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Heteroatom Substituted Organic/Polymeric Semiconductors and their Applications in Field-Effect Transistors Weifeng Zhang, Yunqi Liu, and Gui Yu*
1. Introduction In this research news article, recent progresses in the development of heteroatom substituted organic/polymeric semiconductors with a highlight on the effect of heteroatoms as well as their applications in organic/polymeric field-effect transistors (O/PFETs) is presented. O/PFETs are essential building blocks for the next generation of organic circuits, which have broad potential applications, such as radio frequency identification tags, flexible displays, electronic paper, electronic skin, and sensors.[1] Compared with traditional silicon-based materials, organic/polymeric semiconductors have attracted tremendous attention because of their unique designability of molecular structures, tunability of properties, light-weight, flexibility, and transparency.[2] Since pioneer work in the 1986,[3] many organic/polymeric semiconductors have been synthesized and investigated as active materials in O/PFETs. Among them, pentacene is a benchmark material. However, because of performance limitations and deficiencies, developing new organic/polymeric semiconductors is one of the most important research activities in the past decades. The inherent properties of organic/polymeric semiconductors are the most crucial criteria for O/PFETs performances. To be specific, these properties are considered as follows: 1) Energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied ones (LUMOs).
Dr. W. Zhang, Prof. Y. Liu, Prof. G. Yu Beijing National Laboratory for Molecular Sciences and Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201305297
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Restricted by the need of a small carrier injection barrier from the sourcedrain electrodes into the semiconductor layer, organic/polymeric semiconductors should have suitable HOMO or LUMO energy levels. For example, for ptype materials, the HOMO energy level should be located at −5.1 ± 0.3 eV and for n-type materials, the LUMO energy level should be close to −4.0 eV.[4] 2) Molecular packing mode in solid state. Lamellar packing (2-D π−stacking) mode is important, which can maximize the transfer integrals and transport the charge carriers through the shortest route.[5] 3) Molecular size/weight of polymer. The related research results show that field-effect mobilities improve significantly with increasing molecular weight of polymers.[6a] Polymers with high-molecular weight tend to afford improved thinfilm morphology exhibiting reduced crystallinity and more isotropic films, potentially leading to larger values for chargecarrier mobility.[6b]
Organic/polymeric semiconductors are mainly composed of aromatic systems including phenyl, vinyl, alkynyl, thienyl, and other isoelectric groups, which are constructed of carbon, hydrogen, and so-called ‘hereroatoms’ including chalcogen, nitrogen, and halogen atoms etc. The introduction of heteroatoms could yield different electronic properties by influencing the molecular geometry, the HOMO and LUMO energy levels, intermolecular interactions and so on. In this Research News article, we provide a brief review of the effect of heteroatoms and recent developments in heteroatom substituted organic/polymeric semiconductors, focusing especially on their application in field-effect transistors.
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Nowadays, many approaches are adopted in molecular engineering to tune field-effect properties of π−conjugated systems.[7] One of appealing strategies is to develop new π− conjugated backbones with high planarity, whose essence, actually, is to obtain π−conjugated systems with close π−π stacking and strong intermolecular interaction. Secondly, the introduction of alkyl groups onto the π-conjugated backbone is promising. A suitable alkyl side chain could result in close molecular packing in solid state, and/or make semiconductors to be solution-processable. For polymers, introduction of suitable branched alkyl chains could help to increase the number of monomer units in the π-conjugated backbone of the polymer.[5] When suitable branched alkyl chains are incorporated, the synthesized polymer will have good solubility, which allows polymerization reaction continuing without precipitation. Last but not the least, the incorporation of ‘heteroatoms’, including all elements except from carbon and hydrogen such as chalcogen, nitrogen, and halogen atoms, into the π−conjugated backbone or side chain is found to be useful. This review focuses on the development of heteroatom substituted small-molecule semiconductors and their applications in field-effect transistors, taking analogues of pentacene as examples, such as small-molecule semiconductors containing chalcogen, or nitrogen, and/or halogen atoms. Moreover, heteroatom substituted polymeric semiconductors and their applications in field-effect transistors are also discussed.
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2.1. Small-Molecule Semiconductors Containing Chalcogen Atoms Organic semiconductors containing chalcogen, especially sulfur, play an important role in the development of OFETs. The fused thiophene-ring systems are named as thienoacenes.[8] Many thienoacenes could form close and ordered molecular arrangements, which result in an effectively intra-stack electronic coupling via strong intermolecular π–π, S···S, and CH···π interactions in the solid state, and thus tend to afford good charge carrier mobilties, for example benzothieno[3,2-b] benzothiophene and dinaphtho-[2,3-b:2′,3′-f]thieno[3,2-b]thiophene derivatives.[9] It is believed that the large atomic radius of sulfur leads to a stronger intermolecular interaction and the high electron densities of the sulfur atoms in the HOMO give rise to an effective overlap between the HOMOs of neighboring molecules in the solid state.
