Polylactic acid as a biodegradable material for all-solution-processed organic electronic devices

In this paper we report on the fabrication of spin-coated biodegradable polylactic acid (PLA) thin films to be used as substrates for the realisation of all-solution-processed organic electronic devices. The full mechanical and electrical characterisation of these substrates shows that they exhibit good mechanical and dielectric properties and are therefore suitable for the fabrication of disposable electronics. To demonstrate practically the functionality of such PLA thin films, organic electronic devices were realised on the top of them, exclusively by means of solution-process fabrication techniques and in particular inkjet-printing. Also, a photonic curing procedure is here presented as a means for sintering the conductive inks without heating up the PLA substrates. Two types of organic transistors were fabricated on the top of PLA: organic field-effect transistors (OFETs), where the PLA film was used not only as a substrate but also as the gate dielectric, and all-inkjet-printed organic electrochemical transistors (OECTs). The second typology of transistors exhibited one of the highest transconductance reported so far in the literature (up to 2.75 mS). This study opens an avenue for the fabrication of disposable, low-cost organic electronic devices.     

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.     

Biodegradable Materials for Transient Organic Transistors

Electronic devices that can physically disappear in a controlled manner without harmful by-products unveil a wide range of opportunities in medical devices, environmental monitoring, and next-generation consumer electronics. Their property of transience is indispensable for mitigating the global problem of electronic waste accumulation. Additionally, transient technologies that are biocompatible and can be biologically resorbed are of great potential for applications in temporary medical implants, since it eliminates the need for expensive device recovery surgery. Transistors are the key building blocks of modern electronics, and their fabrication using organic materials is beneficial due to their low cost, unprecedented flexibility and facile processing. This contribution reviews the technological application of biodegradable materials in four major classes of organic transistors, namely organic field-effect transistors (OFETs), organic synaptic transistors, electrolyte-gated OFETs, and organic electrochemical transistors. The fundamental biodegradation mechanism is discussed in detail, followed by a perspective of various biodegradable materials utilized as active semiconductors, dielectrics, electrolytes and substrates in the various types of organic transistor devices. This contribution comprehensively discusses the role and application of biodegradable materials in all of the key modern-day organic transistors, highlighting their unique properties that allow the fabrication of biodegradable, eco-friendly, and sustainable devices.     

A Review of Vertical Organic Transistors

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.     

Ultra-conformable Organic Field-Effect Transistors and circuits for epidermal electronic applications

In this work we report the development of electronic circuits based on low voltage Organic Field-Effect Transistors (OFETs), entirely fabricated on polymer nanosheets acting as sub-micrometric substrates. The overall thickness of the proposed circuits (including the substrate, a 400 nm-thick Parylene C nanosheet) is only 600 nm, thus making them highly flexible, ultra-conformable and light-weighted. A complete characterization of the fabricated devices is reported. Mechanical performances of the nanosheets are thoroughly discussed. Full swing complementary inverters fabricated on same substrate show low noise margins and gains up to 10. Thanks to a carefully designed self-aligned structure, these devices are characterized by a very good frequency response, with a cut-off frequency usually ranging around 100 kHz. The ultra-conformability of such nanosheets allows their transfer and adhesion on complex target surfaces, such as the human skin without a significant change in their electrical performances, representing a step forward to the realization of conformable electronics particularly suited for personal monitoring systems for healthcare and sport.     

Cover Picture: High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing (Adv. Funct. Mater. 15/2007)

A new method for direct patterning of organic optoelectronic/electronic devices using a reconfigurable and scalable printing method is reported by Vladimir Bulovic and co‐workers on p. 2722. The printing technique is applied to the fabrication of high‐resolution printed organic light emitting devices (OLEDs) and organic field effect transistors (OFETs). Remarkably, the final print‐deposited films are evaporated onto the substrate (rather than solvent printed), giving high‐quality, solvent‐free, molecularly flat structures that match the performance of comparable high‐performance unpatterned films. We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Hybrid Nanodielectrics for Low‐Voltage Organic Electronics

