Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene) (original) (raw)
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Advanced Functional Materials, 2004
The optical, structural, and electrical properties of thin layers made from poly(3-hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater.2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X-ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 150 °C. However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue-shift of the solid-state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low-molecular weight P3HT layers.
Applied Physics Letters, 2008
The mobility and contact resistance of transistors based on regioregular poly͑3-hexylthiophene͒ ͑P3HT͒ with Ti/ Pt electrodes were investigated as a function of the molecular weight ͑M W ͒ of P3HT. For an increase in M W from 5.5 to 11 kDa, the mobility increased from 0.04 to 0.16 cm 2 V −1 s −1 , whereas the contact resistance decreased from 1.7 to 0.6 M ⍀. Further increases in M W yielded an apparent saturation in both the mobility and the contact resistance. A nanofibrilar morphology was observed where the width of the nanofibrils increases with M W . A qualitative model based on polymer chain folding is proposed to explain the electrical results.
Synthetic Metals, 2006
The effects of molecular weight and the processing conditions on the polymer supramolecular organisation, its morphology and charge carriers mobility have been investigated for regioregular poly(3-hexylthiophene) (RR-P3HT) thin layers used for the fabrication of field-effect transistors (FETs). The application of three distinctly different processing techniques (dip coating, spin coating and drop casting) together with polymers exhibiting different molecular weights, including previously unexplored molecular weight value (120 kDa by SEC equal to 27 kDa after the MALDI-TOF correction) enabled the separation of the influence of purely macromolecular factors from the morphological ones. It has been demonstrated that the chain length of the deposited polymer is the determining factor in the fabrication of layers with high carriers mobility which changes from 10 −5 cm 2 /(V s) for the shortest chains (Mn corrected = 1 kDa) to 0.04 cm 2 /(V s) for the longest ones (Mn corrected = 27 kDa). The changes of the film morphology cannot explain the dependence of the mobility on Mn. The observed relationship can be rationalized by considering the principal factors, intervening on three different levels: (i) on the molecular level the increase of the conjugation length with Mn is observed which leads to a higher mobility of the carriers along a single chain; (ii) on the supramolecular level the probability of the interchain charge carriers hopping is higher for longer chains since the number of low activation energy pathways for the crossing between chains grows with the increase of the chain length; (iii) on the mesoscopic level the connectivity between aggregations of higher mobility, for example, nanorods observed by AFM, is better assured for longer polymer chains. The product of these contributions results in an enhanced carriers mobility for layers fabricated from high molecular weight polymer fractions. The morphology of the RR-P3HT layers is strongly dependent on the processing method used. For high rate deposition techniques (spin coating) nanorod-type morphologies are obtained for low molecular weight polymers whereas fractions of high molecular weight give films with a granular morphology. For low deposition rate techniques (dip coating) the rod-like morphology persists even for films fabricated from the highest molecular weight fraction. Moreover, for layers obtained from intermediate polymer fractions (from 1.9 to 10.8 kDa) the individual rod width is, within the experimental error, equal to the length of the polymer chain if the all trans conformation is assumed. The above observation implies that polymer chains are oriented perpendicularly to the rod long axis. GIXD investigations fully corroborate this hypothesis. The rod diameter in layers deposited using the highest molecular weight fraction (27 kDa) is significantly lower than the length of the chain in the all trans conformation, suggesting chain folds via all trans-all cis-all trans sequence of conformation changes which would limit the rod diameter. (J.-M. Verilhac), pron@cea.fr (A. Pron).
Macromolecules, 2005
Morphological characterization has been used to explain the previously observed strong correlation between charge carrier mobility measured with thin-film transistors and the number-average molecular weight (MW) of the conjugated polymer regioregular poly(3-hexylthiophene). Atomic force microscopy and X-ray diffraction show that the low-mobility, low-MW films have a highly ordered structure composed of nanorods and the high-mobility, high-MW films have a less ordered, isotropic nodule structure. Modifying the morphology for a constant MW by changing the casting conditions or annealing the samples strongly affects the charge transport and morphology in the low-mobility, low-MW films, but has little effect on the high-MW films. In-plane grazing incidence X-ray scattering shows the in-plane π-stacking peak increases when the mobility increases for a constant MW. When the MW is changed, this correlation breaks down, confirming that in-plane π-stacking does not cause the mobility-MW relationship. We believe a combination of disordered domain boundaries and inherent effects of chain length on the electronic structure cause the mobility-MW relationship.
Journal of the American Chemical Society, 2006
RR-P3HT Synthesis and Characterization: RR-P3HTs with different molecular weights were synthesized through the GRIM method as described earlier. 1,2 A high degree of regioregularity and low polydispersity index (PDI) were confirmed, respectively, by 1 H NMR analysis (Bruker AVANCE 500 MHz NMR spectrometer) and GPC characterization. End group analysis of NMR spectra was used to calculate the number average molecular weight. 3 All NMR samples were dissolved in CDCl 3 . GPC measurements were performed on a Waters 2690 separations module apparatus and a Waters 2487 dual λ absorbance detector with chloroform as the eluent (flow rate 1 mL/min, 35 ºC, λ=254 nm) and a series of three Styragel columns (10 4 , 500, 100 Å; Polymer Standard Services). Toluene was used as an internal standard and calibration based on polystyrene standards was applied for determination of molecular weights. Tapping Mode Atomic Force Microscopy: TMAFM studies were carried out with the aid of a Nanoscope III-M system (Digital Instruments, Santa Barbara, CA), equipped with a J-type vertical engage scanner. The AFM observations were performed at room temperature in air using silicon cantilevers with nominal spring constant of 50 N/m and nominal resonance frequency of 300 kHz (standard silicon TESP probes). A typical value of AFM detector signal corresponding to an r.m.s.