Since these thienoacenes are typically used as p-channel organic semiconductors, the active carrier is a hole, which would migrate through the HOMOs of an array of molecules in the solid state. The HOMO energy level is a very important factor in affecting the charge transport properties of p-type organic semiconductors, in addition to the geometry of HOMO and packing mode in solid state. Pentacene has a high HOMO energy level of −4.6 eV,[10] which makes it sensitive to oxygen and light. Chalcogen atoms could lower the HOMO energy level of related semiconductors, for example, anthra[2,3-b]benzo[d]thiophene (ABT), 1 (Figure 1), 6-methyl-anthra[2,3-b]benzo[d]thiophene (Me-ABT), 2,[11] and dinaphtho[2,3-b:2′,3′-d]thiophene (DNT-V), 3[12] where analogues of pentacene have one thiophene unit, show low HOMO energy levels of −5.35 and −5.68 eV, respectively. ABT and DNT−V-based OFET devices fabricated by vapor-deposited methods afforded hole mobilites of 0.41 and 1.1 cm2 V−1 s−1 (1.5 cm2 V−1 s−1 obtained in single crystal devices), respectively. These results highlighted the importance of HOMO geometry of the π-conjugated backbone exerting great influence on mobilities. Likewise, Me-ABT, 3[13] has an HOMO energy
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2. Heteroatom Substituted Small-Molecule Semiconductors and Their Applications in Field-Effect Transistors
Figure 1. The chemical structure of selected small-molecule semiconductors containing chalcogen, nitrogen, and halogen atoms.
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level of −5.34 eV. It is noted that phototransistors based on the Me-ABT single crystal showed a high mobility of 1.66 cm2 V−1 s−1, a large photoresponsivity of 12 000 A W−1, and a photocurrent/dark current ratio of 6000. An analogue of pentacene containing two thiophene rings, thienoacenes, 4 reported by Takimiya et al.[14] shows the HOMO energy level of −5.8 eV, which is about 0.4 eV lower compared to that of ABT, 1. This thienoacene exhibited low hole mobilites up to 10−2 cm2 V−1 s−1 due to a large hole injection barrier. An analogue of pentacene containing three thiophene rings, namely DBTDT, 5[15] shows a HOMO energy level of −5.6 eV. The thin film transistors based on DBTDT exhibited hole mobilities of 0.51 cm2 V−1 s−1. However, pentathienoacene (PTA), 6 containing five thiophene rings has a HOMO energy level of −5.3 eV.[16] PTA-based devices showed hole mobilities of 0.045 cm2 V−1 s−1. When analogues of pentacene that contain more thiophene rings, their HOMO energy levels show an apparent rising trend. The different trend regarding the mobilities is relatively complex because it is also affected by the molecular geometry and packing in solid state. A similar trend is also found in oligomers containing thiophene units[17] and higher thienoacenes. For example, thienoacenes containing four thiophene units, namely L-DBTTA, 7 and S-DBTTA, 8,[18] reveal HOMO energy levels of −5.62 and −5.70 eV, respectively. Compared to the HOMO energy level of −5.44 eV of DNTT,[19] these two data are obviously decreased. L-DBTTA and S-DBTTA exhibited hole mobilites of 0.15 and 0.047 cm2 V−1 s−1, respectively. Moreover, another series of analogues of pentacene end-capped by thiophene units, such as anthradithiophene[20] and dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene[21] show a decreased trend of HOMO energy levels. In contrast to thienoacenes, the counterparts containing oxygen and selenium atoms are relatively rare. One of the key reasons is that for oxygen, its stronger electronegativity makes the related π−conjugated systems stable with low aromaticity; and for selenium, the lack of methodology makes it difficult to be synthesized. It is noted that the relationship of structure and properties of organic semiconductors has been studied through examining thienoacenes. For example, utilizing the rotation of isopropyl groups, two kind single crystals of thienoacene with different molecular conformation had been obtained. The large difference in predicted mobilities of the two single crystals demonstrate that the conformational control of the organic semiconductor is very important to achieve high charge carrier mobilities.[22]
2.2. Small-Molecule Semiconductors Containing Nitrogen Atoms The nitrogen atom is a versatile heteroatom. It can be used in its sp2 hybridization form, such as pyrrole-nitrogen and pyridine-nitrogen, sp3 and sp hybridization forms. Because the lone pair electrons of the pyrrole-nitrogen are delocalized in aromatic system, the related analogues of pentacene have high HOMO and LUMO energy levels, thus tend to exhibit p-type transport characteristics. This type of semiconductors consist of phthalocyanine, porphyrin, indole, and carbazole derivatives.[23] For example, 9[24] possesses a close π−π stacking with an 6900
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intermolecular distance of 3.38 Å. The single crystal transistors afforded a hole mobility of 0.084 cm2 V−1 s−1. However, diimide derivatives are also composed of pyrrole-nitrogen, this kind of semiconductors always display n-type characteristics which can be attributed to the stronger electronegativity of oxygen atoms making the diimide group act as a strong electron-withdrawing unit. Because the lone pair electrons of pyridine-nitrogen does not contribute to the π-electron system, and the nitrogen atom has a stronger electronegativity, the resulting semiconductors often have low HOMO and LUMO energy levels, thus could exhibit n-type characteristics. This type of semiconductors include pyrazine, oxadiazole, thiazole, and benzobis(thiadiazole) derivatives.[25] Miao et al. reported pyrazine derivative 10 (Figure 1)[26] which has a low LUMO energy level of −4.01 eV. The 10-based devices revealed high electron mobilities of 1.0−3.3 cm2 V−1 s−1, which decreased about 80−90% to 0.3–0.5 cm2 V−1 s−1 when tested in ambient air conditions. However, more often, other electron-withdrawing groups were combined with pyridine-nitrogen to get air stable n-type semiconductors.[27] Two ditrifluoromethyl-triphenodioxazines (11 and 12) were developed.[28] Molecule 11 possesses a strong π−π stacking with an intermolecular distance of 3.44 Å. The OFET devices based on compounds 11 and 12 exhibited electron mobilities of 0.07 and 0.03 cm2V−1s−1, respectively. The close molecular packing resulting from trifluoromethyl groups could prevent oxygen intrusion and could be responsible for the high stability of these OFET devices.
