As high-performance organic semiconductors, π-conjugated polymers have attracted much attention due to their charming advantages including low-cost, solution processability, mechanical flexibility, and tunable optoelectronic properties. During the past several decades, the great advances have been made in polymers-based OFETs with p-type, n-type or even ambipolar characterics. Through chemical modification and alignment optimization, lots of conjugated polymers exhibited superior mobilities, and some mobilities are even larger than 10 cm2 V-1 s-1 in OFETs, which makes them very promising for the applications in organic electronic devices. This review describes the recent progress of the high performance polymers used in OFETs from the aspects of molecular design and assembly strategy. Furthermore, the current challenges and outlook in the design and development of conjugated polymers are also mentioned.
For small organic conjugated molecules the exciton binding energy can be calculated treating molecules as conductor, and is given by a simple relation BE \approx e2/(4πε0εR), where ε is the dielectric constant and R is the equivalent radius of the molecule. However, if the molecule deviates from spherical shape, a minor correction factor should be added.
Light emitting electrospun nanofibers of poly-[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N'-diphenyl)-N,N'-di(p-butyl-oxy-phenyl)-1,4-diaminobenzene)] (PFO-PBAB) are produced by electrospinning under different experimental conditions. In particular, uniform fibers with average diameter of 180 nm are obtained by adding an organic salt to the electrospinning solution. The spectroscopic investigation assesses that the presence of the organic salt does not alter the optical properties of the active material, therefore providing an alternative approach for the fabrication of highly emissive conjugated …
Two dimensional pi conjugated metal porphyrin covalent organic frameworks were produced in aqueous solution on an iodine-modified Au(111) surface by on site azomethine coupling of Fe(III) 5,10,15,20 tetrakis(4 aminophenyl)porphyrin (FeTAPP) with terephthal dicarboxaldehyde and investigated in detail using in-situ scanning tunneling microscopy. Mixed covalent organic porphyrin frameworks consisting of FeTAPP and metal-free TAPP (H2TAPP) were prepared through simultaneous adsorption in a mixed solution as well as partial replacement of FeTAPP by H2TAPP in an as prepared metal porphyrin framework…
In organic field-effect transistors, the structure of the constituent molecules can be tailored to minimize the disorder experienced by charge carriers. Experiments on two perylene derivatives show that disorder can be suppressed by attaching longer core substituents - thereby reducing potential fluctuations in the transistor channel and increasing the mobility at low temperature - without altering the intrinsic transport properties.
We report on organic field-effect transistors with unprecedented resistance against gate bias stress. The single crystal and thin-film transistors employ the organic gate dielectric Cytop(TM). This fluoropolymer is highly water repellent and shows a remarkable electrical breakdown strength. The single crystal transistors are consistently of very high electrical quality: near zero onset, very steep subthreshold swing (average: 1.3 nF V/(dec cm2)) and negligible current hysteresis. Furthermore, extended gate bias stress only leads to marginal changes in the transfer characteristics. It appears t…
We present a simple but powerful method to determine the thicknesses of the accumulation and depletion layers and the distribution curve of injected carriers in organic field effect transistors. The conductivity of organic semiconductors in thin film transistors was measured in-situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. Using this method, the thicknesses of the accumulation and depletion layers of pentacene were determined to be 0.9 nm (VG=-15 V) and 5 nm (VG=15 V).
