Organic Ferroelectric Field-effect Transistors Research Articles

Understanding Quasi-Static and Dynamic Characteristics of Organic Ferroelectric Field Effect Transistors.

Leveraging poly(vinylidene fluoride-trifluoroethylene) [(PVDF-TrFE)] as the dielectric, we fabricated organic ferroelectric field-effect transistors (OFe-FETs). These devices demonstrate quasi-static transfer characteristics that include a hysteresis window alongside transient phenomena that bear resemblance to synaptic plasticity-encapsulating excitatory postsynaptic current (EPSC) as well as both short-term and long-term potentiation (STP/LTP). We also explore and elucidate other aspects such as the subthreshold swing and the hysteresis window under dynamic state by varying the pace of voltage sweeps. In addition, we developed an analytical model that describes the electrical properties of OFe-FETs, which melds an empirical formula for ferroelectric polarization with a compact model. This model agrees well with the experimental data concerning quasi-static transfer characteristics, potentially serving as a quantitative tool to improve the understanding and design of OFe-FETs.

Rubrene/CuInP2S6 organic/inorganic heterojunctions for ferroelectric organic transistor memory devices

Ferroelectric organic field-effect transistor (Fe-OFET) memory devices exhibit low power consumption, low cost and good flexibility, which are indispensable to next generation flexible and wearable memory devices. However, up until the present, the ferroelectric materials used in Fe-OFETs have been limited to ferroelectric polymers, i.e. poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], and suffer from large surface roughness and high-risk of depolarization. This work integrates a new two-dimensional CuInP2S6 crystal into Fe-OFET, which can introduce excellent ferroelectric property due to its single crystalline smoothness and noncentrosymmetric crystal structure. Meanwhile, the high-mobility rubrene crystal is selected as the channel material to construct rubrene/CuInP2S6 organic/inorganic van der Waals heterojunctions. Associated with synergetic effect of single-crystalline nature of the heterojunction and atomically flat/defect-free interface, the memory devices possess a high programming speed of 0.5 s and long retention time of 104 s with a current ratio larger than 103. Furthermore, the performance of flexible devices is also demonstrated, including a programming/erase cycle of 1000 with a current on/off ratio approaching 103, data retention time of 104 s with a current on/off ratio of 103 and a current on/off ratio maintained at 102 even after 100 bending cycles under a bending radius of 25 mm, which are qualified for memory buffer usage or short-term data storage especially when they are integrated with other flexible electronics. The proposed configuration of memory transistors using organic/inorganic heterojunctions provides a rational design of device structure toward flexible Fe-OFET non-volatile memory applications.

High-Mobility, Low-Voltage Programmable/Erasable Ferroelectric Polymer Transistor Nonvolatile Memory Based on a P(VDF-TrFE)/PMMA Bilayer Gate Insulator

Low mobility and high operating voltages are bottlenecks in the practical application of ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memory (NVM). In this article, we demonstrate a high-mobility polymer semiconductor-based Fe-OFET NVM that can well operate at low programming/erasing voltages ( V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> /V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> ). The issue that hinders the reduction of the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> /V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> in Fe-OFET NVMs is discussed. We develop a route to resolve the issue by building a thin bilayer gate insulator consisted of an ultrathin poly(methyl methacrylate) (PMMA) and a thin poly(vinylidenefluoride-trifluoroethylene) films. The processing compatibility, that is, the tri-layered core architecture consisting of polymer semiconductor and insulator films is prepared by a full-solution technology, is confirmed. The mechanism of the performance improvements by using the ultrathin PMMA in the Fe-OFET NVMs is investigated. As a result, the Fe-OFET NVM exhibits excellent performances at low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> /V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> of ±15 V, with a high mobility up to 1.75 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, reliable memory endurance over 400 cycles, and stable memory retention over 65000 s.

Multiresponsive Nonvolatile Memories Based on Optically Switchable Ferroelectric Organic Field-Effect Transistors.

