Organic field-effect transistors with ultrathin gate insulator

We have used a commercially available Mylar film coated with a thin (≈60 nm) layer of aluminium and an ultrathin (≈3.5 nm) SiO₂ layer as flexible substrate for the manufacture of bottom-gate organic field-effect transistors (OFETs). We show that the SiO₂ layer has insulating properties with a breakdown voltage of 1.6 V and a capacitance of ≈1 μF/cm². We have manufactured organic field-effect transistors using this substrate, regioregular poly(3-hexylthiophene) (rrP3HT) as a p-type semiconductor, and gold source and drain contacts. This results in OFETs that operate with voltages on the order 1 V.     

High ON/OFF ratio and stability of amorphous organic field-effect transistors based on spiro-linked compounds

We report high ON/OFF ratios for organic field-effect transistors (OFETs) based on amorphous thin films of spiro-linked compounds. Bottom-contact OFET structures are fabricated, using 2,2′,7,7′-tetra-(m-tolyl-phenylamino)-9,9′-spirobifluorene (Spiro-TPD) and 2,2′,7,7′-tetrakis-(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD) as active materials. The field-effect mobility of holes in Spiro-TPD and Spiro-TAD thin films is 7 × 10⁻⁵ cm²/Vs. We obtained ON/OFF ratios up to 3.6 × 10⁶ with low OFF currents in the pA range. Time-dependent measurements show that the transistor characteristics do not change significantly over 9 months.     

Soluble functionalised fullerenes for photovoltaics

Two mono-adducts of fullerene C₆₀ have been synthesised using new aryl azides in good yields. These were characterized with spectroscopic techniques and then used to fabricate bulk heterojunction solar cells with regioregular poly (3-hexylthiophene) (RR-P3HT). A photocurrent density of 3.89 mA/cm², open-circuit voltage of 0.53 V and power conversion efficiency of 0.89% for adduct I and photocurrent density of 2.93 mA/cm², open-circuit voltage of 0.53 V and power conversion efficiency of 0.71% for adduct II have been achieved.     

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.     

New fluorene–bithiophene-based trimers as stable materials for OFETs

We report the synthesis of five new 5,5′-bis(9,9′-dialkylfluorene-2-yl)-2,2′-bithiophenes 4a–e as active materials for the use in OFETs. Depending on the type of alkyl substituents crystalline or amorphous materials are obtained. Cyclovoltammetry shows that the materials are electrochemically stable and have a HOMO level at −5.3 eV. The different morphologies of thin films of 4a–c have great influence on the performance of the materials in OFETs. The field effect mobilities are in the range of 10⁻⁵ cm²/Vs in an amorphous film of 4c to 3 × 10⁻³ cm²/Vs in a polycrystalline film of 4a. This high mobility remains constant after 3 months at ambient conditions, which proofs the high environmental stability of this class of materials.     

Z-scan measurements of third-order optical non-linearities in poly(phenylacetylenes)

Poly(phenylacetylene) (PPA) and poly(p-methoxyphenylacetylene) (PMOPPA) were synthesized by catalytic polymerization of monosubstituted alkynes and were fully characterized by conventional techniques. Linear and third-order non-linear optical properties of polymers solutions were investigated at λ=780 nm using a Z-scan set-up equipped with a femtosecond laser source at different operating regimes. A high repetition rate (76 MHz) regime was used to measure the linear and non-linear absorption coefficients by means of thermo-optical effects. Values of the linear and non-linear absorption coefficients were α=4.9×10⁻³ cm⁻¹ and β=1.1×10⁻¹¹ cm W⁻¹ for PPA and α=2×10⁻² cm⁻¹ and β=2.1×10⁻¹⁰ cm W⁻¹ for PMOPPA. Very low repetition rate (14 Hz) excitation was used to evidence the purely-optical non-linear refractive indexes, that were γ=6×10⁻¹⁸ cm² W⁻¹ and γ=11×10⁻¹⁸ cm² W⁻¹, respectively, for PPA and PMOPPA.     

