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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

Organic electrochemical transistors (OECTs) are highly versatile in terms of their form factor, fabrication approach that can be applied, and freedom in the choice of substrate material. Their ability to transduce ionic into electric signals and the use of bio-compatible organic materials makes them ideally suited for a wide range of applications, in particular in areas where electronic circuits are interfaced with biologic matter. OECT technology has attracted widespread interest in recent years, which has been accompanied by a steady increase in its performance. However, this progress was mainly driven by device optimization and less by targeting the design of new device geometries and OECT materials. To narrow this gap, this review provides an overview on the different device models that are used to explain the underlying physics governing the steady and transient behavior of OECTs. We show how the models can be used to identify synthetic targets to produce higher performing OECT materials and summarize recently reported materials classes. Overall, a road-map of future research in new device models and material design is presented summarizing the most pressing open questions in the understanding of OECTs.

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.

Organic Field Effect Transistors (OFETs), while showing a lot of promise, currently suffer from a number of limitations. Organic doping can help to overcome these limitations. It opens up a number of new possibilities by offering a way to define majority charge carriers, control the charge carrier density, threshold voltage etc. precisely and produce devices with better performance, stability, and reproducibility. The doping techniques explored in OFETs thus far have been in the range of a few wt.%, which has limited the use of doping to contact doping or a thin doped layer at the gate dielectric interface. Furthermore, the high doping concentrations used place serious limitations on the doping efficiency that can be achieved. Here we demonstrate the successful use of low doping in the 100ppm range throughout the bulk of the organic semiconductor layer of an OFET with the use of a rotating shutter.

Organic field-effect transistors (OFETs) have found a wide range of uses due to their attractive properties. A great deal of effort has been expended on boosting their mobilities, which tend to be low. Given this, accurate estimation of the mobility is crucial. We have developed a web application that automates or simplifies several of the steps required to estimate the mobility from experimental data. The app can be accessed at ofetanalysisapp.shinyapps.io/ofetanalysisapp. The app takes as inputs a file with the data and pieces of information like the number of OFETs and their channel lengths. The app has features that enable the user to mark OFETs as outliers, which are excluded from subsequent calculations. It fits nonlinear regression models to compute estimates of the mobility as well as the threshold voltage. The app provides several visualizations that give the user insight into the nature of the data. The estimates computed by the app can be downloaded in an Excel file so the user can perform further analysis. The use of the app is illustrated with a dataset from one of our OFET experiments.

Organic permeable base transistors (OPBTs) have long been investigated as an alternative to conventional OFETs. Their vertical architecture allows for simple low-resolution fabrication techniques, ...

Doping organic semiconductors has become a key technology to increase the performance of organic light-emitting diodes, solar cells, or field-effect transistors (OFETs). However, doping can be used...

Research in organic permeable base transistors (OPBTs) has led to a significant increase in their performance. However, despite this progress, understanding of the working mechanism of OPBTs is still limited. Although first numerical models of OPBTs are able to describe the switching mechanism of OPBTs correctly, they neglect currents injected at the base electrode, which leads to unrealistically low off-currents and high ON/OFF ratios. Here, a tunneling model is developed that is capable of describing injection of charges through a thin oxide layer formed around the base electrode of OPBTs. With the help of this injection model, the performance of the base–collector diode of OPBTs is discussed. In particular, the model is used to explain the reduction in backward currents due to an exposure to ambient air by an increase in the thickness of the oxide layer. Furthermore, the tunnel model is used to show that the reduction in backward currents of the base–collector diode leads to a decrease in off-currents of complete OPBTs, which in turn leads to an increase in their ON/OFF ratio.

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.