Organic transistors are key elements for flexible, wearable, and biocompatible logic applications. Multiresponsivity is highly sought-after in organic electronics to enable sophisticated operations and functions. Such a challenge can be pursued by integrating more components in a single device, each one responding to a specific external stimulus. Here, the first multiresponsive organic device based on a photochromic-ferroelectric organic field-effect transistor, which is capable of operating as nonvolatile memory with 11 bit memory storage capacity in a single device, is reported. The memory elements can be written and erased independently by means of light or an electric field, with accurate control over the readout signal, excellent repeatability, fast response, and high retention time. Such a proof of concept paves the way toward enhanced functional complexity in optoelectronics via the interfacing of multiple components in a single device, in a fully integrated low-cost technology compatible with flexible substrates.

Stochastic resonance in synapse-mimicking ferroelectric organic field-effect transistor

Ferroelectric organic field-effect transistors (FE-OFETs) are fabricated to implement the functions of a neural synapse, including one-directional signal transmission and synaptic plasticity. Stochastic resonance (SR) experiments are conducted with FE-OFETs that use regioregular poly(3-hexylthiophene-2,5-diyl) as the semiconductor layer and poly(vinylidene fluoride-co-hexafluoropropylene) as the insulator layer. The input–output correlation is observed to increase as the external noise intensity rises, with a cutoff frequency of 1 Hz, demonstrating the experimental occurrence of SR in the fabricated FE-OFETs. Furthermore, vibrational resonance experiments are performed to clarify the frequency-dependence of the SR behavior. The peak input–output correlation shifts toward higher amplitudes of a secondary sine wave that is applied instead of external noise as the frequency of the input signal rises from = 0.5–1 Hz. The FE-OFETs exhibit more adaptive behavior than paraelectric OFETs, with a threshold voltage that changes dynamically according to the frequency characteristics of the input signal and/or external noise. Furthermore,the immediate response to abrupt environmental changes via the SR phenomenon and flexible behavior based on past experience via hysteresis effects can be implemented by a single FE-OFET element.

High-performance polymer semiconductor-based ferroelectric transistor nonvolatile memory with a self-organized ferroelectric/dielectric gate insulator

Ferroelectric organic field-effect transistor nonvolatile memories (Fe-OFET-NVMs) offer attractive features for future memory applications, such as flexible and wearable electronics. Polymer semiconductor-based top-gate Fe-OFET-NVMs possess natural advantages in the device structure and processing manufacturing, compared to small-molecule semiconductor-based bottom-gate Fe-OFET-NVMs. However, their performances, such as mobility and operating voltages, should be further improved to be comparable to those of the latter. In this Letter, we develop a route to achieve high-performance top-gate Fe-OFET-NVMs, by employing a polymer semiconductor channel and self-organized ferroelectric/dielectric gate insulators, which were processed by a solution spin-coating technique. The optimal Fe-OFET-NVM exhibits a high mobility of 1.96 cm2/V s on average, a reliable endurance over 400 cycles, a stable retention capability over 6 × 104 s, and a life more than one year. Furthermore, the operating voltage of the Fe-OFET-NVM is reduced to ±20 V by scaling down the thickness of the ferroelectric/dielectric gate insulator. The whole performances of our memories are comparable to or better than those of the previous Fe-OFET-NVMs.

Tuning Charge Transport in PVDF-Based Organic Ferroelectric Transistors: Status and Outlook.

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor-acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

Organic ferroelectric field‐effect transistor memories with poly(vinylidene fluoride) gate insulators and conjugated semiconductor channels: a review

AbstractOrganic electronics have attracted considerable interest because of their low temperature solution processability, versatile material synthesis and compatibility with a wide range of traditional fabrication methods such as rod‐coating, inkjet and roll‐to‐roll printing techniques. Organic ferroelectric field‐effect transistors (OFeFETs) with a three‐terminal device architecture have ferroelectric thin films as gate insulator layers and semiconductor films as charge transport channel layers. OFeFETs have been demonstrated with non‐volatile memory functions that can retain their high or low conductive states in the semiconductor channel when the power is turned off. The reversible polarization switching in the ferroelectric gate insulator layer between two stable polarized states provides the basis for binary coded memory functions. In this review, we summarize recent development of OFeFETs based on ferroelectric poly(vinylidene fluoride) dielectrics and organic semiconductor channels. © 2020 Society of Chemical Industry

Solvent-Dependent Electrical Characteristics and Mechanical Stability of Flexible Organic Ferroelectric Field-Effect Transistors.

