Outlook and emerging semiconducting materials for ambipolar transistors.

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Outlook and Emerging Semiconducting Materials for Ambipolar Transistors Satria Zulkarnaen Bisri,* Claudia Piliego, Jia Gao, and Maria Antonietta Loi*

1. Introduction This progress report discusses the progress on the fabrication of highly performing ambipolar field-effect transistors and their future prospects. Transistors have become ubiquitous in our daily life, as they are embedded in all communication, computation, and automation appliances. This has been driven by the continuous miniaturization of the electronic devices, minimizing device architectures to make them more compact and efficient. However, to date most of the transistors work only with the transport of one charge carrier type,

Dr. S. Z. Bisri, Dr. C. Piliego, Dr. J. Gao, Prof. M. A. Loi Photophysics and Optoelectronics Group Zernike Institute for Advanced Materials University of Groningen Nijenborgh 4, Groningen 9747 AG, The Netherlands E-mail: [email protected]; [email protected]

DOI: 10.1002/adma.201304280

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despite the fact that in many semiconductors, in agreement with basic semiconductor physics, both electrons and holes can be transported. Ambipolar transistors, transporting holes and electrons simultaneously, allow fabrication of more compact CMOS devices. Moreover, ambipolar field-effect transistors have the potential to be at the heart of the next generation of light-emitting devices, namely, lightemitting transistors that can integrate the light-emission capability and current modulation in a single device architecture.[1] After an introductory discussion about the fundamentals of ambipolar transistors, the important factors that contribute to the efficient functioning of this class of devices are elucidated. In particular, the role of the active material, the charge carrier injection, as well as the interface properties are discussed. We then proceed to review briefly the recent successes in fabrication of ambipolar transistor with solution processable materials, such as organic semiconductors, colloidal semiconductors, and carbon nanotubes. In these materials systems, despite the many remaining challenges, the physics of the devices is well understood and many applications have started to emerge. Novel gating techniques involving the use of electric double-layer gating will be argued as a new tool to realize ambipolar transistors on challenging materials and to gain deeper understanding of the physics of the active semiconductor. Finally, we give an outlook of the physics and application prospects for ambipolar transistors, in the miniaturizations and printed electronics and optoelectronics.

Ambipolar or bipolar transistors are transistors in which both holes and electrons are mobile inside the conducting channel. This device allows switching among several states: the hole-dominated on-state, the off-state, and the electron-dominated on-state. In the past year, it has attracted great interest in exotic semiconductors, such as organic semiconductors, nanostructured materials, and carbon nanotubes. The ability to utilize both holes and electrons inside one device opens new possibilities for the development of more compact complementary metal-oxide semiconductor (CMOS) circuits, and new kinds of optoelectronic device, namely, ambipolar light-emitting transistors. This progress report highlights the recent progresses in the field of ambipolar transistors, both from the fundamental physics and application viewpoints. Attention is devoted to the challenges that should be faced for the realization of ambipolar transistors with different material systems, beginning with the understanding of the importance of interface modification, which heavily affects injections and trapping of both holes and electrons. The recent development of advanced gating applications, including ionic liquid gating, that open up more possibility to realize ambipolar transport in materials in which one type of charge carrier is highly dominant is highlighted. Between the possible applications of ambipolar field-effect transistors, we focus on ambipolar light-emitting transistors. We put this new device in the framework of its prospective for general lightings, embedded displays, current-driven laser, as well as for photonics–electronics interconnection.

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2. Fundamentals of Ambipolar Transistors Ambipolar transistors are field-effect transistor (FET) devices (Figure 1a), which can simultaneously accumulate electrons and holes.[2–6] An FET device structure consists of a metal– insulator–semiconductor (MIS) junction and two additional electrodes on the semiconductor part.[7] The MIS junction drives the charge carrier accumulation at the semiconductor/ insulator (dielectric) interface by the application of a gate voltage. The other two electrodes that are in contact with the semiconductor surface act as source and drain electrodes, from

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1. Unipolar region with V D ≤ (V G − V th,e ) . Here the drain current is defined as

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which charge carriers will be injected to the semiconductor where the accumulation layer is formed. The charge carriers will be drifted by the voltage difference between these electrodes. Therefore, by changing the gate voltage, the amount of accumulated charge carriers can be varied, resulting in a variation of the conductivity at the semiconductor/insulator interface. Because of its switching capabilities, this three-terminal device structure has become the backbone of modern electronics. Figure 1b shows the schematic of the charge carrier accumulation in a basic MIS junction. From the original concept of field-effect charge carrier accumulation, any field-effect transistors should be able to accumulate both holes and electrons, depending on the applied gate voltage.[7] However, this only happens in ideal cases, or at low temperature.[4,6] Most of the field effect transistors work with accumulation of one type of charge carriers, i.e., as unipolar devices by forming either an n-channel or a p-channel. This is due to the difficulties in accumulating the minority carrier, which is known as strong inversion condition. Nevertheless, the existence of traps for minority carriers inside the semiconductor as well as at the semiconductor/insulator interface hampers the realization of the inversion condition. In addition, even in intrinsic semiconductors the difference of the bandwidth of the conduction band and valence band gives asymmetry in their hole and electron transport, thus one type of charge carrier is more preferably accumulated than the other. In the twenty-first century, scientists started to intensively tackle the problems to find a beneficial strong inversion condition with many different strategies to realize ambipolar transistors.[9–11] Ambipolar FET can be perceived as an interfacial p–n junction,[12] in which the charge carrier doping is governed by how much each carrier type can be accumulated at the insulator/ semiconductor interface. In the ambipolar operation regime, both hole and electron should contribute to the total current flowing in the device. Several models have been proposed to describe the ambipolar FET working mechanism. The oldest one is based on a gradual channel approximation (Pao-Sah description),[13] which is conventional for unipolar FETs.[14] However, this model did not consider any possibility of hole and electron recombination as well as a non-ohmic nature of at least one of the contacts.[15] Paasch and Schmechel[8] later on separately proposed improved models. The former model considers electron–hole recombination and the nature of the contacts, and was applied to understand the behavior of early bilayer ambipolar organic FET devices. The latter model considers the equivalent circuit consisting of a resistor-capacitor network and forced ohmic contacts for both electrodes. This model successfully explained experimental results where both hole and electron injection are ohmic. Generalizing from these two models, ambipolar FET accumulates both hole and electron at the same time, therefore are necessary two different threshold voltages: one for electron accumulation (Vth,e) and another for holes accumulation (Vth,h). Thus, the operation scheme of the ambipolar transistor could be distinguished into three different regions, which are:

Satria Zulkarnaen Bisri obtained his B.Sc./S.Si. in physics in 2006 from Institut Teknologi Bandung, Indonesia. He received his M.Sc. in 2008 and his Ph.D. in physics in 2011 from Tohoku University, Japan, under the guidance of Prof. Yoshihiro Iwasa; his dissertation was on ambipolar light-emitting transistors of organic single crystals. He was also affiliated with the Institute for Materials Research, Tohoku University and Department of Applied Physics, Waseda University. Since 2011, he has been a postdoctoral researcher in the photophysics and optoelectronics group of the Zernike Institute for Advanced Materials, University of Groningen, the Netherlands. His research focuses on optoelectronic devices based on emerging materials. Maria Antonietta Loi studied physics at the University of Cagliari in Italy, where she received her Ph.D. in 2001 with a thesis entitled: “Photoexcitations and intermolecular interactions in conjugated oligomers and polymers” carried out under the supervision of Prof. G. Bongiovanni and Prof. A. Mura. In the same year, she joined the group of Prof. N.S. Sariciftci at the University of Linz, Austria, as a postdoctoral researcher. Later, she worked as researcher at the Institute for Nanostructured Materials of the Italian National Research Council in Bologna, Italy. In 2006, she became assistant professor at the Zernike Institute for Advanced Materials of the University of Groningen, the Netherlands. She became associate professor at the same institution in 2010, where she is now the chair of the photophysics and optoelectronics group. Her research activities focus on the photophysics of unconventional semiconductors and in their use to fabricate optoelectronic devices.

ⱍI D ⱍ =

WC 1 :n (V G − V th,n ) − V D L 2

WC ins 1 ) ( ⱍI D ⱍ = :e V G − V th,e − V D L 2

(1)

2. Unipolar saturation region with V D ≥ (V G − V th,e ), but V D ≤ (V G − V th,h ). Here only one carrier is accumulated in the channel and the drain current is govern by



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W: n C ins (V G − V th,n)2 2L W: e C ins (V G − V th,e )2 ⱍI D ⱍ = 2L ⱍI D ⱍ =

(2)

3. Ambipolar region where both charge carrier are injected and accumulated. In addition to the condition for the other regimes, for V D ≥ (V G − V th,h ) the drain current is defined as ⱍID ⱍ =

WCox : n (VG − Vth,n)2 + : p VD − (VG − Vth,p ) 2L

ⱍID ⱍ =

WCins : e (VG − Vth,e )2 + : h VD − (VG − Vth,h ) 2L

(3)

An ideal transfer curve of an ambipolar transistor should exhibit a unique V-shape characteristic (Figure 1d). The negative voltage side of the V characteristics indicates the holes transport, while the positive gate voltage side is determined by the electron transport. The properties of the semiconductor material and the interface with the other building blocks are keys for a working ambipolar transistor. We can numbered the criteria that determine the ambipolar operation as[16]: i) the intrinsic properties of the

semiconductor (energy band structure allows comparable electron and hole conduction); ii) semiconductor/dielectric interface that is able to support the transport of both charge carriers; and iii) interface between the metal electrodes and the semiconductor active layer capable to inject holes and electrons into the conducting channel. The quality of the charge carrier injection from the metal electrode into semiconductor, which is parameterized by the contact resistance, is determined by the energy level alignment between the electrode work function and the conduction band (EC) or valence band (EV) of the semiconductor.[17] Studies in organic transistor devices revealed that the Fermi level alignments between the metal electrodes and the organic semiconductors do not occur efficiently, as in inorganic semiconductors. Several factors can modify the Mott–Schottky-type band bending at the metal/organic interface, such as the imagecharge effects and the formation of interface dipole layers. The image-charge effect is widely believed to be the source of the band-bending in the inorganic semiconductor devices. However, this effect is only thought to appear as a narrowing of the bandgap in the case of organic semiconductors that reduce the Schottky barrier height.[17–20] However, the interfacial electric dipole layer is spontaneously formed at the metal/organic interfaces with the consequence that vacuum level shifts, changing

b

a

Vacuum level

Vacuum level

Electron injector

n

Dielectric

VG

EF

V0

EV

EF insulator

VD

Semiconductor

p

electron

EF

insulator

VS

Hole injector

semiconductor

semiconductor

metal

c

d

ID

hole enhancement

electron enhancement

VG

Figure 1. a) Schematic of the working mechanism of an ambipolar field-effect transistor (FET). b) Band alignment in a metal-insulator-semiconductor (MIS) junction showing electron and holes accumulation at the semiconductor interface. The accumulation of the minority charge carriers is perceived as strong inversion. However, some materials can have nearly identical holes and electron accumulation characteristics. c) Idealized ID–VD output and d) ID–VG characteristics of ambipolar FET. Reproduced with permission.[8] Copyright 2005, American Institute of Physics.