Nanoscale hybrid dielectrics composed of an ultra‐thin polymeric low‐κ bottom layer and an ultra‐thin high‐κ oxide top layer, with high dielectric strength and capacitances up to 0.25 μFcm^?2, compatible with low‐voltage, low‐power, organic electronic circuits are demonstrated. An efficient and reliable fabrication process, with 100% yield achieved on lab‐scale arrays, is demonstrated by means of pulsed laser deposition (PLD) for the fast growth of the oxide layer. With this strategy, high capacitance top gate (TG), n‐type and p‐type organic field effect transistors (OFETs) with high mobility, low leakage currents, and low subthreshold slopes are realized and employed in complementary‐like inverters, exhibiting ideal switching for supply voltages as low as 2 V. Importantly, the hybrid double‐layer allows for a neat decoupling between the need for a high capacitance, guaranteed by the nanoscale thickness of the double layer, and for an optimized semiconductor–dielectric interface, a crucial point in enabling high mobility OFETs, thanks to the low‐κ polymeric dielectric layer in direct contact with the polymer semiconductor. It is shown that such decoupling can be achieved already with a polymer dielectric as thin as 10 nm when the top oxide is deposited by PLD. This paves the way for a very versatile implementation of the proposed approach for the scaling of the operating voltages of TG OFETs with very low level of dielectric leakage currents to the fabrication of low‐voltage organic electronics with drastically reduced power consumption.     

High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing

We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Room‐Temperature Printing of Organic Thin‐Film Transistors with π‐Junction Gold Nanoparticles

Printing semiconductor devices under ambient atmospheric conditions is a promising method for the large‐area, low‐cost fabrication of flexible electronic products. However, processes conducted at temperatures greater than 150 °C are typically used for printed electronics, which prevents the use of common flexible substrates because of the distortion caused by heat. The present report describes a method for the room‐temperature printing of electronics, which allows thin‐film electronic devices to be printed at room temperature without the application of heat. The development of π‐junction gold nanoparticles as the electrode material permits the room‐temperature deposition of a conductive metal layer. Room‐temperature patterning methods are also developed for the Au ink electrodes and an active organic semiconductor layer, which enables the fabrication of organic thin‐film transistors through room‐temperature printing. The transistor devices printed at room temperature exhibit average field‐effect mobilities of 7.9 and 2.5 cm² V^?1 s^?1 on plastic and paper substrates, respectively. These results suggest that this fabrication method is very promising as a core technology for low‐cost and high‐performance printed electronics.     

Cyanoethylated pullulan as a high-k solution processable polymer gate dielectric for SWCNT TFTs

Cyanoethylated pullulan (CEP), a high-k solution processable polymer gate dielectric, is used to fabricate bottom gated single wall carbon nanotube (SWCNT) network thin film transistors (TFTs). Both aqueous and organic dispersions of highly semiconducting enriched SWCNTs are used as the channel material. Use of CEP as the dielectric enables fabrication of devices operating at low voltage (<3 V) with high on-state currents, good on/off ratios (∼10⁵), low subthreshold swings (∼200 mV/decade) and minimal hysteresis (<1 V). However, despite high apparent mobilities extracted from gate voltage sweeps, driving these devices at even modest frequencies (>1 Hz) is found to significantly decrease the transconductance. This is shown to be related to a significant frequency dependence of the capacitance associated with a slow polarization response of the dielectric. Despite this limitation, CEP could be a useful dielectric in SWCNT TFTs for applications such as sensors and low frequency amplifiers.     

Large scale,flexible organic transistor arrays and circuits based on polyimide materials

Polyimide (PI) materials are lightweight, flexible, resistant strongly to heat and chemicals, and have been widely used in electronics industry such as working as electronic packaging materials in large-scale integrated circuits. In this letter, PI materials, for the first time, are introduced into organic field-effect transistors (OFETs) and circuits as insulator layers in order to be compatible with the photolithography process. Moreover, a novel method is developed to make the PI films strong enough to endure the critical processes of photolithography (e.g., the influence of developer on polyimide layer). Based on the intact PI insulator and the modified photolithographic technique, large scale, flexible transistor arrays and circuits were fabricated with high resolution and high performance (mobility up to 0.55 cm² V⁻¹ s⁻¹ for bottom-contact bottom-gate OFETs). It provides a new way for the fabrication of large-area organic devices and circuits beyond solution printed techniques, especially for the application of organic semiconductors with poor solubility, e.g., pentacene.     

