Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Organic semiconductors have sparked interest as flexible, solution processable, and chemically tunable electronic materials. Improvements in charge carrier mobility put organic semiconductors in a competitive position for incorporation in a variety of (opto‐)electronic applications. One example is the organic field‐effect transistor (OFET), which is the fundamental building block of many applications based on organic semiconductors. While the semiconductor performance improvements opened up the possibilities for applying organic materials as active components in fast switching electrical devices, the ability to make good electrical contact hinders further development of deployable electronics. Additionally, inefficient contacts represent serious bottlenecks in identifying new electronic materials by inhibiting access to their intrinsic properties or providing misleading information. Recent work focused on the relationships of contact resistance with device architecture, applied voltage, metal and dielectric interfaces, has led to a steady reduction in contact resistance in OFETs. While impressive progress was made, contact resistance is still above the limits necessary to drive devices at the speed required for many active electronic components. Here, the origins of contact resistance and recent improvement in organic transistors are presented, with emphasis on the electric field and geometric considerations of charge injection in OFETs.     

Electrical Double‐Slope Nonideality in Organic Field‐Effect Transistors

The field effect transistor (FET) is arguably one of the most important circuit elements in modern electronics. Recently, a need has developed for flexible electronics in a variety of emerging applications. Examples include form‐fitting healthcare‐monitoring devices, flexible displays, and flexible radio frequency identification tags. Organic FETs (OFETs) are viable candidates for producing such flexible devices because they incorporate semiconducting π‐conjugated materials, including small molecules and conjugated polymers, which are intrinsically soft and mechanically compatible with flexible substrates. For OFETs to be industrially viable, however, they must achieve not only high charge carrier mobility, but also ideal and comprehensible electrical characteristics. Most recently, nonideal double‐slope characteristics in the transfer curves of OFETs (i.e., high slope at low gate voltage and low slope at high gate voltage), have stirred debate, which has led to different mechanistic rationales in the literature. This review focuses on the general observations, mechanistic understanding, and possible solutions associated with phenomena that result in FETs with double‐slope characteristics. By surveying and systematically summarizing in a single source relevant literature that deals with the issue of double slope, the experimental framework and theoretical basis for interpreting and avoiding this electrical nonideality in OFETs is provided.     

Enhanced Performance of Fullerene n‐Channel Field‐Effect Transistors with Titanium Sub‐Oxide Injection Layer

Enhanced performance of n‐channel organic field‐effect transistors (OFETs) is demonstrated by introducing a titanium sub‐oxide (TiO_x) injection layer. The n‐channel OFETs utilize [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PC₆₁BM) or [6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM) as the semiconductor in the channel. With the TiO_x injection layer, the electron mobilities of PC₆₁BM and PC₇₁BM FET using Al as source/drain electrodes are comparable to those obtained from OFETs using Ca as the source/drain electrodes. Direct measurement of contact resistance (R_c) shows significantly decreased R_c values for FETs with the TiO_x layer. Ultraviolet photoelectron spectroscopy (UPS) studies demonstrate that the TiO_x layer reduces the electron injection barrier because of the relatively strong interfacial dipole of TiO_x. In addition to functioning as an electron injection layer that eliminates the contact resistance, the TiO_x layer acts as a passivation layer that prevents penetration of O₂ and H₂O; devices with the TiO_x injection layer exhibit a significant improvement in lifetime when exposed to air.     

Optimal Structure for High‐Performance and Low‐Contact‐Resistance Organic Field‐Effect Transistors Using Contact‐Doped Coplanar and Pseudo‐Staggered Device Architectures

A low contact resistance achieved on top‐gated organic field‐effect transistors by using coplanar and pseudo‐staggered device architectures, as well as the introduction of a dopant layer, is reported. The top‐gated structure effectively minimizes the access resistance from the contact to the channel region and the charge‐injection barrier is suppressed by doping of iron(III)trichloride at the metal/organic semiconductor interface. Compared with conventional bottom‐gated staggered devices, a remarkably low contact resistance of 0.1–0.2 kΩ cm is extracted from the top‐gated devices by the modified transfer line method. The top‐gated devices using thienoacene compound as a semiconductor exhibit a high average field‐effect mobility of 5.5–5.7 cm² V^?1 s^?1 and an acceptable subthreshold swing of 0.23–0.24 V dec^?1 without degradation in the on/off ratio of ≈10⁹. Based on these experimental achievements, an optimal device structure for a high‐performance organic transistor is proposed.     