2.3. Small-Molecule Semiconductors Containing Halogen Atoms In stark contrast to nitrogen and chalcogen atoms, halogen atoms cannot be inserted into the π-conjugated backbone because of its strict sp3 hybridization character. As a result, halogen atoms can only be used as electron-withdrawing substituents to strengthen the intramolecular interaction and to lower the HOMO and LUMO energy levels, and thus to tune the charge carrier mobilities. The fluorine atom has an atomic radius smaller than that of hydrogen; therefore, it can be used to tune the molecular properties without significantly changing the molecular geometry. An additional function of halogen substituents is to invert the charge carrier type of semiconductors from p-type to ambipolar or n-type. Since ambipolar and n-type organic/polymeric semiconductors with high performance are relatively rare compared to their p-type counterparts, the introduction of halogen becomes an important tool in the development of n-type organic semiconductors. A typical example is 13 (Figure 1) whose evaporated film-based OFET device afforded electron mobilities of 0.11 cm2 V−1 s−1 while pentacene films revealed hole mobilities of 0.45 cm2V−1s−1 under the same conditions.[29] M. Tang et al. utilized halogen atoms to tune the molecular energy levels affording almost 20 molecules, for example, 14 and 15. Based on these molecules, they experimentally set up the correlation between charge carrier type in the OTFT geometry and the HOMO/LUMO energy levels.[30] Our group also introduced chlorine atoms to TIPS-ABT, 16.[22] The resulting TIPS-CABT, 17 has lower HOMO and LUMO energy levels with a hole
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3. Heteroatom Substituted Polymeric Semiconductors and Their Applications in FieldEffect Transistors In the past two years, great achievements have been made in heteroatom substituted polymeric semiconductors exhibiting mobilities over 10 cm2 V−1 s−1, which can be competitive with small molecular counterparts and amorphous silicon semiconductors.[32] Because heteroatom substituted polymeric semiconductors are always composed of more complex donor-acceptor (D-A) systems and often incorporated with several types of heteroatoms, it is sometimes difficult to identify the type and the position of heteroatoms that play a key role in the process of charge transport. However, one thing is certain that the effects caused by heteroatoms on polymeric semiconductors are similar to those in the small molecular systems. The application of heterocyclic electron-deficient dye, such as diketopyrrolopyrrole (DPP),[33] naphthalenediimide (NDI),[34] isoindigo (II),[35] benzobisthiadiazole (BBT),[36] benzothiadiazole (BT),[37] and benzodifurandione-PPV (BDPPV)[38] in PFETs has afforded high hole and/or electron mobilities. Particularly in DPP and BDPPV-based copolymers, the formation of intramolecular hydrogen-oxygen interactions between the oxygen atoms of DPP and hydrogen atoms of thiophene, just like conformational locks, ensures their planarity and shape persistency.[39] However, the electron-rich units are mainly limited to the thiophene-based derivatives. In 2012, our group firstly introduced an electron-donating unit, (E)-2-(2-(thiophen-2-yl)vinyl)thiophene, coupled with a DPP unit affording copolymers PDVT-8, 18 and PDVT-10, 19 (Figure 2).[40] The PFETs based on polymers 18 exhibited a high hole mobility up to 4.5 cm2 V−1 s−1 with a high current on/off ratio of 105−107. And the corresponding PFETs based on polymers 19 exhibited a high hole mobility up to 8.2 cm2 V−1 s−1
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with a high current on/off ratio of 105−107) (Figure 3a,b and c), which is one of the highest mobilities up to date in PFETs. The high performance resulted from the strong intermolecular interactions: 1) corn-leaf like interconnected networks were formed for thin films of 19 after they were annealed at 180 °C (Figure 3b); 2) ordered lamellar structures with interlayer distances of 21.11 Å, and small π−π stacking distances of 3.66 Å (Figure 3e,f). In addition, this copolymer takes predominantly edge-on orientation which has been regarded as one major reason. Subsequently, Kang et al. reported a similar DPP-based copolymer containing (E)-2-(2-(selenophen-2-yl) vinyl)selenophene, namely PDPPDTSE, 20.[41] In their work, the device-based on copolymer containing Se atoms exhibited hole mobilities higher than that of the copolymer containing S atoms under the same conditions. In particular, 20-based PFET devices showed a hole mobility value of 4.97 cm2 V−1 s−1, compared to a value of 2.77 cm2 V−1 s−1 for the counterparts containing S atoms. The improved performance was attributed to the fact that the lone pair electrons of selenophene atom are more mobile than that of thiophene, which could further enhance the interaction between neighboring polymer chains. More recently, inspired by the works on side-chain engineering, the π−conjugated backbones were further modified with a longer alkyl spacer (C6) between the branching point and the backbone.[42] The corresponding polymers P-29-DPPDBTE, 21 and P-29-DPPDTSE, 22-based devices afforded again high mobilites of 10.54 and 12.04 cm2 V−1 s−1, respectively. Such high mobilities can arise from a denser main chain packing and stronger intermolecular interaction with extended side chain. The in-plane XRD analyses showed that the π−π stacking distance was 3.62 Å for 21 and 3.58 Å for 22. These results also verified the advantages of the π−conjugated backbones and the importance of side chain as illustrated before.[35a] On the other hand, Oh et al. introduced siloxane-terminated side chains to DPP-selenophene copolymers named PTDPPSeSiC5, 23.[43] By utilizing a solution-shearing method that forms aligned nanofibrillar films, 23-based PFET device afforded high hole and electron mobilities of 8.84 and 4.8 cm2 V−1 s−1, respectively. The effective charge transport was attributed to two reasons: 1) the hybrid side chains contribute to the formation of efficient π−π stacking (π-stack distance of ∼3.6 Å) with face-on orientations and 2) the thin films adopt 3-D conduction channels that would enhance charge transport. Our group also developed a series of naphthalenediimide-based copolymers, namely PNVT-8, 24 and PNVT-10, 25.