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.     

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.     

Reconfigurable Multifunctional Ambipolar Polymer-Blend Transistors with Improved Switching-Off Capability

Ambipolar field-effect transistors allowing both holes and electrons transport can work in different states, which are attractive for simplifying the manufacture of circuits and endowing the circuits with reconfigurable multi-functionalities. However, conventional ambipolar transistors intrinsically suffer from poor switching-off capability because the gate electrode is not able to simultaneously deplete holes and electrons across the entire transport channel, which hurdles the practical applications. This study shows that the switching-off capability of polymer ambipolar transistor is significantly improved by up to three orders by introducing non-uniformly distributed compensation potentials along the channel to synchronically tune the charge transport at different channel locations. The non-uniform compensation potential is experimentally generated by the non-uniformly distributed electret charges, which are pre-injected into the insulators from source and drain electrodes. By this method, both n-type and p-type operations with high mobility (2.2 and 0.8 cm²s⁻¹V⁻¹ respectively) and high on/off ratio (10⁵) are obtained in the same device, and the different states are reversibly switchable. Moreover, this method endows the device with diverse device characteristics and reconfigurable multi-functionalities, which promotes the application of ambipolar transistors in complementary metal-oxide semiconductors-like circuits and some emerging electronics, including reconfigurable devices, multi-level memories, and artificial synapses.     

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.     

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.     

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.     

Half‐Fused Diketopyrrolopyrrole‐Based Conjugated Donor–Acceptor Polymer for Ambipolar Field‐Effect Transistors

A novel building block, denoted as half‐fused diketopyrrolopyrrole (DPP) (9‐(3‐octadecylhenicosyl)‐8‐(thiophen‐2‐yl)‐7H‐pyrrolo[3,4‐a]thieno[3,2‐g]indolizine‐7,10(9H)‐dione), in which one of the flanking thiophene units is fused to one of the DPP rings via a carbon‐carbon double bond at the N‐position is reported. The half‐fused DPP is successfully utilized as an electron acceptor to prepare the conjugated donor–acceptor polymer PTFDFT , which exhibits ambipolar semiconducting behavior in ambient air. Theoretical calculations and absorption spectral studies show that the backbone of PTFDFT is more planar compared to the reference polymer with conventional DPP units. As a result, PTFDFT shows a narrow bandgap and low lowest unoccupied molecular orbital level. The more planar backbone with fewer side chains favors the dense packing of the polymer chains of PTFDFT with a short π–π stacking distance (3.49 Å). Grazing‐incidence wide‐angle X‐ray scattering data further confirm the predominant edge‐on packing mode of the PTFDFT polymer chains on the substrate. As expected, the PTFDFT thin film shows excellent ambipolar semiconducting properties under ambient conditions, reaching 2.23 and 1.08 cm² V^?1 s^?1 for the n‐ and p‐channels, respectively. In addition, complementary‐like inverter with gain value as high as 141 is successfully constructed using the PTFDFT thin film.     

Heterojunction Ambipolar Organic Transistors Fabricated by a Two‐Step Vacuum‐Deposition Process

Ambipolar organic field‐effect transistors (OFETs) are produced, based on organic heterojunctions fabricated by a two‐step vacuum‐deposition process. Copper phthalocyanine (CuPc) deposited at a high temperature (250 °C) acts as the first (p‐type component) layer, and hexadecafluorophthalocyaninatocopper (F₁₆CuPc) deposited at room temperature (25 °C) acts as the second (n‐type component) layer. A heterojunction with an interpenetrating network is obtained as the active layer for the OFETs. These heterojunction devices display significant ambipolar charge transport with symmetric electron and hole mobilities of the order of 10^–4 cm² V^–1 s^–1 in air. Conductive channels are at the interface between the F₁₆CuPc and CuPc domains in the interpenetrating networks. Electrons are transported in the F₁₆CuPc regions, and holes in the CuPc regions. The molecular arrangement in the heterojunction is well ordered, resulting in a balance of the two carrier densities responsible for the ambipolar electrical characteristics. The thin‐film morphology of the organic heterojunction with its interpenetrating network structure can be controlled well by the vacuum‐deposition process. The structure of interpenetrating networks is similar to that of the bulk heterojunction used in organic photovoltaic cells, therefore, it may be helpful in understanding the process of charge collection in organic photovoltaic cells.     