Structuring of organic layers is essential for large-scale integration and manufacturing of organic devices. A novel photo-patterning method for organic semiconductors is presented, which allows to use dry-etching to transfer resist patterns into metal layers on top of organic layers. Apart from the general advantages of dry-etching, such as reduced processing times and high pattern fidelity, it is shown that the new processing protocol leads to a high yield and reproducibility. Furthermore, in contrast to wet-chemical etching, the presented patterning method is versatile and can as well be used for materials known to be unintentionally doped by wet-chemical etching such as pentacene. Comparing pentacene transistors processed by shadow masking and lithography shows no obvious deterioration, which indicates that the protocol proposed here is compatible with a wide range of materials.

Understanding charge trapping in organic semiconductors is key to the development of highly efficient organic devices. Here, charge trapping in organic p-i-n Zener diodes is studied and an analytical model able to describe the influence of the trapped charges on the electrical performance is proposed. The model shows a good agreement with the experiment and allows quantifying the density of trapped charges and the molecular doping efficiency from capacitance spectroscopy of systematically stressed devices. This work is expected to provide a platform to understand and quantify charge trapping in organic electronic devices, especially two-terminal p-i-n devices such as organic light-emitting diodes and organic solar cells.

Fullerenes and their derivatives are among the most commonly used electron transport materials. However, the use of fullerenes is limited by their low stability in ambient air. Here, we test the hypothesis that the air-stability of fullerene-based electron transport materials can be improved by adding fluorinated functional groups, making the materials less susceptible to degradation by interaction with oxygen and moisture. As proof for the concept, C60CF2 based Organic Field-Effect Transistors (OFETs) are studied. An average mobility of μ = ( 0.83 ± 0.52 ) c m 2 V − 1 s − 1 is found, with a maximum mobility of μ = 2.02 c m 2 V − 1 s − 1 . Compared to identical C60 OFETs, C60CF2 based OFETs degrade at a much lower rate and even sustain exposure times of up to 60 h in ambient air.

Molecular doping is a key technique for flexible and low-cost organic complementary semiconductor technologies that requires both efficient and stable p- and n-type doping. However, in contrast to molecular p-dopants, highly efficient n-type dopants are commonly sensitive to rapid degradation in air due to their low ionization energies (IEs) required for electron donation, e.g., IE = 2.4 eV for tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W2(hpp)4). Here, the air stability of various host:W2(hpp)4 combinations is compared by conductivity measurements and photoemission spectroscopy. A partial passivation of the n-doping against degradation is found, with this effect identified to depend on the specific energy levels of the host material. Since host-W2(hpp)4 electronic wavefunction hybridization is unlikely due to confinement of the dopant highest occupied molecular orbital (HOMO) to its molecular center, this finding is explained via stabilization of the dopant by single-electron transfer to a host material whose energy levels are sufficiently low for avoiding further charge transfer to oxygen–water complexes. Our results show the feasibility of temporarily handling n-doped organic thin films in air, e.g., during structuring of organic field effect transistors (OFETs) by lithography.

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.

Organic Permeable Base Transistors (OPBTs) reach a very high transit frequency and large on-state currents. However, for a later commercial application of this technology, a high operational stability is essential as well. Here, the stability of OPBTs during continuous cycling and during base bias stress is discussed. It is observed that the threshold voltage of these transistors shifts toward more positive base voltages if stressed by applying a constant potential to the base electrode for prolonged times. With the help of a 2D device simulation, it is proposed that the observed instabilities are due to charges that are trapped on top of an oxide layer formed around the base electrode. These charges are thermally released after removing the stress, and the device reaches its initial performance after around 24–48 h.Organic Permeable Base Transistors (OPBTs) reach a very high transit frequency and large on-state currents. However, for a later commercial application of this technology, a high operational stability is essential as well. Here, the stability of OPBTs during continuous cycling and during base bias stress is discussed. It is observed that the threshold voltage of these transistors shifts toward more positive base voltages if stressed by applying a constant potential to the base electrode for prolonged times. With the help of a 2D device simulation, it is proposed that the observed instabilities are due to charges that are trapped on top of an oxide layer formed around the base electrode. These charges are thermally released after removing the stress, and the device reaches its initial performance after around 24–48 h.

