Conjugated Polymer Field-effect Transistors Research Articles

Solid-State Ionic Liquid Additive Enhances Mobility in Conjugated Polymer Field-Effect Transistors

Solid-State Ionic Liquid Additive Enhances Mobility in Conjugated Polymer Field-Effect Transistors

Achieving Ideal and Environmentally Stable n-Type Charge Transport in Polymer Field-Effect Transistors.

Realizing ideal charge transport in field-effect transistors (FETs) of conjugated polymers is crucial for evaluating device performance, such as carrier mobility and practical applications of conjugated polymers. However, the current FETs using conjugated polymers as the active layers generally show certain non-ideal transport characteristics and poor stability. Here, ideal charge transport of n-type polymer FETs is achieved on flexible polyimide substrates by using an organic-inorganic hybrid double-layer dielectric. Deposited conjugated polymer films show highly ordered structures and low disorder, which are supported by grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure, and molecular dynamics simulations. Furthermore, the organic-inorganic hybrid double-layer dielectric provides low interfacial defects, leading to excellent charge transport in FETs with high electron mobility (1.49 ± 0.46 cm2V-1s-1) and ideal reliability factors (102 ± 7%). Fabricated polymer FETs show a self-encapsulation effect, resulting in high stability of the FET charge transport. The polymer FETs still work with high mobility above 1 cm2V-1s-1 after storage in air for more than 300 days. Compared with state-of-the-art conjugated polymer FETs, this work simultaneously achieves ideal charge transport and environmental stability in n-type polymer FETs, facilitating rapid device optimization of high-performance polymer electronics.

Improving OFF-State Bias-Stress Stability in High-Mobility Conjugated Polymer Transistors with an Antisolvent Treatment.

Conjugated polymer field-effect transistors are emerging as an enabling technology for flexible electronics due to their excellent mechanical properties combined with sufficiently high charge-carrier mobilities and compatibility with large-area, low-temperature processing. However, their electrical stability remains a concern. ON-state (accumulation mode) bias-stress instabilities in organic semiconductors have been widely studied, and multiple mitigation strategies have been suggested. In contrast, OFF-state (depletion mode) bias-stress instabilities remain poorly understood despite being crucial for many applications in which the transistors are held in their OFF-state for most of the time. Here, a simple method of using an antisolvent treatment is presented to achieve significant improvements in OFF-state bias-stress and environmental stability as well as general device performance for one of the best performing polymers, solution-processable indacenodithiophene-co-benzothiadiazole (IDT-BT). IDT-BT is weakly crystalline, and the notable improvements to an antisolvent-induced, increased degree of crystallinity, resulting in a lower probability of electron trapping and the removal of charge traps is attributed. The work highlights the importance of the microstructure in weakly crystalline polymer films and offers a simple processing strategy for achieving the reliability required for applications in flexible electronics.

Fluorination-Induced Charge Trapping and Operational Instability in Conjugated-Polymer Field-Effect Transistors.

Fluorination of a conjugated polymer backbone is an effective strategy to control the microstructure and electronic structure of a conjugated polymer. Although fluorination has been widely reported to increase charge carrier mobility, its effect on the operational stability of electronic devices has not been extensively investigated. Here, the effect of fluorination of a conjugated polymer backbone on charge trapping and the operational stability of organic field-effect transistors is investigated. The results show that the device based on a fluorinated conjugated polymer exhibits relatively poor operational stability despite its greater charge carrier mobility compared with that in the device based on its nonfluorinated polymer counterpart. Experimental results reveal that the low stability originates from the greater degree of shallow trapping of charge carriers within the fluorinated polymer thin film and that the shallow trapping is closely related to the presence of minority charge carriers. A mechanism of charge trapping is proposed.

