Polymer-based Organic Field-effect Transistors Research Articles

Revealing two-dimensional electric field crowding effect in breakdown performance of DPPT-TT polymer-based OFETs

The diketopyrrolopyrrole-based polymer (DPPT-TT) has been employed in organic power field effect transistors due to its exceptional off-state breakdown performance. The impact of organic semiconductor layer thickness on the breakdown performance has not been explored. In this study, we investigate the impact of DPPT-TT layer thickness on the breakdown voltage (BV) by fabricating organic field effect transistors (OFETs) with various DPPT-TT layer thicknesses. Our findings reveal that the devices' BV is a strong function of DPPT-TT layer thickness, and reducing the DPPT-TT layer thickness from 68 to 15 nm results in a decrease in BV from 291 to 86 V, attributed to the two-dimensional (2D) electric field crowding effect. An analytical model utilizing the 2D Poisson equation reveals an electric field at the DPPT-TT layer's surface. Thinner DPPT-TT layer exhibits larger electric field peak, leading to premature breakdown near the drain electrode. The relationship between breakdown electric field and DPPT-TT layer thickness was established by fitting the experimental data to the model, revealing an average BV error of only 8.8%. This phenomenon is validated to be ubiquitous in polymer based OFETs via DPPT-TT-based and P3HT-based devices. According to the proposed model, this 2D electric field crowding effect can be mitigated by adjusting the dielectric layer thickness (tD) and/or the dielectric material.

Toward Efficient Charge Transport of Polymer-Based Organic Field-Effect Transistors: Molecular Design, Processing, and Functional Utilization

ConspectusPolymer-based organic field-effect transistors (OFETs) have attracted great attention owing to their significantly improved performance and the recently emerged prospects for broad applications. The charge carrier mobility of polymer-based OFETs has been improved from 10–5 to more than 10 cm2 V–1 s–1 over the past three decades. With the carrier mobility close to or higher than that of amorphous silicon, the application fields of polymer-based OFETs have been extended from sensing to logic or drive circuits. During OFET operation, the charge carriers are successively injected in and then transferred across the semiconducting channel under an external electric field. The behavior of the charge carriers is determined by both charge injection and charge transport. Well-matched frontier molecular orbital (FMO) energy levels and optimized hierarchical structures of semiconducting polymers are essential for the realization of high-performance polymer-based OFETs.In this Account, we mainly demonstrate our recent progress in molecular design strategies and fabrication processing methods for high-performance semiconducting polymers. Moreover, we provide some examples of integrated applications of polymer-based OFETs. The convenient FMO energy level modulation and excellent molecular designability of donor–acceptor (D–A) conjugated polymers facilitate studies on their performance enhancement in OFETs. Proper molecular design of the D–A polymer backbone and side chain can significantly contribute to charge injection enhancement, transport pathway establishment, and trap reduction, thereby resulting in efficient charge transport. Moreover, special fabrication processes of OFET devices can further enhance the charge transport by optimizing the hierarchical structures of semiconducting polymers, thereby enhancing their carrier mobility and stability. By employing high-performance semiconducting polymers as the semiconducting channel of OFETs, various applications can be implemented. These applications range from small-scale logic signal processing and periodic signal generation to complex visual perception systems, indicating broad application prospects in human life. The works on performance improvement and functional utilization of the semiconducting polymers in OFETs might provide insights for molecular design strategies and applications for future plastic electronics.

Effect of post-deposition annealing temperature on the charge carrier mobility and morphology of DPPDTT based organic field effect transistors

There is a considerable research interest in enhancing the charge carrier mobility of polymer-based organic field effect transistors (OFETs) through processing techniques. Herein, we investigate the effect of post-deposition annealing temperature on morphology and mobility of solution processible polymer Poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)], (DPPDTT) based OFETs. The resulting change in the grain size is proportional to the mobility data. At temperatures higher than 100 °C, grain size decreased with obvious desorption due to low Van der Waals forces in the polymer. Our results will hence contribute towards understanding the annealing mechanism in polymers to optimize the performance of DPPDTT based devices.

Design and Synthesis of Air-Stable p-Channel-Conjugated Polymers for High Signal-to-Drift Nitrogen Dioxide and Ammonia Sensing.

