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With growing interest in organic phototransistors, as not only sensors but also neuromorphic computing elements, the vast majority of research investigates structures comprising Ohmic source/drain contacts. Here, it is shown how source‐gated transistors (SGTs), in which a source contact barrier dominates electrical characteristics, can be implemented as phototransistors. Organic photo‐SGTs (OPSGTs) based on vacuum‐processed small‐molecule dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐]thiophene (DNTT) demonstrate low saturation voltage, exceptional tolerance to channel length variation, and photo‐to‐dark current ratio (PDCR) peaks over 106 for 819 µW broad spectrum incident light power. At zero gate‐source voltage, the PDCR reaches 104, showing promise for simple sensor circuit implementation in medical and wellbeing applications. [ABSTRACT FROM AUTHOR]

The development of organic thin-film transistors (TFTs) for high-frequency applications requires a detailed understanding of the intrinsic and extrinsic factors that influence their dynamic performance. This includes a wide range of properties, such as the device architecture, the contact resistance, parasitic capacitances, and intentional or unintentional asymmetries of the gate-to-contact overlaps. Here, we present a comprehensive analysis of the dynamic characteristics of the highest-performing flexible organic TFTs reported to date. For this purpose, we have developed the first compact model that provides a complete and accurate closed-form description of the frequency-dependent small-signal gain of organic field-effect transistors. The model properly accounts for all relevant secondary effects, such as the contact resistance, fringe capacitances, the subthreshold regime, charge traps, and non-quasistatic effects. We have analyzed the frequency behavior of low-voltage organic transistors fabricated in both coplanar and staggered device architectures on flexible plastic substrates. We show through S-parameter measurements that coplanar transistors yield more ideal small-signal characteristics with only a weak dependence on the overlap asymmetry. In contrast, the high-frequency behavior of staggered transistors suffers from a more pronounced dependence on the asymmetry. Using our advanced compact model, we elucidate the factors influencing the frequency-dependent small-signal gain and find that even though coplanar transistors have larger capacitances than staggered transistors, they benefit from substantially larger transconductances, which is the main reason for their superior dynamic performance. [ABSTRACT FROM AUTHOR]

Organic thin‐film transistors are field‐effect transistors comprising a semiconductor in the form of a thin, typically polycrystalline layer of conjugated organic molecules. Since organic transistors can often be fabricated at temperatures no higher than about 100 °C, they are potentially useful for flexible, large‐area electronics applications. An important performance parameter of organic transistors is the transit frequency, which is the highest frequency at which the transistors can be operated. The transit frequency of organic transistors is determined in large part by the channel length and the parasitic gate‐to‐source and gate‐to‐drain overlap lengths. How small these dimensions can be made depends (at least in the case of transistors fabricated in the lateral device architecture) greatly on the resolution of the lithography method that is utilized for the patterning of the gate electrodes and the source and drain contacts. Patterning methods that have yielded organic transistors with lateral dimensions sufficiently small to provide transit frequencies above 10 MHz include photolithography, laser lithography, stencil lithography, and electron‐beam lithography. In this review, these four lithography methods and their roles in the fabrication of high‐frequency organic transistors, as well as their prospects for future improvements in the dynamic performance of organic transistors, will be illuminated. [ABSTRACT FROM AUTHOR]

Low saturation voltages and extremely high intrinsic gain can be achieved in contact‐controlled thin‐film transistors (TFTs) with staggered device architecture, enabled by the energy barrier introduced at the source contact. The resulting device, the source‐gated transistor (SGT), is limited in its usefulness by the high temperature dependence of the drain current induced by the source energy barrier. Here, the interaction between the thermal characteristics of the source contact and the semiconductor to show drastically reduced temperature dependence for SGTs based on organic semiconductors (OSGTs) is exploited. This extraordinarily weak temperature dependence of the drain current is observed regardless of the height of the source energy barrier (27.8% in OSGTs with Ti contacts compared to 22.1% when using Au contacts, over a 34 K range). The reduction in mobility of the semiconductor offsets an increase in thermionic‐field emission of charge carriers at the source. This is a first for SGTs and provides a route to removing one of the last hurdles to their wider adoption. The OSGTs with Ti contacts also demonstrate: drain‐current saturation at very low drain‐source voltages (saturation factor of 0.22); noteworthy stability after 70 days; and minimal drain‐current variation with channel length or illumination. [ABSTRACT FROM AUTHOR]

