Electrical characterization of diodes based on regioregular poly(3-hexylthiophene)

Conjugated polymer films were sandwiched between thin aluminium and gold electrodes in orderto determine current-voltagecharacteristics of the three-layer device. Poly (3-hexylthiophene) fractions different in their degrees of polymerization were used as the representatives of conjugated systems with different chain lengths. The analysis of the data showed that morphology of the polymer film and length of the chain influence the operation voltage of the device. Moreover, due to the position of the Fermi levels of the electrodes and HOMO (LUMO) levels, the device works in single-carrier current regime and space-charge limitation of current is present.     

Comparative study: The effects of solvent on the morphology,optical and structural features of regioregular poly(3-hexylthiophene):fullerene thin films

In this report the effect of solvent to control the degree of mixing of the polymer and fullerene components, as well as the domain size and charge transport properties of the blends were investigated in detail using P3HT:C₆₀ films. The polymer blend films spin coated from faster evaporating, non-aromatic solvents demonstrated an improved ordering and optical absorption in P3HT and blended films, indicating that the limited solubility of P3HT:C₆₀ in a marginal solvent can lead directly to optimal morphologies on the films. The PL quenched by a factor of 3 after blending the P3HT with C₆₀ in a 1:1 wt. ratio using CB, xylene, DCB, and toluene as solvents, indicating a partially charge transfer from P3HT to C₆₀. A complete reduction in the PL intensity was observed in the film spin-coated from chloroform.     

Anisotropic conductivity–temperature characteristic of solution-cast poly(3-hexylthiophene) films

For the purpose of developing a positive temperature coefficient (PTC) device, thin films of poly(3-hexylthiophene) (P3HT) were prepared by solution-cast from chloroform. In the present work, the P3HTs of high molecular weight (M_w) with various regioregularities were employed, and the structure anisotropy and the temperature dependence of conductivities in directions parallel (σ_∥) and perpendicular (σ_⊥) to the film surface were investigated. For highly orientated P3HT films, the temperature dependence of σ_⊥ was found to reflect an amorphous character of charge hopping along this direction. However, the σ_∥ decreased greatly above 50 °C, which was attributed to a decrease in the in-plane π-stacking caused both by the melting of crystallized side chains and the enhanced side-chain disturbance with increasing temperature. Different charge transport mechanisms were proposed to explain the anisotropic conductivity–temperature characteristics observed in the two directions. The high conductivity in the parallel direction was deteriorated significantly by thermal recycles, probably due to a reduction of the orientation degree of P3HT crystallites in the bulk film, rather than a change of conjugated length of polymer backbone. A reversible PTC effect was observed for the P3HT film with a high M_w and medium regioregularity in the perpendicular direction, which suggested that the high M_w was more important than the high regioregularity to design the thin film PTC devices based on the soluble P3HT films.     

On the electronic transport mechanism in thin films of some new poly(azomethine sulfone)s

The studied polymers, poly(azomethine sulfone)s, were prepared by the reaction of bis(4-chlorophenyl)sulfone with a mixture of bisphenols: 2,2-bis(p-hydroxyphenyl)propane (bisphenol A) and 4,4′-bis(4-hydroxybenzylideneiminophenoxy)biphenyl in various molar ratios. The temperature dependences of the electrical conductivity and Seebeck coefficient of polymers were investigated using thin-film samples deposited from chloroform solutions (spin coating method) onto glass substrates. It was found that the respective polymers show typical semiconducting properties. Some correlations between these properties and the chemical structures of the polymers were established. The mechanism of electronic transport in the films studied is discussed. The study of optical absorption (in spectral range, 300–1400 nm) evidenced direct bandgaps ranged between 1.30 and 1.80 eV.     

Thermochromic and solvatochromic effects in poly(3-hexylthiophene)

We report thermochromic and solvatochromic effects in the soluble conductive polymer poly(3-hexylthiophene). Thin solid films of the polymer change colour at higher temperatures. The colour of solutions of the polymer can be changed by varying the composition of the solvent. Optical absorption in good solvents is similar to that of thin solid films at higher temperatures, while in poor solvents, absorption features similar to those found in solid films at low temperatures are observed. We discuss these effects with reference to the existence of a hitherto disregarded type of conformational defect, conformons. These are rotational defects on the polymer chain, leading to loss of planarity. High vacuum studies of the thermochromic effect in thin films using X-ray photoemission spectroscopy support this interpretation. The standard series of defects in conductive polymers, solitons, polarons and bipolarons, may have to be extended to include conformons.     

Effect of macromolecular parameters and processing conditions on supramolecular organisation,morphology and electrical transport properties in thin layers of regioregular poly(3-hexylthiophene)

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 cm²/(V s) for the shortest chains (Mn corrected = 1 kDa) to 0.04 cm²/(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.     