The development of high-performance-conjugated polymer-based gas sensors involves detailed structural tailoring such that high sensitivities are achieved without compromising the stability of the fabricated devices. In this work, we systematically developed a series of diketopyrrolopyrrole (DPP)-based polymer semiconductors by modifying the polymer backbone to achieve and rationalize enhancements in gas sensitivities and electronic stability in air. NO2- and NH3-responsive polymer-based organic field-effect transistors (OFETs) are described with improved air stability compared to all-thiophene conjugated polymers. Five DPP-fluorene-based polymers were synthesized and compared to two control polymers and used as active layers to detect a concentration of NO2 at least as low as 0.5 ppm. The hypothesis that the less electron-donating fluorene main-chain subunit would lead to increased signal/drift compared to thiophene and carbazole subunits was tested. The sensitivities exhibited a bias voltage-dependent behavior. The proportional on-current change of OFETs using a dithienyl DPP-fluorene polymer reached ∼614% for an exposure to 20 ppm of NO2 for 5 min, testing at a bias voltage of -33 V, among the higher reported NO2 sensitivities for conjugated polymers. Electronic and morphological studies reveal that introduction of the fluorene unit in the DPP backbone decreases the ease of backbone oxidation and induces traps in the thin films. The combination of thin-film morphology and oxidation potentials governs the gas-absorbing properties of these materials. The ratio of responses on exposure to NO2 and NH3 compared to drifts while taking the device through repeated gate voltage sweeps is the highest for two polymers incorporating electron-donating linkers connecting the DPP and thiophene units in the backbone, in this category of organic semiconductors. The responses to NO2 were much larger than that to NH3, indicating increased susceptibility to oxidizing vs reducing gases, and that the capability of oxidizing gases to induce additional charge density has a more dramatic electronic effect than when reducing gases create traps. This work demonstrates the capability of achieving improved stability with the retention of high sensitivity in conjugated polymer-based OFET sensors by modulating redox and morphological properties of polymer semiconductors by structural control.

In Situ Observation of Alignment Templating by Seed Crystals in Highly Anisotropic Polymer Transistors.

Due to the highly directional nature of transport in polymer-based organic field-effect transistors (OFETs), alignment of the polymer backbone can significantly affect device performance. While many methods of alignment have been detailed, the mechanism of alignment is rarely revealed-especially in cases of flow-induced alignment. Polymer aggregates are often observed in highly aligned systems, but their role is similarly unclear. Here, we present a comprehensive characterization of blade-coated P(NDI2OD-T2) (N2200) for OFET applications, including a rigorous, multimodal characterization of its in-plane alignment. Film thickness follows the expected power-law dependence on coating speed, while bulk polymer backbone orientation transitions from perpendicular to parallel to the coating direction as speed is increased. Charge carrier mobility >2 cm2/(V s) is achieved parallel to the coating direction for aligned N2200 coated at 5 mm/s and is found to be strongly correlated with the in-plane alignment of the fibrillar morphology at the film's surface, characterized with atomic force microscopy and near-edge X-ray absorption. We develop a model of N2200 crystal anisotropy through rotational scans of grazing incidence wide-angle X-ray scattering (GIWAXS) and use it to analyze simultaneous in situ GIWAXS and UV-vis reflectance data from polymer solutions coated at 5 mm/s. A small population of crystals align early in the drying process, but bulk alignment occurs very late in the drying process, likely mediated by a lyotropic liquid crystal phase transition templated by the aligned crystals. Our characterization also suggests that the majority of material in N2200 thin films is noncrystalline at these conditions.

Implications of doping and depletion on the switching characteristics in polymer-based organic field-effect transistors

Abstract An effective approach for tuning the electronic characteristics of polymer-based organic field-effect transistors (OFETs) consisted of poly(3-hexylthiophene) (P3HT) doped with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and depleting layer formed by aluminum (Al) coating is being reported. Doping of P3HT was employed to accumulate and tune the carrier concentration while implementation of depletion layer led to reduce the channel conductance. It has been demonstrated that on-currents in the OFETs were enhanced with the extent of TCNQ doping from 0% to 20% in the P3HT. Upon introduction of an ultra-thin Al led to the formation of depletion region resulting in to further decrease in the off-state current along with the improvement in the subthreshold characteristics. These two procedures provide counter-effects in terms of the on-state characteristics by doping and the off-state characteristics by depletion, which promotes switching performance in OFETs indicating the potentiality of doping technology for organic electronics.

Synthesis and characterization of thieno-isoindigo derivative-based near-infrared conjugated polymer for ambipolar field-effect transistors and photothermal conversion

A new thieno-isoindigo derivative, (3E,7E)-3,7-bis(4-(2-decyltetradecyl)-4H-thieno[3,2-b]pyrrole-5,6-dione)-5,7-dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione (BTPDI), was designed and synthesized. A donor−acceptor conjugated polymer (PBTPDI-TT) was also synthesized with this new unit as the acceptor and thieno[3,2-b]thiophene as the donor. The microstructure, photophysical, electrochemical, field-effect properties and photothermal performances were investigated. The polymer showed a broad absorption spectrum that spanned across the near-infrared (NIR) region (780–1300 nm), with a very low bandgap (∼0.95 eV), deep lowest unoccupied molecular orbital, and suitable highest occupied molecular orbital levels. As a result, the polymer-based organic field-effect transistors showed highly balanced hole and electron transport characteristics with a hole mobility of 0.027 cm2 V−1s−1 and an electron mobility of 0.022 cm2 V−1s−1. The polymer nanoparticles showed good reusability under NIR (980 nm) irradiation and could also effectively convert NIR light to heat at an excellent efficiency of 20.3%.

