Charge transport and density of trap states in balanced high mobility ambipolar organic thin-film transistors

We report on charge transport and density of trap states (trap DOS) in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer thin-film transistors. This semiconductor possesses high electron and hole field-effect mobilities of up to 0.6 cm²/V-s. Temperature and gate-bias dependent field-effect mobility measurements are employed to extract the activation energies and trap DOS to understand its unique high mobility balanced ambipolar charge transport properties. The symmetry between the electron and hole transport characteristics, parameters and activation energies is remarkable. We believe that our work is the first charge transport study of an ambipolar organic/polymer based field-effect transistor with room temperature mobility higher than 0.1 cm²/V-s in both electrons and holes.     

Recent Advances in Ambipolar Transistors for Functional Applications

Ambipolar transistors represent a class of transistors where positive (holes) and negative (electrons) charge carriers both can transport concurrently within the semiconducting channel. The basic switching states of ambipolar transistors are comprised of common off‐state and separated on‐state mainly impelled by holes or electrons. During the past years, diverse materials are synthesized and utilized for implementing ambipolar charge transport and their further emerging applications comprising ambipolar memory, synaptic, logic, and light‐emitting transistors on account of their special bidirectional carrier‐transporting characteristic. Within this review, recent developments of ambipolar transistor field involving fundamental principles, interface modifications, selected semiconducting material systems, device structures, ambipolar characteristics, and promising applications are highlighted. The existed challenges and prospective for researching ambipolar transistors in electronics and optoelectronics are also discussed. It is expected that the review and outlook are well timed and instrumental for the rapid progress of academic sector of ambipolar transistors in lighting, display, memory, as well as neuromorphic computing for artificial intelligence.     

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.     

Phototuning Selectively Hole and Electron Transport in Optically Switchable Ambipolar Transistors

One of the grand challenges in organic electronics is to develop multicomponent materials wherein each component imparts a different and independently addressable property to the hybrid system. In this way, the combination of the pristine properties of each component is not only preserved but also combined with unprecedented properties emerging from the mutual interaction between the components. Here for the first time, that tri‐component materials comprised of an ambipolar diketopyrrolopyrrole‐based semiconducting polymer combined with two different photochromic diarylethene molecules possessing ad hoc energy levels can be used to develop organic field‐effect transistors, in which the transport of both, holes and electrons, can be photo‐modulated. A fully reversible light‐switching process is demonstrated, with a light‐controlled 100‐fold modulation of p‐type charge transport and a tenfold modulation of n‐type charge transport. These findings pave the way for photo‐tunable inverters and ultimately for completely re‐addressable high‐performance circuits comprising optical storage units and ambipolar field‐effect transistors.     

Gradual Controlling the Work Function of Metal Electrodes by Solution‐Processed Mixed Interlayers for Ambipolar Polymer Field‐Effect Transistors and Circuits

In this paper, a technique using mixed transition‐metal oxides as contact interlayers to modulate both the electron‐ and hole‐injections in ambipolar organic field‐effect transistors (OFETs) is presented. The cesium carbonate (Cs₂CO₃) and vanadium pentoixide (V₂O₅) are found to greatly and independently improve the charge injection properties for electrons and holes in the ambipolar OFETs using organic semiconductor of diketopyrrolopyrrolethieno[3,2‐b]thiophene copolymer (DPPT‐TT) and contact electrodes of molybdenum (Mo). When Cs₂CO₃ and V₂O₅ are blended at various mixing ratios, they are observed to very finely and constantly regulate the Mo's work function from ?4.2 eV to ?4.8 eV, leading to high electron‐ and hole‐mobilities as high as 2.6 and 2.98 cm² V^?1 s^?1, respectively. The most remarkable finding is that the device characteristics and device performance can be gradually controlled by adjusting the composition of mixed‐oxide interlayers, which is highly desired for such applications as complementary circuitry that requires well matched n‐channel and p‐channel device operations. Therefore, such simple interface engineering in conjunction with utilization of ambipolar semiconductors can truly enable the promising low‐cost and soft organic electronics for extensive applications.     

