Controlled one-dimensional nanostructures in poly(3-hexylthiophene) thin film for high-performance organic field-effect transistors

With the aim of improving the field-effect mobilities in poly(3-hexylthiophene) (P3HT) thin film transistors, we controlled the nanostructures of P3HT thin film by changing the solvent vapor pressure in a spin-coating chamber during solidification. The transistors with P3HT thin films spin-coated under a high solvent vapor pressure (56.5 KPa), showing the one-dimensional nanowire morphologies, resulted in the relatively high field-effect mobilities (0.02 cm2/(V.s)) that are typically more than 1 order of magnitude higher than those prepared under ambient conditions, showing the featureless morphologies. This can be attributed to the higher solvent vapor pressure during film formation, providing the solvent is allowed to evaporate slowly and the degree of ordering within the P3HT crystalline domains is dramatically improved.     

Fabrication of polymer Langmuir-Blodgett films containing regioregular poly(3-hexylthiophene) for application to field-effect transistor

Semiconducting thin films consisting of regioregular poly(3-hexylthiophene) (RR-PHT) and poly(N-dodecylacrylamide) (pDDA) were constructed by the Langmuir-Blodgett (LB) technique. A mixture of RR-PHT and pDDA spread from a chloroform solution on a water surface forms a stable monolayer, which can be transferred onto solid substrates by the LB method, yielding a well-defined polymer LB film. Surface morphology studies of the LB film indicate that the RR-PHT is dispersed uniformly throughout the surface. The polymer thin film was chemically doped by contacting with FeCl3 acetonitrile solution, and a conductivity of 5.6 S/cm was achieved. Further, the LB film was utilized as the semiconducting film in the field-effect transistor (FET), and mobilities of 2.2 x 10(-4) and 4.4 x 10(-4) cm2 V(-1) s(-1) were obtained by analyzing the saturated and linear regions of the current-voltage characteristic, respectively.     

Modeling disorder in polymer aggregates: the optical spectroscopy of regioregular poly(3-hexylthiophene) thin films

Absorption and emission in polymer aggregates is studied theoretically, taking into account excitonic (intermolecular) coupling, exciton-phonon (EP) coupling, and disorder, all treated on equal footing within a generalized Holstein Hamiltonian with numerically generated eigenmodes and energies. The disorder is modeled as a Gaussian distribution of molecular transition frequency offsets of width sigma and spatial correlation length l(0). Both herringbone (HB) and lamellar aggregate morphologies are considered. The emission spectral line shape is shown to undergo marked changes in response to increasing disorder, with the intensity of the ac-polarized 0-0 emission peak generally increasing relative to the replica intensities (0-1,0-2,[ellipsis (horizontal)]) as sigma increases and/or as l(0) decreases. This is contrary to the behavior of the b-polarized component of the 0-0 intensity, which, in HB aggregates, decreases with increasing disorder. Comparisons are made to analogous trends in oligomer aggregates. Analytical results are obtained in the strong EP coupling regime appropriate for conjugated polymers while treating the disorder perturbatively. A method for uniquely determining sigma and l(0) from the emission and absorption spectra is presented. Applications are made to absorption and low-temperature emission in thin films of regioregular poly(3-hexylthiophene), with excellent agreement between theory and experiment obtained for a spatial correlation length of only 3-4 molecules.     

Time-temperature superposition for viscoelastic properties of regioregular poly(3-hexylthiophene) films

Shear moduli were determined for chemically polymerized and solvent cast regioregular poly(3-hexylthiophene) films, using thickness shear mode acoustic wave resonators. The results are strikingly different to those for electropolymerized regiorandom poly(3-hexylthiophene) films. The time scale of the measurement was varied directly by use of higher harmonics of the acoustic wave resonator and indirectly via temperature. The significant variations in shear modulus with effective time scale can be "normalized"onto a stress master relaxation curve by using the concept of time-temperature superposition; this is the first time this has been demonstrated for electroactive films. The shift factors required to effect this normalization do not follow the classical Williams-Landel-Ferry (WLF) equation developed for long-range backbone motions of bulk polymers. Instead, they follow an Arrhenius-like behavior, commonly used to describe secondary motions of polymer side-chains. The activation enthalpy associated with this is independent of applied potential, is the same as for as cast (undoped) films, and is similar to that for rotation about a carbon-carbon single bond. These all point to the hexyl side-chains as the origins of the observed phenomena, consistent with the "melting point" separating two temperature-dependent phases and with the different molecular packing arrangements that would necessarily apply to regioregular and regiorandom materials.     