Picosecond time-resolved infrared absorption study on photoexcited dynamics of regioregular poly(3-hexylthiophene)

Picosecond time-resolved infrared absorption spectra of the photoexcited states of a spin-coated film of regioregular poly(3-hexylthiophene) (RR-P3HT) have been recorded at 77 K. The time-resolved photoinduced infrared absorptions due to electronic transitions at 2500 cm^?1 and vibrational transitions at 1575 cm^?1 have fast and slow decay components. By comparing the picosecond time-resolved infrared absorption spectrum of long-lived (slow decay component) transient species with the doping-induced infrared difference spectrum of RR-P3HT, the long-lived transient species are assigned to polarons. The short-lived transient species are assigned to singlet excitons and/or polaron pairs.     

Theoretical and experimental study of photocurrent action spectrum of ITO/sexithiophene/Al structure

We have studied photovoltaic cells using sexithiophene in (ITO/6T/Al) structure, and we have measured the action spectrum obtained by illumination through the aluminum side which is compared to the measured absorption spectrum. Two models were used to interpret this experiment: the Ghosh model using the carriers diffusion property in the bulk, and the kinetic model describing the dynamic behavior of the charge carriers in the device. The confrontation of these models with the experimental spectra allows us to reach the diffusion length of electron–hole pair L=2×10⁻⁶ cm. We also give the mobility of carriers (μ_n≈1.5×10⁻⁴ cm²/V s, and μ_p≈1.5×10⁻⁵ cm²/V s) and their diffusion coefficients (D_n≈4×10⁻⁶ cm²/s, and D_p≈4×10⁻⁷ cm²/s).     

Laser doping of chromium as a double acceptor in silicon carbide with reduced crystalline damage and nearly all dopants in activated state

Chromium, a p-type dopant, has been incorporated into silicon carbide by laser doping. Secondary ion mass spectrometric data revealed enhanced solid solubility (2.29 × 10¹⁹ cm⁻³ in 6H–SiC and 1.42 × 19¹⁹ cm⁻³ in 4H–SiC), exceeding the equilibrium limit (3 × 10¹⁷ cm⁻³ in 6H–SiC above 2500 °C). The roughness, surface chemistry and crystalline integrity of the doped sample were examined by optical interferometry, energy dispersive X-ray spectrometry and transmission electron microscopy, respectively, and showed no crystalline disorder due to laser heating. Deep-level transient spectroscopy confirmed Cr as a deep-level acceptor with activation energies E_v + 0.80 eV in 4H–SiC and E_v + 0.45 eV in 6H–SiC. The Hall effect measurements showed that the hole concentration (1.942 × 10¹⁹ cm⁻³) is almost twice the average Cr concentration (1 × 10¹⁹ cm⁻³), confirming that almost all of the Cr atoms were completely activated to the double acceptor state by the laser-doping process without requiring any additional annealing step.     

N-channel operation of pentacene thin-film transistors with ultrathin polymer gate buffer layer

N-channel operation of pentacene thin-film transistors with ultrathin poly(methyl methacrylate) (PMMA) gate buffer layer and gold source–drain electrode was observed. We prepared pentacene thin-film transistors with an 8-nm thick PMMA buffer layer on SiO₂ gate insulators and obtained electron and hole field-effect mobilities of 5.3 × 10^?2 cm²/(V s) and 0.21 cm²/(V s), respectively, in a vacuum of 0.1 Pa. In spite of using gold electrodes with a high work function, the electron mobility was considerably improved in comparison with previous studies, because the ultrathin PMMA film could decrease electron traps on SiO₂ surfaces, and enhance the electron accumulation by applied gate voltages.     

