Stability Of Organic Field-effect Transistors Research Articles

Protonated Organic Semiconductors: Origin of Water-Induced Charge-Trap Generation.

Despite dramatic improvements in the electronic characteristics of organic semiconductors, the low operational stability of organic field-effect transistors (OFETs) hinders their direct use in practical applications. Although the literature contains numerous reports on the effects of water on the operational stability of OFETs, the underlying mechanisms of trap generation induced by water remain unclear. Here, a protonation-induced trap generation of organic semiconductors is proposed as a possible origin of the operational instability in organic field-effect transistors. Spectroscopic and electronic investigation techniques combined with simulations reveal that the direct protonation of organic semiconductors by water during operation may be responsible for the trap generation induced by bias stress; this phenomenon is independent of the trap generation at an insulator surface. In addition, the same feature occurred in small-bandgap polymers with fused thiophene rings irrespective of their crystalline ordering, implying the generality of protonation induced trap generation in various polymer semiconductors with a small bandgap. The finding of the trap-generation process provides new perspectives for achieving greater operational stability of organic field-effect transistors.

Suppressing the Intrinsic Photoelectric Response of Organic Semiconductors for Highly-Photostable Organic Transistors.

Suppressing the photoelectric response of organic semiconductors (OSs) is of great significance for improving the operational stability of organic field-effect transistors (OFETs) in light environments, but it is quite challenging because of the great difficulty in precisely modulating exciton dynamics. In this work, photostable OFETs are demonstrated by designing the micro-structure of OSs and introducing an electrical double layer at the OS/polyelectrolyte dielectric interface, in which multiple exciton dynamic processes can be modulated. The generation and dissociation of excitons are depressed due to the small light-absorption area of the microstripe structure and the excellent crystallinity of OSs. At the same time, a highly efficient exciton quenching process is activated by the electrical double layer at the OS/polyelectrolyte dielectric interface. As a result, the OFETs show outstanding tolerance to the light irradiation of up to 306 mW·cm-2 , which far surpasses the solar irradiance value in the atmosphere (≈138 mW·cm-2 ) and achieves the highest photostability ever reported in the literature. The findings promise a general and practicable strategy for the realization of photostable OFETs and organic circuits.

Strain-Balanced Organic Semiconductor Film for Improving the Stability of Organic Field-Effect Transistors.

Strain-induced aggregate state instability in organic semiconductor (OSC) films is a critical and bottleneck issue in the practicalization process of organic field-effect transistors (OFETs), but this issue lacks deep insight and effective solutions for a long time. Herein, we developed a novel and general strain balance strategy for stabilizing the aggregate state of OSC films and enhancing the robustness of OFETs. The charge transport zone in OSC films located at the OSC/dielectric interface always suffers from the intrinsic tensile strain induced by substrates and tends to dewet. By introducing a compressive strain layer, the tensile strain can be well balanced and OSC films attain a highly stable aggregate state. Consequently, the OFETs based on strain-balanced OSC heterojunction films exhibit excellent operational and storage stability. This work provides an effective and general strategy to stabilize OSC films and gives guidance in constructing highly stable organic heterojunction devices.

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.

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.

Charge Trapping in a Low-Crystalline High-Mobility Conjugated Polymer and Its Effects on the Operational Stability of Organic Field-Effect Transistors.

The effects of the microstructure of conjugated polymer thin films on charge trapping and operational stability of organic field-effect transistors (OFETs) are investigated. Device characteristics of OFETs based on two model conjugated polymers, poly(3-hexylthiophene) (P3HT) and a random 3-hexylthiophene-thiophene copolymer (RP33), are compared. P3HT films have high crystallinity and long-range molecular order, whereas RP33 films have low crystallinity and short-range molecular order as well as enhanced polymer backbone planarity. Experimental evidence shows that although the microstructure of the RP33 film provides efficient charge transport pathways, its high degree of structural disorder causes severe shallow trapping of charge carriers, which results in its inferior stability under bias stress. This study demonstrates that low-crystalline conjugated polymers with short-range order can provide a high charge-carrier mobility but at the same time be inappropriate for practical OFETs because of their poor intrinsic operational stability.

Enhanced gate-bias stress stability of organic field-effect transistors by introducing a fluorinated polymer in semiconductor/insulator ternary blends

Solution-processed polymer semiconductors are key materials in the fabrication of lightweight, low-cost, and flexible electronic devices without using the high-vacuum process. For practical applications, reliable device operations based on these materials are required. In this study, we propose a strategy to improve the operation stability of organic field-effect transistors (OFETs) using solution-processed polymer semiconductor/insulator blends as the active channel prepared by introducing a fluorinated insulating polymer in the blends. The semiconducting polymer forms nanowire networks in spin-coated ternary blend films, which serve as charge transport pathways in the insulating polymer matrix consisting of the fluorinated polymer and polystyrene. Owing to its high surface potential attributed to the strongly electron-withdrawing structure, the fluorinated polymer provides a large energy barrier for suppression of the hole trapping at the semiconductor/insulator interface. Consequently, OFETs based on the ternary blend films with an optimized polymer composition exhibit almost hysteresis-free transfer and output characteristics and superior electrical stabilities under sustained gate-bias stresses in both N2 and air atmospheres. We believe that our study provides a practical route to the fabrication of OFETs based on polymer semiconductor/insulator blend systems with high operation stabilities.