Semicrystalline morphology in thin films of poly(3-hexylthiophene)

The thermodynamic phase behavior and the morphology in thin films of poly(3-hexylthiophene) (P3HT) has been studied using calorimetry, X-ray scattering, and scanning force microscopy (AFM). Around 225 °C a phase transition from the crystalline state to a layered, liquid crystalline structure occurs in regioregular P3HT, while the regiorandom counterpart material is disordered at all temperatures and displays a glass transition temperature T_g–3 °C. Regioregular P3HT is semicrystalline and forms needle or plate like crystallites which in solution cast thin films are oriented with respect to the substrate. Films produced by spin coating display a non-equilibrium structure with reduced order and orientation. Annealing of these films in the liquid crystalline state leads to the formation of a morphology similar to the one observed in solution cast films.

Impact of the anion on electrochemically doped regioregular and regiorandom poly(3-hexylthiophene)

Chemical and electrochemical doping of π-conjugated polymers is an important aspect in determining the performance and enabling the operation of many organic electronic devices, from organic light emitting diodes and thermoelectrics to organic electrochemical transistors. In both chemical doping and electrochemical doping an ionized dopant or counterion is present along with the doped π-conjugated polymer. This dopant or counterion is not a benign spectator, rather, its presence can significantly impact the optical, electronic, and thermoelectric properties of the resulting material. Here, we investigate how counterion structure impacts the electrochemical doping ability, oxidation potential, ionization energy, and polaron absorbance of regioregular (rr) and regiorandom (rra) P3HT. We find that in most cases the anion has a small effect on the polymer oxidation potential, except for in the case of rr-P3HT with the large tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion. We propose that this large anion is excluded from the crystalline regions and thus the oxidation potential is similar to that of rra-P3HT. The anions also result in significant differences in polaron absorbance and ionization energies, thereby emphasizing the important role of the counterion in determining the optical and electronic properties of doped π-conjugated polymers.     

Crystalline-crystalline block copolymers of regioregular poly(3-hexylthiophene) and polyethylene by ring-opening metathesis polymerization

Block copolymers of regioregular poly(3-hexylthiophene) (P3HT) and polyethylene (PE) were synthesized through the chain transfer of olefin-terminated P3HT in the presence of cyclooctene via ring-opening metathesis polymerization (ROMP). Subsequent hydrogenation of the poly(cyclooctene) block yielded high molecular weight, crystalline-crystalline P3HT-PE block copolymers, which are thermally stable and resistant to solvents under ambient conditions. These copolymers were characterized by 1H NMR, DSC, and WAXS and represent the first materials of a class of crystalline-crystalline semiconducting-insulating block copolymers.     

Structure of friction-transferred highly oriented poly(3-hexylthiophene) thin films

Oriented thin films of P3HT were obtained by a friction-transfer technique. The morphology and structure of the film were studied by means of optical microscopy, atomic force microscopy and transmission electron microscopy. Optical microscopy observation indicates that large size well-ordered P3HT thin films can be produced by a friction-transfer technique. Highly ordered lamellae were observed in P3HT friction-transferred films by electron microscopy. Electron diffraction results confirm the existence of high orientation with the a- and c-axes of P3HT crystals aligned in the film plane while the c-axis parallel to the friction-transfer direction. The atomic force microscopy observation of the as-prepared P3HT thin film shows, however, a featureless top surface morphology, indicating the structure inhomogeneity of the obtained film. To get highly oriented P3HT thin films with homogenous structure, high temperature annealing, solvent vapor annealing and self-seeding recrystallization of the friction-transferred film were performed. It is confirmed that solvent vapor annealing and self-seeding recrystallization methods are efficient in improving the surface morphology and structure of the frictiontransferred P3HT thin film. Highly oriented P3HT films with unique structure can be obtained through friction-transfer with subsequent solvent vapor annealing and self-seeding recrystallization.     

