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Devices made using thin-film semiconductors have attracted much interest recently owing to new application possibilities. Among materials systems suitable for thin-film electronics, organic semiconductors are of particular interest; their low cost, biocompatible carbon-based materials and deposition by simple techniques such as evaporation or printing enable organic semiconductor devices to be used for ubiquitous electronics, such as those used on or in the human body or on clothing and packages1–3. The potential of organic electronics can be leveraged only if the performance of organic transistors is improved markedly. Here we present organic bipolar transistors with outstanding device performance: a previously undescribed vertical architecture and highly crystalline organic rubrene thin films yield devices with high differential amplification (more than 100) and superior high-frequency performance over conventional devices. These bipolar transistors also give insight into the minority carrier diffusion length—a key parameter in organic semiconductors. Our results open the door to new device concepts of high-performance organic electronics with ever faster switching speeds.

Due to their nm-range channel length, vertical organic transistors are gaining scientific interest to overcome the frequency limitations of lateral organic transistor. Especially, vertical organic permeable base transistors (OPBTs) with a structure resembling a vacuum-tube-triode stand out due to their excellent electrical performance. Recently, n-type OPBTs have been demonstrated with excellent performance and great potential to be applied in high-frequency logic circuits, display driving circuits, and light-emitting devices. However, achieving adequate p-type OPBTs to catch up with the performance of n-type devices is still challenging. Here, we report on the peculiar finding that the extended exposure of p-type OPBTs to ambient air during the fabrication has a positive effect on the electrical performance and tremendously improves the yield of devices to 100%. Upon exposing the fresh device to air, oxygen diffuses into the semiconductor layer interacting with the base electrode, which facilitates the formation of pinholes in the base and creates an insulator layer around the base. Moreover, as oxygen acts as a p-dopants to the p-type semiconductors used in this work, the device performance is further improved, manifesting in an increased on-state current. However, as the exposure time increases, different p-type organic semiconductors show different behavior depending on their ionization potential. Comparing two semiconductor materials with a difference in their ionization potential of 0.4 eV, we show that long-term degradation could be effectively suppressed.Therefore, the positive effect of air exposure could be utilized to optimize p-type OPBTs, which offers a simple and universal way to construct high performance p-type OPBTs and possibly also complementary circuits.

Entirely photopatternable solid organic electrochemical transistors were fabricated and their excellent performance and pronounced hysteretic behavior studied in detail.

Besides charge carrier mobility and contact resistance, parasitic overlap capacitance is a key factor limiting the dynamic behavior of organic thin-film transistors. The most effective away to redu...

We demonstrate the key role of charge-transfer complexes in surface doping as a successful methodology for improving channel field-effect mobility and reducing the threshold voltage in organic field-effect transistors (OFETs), as well as raising the film conductivity. Demonstrated here for 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) doped with 2,2'-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ), channel doping by sequential deposition is consistently rationalized by the development of a cocrystalline structure that forms and evolves from the surface of the organic semiconductor film without trading the thin-film structure integrity. This scenario brings higher benefits for the device operation than doping by codeposition, where a decrease in the field-effect mobility of the device, even for a dopant content of only 1 mol %, makes codeposition less suitable. Insight into the structural and electronic properties of the interface satisfactorily explains the improved performance of OFETs upon the incorporation of the dopant and provides an understanding of the mechanism of doping in this system., This work was funded by the projects PID2019-110907GB-I00 and FUNFUTURE CEX2019-000917-S from MCIN/AEI/10.13039/501100011033. A.B. thanks the MCIN for the financial support through the BES-2016-077519 fellowship. This work has been carried out within the framework of the doctoral program of Material Science (Department of Physics) of the Universitat Autònoma de Barcelona (UAB). GIWAXS experiments were performed at the beamline BL11─NCD-SWEET at ALBA Synchrotron with the collaboration of ALBA staff. We would like to thank Stefanie Buchholtz for helping with the UPS measurements., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Lateral-channel dual-gate organic thin-film transistors have been used in pseudo complementary metal–oxide–semiconductor (CMOS) inverters to control switching voltage. However, their relatively long channel lengths, combined with the low charge carrier mobility of organic semiconductors, typically leads to slow inverter operation. Vertical-channel dual-gate organic thin-film transistors are a promising alternative because of their short channel lengths, but the lack of appropriate p- and n-type devices has limited the development of complementary inverter circuits. Here, we show that organic vertical n-channel permeable single- and dual-base transistors, and vertical p-channel permeable base transistors can be used to create integrated complementary inverters and ring oscillators. The vertical dual-base transistors enable switching voltage shift and gain enhancement. The inverters exhibit small switching time constants at 10 MHz, and the seven-stage complementary ring oscillators exhibit short signal propagation delays of 11 ns per stage at a supply voltage of 4 V. Organic n- and p-type vertical transistors, with considerably shorter channel lengths than their planar counterparts, can be used to create complementary metal–oxide–semiconductor (CMOS)-like inverters and ring oscillators that operate in the megahertz frequency range.