[44] With introduction of vinyl groups into polymer backbones, 24 and 25 reveal LUMO energy levels of −3.93 and −3.90 eV, and HOMO energy levels of −5.61 and −5.62 eV, respectively. 25-based devices exhibited good ambipolar characteristics in ambient conditions (20∼40% air humidity) with hole and electron mobilities of 0.30 and 1.57 cm2 V−1 s−1, respectively. GIXRG analyses showed that the strong intermolecular interactions exist in the 24 and 25 thin films. These mobilities are among the highest values observed to date for naphthalenediimide (NDI)-based polymers. As seen for PEFT, both the edge-on oriented and the face-on oriented packing modes relative to the substrates afforded high charge transport. Halogen atoms, especially fluorine atoms, are also a useful tool to tune the charge-transport behavior of polymeric
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mobility up to 0.012 cm2 V−1 s−1. Though no change of charge carrier type was found, the great improvement of hole mobility in comparison with TIPS-ABT showed that the introduction of chlorine atoms could effectively enhance the charge transport. Besides the fluorine atom itself, fluorinated alkyl chains are also used as electron withdrawing groups to change the charge carrier type from p-type to ambipolar, and even to n-type, while the performance and stability of the related devices could also be improved, most likely due to the closer π−π stacking between the molecules. Although the influence of heteroatoms on the molecular properties and field-effect transistor behavior becomes clear, there are still some challenges. One of the problems is how to selectively construct a heteroatom substituted π−conjugated backbone as desired. Because the carbons on the aromatic ring have much similar reactivity, there is no easy way to resolve it. For example, regioselective bromination, especially for large and complex aromatic systems is a useful method to construct a new heteroatom substituted π−conjugated backbone. More attention should be paid in developing new synthetic methodologies for the selective functionalization of large π−conjugated system additionally to methods that have already been introduced.[31]
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Figure 2. The chemical structure of selected polymeric semiconductors.
semiconductors. Park et al. investigated fluorinated DPP-based D-A copolymers, namely DPPPhF1–4[45] and found that as the number of fluorine substitutions increased to 4, the LUMO energy level of DPPPhF4 decreased to −4.18 eV, and the chargetransport behavior in the transistor changed from p-type to n-type. DPPPhF4 exhibited a high n-type charge-transport behavior with an electron mobility of 2.36 cm2 V−1 s−1, when the face-on orientation was favorable in the thin film. Pei et al. also reported a fluorinated isoindigo-based polymer, namely PFII2T,[46] which showed lower HOMO and LUMO energy levels than those of polymer PII2T. The PFETs based on PFII2T exhibited an electron mobility of 0.43 cm2 V−1 s−1 and high hole mobilities of 1.85 cm2 V−1 s−1.
4. Conclusions In summary, we have briefly reviewed the strong effect of heteroatoms on tuning the properties of organic/polymeric semiconductors and recent developments in heteroatom substituted organic/polymeric semiconductors, focusing especially their applications in field-effect transistors. The introduction of heteroatoms including chalcogen, nitrogen, and halogen atoms into
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the π-conjugated backbone or side chain has been proven to be a useful tool to tune the molecular geometry, the HOMO and LUMO energy levels, intermolecular interactions, and packing mode in solid state, and thus the charge injection/transporting properties, and stability of organic semiconductors. Remarkable progress has been made in developing heteroatom substituted organic/polymeric semiconductors in the past decade. Some of them exhibited high performance with mobilities exceeding 10 cm cm2/Vs. However, the overall development of organic semiconductors still lags behind the demand in the organic electronics field. For p-type semiconductors, design and synthesis of new π-conjugated systems incorporating heteroatoms is still one of the most important issues. Development of novel facile and effective synthetic methodologies related to the introduction of heteroatoms is still of major importance. For n-type semiconductors, using electron-withdrawing group/side chains containing heteroatoms to lower the LUMO energy levels, and to tune the intermolecular interactions and packing mode in solid state of high-performance p-type semiconductors is a cost-efficient approach for further advancement. Once, useful small π-conjugated systems have been developed, we can introduce them to a D-A polymeric system.
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RESEARCH NEWS Figure 3. (a) Devices structure of the polymer FETs with a PMMA encapsulation layer. (b) AFM topography images of polymer PDVT-10 films annealed at 180 °C on OTS-modified SiO2/Si substrates. (c) Output, and (d) transfer characteristics of the PDVT-10-based FET device. (e) Grazing incidence X-ray scattering pattern (out-of-plane) of polymer PDVT-8 and PDVT-10 films annealed at 180 °C. (f) In-plane X-ray scattering pattern of polymer PDVT-8 and PDVT-10 films annealed at 180 °C. Copyright 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Therefore, billions of permutation and combination of carbon, hydrogen and heteroatoms pose difficulties to discover stable organic semiconductors with high performance; however, the strong influence of heteroatoms on tuning the properties of organic/polymeric semiconductors provide a bright future for the development of the next generation of cheap and flexible organic circuits.