Flexible organic ferroelectric field-effect transistors (Fe-FETs) have attracted attention for next-generation memory applications. A fundamental understanding of the electrical properties and mechanical stability of transistors is a prerequisite to realizing practical flexible electronics. Here, we demonstrate the solvent-dependent electrical characteristics and mechanical stability of flexible Fe-FETs. Poly(vinylidene fluoride-trifluoro-ethylene) (P(VDF-TrFE)) based Fe-FETs were fabricated by using dimethylformamide (DMF) and methyl ethyl ketone (MEK) solvents on a polyimide substrate. P(VDF-TrFE) from DMF formed a smoother surface than a surface from MEK; the surface property greatly affected the electrical properties and mechanical stability of the devices. Larger hysteresis and higher mobility were obtained from Fe-FET using DMF compared to those characteristics from using MEK. Furthermore, Fe-FET using DMF showed lower degradation of on-current and mobility under repetitive mechanical stress than an MEK-based Fe-FET, due to its excellent semiconductor-insulator interface. These results will guide appropriate solvent selection and contribute to the improvement of flexible Fe-FET electrical properties and mechanical stability in the next generation of memory devices.

Characterizations of Blended PVDF/P(VDF-TrFE) Films for Ferroelectric Organic Field-Effect Transistors

In this study, we fabricated blended PVDFP(VDF-TrFE) solutions at different concentrations and formed ferroelectric films via the sol gel method. We then worked with organic ferroelectric field-effect transistors featuring ferroelectric polymer (PVDF/P(VDF-TrFE)) blended films and a poly-3-hexylthiophene(P3HT) organic channel on a TiN substrate. Finally, electrical and ferroelectric properties were compared to those of pure P(VDFTrFE) and blended PVDF/P(VDF-TrFE) films.

PJ-Level Energy-Consuming, Low-Voltage Ferroelectric Organic Field-Effect Transistor Memories.

Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted considerable attention because of their promising potential for memory applications, while a critical issue is the large energy consumption mainly caused by a high operating voltage and slow data switching. Here, we employ ultrathin ferroelectric polymer and semiconducting molecular crystals to create low-voltage Fe-OFET memories. Devices require only pJ-level energy consumption. The writing and erasing processes require ∼1.2 and 1.6 pJ/bit, respectively, and the reading energy is ∼1.9 pJ/bit (on state) and ∼0.2 fJ/bit (off state). Thus, our memories consume only <0.1% of the energy required for devices using bulk functional layers. Besides, our devices also exhibit low contact resistance and steep subthreshold swing. Therefore, we provide a strategy that opens up a path for Fe-OFETs toward emerging applications, such as wearable electronics.

Vertical Organic Ferroelectric Field Effect Transistor (VO-FeFET) for Low Threshold Voltage and High On/Off Current Ratio

Vertical Organic Ferroelectric Field Effect Transistor (VO-FeFET) for Low Threshold Voltage and High On/Off Current Ratio

Low-voltage programmable/erasable high performance flexible organic transistor nonvolatile memory based on a tetratetracontane passivated ferroelectric terpolymer