Research on organic thin-film transistors tends to focus on improvements in device performance, but very little is understood about the ultimate limits of these devices, the microscopic physical mechanisms responsible for their limitations, and, more generally, the intrinsic transport properties of organic semiconductors. These topics are now being investigated through the study of transport in organic transistors realized using molecular single crystals of unprecedented chemical purity and structural quality. These studies are elucidating detailed microscopic aspects of the physics of organic…
We study the influence of polarity on the binding and diffusion of single conjugated organic molecules on the inorganic (10-10) zinc oxide surface by means of all-atom molecular dynamics simulations at room temperature and above. In particular, we consider the effects of partial fluorination of the para-sexiphenyl (p-6P) molecule with chemical modifications of one head group (p-6PF2) or both (symmetric) head and tail (p-6PF4). Quantum-mechanical and classical simulations both result in consistent and highly distinct dipole moments and densities of the fluorinated molecules, which interestingly…
High-performance $n$-type organic field-effect transistors were developed with ionic-liquid gates and N,N$^"$-bis(n-alkyl)-(1,7 and 1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide)s single-crystals. Transport measurements show that these devices reproducibly operate in ambient atmosphere with negligible gate threshold voltage and mobility values as high as 5.0 cm$^2$/Vs. These mobility values are essentially identical to those measured in the same devices without the ionic liquid, using vacuum or air as the gate dielectric. Our results indicate that the ionic-liquid and $n$-type organic semico…
We report on the fabrication and characterization of single crystal field-effect transistors (FETs) based on diphenylbenzo diselenophene (DPh-BDSe). These organic field-effect transistors (OFETs) function as p-channel accumulation-mode devices. At room temperature, for the best devices, the threshold voltage is less than -7V and charge carrier mobility is nearly gate bias independent, ranging from 1cm2/Vs to 1.5 cm2/Vs depending on the source-drain bias. Mobility is increased slightly by cooling below room temperature and decreases below 280 K.
Hall effect is detected in organic field-effect transistors, using appropriately shaped rubrene (C42H28) single crystals. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The presence of the normal Hall effect means that the electromagnetic character of the surface charge is not of hopping carriers but resembles that of a two-dimensional hole-gas system.
A series of solid organic acids were used to p-dope carbon nanotubes. The extent of doping is shown to be dependent on the pKa value of the acids. Highly fluorinated carboxylic acids and sulfonic acids are very effective in shifting the threshold voltage and making carbon nanotube field effect transistors to be more p-type devices. Weaker acids like phosphonic or hydroxamic acids had less effect. The doping of the devices was accompanied by a reduction of the hysteresis in the transfer characteristics. In-solution doping survives standard fabrication processes and renders p-doped carbon nanotu…
We report on the simple fabrication of hysteresis-free and electrically stable organic field-effect transistors (OFETs) and inverters operating at voltages <1-2 V, enabled by the almost trap-free interface between the organic semiconductor and an ultra-thin (<20 nm) and highly insulating single-layer fluoropolymer gate dielectric (Cytop). OFETs with PTCDI-C13 (N,N'-ditridecylperylene-3,4,9,10-tetracarboxylicdiimide) as semiconductor exhibit outstanding transistor characteristics: very low threshold voltage (0.2V), onset at 0V, steep subthreshold swing (0.1-0.2 V/decade), no hysteresis and exce…
We developed a novel method for obtaining the distribution of trapped carriers over their degree of localization in organic transistors, based on the fine analysis of electron spin resonance spectra at low enough temperatures where all carriers are localized. To apply the method to pentacene thin-film transistors, we proved through continuous wave saturation experiments that all carriers are localized at below 50 K. We analyzed the spectra at 20 K and found that the major groups of traps comprise localized states having wave functions spanning around 1.5 and 5 molecules and a continuous distri…
