RR-P3HT Thin Films Research Articles

Clarifying the Dominant Role of Crystallinity and Molecular Orientation in Differently Processed Thin Films of Regioregular Poly(3-hexylthiophene).

Conjugated polymers (CPs) offer the potential for sustainable semiconductor devices due to their low cost and inherent molecular self-assembly. Enhanced crystallinity and molecular orientation in thin films of solution-processable CPs have significantly improved organic electronic device performance. In this work, three methods, namely spin coating, dip coating, and unidirectional floating-film transfer method (UFTM), were utilized with their parametric optimization for fabricating RR-P3HT films. These films were then utilized for their characterization via optical and microstructural analysis to elucidate dominant roles of molecular orientation and crystallinity in controlling charge transport in organic field-effect transistors (OFETs). OFETs fabricated by RR-P3HT thin films using spin coating and dip coating displayed field-effect mobility (μ) of 8.0 × 10-4 cm2V-1s-1 and 1.3 × 10-3 cm2V-1s-1, respectively. This two-time enhancement in µ for dip-coated films was attributed to its enhanced crystallinity. Interestingly, UFTM film-based OFETs demonstrated μ of 7.0 × 10-2 cm2V-1s-1, >100 times increment as compared to its spin-coated counterpart. This superior device performance is attributed to the synergistic influence of higher crystallinity and molecular orientation. Since the crystallinity of dip-coated and UFTM-thin films are similar, ~50 times improved µ of UFTM thin films, this suggests a dominant role of molecular orientation as compared to crystallinity in controlling the charge transport.

Polaron absorption in aligned conjugated polymer films: breakdown of adiabatic treatments and going beyond the conventional mid-gap state model.

This study provides the first experimental polarized intermolecular and intramolecular optical absorption components of field-induced polarons in regioregular poly(3-hexylthiophene-2,5-diyl), rr-P3HT, a polymer semiconductor. Highly aligned rr-P3HT thin films were prepared by a high temperature shear-alignment process that orients polymer backbones along the shearing direction. rr-P3HT in-plane molecular orientation was measured by electron diffraction, and out-of-plane orientation was measured through series of synchrotron X-ray scattering techniques. Then, with molecular orientation quantified, polarized charge modulation spectroscopy was used to probe mid-IR polaron absorption in the ℏω = 0.075 - 0.75 eV range and unambiguously assign intermolecular and intramolecular optical absorption components of hole polarons in rr-P3HT. This data represents the first experimental quantification of these polarized components and allowed long-standing theoretical predictions to be compared to experimental results. The experimental data is discrepant with predictions of polaron absorption based on an adiabatic framework that works under the Born-Oppenheimer approximation, but the data is entirely consistent with a more recent nonadiabatic treatment of absorption based on a modified Holstein Hamiltonian. This nonadiabatic treatment was used to show that both intermolecular and intramolecular polaron coherence break down at length scales significantly smaller than estimated structural coherence in either direction. This strongly suggests that polaron delocalization is fundamentally limited by energetic disorder in rr-P3HT.

Conjugated polymers mediate intracellular Ca2+ signals in circulating endothelial colony forming cells through the reactive oxygen species-dependent activation of Transient Receptor Potential Vanilloid 1 (TRPV1)

Endothelial colony forming cells (ECFCs) represent the most suitable cellular substrate to induce revascularization of ischemic tissues. Recently, optical excitation of the light-sensitive conjugated polymer, regioregular Poly (3-hexyl-thiophene), rr-P3HT, was found to stimulate ECFC proliferation and tube formation by activating the non-selective cation channel, Transient Receptor Potential Vanilloid 1 (TRPV1). Herein, we adopted a multidisciplinary approach, ranging from intracellular Ca2+ imaging to pharmacological manipulation and genetic suppression of TRPV1 expression, to investigate the effects of photoexcitation on intracellular Ca2+ concentration ([Ca2+]i) in circulating ECFCs plated on rr-P3HT thin films. Polymer-mediated optical excitation induced a long-lasting increase in [Ca2+]i that could display an oscillatory pattern at shorter light stimuli. Pharmacological and genetic manipulation revealed that the Ca2+ response to light was triggered by extracellular Ca2+ entry through TRPV1, whose activation required the production of reactive oxygen species at the interface between rr-P3HT and the cell membrane. Light-induced TRPV1-mediated Ca2+ entry was able to evoke intracellular Ca2+ release from the endoplasmic reticulum through inositol-1,4,5-trisphosphate receptors, followed by store-operated Ca2+ entry on the plasma membrane. These data show that TRPV1 may serve as a decoder at the interface between rr-P3HT thin films and ECFCs to translate optical excitation in pro-angiogenic Ca2+ signals.