3,7-Bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT) based polymer with balanced ambipolar charge transport performance

A novel acceptor building block, 3,7-bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT), is developed to construct a donor-acceptor polymer PIBDTBT-40. This polymer has favorable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for balanced ambipolar charge transport. Organic thin film transistors (OTFTs) based on this polymer shows well-balanced ambipolar characteristics with electron mobility of 0.14 cm² V⁻¹ s⁻¹ and hole mobility of 0.10 cm² V⁻¹ s⁻¹ in bottom-gate bottom-contact devices. This polymer is a promising semiconductor for solution processable organic electronics such as CMOS-like logic circuits.     

Pressure‐Tunable Ambipolar Conduction and Hysteresis in Thin Palladium Diselenide Field Effect Transistors

Few‐layer palladium diselenide (PdSe₂) field effect transistors are studied under external stimuli such as electrical and optical fields, electron irradiation, and gas pressure. The ambipolar conduction and hysteresis are observed in the transfer curves of the as‐exfoliated and unprotected PdSe₂ material. The ambipolar conduction and its hysteretic behavior in the air and pure nitrogen environments are tuned. The prevailing p‐type transport observed at atmospheric pressure is reversibly turned into a dominant n‐type conduction by reducing the pressure, which can simultaneously suppress the hysteresis. The pressure control can be exploited to symmetrize and stabilize the transfer characteristics of the device as required in high‐performance logic circuits. The transistors are affected by trap states with characteristic times in the order of minutes. The channel conductance, dramatically reduced by the electron irradiation during scanning electron microscope imaging, is restored after an annealing of several minutes at room temperature. The work paves the way toward the exploitation of PdSe₂ in electronic devices by providing an experiment‐based and deep understanding of charge transport in PdSe₂ transistors subjected to electrical stress and other external agents.     

Nonvolatile Floating‐Gate Memories Based on Stacked Black Phosphorus–Boron Nitride–MoS2 Heterostructures

Research on van der Waals heterostructures based on stacked 2D atomic crystals is intense due to their prominent properties and potential applications for flexible transparent electronics and optoelectronics. Here, nonvolatile memory devices based on floating‐gate field‐effect transistors that are stacked with 2D materials are reported, where few‐layer black phosphorus acts as channel layer, hexagonal boron nitride as tunnel barrier layer, and MoS₂ as charge trapping layer. Because of the ambipolar behavior of black phosphorus, electrons and holes can be stored in the MoS₂ charge trapping layer. The heterostructures exhibit remarkable erase/program ratio and endurance performance, and can be developed for high‐performance type‐switching memories and reconfigurable inverter logic circuits, indicating that it is promising for application in memory devices completely based on 2D atomic crystals.     

High mobility ambipolar organic field-effect transistors with a nonplanar heterojunction structure

Ambipolar organic field-effect transistors (OFETs) based on a bilayer structure of highly crystalline small molecules, n-type α,ω-diperfluorohexylquaterthiophene (DFH-4T) and p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), are investigated. By employing DFH-4T/DNTT as the bottom/top layers and appropriate high work function (WF) electrodes in a bottom-gate, top-contact configuration, the superior ambipolar characteristics with matched electron and hole mobilities of 1–1.1 cm² V⁻¹ s⁻¹ are achieved. Intriguingly, this high-performance device exhibits a unique feature of an extremely rough, nonplanar heterojunction in the DFH-4T/DNTT combination and a large electron injection barrier from the high WF electrodes to DFH-4T, suggesting some underlying mechanisms for the effective charge transport and injection. The electrical and structural analyses reveal that the crystal packing of the bottom DFH-4T layer supports the growth of a high-quality DNTT crystal network for high-mobility hole transport upon the nonplanar heterojunction, and also enables the formation of an enlarged organic/metal contact surface for efficient electron injection from the high WF electrodes, as the key attributes leading to an overall excellent ambipolar behavior. The effect of intrinsic charge accumulation at the heterojunction interface on the ambipolar conduction is also discussed. Furthermore, a complementary-like inverter constructed with two DFH-4T/DNTT ambipolar OFETs is demonstrated, which shows a gain of 30.     