We have studied charge injection across the metal/organic semiconductor interface in bottom-contact poly(3-hexylthiophene) (P3HT) field-effect transistors, with Au source and drain electrodes modified by self-assembled monolayers (SAMs) prior to active polymer deposition. By using the SAM to engineer the effective Au work function, we markedly affect the charge injection process. We systematically examine the contact resistivity and intrinsic channel mobility, and show that chemically increasing the injecting electrode work function significantly improves hole injection relative to untreated A…
We report detailed studies of the slow relaxation of the photoinduced excess charge carriers in organic metal-insulator-semiconductor field effect transistors consisting of poly(3-hexylthiophene) as the active layer. The relaxation process cannot be physically explained by processes, which lead to a simple or a stretched-exponential decay behavior. Models based on serial relaxation dynamics due to a hierarchy of systems with increasing spatial separation of the photo-generated negative and positive charges are used to explain the results. In order to explain the observed trend, the model is fu…
A model describing the low density carrier state in an organic single crystal FET with high-$κ$ gate dielectrics is studied. The interplay between charge carrier coupling with inter-molecular vibrations in the bulk of the organic material and the long-range interaction induced at the interface with a polar dielectric is investigated. This interplay is responsible for the stabilization of a polaronic state with an internal structure extending on few lattice sites, at much lower coupling strengths than expected from the polar interaction alone. This effect could give rise to polaron self-trappin…
We report a detailed comparison on the role of a self-assembled monolayer (SAM) of dipolar molecules on the threshold voltage and charge carrier mobility of organic field-effect transistor (OFET) made of both amorphous and polycrystalline organic semiconductors. We show that the same relationship between the threshold voltage and the dipole-induced charges in the SAM holds when both types of devices are fabricated on strictly identical base substrates. Charge carrier mobilities, almost constant for amorphous OFET, are not affected by the dipole in the SAMs, while for polycrystalline OFET (pent…
As the dimensions of electronic devices approach those of molecules, the size, geometry and chemical composition of the contact electrodes play increasingly dominant roles in device functions. It is shown here that single-walled carbon nanotubes (SWNT) can be used as quasi one-dimensional (1D) electrodes to construct organic field effect transistors (FET) with molecular scale width (~2 nm) and channel length (1-3 nm). An important feature owing to the quasi 1D electrode geometry is the favorable gate electrostatics that allows for efficient switching of ultra-short organic channels. This affor…
In this paper, the dynamics of bias stress phenomenon in Sexithiophene (T6) Field Effect Transistors (FETs) has been investigated. T6 FETs have been fabricated by vacuum depositing films with thickness from 10 nm to 130 nm on Si/SiO2 substrates. After the T6 film structural analysis by X-Ray diffraction and the FET electrical investigation focused on carrier mobility evaluation, bias stress instability parameters have been estimated and discussed in the context of existing models. By increasing the film thickness, a clear correlation between the stress parameters and the structural properties …
We present photoluminescence studies as a function of temperature from a series of conjugated polymers and a conjugated molecule with distinctly different backbone conformations. The organic materials investigated here are: planar methylated ladder type poly para-phenylene, semi-planar polyfluorene, and non-planar para hexaphenyl. In the longer-chain polymers the photoluminescence transition energies blue shift with increasing temperatures. The conjugated molecules, on the other hand, red shift their transition energies with increasing temperatures. Empirical models that explain the temperatur…
We show that infrared imaging of the charge density profile in organic field-effect transistors (FETs) can probe transport characteristics which are difficult to access by conventional contact-based measurements. Specifically, we carry out experiments and modeling of infrared spectromicroscopy of poly(3-hexylthiophene) (P3HT) FETs in which charge injection is affected by a relatively low resistance of the gate insulators. We conclude that the mobility of P3HT has a power-law density dependence, which is consistent with the activated transport in disorder-induced tails of the density of states.
