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Keywords:

  • organic field-effect transistor;
  • n-type;
  • ambipolar;
  • electron transporting

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information

The advantages of organic field-effect transistors, such as low cost, mechanical flexibility and large-area fabrication, make them potentially useful for electronic applications such as flexible switching backplanes for video displays, radio frequency identifications and so on. A large amount of molecules were designed and synthesized for electron transporting (n-type) and ambipolar organic semiconductors with improved performance and stability. In this review, we focus on the advances in performance and molecular design of n-type and ambipolar semiconductors reported in the past few years.


1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information

Due to the potential applications in integrated circuits for large-area, flexible, and ultralow-cost electronics, organic field-effect transistors (OFETs) have received focus attention all over the world since the invention of the first OFETs in 1986 by Tsumura and coworkers.1 Compared to the inorganic semiconductors, molecules of organic semiconductor (OSCs) can be designed and modified to obtain OSCs with optimized properties for charge transport, light absorption, energy level and so on.2–9 OSCs are also relatively easily scaled up and isolated in high purity. These advantages are not only important for industrial purposes, but also allow us to investigate the relationships between structures and properties. The performance of OFETs has improved immensely over the past decades, with FET mobility in some cases exceeding those of amorphous silicon FETs (α-Si:H, mobility ∼1.0 cm2V−1s−1).10–19 However, the overall development of n-type organic semiconductors still lags behind their p-type counterparts in terms of mobility, ambient stability, and so on. Considering n-type and ambipolar OSCs playing significant roles in ambipolar transistors and complementary circuits, development of novel n-type and ambipolar semiconducting materials with high performance is still a critical focus in organic electronics.

High performance for OFETs means high charge carrier mobility, large on/off ratio, and low threshold voltage. Apart from these, high ambient stability is essential for applications, which is another main property for OSCs. For low cost purposes, many attempts have been made to obtain solution-processible OSCs because they could be readily processed and easily printable, removing the conventional photolithography for patterning and minimize the costly vacuum-based fabrication. To meet these requirements, the development of new organic semiconductors is highly desirable. In this review, n-type and ambipolar semiconducting materials that are suitable for high-performance OFETs are classified by chemical structure. The relationships between the chemical structure, energy level, transistor characteristics, stability, etc. are also discussed.

2. n-Type Semiconductors for OFETs

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information

The first observation of n-channel OFETs was reported by Guillaud et al. with lutetium and thulium bisphthalocyanines (Pc2Lu and Pc2Tm) as semiconducting materials without any dopant in 1990.20 The performance of n-type OFETs has improved remarkably with the FET mobility being higher than 1.0 cm2V−1s−1 in some cases.18, 21–23 However, compared to p-type semiconductors, few semiconductors show high electron mobility and many n-type materials are unstable in air. For an ideal n-type organic semiconductor, the mobility should be high together with a large on/off ratio, low threshold voltage, good air stability and solution processability.24–26

To achieve such ideal n-type OSCs, high electron affinities (related to the lowest unoccupied molecular orbital (LUMO)) to facilitate charge injection and ambient stability are necessary. Strong intermolecular orbital overlap is also a basic requirement to achieve high electron mobilities3, 27, 28 For stabilities, besides decreasing the LUMO energy level of materials below the threshold for thermodynamically favorable electrochemical oxidation, introducing dense molecular packing to form kinetic barriers for preventing the penetration of water and oxygen may also be a practicable way.7, 29–33

LUMO energy levels and morphology of semiconducting layer are among the most important issues for n-type OSCs and this will be discussed in detail.

Energy levels:

  • i
    For electron injection. Gold is the most commonly used metal for source and drain electrodes in OFETs with high work function (around 5.1 eV). For most n-type OSCs, the LUMO levels are in the range of −3.0 eV to −4.0 eV. This high lying LUMO levels make it hard to inject electrons into the semiconducting layer from Au electrodes.34 Although the LUMO energy levels do not impact the intrinsic mobility directly, this high injection barrier may cause the reduction of the measured mobility. Low work function metals like calcium, magnesium or aluminum are used to help electron injection. However, these metals are not environmentally stable.
  • ii
    For stability. It is first discussed by deLeeuw et al. and now generally believed that for the n-type OFETs, air instability usually is not due to the degradation of intrinsically chemically unstable materials, but arises from the charge carrier trapping under ambient conditions by H2O or O2.35 When operated in ambient air, the charge carriers (electrons for n-type) may easily be trapped by H2O or O2 for OFETs based on high LUMO semiconductors. The mobilities of these OFETs can be one or even several orders of magnitude higher when taking precautionary measures to exclude atmospheric oxygen and water in vacuum or inert atmosphere, which should be avoid for the consideration of cost. Decreasing LUMO energy level of OSCs is a promising way to improve the stability of n-type OSCs and it is generally believed that the appropriate LUMO energy level of n-type OSCs should below −4.0 eV with acceptable charge carrier injection and ambient stability.7, 27, 31

Introducing electron-withdrawing groups such as cyano, carbonyls, etc. into the semiconductors is the most common strategy for lowering the LUMO energy levels of OSCs, no matter the electron-withdrawing groups are used as part of the π-system, or as the attachment groups.

Morphology

Morphology, also termed as microstructures of the film, plays a key role in the performance as well as stability of the OFETs. Different fabrication methods, fabrication conditions, annealing temperature and time or even the substrates themselves can result in different molecules packing mode and/or crystallinity, which will influence the charge carrier mobility and hence the transistor behavior.36–39

What Influences the Morphology:

  • i
    The chemical structures of OSCs. Proper engineering of the organic semiconductor's chemical structure is one important tool to control the film morphology. Molecules with different functional groups and side alkyl chains may result in different morphology.40 Even for the same length alkyl chains, the different position of the chain branched may lead to different molecule packing.41
  • ii
    Processing techniques. For an OSC molecule, the morphology varies from different processing techniques, including vacuum evaporation, spin-coating, ink-jet printing, drop casting, solution shearing, etc.42–44 The growth modes with different fabrication methods may be totally different, resulting in various morphologies. Vacuum evaporation is one of the most commonly used processing techniques for some small molecule OSCs where the source material is heated and evaporated in vacuum by various hot sources. The vacuum allows vapor particles to travel directly to the relatively cold substrate, where they condense back to a solid state to form the film. Spin coating is another widely used technique to deposit uniform thin films on flat substrates. A small amount of OSC solution is applied on the center of the substrate and then spun. During the spinning process, the solvent rapidly evaporates, leaving behind a homogeneous thin film. For these two most commonly used techniques, measures are taken to control the morphology like substrate heating and/or optimized disposition speed in vacuum evaporation and optimized rotation speed and/or concentration of the solution in spin-coating. New techniques such as solution shearing etc. have been developed to control the crystallinity or even directional arrangement of the OSC molecules.
  • iii
    Substrate modification. A few kinds of self-assembled monolayers (SAM), e.g. octyltrichlorosilane (OTS-8), octadecyltrichlorosilane (OTS-18), octadecyltrimethoxysilane (OTMS), hexamethyldisilazane (HMDS) etc. are used to tune the surface energy of the dielectric layer.45–47 Research results indicate that the substrate modification can also be used to control the film morphology, especially for solution processed thin film. In some works, even the modified electrodes can induce the formation of large grains near the electrodes.48

What the Morphology Influences:

  • i
    Mobility. In many works, close intermolecular π-stacking is crucial in achieving high performance FETs. Extensive and tight π-stacking may lead to better molecular orbital overlap between neighboring chains and hence the better carrier transfer. Note that there are still some cases of molecules that have effective overlap without extensive π-stacking and likewise there are cases of molecules with strong π-stacking but which lack effective wave function overlap.3, 49 For some polymer based OFETs, charge carriers transporting along the backbones are observed,37 indicate that it is not a universal requirement for all organic compounds to reach tight π-stacking in order to achieve sufficient interchain charge transfer. The π-stacking distances are usually in the range of 3.3 − 3.6 Å for small molecule semiconductors and 3.6 − 4.0 Å for polymer semiconductors.
  • ii
    Stability. It is reported that the close packing of the π-planes in some OFETs can inhibit the ingress of oxygen, which reduces the occurrence of electron trapping. This phenomenon is also observed in some OFETs based on OSCs with long side alkyl chains.25 This may be the reason for some air-stable n-type OFETs.

2.1. Small Molecule OSCs for n-Type OFETs

2.1.1. Rylene and Other Aromatic Diimide

Rylene and other aromatic diimides are one of the classical n-type OSCs with high electron affinities, high mobilities and excellent stabilities.27, 50–52 These materials are electron deficient due to the substitution of an aromatic core with two sets of strong electron-withdrawing carboxylic imide rings. The alkyls or other groups substituted at the nitrogen-positions of diimides also provide a way to increase and/or adjust the solubility of OSCs. Rylene, especially naphthalene and perylene tetracarboxylic diimides (NDI and PDI) are the most valuable building blocks for obtaining high performance electron transporting materials to close the gap in comparison to their p-channel counterparts.