Solution‐Processed Small‐Molecule Bulk Heterojunction Ambipolar Transistors

Solution‐processed small‐molecule bulk heterojunction (BHJ) ambipolar organic thin‐film transistors are fabricated based on a combination of [2‐phenylbenzo[d,d']thieno[3,2‐b;4,5‐b']dithiophene (P‐BTDT) : 2‐(4‐n‐octylphenyl)benzo[d,d ']thieno[3,2‐b;4,5‐b']dithiophene (OP‐BTDT)] and C₆₀. Treating high electrical performance vacuum‐deposited P‐BTDT organic semiconductors with a newly developed solution‐processed organic semiconductor material, OP‐BTDT, in an optimized ratio yields a solution‐processed p‐channel organic semiconductor blend with carrier mobility as high as 0.65 cm² V^?1 s^?1. An optimized blending of P‐BTDT:OP‐BTDT with the n‐channel semiconductor, C₆₀, results in a BHJ ambipolar transistor with balanced carrier mobilities for holes and electrons of 0.03 and 0.02 cm² V^?1 s^?1, respectively. Furthermore, a complementary‐like inverter composed of two ambipolar thin‐film transistors is demonstrated, which achieves a gain of 115.     

Ambipolar Light‐Emitting Transistors of a Tetracene Single Crystal

The first ambipolar light‐emitting transistor of an organic molecular semiconductor single crystal, tetracene, is demonstrated. In the device configuration, electrons and holes injected from separate magnesium and gold electrodes recombined radiatively within the channel. By varying the applied voltages, the position of the recombination/emission zone could be moved to any position along the channel. Because of the changes made to the device structure, including the use of single crystals and polymer dielectric layers and the adoption of an inert‐atmosphere fabrication process, the set of materials that can be used for light‐emitting transistors has been expanded to include monomeric molecular semiconductors.     

Control of Ambipolar and Unipolar Transport in Organic Transistors by Selective Inkjet‐Printed Chemical Doping for High Performance Complementary Circuits

The selective tuning of the operational mode from ambipolar to unipolar transport in organic field‐effect transistors (OFETs) by printing molecular dopants is reported. The field‐effect mobility (μ_FET) and onset voltage (V_on) of both for electrons and holes in initially ambipolar methanofullerene [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) OFETs are precisely modulated by incorporating a small amount of cesium fluoride (CsF) n‐type dopant or tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) p‐type dopant for n‐channel or p‐channel OFETs either by blending or inkjet printing of the dopant on the pre‐deposited semiconductor. Excess carriers introduced by the chemical doping compensate traps by shifting the Fermi level (E_F) toward respective transport energy levels and therefore increase the number of mobile charges electrostatically accumulated in channel at the same gate bias voltage. In particular, n‐doped OFETs with CsF show gate‐voltage independent Ohmic injection. Interestingly, n‐ or p‐doped OFETs show a lower sensitivity to gate‐bias stress and an improved ambient stability with respect to pristine devices. Finally, complementary inverters composed of n‐ and p‐type PCBM OFETs are demonstrated by selective doping of the pre‐deposited semiconductor via inkjet printing of the dopants.     

A Graphene‐Based Vacuum Transistor with a High ON/OFF Current Ratio

A graphene‐based vacuum transistor (GVT) with a high ON/OFF current ratio is proposed and experimentally realized by employing electrically biased graphene as the electron emitter. The states of a GVT are switched by tuning the bias voltage applied to the graphene emitter with an ON/OFF current ratio up to 10⁶, a subthreshold slope of 120 mV dec^?1 and low working voltages of <10 V, exhibiting switching performances superior to those of previously reported graphene‐based solid‐state transistors. GVTs are fabricated and integrated using silicon microfabrication technology. A perfectly symmetric ambipolar device is achieved by integrating two GVTs, implying the potential of realizing vacuum integrated circuits based on GVTs. GVTs are expected to find applications in extreme environments such as high temperature and intense irradiation.     