A printing process for high-resolution transfer of all components for organic electronic devices on plastic substrates has been developed and demonstrated for pentacene (Pn), poly (3-hexylthiophene) and carbon nanotube (CNT) thin-film transistors (TFTs). The nanotransfer printing process allows fabrication of an entire device without exposing any component to incompatible processes and with reduced need for special chemical preparation of transfer or device substrates. Devices on plastic substrates include a Pn TFT with a saturation, field-effect mobility of 0.09 cm^2 (Vs)^-1 and on/off ratio …
The conductivity of organic semiconductors is measured {\it in-situ} and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. The depletion layer thickness can be directly determined as a shift of the threshold thickness at which electric current began to flow. The {\it in-situ} and continuous measurement can also determine qualitatively the accumulation layer thickness together with the distribution function of injected carriers. The accumulation layer thickness is a few mono layers, and it does not depend on gate voltages, rather depends…
In recent years, thin-film organic field-effect transistors (OFETs) have begun to be considered as a possible alternative to the hydrogenated amorphous silicon thin-film transistors (a-Si:H TFT's) used in active matrix flat panel displays and other large-area electronics applications. Low-temperature processability, low-cost fabrication and compatibility with arbitrary substrates are some of the promising advantages of OFETs, among others. Of the many organic materials available, pentacene, in particular, is one of the leading candidates for use in current thin-film OFET architectures - this b…
We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap (trap DOS) of small molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited-current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and…
The surface conductivity is measured by a four-probe technique for pentacene and rubrene single-crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V/decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ~10^12 cm^-2 at the surface of the organic single cryst…
In this paper, two dimensional modulation of the potential in sexithiophene (T6) / N,N-bis(n-octyl)-dicyanoperylenediimide (PDI-8CN2) heterojunction field effect transistors due to the specific microstructure at the interface is used to explain the negative transconductance effect (NTC) experienced in sexithiophene (T6) / N,N-bis(n-octyl)-dicyanoperylenediimide (PDI-8CN2) heterojunction field effect transistors. The NTC effect has been experienced in tunnel devices, such as the tunnel diode, the resonant tunneling field effect transistors (RT-FETs), resonant tunneling double barrier devices. I…
Single-crystal organic field-effect transistors (OFETs) based on p-channel molecular semiconductors have led to breakthrough carrier mobilities and to the observation of band-like transport. These results represent the limit in our quest for the ultimate OFET performance. However, band-like transport has not been reported for n-channel OFETs and, for p-channel transistors, it is not understood why it occurs only for certain molecular materials. Here we report band-like electron transport for n-channel OFETs based on PDIF-CN2 single-crystals. Devices with different gate dielectrics - vacuum, Cy…
We measure the complex impedance between source/drain electrodes and the gate electrode of a pentacene thin-film transistor (TFT) at frequencies 50 Hz < omega/2pi < 20 kHz. Modeling the TFT as a distributed RC network (RC transmission line), we find that the data cannot be explained by a model including only a real, frequency-independent sheet conductivity. Instead, we use the RC transmission line model to extract the frequency-dependent complex sheet conductivity sigma(omega) = sigma'(omega) + jsigma"(omega) of the pentacene film. At high frequencies, sigma(omega) increases with frequency, si…
Synergistic Effect of Multi-Walled Carbon Nanotubes and Ladder-Type Conjugated Polymers on the Performance of N-Type Organic Electrochemical Transistors
We investigate electron transport through single conjugated molecules - including benzenedithiol, oligo-phenylene-ethynylenes of different lengths, and a ferrocene-containing molecule sandwiched between two gold electrodes with different contact structures - by using a single-particle Green function method combined with density functional theory calculation. We focus on the effect of the basis set in the ab initio calculation. It is shown that the position of the Fermi energy in the transport gap is sensitive to the molecule-lead charge transfer which is affected by the size of basis set. This…
Hybrid conjugated polymer/fullerene filaments based on MEH-PPV/PVP/PCBM are prepared by electrospinning, and their properties assessed by scanning electron, atomic and lateral force, tunnelling, and confocal microscopy, as well as by attenuated total reflection Fourier transform-infrared spectroscopy, photoluminescence quantum yield and spatially-resolved fluorescence. Highlighted features include ribbon-shape of the realized fibers, and the persistence of a network serving as a template for heterogeneous active layers in solar cell devices. A set of favorable characteristics is evidenced in t…
The present work is focused on theoretical and experimental study of localised levels in organic materials suitable for light-emitting devices and field effect transistors by means of thermal techniques. In our work we focused on low molecular compounds as well as on polymers, especially of two classes of materials: oxadiazoles and quinoxalines. Both organic compounds are well know as electron transport materials in OLEDs.