A single dopant molecule can exchange more than one charge with the hosting polymer semiconductor, doubling the maximum doping efficiency achievable.

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.

A typical human being carries billions of silicon-based field-effect transistors in his/her pockets. What makes these transistors work is Fermi level control, both by doping and field effect. Organic semiconductors are the core of a novel flexible electronics age, but the key effect of doping is still little understood. Here, precise handling is demonstrated for molar doping ratios as low as 10−5 in p- and n-doped organic thin-films by vacuum co-sublimation, allowing comprehensive studying of the Fermi level control over the whole electronic gap of an organic semiconductor. In particular, dopant saturation and reserve regimes are observed for the first time in organic semiconductors. These results will allow for completely new design rules of organic transistors with improved long term stability and precise parameter control.

Understanding the influence of residual gases present during vacuum deposition of organic light-emitting diodes (OLEDs) and their effect on the device lifetime and the electrical characteristics of OLEDs is crucial for advancing industrial fabrication. In order to gain a more in-depth understanding, the influence of residual nitrogen, oxygen, and water vapor on lifetime and electrical characteristics is investigated. This is achieved by introducing the residual gases into the evaporation chamber through a needle valve while monitoring the partial pressures with the help of a mass spectrometer. We find that water vapor introduces a series resistance to the OLED while the other gases do not influence the electric characteristics. The presence of oxygen or nitrogen impacts the lifetime of the OLEDs by the same amount. Water vapor introduces an additional, even faster degradation process within the first hours of OLED operation. The electrically stressed OLEDs are analyzed by laser desorption/ionization time-of-flight mass spectroscopy. We identify the dimerisation of BPhen as well as the complexation reaction of α -NPD with an Ir(piq)2 fragment as sources of device degradation.

B.L. acknowledges financial support from the Binational Science Foundation under grant No 2014396 and from the National Science Foundation under grant No 1639073. C.K. acknowledges funding from the Kent State University Internal Post-Doctoral Competition. M.C.K. acknowledges funding from the Belgian Agentschap voor innovatie door wetenschap en techniek under grant No IWT 131498. 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. Publisher PDF

In this work, we demonstrate color-stable, ITO-free white organic light-emitting diodes (WOLEDs) with enhanced efficiencies by combining the high-conductivity conducting polymer PEDOT:PSS as transparent electrode and a nanoparticle-based scattering layer (NPSL) as the effective optical out-coupling layer. In addition to efficiency enhancement, the NPSL is also beneficial to the stabilization of electroluminescent spectra/colors over viewing angles. Both the PEDOT:PSS and the NPSL can be fabricated by simple, low-temperature solution processing. The integration of both solution-processable transparent electrodes and light extraction structures into OLEDs is particularly attractive for applications since they simultaneously provide manufacturing, cost and efficiency advantages. Postprint

We demonstrate that electrical dopants can be modified with self-assembling groups to form injection layers in organic electronic devices. A strong p-type dopant has been modified in a one-step synthesis to form self-assembled monolayers on gold electrodes from solution. F 4 TCNQ was functionalized with sulfur-containing triazole anchor groups via nucleophilic substitution in a simple substitution reaction. Assembly of functionalized dopants on gold surfaces was verified by ATR-FT-IR measurements. Pentacene-based bottom gate, bottom contact organic thin film transistors show a strongly improved performance. The effective mobility could be increased by a factor of three when electrodes were covered with a SAM of functionalized dopant.

We demonstrate that direct charge transfer (CT) from trap states of host molecules to the p-dopant molecules raises the doping effect of organic semiconductors (OS). Electrons of the trap states in 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) (EHOMO = 6.1 eV) are directly transferred to the p-dopant, 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) (ELUMO = 5.4 eV). This doping process enhances the conductivity of doped OS by different ways from the ordinary doping mechanism of generating free hole carriers and filling trap states of doped OS. Trap density and trap energy are analysed by impedance spectroscopy and it is shown that the direct charge transfer from deep trap states of host to dopants enhances the hole mobility of doped OS and the I–V characteristics of hole only devices.

