ≈ 10 nm), where out-of-plane interaction between P3DDT molecules and evaporation rate of solvent also become important. Slow evaporation of TL creates better ordering in that multilayer film. SVA of the films although reduces the defects, forms small size grains with relatively less ordered structure due to near dissolution (or coil-like conformation) of the P3DDT at saturated vapor of good solvent and subsequent hindrance in the recovery of rod-like conformation and its ϖ-ϖ stacking under fast solvent evaporation. The change in the grain-size is only effective on the monolayer films, where the initial grain-size was large, while the change in the ordering is mainly effective on the TL evaporated multilayer film, where the initial ordering was better. Further thermal annealing of the films only recovers the size of the grains and the ordering of the P3DDT molecules to a very small extent. However, TA in presence of solvent vapor on the as-deposited multilayer film show remarkable improvement of EO ordering. It is likely that the temperature first restrict the amount of solvent vapor and then such limited vapor pressure in presence of an intermediate temperature helps the reorganization of P3DDT molecules to form EO structure [published in RSC Adv. 7, 2563 (2017)].
  Structural evolution of solution-aged poly(3-hexylthiophene) [P3HT] thin films during TA was studied using complementary in-situ XR and ex-situ AFM and optical absorption (UV-Vis) techniques to understand the possibility of obtaining enhanced EO ordering for the better device properties. The presence of P3HT nanofibers (NFs), which were formed through π−π stacking within solution during aging, is evident in the films. Such NFs are well-organized near the film-substrate interface and less-organized near the film-air interface due to the respective slow and fast evaporation rates of the solvent during spin-coating. Accordingly, prominent EO ordering (i.e. the electron density contrast between polymer backbone and side chains is maximum, Δρ ≈ Δρ_m) near the substrate and negligible ordering (i.e. Δρ → 0) near the top surface took place following the standard decay function: Δρ(z) = Δρ_mexp(−z/ζ), where the critical decay length, ζ, is the measure of the out-of-plane ordering. TA fails to improve the Δρ_m-value, i.e. the EO ordering near the substrate and also the total crystalline aggregates or NFs, rather deteriorates both, when annealed near the melting temperature of P3HT. TA improves the ζ-value, i.e. the EO ordering of more out-of-plane region due to thermal energy induced alignment of the NFs, however, lack of improvement of the EO ordering near the substrate is of concern. A relatively low viscous polymer solution and low spin-coating speed play important roles in the formation of a smooth film-substrate interface and better EO ordering near that interface. Though SVA fails to improve the structure, the combination of SVA and TA, i.e. SVTA, improves the in-plane EO ordering near the substrate (i.e. the Δρ_m-value) along with the out-of-plane ordering (i.e. the ζ-value) of the film. Such improvements, which are probably through the alignment and growth of NFs, promoted by SVTA induced proper diffusion, are of immense importance for obtaining better device properties [published in J. Mater. Chem. C 8, 8804 (2020)].