The effect of the dielectric end groups on the positive bias stress stability of N2200 organic field effect transistors

Bias stress degradation in conjugated polymer field-effect transistors is a fundamental problem in these disordered materials and can be traced back to interactions of the material with environmental species,1,2,3 as well as fabrication-induced defects.4,5 However, the effect of the end groups of the polymer gate dielectric and the associated dipole-induced disorder on bias stress stability has not been studied so far in high-performing n-type materials, such as N2200.6,7 In this work, the performance metrics of N2200 transistors are examined with respect to dielectrics with different end groups (Cytop-M and Cytop-S8). We hypothesize that the polar end groups would lead to increased dipole-induced disorder, and worse performance.1,9,10 The long-time annealing scheme at lower temperatures used in the paper is assumed to lead to better crystallization by allowing the crystalline domains to reorganize in the presence of the solvent.11 It is hypothesized that the higher crystallinity could narrow down the range at which energy carriers are induced and thus decrease the gate dependence of the mobility. The results show that the dielectric end groups do not influence the bias stress stability of N2200 transistors. However, long annealing times result in a dramatic improvement in bias stress stability, with the most stable devices having a mobility that is only weakly dependent on or independent of gate voltage.

Alignment of linear polymeric grains for highly stable N-type thin-film transistors

Summary Temperature-insensitive properties are attractive for most electronics, including polymeric semiconducting devices. Especially, polymeric field-effect transistors (FETs) with high mobility have been important research targets due to their broad applications. However, polymeric FETs with stable charge transport operating at extremely cold or hot zones are faced with enormous challenges. In this study, the polyacrylonitrile was found to significantly tune the sizes of pre-aggregates of polymers in solutions and the crystallinity of the polymeric films. The orientation of 5–25 μm linear grains in the films were prepared through the bar-coating process with polyacrylonitrile as an additive, which stabilized the electron mobility over a wide range of temperatures. The linear-grain morphology of the film contributed to reducing the holes and grain boundaries in the transport paths of carriers. Typically, the top-gate FETs based on P(NDI2OD-T2) displayed a stable electron transporting behavior from 200 to 460 K, with mobility greater than 3.5 cm2V−1s−1.

Utilizing the Diffusion of Fluorinated Polymers to Modify the Semiconductor/Dielectric Interface in Solution-Processed Conjugated Polymer Field-Effect Transistors.

It has been demonstrated that tailoring the properties of semiconductor/dielectric interfaces with fluorinated polymers yields better performance for organic field-effect transistors (OFETs). However, it remains a challenge to fabricate bottom-gate OFET devices on fluorinated dielectrics using solution-processed methods due to the poor wettability of fluorinated dielectrics. Here, we utilized the diffusion of fluorinated poly(methyl methacrylate) (PMMA) to construct the fluorine-rich semiconductor/dielectric interface to achieve the fabrication of bottom-gate OFETs with a solution-processed poly(3-hexylthiophene) (P3HT) semiconductor layer. The consequences indicate that the fluorinated dielectrics can effectively decrease the charge traps density at the semiconductor/dielectric interface and promote the edge-on orientation of P3HT on the dielectric surface. Thus, the devices based on fluorinated PMMA modified dielectrics exhibit higher carrier mobility and electrical stability than those of the fluorine-free devices. Our investigation affords a new strategy for the design and interface optimization of devices, which may further advance the performance of OFET devices.

Field-Effect Charge Transport in Doped Polymer Semiconductor-Insulator Alternating Bulk Junctions with Ultrathin Transport Layers.

Conjugated-polymer field-effect transistors are attractive for flexible electronics. However, relatively high chemical doping (oxidation) concentration of p-type polymer semiconductors is usually not compatible with good transistor performance, due to poor switching-off capability and short-channel performance. Here, we propose a combined simulation and experimental investigation on charge transport in a semiconductor-insulator alternating bulk junction composed of repeating semiconductor and insulator regions, which shows better transistor performance at higher doping levels, as compared with traditional planar transistors. Moreover, the doped semiconductor transport layers in the junction should be less than 2 nm thick to ensure sufficient pinch-off capability. Using some semiconductors including poly(3-hexylthiophene), we utilize a fast solvent evaporation approach to obtain semiconductor-insulator alternating bulk junctions with ultrathin (thickness < 2 nm) semiconductor crystallites and with vertical gradients of both morphology and electronic properties. Doping with a concentration of up to 1019 cm-3 simultaneously induces the improvement of field-effect mobility, on/off ratio, and subthreshold swing, which leads to long-term (>1 year) stability, without lowering the short-channel performance. Moreover, these heterojunctions are optically transparent, nearly colorless, and flexible, thus could be exploited for wide electronic and photonic applications.