Vertical Organic Field‐Effect Transistors

Field‐effect transistors are the fundamental building blocks for electronic circuits and processors. Compared with inorganic transistors, organic field‐effect transistors (OFETs), featuring low cost, low weight, and easy fabrication, are attractive for large‐area flexible electronic devices. At present, OFETs with planar structures are widely investigated device structures in organic electronics and optoelectronics; however, they face enormous challenges in realizing large current density, fast operation speed, and outstanding mechanical flexibility for advancing their potential commercialized applications. In this context, vertical organic field‐effect transistors (VOFETs), composed of vertically stacked source/drain electrodes, could provide an effective approach for solving these questions due to their inherent small channel length and unique working principles. Since the first report of VOFETs in 2004, impressive progress has been witnessed in this field with the improvement of device performance. The aim of this review is to give a systematical summary of VOFETs with a special focus on device structure optimization for improved performance and potential applications demonstrated by VOFETs. An overview of the development of VOFETs along with current challenges and perspectives is also discussed. It is hoped that this review is timely and valuable for the next step in the rapid development of VOFETs and their related research fields.     

Solution‐Processed Ambipolar Field‐Effect Transistor Based on Diketopyrrolopyrrole Functionalized with Benzothiadiazole

Ambipolar charge transport in a solution‐processed small molecule 4,7‐bis{2‐[2,5‐bis(2‐ethylhexyl)‐3‐(5‐hexyl‐2,2′:5′,2″‐terthiophene‐5″‐yl)‐pyrrolo[3,4‐c]pyrrolo‐1,4‐dione‐6‐yl]‐thiophene‐5‐yl}‐2,1,3‐benzothiadiazole (BTDPP2) transistor has been investigated and shows a balanced field‐effect mobility of electrons and holes of up to ～10^?2 cm² V^?1 s^?1. Using low‐work‐function top electrodes such as Ba, the electron injection barrier is largely reduced. The observed ambipolar transport can be enhanced over one order of magnitude compared to devices using Al or Au electrodes. The field‐effect mobility increases upon thermal annealing at 150 °C due to the formation of large crystalline domains, as shown by atomic force microscopy and X‐ray diffraction. Organic inverter circuits based on BTDPP2 ambipolar transistors display a gain of over 25.     

Air Effect on the Ideality of p‐Type Organic Field‐Effect Transistors: A Double‐Edged Sword

Organic field‐effect transistors (OFETs) often deviate from ideal behaviors in air, which masks their intrinsic properties and thus significantly impedes their practical applications. A key issue of how the presence of air affects the ideality of OFETs has not yet been fully understood. It is revealed that air atmosphere may exert a double‐edged sword effect on the active semiconductor layer when determining the ideality of OFETs fabricated from p‐type crystalline organic semiconductors. Upon exposing the as‐fabricated device to air, water and oxygen mainly function as efficient p‐type dopants for the active layer in the contact regions, enhancing charge carrier injection and consequently improving device ideality. Nevertheless, as the exposure time increases, the trapping centers for the injected minority charge carriers appear in the channel region, leading to degradation of device ideality. Inspired by the double‐edged sword behavior of air, a near‐ideal OFET is achieved by ingeniously utilizing the doping/positive effect and eliminating the trapping/negative effect. The effect of air on the ideality of p‐type OFETs is clarified, which not only illuminates some common observations of OFETs in air but also offers useful guidance for the construction of high‐performance ideal OFETs.     

Accurate Extraction of Charge Carrier Mobility in 4‐Probe Field‐Effect Transistors

Charge carrier mobility is an important characteristic of organic field‐effect transistors (OFETs) and other semiconductor devices. However, accurate mobility determination in FETs is frequently compromised by issues related to Schottky‐barrier contact resistance, that can be efficiently addressed by measurements in 4‐probe/Hall‐bar contact geometry. Here, it is shown that this technique, widely used in materials science, can still lead to significant mobility overestimation due to longitudinal channel shunting caused by voltage probes in 4‐probe structures. This effect is investigated numerically and experimentally in specially designed multiterminal OFETs based on optimized novel organic‐semiconductor blends and bulk single crystals. Numerical simulations reveal that 4‐probe FETs with long but narrow channels and wide voltage probes are especially prone to channel shunting, that can lead to mobilities overestimated by as much as 350%. In addition, the first Hall effect measurements in blended OFETs are reported and how Hall mobility can be affected by channel shunting is shown. As a solution to this problem, a numerical correction factor is introduced that can be used to obtain much more accurate experimental mobilities. This methodology is relevant to characterization of a variety of materials, including organic semiconductors, inorganic oxides, monolayer materials, as well as carbon nanotube and semiconductor nanocrystal arrays.     