FePc induced highly oriented PIID-BT conjugated polymer semiconductor with high bias-stress stability

Polymer semiconductors with high crystallinity and high molecular orientation have been demonstrated to be in favor of improving the bias-stress stability of organic field-effect transistors (OFETs). The isoindigo (IID)-bithiophene (BT) based conjugated polymer (PIID-BT) is a typical donor–acceptor polymer with higher hole mobility and can be used for the bias-stress stability study. In this work, we use a small organic molecule of FePc to optimize the morphology and structure of the PIID-BT semiconductor to improve the bias-stress stability of OFET devices. The high crystallinity and ordered morphology of the FePc-doped PIID-BT film are realized, and this as-obtained FePc-doped PIID-BT OFET shows more outstanding bias-stress stability, with a lower drain current decay of only 12% over a stressing time of 1000 s than that of ca. 50% for the pristine PIID-BT devices. The electronic structure features reveal the bind between FePc and PIID-BT molecules via the Fe-O coordination interaction, which would be responsible for the efficiently oriented growth of the PIID-BT polymer and eventually promote the bias-stress stability of PIID-BT based OFET devices.

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostatically Interlocked 2D Molecular Packing

Thermal instability is one of the major obstacles for the application of organic devices. We report herein for

Conjugated Side Chain Tuning Effect of Indacenodithieno[3,2‐b]thiophene and Fluoro‐Benzothiadiazole‐Based Regioregular Copolymers for High‐Performance Organic Field‐Effect Transistors

AbstractTwo regioregular indacenodithieno[3,2‐b]thiophene and fluoro‐benzothiadiazole‐based donor–acceptor (D–A) type copolymers with different conjugated side chains of hexyl‐phenyl or hexyl‐thienyl are designed and synthesized to study the effects of the side chain on performance of organic field‐effect transistors (OFETs). The effect of the different conjugated side chains on the energy levels, optoelectronic properties, film morphology, and transistors performance as well as the solvent effect on the performance of both D–A copolymers are thoroughly studied. Top‐gate/bottom‐contact OFETs with a hexyl‐thienyl substituted polymer show higher hole mobility up to 0.211 cm2 V−1 s−1 than 0.086 cm2 V−1 s−1 observed for the polymer with hexyl‐phenyl side chains. In addition, the hexyl‐thienyl substituted side chain polymer shows relatively better stability under bias stress compared to hexyl‐phenyl conjugated side chain. This result sheds light on the impact of conjugated side chain engineering on the performance and stability of OFETs with D–A copolymers.

Improving Mobility and Stability of Organic Field-Effect Transistors by Employing a Tetratetracontane Modifying PMMA Dielectric

Devise stability is another important problem for hindering the commercial applications of organic field-effect transistors (OFETs) besides the mobility. In this paper, an abnormal instability was observed in the OFET with a pure polymer poly(methyl methacrylate) (PMMA) dielectric due to the minority carriers trapped at the pentacene/PMMA interface. By inserting a tetratetracontane modification layer between PMMA and pentacene, not only the morphology of the deposited pentacene was greatly improved, but also the trap density at the pentacene/dielectric interface was reduced, demonstrated by the investigation of the atomic force microscopy, and the analysis of the subthreshold swing and flicker noise, respectively. As a result, both the stability as well as the mobility of the OFET was obviously improved.

Organic field-effect transistors as high performance humidity sensors with rapid response, recovery time and remarkable ambient stability

The stability of organic field-effect transistors (OFETs) under very high humidity condition has been the bottleneck in many applications. In this work, remarkable enhancement in ambient stability and performance of CuPc based OFETs are observed by exploiting the polarization of hydroxyl groups in Poly (vinyl alcohol) (PVA) dielectric layer, which is sandwiched between Al2O3 and Poly (methyl methacrylate) (PMMA) layer. The devices are used to fabricate OFETs based humidity sensors, which show exceptional ambient stability and rapid response to the water molecules in moisture. In a controlled experiment, the sensors demonstrate 0.73 s as response time and 0.52 s as recovery time. Such results are the fastest responses observed on humidity sensors fabricated based on OFETs. The enhanced responses of the sensors are due to the systematic polarization of the hydroxyl groups present in PVA layer by the additional dipole field arising from the adsorbed water molecules, which are also polarized under gate-field. The devices show no variation in threshold voltage as well as field-effect carrier mobility, measured throughout a year under ambient exposure. The specific design of the sensors with tri-layer dielectric system plays crucial role in enhancing the stability and moisture sensitivity, which can make the devices technologically very relevant.