Photo- and electroluminescence of ambipolar,high-mobility,donor-acceptor polymers

Donor-acceptor polymers with narrow bandgaps are promising materials for bulk heterojunction solar cells and high-mobility field-effect transistors. They also emit light in the near-infrared. Here we investigate and compare the photoluminescence and electroluminescence properties of different narrow bandgap (<1.5 eV) donor-acceptor polymers with diketopyrrolopyrrole (DPP), isoindigo (IGT) and benzodipyrrolidone (BPT) cores, respectively. All of them show near-infrared photoluminescence quantum yields of 0.03–0.09% that decrease with decreasing bandgap. Bottom-contact/top-gate field-effect transistors show ambipolar charge transport with hole and electron mobilities between 0.02 and 0.7 cm² V⁻¹ s⁻¹ and near-infrared electroluminescence. Their external quantum efficiencies reach up to 0.001%. The effect of polaron quenching and other reasons for the low electroluminescence efficiency of these high mobility polymers are investigated.     

Charge Transport in High-Mobility Field-Effect Transistors Based on Inkjet Printed Random Networks of Polymer Wrapped Single-Walled Carbon Nanotubes

Printed random networks of polymer-wrapped multi-chiral semiconducting carbon nanotubes (s-SWCNTs) are an opportunity for mass-manufacturable, high-performance large-area electronics. To meet this goal, a deeper understanding of charge-transport mechanisms in such mixed networks is crucial. Here, charge transport in field-effect transistors based on inkjet-printed s-SWCNTs networks is investigated, obtaining direct evidence for the phases probed by charge in the accumulated channel, which is critical information to rationalize the different transport properties obtained for different printing conditions. In particular, when the fraction of nanotubes with smaller bandgaps is efficiently interconnected, the sparse network provides efficient charge percolation for band-like transport, with a charge mobility as high as 20.2 cm² V⁻¹ s⁻¹. However, when the charges are forced by a less efficient morphology, to populate also higher bandgap nanotubes and and/or the wrapping polymer, thermally activated transport takes place and mobility drops. As a result, a trade-off between network density and charge transport properties is identified for device current optimization, in both p- and n-type regimes. These findings shed light on the fundamental aspects related to charge transport in printed s-SWCNT mixed networks and contribute to devise appropriate strategies for the formulation of inks and processes towards cost-effective mass production schemes of high-performance large-area electronics.     

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.     

Improved ambipolar charge injection in organic field-effect transistors with low cost metal electrode using polymer sorted semiconducting carbon nanotubes

Solution-processed thin film transistors can be implemented using simple and low cost fabrication, and are the best candidates for commercialization due to their application to a range of wearable electronics. We report an ambipolar charge injection interlayer that can improve both hole and electron injection in organic field-effect transistors (OFETs) with inexpensive source-drain electrodes. The solution processed ambipolar injection layer is fabricated by selective dispersion of semiconducting single walled carbon nanotubes using poly(9,9-dioctylfluorene). OFETs with molybdenum (Mo) contacts and interlayer (Mo/interlayer OFETs) exhibit superior performance, including higher hole and electron mobilities, device yield, lower threshold voltages, and lower trap densities than those of bare transistors. While OFETs with Mo contacts show unipolar p-type behaviour, Mo/interlayer OFETs display ambipolar transport due to significant enhancement of electron injection. In the p-type region, transistor performance is comparable to devices with gold (Au). Hole mobility is increased approximately ten-fold over devices with only Mo contacts. The electron mobility of Mo/interlayer OFETs is 0.05 cm²V⁻¹s⁻¹, which is higher than devices with Au electrodes. The p-type contact resistances of Mo/interlayer OFETs are half those of OFETs with Mo contacts. Trap density in Mo/interlayer OFETs is one order magnitude lower than that of pristine devices. We also demonstrate that this approach is extendible to other metals (nickel) and n-type semiconductors with different energy levels. Injection by tunnelling is suggested as the mechanism of ambipolar injection.     