Acknowledgements This research was financially supported by the National Science Foundation of China: (20825208 and 21021091) and the Major State Basic Research Development Program (2011CB808403, 2011CB932303), and Chinese Academy of Sciences. Received: October 25, 2013 Revised: December 16, 2013 Published online: February 28, 2014
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Heteroatom Substituted Organic/Polymeric Semiconductors and their Applications in Field-Effect Transistors Weifeng Zhang, Yunqi Liu, and Gui Yu*
1. Introduction In this research news article, recent progresses in the development of heteroatom substituted organic/polymeric semiconductors with a highlight on the effect of heteroatoms as well as their applications in organic/polymeric field-effect transistors (O/PFETs) is presented. O/PFETs are essential building blocks for the next generation of organic circuits, which have broad potential applications, such as radio frequency identification tags, flexible displays, electronic paper, electronic skin, and sensors.[1] Compared with traditional silicon-based materials, organic/polymeric semiconductors have attracted tremendous attention because of their unique designability of molecular structures, tunability of properties, light-weight, flexibility, and transparency.[2] Since pioneer work in the 1986,[3] many organic/polymeric semiconductors have been synthesized and investigated as active materials in O/PFETs. Among them, pentacene is a benchmark material. However, because of performance limitations and deficiencies, developing new organic/polymeric semiconductors is one of the most important research activities in the past decades. The inherent properties of organic/polymeric semiconductors are the most crucial criteria for O/PFETs performances. To be specific, these properties are considered as follows: 1) Energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied ones (LUMOs).
Dr. W. Zhang, Prof. Y. Liu, Prof. G. Yu Beijing National Laboratory for Molecular Sciences and Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201305297
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Restricted by the need of a small carrier injection barrier from the sourcedrain electrodes into the semiconductor layer, organic/polymeric semiconductors should have suitable HOMO or LUMO energy levels. For example, for ptype materials, the HOMO energy level should be located at −5.1 ± 0.3 eV and for n-type materials, the LUMO energy level should be close to −4.0 eV.[4] 2) Molecular packing mode in solid state. Lamellar packing (2-D π−stacking) mode is important, which can maximize the transfer integrals and transport the charge carriers through the shortest route.[5] 3) Molecular size/weight of polymer. The related research results show that field-effect mobilities improve significantly with increasing molecular weight of polymers.[6a] Polymers with high-molecular weight tend to afford improved thinfilm morphology exhibiting reduced crystallinity and more isotropic films, potentially leading to larger values for chargecarrier mobility.[6b]
Organic/polymeric semiconductors are mainly composed of aromatic systems including phenyl, vinyl, alkynyl, thienyl, and other isoelectric groups, which are constructed of carbon, hydrogen, and so-called ‘hereroatoms’ including chalcogen, nitrogen, and halogen atoms etc. The introduction of heteroatoms could yield different electronic properties by influencing the molecular geometry, the HOMO and LUMO energy levels, intermolecular interactions and so on. In this Research News article, we provide a brief review of the effect of heteroatoms and recent developments in heteroatom substituted organic/polymeric semiconductors, focusing especially on their application in field-effect transistors.
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Nowadays, many approaches are adopted in molecular engineering to tune field-effect properties of π−conjugated systems.[7] One of appealing strategies is to develop new π− conjugated backbones with high planarity, whose essence, actually, is to obtain π−conjugated systems with close π−π stacking and strong intermolecular interaction. Secondly, the introduction of alkyl groups onto the π-conjugated backbone is promising. A suitable alkyl side chain could result in close molecular packing in solid state, and/or make semiconductors to be solution-processable. For polymers, introduction of suitable branched alkyl chains could help to increase the number of monomer units in the π-conjugated backbone of the polymer.[5] When suitable branched alkyl chains are incorporated, the synthesized polymer will have good solubility, which allows polymerization reaction continuing without precipitation. Last but not the least, the incorporation of ‘heteroatoms’, including all elements except from carbon and hydrogen such as chalcogen, nitrogen, and halogen atoms, into the π−conjugated backbone or side chain is found to be useful. This review focuses on the development of heteroatom substituted small-molecule semiconductors and their applications in field-effect transistors, taking analogues of pentacene as examples, such as small-molecule semiconductors containing chalcogen, or nitrogen, and/or halogen atoms. Moreover, heteroatom substituted polymeric semiconductors and their applications in field-effect transistors are also discussed.
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2.1. Small-Molecule Semiconductors Containing Chalcogen Atoms Organic semiconductors containing chalcogen, especially sulfur, play an important role in the development of OFETs. The fused thiophene-ring systems are named as thienoacenes.[8] Many thienoacenes could form close and ordered molecular arrangements, which result in an effectively intra-stack electronic coupling via strong intermolecular π–π, S···S, and CH···π interactions in the solid state, and thus tend to afford good charge carrier mobilties, for example benzothieno[3,2-b] benzothiophene and dinaphtho-[2,3-b:2′,3′-f]thieno[3,2-b]thiophene derivatives.[9] It is believed that the large atomic radius of sulfur leads to a stronger intermolecular interaction and the high electron densities of the sulfur atoms in the HOMO give rise to an effective overlap between the HOMOs of neighboring molecules in the solid state.