2.1.1.1. Naphthalene Diimides (NDIs):

NDIs based n-Channel OFETs were first observed in 1996 with the electron mobility on the order of 10−4 cm2V−1s−1.53 Figure 1 shows the chemical structure of some semiconducting NDIs, and Table 1 summarizes their principal FET performance parameters.

thumbnail image

Figure 1. Chemical structures of some NDI-based small molecule semiconductors.

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Table 1. OFET device data for NDIs based semiconductors.
 LUMO [eV]Deposition ProcessMax μe [cm2V−1s−1]Ion/IoffVTh [V]Device structurea)Ref.
  • a)

    a)device architectures; S/D electrodes; gate/dielectric; modification of substrate; measurement condition;

  • b)

    b)OTMS: octadecyltrimethoxysilane;

  • c)

    c)PαMS: poly-α-methyl styrene;

  • d)

    d)β-PTS: β-phenethyltrichlorosilane.

1a evaporation6.210858TC; gold; Si/SiO2; OTS-18; tested in Ar21
1b evaporation0.710655TC; gold; Si/SiO2; OTS-18; tested in Ar21
1c−3.77zone-cast0.18104 BC/TG; gold; Ag/parylene C; tested in air43
1d−3.92spin coating9.1 × 10−310518BC/TG; Ag/CYTOP; tested in air8
1e−3.71evaporation0.701063 – 13TC; gold; Si/SiO2; OTS-18; tested in air24
1f−3.72evaporation0.34 (N2) 0.27 (Air)10719TC; gold; Si/SiO2; OTS-18; tested in N2 or air54
1f−4.02single crystal0.71042.2TC; gold; Si/SiO2; OTS-18; tested in vacuum55
1g evaporation0.57107 TC; gold; Si/SiO2; OTMSb); tested in air56
1h evaporation2.5 × 10−2105 BC; gold; Si/SiO2; PαMSc)57
1h evaporation0.87107 TC; gold; Si/SiO2; PαMS57
1h evaporation0.3110731 – 52TC; gold; Si/SiO2; OTS-18; tested in air58
1i−4.01evaporation0.9110625TC; gold; Si/SiO2; bare; tested in air54
1i Solution shearing0.951058TC; gold; Si/SiO2; bare42
   4.2610618  
1j−4.01evaporation1.4310723TC; gold; Si/SiO2; OTS-18; tested in air54
1k−3.79evaporation0.35 (vacuum) 0.10 (air)10628TC; gold; Si/SiO2; HMDS59
    10750  
1l−3.95evaporation0.11030TC; Ca; Si/SiO2; β-PTSd); tested in vacuum,60
1m−4.3spin coating1.2108–4.8 – 6.2BC; gold; Si/SiO2; OTS-18; tested in air25
1n spin coating0.34107–5.1BC; gold; Si/SiO2; OTS-18; tested in air26
1o spin coating3.5108–2.5BC; gold; Si/SiO2; OTS-18; tested in air26
1p spin coating0.251077.07BC; gold; Si/SiO2; OTS-18; tested in air26
1q−4.32spin coating0.71077TC; gold; Si/SiO2; OTS-18; tested in air61
1r ink-jet printing0.171041.7BC/TG; gold; Al/CYTOP; tested in N262
1s spin coating1.2 13BC/TG; gold; Al/CYTOP; tested in N263

For the alkyls or other groups substituted at the nitrogen-positions of diimides, the compound 1a is one of the most successful n-type OSCs. 1a based OFETs with high electron mobility of 6.2 cm2V−1s−1 and current on/off ratio of 6 × 108 was reported by D. Shukla and coworkers when measured under a continuous stream argon.21 The OFETs based on 1a showed significantly lower but still acceptable mobility of 0.41 cm2V−1s−1 when tested in ambient air. The compound 1b with alkyl substituents on the N atoms in the same work exhibits a lower FET mobility of 0.7 cm2V−1s−1. 1c with alkylphenyl substituent was also reported and showed air-stable electron transport property. Zone cast method and parylene C dielectric layer was used to fabricate 1c OFETs, which showed mobility of 0.18 cm2V−1s−1.43 Besides alkyls and alkylphenyls, some other groups were also introduced to substitute at the nitrogen-positions of diimides like compound 1d with alkylthienyl group. Relatively low mobility of 9.1 × 10−3 cm2V−1s−1 was obtained for 1d top gate OFETs in ambient air.8

NDI OSCs with fluorinated substituents at the nitrogen-positions were also investigated and exhibit improved air stability duo to the lowered LUMO energy levels. For instance, compound 1e was reported by B. J. Jung and coworkers, showing electron mobility of 0.7 cm2V−1s−1 in air.24 Another fluoroalkyl substituted NDI OSCs 1f with a shorter fluoroalkyl side-chain showed lower mobility of 0.34 cm2V−1s−1 when tested in N2 and 0.27 cm2V−1s−1 in air.54 The OFETs based on single crystal of 1f was also reported and the mobility is 0.7 cm2V−1s−1 when tested in vacuum.55 Fluoroalkylphenyl substituted NDI OSCs 1g also exhibits air-stable electron transport performance. With OSC layer thermal evaporated on bare and OTS-18 treated Si/SiO2 surface, 1g OFETs showed comparable mobilities of 0.39 cm2V−1s−1 and 0.57 cm2V−1s−1 respectively.56 Compound 1h with perfluorophenyl substituents showed a low mobility of 0.025 cm2V−1s−1 with bottom gate (BG) bottom contact (BC) geometry. However, much higher mobility of 0.87 cm2V−1s−1 was obtained using bottom gate top contact (TC) geometry.57 Another work reported a mobility of 0.31 cm2V−1s−1 when measured in ambient air using the same OSCs of 1h.58

Compounds 1i and 1j with chlorine substituted at the naphthalene core were synthesized and showed excellent electron transport properties in ambient air with mobilities of 0.91 cm2V−1s−1 for 1i, 1.43 cm2V−1s−1 for 1j.54 Furthermore, for the OFETs based on the solution shearing 1i film, an increase in effective charge carrier mobility of up to 4.26 cm2V−1s−1 has been observed under bias stress after one thousandth cycles.42

1k and 1l are two NDIs derivatives with backbone catenation. For 1k, FET mobility of 0.35 cm2V−1s−1 was obtained when tested in vacuum and 0.1 cm2V−1s−1 in air based on the thermal evaporated 1k layer. However, a much lower mobility of 3 × 10−3 was obtained with solution-processed films.59 1l based OFETs exhibited highest mobility of 0.1 cm2V−1s−1 with Ca source/drain electrode and β-phenethyltrichlorosilane (β-PTS) treated SiO2 dielectric layer.60

Core-expanded NDIs is another kind of NDIs based n-type OSCs with high performance and stability (1m1p). The expanded planar π-conjugation which promotes intermolecular π−π stacking, and the large degree of π-electron deficiency imparted by the malonitrile-based moieties which can depress LUMO energy levels are the reasons for the excellent OFET performance and stability of 1m1p. For compound 1m, OFETs with spin coated 1m film exhibit mobility up to 1.2 cm2V−1s−1, current on/off ratio of 108.25 Note that the 1m exhibited excellent stability with annealing process taken in ambient air. The low LUMO energy level (−4.3 eV), associate with close π-packing and long side alkyl chains which can inhibit the ingress of oxygen, are the reason for the excellent stability. Recently, three similar compounds (1n1p) were synthesized and all show air-stable electron transporting properties. The obtained mobility of 3.5 cm2V−1s−1 based on 1o is one of the highest mobility for solution processed air stable n-type OFETs.26 Core-expanded NDIs with different functional groups (1q) was also reported and showed similar performance.61

Molecule 1r with two NDIs linked by a π-bridge was synthesized and used in OFETs. Top gate OFETs with CYTOP and Al2O3 as dielectric layer were fabricated with various semiconductor processing techniques and the highest mobility obtained in N2 was 0.17 cm2V−1s−1.62 1s is another compound with two NDIs linked by a π-bridge. High electron mobility up to 1.5 cm2V−1s−1 was observed in N2 with low hole mobility of 0.01 cm2V−1s−1.63

2.1.1.2. Perylene Diimides (PDIs):

Perylene tetracarboxylic diimides (PDIs) were used as one of the n-type OSCs from 1996 by Horowitz et al.64 Table 2 provides a summary of selected n-channel OFET data for the small-molecule PDIs in Figure 2.

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Figure 2. Chemical structures of some PDI and Other aromatic diimides based small molecule semiconductors.

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Table 2. OFET device data for PDIs and ther aromatic diimides based semiconductors.
 LUMO [eV]Deposition ProcessMax μe [cm2V−1s−1]Ion/IoffVTh [V]Device structurea)Ref.
  • a)

    a)device architectures; S/D electrodes; gate/dielectric; modification of substrate; measurement condition;

  • b)

    b)F-SAM: triethoxy-1H,1H,2H,2 H-tridecafluoro-n-octylsilane self-assembled monolayers;

  • c)

    c)OTMS: octadecyltrimethoxysilane.