Analog Circuit Applications Based on All‐2D Ambipolar ReSe2 Field‐Effect Transistors

Complementary circuits based on 2D materials show great promise for next‐generation electronics. An ambipolar all‐2D ReSe₂ field‐effect transistor (FET) with a hexagonal boron nitride gate dielectric is fabricated and its electronic characteristics are comprehensively studied by temperature dependence and noise measurements. Ambipolar transfer characteristics are achieved owing to the tunable Fermi level of the graphene contact and negligible and 30 meV Schottky barrier heights for the n‐ and p‐channel regimes, respectively. An inverter is also fabricated to demonstrate ambipolar ReSe₂ FET operation in a logic circuit. Furthermore, a p/n switchable unipolar FET is designed and shows potential for building complimentary circuits from a signal device. This work demonstrates the potential of all‐2D ReSe₂ FETs and makes available new approaches for designing next‐generation devices.     

Emission Color Tuning in Ambipolar Organic Single‐Crystal Field‐Effect Transistors by Dye‐Doping

The effect of dye‐doping in ambipolar light‐emitting organic field‐effect transistors (LE‐OFETs) is investigated from the standpoint of the carrier mobilities and the electroluminescence (EL) characteristics under ambipolar operation. Dye‐doping of organic crystals permits not only tuning of the emission color but also significantly increases the efficiency of ambipolar LE‐OFETs. A rather high external EL quantum efficiency (～0.64%) of one order of magnitude higher than that of a pure p‐distyrylbenzene (P3V2) single crystal is obtained by tetracene doping. The doping of tetracene molecules into a host P3V2 crystal has almost no effect on the electron mobility and the dominant carrier recombination process in the tetracene‐doped P3V2 crystal involves direct carrier recombination on the tetracene molecules.     

Solution‐Deposited Zinc Oxide and Zinc Oxide/Pentacene Bilayer Transistors: High Mobility n‐Channel,Ambipolar, and Nonvolatile Devices

A solution processed n‐channel zinc oxide (ZnO) field effect transistor (FET) was fabricated by simple dip coating and subsequent heat treatment of a zinc acetate film. The field effect mobility of electrons depends on ZnO grain size, controlled by changing the number of coatings and zinc acetate solution concentration. The highest electron mobility achieved by this method is 7.2 cm² V^?1 s^?1 with On/Off ratio of 70. This electron mobility is higher than for the most recently reported solution processed ZnO transistor. We also fabricated bilayer transistors where the first layer is ZnO, and the second layer is pentacene, a p‐channel organic which is deposited by thermal evaporation. By changing the ZnO grain size (or thickness) this type of bilayer transistor shows p‐channel, ambipolar and n‐channel behavior. For the ambipolar transistor, well balanced electron and hole mobilities are 7.6 × 10^?3 and 6.3 × 10^?3 cm² V^?1 s^?1 respectively. When the ZnO layer is very thin, the transistor shows p‐channel behavior with very high reversible hysteresis. The nonvolatile tuning function of this transistor was investigated.     

Controlling Ambipolar Charge Transport in Isoindigo‐Based Conjugated Polymers by Altering Fluorine Substitution Position for High‐Performance Organic Field‐Effect Transistors

A molecular design strategy to achieve highly balanced ambipolar charge transport for donor–acceptor (D–A) isoindigo (IIG)‐based copolymer through systematic selection of fluorination positions is reported. To study fluorine substitution site effects on electronic and structural properties, two fluorinated IIG‐based copolymers (PIIG‐iFT2 and PIIG‐oFT2) are synthesized, which contain two fluorine atoms at the bithiophene (T2) inner and outer site and compare them with a nonfluorinated copolymer of IIG and T2 (PIIG‐T2) as the reference polymer. Fluorination at the outer site of T2 in PIIG‐oFT2 polymer effectively lowers molecular energy levels and increases molecular planarity more than fluorination at the T2 inner site. PIIG‐oFT2 organic field‐effect transistors show highly balanced ambipolar mobility, hole mobility (μ_h)/electron mobility (μ_e) = 1 by increasing electron mobility, whereas PIIG‐T2 (μ_h/μ_e = 9.0) and PIIG‐iFT2 (μ_h/μ_e = 2.4) exhibit unbalanced ambipolar transport. The ambipolar complementary‐like inverter is also demonstrated by simple one‐time coating of PIIG‐oFT2 with gain = 21.     