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3. Recent Progress in Selected Materials 3.1. Polymers and Small Molecular Thin Films A challenge in fabricating ambipolar transistors using a single component active layer consists in getting efficient injection of both charge carriers from the same metal electrode. As discussed in the previous paragraph, optimal charge injection takes place when the work function of the metal electrode lines up with the HOMO level of the semiconductor for holes injection and with the LUMO level for electron injection. As most of the standard organic semiconductors have band gaps of 2–3 eV, the injection of at least one carrier will be contact limited for any given electrode material. The issue of the injection from the same metal can be solved by using active materials with a narrow band gap (i.e., lower than 1.8 eV). This will lower the injection barrier for both charge carriers allowing their efficient injection. The possibility to obtain ambipolar behavior in organic field-effect transistors by combining two different semiconductors, n-type and p-type materials, in the active layer was first demonstrated in systems in which both layers were deposited by evaporation. Dodabalapur and co-workers combined layers of two small molecules, the hole-conducting R-hexathienylene (R-6T) and the electron-conducting C60, and observed both hole and electron transport in these devices, although with lower mobility values than those of the single component.[22] Both the relative position of HOMO and LUMO levels of the two materials as well as the deposition order were found to be important for achieving good bipolar characteristics. This approach has led to the investigation of devices fabricated with different combination of hole-conducting and electron-conducting small molecules. Dinelli et al. showed that layered structures of small molecules can achieve balanced ambipolar transport. For this application, α,ω-dihexyl-quaterthiophene DH4T and N,N′ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) were used for the p- and n-type layer, respectively (Figure 2a).[23] The

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mobility values reported are as large as 3 × 10−2 cm2 V−1 s−1, which are among the highest reported for bipolar bilayer FET. Morphological analysis by confocal PL microscopy indicates that “growth” compatibility is required in order to form a continuous interface between the two organic films. The authors concluded that the optimum performance is not necessarily achieved by using materials with the highest mobility values in the single layer devices but by combining materials that are compatible and able to form a defined interface when evaporated one on top of the other. In the bilayer architecture, depending on the device configuration and on the materials used, charge accumulation and transport of holes and electrons can occur in different layers. However, at least one of the two accumulation zones will form at the interface between the two organic layers and therefore charge transport will depend on the quality of this interface. Therefore, an accurate control of the growth conditions of the evaporated films is necessary. A similar quality is difficult to achieve by solution-processed semiconductors. Several strategies have been used to solve this problem. They include using an intermediate layer (TiOx) that is inserted between the semiconductor materials, thus electronically separating the p- from the n-channel.[26] Another alternative is to use the contact-film-transfer method,[27] in which two different polymer films are deposited on two different substrates and one of them is transferred afterwards on top of the other. Another method is the sequential spin coating from orthogonal solvents;[28] in this way, it is possible to deposit one layer directly on top of the other without affecting the performance of the first layer. Recently, another approach in fabricating ambipolar FET using multi-components organic semiconductors was demonstrated to address the lack of control on the lateral size and position of the domains of the active materials in multilayer devices. Cavallini et al. developed a new architecture called multi-stripe OFET[24] (Figure 2b). In this configuration, the current flows through a planar array of nanometer-width stripes of pentacene (p-type) and perylene derivative (n-type). The structure was obtained using lithographically controlled wetting to pattern one of the material in submicron wires, followed by the sublimation of the second material that adapts to the patterned structure. This device demonstrated that each stripe can work independently and showed balanced ambipolar characteristics. However, the values of the charge carrier mobilities are still lower than those of multilayer devices. The physical understanding of this device configuration is still in its infancy, but from the fabrication viewpoints, this device configuration can bring to interesting applications. Another way to obtain ambipolar performance is to blend nand p-type organic semiconductors, combining the properties of the two components in a single mixture. By blending organic materials, we can fabricate a bulk heterojunction-like FETs. This can be realized by co-evaporation of organic small molecules, or by solution processing of small molecules/polymers if the materials are soluble in a common solvent. From the fabrication viewpoint, bulk heterojunction FETs is perceived to be simpler to prepare than the previously aforementioned FET devices. However, the challenges in controlling the morphology of the ambipolar semiconductor blends are big.



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the barrier height.[18,21] Therefore, the height of the Schottky barrier in organic material junctions is only given, but not limited, by the difference between the metal work function and the edge of the semiconductor band into which the carriers are injected. Good ohmic contact between the metal electrode and the semiconductor can be achieved when the work function of the metal is closely aligned with the LUMO or HOMO of the semiconductor for the injection of electrons and holes, respectively. Non-ohmic contacts result in irregularities in the linear regime of the output characteristics of the transistor (i.e., primary suppression and then super-linear current increase). High-contact resistance has a similar effect, because of the source-drain voltage drop at the contacts and not across the channel. The potential barrier between the electrode metal and the semiconductor can be reduced by choosing the more appropriate injecting electrode or introducing dipoles at the metal surface with self-assembled monolayers that are able to shift (increase or decrease) the metal work function.[18,19]

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a 40V

-20V A Drain

Source A

P13

-

e

+

h

SiO SiO

2

Gate

P1 D -4

-30V

-10V

DH4T

0V

0V

30V 20V

-40V

10V

c

b

Figure 2. a) A schematic of an OFET device based on a DH4T-P13 bilayer and related p- and n-channel ID–VD output characteristics. Reproduced with permission.[23] b) Ambipolar organic field-effect transistors (OFETs) based on molecular multi-wires of perylene derivative and pentacene. Reproduced with permission.[24] c) Structure of ambipolar transistor with semiconducting polymer blend of P(NDI2OD-T2)/rr-P3HT and its film morphology as well as ID–VG transfer characteristics. Reproduced with permission.[25] Copyright 2010, Royal Society of Chemistry.

Rost et al. demonstrated that by co-evaporating PTCDIC13H27 and α-quinquethiophene with equal fractions results in good ambipolar characteristics with hole and electron mobilities of 10−4 cm2 V−1 s−1 and 10−3 cm2 V−1 s−1, respectively.[9] Those mobility values were smaller than those

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for unipolar transistor of the neat film of each constituent material. This is a general trend in blends of semiconductors. However, the mobility values of hole and electron can be tuned by varying the ratio of the constituent materials.[29]


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consisting of NDI and thienylene-vinylene-thienylene (TVT) units, which have a vinyl linkage between adjacent thiophene units. This PNDI-TVT demonstrated high ambipolar mobility, with 1.4 cm2 V−1 s−1 for electrons and 0.14 cm2 V−1 s−1 for holes in transistors device with Au electrodes.[33] These high-mobility values are achieved because of the improved crystallinity with a highly interdigitated lamellar structure, mixed face-on and edge-on orientation of the core group as well as the extended π-conjugation. The 1,4-diketopyrrolo[3,4-c]-pyrrole (DPP) core unit can be sandwiched between two five-membered aromatic groups to form a coplanar molecular backbone, which allows extended delocalization of the π-electrons. The DPP core usually can be flanked by thiophene, furan, selenophene, and thieno[3,2-b] thiophene to form a larger building blocks, namely, DBT, DBF, DBS, and DBTT, respectively. Bijleveld et al.[34] reported the first polymer consisting of DBT as an acceptor and thiophene as a donor, which exhibited ambipolar transport with 0.04 cm2 V−1 s−1 and 0.01 cm2 V−1 s−1 holes and electron mobility, respectively. By changing the thiophene donor unit with selenophene, higher ambipolar mobility values were achieved (1.62 cm2 V−1 s−1 and 0.14 cm2 V−1 s−1 for holes and electrons, respectively).[35] A well-balanced hole and electron mobility (μhole/μelectron = (0.36 cm2 V−1 s−1)/(0.41 cm2 V−1 s−1)) was demonstrated from DPP derivative with polyazine as the acceptor moieties.[36] It was found that annealing the polymer thin films is crucial for achieving high mobilities. Ambipolar transistors with high on/off ratio up to 106 was achieved by polymers consisting of 2,6-di(thienyl)naphthalene (T2NAP) and DPP with 2-octyldodecyl (OD) branching alkyl chains.[37] This device also demonstrated high hole and electron mobilities 1.3 cm2 V−1 s−1 and 0.1 cm2 V−1 s−1, respectively. Very recently, Lee et al.[38] reported very high ambipolar mobility from a solution processable D–A copolymers consisting DPP acceptor with siloxane solubilizing groups and selenophene donor, named PTDPPSe-Si. Mobilities as high as 3.97 cm2 V−1 s−1 and 2.20 cm2 V−1 s−1 for holes and electrons were achieved, respectively. Modifying the alkyl side chain is known to induce denser molecular packing. PTDPPSeSi modified with pentyl chain demonstrated ambipolar transport with unprecedented high hole and electron mobilities of 8.84 cm2 V−1 s−1 and 4.34 cm2 V−1 s−1, respectively, which are the highest values for ambipolar polymer FET.[39] The development of new D–A copolymers is demonstrated as a successful strategy towards high-mobility ambipolar field effect transistors, more success in this field is expected.