Organic thin‐film transistors (TFTs) that provide subthreshold swings near the theoretical limit together with large on/off current ratios at very low operating voltages require high‐capacitance gate dielectrics with a vanishingly small defect density. A promising approach to the fabrication of such dielectrics at temperatures sufficiently low to allow TFT fabrication on polymeric substrates are hybrid dielectrics consisting of a thin metal oxide layer in combination with a molecular self‐assembled monolayer (SAM). Here, the electrical and surface properties of titanium oxide produced by the plasma‐assisted oxidation of the surface of vacuum‐deposited titanium gate electrodes and its use as the first component of a hybrid TiOx/SAM gate dielectric in flexible organic TFTs are investigated. These transistors have a gate‐dielectric capacitance of about 1 µF cm−2, a subthreshold swing of 59 mV decade−1 (within measurement error of the physical limit at room temperature) for a wide range of channel lengths as small as 0.7 µm, and an on/off current ratio of 107 for a gate‐source‐voltage range of 1 V. [ABSTRACT FROM AUTHOR]

Contact‐controlled transistors are rapidly gaining popularity. However, simply using a rectifying source contact often leads to unsatisfactory operation, merely as a thin‐film transistor with low drain current and reduced effective mobility. This may cause otherwise promising experiments to be abandoned. Here, data from literature is analyzed in conjunction with devices that have been recently fabricated in polysilicon, organic and oxide semiconductors, highlighting the main factor in achieving good saturation, namely keeping saturation coefficient γ well below 0.3. Secondary causes of suboptimal electrical characteristics are also discussed. Correct design of these alternative device structures will expedite their adoption for high gain, low‐frequency applications in emerging sensor circuits. [ABSTRACT FROM AUTHOR]

Herein, we describe a gold‐catalyzed cascade cyclization of Boc‐protected benzylamines bearing two tethered alkyne moieties in a domino reaction initiated by a 6‐endo‐dig cyclization. The reaction was screened intensively, and the scope was explored, resulting in nine new Boc‐protected dihydrobenzo[c]phenanthridines with yields of up to 98 %; even a π‐extension and two bidirectional approaches were successful. Furthermore, thermal cleavage of the Boc group and subsequent oxidation gave substituted benzo[c]phenanthridines in up to quantitative yields. Two bidirectional approaches under the optimized conditions were successful, and the resulting π‐extended molecules were tested as organic semiconductors in organic thin‐film transistors. [ABSTRACT FROM AUTHOR]

Herein we describe a gold‐catalyzed bidirectional synthesis of N‐heteropolycyclic compounds bearing carbazole moieties – namely π‐extended benzodicarbazoles and π‐extended indolocarbazoles. Overall, four previously unknown core structures were synthesized. This approach is convergent, modular and the gold‐catalyzed key step comprises of a cascade reaction starting from stable di‐azido compounds. The obtained molecules were fully characterized and their optical and electronic properties as well as their performance in organic thin‐film transistors generated by vacuum deposition were studied. Charge‐carrier mobilities of up to 0.3 cm2/Vs were measured. [ABSTRACT FROM AUTHOR]

A floating-gate (FG) cell as a circuit-level approach to control the threshold voltage of organic thin-film transistors (TFTs) operated in the transdiode configuration is presented. Charging and discharging of the FG are achieved by controlling the charge leakage through the gate dielectric of one of the organic TFTs that constitute the FG cell. Using programming voltages not exceeding 4 V, systematic tuning of the threshold voltage to values between −0.5 and 2.6 V was achieved. The versatility of the concept is demonstrated by employing organic-TFT-based FG cells as transdiodes with programmable threshold voltage in passive rectifiers and diode-load inverters fabricated on flexible, transparent plastic substrates. Rectifiers with programmable FG cells show flatter frequency response, improved 3-dB point, and reduced ripple compared to conventional rectifiers. Inverters with programmable FG-transdiode load have larger small-signal gain, larger output-voltage swing, and larger noise margins than conventional diode-load inverters. [ABSTRACT FROM AUTHOR]