High‐Speed Organic Single‐Crystal Transistor Responding to Very High Frequency Band

Although high carrier mobility organic field‐effect transistors (OFETs) are required for high‐speed device applications, improving the carrier mobility alone does not lead to high‐speed operation. Because the cut‐off frequency is determined predominantly by the total resistance and parasitic capacitance of a transistor, it is necessary to miniaturize OFETs while reducing these factors. Depositing a dopant layer only at the metal/semiconductor interface is an effective technique to reduce the contact resistance. However, fine‐patterning techniques for a dopant layer are still challenging especially for a top‐contact solution‐processed OFET geometry because organic semiconductors are vulnerable to chemical damage by solvents. In this work, high‐resolution, damage‐free patterning of a dopant layer is developed to fabricate short‐channel OFETs with a dopant interlayer inserted at the contacts. The fabricated OFETs exhibit high mobility exceeding 10 cm² V^?1 s^?1 together with a reasonably low contact resistance, allowing for high frequency operation at 38 MHz. In addition, a diode‐connected OFET shows a rectifying capability of up to 78 MHz at an applied voltage of 5 V. This shows that an OFET can respond to the very high frequency band, which is beneficial for long‐distance wireless communication.     

Interface Engineering for Organic Electronics

The field of organic electronics has been developed vastly in the past two decades due to its promise for low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, and ease of processing. The performance and lifetime of these devices, such as organic light‐emitting diodes (OLEDs), photovoltaics (OPVs), and field‐effect transistors (OFETs), are critically dependent on the properties of both active materials and their interfaces. Interfacial properties can be controlled ranging from simple wettability or adhesion between different materials to direct modifications of the electronic structure of the materials. In this Feature Article, the strategies of utilizing surfactant‐modified cathodes, hole‐transporting buffer layers, and self‐assembled monolayer (SAM)‐modified anodes are highlighted. In addition to enabling the production of high‐efficiency OLEDs, control of interfaces in both conventional and inverted polymer solar cells is shown to enhance their efficiency and stability; and the tailoring of source–drain electrode–semiconductor interfaces, dielectric–semiconductor interfaces, and ultrathin dielectrics is shown to allow for high‐performance OFETs.     

High‐Mobility and Low Turn‐On Voltage n‐Channel OTFTs Based on a Solution‐Processable Derivative of Naphthalene Bisimide

In organic electronics solution‐processable n‐channel field‐effect transistors (FETs) matching the parameters of the best p‐channel FETs are needed. Progress toward the fabrication of such devices is strongly impeded by a limited number of suitable organic semiconductors as well as by the lack of processing techniques that enable strict control of the supramolecular organization in the deposited layer. Here, the use of N,N′‐bis(4‐n‐butylphenyl)‐1,4,5,8‐naphthalenetetracarboxylic‐1,4:5,8‐bisimide (NBI‐4‐n‐BuPh) for fabrication of n‐channel FETs is described. The unidirectionally oriented crystalline layers of NBI‐4‐n‐BuPh are obtained by the zone‐casting method under ambient conditions. Due to the bottom‐contact, top‐gate configuration used, the gate dielectric, Parylene C, also acts as a protective layer. This, together with a sufficiently low LUMO level of NBI‐4‐n‐BuPh allows the fabrication and operation of these novel n‐channel transistors under ambient conditions. The high order of the NBI‐4‐n‐BuPh molecules in the zone‐cast layer and high purity of the gate dielectric yield good performance of the transistors.     