Blending organic molecules to tune their energy levels is currently being investigated as an approach to engineer the bulk and interfacial optoelectronic properties of organic semiconductors. It has been proven that the ionization energy and electron affinity can be equally shifted in the same direction by electrostatic effects controlled by blending similar halogenated derivatives with different energetics. Here we show that the energy gap of organic semiconductors can also be tuned by blending. We use oligothiophenes with different numbers of thiophene rings as an example and investigate their structure and electronic properties. Photoelectron spectroscopy and inverse photoelectron spectroscopy show tunability of the single-particle gap, with the optical gaps showing similar, but smaller, effects. Theoretical analysis shows that this tuning is mainly caused by a change in the dielectric constant with blend ratio. Further studies will explore the practical impact of this energy-level engineering strategy for optoelectronic devices.

We investigate the charge and thermoelectric transport in modulation-doped large-area rubrene thin-film crystals with different crystal phases. We show that modulation doping allows achieving superior doping efficiencies even for high doping densities, when conventional bulk doping runs into the reserve regime. Modulation-doped orthorhombic rubrene achieves much improved thermoelectric power factors, exceeding 20 μW m −1 K −2 at 80°C. Theoretical studies give insight into the energy landscape of the heterostructures and its influence on qualitative trends of the Seebeck coefficient. Our results show that modulation doping together with high-mobility crystalline organic semiconductor films is a previosly unexplored strategy for achieving high-performance organic thermoelectrics.

The main advantage of organic transistors with dual gates/bases is that the threshold voltages can be set as a function of the applied second gate/base bias, which is crucial for the application in logic gates and integrated circuits. However, incorporating a dual gate/base structure into an ultra-short channel vertical architecture represents a substantial challenge. Here, we realize a device concept of vertical organic permeable dual-base transistors, where the dual base electrodes can be used to tune the threshold voltages and change the on-currents. The detailed operation mechanisms are investigated by calibrated TCAD simulations. Finally, power-efficient logic circuits, e.g. inverter, NAND/AND computation functions are demonstrated with one single device operating at supply voltages of, The development of vertical organic transistors with controllable threshold voltage is highly desirable for integrated circuit-based displays and sensors. Here, the authors report vertical organic permeable dual-based transistors with independently tunable on-currents and threshold voltages.

Organic electrochemical transistors (OECTs) based on Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) are a benchmark system in organic bioelectronics. In particular, the superior mechanical properties and the ionic-electronic transduction yield excellent potential for the field of implantable or wearable sensing technology. However, depletion-mode operation PEDOT:PSS-based OECTs cause high static power dissipation in electronic circuits, limiting their application in electronic systems. Hence, having control over the threshold voltage is of utmost technological importance. Here we demonstrate PEDOT:PSS-based dual-gate OECTs with solid-state electrolyte where the threshold voltage is seamlessly adjustable during operation. We show that the degree of threshold voltage tuning linearly depends on the gate capacitance, which is a straightforward approach for circuit designers to adjust the threshold voltage only by the device dimensions. The PEDOT:PSS-based dual-gate OECTs show excellent device performance and can be pushed to accumulation-mode operation, resulting in a simplified and relaxed design of complementary inverters.

Organic electrochemical transistors (OECTs) can be distinguished from conventional organic thin film transistors (OTFTs) by the use of an electrolyte in their gate stack. This results in a unique set of device properties, most notably, mixed ionic and electronic transport. Modeling of these devices has mainly been realized using MOSFET-like equations (often called the ‘Bernards model’) and extensions, which use a circuit diagram to include the different parasitics and capacitances involved [1]. While these models provide qualitative agreements with OECT devices, their quantitative agreement may not be sufficient for realizing circuit simulations. Physically, the discrepancy between the model and the devices are likely to stem from the assumption that the electrolyte behaves as a parallel plate capacitor, as in a conventional MOSFET. Recently, another model based on thermodynamics has been proposed that treats the OECT as a thermodynamic binary system with entropic mixing as the driving force for device operation [2]. Here we explore these different models and then show how they can be used to understand circuit behavior of OECTs. The authors thank the ANR and the Bundesministerium für Bildung und Forschung (BMBF) for funding of the BAYOEN project under contract ANR-21-FAI1-0006-01 and 01IS21089. [1] J Rivnay et al Organic electrochemical transistors. Nature Reviews Materials, 3(2):1–14, 2018 [2] M. Cucchi et al ‘Thermodynamics of Organic Electrochemical Transistors’, accessed on researchsquare.com, https://doi.org/10.21203/rs.3.rs-1143463/v1 Figure 1