Performance Improvements in Conjugated Polymer Devices by Removal of Water-Induced Traps.

The exploration of a wide range of molecular structures has led to the development of high-performance conjugated polymer semiconductors for flexible electronic applications including displays, sensors, and logic circuits. Nevertheless, many conjugated polymer field-effect transistors (OFETs) exhibit nonideal device characteristics and device instabilities rendering them unfit for industrial applications. These often do not originate in the material's intrinsic molecular structure, but rather in external trap states caused by chemical impurities or environmental species such as water. Here, a highly efficient mechanism is demonstrated for the removal of water-induced traps that are omnipresent in conjugated polymer devices even when processed in inert environments; the underlying mechanism is shown, by which small-molecular additives with water-binding nitrile groups or alternatively water-solvent azeotropes are capable of removing water-induced traps leading to a significant improvement in OFET performance. It is also shown how certain polymer structures containing strong hydrogen accepting groups will suffer from poor performances due to their high susceptibility to interact with water molecules; this allows the design guidelines for a next generation of stable, high-performing conjugated polymers to be set forth.

High operational and environmental stability of high-mobility conjugated polymer field-effect transistors through the use of molecular additives.

Due to their low-temperature processing properties and inherent mechanical flexibility, conjugated polymer field-effect transistors (FETs) are promising candidates for enabling flexible electronic circuits and displays. Much progress has been made on materials performance; however, there remain significant concerns about operational and environmental stability, particularly in the context of applications that require a very high level of threshold voltage stability, such as active-matrix addressing of organic light-emitting diode displays. Here, we investigate the physical mechanisms behind operational and environmental degradation of high-mobility, p-type polymer FETs and demonstrate an effective route to improve device stability. We show that water incorporated in nanometre-sized voids within the polymer microstructure is the key factor in charge trapping and device degradation. By inserting molecular additives that displace water from these voids, it is possible to increase the stability as well as uniformity to a high level sufficient for demanding industrial applications.

Remarkable enhancement of charge carrier mobility of conjugated polymer field-effect transistors upon incorporating an ionic additive.

Organic semiconductors with high charge carrier mobilities are crucial for flexible electronic applications. Apart from designing new conjugated frameworks, different strategies have been explored to increase charge carrier mobilities. We report a new and simple approach to enhancing the charge carrier mobility of DPP-thieno[3,2-b]thiophene-conjugated polymer by incorporating an ionic additive, tetramethylammonium iodide, without extra treatments into the polymer. The resulting thin films exhibit a very high hole mobility, which is higher by a factor of 24 than that of thin films without the ionic additive under the same conditions. On the basis of spectroscopic grazing incidence wide-angle x-ray scattering and atomic force microscopy studies as well as theoretical calculations, the remarkable enhancement of charge mobility upon addition of tetramethylammonium iodide is attributed primarily to an inhibition of the torsion of the alkyl side chains by the presence of the ionic species, facilitating a more ordered lamellar packing of the alkyl side chains and interchain π-π interactions.

