Morphology optimization for the fabrication of high mobility thin-film transistors

Retina-inspired organic neuromorphic vision sensor with polarity modulation for decoding light information

The neuromorphic vision sensor (NeuVS), which is based on organic field-effect transistors (OFETs), uses polar functional groups (PFGs) in polymer dielectrics as interfacial units to control charge carriers. However, the mechanism of modulating charge transport on basis of PFGs in devices is unclear. Here, the carboxyl group is introduced into polymer dielectrics in this study, and it can induce the charge transfer process at the semiconductor/dielectric interfaces for effective carrier transport, giving rise to the best device mobility up to 20 cm2 V-1 s-1 at a low operating voltage of -1 V. Furthermore, the polarity modulation effect could further increase the optical figures of merit in NeuVS devices by at least an order of magnitude more than the devices using carboxyl group-free polymer dielectrics. Additionally, devices containing carboxyl groups improved image sensing for light information decoding with 52 grayscale signals and memory capabilities at an incredibly low power consumption of 1.25 fJ/spike. Our findings provide insight into the production of high-performance polymer dielectrics for NeuVS devices.

Recent Progress in Aromatic Polyimide Dielectrics for Organic Electronic Devices and Circuits

Polymeric dielectrics play a key role in the realization of flexible organic electronics, especially for the fabrication of scalable device arrays and integrated circuits. Among a wide variety of polymeric dielectric materials, aromatic polyimides (PIs) are flexible, lightweight, and strongly resistant to high-temperature processing and corrosive etchants and, therefore, have become promising candidates as gate dielectrics with good feasibility in manufacturing organic electronic devices. More significantly, the characteristics of PIs can be conveniently modulated by the design of their chemical structures. Herein, from the perspective of structure optimization and interface engineering, a brief overview of recent progress in PI-based dielectrics for organic electronic devices and circuits is provided. Also, an outlook of future research directions and challenges for polyimide dielectric materials is presented.

A Centimeter-Scale Inorganic Nanoparticle Superlattice Monolayer with Non-Close-Packing and its High Performance in Memory Devices

Due to the near-field coupling effect, non-close-packed nanoparticle (NP) assemblies with tunable interparticle distance (d) attract great attention and show huge potential applications in various functional devices, e.g., organic nano-floating-gate memory (NFGM) devices. Unfortunately, the fabrication of device-scale non-close-packed 2D NPs material still remains a challenge, limiting its practical applications. Here, a facile yet robust "rapid liquid-liquid interface assembly" strategy is reported to generate a non-close-packed AuNP superlattice monolayer (SM) on a centimeter scale for high-performance pentacene-based NFGM. The d and hence the surface plasmon resonance spectra of SM can be tailored by adjusting the molecular weight of tethered polymers. Precise control over the d value allows the successful fabrication of photosensitive NFGM devices with highly tunable performances from short-term memory to nonvolatile data storage. The best performing nonvolatile memory device shows remarkable 8-level (3-bit) storage and a memory ratio over 10⁵ even after 10 years compared with traditional devices with a AuNP amorphous monolayer. This work provides a new opportunity to obtain large area 2D NPs materials with non-close-packed structure, which is significantly meaningful to microelectronic, photovoltaics devices, and biochemical sensors.

Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures

Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a "lying-down" pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional "standing-up" crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications.

Three-Component Integrated Ultrathin Organic Photosensors for Plastic Optoelectronics

By three-component integration, an integrated organic photosensor is presented using common organic dyes as building blocks. Gray-scale photosensing and signal amplification are achieved in the device within a wide range of light intensities. Moreover, with ultrathin film techniques, 470 nm thick devices are realized and continue to work when harshly bent.

Organic thin film transistors with novel photosensitive polyurethane as dielectric layer

The solution-processable photosensitive polyurethane dielectric film is a promising candidate for the exploration of organic thin-film transistors (OTFTs).

