Performance of Organic Electrochemical Transistors with Ionic Liquid Crystal Elastomers as Solid Electrolytes.

Organic electrochemical transistors (OECTs) have emerged as attractive devices for bioelectronics, wearable electronics, soft robotics, and energy storage devices. The electrolyte, being a fundamental component of OECTs, plays a crucial role in their performance. Recently, it has been demonstrated that ionic liquid crystal elastomers (iLCEs) can be used as a solid electrolyte for OECTs. Their capabilities, however, have only been shown for relatively large size substrate-free OECTs. Here, we study the influence of the different alignments of iLCEs on steady state and transient behavior of OECTs using a lateral geometry with source, drain, and gate in the same plane. We achieve excellent electrical response with an ON/OFF switching ratio of >105 and minimal leakage current. The normalized maximum transconductance gm/w of the most sensitive iLCE was found to be 33 S m-1, which is one of the highest among all solid-state-based OECTs reported so far. Additionally, iLCEs show high stability and can be removed and reattached multiple times to the same OECT device without decreasing performance.

Approaching the Intrinsic Limits of Short Channel Vertical Organic Electrochemical Transistors.

Vertical architectures for organic electrochemical transistors (OECTs), due to their submicrometer channel lengths, have presented themselves as a straightforward design approach for achieving high gm/τ ratios, a figure of merit that assesses the performance of the devices by virtue of their transconductance (gm = dID/dVGS) and switching time constant (τ). However, as the practical limitations of the geometries are overcome, the influence of parasitic phenomena becomes more dominant and limits the performance of the device. One approach to reduce the detrimental effects of parasitic resistance in the drain-source circuit is to use a four-point sourcing technique. Here, vertical OECTs are fabricated with four-point structures to approach the intrinsic limit of these devices. It is shown that this approach improves the saturation behavior of the devices, closing the gap between measured gm and intrinsic transconductance gmi at their peak values. Overall, the results discussed here provide insight into the effects of parasitic resistance on OECTs, which in contrast to field-effect transistors, are not as extensively documented.

Influence of Injection Barrier on Vertical Organic Field Effect Transistors.

Organic field-effect transistors (OFETs) have shown great potential for applications that require low temperature deposition on large and flexible substrates. To increase their performance, in particular a high transconductance and transit frequency, the transistor channel length has to be scaled into the submicrometer regime, which can be easily achieved in vertical organic field effect transistors (VOFETs). However, despite high performance observed in VOFETs, these transistors usually suffer from short channel effects like weak saturation of the drain current and direct source-drain leakage resulting in large off currents. Here, we study the influence of the injection barrier at the source electrode on the OFF currents, on/off ratio, and transconductance of vertical OFETs. We use two semiconducting materials, 2,6-diphenyl anthracene (DPA), and C60 to vary the injection barrier at the source electrode and are able to show that increasing the Schottky barrier at the source electrode can decrease the direct source/drain leakage by 3 orders of magnitude. However, the increased injection barrier at the source electrode comes at the expense of an increased contact resistance, which in turn will decrease its transconductance and transit frequency. With the help of a 2D drift-diffusion simulation we show that the trade-off between low off currents and high transconductance is inherent to the current VOFET device setup and that new approaches have to be found to design VOFETs that combine good switching properties with high performance.

Stainless-Steel Antenna on Conductive Substrate for an SHM Sensor System with High Power Demand.

This paper presents the novel concept of structuring a planar coil antenna structured into the outermost stainless-steel layer of a fiber metal laminate (FML) and investigating its performance. Furthermore, the antenna is modified to sufficiently work on inhomogeneous conductive substrates such as carbon-fiber-reinforced polymers (CFRP) independent from their application-dependent layer configuration, since the influence on antenna performance was expected to be configuration-dependent. The effects of different stack-ups on antenna characteristics and strategies to cope with these influences are investigated. The purpose was to create a wireless self-sustained sensor node for an embedded structural health monitoring (SHM) system inside the monitored material itself. The requirements of such a system are investigated, and measurements on the amount of wireless power that can be harvested are conducted. Mechanical investigations are performed to identify the antenna shape that produces the least wound to the material, and electrical investigations are executed to prove the on-conductor optimization concept. Furthermore, a suitable process to fabricate such antennas is introduced. First measurements fulfilled the expectations: the measured antenna structure prototype could provide up to 11 mW to a sensor node inside the FML component.

Flexo-Ionic Effect of Ionic Liquid Crystal Elastomers.

The first study of the flexo-ionic effect, i.e., mechanical deformation-induced electric signal, of the recently discovered ionic liquid crystal elastomers (iLCEs) is reported. The measured flexo-ionic coefficients were found to strongly depend on the director alignment of the iLCE films and can be over 200 µC/m. This value is orders of magnitude higher than the flexo-electric coefficient found in insulating liquid crystals and is comparable to the well-developed ionic polymers (iEAPs). The shortest response times, i.e., the largest bandwidth of the flexo-ionic responses, is achieved in planar alignment, when the director is uniformly parallel to the substrates. These results render high potential for iLCE-based devices for applications in sensors and wearable micropower generators.