Electroluminescence in Ion-Gel Gated Conjugated Polymer Field-Effect Transistors

We report electroluminescence from ion-gel gated, field-effect transistors based on the conjugated polymer, poly(9,9′-dioctylfluorene-co-benzothiadiazole) gated by an 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide/poly (styrene-block-ethylene oxide-block-styrene) ion gel, and investigate the mechanism for light emission. The devices emit light from near the electron-injecting drain electrode when the drain source voltage exceeds approximately the energy gap of the polymer (Vds> Eg/e). Charge accumulation spectroscopy is used to demonstrate the significant penetration of the negative TFSI– ions into the F8BT assisted by the application of negative gate voltages, where they lead to significant p-type doping of the bulk of the F8BT film. In contrast, no evidence for diffusion of positive ions with positive gate voltages is observed, and this is consistent with the location of the recombination zone in the proximity of electron injecting electrode and the absence of a comparable electron curre...

Origin of the different transport properties of electron and hole polarons in an ambipolar polyselenophene-based conjugated polymer

Understanding the mechanisms limiting ambipolar transport in conjugated polymer field-effect transistors (FETs) is of both fundamental and practical interest. Here, we present a systematic study comparing hole and electron charge transport in an ambipolar conjugated polymer, semicrystalline poly(3,3${}^{\ensuremath{'}\ensuremath{'}}$-di-$n$-decylterselenophene) (PSSS). Starting from a detailed analysis of the device characteristics and temperature/charge-density dependence of the mobility, we interpret the difference between hole and electron transport through both the Vissenberg-Matters and the mobility-edge model. To obtain microscopic insight into the quantum mechanical wave function of the charges at a molecular level, we combine charge modulation spectroscopy (CMS) measuring the charge-induced absorption signatures from positive and negative polarons in these ambipolar FETs with corresponding density functional theory (DFT) calculations. We observe a significantly higher switch-on voltage for electrons than for holes due to deep electron trap states, but also a higher activation energy of the mobility for mobile electrons. The CMS spectra reveal that the electrons that remain mobile and contribute to the FET current have a wave function that is more localized onto a single polymer chain than that of holes, which is extended over several polymer chains. We interpret this as evidence that the transport properties of the mobile electrons in PSSS are still affected by the presence of deep electron traps. The more localized electron state could be due to the mobile electrons interacting with shallow trap states in the vicinity of a chemical, potentially water-related, impurity that might precede the capture of the electron into a deeply trapped state.

Electron transport along semiconducting polymer heterojunctions

Most conjugated polymer field-effect transistors rely on charge accumulation between an organic semiconductor and a gate dielectric. Much less use has been made of devices based on heterojunctions between two semiconducting polymers. Here we have realized ambipolar polymer field-effect transistors based on bilayers of two polyfluorene-based semiconducting polymers, poly(9,9-dioctylfluorene- alt- N-(4-butylphenyl)diphenylamine) and poly(9,9-dioctylfluorene- alt-benzothiadiazole). We show that the energy offset between the lowest unoccupied molecular orbitals of the two polymers of 1–1.5 eV is sufficient to confine electrons at the bi-layer interface for n-type operation. However, the transfer current–voltage characteristics exhibit an unusual plateau. This is shown to be due to hole charge trapping in donor-like states in the TFB layer during p-type operation.

Exciton dissociation in polymer field-effect transistors studied using terahertz spectroscopy

We have used terahertz time-domain spectroscopy to investigate photoinduced charge generation in conjugated polymer field-effect transistors. Our measurements show that excitons dissociate in the accumulation layer under the application of a gate voltage, with a quantum efficiency of $\ensuremath{\sim}0.1$ for an average gate field of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{8}\phantom{\rule{0.3em}{0ex}}\mathrm{V}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$. The transistor history is found to affect the exciton dissociation efficiency, which decreases as holes are increasingly trapped in the accumulation layer. The quantum efficiency of charge formation from excitons is compared with the two contrasting models proposed by Onsager and Arkhipov based on the assumption that field-induced exciton dissociation is assisted by the Brownian diffusive motion or an initial excess energy supplied by excited vibrational modes, respectively.