Synthesis and characterization of poly(benzodithiophene) derivative for organic thin film transistors

Poly{2,6‐bis(3‐dodecylthiophen‐2‐yl) benzo[1,2‐b;4,5‐b′]dithiophene} (PTBT) was synthesized, via oxidative polymerization by oxidative agent (FeCl₃). The mole ratio of FeCl₃ and monomer (3.5:1), and keeping low temperature during the dropping of diluted catalyst were very important for the polymerization without crosslinking. The PTBT was confirmed by ¹H NMR, FTIR spectra, and elemental analysis. The PTBT has very good solubility in organic solvents such as chloroform, tetrahydrofuran, etc, and good thermal stability with T_g of 164 °C. The PTBT shows UV‐optical absorption at 406 nm and photoluminescence (PL) spectroscopy at 504 nm in a film. The highest occupied molecular orbital (HOMO) energy of the polymer is ?5.71 eV by measuring cyclic voltammetry (CV). A solution‐processed polymer thin film transistor device shows a mobility of 3 × 10^?5 – 8 × 10^?5 cm² V^?1 s^?1, and an on/off current ratio of 10⁴. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5277–5284, 2007     

Structure and morphology control in thin films of regioregular poly(3‐hexylthiophene)

This review focuses on the structural control in thin films of regioregular poly(3‐hexylthiophene) (P3HT), a workhorse among conjugated semiconducting polymers. It highlights the correlation existing between processing conditions and the resulting structures formed in thin films and in solution. Particular emphasis is put on the control of nucleation, crystallinity and orientation. P3HT can generate a large palette of morphologies in thin films including crystalline nanofibrils, spherulites, interconnected semicrystalline morphologies and nanostructured fibers, depending on the elaboration method and on the macromolecular parameters of the polymer. Effective means developed in the recent literature to control orientation of crystalline domains in thin films, especially by using epitaxial crystallization and controlled nucleation conditions are emphasized. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1218–1233, 2011     

Structure and morphology optimization of poly(3-hexylthiophene) thin films onto silanized silicon oxide

The influence of the preparation conditions, including substrate functionalization with common silanizers, onto structure/morphology of the overlying poly(3-hexylthiophene) thin films has been investigated by using both grazing incidence X-ray diffraction and atomic force microscopy. The factors determining the formation of spin-coated films suitable for applications in field effect transistors, i.e. concentration, spin-speed, and thermal treatment are addressed. We have established, by a tuning of the preparation and post-deposition treatments, the optimal conditions to get films with the required structural/morphologic features. Moreover we have shown that the macromolecules orient and organize at the interface zone (?10 nm from the interface) better than in the upper layers, i.e. far away from the interface.     

Structural transitions of nanocrystalline domains in regioregular poly(3‐hexyl thiophene) thin films

The effects of solution processing and thermal annealing on thin film morphology and crystalline structures of regioregular poly(3‐hexyl thiophene) (RR P3HT) are studied in terms of molecular weight (M_w). Using grazing‐incidence X‐ray diffraction, π‐conjugated planes in drop‐cast films from chloroform solutions are found to be preferentially oriented parallel to the substrates regardless of M_w. However, the mesoscale nanocrystalline morphology of the drop‐cast films is significantly affected by M_w, exhibiting a distinctive morphological transition from short nanorods to long nanofibrils above a critical number‐averaged M_w (～ 3.6 kDa). This is probably due to the change in a conformation change from an extended‐chain to a folded‐chain, as M_w of RR P3HT increases. In contrast, spin‐casting of high M_w RR P3HT produces less ordered films with a lower crystallinity and mixed parallel/perpendicular orientations of π‐conjugated planes. The crystallinity and parallel π‐conjugated orientation of RR P3HT in spin‐cast films could be improved by thermal treatments at high‐temperatures either (1) above the glass transition temperature or (2) above the melting temperature of RR 3PHT followed by recrystallization upon cooling under vacuum. However, the charge mobility of the spin‐cast films for a field‐effect transistor application is still lower than that of the drop‐cast films. This would be attributed to the chain oxidation and the development of distinct grain boundaries between RR P3HT nanofibrils during the thermal treatments. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1303–1312, 2007     

Measurements of HOMO-LUMO levels of poly(3-hexylthiophene) thin films by a simple electrochemical method

Journal of Solid State Electrochemistry - Though poly(3-hexylthiophene) (P3HT) is one of the most commonly used polymers in organic solar cells, a broad range of values, derived from cyclic...     