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Thin films of solution processable polymer/small molecule composites are more appealing for the realization of integrated circuits on large area due to the easy processing. However, controlling the morphology of polymer blend devices is challenging. There are only few reports based on polymer-polymer composite bipolar devices. To date, the best ambipolar blends using polymers have been obtained by Shkunov et al., with a mixture of thieno[2,3-b]thiophene terthiophene polymer and phenyl C61 butyric acid methyl ester on OTS-treated substrates. These devices displayed electron mobility of 9 × 10−3 cm2 V−1 s−1 and hole mobility of 4 × 10−3 cm2 V−1 s−1.[30] The lack of the ambipolar transport in binary blends of polymers for most of the compositions shows that controlling the thin film morphology is a crucial factor for obtaining continuous path for both charge carriers. Therefore, the key issue is to achieve interpenetrating and bi-continuous networks of binary polymer blends in order to establish ambipolar charge transport. The best all-polymer bulk heterojunction bipolar FETs were demonstrated by Szendrei et al.[25] The limited number of polymer blend bipolar FETs is due to the scarcity of highperforming solution processable n-type polymers. The recent discovery of the n-type polymer poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′bithiophene)} (P(NDI2OD-T2)) exhibiting large electron mobility in ambient conditions, opened up the way towards the development of solution processed efficient polymer-based bulk heterojunctions. By using P(NDI2OD-T2) as the n-type component and regioregular poly(3-hexylthiophene) (rr-P3HT), Szendrei et al. demonstrated an ambipolar FET with high and balanced hole and electron mobilities (Figure 2c). The devices fabricated in a bottom gate/bottom contact (Au) configuration where the polymer blend (typically in the proportion of 1:1 by weight) is spin-coated from 1,2-orthodichlorobenzene (ODCB) and annealed overnight at 110 °C in a vacuum oven. The saturation field-effect electron and hole mobilities were calculated to be 4 × 10−3 cm2 V−1 s−1 at VDS = +30 V and a p-type mobility of 2 × 10−3 cm2 V−1 s−1 at VDS = –30 V. These mobilities are the highest balanced mobilities reported so far for solution processed all polymer bulk heterojunction ambipolar FETs. In the past couple of years, progress in the use of conjugated polymer based on electron-donor and acceptor co-polymers (D-A copolymers) has been reported. The interchain D-A interaction strengthens intermolecular interaction that reduces the π–π stacking distance increasing the self-assembly capability. Among the acceptor units, naphthalene diimide (NDI) derivatives and diketopyrrolopyrrole (DPP) derivatives are the most promising in achieving high-mobility ambipolar transport. The electron-accepting NDI unit has strong π–π interactions that provide better charge-transporting properties between chains, the diimide group pulls down the LUMO level to make the n-type transport air stable. Copolymers of dialkyloxydithiophene and NDI result in ambipolar charge transport in transistors, with electron mobility as high as 0.02 cm2 V−1 s−1 and hole mobility still in the range of 10−3 cm2 V−1 s−1.[31] By covalently connecting benzothiadiazole (BZ) units through thiophene linkers to the NDI unit demonstrated significant improvements of the balance of the carrier mobility values (0.05 cm2 V−1 s−1 for electron and 0.1 cm2 V−1 s−1 for hole) with good air stability.[32] Recently, Kim et al.[33] prepared an alternating structure

3.2. Organic Single Crystals Organic single crystals (OSCs) are commonly obtained by the crystallization of oligomers. Van der Waals interactions keep the molecules in their order within the crystal.[40] This attractive force comes from the interaction of dipole moments fluctuation, due to the electron motion within the molecules. The molecular orientations of one molecule respect to the other are significant in determining the van der Waals interaction energy, and affecting ultimately the π-orbital overlap. Because of the weak van der Waals intermolecular interaction, organic molecular crystals retain the electronic properties



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of its constituent molecules.[41,42] In this respect, they are different from the inorganic counterpart, in which the electronic structure of highly ordered atom arrangements will exhibit collective properties. The ground states of molecular crystals are built up of ground states of the individual molecules. When the molecule stacks in a periodic lattice forming an organic crystal, the energy level of the crystal will form a small conduction and valence band that is almost at the same energy than the frontier orbitals energy levels (HOMO and LUMO, respectively) of the constituent molecule.[41,43] Most of the OSC FETs shows unipolar characteristics, in which transport is dominated by hole transport. This is in contrast to the findings obtained with photoconductivity, time of flight (ToF), and time-resolved microwave conductivity (TRMC)[44] measurements that show both holes and electrons are mobile in OSCs. Moreover, it is widely accepted that most of organic semiconductors are intrinsic semiconductors. This apparent contradiction is due to the fact that in a device, the carrier transport is not solely determined by the characteristics of the active material but is also profoundly affected by the other building blocks of the device, i.e., electrodes for charge carrier injection and oxide semiconductor interface. Because of the weak Fermi-level pinning of organic semiconductor on metal, the energy level alignment of the metal work function and the semiconductor HOMO and LUMO level is crucial in determining the type of injected charge carrier. Au and Al electrodes are conventionally used in organic semiconductor devices, when the HOMO of the organic crystal is closer to the Au work function, holes are easier to be injected thus the devices became p-type (i.e., rubrene, pentacene, etc.). However, when the LUMO is closer to the metal work function, the electron is more easily injected, and the device behaves as an n-type semiconductor, as in TCNQ or perylene derivatives single crystal FET[45,46] (Figure 3a). Charge carrier transport in organic semiconductor single crystals is at the crossover between band-type transport and incoherent hopping. This has, as its physical origin, the poor overlap of the molecular orbitals between neighboring molecules, which determines that in organic materials the bandwidth is much narrower than that of inorganic materials. Therefore, charge carriers are not completely delocalized and interact with the local environment of the crystal. The polarizations due to the moving charges in a low dielectric constant system as well as the vibrational modes of the molecules affect the charge carrier mobility, which then have the properties of polarons.[50] From field-effect measurements, it can be determined that most of the organic semiconductor crystals exhibit charge carrier mobility, which decreases with decreasing temperature, indicating thermally activated charge transport or hopping transport.[50] This is in contrast with the results of time of flight (ToF) on very pure crystals in which reported mobility values are up to 400 cm2 V−1 s−1.[50–52] Only recently, Hall-effect measurements on rubrene and several other organic crystals confirmed dominant band-transport in ultrapure single crystals.[53–57] This confirmed initial findings of mobility anisotropy following the crystalline axes,[58] results on temperaturedependent mobility[59] and the space-charge limited current


measurements.[60] The band-like transport behavior was also confirmed by infrared optical spectroscopy,[61] photoconductivity measurement,[62] pump-probe spectroscopy,[63] photoelectron spectroscopy,[64] electron-spin resonance spectroscopy,[65] as well as Seebeck effect measurements.[66] All of these results were obtained on p-type OSCs, but recently Hall-effect measurements have been reported for n-type single crystals.[67] Nevertheless, a challenging question remains open whether those OSCs that exhibit band transport for one of the charge carrier types can also demonstrate band transport for the other charge carrier type. Ambipolar transport characteristics in OSC transistors were first demonstrated in the narrow bandgap Cu- and Fephthalocyanine,[68] followed by the standard p-type OSCs and rubrene[48] (Figure 3c). Since then, ambipolar transport has been reported in many other OSCs, including those with wider bandgaps.[47,69–72] Besides the intrinsic properties of the single crystals that support both hole and electron transport, the keys to the demonstration of ambipolar transport in these single crystals are the minimization of the electron traps at the interface between the single crystal and the oxides gate dielectric through passivation using thin non-hydroxyl polymers as a buffer layer,[73] such as PMMA and crosslinked PVP. The surface morphology as well as solvent residual impurity of the polymer buffer layer strongly influence the number of remaining electron-deep-traps at the conducting interface of the transistor, as observed from the electron threshold voltage values.[74] The selection of metals for improved electron injection and the configuration of the electrodes are very crucial, in particular for semiconductor materials with wide bandgap so the energy level mismatch between the standard electrodes (Au, Ag, or Al) and the LUMO level was more than 1 eV.[75] So far, only top contact electrode configuration has been successfully demonstrated in injecting electron into OSCs. A possible explanation being that the defects created by metal evaporation can assist the electron injection. Metals with low work-function, despite their instability towards ambient atmosphere, have been used as electrodes to obtain effective electron injection. Evaporation of thick Ca top contact electrodes resulted in nearly ohmic-like electron injection into rubrene and several other OSCs[49,70,76] (Figure 3d). Therefore most of the ambipolar FETs reported with OSCs used a configuration where both Au and Ca are used for source and drain electrodes, respectively (Figure 3e).[47,69–71,77–79] The highest electron mobility in ambipolar OSC transistors was achieved with ultrapure rubrene single crystal (μ = 0.81 cm2 V−1 s−1).[49] This value is actually very low compared with the highest achieved hole mobility (μ = 43 cm2 V−1 s−1),[80] and in contrast with theoretical prediction that the ratio between electron mobility and hole mobility values should be around 0.47 on the basis of the HOMO/LUMO bandwidths.[81] The big discrepancy is due to the different device configuration. The high hole mobility is obtained with fourprobe measurements[53,80,82–85] and in most cases using air gap transistor configuration;[45] meanwhile, the highest two probe hole mobilities (on PMMA passivated substrate) was 2.0 cm2 V−1 s−1.[75,86,87] This suggests that challenges still remain in obtaining ambipolar FET of OSC with co-existing high


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S

S

S

500 500 µm µm

b

TCNQ n-type

Rubrene p-type LUMO

LUMO HOMO

5.24 eV

= 5.1 eV Au

HOMO

3.15 eV 5.36 eV

8.34 eV

c

d

(c ) 12

Drain current (nA)

10 8

e V G = 145 V 130 V

Ca

6 4 2 0

300 m Ca Au Au

115 V 100 V 0 0. 5 1 Drain voltage (V )

f

Figure 3. a) A BP3T single crystal, one of organic single crystals that demonstrated ambipolar transistor characteristics. Reproduced with permission.[47] b) Energy-level diagram for the acceptor molecule TCNQ and donor molecule rubrene. The difference of the energy levels with the Au metal work function determines whether one molecule is considered as a p-type or n-type organic semiconductor from the viewpoint of which charge carrier will be easier to inject. c) The first ambipolar transistor