Field effect transistors based on poly (3-hexylthiophene) at different length scales
2004
In this paper we report on thin film transistors based on drop casting solutions of regioregular poly(3-hexylthiophene) (P3HT) over prefabricated gold electrodes. This polymer is known to self-organize into a lamellar structure in chloroform resulting in high field-effect mobilities. We studied the dependency of the charge carrier mobility of devices prepared from solution in chloroform with electrode spacings ranging from 5 µm to 20 nm. It was found that the overall trend was that the mobility decreased as the electrode spacing was made smaller, indicating that the transport properties on closely spaced electrodes were dominated by the contacts. Applying an ac voltage during the preparation of such films resulted in lower mobilities. However, P3HT in p-xylene forms fibres, which were aligned between the electrodes by applying an ac field. Films of aligned fibres with mobilities as high as 0.04 cm 2 V −1 s −1 were prepared.
Fabrication of regioregular poly(3-hexylthiophene) field-effect transistors by dip-coating
Synthetic Metals, 2004
We report the influence of dip-coating speed and concentration of the polymer solution on the characteristics of field-effect transistors (FETs) fabricated in the bottom-contact structure with regioregular poly(3-hexylthiophene) (RR-P3HT) as the active semiconducting material. For each concentration of the polymer in solution, there is an optimum dip coating speed for film deposition with highest field effect mobility; for example, with chloroform as solvent the optimum speed is 0.5 mm/min for a solution containing 1.0 mg/ml and 1.0 mm/min for a solution containing 2.5 mg/ml. Based upon AFM studies of the resulting film morphology, we conclude that the formation of a "rod-like" morphology is the origin of the improved carrier transport in the FET channel.
Progress in Polymer Science, 2013
ABSTRACT Electronic properties of organic semiconductors are often critically dependent upon their ability to order from the molecular level to the macro-scale, as is true for many other materials attributes of macromolecular matter such as mechanical characteristics. Therefore, understanding of the molecular assembly process and the resulting solid-state short- and long-range order is critical to further advance the field of organic electronics. Here, we will discuss the structure development as a function of molecular weight in thin films of a model conjugated polymer, poly(3-hexylthiophene) (P3HT), when processed from solution and the melt. While focus is on the microstructural manipulation and characterization, we also treat the influence of molecular arrangement and order on electronic processes such as charge transport and show, based on classical polymer science arguments, how accounting for the structural complexity of polymers can provide a basis for establishing relevant processing/structure/property-interrelationships to explain some of their electronic features. Such relationships can assist with the design of new materials and definition of processing protocols that account for the molecular length, chain rigidity and propensity to order of a given system.
Journal of the Ceramic Society of Japan, 2010
In this study, we fabricated Organic Field Effect Transistors (OFETs) using an Au/P3HT/SiO 2 /n ++-Si structure. The organic poly(3-hexylthiophene) (P3HT) films with various thickness, which were controlled by changing weight concentration of P3HT in chloroform (CHCl 3) solvent, have been fabricated using a solgel method. The correlations of mobility and on/off current ratio depend on various thickness of P3HT films are revealed. The mobility of the P3HT films were about 1.1, 2.2, and 2.8 © 10 ¹3 [cm 2 V ¹1 s ¹1 ] for 0.4, 0.7, and 1.0 wt %, respectively. We also observed the trade off relation on mobility and on/off ratio with increasing anneal temperature from 100 to 140°C. The surface morphology with various thicknesses was scanned by using atomic force microscopy (AFM) in order to verify the relations between the thickness of film and device performance. We observed the increase of on current with thickness of active layer. These results indicate that the accumulated carriers between semiconductor and insulator are strongly affected by the degree of molecular packing and size of molecular bonding.
Advanced Materials, 2010
In recent years, semiconducting polymers have been widely studied for their potential applications in low-cost, printed, and flexible electronic devices. Carrier mobility in these materials has been steadily increasing, approaching that of hydrogenated amorphous silicon. The best-performing polymeric semiconductors exhibit a high degree of order and are typically semicrystalline. Charge transport in semicrystalline polymers is controlled at several length scales. In the ordered regions of the material, conjugated polymer chains stack in lamellar sheets with p-p interactions between neighboring chains. In addition to ordered crystallites, the microstructure of semicrystalline polymers comprises disordered regions. The spatial arrangement of the crystallites and the disordered regions affect transport via trapping at defects and the percolation properties of the crystalline and disordered networks. Therefore, in order to develop accurate models of charge transport, it is important to understand the relationship between the morphology of the film, its microstructure and its electronic properties. Identifying transport mechanisms and bottlenecks is of particular relevance to the design of materials with improved performance.