Wavelength dependence of nanoscale photodegradation in poly(3-octylthiophene) thin films

The use of conducting polymers in optoelectronic devices is subject to the understanding of the electro-optical processes that take place at the nanoscale. One of the photo-induced processes that limit their application is the photodegradation, which reduces the device working life. In this work the photodegradation of poly(3-octylthiophene) thin films was studied by combining Kelvin probe microscopy and optical microscopy. In this way, a direct correlation between morphological and contact potential changes with optical density changes as a function of the irradiation wavelength and intensity can be made. These results, complemented with Raman spectra help to clarify the degradation processes that are taking place. We find that the photodegradation strongly depends on the irradiation wavelength blue light being much more aggressive than UV. In addition, the optical properties change abruptly before any substantial change in the morphology is observed.     

Interfacial properties of free-standing poly(3-hexylthiophene) films

Using full atomistic classical molecular dynamics simulations, the interfacial properties of free-standing poly(3-hexylthiophene) (P3HT) films have been investigated. The orientations of different parts of the P3HT chain and the surface tensions of the films were calculated in a temperature range of 540 K-600 K. At the liquid/vacuum interface, the P3HT chain shows ordering by exposing hexyl groups at the interface, while the chain backbone lays flat with the thiophene ring preferentially tilt toward the surface. At the interface, the terminal methyl groups of hexyl side chains are in excess compared to the methylene groups or thiophene rings. The surface tension of P3HT in its melt state shows similar temperature dependence to that of polymers that have long alkyl side chains. The surface tension values are comparable to those polymers that expose methyl or methylene groups on the surface. The surface tension values determined for the melt state are lower than the experimental reported values for crystalline P3HT films, as expected.     

Air-tolerant poly(3-hexylthiophene) synthesis via catalyst-transfer polymerization

The discovery of catalyst-transfer polymerization and its further developments have led to unprecedented control over the length and sequence of conjugated polymers. However, the methods themselves are technically challenging to perform due to the air- and moisture-sensitivities of the monomers and catalysts. Herein, we report a catalyst-transfer polymerization method that affords poly(3-hexylthiophene) in high yields without using an inert atmosphere. The synthesis capitalizes on a rapid Negishi cross-coupling using a moisture-tolerant organozinc monomer mediated by an air-stable Pd precatalyst. This simple method should make conjugated polymer synthesis more accessible to a broader range of researchers and may be generalizable to other monomer scaffolds.     

High mobility of dithiophene-tetrathiafulvalene single-crystal organic field effect transistors

Single-crystal field effect transistors of the organic semiconductor dithiophene-tetrathiafulvalene (DT-TTF) were prepared by drop casting. Long, thin crystals connected two microfabricated gold electrodes, and a silicon substrate was used as a back gate. The highest hole mobility observed was 1.4 cm2/Vs, which is the highest reported for an organic semiconductor not based on pentacene. A high ON/OFF ratio of at least 7 x 105 was obtained for this device.     

Patterning organic semiconductors using "dry" poly(dimethylsiloxane) elastomeric stamps for thin film transistors

This communication demonstrates a method of transferring unreacted low molecular weight (LMW) siloxane oligomers from freshly prepared "dry" PDMS stamps for patterning organic semiconductors and conducting polymers into functional devices via selective wetting. The semiconductors were patterned onto the modified surfaces via dip-coating with well-resolved feature sizes as small as 1 mum. Functional transistor arrays exhibited field-effect mobilities as high as 0.07 cm2/Vs. The proposed printing method eliminates the need to ink SAMs for fabricating patterns and results in a simple, fast, and highly reproducible method of patterning organic semiconductors from solution. The method herein also produced a flexible transistor composed of patterned PEDOT source-drain electrodes.     

Surface-induced conformational changes in poly(3-hexylthiophene) monolayer films

With the aim of elucidating the surface-induced molecular ordering in regioregular poly(3-hexylthiophene) (P3HT) monolayer films, we have controlled the intermolecular interactions at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with two kinds of groups (-NH2 and -CH3). We have found that, depending on the surface properties of such modified insulator substrates, the P3HT chains in the monolayer films can adopt two different conformations (edge-on and face-on). This surprising variation in chain conformation arises because of the specific interactions of the P3HT chains with the modified insulator substrates, which can be explained in terms of the following factors: the unshared electron pairs of the SAM end groups (in the -NH2 system), the pi-H interactions between the thienyl backbone bearing pi systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs (in the -NH2 system).