Stable Organic Field-effect Transistors Research Articles

Ambient stable solution-processed organic field effect transistors from electron deficient planar aromatics: effect of end-groups on ambient stability.

Ambient stable solution processed n-channel organic field effect transistors (OFETs) are essential for next-generation low-cost organic electronic devices. Several molecular features, such as suitable orbital energy levels, easy synthetic steps, etc., must be considered while designing efficient active layer materials. Here, we report a case of improved ambient stability of solution-processed n-type OFETs upon suitable end-groups substitution of the active layer materials. A pair of core-substituted napthalenediimide (NDIFCN2 and EHNDICN2) derivatives with alkyl and perfluorinated end groups are considered. The transistor devices made out of these two derivatives exhibited largely different ambient stability behavior. The superior device stability (more than 25 days under ambient conditions) of one of the derivatives (NDIFCN2) was ascribed to the presence of fluorinated end groups that function as hydrophobic guard units inhibiting moisture infiltration into the active layer, thereby achieving ambient stability under humid conditions (>65% relative atmospheric humidity). Molecular level optical and electrochemical properties, thermal stability, and the solution-processed (spin coat and drop cast active layers) device characteristics are described in detail. Our findings highlight the requirement of hydrophobic end groups or sidechains for ambient stability of active layer materials, along with deep LUMO levels for ambient stability.

Enhancement of Stability in n-Channel OFETs by Modulating Polymeric Dielectric.

In this study, a high-K material, aluminum oxide (AlOx), as the dielectric of organic field-effect transistors (OFETs) was used to reduce the threshold and operating voltages, while focusing on achieving high-electrical-stability OFETs and retention in OFET-based memory devices. To achieve this, we modified the gate dielectric of OFETs using polyimide (PI) with different solid contents to tune the properties and reduce the trap state density of the gate dielectric, leading to controllable stability in the N, N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13)-based OFETs. Thus, gate field-induced stress can be compensated for by the carriers accumulated due to the dipole field created by electric dipoles within the PI layer, thereby improving the OFET's performance and stability. Moreover, if the OFET is modified by PI with different solid contents, it can operate more stably under fixed gate bias stress over time than the device with AlOx as the dielectric layer only can. Furthermore, the OFET-based memory devices with PI film showed good memory retention and durability. In summary, we successfully fabricated a low-voltage operating and stable OFET and an organic memory device in which the memory window has potential for industrial production.

Operationally Stable Ultrathin Organic Field Effect Transistors Based on Siloxane Dimers of Benzothieno[3,2‐b][1]Benzothiophene Suitable for Ethanethiol Detection

AbstractUltrathin organic field effect transistors (OFETs) demonstrate great potential as highly sensitive gas sensors since its electrical performance strongly depends on the environment. However, fabrication of high performance OFETs with reliable operational stability for continuous measurements by fast, rather simple, and inexpensive technique is still a challenge. Herein, electrical and sensing properties of ultrathin OFETs based on siloxane dimers of benzothieno[3,2‐b][1]benzothiophene (BTBT) with different aliphatic spacer lengths fabricated by Langmuir–Blodgett, Langmuir–Schaefer (LS) or spin‐coating techniques are studied, compared and optimized. Investigation of the influence of interface dielectric layer on electrical performance and operational stability of the devices allowed obtaining uniform low‐defect ultrathin semiconducting layers responsible for improved electrical performance. Field‐effect mobility up to 0.47 cm2 V−1 s−1 is achieved for the devices based on the dimer with undecylenic spacer between the BTBT core and disiloxane central fragment fabricated by LS method on the top of poly(methyl methacrylate) interface layer. Promising operational stability lead to advanced sensory properties demonstrated by sensing of ethanethiol with the limit of detection of 30 ppb in the humid air, which is a record value for portable sensing technologies.