Organic field-effect transistors (OFET) are important elements in thin-film electronics, being considered for flat-panel or flexible displays, radio frequency identification systems, and sensor arrays. To optimize the devices for high-frequency operation, the channel length, defined as the horizontal distance between the source and the drain contact, can be scaled down. Here, an architecture with a vertical current flow, in particular the Organic Permeable-Base Transistors (OPBT), opens up new opportunities, because the effective transit length in vertical direction is precisely tunable in the nanometer range by the thickness of the semiconductor layer. We present an advanced OPBT, competing with best OFETs while a low-cost, OLED-like fabrication with low-resolution shadow masks is used (Klinger et al., Adv. Mater. 27, 2015). Its design consists of a stack of three parallel electrodes separated by two semiconductor layers of C60 . The vertical current flow is controlled by the middle base electrode with nano-sized openings passivated by an native oxide. Using insulated layers to structure the active area, devices show an on/off ratio of 10⁶ , drive 11 A/cm² at an operation voltage of 1 V, and have a low subthreshold slope of 102 mV/decade. These OPBTs show a unity current-gain transit frequency of 2.2 MHz and off-state break-down fields above 1 MV/cm. Thus, our optimized setup does not only set a benchmark for vertical organic transistors, but also outperforms best lateral OFETs using similar low-cost structuring techniques in terms of power efficiency at high frequencies.

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.

The realization and performance of a novel organic field-effect transistor—the organic junction field-effect transistor (JFET)—is discussed. The transistors are based on the modulation of the thickness of a depletion layer in an organic pin junction with varying gate potential. Based on numerical modeling, suitable layer thicknesses and doping concentrations are identified. Experimentally, organic JFETs are realized and it is shown that the devices clearly exhibit amplification. Changes in the electrical characteristics due to a variation of the intrinsic and the p-doped layer thickness are rationalized by the numerical model, giving further proof to the proposed operational mechanism.

An effective method is presented for enhancing the outcoupling efficiency of translucent/bi-directional organic light-emitting diodes (TL/BD-OLEDs) with a bottom indium tin oxide (ITO) anode and a top cathode comprised of a thin Ag layer covered with an organic capping layer. Upon insertion of a nanoparticle (NP)-based scattering layer (NPSL) between the substrate and the ITO anode, the TL/BD-OLEDs exhibit significantly enhanced external quantum efficiency (EQE) in both emission directions. Furthermore, the NPSL improves the color stability of the TL/BD-OLEDs over a wide range of viewing angles. Simulations based on geometrical and statistical optics are performed to elucidate the mechanism by which the efficiency is enhanced and to establish strategies for further optimization. Simulations performed on the scattering layers with varying NP volume percentage reveal that the bottom-side emission is governed by competition between waveguide-mode extraction and backward scattering by NPs in the film, while the top-side emission is largely dominated by the latter. Optimized bi-directional OLEDs achieve a 1.64-fold enhanced EQE compared to reference devices without NPSL.

We investigated the control of micro-cavity (MC) effects in organic light-emitting diodes (OLEDs) with the introduction of a striped thin metal layer between the indium tin oxide (ITO) layer and the hole transporting layer (HTL). With an enhanced MC effect obtained through the inserted metal layer, the forward emission of the OLED became stronger and the angular distribution became more forward-directed, leading to a current efficiency (CE) that was nearly 1.45 times higher than that of the reference device without the inserted metal layer. The net CE of the OLEDs with a striped metal layer was found to be determined by the area-weighted average of the CE’s of full-cavity-enhanced OLEDs and non-cavity OLEDs. It was also observed that the trade-off between resonance enhancement in efficiency and angle-dependent color stability, often found problematic in MC-based OLEDs, could be mitigated in a straight-forward manner by changing the relative portion of the metal-covered area.