2a evaporation1.710710 − 15BC; Ag; Si/SiO2; PαMS; tested in 10−4 Torr of H222
2a  0.36(PMMA)10525TC; gold; Si/SiO2; PMMA or COC; tested in air65
   0.67 (COC)10521  
2b−3.4evaporation0.5810744TC; Cr; Si/SiO2; OTS-18; tested in inert atmosphere67
2c−4.1evaporation0.1110529TC; gold; Si/SiO2; OTS-18; tested in air70
2c nanowire0.24 1.4 (single)106−20 − 14BC; gold; Si/SiO2; OTS-18; tested in air71
2d−4.27 0.066 (vacuum) 0.052 (air)  TC; gold; Si/SiO2; OTS-18; tested in vacuum or air72
2e−3.79evaporation0.62 (vacuum)1065 − 14TC; gold; Si/SiO2; OTS-18; tested in vacuum or air18
   0.37 (air)1077 − 19  
2f−3.85evaporation1.44 (vacuum)10514 − 29TC; gold; Si/SiO2; OTS-18; tested in vacuum or air18
   1.24 (air)10628 − 43  
2g−3.84evaporation0.061  TC; gold; Si/SiO2; OTS-18; tested in air73
2h evaporation0.016105 BC; gold; Si/SiO2; bare; tested in vacuum or air74
   0.0029104   
2i Inkjet printing0.056106−1.4BC; gold; Si/SiO2; bare; tested in N282
2j−4.3solution processed1.3 −8TC; gold; Si/SiO2; F-SAM b); tested in air77
2j crystal6(vacuum) 3(air)  BC; gold; Si/SiO2;PMMA; tested in vacuum or air78
2j crystal5.1 (290 K) 10.8 (230 K)  vacuum gap;79
2k−4.33evaporation0.86  TC; gold; Si/SiO2; OTS-18; tested in air73
2l−3.9evaporation0.1810636TC; gold; Si/SiO2; OTS-18; tested in air70
2m−4.23evaporation0.8210828TC; gold; Si/SiO2; OTS-18; tested in air83
2n−3.88evaporation0.66 (vacuum)10710 − 24TC; gold; Si/SiO2; OTS-18; tested in vacuum or air18
   0.61 (air)10626 − 34  
2o−3.93evaporation0.058 (vacuum)1051 − 18TC; gold; Si/SiO2; OTS-18; tested in vacuum or air18
   0.056 (air)1065 − 37  
2p-3.88evaporation0.15 (vacuum)10628TC; gold; Si/SiO2; HMDS59
   0.08 (air)10750  
3a−3.22evaporation0.0210645TC; gold; Si/SiO2; HMDS; tested in vacuum84
3b−4.07evaporation0.03 (vacuum)10617TC; gold; Si/SiO2; HMDS; tested in vacuum or air84
   0.02 (air)10722  
3c−4.20evaporation0.06 (vacuum)10510TC; gold; Si/SiO2; HMDS; tested in vacuum or air84
   0.04 (air)10412  
3d−3.90spin coating1.0 (vacuum)105−15TC; gold; Si/SiO2; OTMSc); tested in vacuum or air85
   0.51 (air)1044 − 40  

PDIs with N-substituted alkyl chains CxH2x+1 have received considerable attention in OFETs. Compound 2a with C8H17 and 2b with C13H27 alkyl chains are among the most famous n-type OSCs.22, 33, 65–69 The highest FET mobility with 2a as active layer is 1.7 cm2V−1s−1 when tested under a partial pressure of H2 (10−4 Torr).22 When measured in ambient air, 2a based OFETs exhibit mobilities of 0.36 cm2V−1s−1and 0.67 cm2V−1s−1 on polymethylmethacrylate (PMMA) and cyclic olefin copolymer (COC) coated Si/SiO2 substrates.65 Compound 2b is another famous n-type OSC investigated in many works.66–69 Mobilities of 0.58 cm2V−1s−1 was obtained with Cr as source and drain (S/D) electrodes, which is higher than the gold S/D electrodes based OFETs (0.21 cm2V−1s−1).67 PDIs with phenylethyl substituent (2c) is an air stable OSCs with thin film mobility of 0.11 cm2V−1s−1.70 2c is also used in nanowire OFETs, showed a mobility of 1.4 cm2V−1s−1 with individual wires of 2c.71 In 2007, a series of air stable n-type OSCs based on fluorinated derivatives of PDIs were designed and synthesized by Chen et al.72 Among them, 2d with perfluorinated phenyl substituent exhibits excellent stability and highest performance with mobilities of 0.066 cm2V−1s−1 in nitrogen and 0.052 cm2V−1s−1 in air. Compared with 2c and 2d, compound 2e with perfluorinated phenyl substituents showed higher mobilities of 0.62 cm2V−1s−1 tested in vacuum and 0.37 cm2V−1s−1 in air.18 In the same work, fluorinated alkyl substituted PDIs 2f was also reported with excellent electron transporting and air stability. With 2f as active layer and OTS-18 modified Si/SiO2 as substrate, high electron mobilities of 1.44 cm2V−1s−1 and 1.24 cm2V−1s−1 were obtained when tested in vacuum and in air respectively. Another partial fluorinated alkyl substituted PDIs 2g was used to investigate the effects of the interplay between energetic and kinetic factors on the air stability of n-channel OFET.73 2g OFETs (LUMO −3.84 eV) were not air stable until the thickness of semiconducting layer increased up to ∼10 monolayers and mobility of 0.061 cm2V−1s−1was obtained in ambient air with thickness of 10.5 ± 0.1 monolayers. However, another PDI OSCs of 2k (LUMO −4.33 eV) showed much better air stability. 2k OFETs with thickness of 3.6 ± 0.2 monolayers showed air stable mobility of 0.86 cm2V−1s−1, the same with tested in vacuum. These result showed that the LUMO energy level of 2g is at the onset of air-stable region. The air stability of 2g increased drastically with increasing nominal thickness up to 10 monolayers, indicating that the thicker film acts as a kinetic barrier to the diffusion of ambient oxidants.73 Some other substituents were also introduced to PDIs, such as 2h. Relatively low electron mobilities of 0.016 cm2V−1s−1 (in vacuum) and 0.0029 cm2V−1s−1 (in air) were obtained on bare Si/SiO2 substrate.74

1,7-Dicyano PDIs are usually air stable n-type OSCs due to the reduced LUMO energy level by cyano group. 2j is one of the most successful PDI based n-type semiconductors with high mobility and excellent stability.75–77 The low LUMO energy level of −4.3 eV associate with the dense packed cores and fluoroalkyl chains which can inhibit the ingress of oxygen, are the reason for the excellent stability. Highly crystalline films on triethoxy-1H,1H,2H,2H-tridecafluoron-octylsilane treated gate dielectrics with carrier mobility of 1.3 cm2V−1s−1 was achieved by a solution processed method with 2j as semiconductor in ambient air.77 Furthermore, 2j is also used in single-crystal OFETs with excellent performance of 6 cm2V−1s−1 in vacuum and 3 cm2V−1s−1 in ambient air.78 In another work, high electron mobility of 5.1 cm2V−1s−1 was obtained with a vacuum-gap single-crystal 2j OFET. And the mobility increased from 5.1 cm2V−1s−1 at T = 290 K to 10.8 cm2V−1s−1 at T = 230 K, showing a band-like electron transport. For the n-channel OFETs, this is the first in which transport occurs in band-like regime, indicating that the quality is comparable to that of the very best p-channel organic transistors.79

Most of studies of PDI-based FETs have been carried out using films deposited by vacuum evaporation. However, recent studies demonstrate the great potential of these materials for solution-processed FETs. 2i is one of the most widely used solution processible PDI OSCs. For instance, ring oscillators operating at a frequency of ∼2 kHz were fabricated on Si/SiO2 substrates using solution-deposited films of 2i for the n-channel FETs and poly-3-hexylthiophene (P3HT) for the p-channel FETs.80 Another solution-processed 2i OFET was reported by Facchetti et al. with mobility of ∼0.08 cm2V−1s−1.81 Inkjet printed 2i OFETs were also fabricated, showed highest mobility of 0.056 cm2V−1s−1.82

2l and 2m are two chloro-substituted PDIs for stable n-type OSCs. For the tetrachloro-substituted 2l, LUMO level of −3.9 eV and mobility of 0.18 cm2V−1s−1 were obtained and the mobilities decreased slightly from 0.16 and 0.18 cm2V−1s−1 to 0.08 and 0.04 cm2V−1s−1 after storage in air for 80 days.70 Octachloro-substituted PDIs 2m showed much lower LUMO of −4.23 eV compared with the core-unsubstituted (−3.7 eV) and tetrachloro-substituted PDIs (−3.9 eV) which is clearly in the range of air-stable n-channel semiconductors.83 Mobilities of 0.91 and 0.82 cm2V−1s−1 were obtained in vacuum and in ambient air. Furthermore, these excellent charge carrier mobilities and on/off ratios are almost unchanged even after exposing the devices to air for about 20 months (0.60 cm2V−1s−1, Ion/Ioff ∼107). The air-stability of these n-channel transistors is probably due to the energetically low-lying LUMO and the high packing density of 2m.