Sensitive and Selective NO2 Sensing Based on Alkyl- and Alkylthio-Thiophene Polymer Conductance and Conductance Ratio Changes from Differential Chemical Doping.

NO2-responsive polymer-based organic field-effect transistors (OFETs) are described, and room-temperature detection with high sensitivity entirely from the semiconductor was achieved. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecylthioquaterthiophene) (PQT12 and PQTS12, respectively), were used as active layers to detect a concentration at least as low as 1 ppm of NO2. The proportional on-current change of OFETs using these polymers reached over 400% for PQTS12, which is among the highest sensitivities reported for a NO2-responsive device based on an organic semiconducting film. From measurements of cyclic voltammetry and the electronic characteristics, we found that the introduction of sulfurs into the side chains induces traps in films of the PQTS12 and also decreases domain sizes, both of which could contribute to the higher sensitivity of PQTS12 to NO2 gas compared with PQT12. The ratio of responses of PQTS12 and PQT12 is higher for exposures to lower concentrations, making this parameter a means of distinguishing responses to low concentrations for extended times from exposures to high concentrations from shorter times. The responses to nonoxidizing vapors were much lower, indicating good selectivity to NO2 of two polymers. This work demonstrates the capability of increasing selectivity and calibration of OFET sensors by modulating redox and aggregation properties of polymer semiconductors.

Fabrication of polymer-based transistors with source–drain electrodes made of carbon nanotubes and silver nanoparticles by soft lithography techniques

In this study, we have developed multilayer deposition and patterning processes with a resolution of 1μm that can be used to fabricate all-printed, polymer-based organic field-effect transistors (p-OFETs) on the basis of vacuum-free, solution-processable soft lithography techniques. We have used regioregular poly(3-hexylthiophene) (P3HT) as a soluble polymer semiconductor and poly(methylmethacrylate) as soluble polymer gate insulators. We have compared the electrical properties of p-OFETs with multi-walled carbon nanotubes (MWNTs), silver nanoparticles (NPs), and their composites as printed source–drain (S–D) electrodes in order to fabricate vacuum-free, all-printed p-OFETs. The p-OFETs with MWNT S–D electrodes exhibited higher hole mobility and on/off ratios than the devices with Ag NP S–D electrodes owing to better contact at the MWNT/P3HT interface. The high sheet resistance of MWNT electrodes was considerably reduced by mixing with Ag NPs. Both the contact resistance and channel resistance were maintained at low values, resulting in improved electrical properties of p-OFETs.

Fabrication of Polymer-Based Transistors with Carbon Nanotube Source Drain Electrodes Using Softlithography Techniques

In this study, we have developed the multilayer deposition and patterning processes with a resolution of 1 µm for fabricating polymer-based organic field effect transistors (p-OFETs) based on vacuum-free, solution processable softlithography techniques. We have used regioregular poly(3-hexylthiophene) (P3HT) as the soluble polymer semiconductor, and poly(methyl methacrylate) (PMMA) and polyimide as the soluble and insoluble polymer gate insulators, respectively. We have used multiwalled carbon nanotubes (MWNTs) as the printed source–drain (S–D) electrodes in order to fabricate vacuum-free, all printed OFETs. The p-OFETs with MWNT S–D electrodes exhibit higher hole mobility and on/off ratio than the devices with vacuum-evaporated Au electrodes, probably owing to the better contact of the electrode interface and damage-free transfer of electrodes onto the gate insulator. The mobility was further improved by the crystallization of the P3HT film after heat treatment prior to the pattern transfer of P3HT.

Organic field-effect transistors based on a crosslinkable polymer blend as the semiconducting layer

For fabrication of top-gate polymer-based organic field-effect transistors (OFETs), it is essential that the semiconducting layer remain intact during spin coating of the overlying dielectric layer. This requirement severely limits the applicable solvent and materials combinations. We show here that a crosslinkable polymer blend consisting of a p-type semiconducting polymer {e.g., TFB; poly[9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine]} and an electroactive crosslinkable silyl reagent {e.g., TPDSi2; 4,4′-bis[(p-trichloro-silylpropylphenyl)phenylamino]biphenyl} is effective as the semiconducting layer in a top-gate bottom-contact OFET device. The TFB+TPDSi2 semiconducting blend is prepared by spin-coating in ambient. The crosslinking process occurs during spin-coating in air and is completed by curing at 90 °C, which renders the resulting film insoluble in common organic solvents and allows subsequent deposition of dielectric layers from a wide range of organic solvents. We also show that the presence of TPDSi2 in the semiconductor layer significantly reduces typical TFB-source-drain threshold voltages in bottom-contact devices, likely due to favorable interfacial TPDSi2-gold electrode interactions.