Carbon Nanotube Based Multifunctional Ambipolar Transistors for AC Applications

Field‐effect transistors (FETs) fabricated on large diameter carbon nanotubes (CNTs) present typical ambipolar transfer characteristics owing to the small band‐gap of CNTs. Depending on the DC biasing condition, the ambipolar FET can work in three different regions, and then can be used as the core to realize multifunctional AC circuits. The CNT FET based circuits can work as a high‐efficiency ambipolar frequency doubler in the ambipolar transfer region, and also can function as in‐phase amplifier and inverted amplifier in the linear transfer region. Due to current saturation of the CNT FET, an AC amplifier with a voltage gain of 2 is realized when the device works in the linear transfer region. Achieving an actual amplification and frequency doubling functions indicates that complicated radio frequency circuits or systems can be constructed based on just one kind of device: ambipolar CNT FETs.     

Gate-Controlled Polarity-Reversible Photodiodes with Ambipolar 2D Semiconductors

A photosensor with an amplitude-tunable and polarity-reversible response under gate modulation has potential as a computational photosensor, which can provide more recognition degree of data to enhance signal processing efficiency. Although, the ambipolar 2D semiconductors possess unique gate-tunable properties, the question of how to utilize this property to design polarity-reversible photodiodes for intelligent applications remains unanswered. Here, gate-controllable polarity-reversible photodiodes based on ambipolar 2D semiconductors with an asymmetrically metal-contacted architecture are proposed. By controlling the gate-field, the local carrier type and density profile can be manipulated in the channel due to the partial shielding feature of the asymmetrically metal-contacted architecture, resulting in a polarity-reversible photodiode. The reported WSe₂-based photodiode possesses excellent rectifying behavior with a rectification ratio over 10⁵, photovoltaic performance with 90% external quantum efficiency, and 2.3% power conversion efficiency under gate regulation. Meanwhile, the device exhibits reversible polarity of photovoltage from a negative to positive state under gate control. By utilizing the reversible photovoltage of the WSe₂ photodiode, an optoelectronic switch with a photovoltage polarity signal is demonstrated without a bias voltage. This photovoltage-reversible homodiode paves the way to develop 2D devices with multiple operation modes for potential applications in high-efficiency photovoltaics, intelligent vision sensors, and logic optoelectronics.     

Selectively Fluorinated Furan-Phenylene Co-Oligomers Pave the Way to Bright Ambipolar Light-Emitting Electronic Devices

Linearly conjugated oligomers attract ever-growing attention as promising systems for organic optoelectronics because of their inherent lucky combination of high charge mobility and bright luminescence. Among them, furan-phenylene co-oligomers (FPCOs) are distinguished by outstanding solubility, very bright luminescence, and good hole-transport properties; however, furan-containing organic semiconductors generally lack electron transport, which makes it impossible to utilize them in efficient light-emitting electronic devices, specifically, ambipolar light-emitting transistors. In this work, 1,4-bis(5-phenylfuran-2-yl)benzene (FP5) derivatives are synthesized with the fully/partially fluorinated central and edge phenyl rings. It is shown that the selective fluorination of FPCOs lowers the energies of frontier molecular orbitals, maintaining the bandgap, solubility, and bright luminescence, dramatically improves the photostability, tunes the π-π stacked packing, and allows the first realization of electron transport in FPCOs. It is found that selectively fluorinated 2,2′-(2,3,5,6-tetrafluoro-1,4-phenylene)bis[5-(3,5-difluorophenyl)furan] demonstrates well-balanced ambipolar charge transport and efficient electroluminescence in an organic light-emitting transistor (OLET) with external quantum and luminous efficiencies as high as 0.63% and 5 cdA⁻¹, respectively, which are among the best reported for OLETs. The findings show that “smart” fluorination is a powerful tool to fine-tune the stability and performance of linearly conjugated small molecules for organic optoelectronics.     