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electron and hole mobility values. Air-gap transistor will be the best configuration to avoid any possible traps for electron, if the challenge to fabricate suitable electrode for electron injection can be surpassed (Figure 3f). Because of the difficulties in obtaining balanced ambipolar transport in OSCs,[47,48,69–71,75,77,78,88–90] recently some alternative ways to obtain ambipolar transport have been sought. One of the attempted strategies is using OSCs that have a slightly deeper HOMO–LUMO level. An oligoPPV derivative named P5V4 has a 3.1 eV LUMO that permit effective electron injection from Ca, and a 5.7 eV HOMO level that suppress hole injection from Au. As a result, the ambipolar FET device was dominated by electrons. To make ambipolarity more balanced, a very thin layer of MoOx was inserted as a hole-injecting layer, resulting in reduction of the hole threshold voltage[91] (Figure 4a). By analogy, an insertion of electron injection layer, such as CsF, into ambipolar FET of OSC has also been attempted,[92] which results in slight improvement of electron injection (Figure 4b) in a similar fashion than can be done with polymer ambipolar FET.[93,94] In addition to OSC based on single type of molecule, new efforts to fabricate ambipolar transistors have been made to use co-crystals based on donor–acceptor (D–A) and donor– bridge–acceptor molecules. The first ambipolar FET based on this kind of co-crystal was demonstrated almost a decade ago in (BEDT-TTF)(F2TCNQ) at low temperature.[97] By using the analog charge transfer compounds with different D and A moieties as electrodes, the properties of (DBTTF)(TCNQ) co-crystal FET can be fine-tuned from p-type, ambipolar, to n-type.[98] Recently, Zhang, et al.[99,100] have reported about solution processable heterojunctions based on sulfur-bridged annulene co-crystal, either with TCNQ or with fullerene, that resulted in stable and balance hole and electron mobility in ambient atmosphere. Besides the TTF-TCNQ-based system, only a small number of co-crystal system were demonstrated to give ambipolar transport. Park et al.[96] developed new cocrystal based on isometric distryrylbenzene and dicyanobenzene based molecules (4M-DSB)(CN-TFPA) that gave good ambipolar transport (Figure 4c). To date, the highest carrier mobility values that can be achieved in co-crystals are still in the order of 10−1 cm2 V−1 s−1. Despite the recent progress and the prospect of combining the properties of the constituent compounds, co-crystallization of organic molecules in form of single crystals remains a big challenge as an alternative to the device structural improvements.

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a

of organic single crystal made of rubrene single crystal with Ag paste electrode and PMMA buffer layer to terminate the interfacial electron traps. The ambipolar transport appeared only when the device measured in vacuum or in inert atmosphere. Reproduced with permission.[48] Copyright 2006, American Institute of Physics. d) Low bias ID–VD output characteristics of a rubrene ambipolar FET with fast-evaporated thick Ca electrode, where a nearly ohmic electron injection was demonstrated. Reproduced with permission.[49] Copyright 2010, American Institute of Physics. e) Optical micrograph of a BP3T single-crystal FET with asymmetric electrode configuration of Ca and Au. f) Scanning electron micrograph of rubrene single-crystal transistor with free-space gate dielectrics (air-gap transistor) with Au electrodes. Reproduced with permission.[45]



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3.3. Carbon Nanotubes

Figure 4. a) ID–VG transfer characteristics of P5V4 single crystal FET before and after the introduction of a MoOx film. The threshold voltage for hole accumulation was significantly reduced by the insertion of MoOx injection layer. Reproduced with permission.[95] b) Ambipolar FET using BP3T single crystal with CsF buffer layer at the interface between Ca electrode and the single crystal to assist electron injection. The insertion of the buffer layer improved the ohmic characteristics of charge carrier injection and also reduced the electron accumulation threshold voltage. Reproduced with permission.[92] c) Co-crystals consisting of donor and acceptor molecules of (4M-DSB) and (CN-TFPA) from which ambipolar FET was demonstrated. Reproduced with permission.[96] Copyright 2013, American Chemical Society.

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Carbon nanotubes, particularly semiconducting single-walled carbon nanotubes (sSWNTs) have attracted great scientific interests owing to their extraordinary charge carrier mobility. Among the prospective carbon-based nanomaterials for future electronic applications, sSWNTs are the only true semiconductor, as they have bandgaps of ≈0.7–1 eV, depending on their size. Furthermore, sSWNTs are intrinsic and direct bandgap semiconductors, which have in principle the possibility for both hole and electron conduction. The competing carbon-based system graphene is metallic, and significant effort is needed to open even a very small bandgap in it.[101] As the first report of carbon nanotube transistors in 1991,[102,103] extensive progress has been made in this field, and the prospect of the nanotube electronics seems unchallenged by any other materials because of their properties. However, a more careful look shows that two of the biggest challenges for nanotube electronics are selection and purification of semiconducting nanotubes and the device fabrication. The large variability of SWNT properties with their structure and the difficulty in growing nanotubes with a specific structure makes the fabrication of carbon nanotubes based FET not trivial. For this reason, the high performing ambipolar transistor based on sSWNTs with very high carrier mobility (μ ≈ 105 cm2 V−1 s−1) and high on/off ratio (107) is only demonstrated in single-strand carbon nanotubes.[4,104,105] To fabricate this device, it is necessary to spatially select the individual nanotube from, e.g., the CVD grown nanotube forest and lithographically pattern the electrodes. These processes are difficult, time- and resource-consuming as well as far from the compatibility and reproducibility required for future electronic devices mass production. Another approach to make SWNT FETs is to use carbon nanotube networks as an active medium instead of a singlestrand nanotube. The carbon nanotube networks can be fabricated by using different methods. When CVD-grown nanotubes are used devices display relatively high carrier mobility (50 cm2 V−1 s−1 < μ < 200 cm2 V−1 s−1).[106,107] However, with this technique a random mixture of metallic and semiconducting nanotubes are grown, making the on/off ratio of the transistors relatively low (≤105).[106–108] In addition, the high temperature process of CVD gave difficulties when special device architectures are required. A much simpler process to fabricate the nanotube network films is through solution deposition (i.e. drop casting, spincoating, inkjet printing, etc.).[109–113] For these processes, pure and well-dispersed sSWNT inks are required. Various growth methods result in many different kinds of possible SWNTs contained in the ink. Therefore, the greatest challenges to develop ideal nanotube inks are in the separation of sSWNTs from the metallic one; the necessity to achieve single chirality and length control of the nanotubes in the ink; and the requirement for high concentration of the sSWNTs in specified solvents. Effective SWNT sorting methods must meet the semiconducting electronics requirements: high purity (>99% of semiconducting species in resulting dispersion), scalability (in order to utilize it in mass production), and compatibility with a wide range of SWNT lengths and diameters. To date, there are several


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105. The FET fabricated using sSWNT dispersions purified by DGU method and selective solubilization has achieved tremendous results reaching >106 on/off ratio,[121,132] indicating the high purity of the semiconducting species with minimum metallic content. However, not all these devices are ambipolar, e.g., the DGU methods results in aqueous solution incompatible with air-free processes, or some wrapping polymers used for the selective solubilization method trap one type of charge carrier. Through proper device fabrication processes (Figure 5b), wrapping polymer selection as well as effective enrichment of the semiconducting species, ambipolar FET with high (geometrical) carrier mobilities for both carriers and high on/off ratio values (up to 107) can be achieved[121] (Figure 5c), defying the belief that ambipolar transistors cannot be completely turned off. Moreover, efforts to passivate sSWNT films, nanotube alignment, as well as the use of ionic liquid or ion gel gating have been shown effective to demonstrate ambipolar transistors with high carrier mobility, satisfying on/off ratio as well as low driving voltage operation.[120,133–135] As mentioned previously, not only the selection of semiconducting SWNTs but also the possibility of controlling the nanotube assembly, either with top-down or bottom up methods, have been the most important blocking points towards application of sSWNTs. A very recent work by Kwak et al.[122] demonstrated the possibility to sort and self-assemble semiconducting carbon nanotubes by using DNA-based block copolymers. This block copolymer, (poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO)-b-DNA), takes the advantage of both components (Figure 5d). Although the PFO shows selectivity towards semiconducting SWNTs with high chiral angle and small diameters (0.8–1.1 nm)—which strongly depends on the solvent, the nature of the side chain, and the concentration of the polymers—the single-stranded DNA also shows the ability to disperse carbon nanotube in aqueous solution. By comparing the optical spectra of the dispersed carbon nanotubes and the molecular dynamics simulation results, it was concluded that the hydrophobic PFO block envelops the SWNT surface while the DNA block extends from the nanotube and provides water solubility as well as the possibility for functionalization. Solution-processed field-effect transistors by self-assembling (locking the single-strand DNA with its complementary sequence) dispersed SWNTs with PFOb-DNA show yields as high as 98% and 80% of them show ambipolar electrical characteristics. With the development of the separation and the selfassembly techniques, the realization of commercial products based on carbon nanotubes is expectable in the near future.

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post-synthetic separation techniques claimed to be successful in purifying the sSWNTs, such as electrophoresis,[114,115] density gradient ultracentrifugation (DGU),[111,116,117] gel chromatography,[118] and selective solubilization.[119] DGU exploits the difference in the buoyancy of each carbon nanotubes in a different density gradient medium. Through the application of a centrifugal force, the SWNTs move to the position where their density is equal to the surrounding medium. Therefore, different nanotubes can be fractionized by their electronic types or length. However, the long duration of the DGU process still gave limited yield with a wide distribution of sSWNT diameter. However, gel chromatography has been widely used in biochemistry for material separation, e.g, proteins, DNA, etc. The nanotube solution mixed with surfactants that have different affinity towards metallic and semiconducting nanotubes is placed on top of a gel-filled chromatographic column. On the basis of their size in comparison with the size of the gel pores, the surfactant-covered nanotubes will be sieved by the gels. This chromatography technique has been proved effective for SWNT sorting based on their electronic properties, but limited only for a narrow diameter distribution of the initial nanotube source. Dielectrophoresis utilize the translational motion of different nanotubes along an electric field gradient. This motion is strongly influenced by the difference of the dielectric constant of the metallic tubes (ε ≈ 1000) and semiconducting tubes (ε ≈ 5), as well as the solvent, so the metallic tubes move to the higher field region. Despite the effectiveness of this method, it still suffers from the disadvantage of limited throughput. Another recently invented method to separate and purify the semiconducting carbon nanotubes is by using selective solubilization. This method exploits the non-covalent interactions between the surface of carbon nanotubes and π-conjugated organic polymers. Specific conjugated polymers have shown ability to wrap only some of the carbon nanotubes through this interaction, differentiating the carbon nanotube bundles, metallic tubes, and the desired semiconducting tubes. The semiconducting polymer poly-9,9-di-n-octyl-fluorenyl-2,7-diyl (PFO) was the first polymer used for selective solubilization of various types of carbon nanotube sources, e.g., CoMoCAT and HiPCO.[119,123–125] The mechanism of carbon nanotube selection using this polymer is largely investigated by mean of molecular dynamic simulations as well as spectroscopic studies.[126–129] It has been found that the important factor is not only the interaction between the wall of the SWNTs and the backbone of the conjugated polymers,[126,127,129,130] but also the side chains of the polymer have a very important role in the selection mechanism.[120] This method of selective solubilization has provided the possibility to separate carbon nanotubes in large scale. Recently, this method has been demonstrated to be effective not only with small diameter tubes but also with large diameters sSWNTs by using polymers with long alkyl side chains[120] (Figure 5a). Because of the methods developed to obtain pure sSWNT inks, field effect transistors fabricated with solution processable carbon nanotube network with high performance started to emerge. Sangwan et al. demonstrated random networks of CNT FET with carrier mobilities comparable to the best obtained from CVD grown tubes[131] with on/off ratios of about