Electronic structures and chemical reactions at the interface between Li and regioregular poly (3-hexylthiophene)

Interfaces between metals and π-conjugated polymers play an important role in the organic electronic and optoelectronic devices such as polymer-based light-emitting diodes (PLEDs) and photovoltaic devices. In this study, synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS) have been applied to in situ investigate the chemical reactions and electronic structure during the interface formation of Li on the regioregular poly(3-hexylthiophene) (rr-P3HT) thin films. Upon Li adsorption onto P3HT at 300K, Li dopes electrons into P3HT, inducing the occurrence of the P3HT band bending. Moreover, Li can diffuse into the subsurface and react with both S and C atoms in the thiophene rings, leading to the formation of Li2S and Li–C complex. Compared to the interface of Ca/P3HT, the diffusion/reaction depth of Li is much larger at the Li/P3HT interface. Through the investigation of the evolution of core level and valence band spectra together with secondary electron cutoff an energy level alignment diagram at the Li/rr-P3HT interface is derived.

Study of crystalline structure change of annealing-induced self-organization in polymer field-effect transistors

With the aim of understanding the relationships between polymer self-organization and charge carrier mobility of polymer organic field-effect transistor (OFET), we investigate crystalline microstructure change of annealing-induced self-organization of regioregular poly(3-hexylthiophene) (RR-P3HT) active thin layer in polymer OFET by synchrotron radiation grazing incident X-ray diffraction (GIXRD). The crystalline microstructures of RR-P3HT thin film with different preparation methods (spin-coating and drop-casting) and different concentrations (2.5 mg/ml and 3.5 mg/ml) at various annealing temperatures are studied. These results present that, the crystalline structures of RR-P3HT active layers annealed at 150 ℃ are better and enhanced to charge transport, which tend to pack form the thiophene rings are perpendicular and the π-π interchain stacking parallel to the substrate. Furthermore, we find that an appropriate annealing temperature can facilitate the crystal structure of edge-on form, resulting in field-effect mobility enhancement of polymer OFET.

Interplay of Processing, Morphological Order, and Charge-Carrier Mobility in Polythiophene Thin Films Deposited by Different Methods: Comparison of Spin-Cast, Drop-Cast, and Inkjet-Printed Films

The dependence of morphology and polymer-chain orientation of regioregular poly(3-hexylthiophene) (rrP3HT) thin films on processing conditions have been widely studied. However, their possible variation across the film thickness direction remains largely unknown. We report here a marked difference in the optical dielectric (n,k) spectra between the top and bottom interfaces of spin-cast (sc) rrP3HT films deposited from chlorobenzene solutions. These spectra were obtained from reflection variable-angle spectroscopic ellipsometry using a self-consistent graded optical model with self-imposed Kramers-Krönig consistency. The top interface shows a red-shifted absorption that is characteristic of better order than at the bottom, across a wide range of film thicknesses. This disparity diminishes in drop-cast (dc) and multipass inkjet-printed (ijp) films, and disappears in amorphous films such as those of polystyrene and of a green-emitting phenyl-substituted poly(p-phenylenevinylene). The (n,k) spectra also reveal that crystallinity increases across sc < dc < ijp films. This is supported by cross section scanning electron microscopy of the cleaved edges and measurement of the microroughness of both the film interfaces. Furthermore, optical anisotropy decreases across sc > dc > ijp films. Finally, near-edge X-ray absorption fine structure spectroscopy also shows the frontier chains in ijp and dc films are more isotropically oriented than those in sc films. These results suggest that semicrystalline conjugated polymer films can be produced far from equilibrium. This explains the marked variation in their (opto)electronic properties between the top and bottom surfaces that has sometimes been found depending on the film deposition method. In particular, an unusually pronounced crystallization is induced by ijp. We label this marked ijp-induced crystallization the "ijp morphology", which appears to be general, as it is found also in single-inkjet-droplet films. It appears also to be responsible for the lower field-effect mobility measured for ijp films deposited on a variety of linear and circular electrode arrays. This however can fortuitously be reversed by annealing in solvent vapor. As all films were deposited in the low Peclet-number regime, we can rule out surface skin formation. We attribute the extensive crystallization to the non-uniform drying of picoliter droplets, further promoted by repeated film swelling-deswelling cycles in multipass-ijp films.