A novel polymer as a functional dielectric layer for OTFTs to improve the grain size of the pentacene semiconductor

A series of novel polymers as functional dielectric layers for pentacene thin-film transistors was synthesized and investigated to explore the relationship between the grain size and the charge carrier mobility with a single variable.

Organic thin-film transistors with novel high-k polymers as dielectric layers

A series of novel polymers are promising candidates for the exploration of low-voltage organic thin-film transistors (OTFTs).

Poly(sodium-4-styrene sulfonate) (PSSNa)-assisted transferable flexible, top-contact high-resolution free-standing organic field-effect transistors

Here we demonstrate how, by means of poly(sodium-4-styrene sulfonate), one can successfully transfer the free-standing, flexible, high-resolution top-contact OFETs based on polystyrene insulator to arbitrary substrates.

Novel high-k polymers as dielectric layers for organic thin-film transistors

The novel high-k polymers are promising candidates for the exploration of low-threshold-voltage organic thin-film transistors (OTFTs).

Chemically tunable ultrathin silsesquiazane interlayer for n-type and p-type organic transistors on flexible plastic

In organic field-effect transistors (OFETs), surface modification of the gate-dielectric is a critical technique for enhancing the electrical properties of the device. Here, we report a simple and versatile method for fabricating an ultrathin cross-linked interlayer (thickness ∼3 nm) on an oxide gate dielectric by using polymeric silsesquiazane (SSQZ). The fabricated siloxane film exhibited an ultrasmooth surface with minimal hydroxyl groups; the properties of the surface were chemically tuned by introducing phenyl and phenyl/fluorine pendent groups into the SSQZ. The growth characteristics of two semiconductors-pentacene (p-type) and N,N'-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C13, n-type)-on this ultrathin film were systematically investigated according to the type of pendent groups in the SSQZ-treated gate dielectric. Pentacene films on phenyl/fluorine groups exhibited large grains and excellent crystalline homogeneity. By contrast, PTCDI-C13 films exhibited greater crystalline order and perfectness when deposited on phenyl groups rather than on phenyl/fluorine groups. These microstructural characteristics of the organic semiconductors, as well as the dipole moment of the pendent groups, determined the electrical properties of FETs based on pentacene or PTCDI-C13. Importantly, compared to FETs in which the gate dielectric was treated with a silane-coupling agent (a commonly used surface treatment), the FETs fabricated using the tunable SSQZ treatment showed much higher field-effect mobilities. Finally, surface treatment with an ultrathin SSQZ layer was also utilized to fabricate flexible OFETs on a plastic substrate. This was facilitated by the facile SSQZ deposition process and the compatibility of SSQZ with the plastic substrate.

Organic thermoelectric materials: emerging green energy materials converting heat to electricity directly and efficiently

The abundance of solar thermal energy and the widespread demands for waste heat recovery make thermoelectric generators (TEGs) very attractive in harvesting low-cost energy resources. Meanwhile, thermoelectric refrigeration is promising for local cooling and niche applications. In this context there is currently a growing interest in developing organic thermoelectric materials which are flexible, cost-effective, eco-friendly and potentially energy-efficient. In particular, the past several years have witnessed remarkable progress in organic thermoelectric materials and devices. In this review, thermoelectric properties of conducting polymers and small molecules are summarized, with recent progresses in materials, measurements and devices highlighted. Prospects and suggestions for future research efforts are also presented. The organic thermoelectric materials are emerging candidates for green energy conversion.

"Double exposure method": a novel photolithographic process to fabricate flexible organic field-effect transistors and circuits

A novel process called "double exposure method" has for the first time been developed to utilize common organic materials as insulating layers at low annealing temperature in the process of photolithography. In this method, organic dielectric layer will not dissolve in the final lift-off step by using developer to replace traditional acetone. Bottom-gate bottom-contact (BGBC) OFETs are fabricated on the flexible PET substrates with polystyrene (PS) and pentacene as dielectric layer and semiconductor layer, respectively. Transistors with mobility of 0.36 cm2 V(-1) s(-1) and logic inverter with gain of 9 on the plastic substrates have been fabricated, demonstrating the potential appliction of "double exposure method" in flexible organic electronics.