3.4. Colloidal Semiconducting Nanocrystals Colloidal semiconducting nanocrystals (CNCs) are crystals of semiconducting materials of 2–20 nm size. They combine the robustness of their parent bulk materials, the size-tunable electronic and optical properties, and the bottom-up fabrication simplicity of solution processable materials. The individual nanocrystal exhibit quantized hole and electron states that can be precisely tuned by particle size variations. They are among



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the best candidates for various new electronic and optoelectronic device applications, such as compact and power-efficient photodetectors, photovoltaic cells, and light-emitting devices. Moreover, the CNCs are stabilized with ligands that provide protection to their quantum confinement properties as well as provide their solubility in either aqueous or organic solvents. Therefore, they are compatible with simple solution-based fabrication processes. One important aspect of CNC-based device fabrication is how they assemble to form thin films (Figure 6a). Therefore, the physics of the CNC films is not only determined by their intrinsic individual particle characteristics but also by their collective properties. The assembly of the CNCs will influence the interaction between particles that determine the collective electronic transport. Conduction in CNC solids should involve charge transfer between individual CNCs. Several factors determine the delocalization of the charge carriers in the CNC assembly[140]: i) the homogeneity of the CNC size and shape; ii) periodicity of the CNCs array; and iii) low surface defect concentration; and iv) the strength of the coupling between CNCs. In general, the molecular ligands passivates the dangling bonds and provide solubility, but they also limit the CNCs usage in electronic devices. CNCs do not display efficient charge carrier transport when stabilized with commercially available ligands, such as carboxylic acids, amines, phosphines, phosphine oxides, and alkyl thiols, which are insulators and can also act as trapping sites for at least one type of charge carriers. Only recently, the use of metal chalcogenide ligands in CdSe/CdS CNC assembly allowed to demonstrate carrier mobility up to 16 cm2 V−1 s−1 and band-like transport characteristics, although limited to electron transport[137] (Figure 6b). Therefore, proper combination of CNCs and the surface ligand

Figure 5. a) Selective solubilization to separate carbon nanotubes is obtained by conjugated polymers. Through the usage of different conjugated polymers, either with different molecular backbone or different alkyl chain length, specific semiconducting nanotubes can be selected (shown with yellow hexagons), including those with large diameters. Reproduced with permission.[120] b) Optical absorption spectra of the as-dispersed sSWNTs and the enriched sSWNTs that underwent second centrifugation to remove the excessive wrapping polymer. The enrichment method is very crucial to obtain clean dispersion of semiconducting carbon nanotubes with minimum amount of residual polymer and metallic nanotube bundles. Reproduced with permission.[121] c) p-channel and n-channel ID–VG characteristics of ambipolar FET made from drop-casted random network of semiconducting nanotubes separated by using

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the polymer-wrapping method. Through a proper nanotube purification and optimized device fabrication, high on/off ratio values up to 107 for both holes and electrons were demonstrated, even in a very short channel device (5 μm). Reproduced with permission.[121] d) Programmed assembly of SWNTs on surfaces of nano-electronic devices where PFOb-DNA block copolymer was used to wrap and to assist the assembly of the nanotube. Through this method ambipolar transistors were demonstrated from nearly single-strand carbon nanotubes. Reproduced with permission.[122]


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PROGRESS REPORT Figure 6. a) Colloidal nanocrystals are usually stabilized using long molecular ligands to ensure its solubility and its quantum confinement. To form a nanocrystal solids the long molecular ligands should be replaced with smaller ligands that are able crosslink the nanocrystals. As shown by the TEM images, the distance between the nanocrystals were reduced after the ligand exchange. Reproduced with permission.[136] b) Plots of drain current ID versus drain-source voltage VDS as a function of gate voltage VG for a device using In2Se42−-capped 3.9 nm CdSe nanocrystals; and the temperature dependence of field-effect mobility for the n-channel FET operation, for which band-like transport for electrons in this nanocrystal film is suspected. Reproduced with permission.[137] Copyright 2011, Macmillan Publishers Ltd. c) Ambipolar FETs made with PbS nanocubes on a Kapton flexible substrate and Parylene gate. The PbS nanocubes were crosslinked by using thiocyanate ligands and demonstrated high mobility for both holes and electrons. Reproduced with permission.[138] Copyright 2011, American Chemical Society. d) ID–VD output characteristics of ambipolar FET using PbS nanocrystal gated by using ion gel of [EMIM][TFSI]. The device operated with very small driving voltage demonstrated ambipolar transport with clear saturation behavior and ohmic characteristics for both holes and electrons. Reproduced with permission.[139] e) The utilization of ion gel gating for this nanocrystal device also improved significantly the on/off ratio value, which is much higher than the value can be achieved in the similar transistor with conventional SiO2 gating. Reproduced with permission.[139]

selection is the first basic issues, which should be addressed in CNCs to support both hole and electron transport for obtaining ambipolar transistors.

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The low electronic conductivity in semiconductor CNC solids is due to the small concentration of mobile carriers, in addition to the traps associated with the nanocrystal surface



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dangling bonds. Many efforts have been attempted to increase the concentration of mobile carriers in CNC solids, i.e., by substitutional doping,[141] electrochemical intercalation of alkaline metals,[142] charge transfer dopants,[143] and CNC stoichiometry control.[144] Nevertheless, these approaches do not fully succeed in improving the electronic transport properties of CNCs. At best, they succeed in improving one type of charge carrier, leaving the other much inferior. Despite the electronic band structures of most CNCs actually enable both holes and electrons to be mutually mobile with nearly similar quality[145] ambipolar transport appears challenging to be achieved. From many different kinds of colloidal nanocrystals, ambipolar transistors have only been demonstrated using lead chalcogenide (PbSe, PbTe, and PbS) nanocrystals.[146–148] Their narrow bandgaps allow the easy injection of both holes and electrons. However, most of the early reports on ambipolar transistors with CNCs demonstrated only very low carrier mobility values (μ ≤ 10−3 cm2 V−1 s−1). The exchange of the molecular ligands aiming to improve the coupling between CNCs and the carrier mobilities may also lead to the different dominant charge carriers. Some molecular ligands of choice can shift the properties of the nanocrystal assembly to become p-type or n-type.[149] Moreover, the environmental conditions (the presence of oxygen and moisture) also affect the behavior of the ligands in shifting the electronic properties of the nanocrystal films.[150] The result is that only a few of the known surface treatments can support ambipolar transport. By utilizing large-size PbS nanocubes decorated with thiocyanate ligands, ambipolar FET with high carrier mobility were reported,[138] together with inverters devices on flexible substrate (Figure 6c). Improved ambipolar transport was obtained by applying different type of oxide gate dielectrics,[151] as well as the use of ion gel gating.[139,152] Through these efforts, ambipolar transistors of PbX nanocrystals with μ > 1 cm2 V−1 s−1 have been reported (Figure 6d–e).[138,139] Challenges still remain in realizing ambipolar transistors using nanocrystals other than PbX.

4. Emerging Dielectric Materials for Ambipolar Transistors As mentioned in the previous sections, most of the field-effect transistors use oxides or other solid-state insulators as gate dielectric. The capacitance of the dielectric per unit area can be expressed as C = g rg 0 /d, where εr is the relative permittivity of the insulator, ε0 is the vacuum permittivity and d is the thickness of the dielectric layer.[7] In conventional solid-dielectrics, the capacitance is constrained by the structural thickness of the dielectric film (generally in the order of 10–100 nm). Therefore, it is necessary to use high dielectric constant (high-k) insulators or ultra-thin films of the dielectric (e.g., self-assembled monolayers) in order to enhance the gate capacitance and increase the device conductivity at low voltage. However, these two approaches have several limitations. The use of high-k dielectric negatively affects the charge carrier mobility because of the induced trapping by Fröhlich polaron as reported in organic semiconductor devices.[87] Only select semiconducting polymer materials that are not affected by the dielectric coefficient of

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the insulator.[153] However, the decrease of the dielectric thickness is a challenge because of the chance of increased gate leak current. Most high-k dielectrics are oxide compounds that from the ambipolar transistor viewpoint are not very suitable because their surface is likely to trap electrons. Therefore, much attention is focused on methods to passivate the surface of these high-k dielectrics to remove the possible electron traps, by using polymers or SAMs as had been done for SiO2 surface passivation. SAMs and polymers are also used themselves as new high-k dielectrics. By coating Ta2O5 (ε = 25–27) with poly(vinyl alcohol), ambipolar transistors with pentacene thin films have been demonstrated.[154] Similarly, coating Al2O3 (ε = 9.34) with either Cytop or octadecyplphosphonic acid (ODPA) ambipolar transistors with satisfying ambipolar mobility values and relatively low threshold voltage based on naphthalene diimide derivative[155] and inorganic semiconducting nanowire[156] were obtained. Recently, high-k relaxor polymer–polymer blend dielectrics have received great attention for the use in ambipolar transistor. Naber et al. used high-k (14) poly(vinylidenefluoride-trifluoroethylene) P(VDF-TrFE) on top of a low-k (2.3) polycyclohexylehthylene (PCHE) layer to allow high ambipolar mobility on F8BT LET.[157] Besides the high-k that can allow inducing higher accumulated carrier density, this P(VDF-TrFE) dielectric demonstrates ferroelectric behavior that results in a large bias hysteresis in the transfer plots owing to the dipolar polarization of the β-phase of the film. To reduce the ferroelectric phase formation, mixing the P(VDF-TrFE) with other dielectric polymers, such as PMMA, has been found as crucial to suppress hysteresis in the transfer characteristics. Baeg et al. mixed the P(VDF-TrFE) with PMMA to reduce the ferroelectric domain and demonstrated that the ambipolarity of P(NDI2OD-T2) FETs can be tuned.[158] Besides P(VDF-TrFE), there are several other high-k polymer-polymer blend and inorganic polymer blend dielectrics that are prospective for inducing high carrier density in ambipolar transistors between them: P(VDF) derivatives (i.e., P(VDF-TrFE-CFE));[159] sodium beta alumina[160]; Zr-SAND dielectric[161]; and cross-linked inorganic/organic hybrid blend (CHB).[162] These dielectrics have demonstrated to be effectively applied for both p-type and n-type unipolar transistor. However, to date, their usage on semiconducting materials capable for ambipolar transport has never been shown. Electrolyte dielectrics are alternatives to the conventional dielectrics, including the high-k solid dielectrics, which can accumulate very high carrier density at the interface. Electrolytes are ionic conductors but electron and hole insulators. Therefore, the prompt motion of the positive and the negative ions under electric field can create strong accumulation of space charges at the electrolyte/semiconductor interface, in the form of an electric double layer, known as Helmholtz layer (Figure 7a). This electric double layer acts as nanogap capacitor with huge capacitance and electric field; the capacitance is determined by the shortest distance of the nearest ionic molecules to the ionic-liquid/semiconductor interface (≈1 nm). As a result, a very high concentration (≈1014 cm−2) of free sheet carriers (electron or holes) inside the semiconductor can be induced electrostatically. Therefore, electric double-layer gating is able to accumulate more charge carriers by applying only very low gate