Polyaniline with ω-hydroxyalkoxy pendant substituent on the meta-position

A series of meta-substituted polyaniline derivatives, poly[3-(ω-hydroxyhexoxy)aniline], poly[3-(ω-hydroxyoctoxy)aniline], poly[3-(ω-hydroxydecoxy)aniline] and poly[3-(ω-hydroxydodecoxy)aniline] have been synthesized via chemical oxidative polymerization. The resulting polymers show enhanced solubility in organic solvent. The polymerization yield is increased and the reaction is accelerated by the addition of aniline. The homopolymers can attain conductivity in the range of 10⁻⁶ to 10⁻⁴ S cm⁻¹ after protonic doping with HCl. Upon doping oxidatively with iodine, the conductivity is ca. 10⁻⁵ to 10⁻³ S cm⁻¹. For the copolymers with aniline (15 mol%), the corresponding conductivity is higher, 10⁻⁵ to 10⁻³ S cm⁻¹ upon HCl doping and 10⁻⁴ to 10⁻² S cm⁻¹ upon iodine doping. Thermogravimetry (TG) of the polymer depicts two major weight loss steps corresponding to the degradation of side chain and the thermal breakdown of the backbone, respectively. The polymers are largely amorphous but show an increase in crystallinity with increasing pendant chain length. The polymers were also subjected to FT-IR, UV–VIS and X-ray photoelectron spectroscopy (XPS) studies.     

Excimer-laser micropatterned photobleaching as a means of isolating polymer electronic devices

Results are reported of an investigation into the effects of UV irradiation on the electrical conductivity of poly(3-hexylthiophene) films and on the field-effect mobility in transistors formed from this semiconducting polymer. The UV source used was a pulsed excimer-laser (KrF, 248 nm) in a commercial, excimer-laser micromachining workstation. By limiting the fluence in the pulse to ∼50 mJ/cm² controlled reductions of up to 2 orders of magnitude in both the bulk conductivity and field-effect mobility were achieved before significant ablation took place. Changes in the UV–vis spectrum of the P3HT show that reduced electrical transport is accompanied by an increase in the optical bandgap which is attributed to a reduction in π-conjugation either by chain scission or photo-oxidation of the polymer. It is argued that photobleaching of selected regions of the semiconducting polymer film is a viable technique for isolating individual transistors in a polymer electronic circuit.     

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.     

Analysis of electrochemical impedance of polypyrrole|sulfate and polypyrrole|perchlorate films

PPy|SO₄ and PPy|ClO₄ films have been synthesized and investigated in K₂SO₄, ZnSO₄ and NaClO₄ aqueous solutions by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and electron probe microanalysis (EPMA) methods. On the basis of obtained data and calculated impedance parameters as the potential functions, the role of different processes (diffusion of ions, double-layer charging, adsorption and charge transfer) in oxidized, partially reduced and reduced PPy films is estimated. The lowest pseudocapacitance values (from n × 10⁻⁶ to n × 10⁻⁴ μF cm⁻² for 1 μm film), independent of solution concentration, were established for PPy|SO₄ in ZnSO₄. This phenomenon is related with strongly aggravated film reduction process in the solution of double-charged cations. In the case of PPy|ClO₄ in NaClO₄ and PPy|SO₄ in K₂SO₄,where the mono-charged cations participate in redox process, the capacitance values are in the range from: n × 10⁻³ to n × 10⁻² μF cm⁻² and even somewhat higher for PPy|ClO₄ system at oxidized state. The calculated effective diffusion coefficients of ions D remain inside the range from n × 10⁻¹² to n × 10⁻¹⁴ cm² s⁻¹ for PPy|SO₄ in 0.1 M K₂SO₄ and PPy|ClO₄ in 0.1 M NaClO₄ aqueous solution. In the case of PPy|SO₄ film in ZnSO₄ solution the D values are essentially lower.     

Microwave-assisted synthesis of polythiophenes via the Stille coupling

Microwave radiation has been employed to synthesise soluble polythiophenes 1–4 via the Stille coupling, resulting in the preparation of polythiophene 1 with higher molecular weights (M_n > 15,000) and a lower polydispersity (M_w/M_n ≈ 2) than when synthesised using conventional heating. Field-effect mobilities of greater than 3 × 10⁻² cm²/V s were measured for polymer 1 in an organic field-effect transistors (OFETs) device.     