Ferroelectrics-Electret Synergetic Organic Artificial Synapses with Single-Polarity Driven Dynamic Reconfigurable Modulation

Most current studies of artificial synapses only mimic the static plasticity, which is far from achieving the complex behaviors of the human brain. The few reported dynamic reconfigurable synapses based on ambipolar transistors switch the operating states by voltages with opposite polarity, which impedes the development of highly efficient synaptic readout circuits. To improve the efficiency, flexibility, and biocompatibility of dynamic reconfigurable synapses, here a ferroelectrics-electret synergetic organic synaptic p-type transistor (FESOST) is devised. Owing to the synergetic action of ferroelectric polarization switching and charge capture, FESOST exhibits single-polarity driven dynamic reconfigurable operating states with different synaptic behaviors (potentiation and depression) in response to the same gate pulse in different modes (excitatory and inhibitory). In addition, various single-polarity driven synaptic behaviors including short-term/long-term plasticity, paired-pulse facilitation/depression, spike-rate-dependent plasticity, and spike-number-dependent plasticity are also simulated. Finally, the reconfigurable artificial temperature perception system is simulated for the complex emotions of humans in response to different weather stimuli for people of different constitutions. The novel device architecture represents a major step forward in the development of dynamic, reconfigurable, high-efficiency, organic synapses.     

Single-Component CMOS-Like Logic using Diketopyrrolopyrrole-Based Ambipolar Organic Electrochemical Transistors

Complementary circuits based on organic electrochemical transistors (OECTs) are attractive for the development of inexpensive and disposable point-of-care bioelectronic devices. Ambipolar OECTs, which employ a single channel material, could decrease the fabrication complexity and manufacturing costs of such circuits. An ideal channel material for ambipolar OECTs should be electrochemically stable in aqueous environments, afford facile ion insertion for both cations and anions, and also facilitate high and balanced electron and hole transport. In this study, triethylene glycol functionalized diketopyrrolopyrrole (DPP)-based polymer is proposed for the development of ambipolar OECTs. It is shown that DPP-based OECTs have a high and comparable figure of merit for both n- and p-type operations. Logic NOT, NAND, and NOR operations with corresponding complementary circuits constructed from identical DPP-based OECT devices are demonstrated. This study is an important step toward the development of sophisticated complementary metal–oxide–semiconductor-like logic circuits using single-component OECTs.     

Capacitively Coupled Hybrid Ion Gel and Carbon Nanotube Thin‐Film Transistors for Low Voltage Flexible Logic Circuits

The lamination of a high‐capacitance ion gel dielectric layer onto semiconducting carbon nanotube (CNT) thin‐film transistors (TFTs) that are bottom‐gated with a low‐capacitance polymer dielectric layer drastically reduces the operating voltage of the devices resulting from the capacitive coupling effect between the two dielectric layers sandwiching the CNT channel. As the CNT channel has a network structure, only a compact area of ion gel is required to make the capacitive coupling effect viable, unlike the planar channels of previously reported transistors that required a substantially larger area of ion gel dielectric layer to induce the coupling effect. The capacitively coupled CNT TFTs possess superlative electrical characteristics such as high carrier mobilities (42.0 cm² (Vs)^?1 for holes and 59.1 cm² (Vs)^?1 for electrons), steep subthreshold swings (160 mV dec^?1 for holes and 100 mV dec^?1 for electrons), and low gate leakage currents (<1 nA). These devices can be further integrated to form complex logic circuits on flexible substrates with high mechanical resilience. The layered geometry of the device coupled with scalable solution‐based fabrication has significant potential for large‐scale flexible electronics.