Abstract Future flexible electronic systems require memory devices combining low power consumption and mechanical bendability. However, high programming/erasing (P/E) voltages, which are universally required to switch the storage states in previously reported ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memories (NVMs), severely prevent their practical applications. In this work, we develop a novel route to achieve a low-voltage programmable/erasable flexible Fe-OFET NVM. Ferroelectric terpolymer poly(vinylidene-fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)], rather than the conventional ferroelectric copolymer poly(vinylidene-fluoride-trifluoroethylene) [P(VDF-TrFE)], is used as the gate dielectric. The low coercive field of P(VDF-TrFE-CTFE) is the main contribution to the low-voltage operation in the Fe-OFET NVM, even with a relative thick ferroelectric gate dielectric layer. By depositing a long-chain alkane molecule Tetratetracontane (TTC) as the passivation layer on the surface of P(VDF-TrFE-CTFE) film, the layer-by-layer growth mode of semiconductor pentacene is obtained, which results in a large crystalline grain and good interface morphology at the channel/dielectric. Therefore, the mobility of Fe-OFET NVMs is greatly improved. As a result, a high performance flexible Fe-OFET NVM is achieved, with a low P/E voltage of ±15 V, high mobility up to 0.5 cm2 V−1 s−1, reliable P/E endurance property over 1000 cycles, stable data storage retention capability over 6000 s, and excellent mechanical bending durability without visible degradation after 2000 repetitive tensile bending cycles at a small curvature radius of 4.0 mm.

Switching Time in Ferroelectric Organic Field‐Effect Transistors

Ferroelectric organic field‐effect transistors (Fe‐OFETs) are considered a promising technology to develop flexible and low‐cost nonvolatile memories. Although their switching time between program‐state and erase‐state forms an important aspect of their functionality, it is still unclear as to what parameters dominate the switching time. Two possible parameters that influence the switching time are the channel formation of the semiconductor layer and the ferroelectric polarization in the ferroelectric layer. In this study, the origin of the switching time of the Fe‐OFETs is experimentally clarified. The switching times of Fe‐OFETs and ferroelectric capacitors are independently measured. Programming time of the Fe‐OFETs is limited by the above two times as expected; however, the ferroelectric polarization is not negligible and limited the time in most of cases. Additionally, it is found that switching time from depolarized erase‐state to polarized program‐state in the Fe‐OFETs is comparable to the ferroelectric polarization time in the ferroelectric capacitor. On the other hand, erasing time of the Fe‐OFETs is much slower than their programing time due to the depletion layer in the semiconductor layer. The erasing time can be predicted by taking a series capacitance made by depleted semiconductor layer into consideration.

High Mobility Flexible Ferroelectric Organic Transistor Nonvolatile Memory With an Ultrathin ${\text {AlO}}_{{X}}$ Interfacial Layer

The low mobility is a primary issue to prevent the practical application of the ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memory (NVM). In this paper, we propose a route to resolve this issue by inserting an ultrathin ${\text {AlO}}_{X}$ layer between the semiconductor film and the ferroelectric polymer film. A high mobility of 6.5 $\text{cm}^{2}\cdot \text {V}^{-1}\cdot \text {s}^{-1}$ is achieved in the fabricated flexible Fe-OFET NVM, which also exhibits good memory performances with a large memory window of 17.2 V, a high memory on-off ratio up to $2 \times 10^{5}$ , reliable switching endurance property over 600 cycles, and stable retention capability with the memory on-off ratio larger than 102 over 104 s, at the low programming/erasing voltages of ±15 V.

Low-voltage operating flexible ferroelectric organic field-effect transistor nonvolatile memory with a vertical phase separation P(VDF-TrFE-CTFE)/PS dielectric

Future flexible electronic systems require memory devices combining low-power operation and mechanical bendability. However, high programming/erasing voltages, which are universally needed to switch the storage states in previously reported ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memories (NVMs), severely prevent their practical applications. In this work, we develop a route to achieve a low-voltage operating flexible Fe-OFET NVM. Utilizing vertical phase separation, an ultrathin self-organized poly(styrene) (PS) buffering layer covers the surface of the ferroelectric polymer layer by one-step spin-coating from their blending solution. The ferroelectric polymer with a low coercive field contributes to low-voltage operation in the Fe-OFET NVM. The polymer PS contributes to the improvement of mobility, attributing to screening the charge scattering and decreasing the surface roughness. As a result, a high performance flexible Fe-OFET NVM is achieved at the low P/E voltages of ±10 V, with a mobility larger than 0.2 cm2 V−1 s−1, a reliable P/E endurance over 150 cycles, stable data storage retention capability over 104 s, and excellent mechanical bending durability with a slight performance degradation after 1000 repetitive tensile bending cycles at a curvature radius of 5.5 mm.