Surface confined dehalogenation reactions are versatile bottom-up approaches for the synthesis of carbon-based nanostructures with predefined chemical properties. However, for devices generally requiring low conductivity substrates, potential applications are so far severely hampered by the necessity of a metallic surface to catalyze the reactions. In this work we report the synthesis of ordered arrays of poly(p-phenylene) chains on the surface of semiconducting TiO2(110) via a dehalogenative homocoupling of 4,4"-dibromoterphenyl precursors. The supramolecular phase is clearly distinguished fr…
Bias stress effects in n-channel organic field-effect transistors (OFETs) are investigated using PDIF-CN2 single-crystal devices with Cytop gate dielectric, both under vacuum and in ambient. We find that the amount of bias stress is very small as compared to all (p-channel) OFETs reported in the literature. Stressing the PDIF-CN2 devices by applying 80 V to the gate for up to a week results in a decrease of the source drain current of only ~1% under vacuum and ~10% in air. This remarkable stability of the devices leads to characteristic time constants, extracted by fitting the data with a stre…
Research on conjugated polymers for thermoelectric applications has made tremendous progress in recent years, which is accompanied by surging interest in molecular doping as a means to achieve the high electrical conductivities that are required. A detailed understanding of the complex relationship between the doping process, the structural as well as energetic properties of the polymer films, and the resulting thermoelectric behavior is slowly emerging. This review summarizes recent developments and strategies that permit enhancing the electrical conductivity of p- and n-type conjugated polym…
Two-dimensional atomic crystals are extensively studied in recent years due to their exciting physics and device applications. However, a molecular counterpart, with scalable processability and competitive device performance, is still challenging. Here, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. The crystalline layers are atomically s…
Solution processed polymers are at the core of organic electronics. Polyradicals (polymers in which each repeating unit contains an unpaired spin) are unique alternatives to their p-conjugated, semiconducting counterparts. Unique of polyradicals are tunable charge states localized at their repeating units, which enable electrically switchable charge transport regimes. Tremendous efforts were focused on polyradical memristors and batteries. Notwithstanding recent progress in doping, polyradical field-effect transistors (PR-FETs) have not been reported. Here, we show that vertical architectures,…
We show that the performance of pentacene transistors can be significantly improved by maximizing the interfacial area at single walled carbon nanotube (SWCNT)/pentacene. The interfacial areas are varied by anchoring short SWCNTs of different densities (0-30/μm) to the Pd electrodes. The average mobility is increased three, six and nine times for low, medium and high SWCNT densities, respectively, compared to the devices with zero SWCNT. The current on-off ratio and on-current are increased up to 40 times and 20 times with increasing the SWCNT density. We explain the improved device performanc…
We have used an electro-optic technique to measure the position-dependent infrared absorption of holes injected into a thin crystal of the organic semiconductor, 6,13-bis(triisopropylsilylethynyl)-pentacene incorporated in a field-effect transistor. By applying square-wave voltages of variable frequency to the gate or drain, one can measure the time it takes for charges to accumulate on the surface, and therefore determine their mobility.
Polymer based printable organic thin film transistor (OTFT) is a viable low cost alternative to amorphous silicon based thin film transistors and possesses light-weight and flexibility advantage. In this paper, we report on the hole injecting properties of doped PANI in OLED devices using it as an anode. From these results we conclude that hole doped PANI layers can be used as a low contact resistance source and drain electrode material for polymer OTFTs.
In this paper we present an improved process for producing elastomer transistor stamps and high-mobility organic field-effect transistors (FETs) based on semiconducting acene molecular crystals. In particular, we have removed the need to use a silanized Si wafer for curing the stamps and to handle a fragile micron-thickness polydimethylsiloxane (PDMS) insulating film and laminate it, bubble free, against the PDMS transistor stamp. We find that despite the altered design, rougher PDMS surface, and lamination and measurement of the device in air, we still achieve electrical mobilities of order 1…
Transport properties of a series of poly(3-hexylthiophene) organic field effect transistors with Cr, Cu and Au source/drain electrodes were examined over a broad temperature range. The current-voltage characteristics of the injecting contacts are extracted from the dependence of conductance on channel length. With reasonable parameters, a model of hopping injection into a disordered density of localized states, with emphasis on the primary injection event, agrees well with the field and the temperature dependence of the data over a broad range of temperatures and gate voltages.