Apart from cyano and chloro groups, fluoro groups are also used as substituents at the PDI cores. 2n and 2o are two fluoro-substituted PDIs with same N-substituted alkyl chains with 2f. Compared to 2f, 2n and 2o OFETs showed lower mobilities of 0.66 (vacuum) and 0.61 cm2V−1s−1 (air) for 2n and 0.058(vacuum), 0.056 cm2V−1s−1 (air) for 2o. Though the mobilities decreased, the stability increased due to the lower LUMO levels (2f: −3.85 eV, 2n: −3.88 eV, 2o: −3.93 eV).18

2.1.1.3. Other Aromatic Diimides:

Some other aromatic diimide compounds are also used as OSCs. Take anthracene diimides (ADIs) for example, a family of compounds based on ADI core was synthesized and characterized.84 Due to the electron-withdrawing nature of diimide, –Br, and–CN functionalities, which tune both molecular orbital energetics and solid state self-organization of the anthracene core, electron transport were observed in vacuum and/or ambient air.84 Among them, 3a, 3b and 3c showed relatively high mobilities. For 3a based OFETs, mobility of 0.02 cm2V−1s−1 was obtained in vacuum and no FET performance was observed in ambient air due to the high LUMO energy level of −3.22 eV. For the 3b, 3c with cyano and fluoro substituents, energy levels are much lower, showing air operable FET performance with highest mobilities of 0.06 cm2V−1s−1 (vacuum) and 0.04 cm2V−1s−1 (air) for 3c. Recently, large disc-like ovalene diimides of 3d was synthesized for solution-processible n-type semiconductor for air stable OFETs. Mobilities of 1.0 cm2V−1s−1 in vacuum and 0.51 cm2V−1s−1 in air obtained are among the best performance for solution-processed n-channel OFETs.85

Besides diimides, some other small molecule semiconductors with electron-withdrawing groups like cyano, carbonyl, halogen etc. also exhibit good electron transporting properties. Figure 3 shows their chemical structures and the OFET performance parameters are listed in Table 3.

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Figure 3. Chemical structures of other n-type small molecule semiconductors.

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Table 3. OFET device data for other n-type small molecule organic semiconductors.
 LUMO [eV]Deposition ProcessMax μe [cm2V−1s−1]Ion/IoffVTh [V]Device structurea)Ref.
  • a)

    a)device architectures; S/D electrodes; gate/dielectric; modification of substrate; measurement condition.

4a spin coating0.16104−1.2TC; Si/SiO2; OTS-18; tested in air87
4b−4.2spin coating0.0151055.1TC; Si/SiO2; OTS-8; tested in air88
4d−4.37spin coating0.0141048.3TC; gold; Si/SiO2; OTS-18; tested in air89
5a−4.3drop casting0.9105 TC; gold; Si/SiO2; OTS-18; tested in air90
6a−3.94spin coating0.01610511BC; gold; Si/SiO2; OTS-18; tested in vacuum91
7a−3.96evaporation0.01110625BC; gold; Si/SiO2; HMDS; tested in vacuum92
7b evaporation0.01110627BC; gold; Si/SiO2; bare; tested in vacuum92
8a evaporation0.551069.5TC; gold; Si/SiO2; OTS-18; tested in air93
8b spin coating0.35106−1.2TC; gold; Si/SiO2; OTS-18; tested in air93
9a−2.54evaporation0.310756TC; gold; Si/SiO2; OTS-18; tested in vacuum96
10a evaporation0.014104−15TC; gold; Si/SiO2; bare; tested in vacuum97
11a−3.53evaporation0.161059.2TC; gold; Si/SiO2; PS; tested in vacuum98
12a evaporation1.210763TC; gold; Si/SiO2; OTS-18; tested in vacuum99, 100
13a evaporation0.64 24TC; gold; Si/SiO2; OTS-18; tested in vacuum101
14a−3.1Crystal evaporation3.1 (crystal)  TC; gold; Si/SiO2; bare; tested in vacuum23
   0.6 (film)    
15a−4.1evaporation0.15 (vacuum)10417TC; gold; Si/SiO2; HMDS; tested in vacuum or air102
   0.12 (air)10427  
16a−3.79evaporation0.39 (vacuum)10623TC; gold; Si/SiO2; OTS-18; tested in vacuum103
   0.14 (air)10723  
17a evaporation0.310525TC; gold; Si/SiO2; OTS-18; tested in vacuum104
18a−3.8crystal evaporation0.16 (crystal)10421TC; gold; Si/SiO2;105
   0.03 (film)10411Bare (crystal);OTS(film); 
19a−3.76evaporation0.6110632TC; gold; Si/SiO2; OTS-18; tested in N2106
19b−3.94evaporation2.1410734TC; gold; Si/SiO2; OTS-18; tested in N2106
20a−3.96 4.6  TC; gold; Si/SiO2; PαMS; tested in N2107, 108
21a−3.51evaporation0.18  TC; gold; Si/SiO2; OTMS; tested in air109
21b−3.78evaporation0.12  TC; gold; Si/SiO2; OTMS; tested in air109
22a−4.01evaporation3.3 (vacuum)  TC; gold; Si/SiO2; OTMS; tested in vacuum or air110
   0.5 (air)   111
23a−3.79crystal3.39 (crystal)104 TC/TG; graphite; graphite/parylene; OTS-18; tested in air112
2.1.2. Cyano and/or Carbonyl-Containing Semiconductors

π-Conjugated molecules with electron-withdrawing groups like cyano and/or carbonyl groups are also used as electron transport semiconductors. For instance, 7,7,8,8-tetracyanoquinodimethane (TCNQ)-based compounds have been known as excellent electron conductors for a long time86 and TCNQ itself is a very strong electron acceptor. Thus, π-conjugated molecules with cyano and/or carbonyl groups are expected to be a promising candidate for the n-channel OFETs.

Compounds 4a and 4b are two thienoquinoidal derivatives endcapped with cyanomethylene groups. Spin-coated 4a and 4b OFETs were fabricated and showed mobilities of 0.16 cm2V−1s−1 and 0.015 cm2V−1s−1 in ambient air, respectively.87, 88 A series of pyrrole-containing quinoids with dicyanomethylene groups were synthesized and characterized by D. B. Zhu et al. Among them, 4d showed the highest electron mobilities of 0.014 cm2V−1s−1, with one order of magnitude lower for the 4c and 4e based OFETs.89 Dicyanomethylene-substituted fused tetrathienoquinoid 5a was also synthesized and showed much higher mobility up to 0.9 cm2V−1s−1 for OFETs with drop casting 5a film in ambient air.90 Oligothiophenes containing dicyanomethylene-substituted cyclopenta[b]thiophene 6a and 6b were used to investigate the air stability of n-channel OFETs. The LUMO energy level dropped from −3.94 eV of 6a to −4.18 of 6b, indicate better air stability of 6b.91 6a showed better performance with mobility of 0.016 cm2V−1s−1 when tested in vacuum. However, when tested in air, significant decrease in electron mobility by a factor of 5 was observed. In contrast to 6a, the electron mobility of 6b in air-exposed conditions (2.4 × 10−3 cm2V−1s−1) was retained as compared with that measured under vacuum conditions (5.0 × 10−3 cm2V−1s−1). 7b is another semiconducting dicyanomethylene derivative with mobility of 0.11 cm2V−1s−1 when tested in vacuum.92 Recently, D. B. Zhu et al. reported diketopyrrolopyrrole (DPP)-containing quinoidal with dicyanomethylene groups 8a and 8b, which is the first demonstration of DPP-based small molecules offering only electron transport characteristics. OFETs exhibit maximum electron mobility up to 0.55 cm2V−1s−1 with on/off ratio of 106 for 8a by vapor evaporation, and 0.35 cm2V−1s−1 for 8b by solution process in air.93 Additionally, the intramolecular H bonds lead to a cis-conformation of the double bond linking the thiophene ring is observed (Figure 3, 8a, 8b).

2.1.3. Halogen-Containing Semiconductors

After the early reports of n-channel materials containing halogen substitutions, such as perfluoro copper phthalocyanine (F16CuPc)94 and fluorinated alkyl PDIs or NDIs,95 the halogen atom, especially fluorine atom has been one of the most useful substituent for the n-type organic materials. Many p-type materials were transformed into the n-type materials when halogen atoms were introduced.