Branched Segments in Polymer Gate Dielectric as Intrinsic Charge Trap Sites in Organic Transistors

Charge traps in polymer gate dielectrics determine the electrical stability of organic field-effect transistors (OFETs), and polar alkoxy groups are well-known extrinsic charge traps. However, the actual location of intrinsic charge traps in nonpolar polymer gate dielectrics has been poorly understood yet. Here, we demonstrate that the skeletal structure of polymer chain plays an important role in determining the electrical stability. To verify it, we prepared linear and branched polystyrene (l-PS and b-PS) and blended them, in which branched segments provide much larger free volume than the other segments. The current-insulating performance and field-effect mobility increased with decease of b-PS portion. In particular, the bias-stress stability was remarkably varied according to the change of b-PS portion even though all measurements excluded reactive components such as oxygen and water; the increase of b-PS resulted in time-dependent decay of mobility and threshold voltage under bias stress. This indic...

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...

Fluorinated polyimide gate dielectrics for the advancing the electrical stability of organic field-effect transistors.

Organic field-effect transistors (OFETs) that operated with good electrical stability were prepared by synthesizing fluorinated polyimide (PI) gate dielectrics based on 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI. 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI contain 6 and 18 fluorine atoms per repeat unit, respectively. These fluorinated polymers provided smooth surface topographies and surface energies that decreased as the number of fluorine atoms in the polymer backbone increased. These properties led to a better crystalline morphology in the semiconductor film grown over their surfaces. The number of fluorine atoms in the PI backbone increased, the field-effect mobility improved, and the threshold voltage shifted toward positive values (from -0.38 to +2.21 V) in the OFETs with pentacene and triethylsilylethynyl anthradithiophene. In addition, the highly fluorinated polyimide dielectric showed negligible hysteresis and a notable gate bias stability under both a N2 environment and ambient air.

Operational Stability of Organic Field‐Effect Transistors

Organic field-effect transistors (OFETs) are considered in technological applications for which low cost or mechanical flexibility are crucial factors. The environmental stability of the organic semiconductors used in OFETs has improved to a level that is now sufficient for commercialization. However, serious problems remain with the stability of OFETs under operation. The causes for this have remained elusive for many years. Surface potentiometry together with theoretical modeling provide new insights into the mechanisms limiting the operational stability. These indicate that redox reactions involving water are involved in an exchange of mobile charges in the semiconductor with protons in the gate dielectric. This mechanism elucidates the established key role of water and leads in a natural way to a universal "stress function", describing the stretched exponential-like time dependence ubiquitously observed. Further study is needed to determine the generality of the mechanism and the role of other mechanisms.

Damage-free hybrid encapsulation of organic field-effect transistors to reduce environmental instability

Damage-free hybrid encapsulation layers have been fabricated and optimized to improve the structural reliability and operational stability of organic field-effect transistors (OFETs); this reduction in instability was systematically demonstrated under various environmental conditions. The introduction of a 150 nm thick SiOx layer onto 50 nm thick pentacene OFETs on polymer/SiO2 dielectrics was found to impart strong structural constraints against morphological and polymorphic changes of pentacene crystals in the OFETs exposed to physicochemically active species or elevated temperatures up to 180 °C. In contrast, under the same environmental conditions unpassivated pentacene OFETs were found to undergo a drastic decrease in the quality of the π overlap of pentacene molecules, causing unfavorable charge-carrier transport within the semiconductor. The deposition of either complementary CYTOP™ polymer or AlOx layer was found to successfully prevent the diffusion of H2O and O2 into the pentacene films and thus to enhance the barrier properties of the SiOx layer, in particular, against H2O and O2.

Enhanced Stability of Organic Field-Effect Transistors with Blend Pentacene/Anthradithiophene Films

By taking advantage of the similarity between pentacene (PEN) and anthradithiophene (ADT) in molecular dimension and charge transport property, we have produced organic field-effect transistors (OF...

Influence of the semiconductor oxidation potential on the operational stability of organic field-effect transistors

During prolonged application of a gate bias, organic field-effect transistors show a gradual shift of the threshold voltage towards the applied gate bias voltage. The shift follows a stretched-exponential time dependence governed by a relaxation time. Here, we show that a thermodynamic analysis reproduces the observed exponential dependence of the relaxation time on the oxidation potential of the semiconductor. The good fit with the experimental data validates the underlying assumptions. It demonstrates that this operational instability is a straightforward thermodynamically driven process that can only be eliminated by eliminating water from the transistor.

Solvent-free solution processed passivation layer for improved long-term stability of organic field-effect transistors

In an effort to realize organic field-effect transistors (OFETs) that are stable over long periods of time, we have designed an organic–inorganic hybrid passivation material (TGD622t) prepared via a non-hydrolytic sol–gel process that does not require the use of solvents. Fourier-transform infrared spectroscopy, atomic force microscopy, and UV-visible spectroscopy demonstrated the high density and low porosity of the organic–inorganic hybrid transparent TGD622t film after low-temperature curing (below 100 °C). The dense TGD622t passivation layer, which exhibited a water vapor transmission rate (WVTR) of 0.434 g m−2 per day, effectively protected the poly[9,9-dioctylfluorenyl-2,7-diyl]-co-(bithiophene)]-based OFETs from humidity and oxygen in ambient air, resulting in a much more robust OFET performance with long-term stability relative to the operation of unpassivated devices.