Tuning Contact Resistance in Top‐Contact p‐Type and n‐Type Organic Field Effect Transistors by Self‐Generated Interlayers

Contact resistance significantly limits the performance of organic field‐effect transistors (OFETs). Positioning interlayers at the metal/organic interface can tune the effective work‐function and reduce contact resistance. Myriad techniques offer interlayer processing onto the metal pads in bottom‐contact OFETs. However, most methods are not suitable for deposition on organic films and incompatible with top‐contact OFET architectures. Here, a simple and versatile methodology is demonstrated for interlayer processing in both p‐ and n‐type devices that is also suitable for top‐contact OFETs. In this approach, judiciously selected interlayer molecules are co‐deposited as additives in the semiconducting polymer active layer. During top contact deposition, the additive molecules migrate from within the bulk film to the organic/metal interface due to additive‐metal interactions. Migration continues until a thin continuous interlayer is completed. Formation of the interlayer is confirmed by X‐ray photoelectron spectroscopy (XPS) and cross‐section scanning transmission electron microscopy (STEM), and its effect on contact resistance by device measurements and transfer line method (TLM) analysis. It is shown that self‐generated interlayers that reduce contact resistance in p‐type devices, increase that of n‐type devices, and vice versa, confirming the role of additives as interlayer materials that modulate the effective work‐function of the organic/metal interface.     

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.     

Organic Field‐Effect Transistors and Unipolar Logic Gates on Charged Electrets from Spin‐On Organosilsesquioxane Resins

Controllable shifting of threshold voltage and modulation of current in organic field‐effect transistors (OFETs) is demonstrated, resulting in the formation of unipolar inverters by making use of space‐charge electrets. Prior to the deposition of the organic semiconductor (OSC), negative corona charges are injected and trapped in the bulk of the organosilsesquioxane glass resin gate dielectrics. The effective surface potential is controlled by the corona‐charging and subsequent annealing process. It is found that the shift of the transfer characteristics is governed by the electrostatic induction effects of the charged gate electrets, and this observed shift can be related to the surface potential of the layer next to the transistor channel. The process control, efficiency, and long‐term stability of charge storage in spin‐on organosilsesquioxane glass resins are sufficient to enable the construction of simple unipolar inverters and to allow for circuit tuning. New OFET unipolar inverters with an enhancement‐mode driver and a depletion‐mode load are presented, composed of only two simple OFETs with the same channel dimensions and the same p‐type OSC on charged electrets. This design allows the implementation of full‐swing organic logic circuits and illustrates a potential process simplification for organic electronics.     

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.     

Micromolding in capillaries and microtransfer printing of silver nanoparticles as soft-lithographic approach for the fabrication of source/drain electrodes in organic field-effect transistors

Within the past years there has been much effort in developing and improving new techniques for the processing of advanced functional materials used in promising applications like micro-optics or organic electronics. Much attention has been paid to solution-based techniques, which enable low-cost processing and new possible developments like flexible displays or inkjet printed electronics. An alternative approach to inkjet printing is soft-lithography, which is a collective term for a number of non-photolithographic techniques and has become an important tool for the micron-sized structuring of materials.Here we report on the use of micromolding in capillaries (MIMIC) and microtransfer printing (μTP) as two soft-lithographic techniques for the fabrication of silver source/drain electrodes in well-performing bottom-gate/bottom-contact organic field-effect transistors (OFETs) with poly(3-hexylthiophene) as active layer material.While MIMIC combines solution-processability with high lateral resolution for highly accurate patterns, μTP is the miniaturized counterpart to conventional letterpress printing.The performance of the OFETs fabricated with these techniques is similar to devices based on conventional gold source/drain electrodes with well-defined source-to-drain current saturation and a linear behavior at low drain voltages suggesting a low contact resistance and hence good carrier injection from the silver electrodes into the organic semiconductor.     

Micro-scale twistable organic field effect transistors and complementary inverters fabricated by orthogonal photolithography on flexible polyimide substrate

We fabricated micro-scale organic field effect transistors (OFETs) and complementary inverters on a twistable polyimide (PI) substrate by applying orthogonal photolithography. By applying a highly fluorinated photoresist and development solvent, it becomes possible to create organic electronic devices with a micro-scale channel length without damaging the underlying polymer films. The 3 μm-channel twistable pentacene OFET devices and complementary inverters created using p-type pentacene and n-type copper hexadecafluorophthalocyanine exhibited stable electrical characteristics from flat to twist configurations (angle of up to ∼50°). The realization of twistable micro-scale OFETs and inverter devices on a PI substrate may enable the production of functioning organic devices in practical, flexible configurations.