Perfluorophenyl and perfluoroalkyl are most widely used substituent in the halogen-containing semiconductors. Compounds 9a and 10a are two OSCs with perfluorophenyl substituent. For the tetrathienoacene derivative 9a, mobility as high as 0.30 cm2V−1s−1 and a high on/off ratio of 1.8 × 107 was obtained in vacuum.96 Perfluorophenyl terminated oligothiophene derivatives 10a was also used in OFETs and showed lower mobility of 0.014 cm2V−1s−1 in vacuum.97 For the indenofluorenediones 11a, mobility of 0.16 cm2V−1s−1 was obtained in vacuum. In addition, the electron mobilities of 11a FETs decreased from 0.15 to 0.07 cm2V−1s−1 after 40 min of storage in air, but showed negligible changes, even after 3 months.98

Trifluoromethylphenyl is another most widely used substituent in the halogen-containing n-type OSCs. Compounds 12a19b are molecules synthesized and used in OFETs with trifluoromethylphenyl group. Compounds 12a, 13a and 13b are derivatives with a 2-(4-trifluoromethylphenyl)thiazole unit afforded a high performance FET device. The only difference is the thiophene rings for 12a and the thiazole ring for 13a and 13b. For 12a OFETs, high electron mobility of 0.3 cm2V−1s−1 on the SiO2 substrate was obtained, and the mobility increased up to 1.20 cm2V−1s−1 on the OTS-18 modified substrate.99, 100 The only problem of 12a OFETs is the high threshold voltages ranging from 63 to 67 V. 13a and 13b are then designed and synthesized to lower the LUMO levels by introducing electron-accepting thiazole rings, which are useful in reducing the threshold voltage of the FET device.101 The thiazole rings were also expected to increase the intermolecular interactions by the formation of good π−π stacking as well as heteroatom contacts, which would increase the electron mobility. Although 13a and 13b have similar molecule structure and HOMO and LUMO energies, their FET devices showed very different behaviors. 13b OFETs showed very low mobility of 10−4 cm2V−1s−1 while 13a OFETs showed much higher mobility of 0.12 cm2V−1s−1 with bottom contact geometry and 0.64 cm2V−1s−1with top contact geometry. Relatively low threshold voltage of 24 V was obtained in the high performance devices. 14a and 15a are another two OSCs with trifluoromethylphenyl groups. For 14a based on a trifluoromethyl-substituted alternating (thiophene/phenylene)-co-oligomer, mobility of 0.6 cm2V−1s−1 for thin film OFET and 3.1 cm2V−1s−1 for single crystal OFET were obtained in vacuum on bare Si/SiO2 substrates.23 15a is a derivative of benzo[1,2-b:4,5-b′]-dithiophene-4,8-dione with thiazole rings and trifluoromethylphenyl groups. This derivative has deep LUMO level of −4.1 eV, leading to efficient charge-carrier injection and air stability. Comparable mobilities of 0.15 cm2V−1s−1 in vacuum and 0.12 cm2V−1s−1 in air were obtained.102 Compound 16a with new designed electronegative unit of 4,9-dihydro-s-indaceno[1,2-b:5,6-b′]-dithiazole-4,9-dione (IDD), and trifluoromethyl and carbonyl groups, exhibited mobility of 0.39 cm2V−1s−1 in vacuum and 0.14 cm2V−1s−1 in air. This result demonstrated that IDD is a promising electronegative unit for constructing π-conjugated systems for n-type OFET materials.103

Some OSCs like 17a19b are designed with both cyanovinyl and trifluoromethyl group. For 17a with β-cyanovinyl, OFET mobility of 0.3 cm2V−1s−1 was obtained while for 17b with α-cyanovinyl, OFET mobility is almost one order of magnitude lower of 0.04 cm2V−1s−1.104 With structure similar to 17b, compounds 18a showed similar mobility of 0.03 cm2V−1s−1 for OFET with evaporated film and 0.16 cm2V−1s−1 for single crystal OFET.105 In light of the fact that well-organized crystalline structures and lowered LUMO energy levels improve field-effect electron mobility, two 18a derivatives, 19a and 19b were designed and synthesized by S. W. Yun et al.106 They envision better n-type transistors by enhancing molecular stacking and further decreasing the LUMO level. Electron mobilities as high as 0.61 cm2V−1s−1 for 19a and 2.14 cm2V−1s−1 for 19b and on/off current ratios higher than 106 were demonstrated, showed successful molecule design.

20a20c are three oligothiophene derivatives with carbonyl and/or fluoroalkyl electron-withdrawing groups. 20a with both carbonyl and fluorohexyl groups showed highest mobilities up to 2 cm2V−1s−1 on the polystyrene (PS) modified Si/SiO2 substrate in N2. Compound 20b with fluorohexyl groups showed only electron transport property with much lower mobility of 0.026 cm2V−1s−1. For 20c with only carbonyl group, ambipolar OFET performance were demonstrated with high electron mobility of 0.7 cm2V−1s−1 and very low hole mobility of 0.003 cm2V−1s−1.107 High mobility of 4.6 cm2V−1s−1 was also obtained for the 20a transistors with Au top contacts and the mobility is lower with the easily oxidizable metals Al/LiF and Yb as top-contact electrodes. It was found that the reduced transistor performance of Al/LiF and Yb top-contact transistors is attributed to an electron-transfer reaction between the ketone groups of 20a and Al or Yb, respectively, leading to an insulating interfacial layer.108

2.1.4. n-Type Pentacene Analogues

Pentacene is one of the most widely investigated OSCs and it is also the benchmark p-type semiconductors with excellent hole mobilities. Numerous recent studies have shown that high electron mobilities can also be extracted from the pentacene based molecules.

21a and 21b are two kinds of pentacenequinones derivatives designed and synthesized by Q. Miao et al.109 21a is fluorinated pentacenequinones which showed acceptable mobility up to 0.18 cm2V−1s−1 when measured in vacuum. 21b is a kind of N-heteropentacenequinones exhibits similar performance with 21a (0.12 cm2V−1s−1).109 This result indicated that both fluorine atoms and pyrazine-type nitrogen atoms are able to lower the LUMO energy level of pentacenequinone to yield n-type organic semiconductors. Inspired by the success of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN), compound 22a was designed and synthesized as soluble and stable n-type semiconductors by substituting N-heteropentacenes with silylethynyl groups. High electron mobilities in the range of 1.0–3.3 cm2V−1s−1 measured under vacuum and 0.3–0.5 cm2V−1s−1 in ambient air were obtained for the vacuum-deposited 22a OFETs.110, 111 However, mobilities measured from solution-processed films were 3 orders of magnitude lower. 23a is another pentacene analogues used as active components in n-channel thin film transistors which showed high electron mobility. Single crystal FETs with 23a as the channel material and graphite as S/D electrodes exhibits a very high electron mobility of 3.39 cm2V−1s−1 in ambient conditions.112

2.2. Polymer Semiconductors for n-Type OFETs

Conjugated polymers are composed of alternating single and double bonds between covalently bound carbon atoms, leading to one unpaired electron (the π electron) per carbon atom. In the polymer molecules, the electrons are delocalized along the backbone of the polymer which can provide a pathway for charge transport along the polymer main chain.30 Polymer semiconductors are considered as one of the most appropriate candidates for OFETs due to their unique properties such as readily processible, mechanical flexibility etc. Development of polymer semiconductors has lagged far behind their small molecule counterpart in the past. However, great progress have been made in developing high performance polymer semiconductors recently and excellent hole transporting polymer semiconductors with comparable performance to small molecule have been developed. For electron transporting polymer semiconductors, most n-type semiconducting polymers exhibited low electron mobilities below 0.1 cm2V−1s−1 and few of them are air stable. Despite this, development of n-type polymer semiconductors is still an important and active field and some high performance n-type polymer semiconductors have also been reported recently. Chemical structures of some n-type polymer semiconductors were illustrated in Figure 4, and Table 4 provides a summary of their OFET data.

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Figure 4. Chemical structures of some n-type polymer semiconductors.

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Table 4. OFET device data for n-type polymer semiconductors.
 LUMO [eV]Deposition ProcessMax μe [cm2V−1s−1]Ion/IoffVTh [V]Device structurea)Ref.
  • a)

    a)device architectures; S/D electrodes; gate/dielectric; modification of substrate; measurement condition.