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PROGRESS REPORT Figure 7. a) Schematic of charge accumulation by an EDL formed at the interface between a liquid (ionic conductor: ionic liquid) and the solid (electron conductor: semiconductor). Cations (red circles) and anions (green circles) can be electrostatically accumulated onto the channel surface by applying a positive and negative gate voltage, respectively. The inset shows magnified schematics of the interface between the semiconductor and the ionic liquid. Reproduced with permission.[163] Copyright 2010, Macmillan Publishers Ltd. Different types of electrolyte gating materials: b) Dissolved ionic species in a neutral polymer, e.g., LiClO4 in polyethylene oxide; c) polymer electrolyte where the polymer itself is charged, e.g., P(VPA-AA); d) ionic liquids which is a molten salt such as [EMIM][TFSI] that can be used as it is included into a block co-polymer matrix (e.g. PS-PMMA-PS) to form a ion gel. e) Comparison of the dynamical response to step wise pulses of VG = 2 V in EDLTs gated by ionic liquid, liquid KClO4/PEO (Mw = 200) and solid KClO4/PEO (Mw = 2000). The dynamic response of ionic liquid gating is the fastest among the other EDLTs. Reproduced with permission from.[164]

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voltage in comparison with conventional solid-dielectric gate (i.e., SiO2). To date, there have been several types of electrolytes used for gating, each with their own merits. The first type consists of electrolytes made with small ions such as K+, Li+, ClO4−, BF4−, and PF6− dissolved in organic or aqueous solutions.[168] Initially, this type was not intended for gating but instead as ionsensitive field-effect transistors (ISFET) for chemical sensors, in which these ions act as adsorbates on the semiconductors. However, this was the first time scientists realized that ionic moieties can influence the transistor gating so much.[169] The second type is the polymer electrolyte dielectrics that consist of ionic species,[170] as in the first type, dissolved in polymers such as poly(ethylene oxide) (PEO) (Figure 7b). The polymer electrolyte is more easily integrated into the transistor structure compared with the diluted inorganic ions because of its mechanical properties. The polymer electrolyte can be easily cast from solution to become a thin film where the gate electrode can be deposited on top of the dried electrolyte. Polymer electrolytes based on LiClO4 in PEO have been widely used in many different kinds of transistors such as OSCs,[171,172] smallmolecule thin films, polymers,[173] graphene, and carbon nanotubes[174,175] for low-voltage operation. The third type of electrolyte gate is a polymer electrolyte, which is characterized by the fact that the polymer chains themselves contain immobile anions, and mobile protons (H+) are added to work as polyelectrolytes. One of the notable examples is polyvinylphenol (PVP) and its derivative P(VPA-AA), in which a low-molecular-weight protic solvent, such as water, can be dissolved[176,177] (Figure 7c). However, the drawback of these polymer electrolytes is their slow ionic diffusion before the EDL formation, which is detrimental to the switching speed of the transistors. Moreover, many devices with polymer electrolytes suffer from serious deterioration in carrier mobility due to possible damages at the semiconductor/electrolyte interface. In recent years, ionic liquids have also started to be used as a fourth type of gate. Ionic liquids are highly polar molten binary salts at room temperature (Figure 7d). They typically consist of nitrogen-containing organic cations and inorganic anions, and they are stable and non-reactive in contrast with the other electrolytes. Ionic liquid have negligible vapor pressure and do not require solvents to make them compatible with most material systems. Moreover, they can be exposed to moderate potential differences without undergoing redox reactions. For transistor perspectives, the ionic liquid can form the EDLs at relatively high speed owing to the rapid diffusion of the ions (≈1 μs) (Figure 7e). The use of ionic liquid for EDL gating was initiated by Hebard et al.[178] on InOx, and later this technique was

further developed independently by Frisbie et al.[179] and Iwasa et al.[173] There are two variants of the EDL gating based on ionic liquids. The first one is the ionic liquid gate: here, the ionic liquid droplet is placed on the semiconductor or any materials of interest, and the gate probe is then inserted into the droplet. The second one is the ion gel gate, which is made by dispersing the ionic liquid into a block copolymer matrix and has the stiffness of a solid-state dielectric with ionic-gating capability.[179,180] An ion gel gate shares identical characteristics with the ionic liquid gating in terms of frequency response and capacitance.[181] Ionic liquid gating can accumulate charge carrier up to 8.0 × 1014 cm−2 using [DEME][TFSI] at super-cooled condition and at nearly steady state measurement[164] (Figure 7f–g). However, ionic-liquid-based ion gels can have comparable capacitance, reaching 43 μF.cm−2, and some of them can still have 1 μF.cm−2 at 100 kHz operation.[182] The ability to accumulate very high carrier density has made EDL–FET a method of choice to discover new material physics.[183] In the past five years, there have been high-impact reports on the usage of EDL–FET to realize insulator-to-metal transitions,[164,184,185] field-induced superconductivity,[163,186,187] and field-induced ferromagnetism[188] in oxides,[164,184,186] layered materials,[163,187] and topological insulators.[188] Recently, insulator-to-metal transitions were also reported in organic materials by using this technique.[189] With respect to ambipolar transistors, the EDL-gating technique contributes towards the trap filling that enables the minority carrier to become mobile in the transistor channel. In addition, the high electric field created by the EDL can assist the charge carrier injection of the minority carrier. Ambipolar FET with EDL gating was initially reported by Kang, et al.[152] in PbSe nanocrystal film using EMIM-TFSI ion gel. Despite this, PbSe nanocrystals have shown to be ambipolar even using conventional oxide dielectric,[146] the usage of an ion gel significantly reduces the threshold voltage for both hole and electron accumulations. Since then, many efforts have been made to realize ambipolar characteristics from materials that are by far known to be unipolar. Two of the most notable example are: MoS2 (Figure 7h), which is one of the most well-known 2D material alongside graphene; and Bi2Te3 (Figure 7i), which is a topological insulator. Both of them are supposed to show ambipolarity, but conventional gating methods cannot access it. Using ionic liquid gating of DEME-TFSI, ambipolar FET of both Bi2Te3[167] and MoS2[166] was reported and clarified by Hall effect measurement. Until now, only ionic liquid gating could access the ambipolar characteristics in these kinds of materials.[190,191] Another material systems

f) Comparison of static charging states in [DEME] [TFSI] ionic-liquid/ZnO EDL transistor estimated from Hall measurement and the C–VG integration. There is a boundary in frequency regime between when the charging mechanism occurs electrostatically or electrochemically. Reproduced with permission.[165] Copyright 2010 American Chemical Society. g) Comparison of VG-dependent sheet carrier density on ZnO induced by gating using [DEME] [TFSI] at 300 K and 220 K (super-cooled). Ionic liquid gating can induce sheet carrier density up to 8.0 × 1014 cm−2. Reproduced with permission.[165] Copyright 2010, American Chemical Society. h) Ambipolar transistor made from MoS2 thin flake gated with ionic liquid. By inducing a very high carrier density, a p-type conducting state was created. Reproduced with permission.[166] Copyright 2012, American Chemical Society. i) Transfer characteristics (IDS–VG) of an EDLT based transistor made with ultrathin films of the topological insulator Bi2Te3 at 220 K demonstrating ambipolar transport. Reproduced with permission.[167] Copyright 2011, American Chemical Society. j) Characteristics of two different ambipolar FETs based on random networks of small diameter HiPCO nanotubes and large diameter SO nanotubes gated by [EMIM][TFSI] ion gel. The devices show a clear saturation and high on/off ratio values. Reproduced with permission.[120]

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5.1. Logic Devices

Figure 8. a) Schematic representation and optical micrograph of an ambipolar EDL transistor made with organic single crystals using ion gel gating. Reactive metals such as Ca should be isolated from the ion gel to prevent electrochemical reactions. By using this technique ambipolar EDLTs were demonstrated using pentacene and rubrene single crystals. The threshold voltage for both holes and electron accumulations reflect the relative HOMO and LUMO values of the active semiconductor materials. Reproduced with permission.[192] b) Ambipolar FET of WS2 monolayer gated by ionic liquid. The bandgap of the material can be quantitatively probed by the transistor characteristics. Reproduced with permission.[190] Copyright 2012, American Chemical Society.

that demonstrated ambipolar characteristics by means of ionic liquid or ion gel gating were carbon nanotubes[120,134,135] (Figure 7j), colloidal nanocrystals[139,152] (Figure 6d) and organic semiconductors[192,193] (Figure 8a). The key motivation for electrolyte gating to access ambipolar transport is its ability to accumulate very high carrier density (≈1014 cm−2),[164] which means that it is able to fill all of the available traps for charge carriers (≈1012 cm−2).[194] Therefore, the values of the threshold gate voltage for the charge carrier accumulation is affected only by the edge of valence band.[195] or the edge of the conduction band.[196] Some notable ambipolar transistors that use ionic liquid or ion gel gating demonstrated a very small sub-threshold swing (SS),[120,139,190,192] close to the ultimate limit of SS = 66 meV decade–1 for room temperature operation of conventional semiconductors. These findings indicate the small charge trapping effects in these kinds of transistors allowing the use of ambipolar transistor with ionic liquid gating to quantitatively determine the bandgap of the active semiconductor materials[190,192] (Figure 8b).