Excellent low-voltage operating flexible ferroelectric organic transistor nonvolatile memory with a sandwiching ultrathin ferroelectric film

The high operating voltage is a primary issue preventing the commercial application of the ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memory (NVM). In this work, we propose a novel route to resolve this issue by employing two ultrathin AlOX interfacial layers sandwiching an ultrathin ferroelectric polymer film with a low coercive field, in the fabricated flexible Fe-OFET NVM. The operation voltage of Fe-OFET NVMs decreases with the downscaling thickness of the ferroelectric film. By inserting two ultrathin AlOX interfacial layers at both sides of the ultrathin ferroelectric film, not only the gate leakage is prominently depressed but also the mobility is greatly improved. Excellent memory performances, with large mobility of 1.7 ~ 3.3 cm2 V−1 s−1, high reliable memory switching endurance over 2700 cycles, high stable data storage retention capability over 8 × 104 s with memory on-off ratio larger than 102, are achieved at the low operating voltage of 4 V, which is the lowest value reported to data for all Fe-OFET NVMs. Simultaneously, outstanding mechanical fatigue property with the memory performances maintaining well over 7500 bending cycles at a bending radius of 5.5 mm is also achieved in our flexible FE-OFET NVM.

Speed up Ferroelectric Organic Transistor Memories by Using Two-Dimensional Molecular Crystalline Semiconductors.

Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted intensive attention because of their promising potential in nonvolatile memory devices. The quick switching between binary states is a significant fundamental feature in evaluating Fe-OFET memories. Here, we employ 2D molecular crystals via a solution-based process as the conducting channels in transistor devices, in which ferroelectric polymer acts as the gate dielectric. A high carrier mobility of up to 5.6 cm2 V-1 s-1 and a high on/off ratio of 106 are obtained. In addition, the efficient charge injection by virtue of the ultrathin 2D molecular crystals is beneficial in achieving rapid operations in the Fe-OFETs; devices exhibit short switching time of ∼2.9 and ∼3.0 ms from the on- to the off-state and from the off- to the on-state, respectively. Consequently, the presented strategy is capable of speeding up Fe-OFET memory devices by using solution-processed 2D molecular crystals.

Achieving high mobility, low-voltage operating organic field-effect transistor nonvolatile memory by an ultraviolet-ozone treating ferroelectric terpolymer.

Poly(vinylidene fluoride–trifluoroethylene) has been widely used as a dielectric of the ferroelectric organic field-effect transistor (FE-OFET) nonvolatile memory (NVM). Some critical issues, including low mobility and high operation voltage, existed in these FE-OFET NVMs, should be resolved before considering to their commercial application. In this paper, we demonstrated low-voltage operating FE-OFET NVMs based on a ferroelectric terpolymer poly(vinylidene-fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] owed to its low coercive field. By applying an ultraviolet-ozone (UVO) treatment to modify the surface of P(VDF-TrFE-CTFE) films, the growth model of the pentacene film was changed, which improved the pentacene grain size and the interface morphology of the pentacene/P(VDF-TrFE-CTFE). Thus, the mobility of the FE-OFET was significantly improved. As a result, a high performance FE-OFET NVM, with a high mobility of 0.8 cm2 V−1 s−1, large memory window of 15.4~19.2, good memory on/off ratio of 103, the reliable memory endurance over 100 cycles and stable memory retention ability, was achieved at a low operation voltage of ±15 V.

Solution-processed organic ferroelectric field-effect transistors on ultra-flexible substrates

We fabricated ultra-flexible organic non-volatile ferroelectric field-effect transistors (FeFETs) with poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] on polyimide substrates, which adopted a solution-based top-contact/bottom-gate structure for low cost process without patterning. P(VDF-TrFE) gate dielectric layers and regioregular poly(3-hexylthiophene) active layers were formed by the spin-coating method. The field-effect mobility (μFE) was ∼0.28 cm2/V s, the on/off ratio was approximately 5.6 × 103, and the memory window (threshold voltage shift) was approximately 7 V. In addition, FeFETs were operated even at small bend radii without considerable changes in these values.