Organic printed electronics has proven its potential as an essential enabler for applications related to healthcare, entertainment, energy and distributed intelligent objects. The possibility of exploiting solution-based and direct-writing production schemes further boosts the benefits offered by such technology, facilitating the implementation of cheap, conformable, bio-compatible electronic applications. The result shown in this work challenges the widespread assumption that such class of electronic devices is relegated to low-frequency operation, owing to the limited charge mobility of the …
Excitation energies of light-emitting organic conjugated polymers have been investigated with time-dependent density functional theory (TDDFT) within the adiabatic approximation for the dynamical exchange-correlation potential. Our calculations show that the accuracy of the calculated TDDFT excitation energies largely depends upon the accuracy of the dihedral angle obtained by the geometry optimization on ground-state DFT methods. We find that, when the DFT torsional dihedral angles between two adjacent phenyl rings are close to the experimental dihedral angles, the TDDFT excitation energies a…
Designing molecular organic semiconductors with distinct frontier orbitals is key for the development of devices with desirable properties. Generating defined organic nanostructures with atomic precision can be accomplished by on-surface synthesis. We use this dry chemistry to introduce topological variations in a conjugated poly-para-phenylene chain in the form of meta-junctions. As evidenced by STM and LEED, we produce a macroscopically ordered, monolayer thin zigzag chain film on a vicinal silver crystal. These cross-conjugated nanostructures are expected to display altered electronic prope…
In organic field effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers -that determines the device performance- has been related to the quality of the organic semiconductor. Recently, it has been appreciated that also the nearby dielectric has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms we have studied transport through organic single crystal FETs with different gate insul…
Electronic transport through rubrene single-crystal field effect transistors (FETs) is investigated experimentally in the high carrier density regime (n ~ 0.1 carrier/molecule). In this regime, we find that the current does not increase linearly with the density of charge carriers, and tends to saturate. At the same time, the activation energy for transport unexpectedly increases with increasing n. We perform a theoretical analysis in terms of a well-defined microscopic model for interacting Frohlich polarons, that quantitatively accounts for our experimental observations. This work is particu…
Observation of Anderson localization transitions in a two-dimensional conjugated metal-organic framework
Tianjin University（天津大学）；Key Laboratory of Organic Integrated Circuits, Ministry of Education（教育部有机集成电路重点实验室）；Tianjin Key Laboratory of Molecular Optoelectronic Sciences（天津市分子光电科学重点实验室）；Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)（（天津）化工过程与先进制造国家重点实验室协同创新中心）；Center for Joint Quantum Studies（联合量子研究中心）；Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology（天津市低维材料物理与制备技术重点实验室）
The organic electrochemical transistor (OECT) with a conjugated polymer as the active material is the elementary unit of organic bioelectronic devices. Increased functionalities, such as low power consumption, can be achieved by building complementary circuits featuring two or more OECTs. Complementary circuits commonly combine both p- and n-type transistors to reduce power draw. While p-type OECTs are readily available, n-type OECTs are less common mainly due to poor stability of the n-type active channel material in aqueous electrolyte. Additionally, an OECT based complementary circuit requi…
Role of structure and charge trapping on the bipolaron formation and magnetic-field response of gated conjugated polymers
Institut national de la recherche scientifique (INRS)（国家科学研究学院）；University of Mons（蒙斯大学）；Institute of Chemistry and Processes for Energy Environment and Health (ICPEES), University of Strasbourg, CNRS（斯特拉斯堡大学化学、能源、环境与健康过程研究所（CNRS））；Namur Institute of Structured Matter, Université de Namur（纳慕尔结构物质研究所，纳慕尔大学）