Preparation and characterization of graphene nanosheets/poly(3-hexylthiophene) thermoelectric composite materials

Graphene nanosheets/poly(3-hexylthiophene) (GNs/P3HT) composites were prepared by oxidative polymerization of 3-hexylthiophene in a GNs dispersed chloroform solution. The phase composition of the composite materials was analyzed by X-ray diffraction and Fourier transform infrared spectra. The thermoelectric properties of the cold pressed composite pellets with different GNs loadings were measured at room temperature. As the GNs loading increased from 0 to 30 wt.%, the electrical conductivity of the composites dramatically increased from ∼10⁻⁶ to ∼1.2 S/cm while the Seebeck coefficient slightly increased from 33.15 to 35.46 μV/K. The highest power factor (∼0.16 μW m⁻¹ K⁻²) was obtained in the 30 wt.% GNs/P3HT composite material.     

Elastic characterization of poly(3-n.decylpyrrole) thin films by Brillouin light scattering

The first elastic characterization of processable electrically conductive polymer obtained by Brillouin scattering (BS) is presented. BS measurements are performed on samples of poly(3-n.decylpyrrole) in the form of supported thin films obtained from spinning. Elastic parameters, such as Young's modulus and Poisson's ratio, are derived from BS spectra and discussed in comparison with other related polymeric materials.     

Tantalum-doped hydroxyapatite thin films: Synthesis and characterization

To achieve a good bioactivity, magnetron-sputtered (MS) hydroxyapatite (HA) coatings have to be stoichiometric and crystalline. It has also been suggested that doping HA with metallic elements improves its bioactivity but frequently reduces its Young’s modulus. Therefore, efforts are still necessary to identify adequate growth conditions, good doping elements and finally to grow doped HA films. In a first attempt, HA films have been synthesized by MS. Our conditions enabled us to grow at ～10 nm min^?1 stoichiometric and crystallized HA coatings that presented a strong texture. The latter is discussed in view of the competitive growth of the material under the strong electron and ion bombardments used here. Based on ab initio calculations identifying Ta as a promising doping element, we then investigated the growth of Ta-doped hydroxyapatite (HA:Ta) by magnetron co-sputtering. The HA:Ta films were in situ crystallized. Our data reveals that for Ta contents <4.5 at.%, Ta could substitute Ca in the HA cell. For higher doping contents, a deviation from the stoichiometric compound is observed and CaO appears. By nanoindentation, we have measured an elastic modulus of 120 ± 9 GPa for a Ta content of 3 at.%. This value is very close to the value of 110 GPa calculated by ab initio calculations, supporting the substitution scenario. The elasticity drop may be understood by screening of the ionic interaction between constituents in HA upon Ta incorporation.     

Post-functionalization of poly(3-hexylthiophene) via anodic chlorination

Selective anodic chlorination of a poly(3-hexylthiophene) was successfully carried out by electrochemical polymer reaction. NMR and EDX analyses revealed the selective and sufficient substitution of chlorine atom at the 4-position of the repeating thiophene ring. GPC measurement of the polymer before and after electrolysis indicated that neither decomposition nor propagation of the polymer occurred even after passage of the excess charge. The optical and electrochemical properties of the obtained chlorinated polymer were investigated in detail.     

A comparative study on liquid-state photoelectrochemical cells based on poly(3-hexylthiophene) and a composite film of poly(3-hexylthiophene) and nanocrystalline titanium dioxide

A comparative study was done between liquid-state photoelectrochemical cells (PECs) consisting of photoactive electrodes of poly(3-hexylthiophene), P3HT, and a composite film of nanocrystalline titanium dioxide (nc-TiO₂) and P3HT. The nc-TiO₂/P3HT based device shows better performance with an open-circuit voltage of 0.51 V, a short-circuit current of 0.31 mA/cm², and a fill factor of 0.51 when illuminated with white light intensity of 100 mW/cm². The IPCE% obtained at 550 nm for P3HT based device was 0.18% while for that of the nc-TiO₂/P3HT based device was 4%. In P3HT based devices, P3HT showed its p-type behavior while in nc-TiO₂/P3HT based devices, P3HT acted as a sensitizer to nanocrystalline TiO₂.     

Photoelectrochemical response of poly(3-hexylthiophene) and poly(2,3-diethylquinoxaline-5,8-diyl) in aqueous media

The photoelectrochemical properties of poly(3-hexylthiophene) (P3HT) and poly(2,3-diethylquinoxaline-5,8-diyl) (PDEQx) thin films on ITO (indium-tin oxide) electrodes were investigated in aqueous media. Photoirradiation of the P3HT-coated electrode resulted in cathodic photocurrents in the presence of methyl benzoylformate, whereas the PDEQx-coated electrode generated stable anodic photocurrents in the presence of 2-mercaptoethanol. The action spectra indicate that photoexcitations of the polymers cause photoelectrochemical reactions. The films of P3HT and PDEQx exhibit a typical photochemical response of p- and n-type semiconducting materials, respectively.     