Elucidating the Role of Water‐Related Traps in the Operation of Polymer Field‐Effect Transistors

AbstractConjugated polymers have gained momentum as serious contenders for next‐generation flexible electronics, but their susceptibility to water represents a major problem. Atmospheric water is ubiquitous and its inadvertent diffusion into polymeric devices generates charge carrier traps, reducing their performance and stability. A good understanding of the physical processes associated with the presence of water is therefore necessary in order to be able to suppress the related trapping events and enable stable, high‐performance devices. Here, evidence is shown that water introduces traps in the bandgap of organic semiconductors and the impact of these traps on the electrical properties of polymer organic field‐effect transistors (OFETs) based on indacenodithiophene‐co‐benzothiadiazole (IDT‐BT) is investigated. Monitoring device parameters and the trap density of states (t‐DOS) during moisture extrication reveals the existence of two types of water‐related traps: shallow traps originating from water inhabiting the voids of the polymer film and deeper traps arising from chemisorbed water present at the dielectric/polymer interface. A trap passivation method based on flame‐annealing is introduced to eliminate the interfacial traps. As a result, stable OFETs, with threshold voltage shifts less than ΔVth = −0.3 V and constant mobilities (<10% variation) after three months of storage, are fabricated.

Low-Temperature Packaging of Ion-Sensitive Organic Field-Effect Transistors on Plastic for Multiple Ion Detection

In this work, a low temperature (< 85∘C) packaging approach is proposed to implement highly stable ion-sensitive organic field-effect transistors (ISOFET) for multiple ion detection. It is realized by flip-chip bonding organic field effect transistor (OFET) transducers and sensitive/reference electrodes (SE/RE) on a plastic carrier using silver paste as the conductive adhesive and subsequent sealing with silicone sealant. A hollow structure is adopted to prevent OFETs from damaging by residual solvents in the adhesive and also from the influence of moisture in the air. The packaged OFET maintains high stability during the packaging process and presents good shelf-life performance with excellent bias stressing stability even after 24 days. A flexible tag consisting of three ISOFETs is demonstrated to properly detect Na+, K+, and H+ ions.

Improving Bias-Stress Stability of p-Type Organic Field-Effect Transistors by Constructing an Electron Injection Barrier at the Drain Electrode/Semiconductor Interfaces.

Bias-stress instability has been a challenging problem and a roadblock for developing stable p-type organic field-effect transistors (OFETs). This device instability is hypothesized because of electron-correlated charge carrier trapping, neutralization, and recombination at semiconductor/dielectric interfaces and in semiconductor channels. Here, in this paper, a strategy is demonstrated to improve the bias-stress stability by constructing a multilayered drain electrode with energy-level modification layers (ELMLs). Several organic small molecules with high lowest unoccupied molecular orbital (LUMO) energy levels are experimented as ELMLs. The energy-level offset between the Fermi level of the drain electrode and the LUMOs of the ELMLs is shown to construct the interfacial barrier, which suppresses electron injection from the drain electrode into the channel, leading to significantly improved bias-stress stability of OFETs. The mechanism of the ELMLs on the bias-stress stability is studied by quantitative modeling analysis of charge carrier dynamics. Of all injection models evaluated, it is found that Fowler-Nordheim tunneling describes best the observed experimental data. Both theory and experimental data show that, by using the ELMLs with higher LUMO levels, the electron injection can be suppressed effectively, and the bias-stress stability of p-type OFETs can thereby be improved significantly.

Highly stable flexible organic field-effect transistors with Parylene-C gate dielectrics on a flexible substrate

Abstract Poly(chloro-p-xylene), or Parylene-C, is a polymeric insulating material that has good physical and chemical properties, such as a high dielectric strength, a pin-free surface, and good mechanical/chemical stability, but is difficult to apply to top-contact-structured OFETs since its hydrophobic and very rough surfaces hinder the growth of organic semiconductor crystals and promote the formation of interface traps. Herein, we applied a blend of PS and TIPS-PEN dissolved in 1,2,3,4 tetrahydronaphthalene to overcome these limitations of Parylene-C. To confirm the influence of this system, we analyzed the morphologies of crystals grown on Parylene-C surfaces modified by various organic and polymer materials, including methacryloxypropyltrimethoxysilane, hexamethyldisilazane, and dimethylchlorosilane-terminated polystyrene. Our investigation showed the ability of the PS:TIPS-PEN blend system to be used to overcome the above-described limitations of Parylene-C, and to manufacture top-contact OFETs displaying stable operation under gate-bias stress. Notably, we applied Parylene-C and this blend system in practical flexible OFETs that displayed highly stable properties.