Multifunctional phosphonic acid self-assembled monolayers on metal oxides as dielectrics, interface modification layers and semiconductors for low-voltage high-performance organic field-effect transistors

Insulating and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2 V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source-drain and dielectric surfaces in bottom-contact OFETs, and (3) SAM-FETs based on molecular monolayer semiconductors. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV dec(-1), high I(on)/I(off) ratios of 10(5)-10(7), contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1-4.6 cm(2) V(-1) s(-1), and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors.

Comprehensive approach to intrinsic charge carrier mobility in conjugated organic molecules, macromolecules, and supramolecular architectures

Si-based inorganic electronics have long dominated the semiconductor industry. However, in recent years conjugated polymers have attracted increasing attention because such systems are flexible and offer the potential for low-cost, large-area production via roll-to-roll processing. The state-of-the-art organic conjugated molecular crystals can exhibit charge carrier mobilities (μ) that nearly match or even exceed that of amorphous silicon (1-10 cm(2) V(-1) s(-1)). The mean free path of the charge carriers estimated from these mobilities corresponds to the typical intersite (intermolecular) hopping distances in conjugated organic materials, which strongly suggests that the conduction model for the electronic band structure only applies to μ > 1 cm(2) V(-1) s(-1) for the translational motion of the charge carriers. However, to analyze the transport mechanism in organic electronics, researchers conventionally use a disorder formalism, where μ is usually less than 1 cm(2) V(-1) s(-1) and dominated by impurities, disorders, or defects that disturb the long-range translational motion. In this Account, we discuss the relationship between the alternating-current and direct-current mobilities of charge carriers, using time-resolved microwave conductivity (TRMC) and other techniques including field-effect transistor, time-of-flight, and space-charge limited current. TRMC measures the nanometer-scale mobility of charge carriers under an oscillating microwave electric field with no contact between the semiconductors and the metals. This separation allows us to evaluate the intrinsic charge carrier mobility with minimal trapping effects. We review a wide variety of organic electronics in terms of their charge carrier mobilities, and we describe recent studies of macromolecules, molecular crystals, and supramolecular architecture. For example, a rigid poly(phenylene-co-ethynylene) included in permethylated cyclodextrin shows a high intramolecular hole mobility of 0.5 cm(2) V(-1) s(-1), based on a combination of flash-photolysis TRMC and transient absorption spectroscopy (TAS) measurements. Single-crystal rubrene showed an ambipolarity with anisotropic charge carrier transport along each crystal axis on the nanometer scale. Finally, we describe the charge carrier mobility of a self-assembled nanotube consisting of a large π-plane of hexabenzocoronene (HBC) partially appended with an electron acceptor. The local (intratubular) charge carrier mobility reached 3 cm(2) V(-1) s(-1) for the nanotubes that possessed well-ordered π-stacking, but it dropped to 0.7 cm(2) V(-1) s(-1) in regions that contained greater amounts of the electron acceptor because those molecules reduced the structural integrity of π-stacked HBC arrays. Interestingly, the long-range (intertubular) charge carrier mobility was on the order of 10(-4) cm(2) V(-1) s(-1) and monotonically decreased when the acceptor content was increased. These results suggest the importance of investigating charge carrier mobilities by frequency-dependent charge carrier motion for the development of more efficient organic electronic devices.

Electrochemical doping for lowering contact barriers in organic field effect transistors

By electrochemically p-doping pentacene in the vicinity of the source-drain electrodes in organic field effect transistors the injection barrier for holes is decreased. The focus of this work is put on the influence of the p-doping process on the transistor performance. Cyclic voltammetry performed on a pentacene based transistor exhibits a reversible p-doping response. This doped state is evoked at the transistor injection electrodes. An improvement is observed when comparing transistor characteristics before and after the doping process apparent by an improved transistor on-current. This effect is reflected in the analysis of the contact resistances of the devices.