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Complementary inverters, which invert incoming signals into an outgoing signal, are fundamental element in integrated circuits. High-performance complementary inverters consist of p-type and n-type FETs, which have comparable mobility, threshold voltage values, and response speed. In operation, the p- and n-type transistors are connected at the drain and the gate electrodes serving as output (Vout) and input (Vin) nodes, respectively. The source of the load FET is connected to the power supply (Vsupply), whereas the source of the driver transistor is grounded. The state of the n- and p-type FETs can be varied by changing the input voltage. An important characteristic of complementary inverters is that when the output is in a steady logic state (Vout = 0 or Vout = Vsupply), only one of the transistors is “on”, and the other is in “off” state. This reduces the current flowing into the inverter device to a minimum equivalent to the leakage current that is passing through the transistors, which occur only for very short time during the switching of the transistors. This significantly reduces the power consumption of these circuits with respect to other circuit architecture that uses single-transistor-based inverters. Complementary inverters are usually built on with p-MOSFET and n-MOSFET in single crystal silicon platform, where the p-type and n-type doping coexist but are spatially defined. To build complementary inverters based on FETs of other materials, which mostly are unipolar, the fabrication process is more complicated as two separate devices should be integrated, where the n- and p-type transistor active areas must be spatially separated. Ambipolar transistors provide the opportunity to have p-type and n-type transistor integrated in a single device, thus are a viable option for the development of complementary inverters. The first demonstration of a complementary-like inverter based on an ambipolar FET of polymer semiconductor used a blend of OC1C10-PPV and PCBM.[11] This device demonstrated gain of 10 at VDD = 30 V. This value was adequate considering the low carrier mobility for both charge carriers (in the order of 10−5 cm2 V−1 s−1). Another polymer blend of thieno[2,3-b] thiophene terthiophene polymer and phenyl C61 butyric acid methyl ester on OTS-treated substrate demonstrated maximum gain of 45 owing to the higher mobility.[30] Efforts have been made also to use ambipolar FET made with single component polymers. Sirringhaus et al.[197] successfully demonstrated complementary-like inverter using two identical ambipolar transistors of poly(3-octyl)selenophene. The two ambipolar transistors were fabricated using the same film with a common gate, eliminating the necessity to pattern the semiconducting layer. This inverter achieved gain as high as 86 despite the on/ off ratio of the ambipolar FET was only 103. By using a blend of P(VDF-TrFE) for high-k dielectrics, Noh et al. modified the properties on P(NDI2OD-T2) polymer, which is known to give n-type transistors, to become ambipolar[158]. A flexible and airstable complementary-like inverter and ring oscillator based on the P(NDI2OD-T2) ambipolar FETs were fabricated on top poly(ethylene naphthalate) (PEN) plastic substrate by simple



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5. Application Directions

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printing and demonstrated gain up to 45 operate at 3.5 kHz signal.[158] By using a much simpler bar coating process for a large area device, the same group demonstrated a much faster ring oscillator that is flexible and transparent and that can operate at 25 kHz based on an inverter made of ambipolar FET of functionalized poly(thiennylenevinylene-co-phthalimide) (PTVPhI-Eh) polymer (Figure 9a).[198] Besides polymers, several efforts have been made to realize complementary inverters using ambipolar FET consisting either of heterojunctions of organic small molecule films or single-component small-molecule thin films as the mobility values are generally higher than for polymers. However, those complementary inverters only demonstrate low gain, around 10, which is similar to what was achieved by the earliest polymer complementary inverters.[200–202] The low gain values are attributed to the low on/off ratio of the constituent ambipolar FETs, which is less than 100. Recently complementary inverters based on single-component ambipolar FET of organic small molecule thin film has been successfully demonstrated. The active semiconductor material was indigo, a biodegradable natural pigment (Figure 9b). This complementary inverter was air stable and achieved high gain of 110 at VDD = ±14 V[199], owing to well-balanced carrier mobility values and the relatively high on/off ratio for its kind (103). Uemura et al.[76] demonstrated monolithic complementary inverters with an organic single crystals. By depositing two pairs of electrodes (a pair using Ca and another using Au) on a rubrene single crystal, ambipolar FETs were fabricated with optimized carrier injections and on/ off ratio for both charge carriers. The complementary inverter worked with gain as high as 150 at VDD = 10 V and consuming a power as low as μW in the switching state and nW in the static state (Figure 9c). Owing to its intrinsic ambipolar nature, CNT is among the materials of interest for CMOS inverters. In the early days, the CNT-based inverters were realized by spatially doping the tubes to become p-type and n-type, thus suppressing the intrinsic ambipolar characteristics of the CNT. Yu et al.[203] directly used the ambipolar characteristics of CNT to act simultaneously as p-type transistor and n-type transistor in the inverter. The inverter fabricated from random networks of SWNT on Al2O3 dielectric demonstrated a modest gain value of 4. However, they also proved that this inverter was so adaptive in the integration of CMOS-like logic gates, where NOR and NAND functions were achieved. Ha et al.[134] demonstrated much better performing inverter using ambipolar CNT-FETs through utilization of an ion gel gate. The device was printed on top of a flexible polyimide substrate and achieved gain as high as 58 and functioned well at a 5 kHz input signal.[134] The same authors successfully developed a ring oscillator and NAND circuit with >20 kHz operating frequencies and a stage delay smaller than 5 μs (Figure 9d). It was also found out that the ionic conductivity of the ion gel dielectric strongly influence the switching time of the inverter.[135] Fabricating good complementary-like inverters using ambipolar FET it is a great challenge. Most of the ambipolar FETs cannot be fully switched off as the electron current rises before the hole current completely disappears. Usually, the on/off ratio for both holes and electrons in most of the ambipolar FET is lower than 104. A typical silicon transistor (unipolar) in


Figure 9. a) Voltage gain characteristics and the related noise margin of the complementary inverter circuit made from ambipolar transistor of PTVPhI-Eh polymer prepared by using a simple bar-coating process. Reproduced with permission.[198] b) A complementary inverter circuit built from an ambipolar transistor of indigo material. The gain characteristics reached 110 for VDD = 14 V owing to the relatively high on/off ratio of the ambipolar transistor. Reproduced with permission from.[199] c) A monolithic inverter fabricated on a rubrene single crystal where two ambipolar transistors were made, one with optimized hole injection, and the other with optimized electron injection. The inverter gain performance reached 145 for VDD value as small as 10 V. Reproduced with permission.[76] d) Ring resonator and NAND circuit made from ambipolar transistors of carbon nanotubes gated by ion gel demonstrating frequency response up to 22 kHz. Reproduced with permission.[134,135] Copyright 2010 and 2013, American Chemical Society.


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5.2. Ambipolar Light-Emitting Transistors Ambipolar light-emitting transistors (LETs) are newly developed structures that combine switching functionality (which is typical of transistors) and light-emitting capability in a single device. The light emission results from the electron and holes recombination in the gate-controlled conducting channel.[204] In principle, every ambipolar FET should be able to operate as ambipolar LET. The light emission intensity in the channel will depend on the current density and the quantum efficiency of the semiconducting materials. The ambipolar LET geometry has many advantages over the LED structure: i) higher channel conductivity[1]; ii) gatetunable charge-carrier balance and emission-zone position[205]; iii) minimized exciton losses; iv) higher external quantum efficiency (EQE) than that of the LEDs; and v) planar architecture that allows direct observation of the micrometer-size emission zone.[78,206] The minimized excitonic losses in ambipolar LET fabricated with organic semiconductors have been proved with the higher external quantum efficiency (EQE) stability of the device at higher current density operation than the one of OLEDs. Zaumseil et al.[207] demonstrated that ambipolar LET of conjugated polymer can achieve ambipolar current density up to 50 A cm−2, which is comparable to what was obtained with rubrene single crystal ambipolar LET. EQE stability is observed in device operating at current densities as high as 4 kA cm−2.[86] This is in sharp contrast with values achieved by conventional OLEDs, where the EQE starts to suffer a catastrophic roll-off at current density as low as 1 A cm−2.[208] The direct comparison of the EQE of the ambipolar LET with the one of OLED was a great challenge. Despite intense electroluminescence has been obtain not only from ambipolar LETs with small molecule thin film,[23,95,209,210] but also from polymer thin films[157,205,211] as well as OSCs,[47,69] most of those ambipolar LETs were fabricated with a single organic semiconductor that acts as both the transport and emitting layer. This is in contrast to OLEDs, which usually use specialized layers where electron transport, holes transport, and emission are each optimized in a single layer. Recently, ambipolar LETs have been proved to be capable of reaching much higher EQE values than those of comparable optimized OLEDs. This was done by comparing ambipolar LETs with a trilayer heterostructure (p-channel/light emitter/n-channel) and the identical trilayer heterostructured OLEDs[212] (Figure 10a). The tri-layer ambipolar LET devices reach EQE values as high as 5%, which outperform the OLED that reached only 2.2% EQE. The high