Owing to their low dimensionality, two-dimensional semiconductors, such as monolayer molybdenum disulfide, have a range of properties that make them valuable in the development of nanoelectronics. For example, the electronic bandgap of these semiconductors is not an intrinsic physical parameter and can be engineered by manipulating the dielectric environment around the monolayer. Here we show that this dielectric-dependent electronic bandgap can be used to engineer a lateral heterojunction within a homogeneous MoS2 monolayer. We visualize the heterostructure with Kelvin probe force microscopy and examine its influence on electrical transport experimentally and theoretically. We observe a lateral heterojunction with an approximately 90 meV band offset due to the differing degrees of bandgap renormalization of monolayer MoS2 when it is placed on a substrate in which one segment is made from an amorphous fluoropolymer (Cytop) and another segment is made of hexagonal boron nitride. This heterostructure leads to a diode-like electrical transport with a strong asymmetric behaviour. A lateral heterojunction with diode-like electrical transport can be created in a homogeneous MoS2 monolayer by using a substrate in which one segment is made from an amorphous fluoropolymer and another segment from hexagonal boron nitride.

Organic electronics is the technology enabling truly flexible electronic devices. However, despite continuous improvements in the charge-carrier mobility, devices used for digital circuits based on organic field-effect transistors (OFETs) have still not achieved a commercial breakthrough. A substantial hurdle to the realization of effective digital circuitry is the proper control of the threshold voltage Vth. Previous approaches include doping or self-assembled monolayers to provide the threshold voltage control. However, while self-assembled monolayers-modified OFETs often do not show the level of reproducibility which is required in digital circuit engineering, direct doping of the channel material results in a poor on/off ratio leading to unfavorable power dissipation. Furthermore, direct doping of the channel material in organic semiconductors could cause the formation of trap states impeding the charge-carrier transport. Employing the concept of modulation-doped field-effect transistors (MODFETs), which is well established in inorganic electronics, the semiconductor-dopant interaction is significantly reduced, thereby solving the above-described problems. Here, we present the concept of an organic semiconductor MODFET which is composed of an organic-organic heterostructure between a highly doped wide-energy-gap material and an undoped narrow-energy-gap material. The effectiveness of charge transfer across the interface is controlled by the doping concentration and thickness of an undoped buffer layer. A complete picture of the energy landscape of this heterostructure is drawn using impedance spectroscopy and ultraviolet photoelectron spectroscopy. Furthermore, we analyze the effect of the dopant density on the charge-carrier transport properties. The incorporation of these heterostructures into OFETs enables a precise adjustment of the threshold voltage by using the modulation doping concept.

Lateral-channel dual-gate organic thin-film transistors (OTFTs) are utilized in organic pseudo-CMOS inverters to realize switching voltage control. However, the relatively long channel length will slow the inverter operation. Vertical-channel dual-gate OTFTs are an attractive alternative due to the short channel length. In this work, controllable and reliable complementary inverters are presented using vertical n-channel organic permeable dual-base transistors (OPDBTs) and vertical p-channel organic permeable base transistors (OPBTs). With operating voltages < 2.0 V, the threshold voltages of the n-type OPDBTs are changed across a wide range from 0.12 to 0.82 V by varying the voltage of the additional base. The fabricated tunable organic complementary inverter features switching voltage shift and gain enhancement. In addition, the inverters show very small switching time constants (< 10 ns) at 10 MHz. Our work represents a significant step towards the application of vertical dual-gate/base transistors in power-efficient organic complementary inverters, offering the capability to easily tune the switching voltage of organic complementary inverters. This facilitates the development of high-performance and complex organic digital integrated circuits.

The high degree of order in organic single crystals often leads to optimum charge transport properties and their robustness under high current density which is attractive for solid state lighting and laser technology. However, conventional method of single crystal growth based on physical vapour transport is unsuitable for device integration and large area applications. Here, we present a vacuum processed approach to prepare organic single crystals on inch sized substrates and integrate them into organic light emitting devices (OLED). The device performance are characterized in comparison to standard OLED based on amorphous materials.

In my presentation, I introduce the vertical organic field-effect concept and highlight the development of optimized transistor geometries leading to superior performance with an on/off ratio>10⁷, transition frequency approaching 100MHz and excellent operational stability. In the second part of the contribution, I focus on the potential of VOFETs for active-matrix OLED (AMOLED) display driving. Comparing typical TFT design guidelines for vertical and horizontal TFTs, it can be deduced that the VOFET design leads to a considerable reduction of area consumption within a pixel which ultimately enables a higher display resolution and a lower power consumption. The concept of vertical transistors is not restricted to organic semiconductors. Hence, I discuss in the third part of my presentation the potential of high mobility metal-oxide semiconductors for vertical thin-film transistors for GHz operation.