24a−4.0spin coating0.851075 − 10BC/TG; gold; gold/D2200; tested in air113–120
24b−3.76spin coating0.0761057TC; gold; Si/SiO2; OTS-18; tested in N2121
24c−4.2nanobelt0.007104 BC; gold; Si/SiO2; tested in air122, 123
24d−3.54spin coating0.00261045TC; gold; Si/SiO2; OTS-18; tested in inert atmosphere.125
24e−3.79spin coating0.0510414TC; gold; Si/SiO2; OTS-18; tested in N2124
24f−4.0spin coating0.5105 BC/TG; gold; gold/PMMA; tested in air126
25a−3.75spin coating0.051058TC; gold; Si/SiO2; OTS-18; tested in N2124
25b−4.00spin coating0.0110538TC; gold; Si/SiO2; OTS-18; tested in air127
25c spin coating0.01710613TC; gold; Si/SiO2; OTS-18; tested in N2127
26a−3.47spin coating0.19105 BC/TG; gold; gold/D2000; tested in vacuum128
27a−3.66spin coating3104 BC/TG; gold; Al/CYTOP; tested in N2129
27b−4.18spin coating2.36  TC; gold; Si/SiO2; OTS-18; tested in air130

The NDI and thiophene based polymer semiconductor 24a, 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)), is one of the most widely investigated n-type polymer semiconductor. High performance top-gate OFET based on 24a with mobility up to 0.85 cm2V−1s−1 in ambient air was reported by H. Yan et al. in 2009.113 Spin coating as well as gravure, flexographic and inkjet printing were used to fabricate the semiconducting layer, demonstrating great processing versatility of 24a. Remarkable stability was also demonstrated by monitoring the OFET performance under different relative humidity. First spin-coated and gravure-printed polymeric semiconductor complementary inverters with large gains of 25–60 operated in ambient conditions have been realized. Thereafter, many works have been reported with 24a as active material to investigate the relationship between dielectric layers, morphology (molecule packing) device geometry and electron transport.114–120 The molecular packing and microstructure of 24a was investigated in detail using various methods and unconventional face-on molecular packing was observed.115, 119 Although the edge-on molecular packing was found to be beneficial for several high performance polymer FETs due to the fast two-dimensional charge transport along the chain backbone and the π-stacking direction, such ordering for 24a is detrimental for electron transport reported by S. Fabiano et al. for the first time.117 It was reported that the minimum thickness of 24a semiconductor to act as the channel of a OFET is just one layer and that a change in the conformational order (from edge-on to face-on) may be crucial to improve up to one order the mobility of thin film transistors. These researches have greatly promoted the fundamental studies of electron transport polymers.

Similar to 24a, polymer 24b with one more thiophene unit was designed and synthesized. Electron mobilities as high as 0.076 cm2V−1s−1 were achieved for bottom-gate top-contact OFETs from 24b.121 Note that in the same work, 24a OFETs showed mobility of 0.039 cm2V−1s−1 with this device geometry. Therefore, measurable performance of 24b should be able to exceed that of 24a by employing alternative OFET device architectures such as top-gate bottom-contact and the use of various dielectrics and printing fabrication techniques.

Polymers 24c24f are some other NDI based polymers synthesized and investigated as active components in n-channel OFETs.122–126 Among them, 24c was used for self-assembled nanobelt and tight π-stacking of 3.36 Å was observed. The highest mobilities of the single nanobelt based OFET is 7 × 10−3 cm2V−1s−1.123 Although the mobility is relatively low, 24c based OFETs showed stable and reproducible performance in air for over 4 years. The excellent stability of 24c OFETs are due to the two reasons discuss above: i) the low lying LUMO energy level (−4.2 eV) which enable the charge carriers (electrons) to remain thermodynamically stable in the presence of oxygen and water; ii) highly crystalline morphology and tight π-stacking provide a physical barrier against the diffusion and permeation of oxygen.122, 123 24f is another NDI based polymer semiconductor exhibited high electron mobility (0.5 cm2V−1s−1 for TG/BC architecture) in ambient air.126 Inkjet printed complementary inverters and ring oscillators with 24f as n-type active material showed high performance with voltage gains ∼50 for inverters and oscillation frequencies fosc ∼1.25 kHz for ring oscillators.

25a25c are n-type semiconducting polymers based on PDIs. Polymer 25a based on PDI and phenothiazine was synthesized and reported together with the NDI based polymer 24e. These two copolymers with same phenothiazine groups showed similar OFET performances with mobilities of 0.05 cm2V−1s−1 in nitrogen.124 For the recent reported acceptor-acceptor conjugated copolymer 25b and donor-acceptor copolymer 25c with same PDI unit, mobilities of 0.01 cm2V−1s−1 in air for 25b were obtained while for 25c, OFETs didn't function in air.127 When tested in N2, mobility of 0.017 cm2V−1s−1 was obtained for 25c OFETs. This result indicates that introduction of electron-deficient co-unit to polymer main chain giving acceptor-acceptor type conjugated polymer provides a feasible strategy to enhance the air stability of n-type conjugated polymers. A bithiophene-imide based n-type polymer 26a was used to investigate the influence of molecule weight on film morphology and device performance.128 The high molecule weight batch 26a affords more crystalline film microstructures, hence higher electron mobility. In a TG/BC OFET architecture, high molecule weight 26a OFET achieves electron mobility up to 0.19 cm2V−1s−1.

Very recently, diketopyrrolopyrrole (DPP) based conjugated copolymer 27a and 27b was designed and synthesized for n-channel OFETs. For DPP-DPP copolymer 27a with one of the DPP units functionalized with triethylene glycol (TEG) side chains, spontaneous chain crystallization was induced, providing maximum solubility and allowing the synthesis of high molecular weight DPP−DPP copolymers. Top gate OFETs based on 27a with spin coated CYTOP as dielectric layer were fabricated and showed high electron mobility, which in some “hero” devices exceeded 3 cm2V−1s−1.129 Another polymer 27b also exhibited high electron mobility of 2.36 cm2V−1s−1.130 Both the face-on and edge-on packing orientation on the substrate was observed and due to the low LUMO level of −4.18 eV, the 27b based OFETs exhibited excellent air stability. The electrical characteristics of 27b based OFETs did not change significantly after 7 months storage in ambient atmosphere. These results should be an important step toward the development of high performance n-type polymers.

3. Ambipolar Semiconductors for OFETs

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information

Most of the OFETs that have been fabricated to date show only unipolar behavior (either holes or electrons). However, the fabrication of devices exhibiting ambipolar behavior that can provide both n- and p-channel performance in one device is very important due to their application in large-area manufacturing of complementary integrated circuits without requiring micro-patterning of the individual p- and n-channel semiconductors.34, 131 Another advantage of ambipolar OFETs is that the light emission can be achieved by recombination of holes and electrons within the transistor channel.

There are three major groups of ambipolar transport OFETs which can be classified by the semiconducting layer: bilayer, blend, and single-component transistors.34 For the bilayer ambipolar OFETs, the second semiconducting layer is often fabricated via vacuum deposition as the solution processed bilayer films are difficult to realize without damage to the first layer. For the blend ambipolar OFETs, ambipolar charge transport and the associated carrier mobilities in blend system have a complex dependence on the blend composition and the phase-separated morphology, and therefore are highly dependent on processing conditions. As a result, single-component OFETs with ambipolar transporting semiconductors have great advantage. In this paper, we only focus on the ambipolar OFETs with single-component semiconductor.

For OFETs that can provide both n- and p-channel performance, the semiconducting layer must be suitable for both electron and hole injection and transport. Hence the energy level of HOMO and LOMO are a key factor for the ambipolar transporting semiconductors. The semiconductor is required to have a HOMO energy level below −5.0 eV to achieve stable hole transport and for the stable electron transport, the LUMO level needs to be below or at least close to −4.0 eV.34 Considering the single metal used as both the source and drain electrodes, the energy gap of HOMO and LUMO should not be too large to avoid charge injection barriers. Figure 5 shows the chemical structures of ambipolar organic semiconductors, and Table 5 provides a summary of their OFET data.

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Figure 5. Chemical structures of ambipolar transporting organic semiconductors.

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Table 5. OFET device data for ambipolar transporting organic semiconductors.
 HOMO [eV]LUMO [eV]Deposition ProcessMax μh [cm2V−1s−1]Max μe [cm2V−1s−1]Device structurea)Ref.
  • a)

    a)device architectures; S/D electrodes; gate/dielectric; modification of substrate; measurement condition;

  • b)

    b)TTC: tetratetracontane;

  • c)

    c)DTS: decyltrichlorosilane;

  • d)

    d)Obtained by ultraviolet photoelectron spectroscopy and the UV−vis−NIR absorption.