A new synthetic route to tetraindenopyrene (TIP)—a bowl‐shaped cut‐out structure of C70—is reported. The key step in this approach is a fourfold palladium‐catalyzed C−H activation that increases the yield more than 50 times in comparison to the approach originally described by Scott and co‐workers. Besides examination of its optoelectronic properties and study of its aggregation in solution, TIP was also re‐investigated by dispersion‐corrected DFT methods, which showed that dispersion interactions significantly increase the bowl‐to‐bowl inversion barrier. Furthermore, TIP was used as a semiconductor in p‐channel thin‐film transistors (TFTs). [ABSTRACT FROM AUTHOR]

Rectifier circuits featuring low threshold voltages and high cutoff frequencies based on p-channel organic thin-film transistors (TFTs) have been designed, fabricated and characterized. The TFTs and circuits were fabricated by shadow-mask lithography on flexible plastic substrates using the vacuum-deposited small-molecule organic semiconductor dinaphtho[2,3-b: 2',3'-ƒ]thieno[3,2-b] thiophene (DNTT). The TFTs have a gate dielectric with a thickness of 5.3 nm and a channel length of 10 &#956m. The study considers the frequency characteristics of diode-connected transistors (transdiodes) and adopts circuit techniques from silicon CMOS technology, namely single-stage and multistage dynamic-threshold-compensated differential rectifiers. The characterization of the rectifier circuits indicates cutoff frequencies up to 4.75 MHz at a peak-to-peak input voltage of 3 V for transdiodes, up to 32 MHz at a peak-to-peak input voltage of 1.5 V for single-stage differential rectifiers and up to 7.5 MHz at a peak-to-peak input voltage of 1.5 V for two-stage rectifiers. The efficiency is 25% for a load of 10 M\Ω and below 1% for a load of 1 M\Ω . [ABSTRACT FROM AUTHOR]

The article presents a study related to flexible low-voltage high-frequency organic thin-film transistors (TFTs). It mentions that most organic-TFT technologies show limited dynamic performance unless high operating voltages are applied to mitigate high contact resistances and large parasitic capacitances; and also mentions that TFT enters a contact-limited regime where the contact resistance constitutes a substantial portion of the total resistance of the TFT.

In organic thin‐film transistors (TFTs) fabricated in the inverted (bottom‐gate) device structure, the surface roughness of the gate dielectric onto which the organic‐semiconductor layer is deposited is expected to have a significant effect on the TFT characteristics. To quantitatively evaluate this effect, a method to tune the surface roughness of a gate dielectric consisting of a thin layer of aluminum oxide and an alkylphosphonic acid self‐assembled monolayer over a wide range by controlling a single process parameter, namely the substrate temperature during the deposition of the aluminum gate electrodes, is developed. All other process parameters remain constant in the experiments, so that any differences observed in the TFT performance can be confidently ascribed to effects related to the difference in the gate‐dielectric surface roughness. It is found that an increase in surface roughness leads to a significant decrease in the effective charge‐carrier mobility and an increase in the subthreshold swing. It is shown that a larger gate‐dielectric surface roughness leads to a larger density of grain boundaries in the semiconductor layer, which in turn produces a larger density of localized trap states in the semiconductor. [ABSTRACT FROM AUTHOR]

Despite the large body of research conducted on organic transistors, the transit frequency of organic field‐effect transistors has seen virtually no improvement for a decade and remains far below 1 GHz. One reason is that most of the research is still focused on improving the charge‐carrier mobility, a parameter that has little influence on the transit frequency of short‐channel transistors. By examining the fundamental equations for the transit frequency of field‐effect transistors and by extrapolating recent progress on the relevant device parameters, a roadmap to gigahertz organic transistors is derived. [ABSTRACT FROM AUTHOR]

The pressure dependence of pentacene (C22H14) transistors with solution-processed polyvinylphenol gate dielectric on glass substrates is investigated by applying uniaxial mechanical pressure with a needle. We found that organic thin-film transistors are sensitive to applied pressure inherently. The measurements reveal a reversible current dependence of the transfer characteristics where the drain current is switching between two states. Experimental and simulation results suggest that switch-on voltage and interface resistance are affected. The change takes seconds, hinting at trap states being responsible for the effect. [ABSTRACT FROM AUTHOR]