Since these thienoacenes are typically used as p-channel organic semiconductors, the active carrier is a hole, which would migrate through the HOMOs of an array of molecules in the solid state. The HOMO energy level is a very important factor in affecting the charge transport properties of p-type organic semiconductors, in addition to the geometry of HOMO and packing mode in solid state. Pentacene has a high HOMO energy level of −4.6 eV,[10] which makes it sensitive to oxygen and light. Chalcogen atoms could lower the HOMO energy level of related semiconductors, for example, anthra[2,3-b]benzo[d]thiophene (ABT), 1 (Figure 1), 6-methyl-anthra[2,3-b]benzo[d]thiophene (Me-ABT), 2,[11] and dinaphtho[2,3-b:2′,3′-d]thiophene (DNT-V), 3[12] where analogues of pentacene have one thiophene unit, show low HOMO energy levels of −5.35 and −5.68 eV, respectively. ABT and DNT−V-based OFET devices fabricated by vapor-deposited methods afforded hole mobilites of 0.41 and 1.1 cm2 V−1 s−1 (1.5 cm2 V−1 s−1 obtained in single crystal devices), respectively. These results highlighted the importance of HOMO geometry of the π-conjugated backbone exerting great influence on mobilities. Likewise, Me-ABT, 3[13] has an HOMO energy
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2. Heteroatom Substituted Small-Molecule Semiconductors and Their Applications in Field-Effect Transistors
Figure 1. The chemical structure of selected small-molecule semiconductors containing chalcogen, nitrogen, and halogen atoms.
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level of −5.34 eV. It is noted that phototransistors based on the Me-ABT single crystal showed a high mobility of 1.66 cm2 V−1 s−1, a large photoresponsivity of 12 000 A W−1, and a photocurrent/dark current ratio of 6000. An analogue of pentacene containing two thiophene rings, thienoacenes, 4 reported by Takimiya et al.[14] shows the HOMO energy level of −5.8 eV, which is about 0.4 eV lower compared to that of ABT, 1. This thienoacene exhibited low hole mobilites up to 10−2 cm2 V−1 s−1 due to a large hole injection barrier. An analogue of pentacene containing three thiophene rings, namely DBTDT, 5[15] shows a HOMO energy level of −5.6 eV. The thin film transistors based on DBTDT exhibited hole mobilities of 0.51 cm2 V−1 s−1. However, pentathienoacene (PTA), 6 containing five thiophene rings has a HOMO energy level of −5.3 eV.[16] PTA-based devices showed hole mobilities of 0.045 cm2 V−1 s−1. When analogues of pentacene that contain more thiophene rings, their HOMO energy levels show an apparent rising trend. The different trend regarding the mobilities is relatively complex because it is also affected by the molecular geometry and packing in solid state. A similar trend is also found in oligomers containing thiophene units[17] and higher thienoacenes. For example, thienoacenes containing four thiophene units, namely L-DBTTA, 7 and S-DBTTA, 8,[18] reveal HOMO energy levels of −5.62 and −5.70 eV, respectively. Compared to the HOMO energy level of −5.44 eV of DNTT,[19] these two data are obviously decreased. L-DBTTA and S-DBTTA exhibited hole mobilites of 0.15 and 0.047 cm2 V−1 s−1, respectively. Moreover, another series of analogues of pentacene end-capped by thiophene units, such as anthradithiophene[20] and dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene[21] show a decreased trend of HOMO energy levels. In contrast to thienoacenes, the counterparts containing oxygen and selenium atoms are relatively rare. One of the key reasons is that for oxygen, its stronger electronegativity makes the related π−conjugated systems stable with low aromaticity; and for selenium, the lack of methodology makes it difficult to be synthesized. It is noted that the relationship of structure and properties of organic semiconductors has been studied through examining thienoacenes. For example, utilizing the rotation of isopropyl groups, two kind single crystals of thienoacene with different molecular conformation had been obtained. The large difference in predicted mobilities of the two single crystals demonstrate that the conformational control of the organic semiconductor is very important to achieve high charge carrier mobilities.[22]
2.2. Small-Molecule Semiconductors Containing Nitrogen Atoms The nitrogen atom is a versatile heteroatom. It can be used in its sp2 hybridization form, such as pyrrole-nitrogen and pyridine-nitrogen, sp3 and sp hybridization forms. Because the lone pair electrons of the pyrrole-nitrogen are delocalized in aromatic system, the related analogues of pentacene have high HOMO and LUMO energy levels, thus tend to exhibit p-type transport characteristics. This type of semiconductors consist of phthalocyanine, porphyrin, indole, and carbazole derivatives.[23] For example, 9[24] possesses a close π−π stacking with an 6900
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intermolecular distance of 3.38 Å. The single crystal transistors afforded a hole mobility of 0.084 cm2 V−1 s−1. However, diimide derivatives are also composed of pyrrole-nitrogen, this kind of semiconductors always display n-type characteristics which can be attributed to the stronger electronegativity of oxygen atoms making the diimide group act as a strong electron-withdrawing unit. Because the lone pair electrons of pyridine-nitrogen does not contribute to the π-electron system, and the nitrogen atom has a stronger electronegativity, the resulting semiconductors often have low HOMO and LUMO energy levels, thus could exhibit n-type characteristics. This type of semiconductors include pyrazine, oxadiazole, thiazole, and benzobis(thiadiazole) derivatives.[25] Miao et al. reported pyrazine derivative 10 (Figure 1)[26] which has a low LUMO energy level of −4.01 eV. The 10-based devices revealed high electron mobilities of 1.0−3.3 cm2 V−1 s−1, which decreased about 80−90% to 0.3–0.5 cm2 V−1 s−1 when tested in ambient air conditions. However, more often, other electron-withdrawing groups were combined with pyridine-nitrogen to get air stable n-type semiconductors.[27] Two ditrifluoromethyl-triphenodioxazines (11 and 12) were developed.[28] Molecule 11 possesses a strong π−π stacking with an intermolecular distance of 3.44 Å. The OFET devices based on compounds 11 and 12 exhibited electron mobilities of 0.07 and 0.03 cm2V−1s−1, respectively. The close molecular packing resulting from trifluoromethyl groups could prevent oxygen intrusion and could be responsible for the high stability of these OFET devices.