Fabrication and optical characterization of electrospun poly(3-buthylthiophene) nanofibers

Light-emitting poly(3-buthylthiophene) (P3BT) nanofibers were fabricated via an electrospinning method. Ten weight percent poly(ethylene oxide) (PEO) was included in a P3BT spinning solution to maintain the required viscosity for electrospinning. The formation of P3BT nanofibers was confirmed through high-resolution confocal photoluminescence imaging and spectroscopy and micro-Raman measurements performed on single strands of P3BT nanofibers. Our results show that P3BT nanofibers can be fabricated using the electrospinning method that can produce abundant continuous nanofibers.     

Double-walled carbon nanotube (DWCNT)–poly(3-octylthiophene) (P3OT) composites: Electrical,optical and structural investigations

In the present study, we report experiments based on in situ polymerization of 3-octylthiophene with DWCNT and many hitherto unstudied properties for the first time. The specialty of DWCNT being the inner tube retaining its intrinsic electronic and optical properties, while the outer tube could be involved in the composite formation. From the IR aromatic and out-of-plane C–H stretching frequencies, the interaction between the polymer and the nanotubes is probed. Further to this absorption spectroscopy, photoluminescence and Raman measurements were carried out to characterize the composites. The conductivity contributions of the nanotubes and Hall voltage measurements are used to understand the utility of the composites for solar cell applications.     

Electrical conduction mechanism in N-(p-R-phenacyl)-4,5-diazafluorenium-9-one bromides thin films

The synthesis and the study of temperature dependence of the electrical conductivity, σ, and Seebeck coefficient, S, for some new organic salts, N-(p-R-phenacyl)-4,5-diazafluorenium-9-one bromides, in thin films, is reported.The dependences ln σ(10³/T) and S(10³/T) are typical for p-type semiconducting compounds. The values of some characteristic parameters for the examined compounds (the thermal activation energy of electrical conduction, ΔE, the ratio of carrier mobilities, b, etc.) have been determined. The obtained values of ΔE ranged from 1.71 and 2.01 eV, while those of b lay in the range 0.70–0.87. The analysis of the absorption edge evidenced direct energy gaps, ranged between 3.31 and 3.39 eV, as well the presence of Urbach absorption tails.Some correlations between respective values and molecular configurations of the present organic salts are established. The model based on band gap representation is suitable for the explanation of the electronic transfer mechanism in the investigated compounds.     

Structural,optical and electrical characterization of chemically deposited CdSe thin films

Cadmium selenide (CdSe) thin films have been deposited on glass substrate. CdSe thin films were characterized by various techniques such as X-ray diffraction, scanning electron microscopy and UV–vis–NIR double beam spectrophotometer. The electrical and thermo-electrical properties are also studied. The X-ray diffraction analysis shows that the film samples are in cubic crystal structure. The optical band gap energy (Eg) was found to be 1.7 eV.     

Improved device performance based on crosslinking of poly (3-hexylthiophene)

Diode devices (glass/ITO/polymer/Al) have been fabricated using poly (3-hexylthiophene) (P3HT) crosslinked with two different biaryl crosslinkers. Crosslinking was performed by exposing the thin films with different wt% of crosslinker to UV irradiation and progress of crosslinking was monitored by IR spectroscopy. An increase in hole mobility of two orders of magnitude has been observed after crosslinking.     

Thienylmanganese halides for the preparation of regioregular poly(3-hexylthiophene)

For the first time, thienylmanganese halides have been used for the synthesis of regioregular head-to-tail poly(3-hexylthiophene). A variety of conditions were examined, and the polymerization was successfully completed in the presence of Ni(dppe)Cl₂ at ambient temperature affording the title polymer in high yields with excellent regioregularity.     

Fabrication of regioregular poly(3-hexylthiophene) field-effect transistors by dip-coating

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.     

Electrical bistability and memory applications of poly(p-phenylenevinylene) films

Devices based on oriented and unoriented poly(p-phenylenevinylene) films showed presence of electrical bistability. Depending on the voltage sweep direction, current through the device at any voltage had two values. Switching between a low- and a high-conducting state occurred when a suitable voltage pulse was applied. Impedance spectroscopy identified a change in bulk resistance of the device during the transition between the two states. Role of orientation of the polymer film on switching and electrical bistability has been studied. The bistability in these devices had an associated non-volatile memory. We have demonstrated read-only and random access memory applications in such devices using current–voltage and impedance characteristics.