Effect of Molecular Weight and Annealing of Poly(3-hexylthiophene)s on the Performance of Organic Field-Effect Transistors (original) (raw)
Related papers
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.
Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene)
Macromolecules, 2006
Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. To study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites.
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.
Field-effect transistors based on poly(3-hexylthiophene): Effect of impurities
Organic Electronics, 2007
Organic field-effect transistors (OFETs) based on regioregular poly(3-hexylthiophene)s (P3HT)s have been studied as a function of the amount of impurities in the active polymer. P3HTs have been synthesized via a nickel-initiated cross-coupling polymerization and successively purified by a series of Soxhlet extractions with methanol, hexane and chloroform. At each stage, the amount of impurities was quantified by means of 1 H nuclear magnetic resonance (NMR) spectroscopy, Rutherford backscattering spectroscopy (RBS) and particle induced X-ray emissions (PIXE). Traces of Ni, Cl, Mg, Ca, Fe and Zn could be detected in non-fully purified P3HTs. The presence of impurities in the different fractions of P3HT is shown to affect not only the characteristics of the OFETs but also the photovoltaic performances.
Physical Review B, 2007
We carry out a comprehensive theoretical and experimental study of charge injection in poly͑3-hexylthiophene͒ ͑P3HT͒ to determine the most likely scenario for metal-insulator transition in this system. We calculate the optical-absorption frequencies corresponding to a polaron and a bipolaron lattice in P3HT. We also analyze the electronic excitations for three possible scenarios under which a first-or a second-order metal-insulator transition can occur in doped P3HT. These theoretical scenarios are compared with data from infrared absorption spectroscopy on P3HT thin-film field-effect transistors ͑FETs͒. Our measurements and theoretical predictions suggest that charge-induced localized states in P3HT FETs are bipolarons and that the highest doping level achieved in our experiments approaches that required for a first-order metal-insulator transition.
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.
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).
Journal of Polymer Science Part B: Polymer Physics, 2012
A prototypical semiconducting bicomponent system consisting of a conjugated polymer, that is, poly(3-hexylthiophene) (P3HT), blended with a small thiophene containing conjugated molecule, that is, an alkyl-substituted bisphenyl-bithiophene [phenylene-thiophene-thiophene-phenylene (PTTP)], has been used as an electroactive active layer in field-effect transistors (FETs). The self-assembly of this bicomponent system at surfaces has been studied at different length scales, from the nanoscale to the macroscale, and compared with the behavior of monocomponent films of PTTP and P3HT. The correlation between morphology and electric properties of the semiconducting material is explored by fabricating prototypes of FETs varying the relative concentrations of the two-component blend. The maximum charge carrier mobility value, achieved with a few percent of PTTP component, is not simply due to a uniform dispersion of the molecules in the polymer matrix, but rather to the generation of very long percolation paths, whose composition and electrical properties can be tuned with the PTTP concentration. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 642-649, 2012
Journal of Applied Crystallography, 2013
Organic field-effect transistors (OFETs) were fabricated by depositing a regioregular poly(3-hexylthiophene) (P3HT) active layer using a dip-coating method. The field-effect mobility in OFETs depends on chain orientation and crystallinity and is related to direction and withdrawal speed with respect to the source/drain orientation. In this paper, how to control the structural and transport properties of P3HT films by coating parallel and perpendicular to the dipping direction is demonstrated. X-ray diffraction curves taken in the perpendicular direction exhibit a higher degree of crystalline ordering and edge-on conformation compared with those in the parallel direction; this finding correlates with the directional anisotropy of the OFET mobility. Both structural anisotropy and transport properties are enhanced upon thermal treatment.