Surface treatment of Parylene-C gate dielectric for highly stable organic field-effect transistors

Abstract Poly(chloro-p-xylene) or Parylene-C is used as a polymer gate dielectric in organic field-effect transistors (OFETs). Although pristine Parylene-C possesses good physical and chemical properties, such as high dielectric strength, uniform thickness, pin-free surface morphology, and mechanical flexibility, its hydrophobic and rough surface can degrade the crystalline growth of semiconductor molecules and increase interface trap density. Herein, we investigated the surface treatment of Parylene-C with several organic and polymer materials, including methacryloxypropyltrimethoxysilane, hexamethyldisilazane, and dimethylchlorosilane-terminated polystyrene. This allowed modulation of surface hydrophobicity and roughness, thereby affecting the crystal morphology of the semiconductor layer growing on the gate dielectric surface, as well as the device performance and stability under gate-bias stress of the corresponding OFETs.

Solution-Processed Organic Field-Effect Transistors with High Performance and Stability on Paper Substrates.

High-performance operationally stable organic field-effect transistors were successfully fabricated on a PowerCoat HD 230 paper substrate with a TIPS-pentacene:polystyrene blend as the active layer and poly(4-vinylphenol)/HfO2 as the hybrid gate dielectric. The fabricated devices exhibited excellent p-channel characteristics with a maximum and av field effect mobility of 0.44 and 0.22(±0.11) cm2 V-1 s-1, respectively, av threshold voltage of 0.021(±0.63) V, and current on-off ratio of ∼105 while operating at -10 V. These devices exhibited remarkable stability under effects of gate bias stress and large number of repeated transfer scans with negligible performance spread. In addition, these devices displayed very stable electrical characteristics after long exposure periods to humidity and an excellent shelf life of more than 6 months in ambient environment. Thermal stress at high temperatures however deteriorates the device characteristics because of the generation and propagation of cracks in the active semiconductor crystals. Furthermore, novel paper-based phototransistors have been demonstrated with these devices.

Parylene-Based Double-Layer Gate Dielectrics for Organic Field-Effect Transistors.

We demonstrate high-performance and stable organic field-effect transistors (OFETs) using parylene-based double-layer gate dielectrics (DLGDs). DLGDs, consisting of parylene C as the upper layer and F as the lower layer, are designed to simultaneously provide good interface and bulk gate dielectric properties by exploiting the advantages of each gate dielectric. The structural effects of DLGDs are systematically investigated by evaluating the electrical characteristics and dielectric properties while varying the thickness ratio of each gate dielectric. The OFET with the optimized DLGD exhibits high performance and operational stability. This systematic approach will be useful for realizing practical electronic applications.

Complete Suppression of Bias-Induced Threshold Voltage Shift below 273 K in Solution-Processed High-Performance Organic Transistors.

Despite their importance for future applications, the operational electrical stability of organic thin-film transistors is far from being understood. Even in the most stable organic field-effect transistors (OFETs) operated under vacuum, a hitherto unknown source leads to bias stress. Here, we investigate the electrical characteristics and operational stability of a high-performance diketopyrrolopyrrole- alt-terthiophene organic semiconductor. Even though the OFETs are characterized by a high mobility of 3 cm2 V-1 s-1 and trap-free transport, the threshold voltage shift in all stress modes remains sensitive to the presence of water even when operating devices in high vacuum. Exponential fitting from current bias-stress measurement up to 500 000 s showed a bias-voltage shift of <1 V, which corresponds to the density of the bias-induced trap states at infinite time NT∞ = 7.6 × 1010 cm-2. We have surprisingly found that electrical stress could be completely suppressed when devices are cooled to below 273 K. We present evidence that H3O+ and OH- stemming from the autoionization of liquid water is the hitherto unidentified universal trap (i.e., an extrinsic trap not stemming from the semiconductor itself) causing threshold voltage shift even in the otherwise stable devices. This interpretation would also clarify why in the literature similar NT have been reported in various semiconductors, suggesting that this number is independent of the organic semiconductor, processing and measurement environment but only dependent on residual contaminants-most notably water.