Thickness dependence of surface morphology and charge carrier mobility in organic field-effect transistors

With the aim of understanding the relationships between organic small molecule field-effect transistors (FETs) and organic conjugated polymer FETs, we investigate the thickness dependence of surface morphology and charge carrier mobility in pentacene and regioregular poly (3-hexylthiophene) (RR-P3HT) field-effect transistors. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility is largely related to the morphology of the organic active layer. We observe that the change trends of the surface morphologies (average size and average roughness) of pentacene and RR-P3HT thin films are mutually opposite, as the thickness of the organic layer increases. Further, we demonstrate that the change trends of the field-effect mobilities of pentacene and RR-P3HT FETs are also opposite to each other, as the thickness of the organic layer increases within its limit.

Nanopatterning via Pressure‐Induced Instabilities in Thin Polymer Films

Polymer thin films are the bedrock of modern electronic devices, playing key roles in their fabrication (photoresists) and operation (dielectrics, light emitting layers, sensor films, etc.). Consequently, enormous efforts have been devoted to understanding their fundamental properties and to pattern thin polymer films, especially now that the thickness of the films and the feature sizes are reaching molecular dimensions. The vast bulk of these studies has employed films at thermodynamical equilibrium, but as we will demonstrate here, non-annealed films containing residual stresses from the preparation of the films by spincasting can be harnessed in new patterning strategies. Nanoimprint lithography (NIL) is an extremely powerful technique for rapid largearea nanopatterning. [1,2] Thebasic principle involvesplacing a master with nanoscale topographic features on top of a thin polymer film, and subsequently heating the polymer film above the glass transitions temperature (Tg) while applying pressure, so that the polymerflows tofill therelief pattern ofthemaster.A thin residual layer usually remains beneath the master protrusions, which needs to be removed by a subsequent etching process before the pattern can be transferred into the substrate. Solvent-assisted NIL [3] unexpectedly resulted in pattern replication without any residual film between the features, due to dewetting of the plasticized film. Alternatively, films on completely non-wettable fluorinated surfaces break up during imprinting and heating, leading to individual polymer nano- and microstructures. [4] Scanning-probe lithography has been used extensively to produce nanostructures in self-assembled monolayers on gold by ‘shaving’ the chemisorbed alkanethiols. [5] Interestingly, residue-free nano- to micrometer-scale trenches were generated by scribing poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) thin films with an AFM tip, presumably as a result of the release of residual strain in the films. [6] These examples