Analytic Device Model of Organic Field-Effect Transistors with Doped Channels.

Doping has been shown to not only provide additional degrees of freedom in the design of organic field-effect transistors (OFETs) but to increase their performance and stability as well. An analytical model based on the assumption of a square doping profile inside the channel is presented here that describes the effect of doping on the transfer characteristic of OFETs. The model is validated experimentally by a series of OFETs with varying doping conditions. The precise doping profile in the transistor channel is determined by fitting the capacitance/voltage response of doped metal-insulator-semiconductor (MIS) junctions using an AC small-signal drift-diffusion simulation. It is shown that the real doping profile deviates from the simplifying assumptions of the analytical model, i.e., it is found that the effective doping concentration at the dielectric/semiconductor interface is reduced. However, it is shown that the analytical model is not sensitive to this deviation as only the total density charges per unit area determine the changes in the transistor behavior. Overall, the presented theory provides new design rules that can be used to guide the development of doped OFETs with high performance.

Finding the equilibrium of organic electrochemical transistors.

Organic Electrochemical Transistors are versatile sensors that became essential for the field of organic bioelectronics. However, despite their importance, an incomplete understanding of their working mechanism is currently precluding a targeted design of Organic Electrochemical Transistors and it is still challenging to formulate precise design rules guiding materials development in this field. Here, it is argued that current capacitive device models neglect lateral ion currents in the transistor channel and therefore fail to describe the equilibrium state of Organic Electrochemical Transistors. An improved model is presented, which shows that lateral ion currents lead to an accumulation of ions at the drain contact, which significantly alters the transistor behavior. Overall, these results show that a better understanding of the interface between the organic semiconductor and the drain electrode is needed to reach a full understanding of Organic Electrochemical Transistors.

Poly(ethylene glycol) Diacrylate Based Electro-Active Ionic Elastomer.

Preparation and low voltage induced bending (converse flexoelectricity) of crosslinked poly(ethylene glycol) diacrylate (PEGDA), modified with thiosiloxane (TS) and ionic liquid (1-hexyl-3-methylimidazolium hexafluorophosphate) (IL) are reported. In between 2µm PEDOT:PSS electrodes at 1 V, it provides durable (95% retention under 5000 cycles) and relatively fast (2 s switching time) actuation with the second largest strain observed so far in ionic electro-active polymers (iEAPs). In between 40 nm gold electrodes under 8 V DC voltage, the film can be completely curled up (270° bending angle) with 6% strain that, to the best of the knowledge, is unpreceded among iEAPs. These results render great potential for the TS/PEGDA/IL based electro-active actuators for soft robotic applications.

Electroresponsive Ionic Liquid Crystal Elastomers.

This paper describes the preparation, physical properties, and electric bending actuation of a new class of active materials-ionic liquid crystal elastomers (iLCEs). It is demonstrated that iLCEs can be actuated by low-frequency AC or DC voltages of less than 1 V. The bending strains of the unoptimized first iLCEs are already comparable to the well-developed ionic electroactive polymers. Additionally, iLCEs exhibit several novel and superior features, such as the alignment that increases the performance of actuation, the possibility of preprogrammed actuation patterns at the level of the cross-linking process, and dual (thermal and electric) actuations in hybrid samples. Since liquid crystal elastomers are also sensitive to magnetic fields and can also be light sensitive, iLCEs have far-reaching potentials toward multiresponsive actuations that may have so far unmatched properties in soft robotics, sensing, and biomedical applications.

Doped bottom-contact organic field-effect transistors.

The influence of doping on doped bottom-gate bottom-contact organic field-effect transistors (OFETs) is discussed. It is shown that the inclusion of a doped layer at the dielectric/organic semiconductor layer leads to a significant reduction in the contact resistances and a fine control of the threshold voltage. Through varying the thickness of the doped layer, a linear shift of threshold voltage V T from -3.1 to -0.22 V is observed for increasing thickness of doped layer. Meanwhile, the contact resistance at the source and drain electrode is reduced from 138.8 MΩ at V GS = -10 V for 3 nm to 0.3 MΩ for 7 nm thick doped layers. Furthermore, an increase of charge mobility is observed for increasing thickness of doped layer. Overall, it is shown that doping can minimize injection barriers in bottom-contact OFETs with channel lengths in the micro-meter regime, which has the potential to increase the performance of this technology further.

Tuning charge carrier transport and optical birefringence in liquid-crystalline thin films: A new design space for organic light-emitting diodes.

Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical property of organic thin-film devices as well. In this study, the molecular orientation in a liquid-crystalline semiconductor film is tuned by a novel blading process as well as by different annealing protocols. The altered alignment is verified by cross-polarized optical microscopy and spectroscopic ellipsometry. It is shown that a change in alignment of the liquid-crystalline semiconductor improves charge transport in single charge carrier devices profoundly. Comparing the current-voltage characteristics of single charge carrier devices with simulations shows an excellent agreement and from this an in-depth understanding of single charge carrier transport in two-terminal devices is obtained. Finally, p-i-n type organic light-emitting diodes (OLEDs) compatible with vacuum processing techniques used in state-of-the-art OLEDs are demonstrated employing liquid-crystalline host matrix in the emission layer.

Minority Currents in n-Doped Organic Transistors.

Doping allows us to control the majority and minority charge carrier concentration in organic field-effect transistors. However, the precise mechanism of minority charge carrier generation and transport in organic semiconductors is largely unknown. Here, the injection of minority charge carriers into n-doped organic field-effect transistors is studied. It is shown that holes can be efficiently injected into the transistor channel via Zener tunneling inside the intrinsic pentacene layer underneath the drain electrode. Moreover, it is shown that the onset of minority (hole) conduction is shifted by lightly n-doping the channel region of the transistor. This behavior can be explained by a large voltage that has to be applied to the gate in order to fully deplete the n-doped layer as well as an increase in hole trapping by inactive dopants.

Doped Organic Transistors.

Organic field-effect transistors hold the promise of enabling low-cost and flexible electronics. Following its success in organic optoelectronics, the organic doping technology is also used increasingly in organic field-effect transistors. Doping not only increases device performance, but it also provides a way to fine-control the transistor behavior, to develop new transistor concepts, and even improve the stability of organic transistors. This Review summarizes the latest progress made in the understanding of the doping technology and its application to organic transistors. It presents the most successful doping models and an overview of the wide variety of materials used as dopants. Further, the influence of doping on charge transport in the most relevant polycrystalline organic semiconductors is reviewed, and a concise overview on the influence of doping on transistor behavior and performance is given. In particular, recent progress in the understanding of contact doping and channel doping is summarized.

Contact Resistance Effects in Highly Doped Organic Electrochemical Transistors.

Injection at the source contact critically determines the behavior of depletion-type organic electrochemical transistors (OETs). The contact resistance of OETs increases exponentially with the gate voltage and strongly influences the modulation of the drain current by the gate voltage over a wide voltage range. A modified standard model accounting contact resistance can explain the particular shape of the transconductance.

Vertical organic transistors.

Organic switching devices such as field effect transistors (OFETs) are a key element of future flexible electronic devices. So far, however, a commercial breakthrough has not been achieved because these devices usually lack in switching speed (e.g. for logic applications) and current density (e.g. for display pixel driving). The limited performance is caused by a combination of comparatively low charge carrier mobilities and the large channel length caused by the need for low-cost structuring. Vertical Organic Transistors are a novel technology that has the potential to overcome these limitations of OFETs. Vertical Organic Transistors allow to scale the channel length of organic transistors into the 100 nm regime without cost intensive structuring techniques. Several different approaches have been proposed in literature, which show high output currents, low operation voltages, and comparatively high speed even without sub-µm structuring technologies. In this review, these different approaches are compared and recent progress is highlighted.

Advanced Organic Permeable-Base Transistor with Superior Performance.

An optimized vertical organic permeable-base transistor (OPBT) competing with the best organic field-effect transistors in performance, while employing low-cost fabrication techniques, is presented. The OPBT stands out by its excellent power efficiency at the highest frequencies.

Doped organic transistors operating in the inversion and depletion regime.

The inversion field-effect transistor is the basic device of modern microelectronics and is nowadays used more than a billion times on every state-of-the-art computer chip. In the future, this rigid technology will be complemented by flexible electronics produced at extremely low cost. Organic field-effect transistors have the potential to be the basic device for flexible electronics, but still need much improvement. In particular, despite more than 20 years of research, organic inversion mode transistors have not been reported so far. Here we discuss the first realization of organic inversion transistors and the optimization of organic depletion transistors by our organic doping technology. We show that the transistor parameters--in particular, the threshold voltage and the ON/OFF ratio--can be controlled by the doping concentration and the thickness of the transistor channel. Injection of minority carriers into the doped transistor channel is achieved by doped contacts, which allows forming an inversion layer.

High mobility N-type transistors based on solution-sheared doped 6,13-bis(triisopropylsilylethynyl)pentacene thin films.

An N-Type organic thin-film transistor (OTFT) based on doped 6,13-Bis(triisopropylsilylethynyl)pentacene is presented. A transition from p-type to n-type occurrs with increasing doping concentrations, and the highest performing n-channel OTFTs are obtained with 50 mol% dopant. X-ray diffraction, scanning Auger microscopy, and secondary ionization mass spectrometry are used to characterize the morphology of the blends. The high performance of the obtained transistors is attributed to the highly crystalline and aligned nature of the doped thin films.