This article presents a novel generic method for the extraction of the current–voltage characteristics of organic thin-film transistors (TFTs) that specifically accounts for the individual contributions of the source and drain contacts and their nonlinear behavior. Based on this extraction method, an analytical model has been derived that describes the behavior of the intrinsic TFT and of the source and drain contacts both in the linear and the saturation regimes. The essential feature of the proposed extraction method is that it does not depend on or assume any physical phenomena underlying the electrical characteristics of the contacts. Instead, it extracts the current–voltage characteristics of the contacts which can then be fit to any physics-based TFT model. This is beneficial both for circuit designers and for those developing physics-based TFT models. The proposed extraction method has been applied to organic TFTs fabricated on flexible plastic substrates in the staggered and coplanar device architectures using two different small-molecule organic semiconductors. The compact dc model has been used to simulate organic TFTs with channel lengths ranging from 4 to $100~\mu \text{m}$ and a transimpedance amplifier circuit based on TFTs with different channel lengths, and the results are in good agreement with the measurements. [ABSTRACT FROM AUTHOR]

A dense array of 500 organic TFTs with two different threshold voltages arranged in a checkerboard pattern has been fabricated. The threshold voltages were defined by preparing self-assembled monolayers (SAMs) of either an alkyl or a fluoroalkylphosphonic acid on the gate-oxide surface of each TFT, using a combination of microcontact printing from an elastomeric stamp and dipping into a solution. The threshold voltages are −1.01 ± 0.15 V for the TFTs with the fluoroalkyl SAM and −1.28 ± 0.23 V for the TFTs with the alkyl SAM. ToF-SIMS analysis shows that the two SAMs can be patterned with a pitch of 10 μm and without significant cross-contamination. Cross-sectional TEM and NEXAFS characterization of the SAMs indicate that the properties of the SAMs prepared by microcontact printing and dipping are essentially identical. [ABSTRACT FROM AUTHOR]

Organic thin-film transistors (TFTs) are prepared by vacuum deposition and solution shearing of 2,9-bis(perfluoroalkyl)-substituted tetraazaperopyrenes (TAPPs) with bromine substituents at the aromatic core. The TAPP derivatives are synthesized by reacting known unsubstituted TAPPs with bromine in fuming sulphuric acid, and their electrochemical properties are studied in detail by cyclic voltammetry and modelled with density functional theory (DFT) methods. Lowest unoccupied molecular orbital (LUMO) energies and electron affinities indicate that the core-brominated TAPPs should exhibit n-channel semiconducting properties. Current-voltage characteristics of the TFTs established electron mobilities of up to μn = 0.032 cm2 V−1 s−1 for a derivative which was subsequently processed in the fabrication of a complementary ring oscillator on a flexible plastic substrate (PEN). [ABSTRACT FROM AUTHOR]

We successfully fabricated a large-area flexible strain-sensing system based on a 2-D array of organic self-bias-feedback amplifier with a signal gain of 400. The amplifier system consists of three layers: a self-assembled monolayer (SAM) capacitor matrix, a 2-D array of organic pseudo-CMOS inverters with a floating-gate structure using SAM gate dielectric, and an active matrix of organic thin-film transistors. The amplifier sheet comprises 8 \times 8 amplifier cells, with an effective size of \7 \times \7\ \cm^2. The organic transistors exhibit a mobility of 1.7 \cm^2/\V\cdot \s in the saturation regime at an operation voltage of 2 V. A strain sensor is made of a polymeric piezoelectric [polyvinylidene difluoride (PVDF)] sheet. When a cell of the PVDF sheet is touched (that is, when mechanical pressure is applied), a small signal is generated by intermolecular polarization in the PVDF. These signals are amplified by the organic amplifier circuits from 10 to 150 mV. [ABSTRACT FROM AUTHOR]

In this paper we present User Customizable Logic Paper (UCLP) with a Sea-of Transmission-Gates (SOTG) of 2-V organic CMOS transistors. This can enable users to fabricate custom integrated circuits, by printing 200 \mu\ m wide interconnects with at-home ink-jet printers for the prototyping of large-area electronics and educational purposes. The SOTG reduces the area of the circuits in UCLP by between 11% and 85% compared with a conventional gate array architecture. [ABSTRACT FROM AUTHOR]