Effect of Doping Concentration on Microstructure of Conjugated Polymers and Characteristics in N‐Type Polymer Field‐Effect Transistors

Despite extensive progress in organic field‐effect transistors, there are still far fewer reliable, high‐mobility n‐type polymers than p‐type polymers. It is demonstrated that by using dopants at a critical doping molar ratio (MR), performance of n‐type polymer poly[[N,N9‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,59‐(2,29‐bithiophene)] (P(NDI2DO‐T2)) field‐effect transistors (FETs) can be significantly improved and simultaneously optimized in mobility, on–off ratio, crystallinity, injection, and reliability. In particular, when using the organic dopant bis(cyclopentadienyl)–cobalt(II) (cobaltocene, CoCp₂) at a low concentration (0.05 wt%), the FET mobility is increased from 0.34 to 0.72 cm² V^–1 s^–1, and the threshold voltage was decreased from 32.7 to 8.8 V. The relationship between the MR of dopants and electrical characteristics as well as the evolution in polymer crystallinity revealed by synchrotron X‐ray diffractions are systematically investigated. Deviating from previous discoveries, it is found that mobility increases first and then decreases drastically beyond a critical value of MR. Meanwhile, the intensity and width of the main peak of in‐plane X‐ray diffraction start to decrease at the same critical MR. Thus, the mobility decrease is correlated with the disturbed in‐plane crystallinity of the conjugated polymer, for both organic and inorganic dopants. The method provides a simple and efficient approach to employing dopants to optimize the electrical performance and microstructure of P(NDI2DO‐T2).     

A Noble Metal Dichalcogenide for High‐Performance Field‐Effect Transistors and Broadband Photodetectors

2D layered materials are an emerging class of low‐dimensional materials with unique physical and structural properties and extensive applications from novel nanoelectronics to multifunctional optoelectronics. However, the widely investigated 2D materials are strongly limited in high‐performance electronics and ultrabroadband photodetectors by their intrinsic weaknesses. Exploring the new and narrow bandgap 2D materials is very imminent and fundamental. A narrow‐bandgap noble metal dichalcogenide (PtS₂) is demonstrated in this study. The few‐layer PtS₂ field‐effect transistor exhibits excellent electronic mobility exceeding 62.5 cm² V^?1 s^?1 and ultrahigh on/off ratio over 10⁶ at room temperature. The temperature‐dependent conductance and mobility of few‐layer PtS₂ transistors show a direct metal‐to‐insulator transition and carrier scattering mechanisms, respectively. Remarkably, 2D PtS₂ photodetectors with broadband photodetection from visible to mid‐infrared and a fast photoresponse time of 175 µs at 830 nm illumination for the first time are obtained at room temperature. Our work opens an avenue for 2D noble‐metal dichalcogenides to be applied in high‐performance electronic and mid‐infrared optoelectronic devices.     

Control of Polymorphism and Morphology in Solution Sheared Organic Field‐Effect Transistors

During the last decades, small molecule organic semiconductors have been successfully used as active layer in organic field‐effect transistors (OFETs). Despite the high mobility achieved so far with organic molecules, in order to progress in the field it is crucial to find techniques to process them from solution. The device reproducibility is one of the principal weak points of organic electronics for further commercialization. To achieve a high device‐to‐device reproducibility it is essential to control the morphology and polymorphism of the active layer for OFET application. In this work, the preparation of thin films is reported based on blends of the organic semiconductor dibenzo‐tetrathiafulvalene (DB‐TTF) and polystyrene by a solution shearing technique compatible with upscaling. Here, it is demonstrated that varying the deposition parameters (i.e., speed and temperature) or the solution formulation (i.e., semiconductor/binder polymer ratio) is possible to control the film morphology and semiconductor polymorphism and, hence, the different intermolecular interactions. It is demonstrated that the control of the thermodynamics and kinetics of the crystallization process is key for the device performance optimization. Further, this is the first time that DB‐TTF thin films of the α‐polymorph are reported.     