2.3. Small-Molecule Semiconductors Containing Halogen Atoms In stark contrast to nitrogen and chalcogen atoms, halogen atoms cannot be inserted into the π-conjugated backbone because of its strict sp3 hybridization character. As a result, halogen atoms can only be used as electron-withdrawing substituents to strengthen the intramolecular interaction and to lower the HOMO and LUMO energy levels, and thus to tune the charge carrier mobilities. The fluorine atom has an atomic radius smaller than that of hydrogen; therefore, it can be used to tune the molecular properties without significantly changing the molecular geometry. An additional function of halogen substituents is to invert the charge carrier type of semiconductors from p-type to ambipolar or n-type. Since ambipolar and n-type organic/polymeric semiconductors with high performance are relatively rare compared to their p-type counterparts, the introduction of halogen becomes an important tool in the development of n-type organic semiconductors. A typical example is 13 (Figure 1) whose evaporated film-based OFET device afforded electron mobilities of 0.11 cm2 V−1 s−1 while pentacene films revealed hole mobilities of 0.45 cm2V−1s−1 under the same conditions.[29] M. Tang et al. utilized halogen atoms to tune the molecular energy levels affording almost 20 molecules, for example, 14 and 15. Based on these molecules, they experimentally set up the correlation between charge carrier type in the OTFT geometry and the HOMO/LUMO energy levels.[30] Our group also introduced chlorine atoms to TIPS-ABT, 16.[22] The resulting TIPS-CABT, 17 has lower HOMO and LUMO energy levels with a hole
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3. Heteroatom Substituted Polymeric Semiconductors and Their Applications in FieldEffect Transistors In the past two years, great achievements have been made in heteroatom substituted polymeric semiconductors exhibiting mobilities over 10 cm2 V−1 s−1, which can be competitive with small molecular counterparts and amorphous silicon semiconductors.[32] Because heteroatom substituted polymeric semiconductors are always composed of more complex donor-acceptor (D-A) systems and often incorporated with several types of heteroatoms, it is sometimes difficult to identify the type and the position of heteroatoms that play a key role in the process of charge transport. However, one thing is certain that the effects caused by heteroatoms on polymeric semiconductors are similar to those in the small molecular systems. The application of heterocyclic electron-deficient dye, such as diketopyrrolopyrrole (DPP),[33] naphthalenediimide (NDI),[34] isoindigo (II),[35] benzobisthiadiazole (BBT),[36] benzothiadiazole (BT),[37] and benzodifurandione-PPV (BDPPV)[38] in PFETs has afforded high hole and/or electron mobilities. Particularly in DPP and BDPPV-based copolymers, the formation of intramolecular hydrogen-oxygen interactions between the oxygen atoms of DPP and hydrogen atoms of thiophene, just like conformational locks, ensures their planarity and shape persistency.[39] However, the electron-rich units are mainly limited to the thiophene-based derivatives. In 2012, our group firstly introduced an electron-donating unit, (E)-2-(2-(thiophen-2-yl)vinyl)thiophene, coupled with a DPP unit affording copolymers PDVT-8, 18 and PDVT-10, 19 (Figure 2).[40] The PFETs based on polymers 18 exhibited a high hole mobility up to 4.5 cm2 V−1 s−1 with a high current on/off ratio of 105−107. And the corresponding PFETs based on polymers 19 exhibited a high hole mobility up to 8.2 cm2 V−1 s−1
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with a high current on/off ratio of 105−107) (Figure 3a,b and c), which is one of the highest mobilities up to date in PFETs. The high performance resulted from the strong intermolecular interactions: 1) corn-leaf like interconnected networks were formed for thin films of 19 after they were annealed at 180 °C (Figure 3b); 2) ordered lamellar structures with interlayer distances of 21.11 Å, and small π−π stacking distances of 3.66 Å (Figure 3e,f). In addition, this copolymer takes predominantly edge-on orientation which has been regarded as one major reason. Subsequently, Kang et al. reported a similar DPP-based copolymer containing (E)-2-(2-(selenophen-2-yl) vinyl)selenophene, namely PDPPDTSE, 20.[41] In their work, the device-based on copolymer containing Se atoms exhibited hole mobilities higher than that of the copolymer containing S atoms under the same conditions. In particular, 20-based PFET devices showed a hole mobility value of 4.97 cm2 V−1 s−1, compared to a value of 2.77 cm2 V−1 s−1 for the counterparts containing S atoms. The improved performance was attributed to the fact that the lone pair electrons of selenophene atom are more mobile than that of thiophene, which could further enhance the interaction between neighboring polymer chains. More recently, inspired by the works on side-chain engineering, the π−conjugated backbones were further modified with a longer alkyl spacer (C6) between the branching point and the backbone.[42] The corresponding polymers P-29-DPPDBTE, 21 and P-29-DPPDTSE, 22-based devices afforded again high mobilites of 10.54 and 12.04 cm2 V−1 s−1, respectively. Such high mobilities can arise from a denser main chain packing and stronger intermolecular interaction with extended side chain. The in-plane XRD analyses showed that the π−π stacking distance was 3.62 Å for 21 and 3.58 Å for 22. These results also verified the advantages of the π−conjugated backbones and the importance of side chain as illustrated before.[35a] On the other hand, Oh et al. introduced siloxane-terminated side chains to DPP-selenophene copolymers named PTDPPSeSiC5, 23.[43] By utilizing a solution-shearing method that forms aligned nanofibrillar films, 23-based PFET device afforded high hole and electron mobilities of 8.84 and 4.8 cm2 V−1 s−1, respectively. The effective charge transport was attributed to two reasons: 1) the hybrid side chains contribute to the formation of efficient π−π stacking (π-stack distance of ∼3.6 Å) with face-on orientations and 2) the thin films adopt 3-D conduction channels that would enhance charge transport. Our group also developed a series of naphthalenediimide-based copolymers, namely PNVT-8, 24 and PNVT-10, 25.[44] With introduction of vinyl groups into polymer backbones, 24 and 25 reveal LUMO energy levels of −3.93 and −3.90 eV, and HOMO energy levels of −5.61 and −5.62 eV, respectively. 25-based devices exhibited good ambipolar characteristics in ambient conditions (20∼40% air humidity) with hole and electron mobilities of 0.30 and 1.57 cm2 V−1 s−1, respectively. GIXRG analyses showed that the strong intermolecular interactions exist in the 24 and 25 thin films. These mobilities are among the highest values observed to date for naphthalenediimide (NDI)-based polymers. As seen for PEFT, both the edge-on oriented and the face-on oriented packing modes relative to the substrates afforded high charge transport. Halogen atoms, especially fluorine atoms, are also a useful tool to tune the charge-transport behavior of polymeric