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efficiency value of the ambipolar LET clearly indicates that the photon losses owing to exciton-metal quenching in this device are minimized or virtually non-existing. Recently, ambipolar LETs with EQE higher than 8% have been demonstrated by optimizing the light outcoupling of the device fabricated using fluorescent polymer thin film.[213] With this performance, ambipolar LETs are becoming another viable route towards highperformance organic light-emitting devices with the added switching functionality. If the future technical challenge can be overcome, including the use of phosphorescence materials,[214,215] this device could have direct practical use in display and lighting technology. Ambipolar LETs of organic semiconductors have been demonstrated as one of the most promising routes towards realizing the long-sought electrically driven organic laser. Despite the history of optically pumped lasing in organic semiconductor being as long as that of the laser device itself,[216,217] an electrically pumped laser based on organic semiconductors is still the most challenging topic in organic optoelectronics. The large interest in realizing electrically driven organic laser derives from the huge number of advantages expected over conventional semiconductor lasers: i) higher laser efficiency as the laser threshold in organic semiconductors is orders of magnitude lower than that of inorganic semiconductors; ii) great prospect for having broader spectral range for laser emission; and iii) higher potential for integration of the device in many different platforms. Despite the lower threshold energy needed for lasing than in their inorganic semiconductor counterparts, the organic semiconductors suffer from the intrinsic low carrier mobility, and consequently, in organic devices it is difficult to achieve the current density required to reach the threshold for population inversion.[218] Moreover, the device structure can introduce another possible excitonic loss path that would increase the laser threshold energy. The ambipolar LET structure, which has been demonstrated to display minimum excitonic losses as well as having the capability to operate at a high current density while maintaining its quantum efficiency,[86,207] is attractive for solving the challenges towards electrically driven organic laser. At the same time, ambipolar LETs are attractive to be integrated with other optical structures, such as a distributed feedback (DFB) resonator[219] and DBR microcavity[220] (Figure 10b). Intriguing results in this direction have been demonstrated for ambipolar LETs of OSCs. Two classes of organic molecules thiophene-phenylene co-oligomer (TPCO) and styrylbenzene have been identified as having a high carrier mobility in the single crystal phase while retaining their high luminescence efficiency, defying the general belief that OSCs with a high carrier mobility have a low luminescence efficiency and vice versa. [47,69,70,78,86,95,221,222] Among these materials, α,ω-bis(biphenylyl) terthiophene (BP3T) can be operated as ambipolar LET with very high current density operation.[74,92] The luminescence of this single crystal is self-confined owing to the high refractive index and is emitted only from the crystal edges, which is ideal to enhance the photon-exciton interaction required for lasing[223–225]. Recently, by utilizing these properties, optical directional coupler for ambipolar LET has been developed (Figure 10c).[79] One of the single crystal waveguides acts as the channel in which recombination occurs, whereas the other smaller single crystal with parallel facets acts as an optical



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integrated circuits usually works with on/off ratio of at least 106; this is the reason why the maximum gain values achieved in ambipolar FET-based inverters is still on the order of 100. However, recent progress in the fabrication of ambipolar FET based on carbon nanotube networks[121] and WS2[190] demonstrate against all the previous believes on/off ratio reaching 106–108 for both holes and electrons accumulations. Therefore, we can expect a bright future for high performing monolithic CMOS inverters using ambipolar FETs that will lead to more compact and energy efficient integrated circuits.

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Figure 10. a) A trilayer ambipolar light-emitting transistor consisting of a hole-transporting layer, an emitting layer and an electron-transporting layer, separating the exciton formation region and the carrier transporting region. The electroluminescence external quantum efficiency surpassed the one for the organic light-emitting diode with similar material stacking. Reproduced with permission.[212] Copyright 2010, Macmillan Publishers Ltd. b) The planar structure of ambipolar light-emitting transistors makes them compatible with the integration of optical resonators, including distributed Bragg reflector (DBR) microcavity and distributed feedback (DFB) resonator grating. Depending on where the light is emitted and propagating, one of those two resonator types will be beneficial for amplification as well as for outcoupling. c) An ambipolar light-emitting transistor of organic (BP3T) single crystal integrated with on an optical directional coupler to reduce the laser energy threshold. By operating the device at high current density signatures of current-density-dependent spectral narrowing were observed with a clear threshold behavior. This

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resonator, significantly reducing the laser threshold. Currentdensity-dependent spectral narrowing with a clear threshold was observed from these ambipolar LETs. Despite the spectral narrowing in these ambipolar LETs, the devices still need to be further investigated against the criteria for lasing. There are still many challenges laying ahead for the utilization of ambipolar LET as platforms for electrically driven lasers. The necessary efforts that need to be pursued: the search for new materials[226,227] to obtain materials with high luminescence efficiency coexisting with satisfying ambipolar transport;[228–230] the use of triplet exciton scavenging molecules;[231] and synthesis of new molecules (e.g. possessing thermally activated delayed fluorescence (TADF)[232–234], etc.) to find materials with high density of singlet excitons with a minimum path for triplet excitons creation.[235,236] However, new materials, such as colloidal nanocrystals, are also now appearing as good candidates for light-emitting transistors and electrically driven lasing.[152,166,237] Device structure and operation improvements remain important to enhance the current density, to increase singlet exciton formation, to maximize the photon-exciton interaction and to minimize the singlet exciton losses by triplet exciton. These have being attempted through current crowding,[92] recombination size control,[238–240] AC current operation,[241,242] spin-polarized injection,[243,244] magnetic field effect,[245,246] incorporation of resonators,[247,248] as well as microcavity polariton lasing.[249] The planar structure of the ambipolar LET is also useful for the study of recombination process in the active semiconductor materials by analyzing the recombination zone width (WRZ) and shape,[78,206,250,251] which is free from the influence of the metal electrodes as well as non-emitting transport layers that can create distortions of the observed light emission.[8,205,252–254] The recombination zone in ambipolar LETs, which is also the boundary of charge carrier accumulation zones can also be used to study the charge carrier injection efficiency in the device. For example, some efforts have been made by measuring the second harmonic generation emission from this zone of the transistor.[255–257] However, the ambipolar LET can also be used as a surface sensitive probe. Through imaging of the movement of the light emission zone, not only the structural morphology of the semiconductor materials but also the existence of traps that influence the recombination process can be probed.[206,258–262] This makes ambipolar LETs useful as tools to study the charge carrier transport process in the semiconductor materials[140,263–267] in the near future.

6. Conclusions This article presents an overview of the progress made in the realization and understanding of the underlying working mechanisms, as well as the application directions of ambipolar transistors made with various semiconducting materials. We

device paves the way to the possible realization of current-induced lasing in organic semiconductor. Reproduced with permission.[79] Copyright 2012, Macmillan Publishers Ltd.


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www.advmat.de www.MaterialsViews.com Nano-Hybrids for Photonic Devices (NaPhoD). The authors would like thanks Dr. K. Szendrei for discussions. S.Z.B. would like to thank Prof. Y. Iwasa, Dr. H. T. Yuan for some stimulating insight on the future direction of the field. Received: August 26, 2013 Published online: October 29, 2013

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M.A.L. would like to acknowledge the European Research Council for the ERC-Starting Grant Hybrids Solution Processable optoelectronic devices (Hy-SPOD), STW-DFG “Polymer-wrapped carbon nanotubes for high performance field effect transistors”, and the Nanoscience-Era Project

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conclude this article with a brief summary of the advantages of these devices, their challenges, and their future prospects. The key merit of ambipolar transistors is the capability to operate with both holes and electrons providing three different states: the hole current on state, the off state, and the electron current on state, which is beneficial to construct logic circuits with a lower number of device components. The recent advances in ambipolar transistor of several materials demonstrated that the on/off ratio can be as high as 107, which is comparable to the state-of-the-art unipolar transistor. As high on/off ratios influence the circuit performance (i.e., gain value in complementary inverter), ambipolar FET of those materials will be able to achieve a high figure of merit for further circuit miniaturization. Another intriguing feature of ambipolar FETs is the capability to emit light when the right active material is used, so they work as ambipolar light-emitting transistors. This provide a further miniaturization opportunity combining electronic functionality with optical functionality. Ambipolar FET that work as ambipolar LET has great prospect to bridging the gap between the state-of-the-art electronic computing process and emerging photonic computing. The demonstration of ambipolar transport in FET of many emerging materials, such as layered materials beyond graphene as well as solution processable nanomaterials provide opportunities to utilize the new physical properties of these materials for new functionality. Nevertheless, there are still many challenges to overcome before ambipolar FETs can find their place not only in fundamental physics, chemistry, and material sciences, but also in the perspective of applications. One of the tasks is the realization of ambipolar transport in wide bandgap materials using stable and inert materials to fabricate the injection electrodes. The next issue is that the nature of ambipolar transport in transistors made with many materials is still far from what is generally predicted for the intrinsic properties of the semiconductors. For a more application perspective, a general recipe to obtain combination of high on/off ratio, type of materials, and the required driving voltage value for ambipolar carrier injection still needs to be sought in order to be compatible with the current status of modern electronics. Efforts have been made with the introduction of new kind of gating, including an electric double-layer gating technique. Many challenges are still waiting to be explored in this recently born field. Finally, the ambipolar FETs themselves provide tools to investigate the intrinsic properties of the semiconductor material themselves, in particular in new emerging materials, such as layered materials, topological insulators, colloidal nanocrystal superlattices, semiconducting nanowires, etc. Integrated superconductor switches, interconnecting devices by ambipolar FET/ LET-based laser and complementary circuits are among the future direction of ambipolar transistor research.

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Electrostatically Reversible Polarity of Ambipolar α-MoTe2 Transistors.

High-mobility ambipolar ZnO-graphene hybrid thin film transistors.

Cyanated isoindigos for n-type and ambipolar organic thin film transistors.

Device perspective for black phosphorus field-effect transistors: contact resistance, ambipolar behavior, and scaling.

Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides.

Gas sensors based on semiconducting nanowire field-effect transistors.

Controlled doping of semiconducting titania nanosheets for tailored spinelectronic materials.

Quantum and classical magnetoresistance in ambipolar topological insulator transistors with gate-tunable bulk and surface conduction.

Controlling Conjugation and Solubility of Donor-Acceptor Semiconducting Copolymers for High-Performance Organic Field-Effect Transistors.

Synthesis and properties of semiconducting bispyrrolothiophenes for organic field-effect transistors.

Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures.

Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors.

Leveraging the ambipolar transport in polymeric field-effect transistors via blending with liquid-phase exfoliated graphene.

Enrichment of large-diameter semiconducting SWCNTs by polyfluorene extraction for high network density thin film transistors.

Bioengineered collagens: emerging directions for biomedical materials.

Titanyl phthalocyanine ambipolar thin film transistors making use of carbon nanotube electrodes.

Emerging ceramic-based materials for dentistry.

High Mobility 2D Palladium Diselenide Field-Effect Transistors with Tunable Ambipolar Characteristics.

An outlook review: mechanochromic materials and their potential for biological and healthcare applications.

Morphology control in biphasic hybrid systems of semiconducting materials.

Selective remanent ambipolar charge transport in polymeric field-effect transistors for high-performance logic circuits fabricated in ambient.

Near infrared organic semiconducting materials for bulk heterojunction and dye-sensitized solar cells.

Emerging chitin and chitosan nanofibrous materials for biomedical applications.

Suture materials - Current and emerging trends.