A high purity approach to poly(3-hexylthiophene) diblock copolymers

The combination of superior optoelectronic properties and enhanced solubility for regioregular poly(3-hexylthiophene) (rr-P3HT) has enabled materials based on P3HT to be widely used for organic field-effect transistors, chemical sensors, and photovoltaic solar cells. Significantly, the optoelectronic performance of rr-P3HT thin film has been shown to depend on molecular weight, processing conditions, and end group functionalities which places significant demands on developing synthetic approaches that do not lead to defects in the P3HT chain. This is especially true for the preparation of P3HT block copolymers where control of the polymeric chain ends is essential and the successful incorporation of nonconductive vinyl blocks, such as polystyrene, leads to modification of the mechanical and electronic properties. Starting with the pioneering work of McCullough and coworkers, a variety of groups have prepared rr-P3HT-based block copolymers using a combination of Grignard metathesis method (GRIM) and controlled radical polymerization (i.e., atom transfer radical polymerization, ATRP) or anionic polymerization. More recently, P3HTblock-poly(lactic acid) copolymers have also been synthesized from telechelic P3HT derivatives by ring-opening polymerization. For vinyl-based systems, controlled radical polymerization (CRP) such as ATRP has a number of advantages such as wide monomer selection, tolerance for various functionalities, and simplified synthetic procedures. As demonstrated by several research groups, P3HT-based block copolymers have been successfully synthesized by CRP and in these cases, a monofunctional P3HT macroinitiator is initially prepared containing an initiating fragment at one chain end, for example a 2-bromopropionyl bromide unit for ATRP procedures. In many examples, the second P3HT chain end arises from the 2,5-dibromo-3-hexyl thiophene monomer and the resulting x-bromo chain end is undesirable for a variety of reasons. For example, the thiophene-bromine bond can be active in a variety of side reactions which may lead to the introduction of defects, such as coupling between P3HT chains, either during synthesis of the macroinitiator or during the subsequent ATRP reaction. The presence of small amounts of coupled P3HT having a higher molecular weight and two active initiating chain ends would lead to an ABA block structure that can greatly affect the phase behavior of P3HT block copolymers. Previously, McCullough and coworkers have elegantly Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 8200–8205 (2008)

Performance of Organic Field-Effect Transistor Based on Poly(3-hexylthiophene) as a Semiconductor and Titanium Dioxide Gate Dielectrics by the Solution Process

Organic field-effect transistors (OFETs) were fabricated with a high-dielectric-constant and high-permittivity titanium dioxide (TiO2) as a gate insulator and regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) as the electronically active semiconductor. Positive OFET characteristics were obtained with a low threshold voltage (+3 V) and high field-effect mobility (3.73 ×10-3 cm2 V-1 s-1). The dielectric material (TiO2) was prepared by the sol–gel technique and the gate insulator layer was fabricated by spin coating. The RR-P3HT thin films were fabricated by drop casting with different solution concentrations. The thickness of the thin films was measured using the surface profile measuring system. The fabricated thin film structure was analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD), and UV–visible absorption spectra. The X-ray result shows that the drop-cast RR-P3HT thin film has a high crystallinity on the TiO2 surface, which leads to the high field-effect mobility of the OFET. The results show that the OFET performances are not only dependent on the orientation and crystallinity of the polymer semiconductor, but that also the gate dielectric properties, surface roughness, and interface properties between the gate and semiconductor layers are very important for the efficient performance of the OFET.

The Control of Electronic States Spreading Outside the Conjugated Polymer Surface

ABSTRACTThe surface electronic structures of conjugated regio regular and regio random poly (3- hexylthiophene) (rr-P3HT and rra-P3HT) thin films were studied by near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy and Penning ionization electron spectroscopy (PIES). The distribution of the surface electronic states was controlled on rr-P3HT and rra-P3HT thin films with different molecular ordering by varying the coating process and PIES was adopted to observe the electronic states existing outside the surface.

Effect of Mesoscale Crystalline Structure on the Field-Effect Mobility of Regioregular Poly(3-hexyl thiophene) in Thin-Film Transistors

Regioregular poly(3-hexyl thiophene) (RR P3HT) is drop-cast to fabricate field-effect transistor (FET) devices from different solvents with different boiling points and solubilities for RR P3HT, such as methylene chloride, toluene, tetrahydrofuran, and chloroform. A Petri dish is used to cover the solution, and it takes less than 30 min for the solvents to evaporate at room temperature. The mesoscale crystalline morphology of RR P3HT thin films can be manipulated from well-dispersed nanofibrils to well-developed spherulites by changing solution processing conditions. The morphological correlation with the charge-carrier mobility in RR P3HT thin-film transistor (TFT) devices is investigated. The TFT devices show charge-carrier mobilities in the range of 10 - 4 ∼10 - 2 cm 2 V - 1 s - 1 depending on the solvent used, although grazing-incidence X-ray diffraction (GIXD) reveals that all films develop the same π-π-stacking orientation, where the -axis is normal to the polymer films. By combining results from atomic force microscopy (AFM) and GIXD, it is found that the morphological connectivity of crystalline nanofibrils and the -axis orientation distribution of the π-π-stacking plane with respect to the film normal play important roles on the charge-carrier mobility of RR P3HT for TFT applications.