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mobility up to 0.012 cm2 V−1 s−1. Though no change of charge carrier type was found, the great improvement of hole mobility in comparison with TIPS-ABT showed that the introduction of chlorine atoms could effectively enhance the charge transport. Besides the fluorine atom itself, fluorinated alkyl chains are also used as electron withdrawing groups to change the charge carrier type from p-type to ambipolar, and even to n-type, while the performance and stability of the related devices could also be improved, most likely due to the closer π−π stacking between the molecules. Although the influence of heteroatoms on the molecular properties and field-effect transistor behavior becomes clear, there are still some challenges. One of the problems is how to selectively construct a heteroatom substituted π−conjugated backbone as desired. Because the carbons on the aromatic ring have much similar reactivity, there is no easy way to resolve it. For example, regioselective bromination, especially for large and complex aromatic systems is a useful method to construct a new heteroatom substituted π−conjugated backbone. More attention should be paid in developing new synthetic methodologies for the selective functionalization of large π−conjugated system additionally to methods that have already been introduced.[31]
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Figure 2. The chemical structure of selected polymeric semiconductors.
semiconductors. Park et al. investigated fluorinated DPP-based D-A copolymers, namely DPPPhF1–4[45] and found that as the number of fluorine substitutions increased to 4, the LUMO energy level of DPPPhF4 decreased to −4.18 eV, and the chargetransport behavior in the transistor changed from p-type to n-type. DPPPhF4 exhibited a high n-type charge-transport behavior with an electron mobility of 2.36 cm2 V−1 s−1, when the face-on orientation was favorable in the thin film. Pei et al. also reported a fluorinated isoindigo-based polymer, namely PFII2T,[46] which showed lower HOMO and LUMO energy levels than those of polymer PII2T. The PFETs based on PFII2T exhibited an electron mobility of 0.43 cm2 V−1 s−1 and high hole mobilities of 1.85 cm2 V−1 s−1.
4. Conclusions In summary, we have briefly reviewed the strong effect of heteroatoms on tuning the properties of organic/polymeric semiconductors and recent developments in heteroatom substituted organic/polymeric semiconductors, focusing especially their applications in field-effect transistors. The introduction of heteroatoms including chalcogen, nitrogen, and halogen atoms into
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the π-conjugated backbone or side chain has been proven to be a useful tool to tune the molecular geometry, the HOMO and LUMO energy levels, intermolecular interactions, and packing mode in solid state, and thus the charge injection/transporting properties, and stability of organic semiconductors. Remarkable progress has been made in developing heteroatom substituted organic/polymeric semiconductors in the past decade. Some of them exhibited high performance with mobilities exceeding 10 cm cm2/Vs. However, the overall development of organic semiconductors still lags behind the demand in the organic electronics field. For p-type semiconductors, design and synthesis of new π-conjugated systems incorporating heteroatoms is still one of the most important issues. Development of novel facile and effective synthetic methodologies related to the introduction of heteroatoms is still of major importance. For n-type semiconductors, using electron-withdrawing group/side chains containing heteroatoms to lower the LUMO energy levels, and to tune the intermolecular interactions and packing mode in solid state of high-performance p-type semiconductors is a cost-efficient approach for further advancement. Once, useful small π-conjugated systems have been developed, we can introduce them to a D-A polymeric system.
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RESEARCH NEWS Figure 3. (a) Devices structure of the polymer FETs with a PMMA encapsulation layer. (b) AFM topography images of polymer PDVT-10 films annealed at 180 °C on OTS-modified SiO2/Si substrates. (c) Output, and (d) transfer characteristics of the PDVT-10-based FET device. (e) Grazing incidence X-ray scattering pattern (out-of-plane) of polymer PDVT-8 and PDVT-10 films annealed at 180 °C. (f) In-plane X-ray scattering pattern of polymer PDVT-8 and PDVT-10 films annealed at 180 °C. Copyright 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Therefore, billions of permutation and combination of carbon, hydrogen and heteroatoms pose difficulties to discover stable organic semiconductors with high performance; however, the strong influence of heteroatoms on tuning the properties of organic/polymeric semiconductors provide a bright future for the development of the next generation of cheap and flexible organic circuits.
Acknowledgements This research was financially supported by the National Science Foundation of China: (20825208 and 21021091) and the Major State Basic Research Development Program (2011CB808403, 2011CB932303), and Chinese Academy of Sciences. Received: October 25, 2013 Revised: December 16, 2013 Published online: February 28, 2014
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