The air sensitivity of n-doped layers is crucial for the long-term stability of organic electronic devices. Although several air-stable and highly efficient n-dopants have been developed, the reason for the varying air sensitivity between different n-doped layers, in which the n-dopant molecules are dispersed, is not fully understood. In contrast to previous studies that compared the air stability of doped films with the energy levels of neat host or dopant layers, we trace back the varying degree of air sensitivity to the energy levels of integer charge transfer states (ICTCs) formed by host anions and dopant cations. Our data indicate a universal limit for the ionization energy of ICTCs above which the n-doped semiconductors are air-stable.

Organic light-emitting transistors, three-terminal devices combining a thin-film transistor with a light-emitting diode, have generated increasing interest in organic electronics. However, increasing their efficiency while keeping the operating voltage low still remains a key challenge. Here, we demonstrate organic permeable base light-emitting transistors; these three-terminal vertical optoelectronic devices operate at driving voltages below 5.0 V; emit in the red, green and blue ranges; and reach, respectively, peak external quantum efficiencies of 19.6%, 24.6% and 11.8%, current efficiencies of 20.6 cd A–1, 90.1 cd A–1 and 27.1 cd A–1 and maximum luminance values of 9,833 cd m–2, 12,513 cd m–2 and 4,753 cd m–2. Our simulations demonstrate that the nano-pore permeable base electrode located at the centre of the device, which forms a distinctive optical microcavity and regulates charge carrier injection and transport, is the key to the good performance obtained. Our work paves the way towards efficient and low-voltage organic light-emitting transistors, useful for power-efficient active matrix displays and solid-state lighting. Vertical organic light-emitting transistors are realized by using a porous base electrode in the centre of the device, which improves efficiency and reduces operating voltage by regulating charge transport and forming an optical microcavity.

Blending organic molecules to tune their energy levels is currently being investigated as an approach to engineer the bulk and interfacial optoelectronic properties of organic semiconductors. It has been proven that the ionization energy and electron affinity can be equally shifted in the same direction by electrostatic effects controlled by blending similar halogenated derivatives with different energetics. Here we show that the energy gap of organic semiconductors can also be tuned by blending. We use oligothiophenes with different numbers of thiophene rings as an example and investigate their structure and electronic properties. Photoelectron spectroscopy and inverse photoelectron spectroscopy show tunability of the single-particle gap, with the optical gaps showing similar, but smaller, effects. Theoretical analysis shows that this tuning is mainly caused by a change in the dielectric constant with blend ratio. Further studies will explore the practical impact of this energy-level engineering strategy for optoelectronic devices.

Red and near-infrared light detection is vital for numerous applications, including full-color imaging, optical communication, and machine vision. However, this development is hindered by a limited choice of small band gap and narrow-bandwidth materials. Here, we report a device principle with a simple organic planar heterojunction architecture that enables a selective activation of excitons for tuning the photoresponse spectra to fabricate thin-film, filterless, red-light organic photodiodes. A sequential solution-processed active layer is formed by depositing the top layer of PC

Organic permeable base transistors (OPBTs) are of great interest for flexible electronic circuits, as they offer very large on-current density and a record-high transition frequency. They rely on a vertical device architecture with current transport through native pinholes in a central base electrode. This study investigates the impact of pinhole density and pinhole diameter on the DC device performance in OPBTs based on experimental data and TCAD simulation results. A pinhole density of NPin = 54 μm−2 and pinhole diameters around LPin = 15 nm are found in the devices. Simulations show that a variation of pinhole diameter and density around these numbers has only a minor impact on the DC device characteristics. A variation of the pinhole diameter and density by up to 100% lead to a deviation of less than 4% in threshold voltage, on/off current ratio, and sub-threshold slope. Hence, the fabrication of OPBTs with reliable device characteristics is possible regardless of statistical deviations in thin film formation.

Powerful electronic devices require performant short-channel transistors. For organic electronics, though, promising low-cost and flexible electronic circuits, high processing costs for short channel devices are not acceptable. In this regard, vertical organic transistors (VOTs) are an attractive alternative, and in fact, today they reach the highest transition frequency (40 MHz) and the highest footprint current density (>1 MA cm−2) among all organic transistors. Here, all VOT concepts are reviewed, while discussing device physics, integration approaches, and highlighting the recent developments. The upcoming challenges for the VOT technology are also presented with a guideline for further developments.