28a−5.39−3.35evaporation0.0970.133TC; gold; Si/SiO2; OTS-18; tested in N2132
29a−5.49−3.68evaporation0.22 (air)1.1 (vacuum)TC; gold; Si/SiO2; OTMS;111
30a−5.5−3.53evaporation0.230.21TC; gold; Si/SiO2; OTMS; tested in N2133, 134
31a−5.8−4.0evaporation0.22 (vacuum)0.03 (vacuum)TC; gold; Al/AlOx/TTCb);135
    0.20 (air)0.015 (air)  
31b−5.5−3.8evaporation0.01 (vacuum) 0.01 (air)0.01 (vacuum)TC; gold; Al/AlOx/TTC;136
32a−5.8−2.8single crystal0.120.013 137
32b−5.6−2.7single crystal0.170.09 137
32c−5.7−3.0single crystal0.120.11 137
33c−5.1−4.3spin coating0.030.003BC; gold; Si/SiO2; OTS-18; tested in air138
34a−5.5−4.0spin coating0.10.09TC; Ba; Si/SiO2; OTS-8; tested in N2140
34b−5.2−4.0spin coating0.350.4TC; gold; Si/SiO2; OTS-8; tested in N2141
34b−5.2−4.0spin coating0.530.58BC/TG; gold; D139; tested in vacuum142
34c−5.40−3.68spin coating0.0240.056TC; gold; Si/SiO2; OTS-8; tested in N2143
34d−5.40−3.68spin coating0.0130.010TC; gold; Si/SiO2; OTS-8; tested in N2143
34e−4.55−3.9spin coating0.831.36BC; gold; Si/SiO2; DTSc); tested in N2144
34f−4.55−3.9spin coating1.171.32BC; gold; Si/SiO2; DTS; tested in N2144
34g−5.33−4.07spin coating1.361.56BC/TG; gold; gold/PMMA; tested in N2146
34h−5.09−3.46spin coating1.620.14TC; gold; Si/SiO2; OTS-18; tested in N2147
34i−5.67−4.24spin coating0.360.41BC/TG; gold; Al/CYTOP;148
34j−5.3−3.4spin coating0.110.081TC; gold; Si/SiO2; OTS-18; tested in vacuum149
34k−5.3−3.4spin coating1.30.1TC; gold; Si/SiO2; OTS-18; tested in vacuum149
34l−4.65−3.5spin coating0.290.25BC; gold; Si/SiO2; DTS; tested in N2150
34m−5.37−3.74spin coating0.230.56TC; gold; Si/SiO2; OTS-8; tested in N2151
34n−5.07−3.82solution shearing2.530.43TC; gold; Si/SiO2; OTS; tested in N241
34o−4.91d)−3.65 d)solution shearing3.972.2TC; gold; Si/SiO2; OTS; tested in N241
34p−5.17−3.56solution shearing6.163.07TC; gold; Si/SiO2; OTS; tested in N2152
34q−5.10−3.49solution shearing8.844.34TC; gold; Si/SiO2; OTS; tested in N2152
35a−5.2−3.5spin coating0.540.022BC/TG; gold; Al/PMMA; tested in N2153
36a−5.27−3.79spin coating0.0232.8 × 10−3TC; gold; Si/SiO2;154
37a−4.36−3.8spin coating1.00.7BC; gold; Si/SiO2; DTS; tested in N2155
38a−5.56−3.7spin coating0.040.3BC/TG; gold; gold/PMMA; tested in air156
38b−5.01−3.7spin coating0.0030.03BC/TG; gold; gold/PMMA; tested in air156
39a−5.59−4.31spin coating0.0660.0062BC; gold; Si/SiO2; DTS; tested in air157
39b−5.49−4.17spin coating0.0530.021BC; gold; Si/SiO2; DTS; tested in air157

3.1. Small Molecule OSCs for Ambipolar OFETs

28a30a are acene-based semiconductors used in ambipolar OFETs. Reported in 2008 by Z. Bao et al., compound 28a exhibited balanced ambipolar performance with hole mobility of ∼0.1 cm2V−1s−1 and electron mobility of 0.133 cm2V−1s−1 in nitrogen.132 29a was reported as the constitutional isomer of 22a with only different position of N atom.111 22a showed only high electron mobility while 29a exhibited ambipolar performance with hole mobility of 0.22 cm2V−1s−1 and electron mobility of 1.1 cm2V−1s−1. Note that the high electron performance can only be obtained in vacuum and when tested in ambient air, the electron mobility decreased to the range of 10−3 cm2V−1s−1 due to the electron trapping of oxygen or water. 30a OFETs showed similar performance to 28a,133 and 30a based CMOS-like inverter with high gain up to 180 was also reported.134 All these three acene-based semiconductors showed ambipolar performance only in vacuum or inert atmosphere due to their relative high LUMO level.

31a and 31b were reported with similar chemical structure by the same group. They are a kind of natural material for ambipolar OFETs and circuits. 31a and 31b OFET devices were fabricated on the natural resin shellac substrates and Al was employed as gate electrode while AlOx/tetratetracontane for dielectric layer. Ambipolar performance was observed in vacuum and after extensive exposure to air, n-channel performance showed deterioration in both 31a and 31b OFETs.135, 136 One interesting thing is that apart from tetratetracontane, only evaporated polyethylene films allowed for the observation of ambipolar transport in 31b based OFETs. 31b did not show any semiconductor behavior on other investigated dielectrics, i.e., poly(vinyl alcohol), shellac, melamine, adenine, and guanine as well as plain aluminum oxide.136 Complementary-like inverter circuits were fabricated with both 31a and 31b and highest gain of 255 were obtained, which is one of the highest reported organic ambipolar devices.135

Single crystal OFETs based on highly luminescent oligo(p-phenylenevinylene) derivatives 32a32c were reported by H. Nakanotani et al.137 All the OFETs based on 32a32c showed ambipolar behavior in which 32c based OFETs exhibited balanced hole and electron mobilities both higher than 0.1 cm2V−1s−1. Intense electroluminescence was also observed in the channel of a 32c single crystal based OFETs. 33a33c are recent reported tetrathiafulvalene fused NDI derivatives with low LUMO energy level of −4.3 eV. Ambipolar behaviors were obtained for all the 33a33c based OFETs in ambient air due to their low LUMO level.138 However, the mobility are relatively low of about 10−4 cm2V−1s−1 for 33a and 33b OFETs. Highest mobility up to 0.03 cm2V−1s−1 and 0.003 cm2V−1s−1 were observed for hole and electron respectively for 33c based devices.

3.2. Polymer OSCs for n-Type OFETs

Numerous DPP-containing polymers were recently developed for high performance OFETs applications139 and a very high hole mobility over 10 cm2V−1s−1 was reported for the first time.16 Among these DPP-containing polymers based OFETs, high ambipolar performance were also observed.

As mentioned above, for molecules with DPP and thiophene unit, intramolecular hydrogen bond between the H atom at the thiophene ring and O atom at the DPP unit was observed, indicated planar configuration of the DPP unit and thiophene ring. Due to the existence of the intramolecular hydrogen bond, we suggest that the chemical structure of molecule based on DPP and thiophene unit showed be described like 8a in Figure 3 with H at the thiophene ring close to O at the DPP unit.

Diphenyl-DPP polymers are a kind of OSCs which showed excellent OFET performance in recent years. Diphenyl-DPP polymers polymer 34a is the first reported OSCs for high performance solution processed ambipolar OFETs with both hole and electron mobilities around 0.1 cm2V−1s−1. Near-infrared light emitting of the 34a OFETs was also observed when driven under appropriate bias conditions.140 34b is a diphenyl-DPP polymers with benzothiadiazole unit reported by P. Sonar et al. 34b OFETs with bottom gate architecture exhibited high and balanced ambipolar performance with hole mobility of 0.35 cm2V−1s−1 and electron mobility of 0.4 cm2V−1s−1.141 For the recent reported top gate OFETs based on 34b, even higher mobilities were obtained with both hole and electron mobilities higher than 0.5 cm2V−1s−1.142 Suitable HOMO (−5.2 eV) and LUMO (−4.0 eV) energy levels as well as the highly ordered lamellar packing with small π−π stacking distance (3.73 Å) after thermal annealing are responsible for getting the high and balanced ambipolar performance in 34b based OFETs.

34c and 34d are recently reported diphenyl-DPP polymers without any other unit. Relative large bandgap and low ambipolar performance were obtained for both 34c and 34d OFETs.143 Polymers 34e and 34f consisted of diphenyl-DPP coupled with benzobisthiadiazole (BBT) are synthesized and investigated by J. D. Yuen. et al.144 The strong accepting group of BBT lowers the LUMO values substantially and increases the HOMO slightly, resulting a narrow bandgap of 0.65 eV. Excellent ambipolar performance with equivalent p-type and n-type mobilities above 0.5 cm2V−1s−1 were obtained. In particular, 34f has mobilities exceeding 1 cm2V−1s−1 for both hole and electron, which is one of the highest for ambipolar OFETs reported. Another ambipolar OFETs with balanced performance was reported almost in the same time by Z. Y. Chen et al. Polymer 34g was previous reported polymer semiconductor with only p-channel performance.145 However, ambipolar performance was obtained using top gate and solvent-cleaned gold contact instead of previous reported bottom gate and O2-plasma-cleaned gold contact.146 The main reason for this is that the work function of solvent-cleaned gold (4.7 − 4.9 eV) is more suitable for electron injection compared with O2-plasma-cleaned gold (5 − 5.5 eV). Mobilities of 1.36 cm2V−1s−1 and 1.56 cm2V−1s−1 were obtained for holes and electrons, respectively.