Organic Junction Field‐Effect Transistor

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.     

Solution‐Processed Monolayer Organic Crystals for High‐Performance Field‐Effect Transistors and Ultrasensitive Gas Sensors

This work innovatively develops a dual solution‐shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large‐area and high‐performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm² V^?1 s^?1 among the mobility values in the reported solution‐processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field‐effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band‐like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.     

Super‐ and Ultrathin Organic Field‐Effect Transistors: from Flexibility to Super‐ and Ultraflexibility

Physically flexible electronics offer a wide range of benefits, including the development of next‐generation consumer electronics and healthcare products. The advancement of physical flexibility, typically achieved by the reduction of the total device thickness, including substrates and encapsulation layers, shows great promise for skin‐laminated electronics. Organic electronics—devices relying on carbon‐based materials—offer many advantages over their inorganic counterparts, including the following: significantly lower fabrication temperatures resulting in alternative fabrication techniques, including inkjet and roll‐to‐roll printing, enabling low‐cost and large‐area fabrication; biocompatibility; and spectacular physical flexibility. This article presents a review, spanning the last two decades, of organic field‐effect transistors with the total thickness of just a few microns as well as devices demonstrated in this decade with a total thickness of few hundred of nanometers. A handful of demonstrations of other organic electronic thin film devices are also presented.     

Donor–Acceptor‐Conjugated Polymer for High‐Performance Organic Field‐Effect Transistors: A Progress Report

Polymeric semiconductors have demonstrated great potential in the mass production of low‐cost, lightweight, flexible, and stretchable electronic devices, making them very attractive for commercial applications. Over the past three decades, remarkable progress has been made in donor–acceptor (D–A) polymer‐based field‐effect transistors, with their charge‐carrier mobility exceeding 10 cm² V^?1 s^?1. Numerous molecular designs of D–A polymers have emerged and evolved along with progress in understanding the charge transport physics behind their high mobility. In this review, the current understanding of charge transport in polymeric semiconductors is covered along with significant features observed in high‐mobility D–A polymers, with a particular focus on polymeric microstructures. Subsequently, emerging molecular designs with further prospective improvements in charge‐carrier mobility are described. Moreover, the current issues and outlook for future generations of polymeric semiconductors are discussed.     

Porous Organic Field‐Effect Transistors for Enhanced Chemical Sensing Performances

The thin‐film structures of chemical sensors based on conventional organic field‐effect transistors (OFETs) can limit the sensitivity of the devices toward chemical vapors, because charge carriers in OFETs are usually concentrated within a few molecular layers at the bottom of the organic semiconductor (OSC) film near the dielectric/semiconductor interface. Chemical vapor molecules have to diffuse through the OSC films before they can interact with charge carriers in the OFET conduction channel. It has been demonstrated that OFET ammonia sensors with porous OSC films can be fabricated by a simple vacuum freeze‐drying template method. The resulted devices can have ammonia sensitivity not only much higher than the pristine OFETs with thin‐film structure but also better than any previously reported OFET sensors, to the best of our knowledge. The porous OFETs show a relative sensitivity as high as 340% ppm^?1 upon exposure to 10 parts per billion (ppb) NH₃. In addition, the devices also exhibit decent selectivity and stability. This general and simple strategy can be applied to a wide range of OFET chemical sensors to improve the device sensitivity.     

Fully Integrated Microscale Quasi‐2D Crystalline Molecular Field‐Effect Transistors

Monolithic integration of microscale organic field‐effect transistors (micro‐OFETs) is the only and inevitable path toward low‐cost large‐area electronics and displays. However, to date, such an ultimate technology has not yet evolved due to challenges in positioning and patterning highly crystalline microscale molecular layers as well as in developing micrometer scale integration schemes. In this work, by mastering the local growth of molecular semiconductors on pre‐defined terraces, single‐crystal quasi‐2D molecular layers tens of square micrometers in size are created in dense periodic arrays on a Si substrate. Nondestructive photolithographic processes are developed to pattern micro‐OFETs with mobilities up to 34.6 cm² V^?1 s^?1. This work demonstrates the feasibility to integrate arrays of short‐channel micro‐OFETs into electronic circuitry by highly parallel and size scalable fabrication technologies.     

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.