Bar-Coated Ultrathin Semiconductors from Polymer Blend for One-Step Organic Field-Effect Transistors.

One-step deposition of bi-functional semiconductor-dielectric layers for organic field-effect transistors (OFETs) is an effective way to simplify the device fabrication. However, the proposed method has rarely been reported in large-area flexible organic electronics. Herein, we demonstrate wafer-scale OFETs by bar coating the semiconducting and insulating polymer blend solution in one-step. The semiconducting polymer poly(3-hexylthiophene) (P3HT) segregates on top of the blend film, whereas dielectric polymethyl methacrylate (PMMA) acts as the bottom layer, which is achieved by a vertical phase separation structure. The morphology of blend film can be controlled by varying the concentration of P3HT and PMMA solutions. The wafer-scale one-step OFETs, with a continuous ultrathin P3HT film of 2.7 nm, exhibit high electrical reproducibility and uniformity. The one-step OFETs extend to substrate-free arrays that can be attached everywhere on varying substrates. In addition, because of the well-ordered molecular arrangement, the moderate charge transport pathway is formed, which resulted in stable OFETs under various organic solvent vapors and lights of different wavelengths. The results demonstrate that the one-step OFETs have promising potential in the field of large-area organic wearable electronics.

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer To Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors.

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection, and the addition of a fluorine-containing material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm2/(V·s) compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm2/(V·s)). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from -1.84 V to -0.28 V.

Trends in molecular design strategies for ambient stable n-channel organic field effect transistors

This review highlights recent advancement in developing ambient stable organic molecular semiconductors from the theoretical and experimental perspectives.

(Poly(3,4-ethylenedioxythiophene):Polystyrene Sulfonate):Polytetrafluoroethylene for Use in High-Performance and Stable Bottom-Contact Organic Field-Effect Transistors

We successfully fabricated highly stable pentacene-based bottom-contact organic field-effect transistors (OFETs) with good charge injection properties at the electrode/organic semiconductor interface, obtained by optimizing the composition of solution-processed (poly(3,4-ethylenedioxythiophene):polystyrene sulfonate):polytetrafluoroethylene ((PEDOT:PSS):PTFE)-treated Au source/drain (S/D) electrodes. The (PEDOT:PSS):PTFE layer was deposited on the Au layer by spin-coating a mixture solution. The work function of the electrode increased from 4.84 to 5.21 eV as the PTFE concentration increased, accompanied by an interface dipole at the electrode surface. The optimized (PEDOT:PSS):PTFE (0.95:0.05)-treated electrodes significantly reduced the charge injection barrier at the electrode/semiconductor interface to achieve efficient charge transfer in the OFETs. Bottom-contact OFETs prepared with the optimized (PEDOT:PSS):PTFE-treated S/D electrodes had a field-effect mobility of 0.16 cm2/(V·s), which exceeded tha...

The role of H-bonds in the solid state organization of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) structures: bis(hydroxy-hexyl)-BTBT, as a functional derivative offering efficient air stable organic field effect transistors (OFETs)

The rapid and inexpensive functionalization of the BTBT core with hydroxyl end group functionalized hexyl chains offered a new platform for functionalized self-organising BTBT.