Some other groups like selenophene, azine and naphthalene are also used in the diphenyl-DPP copolymers (34h34k). For the selenophene-DPP copolymer 34h OFETs, imbalanced ambipolar performance was observed with high hole mobilities of 1.62 cm2V−1s−1 and low electron mobilities of 0.14 cm2V−1s−1.147 The polymer 34i with azine unit showed low LUMO level of −4.24 eV and balanced ambipolar performance around 0.4 cm2V−1s−1 for both hole and electron.148 34j and 34k with different alkyl chains are two diphenyl-DPP copolymers with naphthalene unit.149 With similar LUMO levels of 34h, similar performance with high hole mobilities and low electron mobilities were observed for these two polymer OFETs with gold top contact. Problem of the imbalanced hole and electron mobilities was not solved by employing lower work function metal contacts and highest performance was still obtained by the gold contact 34k OFETs with hole mobilities of 1.3 cm2V−1s−1 and electron mobilities of 0.1 cm2V−1s−1. Diphenyl-DPP with highly extended fused ring aromatic moiety copolymer 34l was also reported with balanced performance.150

Furan substituted DPP combined with benzothiadiazole based polymer semiconductor 34m has been synthesized and evaluated as an ambipolar semiconductor in OFETs by P. Sonar et al.151 Hole and electron mobilities as high as 0.20 cm2V−1s−1 and 0.56 cm2V−1s−1, respectively, were achieved for 34m OFETs. 34n and 34o are two DPP polymers reported by Lee and coworkers with different side chains of DPP.41 34n with similar structure of 34h showed almost the same OFET performance with 34h while the active layer was spin coated. By employing solution shearing techniques, much higher but still imbalanced performance was obtained. For 34o with heptamethyltrisiloxyl groups, OFETs exhibited similar performance with mobilities of 1.69 cm2V−1s−1 and 0.20 cm2V−1s−1 for hole and electron, respectively. However, solution sheared 34o OFETs exhibited unprecedentedly high and balanced hole and electron mobilities of 3.97 and 2.20 cm2V−1s−1, respectively.41 Out-of-plane X-ray diffraction (XRD) analyses were performed and a 3.6 Å π-stacking distance with face-on orientations was observed for 34o film. The authors suggested that the enhanced electron mobilities were due to the lower LUMO energy levels enabled by the enhanced molecular packing, and polymer reorganization after phase transition resulting in a microstructure more favorable for electron transport. Very recently, another two DPP–selenophene copolymers similar with 34o were reported by the same group with only different side chains of DPP unit (34p, 34q).152 By tuning the length of the alkyl spacer group in the hybrid side chain, unprecedentedly high hole and electron mobilities were achieved for both 34p and 34q based OFETs. Record high mobilities of 8.84 and 4.34 cm2V−1s−1 for hole and electron respectively were obtained with solution sheared 34p OFETs. These results provide guidelines for the molecular design of semiconducting polymers with hybrid side chains.

Note that for the high and balanced performance ambipolar transporting DPP polymers such as 34b, e−g, i, l, o−q, the bandgaps are not very large with many of them smaller than 1.2 eV and the LUMO levels are usually low for efficient electron injection with some of them close to or even lower than −4.0 eV. The highest performance for DPP polymers OFETs are usually achieved after thermal annealing due to the reason that annealing in an inert atmosphere could possibly reduce the concentration of charge traps, increase the crystallinity of the polymer and improve the contact between the polymer and the source/drain electrodes.

For the other ambipolar transporting polymers without DPP unit, 35a with thienlylenevinylene and phthalimide unit exhibited unbalanced high hole mobilities of 0.54 cm2V−1s−1 and low mobilities of 0.022 cm2V−1s−1 with gold contact.153 This may due to the high LUMO level of −3.5 eV which is not suitable for electron injection from gold electrodes. Cesium (Cs) salts was used as electron-injection and hole-blocking layers and the work function of Au electrode decreased from 4.7 eV to around 4.1 eV after deposition of Cs salt layer. 35a OFET with Au/CsF contact showed mobilities of 0.040 cm2V−1s−1 and 0.26 cm2V−1s−1 for holes and electrons. Solution-processed high speed complementary ring oscillators were also fabricated with selective spray-deposited CsF layer and demonstrated frequencies of 12KHz. 36a with NDI and thiophene unit also exhibited ambipolar behavior with relatively low mobilities.154 Narrow bandgap polymer 37a was reported by J. Fan et al. with HOMO level of −4.36 eV and LUMO level of −3.80 eV.155 Very short π–π stacking distance was observed from in-plane grazing incidence XRD measurement implies strong interactions between “edge-on” orientated polymer chains. Maximum mobilities of 37a OFETs were measured as 1.0 cm2V−1s−1 for holes and 0.7 cm2V−1s−1 for electrons.

H. Usta et al. reported ambipolar transporting polymer 38a and 38b embedding dithienocoronenediimide acceptor unit along with thiophene and 3,3′-dialkoxybithiophene donor moieties.156 With this newly designed dithienocoronenediimide core, ambipolar performance was observed in both 38a and 38b OFETs in ambient environment. Highest mobilities of 0.04 cm2V−1s−1 and 0.3 cm2V−1s−1 for holes and electrons respectively were obtained on the 38a OFETs with top gate bottom contact architecture. It is noteworthy that similar electron/hole mobilities are measured for 38a and 38b devices even after 1 week of storage in ambient, indicating negligible deterioration due to ambient trapping species of O2/H2O. The authors suggested that the observed device stability is due to an interplay of polymer electronic structure, uniform film morphology, and self-encapsulated top-gate FET architecture. 39a and 39b are very recently reported air-operable ambipolar polymers.157 With the strong electron accepting moiety of dipyrrolo[2,3-b:2′,3′-e]pyrazine-2,6(1H,5H)-dione, very low LUMO level of −4.31 eV for 39a and −4.17 eV for 39b were obtained. These low LUMO levels are beneficial for the electron injection and stability of the polymer. Bottom gate bottom contact OFETs with 39b as active layer showed highest mobilities of 0.053 cm2V−1s−1 and 0.021 cm2V−1s−1 for holes and electrons, respectively.

4. Conclusions and Outlook

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information

In this review, we summarized the advances in molecular design and performance of n-type and ambipolar semiconductors reported in the past few years. Great progress has been made in developing n-type and ambipolar organic semiconductors due to the rapid growth in demand in the organic electronics field. The introduction of electron-withdrawing units into π-conjugated systems has been proven to be a promising approach to enhance the electron injection and transporting. Many π-conjugated molecules based on either traditional units like NDIs and PDIs or recently developed DPPs have been rationally designed, synthesized, and investigated in OFETs. Although some of them exhibited high performance with FET mobilities higher than amorphous silicon (∼1.0 cm2V−1s−1), the overall development of n-type and ambipolar OSCs still lags behind p-type OSCs in terms of mobility, ambient stability, and so on.

For electron-transporting OSCs, the LUMO energy level is still one of the most important issues. The LUMO level of molecules not only plays a key role for charge injection, which can influence the measured FET mobility directly, but is also crucial for its ambient stability. Effective overlap between the π-systems on adjacent molecules is another important issue for high mobilities. Besides, molecule packing, electrodes, device architectures, interfaces in devices etc. can also influence the performance of electron transporting OFETs. The ultimate goal for organic semiconductors should be the application in electronics such as silicon, where operational stability and reliability should also be considered besides carrier mobility. Therefore, new π-conjugated system designed for OFETs is still a difficult and complex task. The current family of semiconductors should be extended and new families should be developed. Better scientific understanding of the relationship between molecule and performance and physical principles in OFETs is also necessary. We hope that this review may support the development of n-type and ambipolar transporting organic semiconductors.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information

This article is part of an ongoing series celebrating the 25th anniversary of Advanced Materials. This work was supported by the National Natural Science Foundation of China (60911130231, 51233006, 61101051), The Major State Basic Research Development Program (2013CB733700, 2011CB932303, 2009CB623603) and Chinese Academy of Sciences.

Biographical Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information
Thumbnail image of

Yan Zhao graduated with a B.S. degree in chemistry from Shandong University in 2008 and is presently a Ph.D. student in the Institute of Chemistry, Chinese Academy of Sciences (CAS). His research focuses on the solution processed organic field-effect transistors and organic circuits like inverter and ring oscillator.

Biographical Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information
Thumbnail image of

Yunlong Guo received his B.S. degree in chemistry from Hebei Normal University in 2005, and a Ph.D degree in physical chemistry from the Institute of Chemistry, CAS in 2010. Now, he is an associate professor at the Institute of Chemistry, CAS. His research interest includes fabrication, characterization, and optimization of organic field-effect transistors and functional organic field-effect transistors.

Biographical Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. n-Type Semiconductors for OFETs
  5. 3. Ambipolar Semiconductors for OFETs
  6. 4. Conclusions and Outlook
  7. Acknowledgements
  8. Biographical Information
  9. Biographical Information
  10. Biographical Information
Thumbnail image of

Yunqi Liu graduated in 1975 from the Department of Chemistry, Nanjing University, and received a doctorate from Tokyo Institute of Technology, Japan, in 1991. Presently, he is a professor at the Institute of Chemistry, CAS. His research interests include molecular materials and devices.