Bithienopyrroledione‐Based Copolymers,Versatile Semiconductors for Balanced Ambipolar Thin‐Film Transistors and Organic Solar Cells with V oc > 1 V

Conjugated polymer semiconductors P1 and P2 with bithienopyrroledione (bi‐TPD) as acceptor unit are synthesized. Their transistor and photovoltaic performances are investigated. Both polymers display high and balanced ambipolar transport behaviors in thin‐film transistors. P1‐ based devices show an electron mobility of 1.02 cm² V^?1 s^?1 and a hole mobility of 0.33 cm² V^?1 s^?1, one of the highest performance reported for ambipolar polymer transistors. The electron and hole mobilities of P2 transistors are 0.36 and 0.16 cm² V^?1 s^?1, respectively. The solar cells with PC₇₁BM as the electron acceptor and P1/P2 as the donor exhibit a high V _oc about 1.0 V, and a power conversion efficiency of 6.46% is observed for P1‐ based devices without any additives and/or post treatment. The high performance of P1 and P2 is attributed to their crystalline films and short π–π stacking distance (<3.5 Å). These results demonstrate (1) bi‐TPD is an excellent versatile electron‐deficient unit for polymer semiconductors and (2) bi‐TPD‐based polymer semiconductors have potential applications in organic transistors and organic solar cells.     

Quinoidal Molecules as a New Class of Ambipolar Semiconductor Originating from Amphoteric Redox Behavior

The two small molecules, quinoidal bithiophene (QBT) and quinoidal biselenophene (QBS), are designed based on a quinoid structure, and synthesized via a facile synthetic route. These quinoidal molecules have a reduced band gap and an amphoteric redox behavior, which is caused by an extended delocalization. Due to such properties, organic field‐effect transistors based on QBT and QBS have shown balanced ambipolar characteristics. After thermal annealing, the performances of the devices are enhanced by an increase in crystallinity. The field‐effect hole and electron mobilities are measured to be 0.031 cm² V^?1 s^?1 and 0.005 cm² V^?1 s^?1 for QBT, and 0.055 cm² V^?1 s^?1 and 0.021 cm² V^?1 s^?1 for QBS, respectively. In addition, we investigate the effect of chalcogen atoms (S and Se) on the molecular properties. The optical, electrochemical properties and electronic structures are mainly dominated by the quinoidal structure, whereas molecular properties are scarcely affected by either type of chalcogen atom. The main effect of the chalcogen atoms is ascribed to the difference of crystallinity. Due to a strong intermolecular interaction of the selenophene, QBS exhibits a higher degree of crystallinity, which leads to an enhancement of both hole and electron mobilities. Consequently, these types of quinoidal molecules are found to be promising for use as ambipolar semiconductors.     

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.     

Novel Photoinduced Recovery of OFET Memories Based on Ambipolar Polymer Electret for Photorecorder Application

A new design concept for novel photoresponsive flash organic field‐effect transistor (OFET) memory is demonstrated by employing the carbazoledioxazine polymer (Poly CD) as an electret. Photoactive electrets that can absorb the light effectively rather than photoactive semiconductors are proposed by the “photoinduced recovery” mechanism in the literature; however, the correlation between the chemical structure and photoresponsive electrical performances is ambiguous. In this study, it is reported for the first time that the OFET memory with trapped charges can be optically recovered by a polymer electret and the working mechanism can be explained by the structural design. The highly planar Poly CD electret exhibits photoluminescence quenching in film states, resulting in the generation of sufficient excitons to eliminate trapped charges under light excitation. Additionally, the Poly CD electret with coplanar donor–acceptor moieties is suitable for both p‐channel and n‐channel semiconductors. For p‐type memory devices, a large memory window (82 V) and stable nonvolatile retention performance with high ON/OFF ratio could be obtained. The memories also display good switching reliability for voltage‐programming and light‐erasing cycles. This study provides useful information for the development of polymer‐based photoresponsive flash OFET memories and demonstrates the practical applications of photorecorder and photosensitive smart tag.     

利用MoO3为电极修饰层的F16CuPc/ CuPc异质结的双极性有机场效应晶体管

在本文中,我们利用钛青铜(CuPc)和氟化钛青铜(F16CuPc)作为空穴传输层和电子传输层的制备了具有异质结结构的有机场效应晶体管(OFETs)。与单层的F16CuPc晶体管相比,异质结结构的晶体管的电子迁移率从3.1×10-3cm2/Vs提高至8.7×10-3cm2/vs,然而,空穴的传输行为却没有被观测到。为了提高空穴的注入能力,我们利用MoO3对源-漏电极进行了修饰,有效地改善了空穴注入。并进一步证实了MoO3的引入使得器件的接触电阻变小,平衡了电子和空穴的注入,从而最终实现了器件的双极性传输。