Relative importance of polaron activation and disorder on charge transport in high-mobility conjugated polymer field-effect transistors

The charge transport properties of conjugated polymer semiconductors are governed by strong electron phonon coupling, leading to polaron formation as well as the presence of structural and electronic disorder. However, the relative contribution which polaronic relaxation and disorder broadening make to the temperature activation of the mobility of these materials is not well understood. Here we present a combined study of the temperature and concentration dependences of the field-effect mobility and the optically induced electron-transfer transitions of a series of poly(3-hexylthiohene) field-effect transistors of different molecular weight. We apply a vibronic coupling model to extract the reorganization energy and the strength of electronic coupling from the optical spectra. We observe a transition from a localized to a delocalized transport regime as a function of molecular weight and crystalline quality. Polaron activation is comparable to disorder-induced activation in the low-mobility regime $[\ensuremath{\sim}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕(\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s})]$ and needs to be taken into account when interpreting the field-effect mobility, while disorder becomes the dominant mechanism to limit charge transport in the high-mobility regime with mobilities $g{10}^{\ensuremath{-}2}--{10}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕(\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s})$.

Analysis of the contact resistance in staggered, top-gate organic field-effect transistors

Contact resistance effects are significant in many organic field-effect transistors. Here, we present a detailed analysis of the contact resistance in staggered, top-gate conjugated polymer field-effect transistors. A compact physical model based on the current crowding formalism has been developed. It includes gate modulation of the bulk resistivity of the semiconductor to explain the experimentally observed gate voltage dependence of the contact resistance for different thicknesses of the semiconducting film. The contact resistance is found to be Ohmic. For thick semiconducting films, we have observed a significant asymmetry between source and drain contact resistances with the drain resistances increasing more rapidly with thickness than the source resistance, reflecting the importance of diffusion at the drain contact.

Device modeling of light-emitting ambipolar organic semiconductor field-effect transistors

Recent experiments have demonstrated ambipolar channel conduction and light emission in conjugated polymer field-effect transistors (FETs). The devices have source∕drain contacts fabricated using metals with different work functions. Negative charge carriers are injected from a low work-function metal contact and positive charge carriers from a high work-function contact. In the ambipolar mode of operation, the gate potential lies between the potentials of the electron and hole injecting contacts, so that electrons dominate the channel conductance near the electron injecting contact and holes dominate channel conductance near the hole injecting contact. The injected charge carriers propagate along the FET channel and recombine in regions where both types of carriers are present. The location and intensity of maximum recombination and light emission is controlled by the voltages applied to the transistor terminals. In this paper a device model for ambipolar organic field-effect transistors based on the gradual channel approximation is presented. The model includes the effect of charge carrier trapping through density dependent mobilities. The resulting nonlinear differential equation for the channel potential is solved numerically. The results of the device model are in good agreement with the published experimental data.

Analytic device model for light-emitting ambipolar organic semiconductor field-effect transistors

Recent experiments have demonstrated ambipolar channel conduction and light emission in conjugated polymer field-effect transistors (FETs). Electrons and holes are injected from metal source and drain contacts with different work functions, propagate through the FET channel, and recombine emitting light. The position of maximum light emission is varied by changing the voltages applied to the transistor terminals. Here, we present an analytic device model for ambipolar organic field-effect transistors, based on the gradual channel approximation. Trapping of the injected carriers is found to be important. The model results are in good agreement with the experimental observations.

Mobility enhancement in conjugated polymer field-effect transistors through chain alignment in a liquid-crystalline phase

A method is demonstrated by which liquid-crystalline self-organization in rigid-rod nematic conjugated polymers can be used to control the microstructure of the active semiconducting layer in solution-processed polymer thin-film transistors (TFTs). Enhanced charge carrier mobilities of 0.01–0.02 cm2/V s and good operating stability have been achieved in polyfluorene copolymer TFTs by preparing the polymer in a nematic glassy state and by aligning the polymer chains parallel to the transport direction with the help of an alignment layer. Mobility anisotropies of 5–8 for current flow parallel and perpendicular to the alignment direction have been observed that are of the same order of magnitude as optical dichroic ratios.