Flexible electronic circuits are an essential prerequisite for the development of rollable displays, conformable sensors, biodegradable electronics and other applications with unconventional form factors. The smallest radius into which a circuit can be bent is typically several millimetres, limited by strain-induced damage to the active circuit elements. Bending-induced damage can be avoided by placing the circuit elements on rigid islands connected by stretchable wires, but the presence of rigid areas within the substrate plane limits the bending radius. Here we demonstrate organic transistors and complementary circuits that continue to operate without degradation while being folded into a radius of 100 μm. This enormous flexibility and bending stability is enabled by a very thin plastic substrate (12.5 μm), an atomically smooth planarization coating and a hybrid encapsulation stack that places the transistors in the neutral strain position. We demonstrate a potential application as a catheter with a sheet of transistors and sensors wrapped around it that enables the spatially resolved measurement of physical or chemical properties inside long, narrow tubes. [ABSTRACT FROM AUTHOR]

The bias stress effect in pentacene organic thin-film transistors has been investigated. The transistors utilize a thin gate dielectric based on an organic self-assembled monolayer and thus can be operated at low voltages. The bias stress-induced threshold voltage shift has been analyzed for different drain-source voltages. By fitting the time-dependent threshold voltage shift to a stretched exponential function, both the maximum (equilibrium) threshold voltage shift and the time constant of the threshold voltage shift were determined for each drain-source voltage. It was found that both the equilibrium threshold voltage shift and the time constant decrease significantly with increasing drain-source voltage. This suggests that when a drain-source voltage is applied to the transistor during gate bias stress, the tilting of the HOMO and LUMO bands along the channel creates a pathway for the fast release of trapped carriers. [ABSTRACT FROM AUTHOR]

We report the first organic complementary circuits on flexible substrates. Organic thin-film transistors were fabricated Using pentacene as the semiconductor for the p-channel devices and hexadecafluorocop-perphthalocyanine (F16 CuPc) as the semiconductor for the n-channel devices. Both semiconductors were purchased from commercial sources and deposited by evaporation in vacuum. The pentacene layer was photolithographically patterned to simplify the circuit layout and reduce the circuit area. The transistors and circuits were manufactured on thin, transparent sheets of polyethylene naphthalate. Evaporated metals were used to define all contacts and interconnects, and a 50-nm-thick layer of solution-processed polyvinylphenol was used as the gate dielectric. Transistors and circuits operate at supply voltages as low as 8 V, and ring oscillators have a signal propagation delay as low as 8 μs per stage. To our knowledge, these are the fastest organic complementary circuits reported to date. [ABSTRACT FROM AUTHOR]

Organic thin film transistors (TFTs) are of interest for a variety of large-area electronic applications, such as displays, sensors and electronic barcodes. One of the key problems with existing organic TFTs is their large operating voltage, which often exceeds 20?V. This is due to poor capacitive coupling through relatively thick gate dielectric layers: these dielectrics are usually either inorganic oxides or nitrides, or insulating polymers, and are often thicker than 100?nm to minimize gate leakage currents. Here we demonstrate a manufacturing process for TFTs with a 2.5-nm-thick molecular self-assembled monolayer (SAM) gate dielectric and a high-mobility organic semiconductor (pentacene). These TFTs operate with supply voltages of less than 2?V, yet have gate currents that are lower than those of advanced silicon field-effect transistors with SiO2 dielectrics. These results should therefore increase the prospects of using organic TFTs in low-power applications (such as portable devices). Moreover, molecular SAMs may even be of interest for advanced silicon transistors where the continued reduction in dielectric thickness leads to ever greater gate leakage and power dissipation. [ABSTRACT FROM AUTHOR]