Effect of Au deposition rate on the performance of top-contact pentacene organic field-effect transistors

A study of the influence of the deposition rate of top-contact Au source and drain electrodes deposited by electron-beam evaporation on the electrical performance of pentacene organic field-effect transistors (OFETs) is presented. By adjusting the deposition rate of the Au electrodes to minimize metal diffusion into the semiconductor pentacene layer, the source/drain contact resistance could be reduced. At a Au deposition rate of 10 Å/s, high-performance pentacene p-channel OFETs were obtained with a field-effect mobility of 0.9 cm²/Vs and a normalized channel width resistance of 23 kΩ cm in a device with a channel length of 25 μm.     

Soluble terthiophene-labeled cruciform molecule as a semiconductor for organic field-effect transistor

Cruciform conjugated molecule, 4(HP3T)-benzene bearing terthiophene moieties has been synthesized through Heck coupling reaction using 5-hexyl-5″-vinyl-[2,2′,5′,2″]terthiophene as dendrons and 1,2,4,5-tetrabromo-benzene as the core unit; this molecule has been fully characterized. This terthiophene-based molecule exhibits good solubility in common organic solvents and good self-film forming property. They are intrinsically crystalline as they exhibit well-defined X-ray diffraction patterns from uniform orientations of molecules. Thus, intermolecular interaction can be enhanced to affect the carrier transport phenomena after annealing at 160 °C. The semiconducting property of 4(HP3T)-benzene has been evaluated in organic field-effect transistors. 4(HP3T)-benzene exhibit carrier mobility as high as (3.7 ± 0.5) × 10⁻⁴ cm² V⁻¹ s⁻¹ with an on/off ratio of about 1 × 10³.     

Swelling of poly(3-alkylthiophene) films exposed to solvent vapors and humidity: Evaluation of solubility parameters

White light interferometry was used to determine the swelling of poly(3-alkylthiophene)s (P3ATs), with different head-to-tail regioregularity, exposed to different volatile compounds (VCs): humidity, cyclohexanone, tetrahydrofuran and chloroform. Film expansion increases always from regioregular R-P3DDT, R-P3HT to regiorandom P3BT, P3DDT. Regular solution approach was used to determine Flory–Huggins interaction parameter between P3ATs and VCs. Solubility parameters δ_P ∼ 16.9 ± 0.8, 15.3 ± 0.4, 13.1 ± 0.3, and 12.5 ± 0.2 MPa^1/2 evaluated for P3DDT, P3BT, R-P3HT, and R-P3DDT, respectively, were confirmed by the values extrapolated from surface tension data. Evaluated δ_P values should allow an easier strategy for P3AT solution processing aimed at optimized film structure of P3AT polymers and their blends.     

Controlling charge injection by self-assembled monolayers in bottom-gate and top-gate organic field-effect transistors

We investigate the modulation of the charge injection in organic field-effect transistors with self-assembled monolayers (SAMs) using both a bottom-gate and a top-gate geometry. The current modulation by using SAMs is more pronounced in the top-gate geometry due to the better defined upper surface of the bottom source and drain electrodes. By modifying Ag electrodes with a perfluorinated monolayer an injection barrier as high as 1.6 eV into poly(9,9-dioctylfluorene) can be surmounted, enabling the measurement of the saturated field-effect mobility of 6 × 10^?5 cm² V^?1 s^?1.     

3,6-Di(diphenylamino)-9-alkylcarbazoles: novel hole-transporting molecular glasses

3,6-Di(diphenylamino)-9-alkylcarbazoles are found to constitute a novel family of hole-transporting amorphous molecular materials, as characterized by differential scanning calorimetry and time of flight method. Hole drift mobility of the synthesized materials were in the range of 10⁻⁴ to 10⁻⁵ cm²/Vs at an applied field of 3.6×10⁵ V/cm.