Grafting Fluorinated Polymer Nanolayer for Advancing the Electrical Stability of Organic Field-Effect Transistors

With the goal of achieving high-performance electrically stable organic field-effect transistors (OFETs), we chemically graft a fluorinated polymer nanolayer onto the polar oxide dielectric surfaces via a simple and easy fabrication process in ambient air. The para-fluorine-thiol click reaction between poly(pentafluorostyrene) (PFS) and mercaptopropyltrimethoxysilane is used to synthesize a graftable fluorinated polymer (gPFS). The surface characteristics of the gPFS-treated SiO2 dielectrics and the crystal structure and grain growth of the overlying organic semiconductors are investigated. Various semiconductor materials are employed for the OFETs prepared with gPFS-treated SiO2 dielectrics, including vacuum-processed pentacene, N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide, solution-processed 5,11-bis(triethylsilylethynyl)anthradithiophene, and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2-di(2,2′-bithiophen-5-yl)ethane. Three OFETs are prepared with different diel...

Stable Organic Field-Effect Transistors for Continuous and Nondestructive Sensing of Chemical and Biologically Relevant Molecules in Aqueous Environment

The use of organic field-effect transistors (OFETs) as sensors in aqueous media has gained increased attention for environmental monitoring and medical diagnostics. However, stable operation of OFETs in aqueous media is particularly challenging because of electrolytic hydrolysis of water, high ionic conduction through the analyte, and irreversible damage of organic semiconductors when exposed to water. To date, OFET sensors have shown the capability of label-free sensing of various chemical/biological species, but they could only be used once because their operational stability and lifetime while operating in aqueous environments has been poor, and their response times typically slow. Here, we report on OFETs with unprecedented water stability. These OFETs are suitable for the implementation of reusable chemical/biological sensors because they primarily respond to charged species diluted in an aqueous media by rapidly shifting their threshold voltage. These OFET sensors present stable current baselines and saturated signals which are ideal for detection of low concentration of small or large molecules that alter the pH of an aqueous environment. The overall response of these OFET sensors paves the way for the development of continuous chemical/biological nondestructive sensor applications in aqueous media.

High performance organic field-effect transistors with ultra-thin HfO2 gate insulator deposited directly onto the organic semiconductor

We have produced stable organic field-effect transistors (OFETs) with an ultra-thin HfO2 gate insulator deposited directly on top of rubrene single crystals by atomic layer deposition (ALD). We find that ALD is a gentle deposition process to grow thin films without damaging rubrene single crystals, as results these devices have a negligibly small threshold voltage and are very stable against gate-bias-stress, and the mobility exceeds 1 cm2/V s. Moreover, the devices show very little degradation even when kept in air for more than 2 months. These results demonstrate thin HfO2 layers deposited by ALD to be well suited as high capacitance gate dielectrics in OFETs operating at small gate voltage. In addition, the dielectric layer acts as an effective passivation layer to protect the organic semiconductor.

High Performance and Stable N-Channel Organic Field-Effect Transistors by Patterned Solvent-Vapor Annealing

We report the fabrication of high-performance, printed, n-channel organic field-effect transistors (OFETs) based on an N,N-dialkyl-substituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) derivative, PDI-RCN2, optimized by the solvent-vapor annealing (SVA) process. We performed a systematic study on the influence of solubility and the chemical structure of a solvent used for the SVA process on the ordering and orientation of PDI-RCN2 molecules in the thin film. The PDI-RCN2 film showed improved crystallinity under vapor annealing with the aliphatic 1,2-dichloroethane (DCE) as a marginal solvent. The n-type OFETs with DCE-vapor-annealed PDI-RCN2 show highly improved charge-carrier mobility of ~0.5 cm(2) V(-1) s(-1) and higher stability under gate bias stress than the pristine OFETs. This large performance improvement was mainly attributed to increased crystallinity of the semiconductor thin film, enhancing π-π stacking. We also introduced a new method to pattern crystallinity of a certain region in the semiconducting film by selective exposure to the solvent vapor using a shadow mask. The crystal-patterned PDI-RCN2 OFETs exhibit decreased off-currents by ~10× and improved gate bias stability by minimizing crosstalk, reducing leakage current between devices, and reducing the density of charge trap states of the organic semiconductor.