Pseudo-CMOS inverters operating at 2 V and comprising four p-type organic transistors with ultrahigh gain are fabricated using self-assembled monolayer gate dielectrics. The inverter gain is as large as 302 at an operation voltage of 2 V, whereas the minimum operation voltage is as small as 0.5 V. The oscillation frequency of a five-stage ring oscillator comprising pseudo-CMOS inverters is 4.27 kHz at 2 V, corresponding to 23.4 \mu\s of propagation delay per stage. This is the fastest among organic circuits operating at low voltage. Pseudo-CMOS amplifier circuits show a large gain of 240 for a 3.0-mV input voltage. [ABSTRACT FROM AUTHOR]

We have demonstrated threshold-voltage control of p- and n-channel organic transistors with a floating-gate structure and self-assembled monolayer-based gate dielectrics and applied this technique to tune the switching voltage of organic complementary inverters. The threshold voltages of the p- and n-channel transistors are changed independently and systematically across a wide range from +2.4 to -1 V and from -0.3 to +1.5 V, respectively, when the program voltages of -6 V and +6 V are applied to the p- and n-channel transistors, respectively. Furthermore, we fabricated tunable organic complementary inverters, and ring oscillators whose oscillations are controlled by varying floating-gate charges. [ABSTRACT FROM AUTHOR]

We have fabricated pentacene organic thin-film transistors (TFTs) using self-assembled monolayers (SAMs) based on alkyl-phosphonic acids with five different alkyl chain lengths as the gate dielectric and investigated the relationship between the SAM chain length and the electrical performance and stability of the transistors. A SAM chain length of 14 carbon atoms provides a maximum TFT mobility of 0.7 cm2/V s, along with an on/off current ratio greater than 105. We have also investigated the bias stress effect in these TFTs and found that the change in drain current is substantially less severe than in pentacene TFTs with polymer gate dielectrics. [ABSTRACT FROM AUTHOR]

We have fabricated organic thin film transistors, inverters, and ring oscillators on glass and on flexible polyethylene naphthalate, using the small-molecule hydrocarbon pentacene as the semiconductor and solution-processed polyvinylphenol as the gate dielectric. Depending on the choice of substrate, the transistors have a carrier mobility between 0.3 and 0.7 cm[SUP2]/V s, an on/off current ratio between 10[SUP5] and 10[SUP6], and a subthreshold swing between 0.9 and 1.6 V/decade. To account for the positive switch-on voltage of the transistors, circuits were designed to operate with integrated level shifting. Depending on the type of substrate, ring oscillators have a signal propagation delay as low as 15 μs per stage. [ABSTRACT FROM AUTHOR]

A charge-based compact model for the bias-dependent low-frequency noise in organic thin-film transistors is presented. The model combines the carrier-number fluctuation effect and the fluctuation of the carrier mobility according to the Hooge model to predict the drain-current noise. The model has been verified using experimental data from a large number of organic transistors with two different channel lengths fabricated in the inverted staggered device architecture on a flexible polymeric substrate. [Display omitted] • Charge-based compact model for the low-frequency noise in organic TFTs. • The model is using the carrier-number fluctuation and mobility-fluctuation effects. • Organic TFTs fabricated in the inverted staggered device architecture were tested. • The model is implemented in Verilog-A. [ABSTRACT FROM AUTHOR]

Low-voltage p-channel organic thin-film transistors based on [n]phenacene (n = 5, 6 or 7) were fabricated on glass and on flexible poly(ethylene 2,6-naphthalate) (PEN) substrates. For the first time, these phenacenes were combined with two ultrathin gate dielectrics based on aluminium oxide and a monolayer of octadecyl-phosphonic acid in three different transistor structures. Regardless of the substrate and the transistor structure, the field-effect mobility is found to increase with increasing length of the conjugated [n]phenacene core, leading to the best performance for [7]phenacene. The largest average field-effect mobility we have obtained is 0.27 cm2/V⋅s for transistors on glass and 0.092 cm2/V⋅s for transistors on flexible PEN. Image 1 • Fabricated low-voltage organic transistors based on [5], [6] and [7]phenacenes. • Two bi-layer gate dielectrics (AlO x functionalised with phosphonic acid) used. • The field-effect mobility increases with increasing length of phenacene core. • For transistors on glass the highest average field-effect mobility is 0.27 cm2/V⋅s. • For transistors on flexible PEN the highest average field-effect mobility is 0.092 cm2/V⋅s. [ABSTRACT FROM AUTHOR]