Organic Donor–Acceptor Complexes as Novel Organic Semiconductors

Jun 13, 2017 - ... University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China ... Energy Interconnection Research Institute, Beij...
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Organic Donor−Acceptor Complexes as Novel Organic Semiconductors Jing Zhang,† Wei Xu,*,‡,∥ Peng Sheng,§ Guangyao Zhao,§ and Daoben Zhu*,‡,∥ †

Key Laboratory for Organic Electronics and Information Displays and Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Material Laboratory of State Grid Corporation of China, State Key Laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute, Beijing 102211, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China CONSPECTUS: Organic donor−acceptor (DA) complexes have attracted wide attention in recent decades, resulting in the rapid development of organic binary system electronics. The design and synthesis of organic DA complexes with a variety of component structures have mainly focused on metallicity (or even superconductivity), emission, or ferroelectricity studies. Further efforts have been made in high-performance electronic investigations. The chemical versatility of organic semiconductors provides DA complexes with a great number of possibilities for semiconducting applications. Organic DA complexes extend the semiconductor family and promote charge separation and transport in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In OFETs, the organic complex serves as an active layer across extraordinary charge pathways, ensuring the efficient transport of induced charges. Although an increasing number of organic semiconductors have been reported to exhibit good p- or n-type properties (mobilities higher than 1 or even 10 cm2 V−1 s−1), critical scientific challenges remain in utilizing the advantages of existing semiconductor materials for more and wider applications while maintaining less complicated synthetic or device fabrication processes. DA complex materials have revealed new insight: their unique molecular packing and structure−property relationships. The combination of donors and acceptors could offer practical advantages compared with their unimolecular materials. First, growing crystals of DA complexes with densely packed structures will reduce impurities and traps from the self-assembly process. Second, complexes based on the original structural components could form superior mixture stacking, which can facilitate charge transport depending on the driving force in the coassembly process. Third, the effective use of organic semiconductors can lead to tunable band structures, allowing the operation mode (p- or n-type) of the transistor to be systematically controlled by changing the components. Finally, theoretical calculations based on cocrystals with unique stacking could widen our understanding of structure− property relationships and in turn help us design high-performance semiconductors based on DA complexes. In this Account, we focus on discussing organic DA complexes as a new class of semiconducting materials, including their design, growth methods, packing modes, charge-transport properties, and structure−property relationships. We have also fabricated and investigated devices based on these binary crystals. This interdisciplinary work combines techniques from the fields of self-assembly, crystallography, condensed-matter physics, and theoretical chemistry. Researchers have designed new complex systems, including donor and acceptor compounds that self-assemble in feasible ways into highly ordered cocrystals. We demonstrate that using this crystallization method can easily realize ambipolar or unipolar transport. To further improve device performance, we propose several design strategies, such as using new kinds of donors and acceptors, modulating the energy alignment of the donor (ionization potential, IP) and acceptor (electron affinity, EA) components, and extending the π-conjugated backbones. In addition, we have found that when we use molecular “doping” (2:1 cocrystallization), the charge-transport nature of organic semiconductors can be switched from hole-transport-dominated to electron-transport-dominated. We expect that the formation of cocrystals through the complexation of organic donor and acceptor species will serve as a new strategy to develop semiconductors for organic electronics with superior performances over their corresponding individual components.

1. INTRODUCTION Organic donor−acceptor complexes are crucial tools for the construction and regulation of the electronic structures of © 2017 American Chemical Society

Received: March 13, 2017 Published: June 13, 2017 1654

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Accounts of Chemical Research 2.1. Design of DA complexes

organic functional materials. In early stages of research, studies mostly focused on high conductivity or superconductivity originating from organic charge-transfer (CT) salts.1,2 Recently, donor−acceptor supramolecular cocrystals have been shown to be good candidates for organic ferroelectrics because of the possible long-range orientation of their CT dipoles existing in highly ordered networks.3,4 Moreover, ferroelectrics with Curie temperatures above room temperature can also be obtained. Besides, composite organic structures formed by the combination of two or three components have been observed to be highly emissive and sometimes can be tailored.5 For organic semiconductor applications, organic complexes have only gained attention from researchers very recently. Benefiting from molecular-structure design and processingtechnique improvements, significant progress in organic semiconductors has been made, for both p- and n-type transport.6,7 However, a large number of organic semiconductors are faced with relatively poor performance (mobilities lower than 0.01 or even 0.001 cm2 V−1 s−1). In addition, ambipolar semiconductors and n-type materials with good performance are still rare.8 There are critical problems due to the high energies of the lowest unoccupied molecular orbitals (LUMOs), which easily result in degradation or even absence of electron-transport characteristics under ambient conditions. In view of the complicated synthesis procedure, a novel easy-to-process or “green” approach to produce efficient ambipolar or electron transport is highly demanded. DA complexes, obtained from facile mixing processes, have rarely been investigated as semiconductors. With physical properties significantly different from the corresponding pristine components, organic complexes have only recently been identified for use as organic semiconductors.9 The ambipolar transport behavior DA complex (BEDTTTF)(F 2 TCNQ) (BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene; F2TCNQ is 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane) at low temperature was first reported in 2004 by Hasegawa et al.10 Then Brédas and co-workers first predicted the remarkable ambipolar charge-transport characteristics in mixed-stack CT complex crystals in 2012, showing that they could rival or even surpass those in the best single-component organic crystals.11 To date, a series of organic DA complexes with different alternative stacking modes have been prepared to generate a strong superexchange effect (the electronic coupling for holes/electrons results from the mixing of the frontier orbitals of two closest donor/acceptor molecules with the orbitals of the “bridging” acceptor/donor molecule) and/or good energy level alignments that facilitate charge transport via varied self-organization methods. In this Account, we focus on the design, synthesis, packing mode, and structure−property relationship of recently studied DA complexes as novel organic semiconductors. The p-type, ambipolar, or n-type charge-transport properties of this complex family are presented. The photoresponsive behaviors and photovoltaic properties of DA complexes are also mentioned, as such cocrystal complexes can be viewed as ideal bulk heterojunctions with molecular-level well-defined structures.

The control of the supramolecular organization by varying the πconjugated backbone, symmetry and energy levels (the ionization energy of the donor and the electron affinity of the acceptor) of the constituents has been shown to be a key factor affecting the intermolecular interactions, frontier orbital overlaps and electronic couplings.13 Generally, supramolecular nonbonding interactions, including π−π-stacking, CT, and halogen- and hydrogen-bonding interactions, act as driving forces in the assembly of organic molecules. As one of the most investigated acceptor systems (Figure 1), TCNQ and its

Figure 1. Typical acceptor molecules used in the construction of DAcomplex-type semiconductors.

derivatives have been identified as one of the best components for the construction of DA complexes for semiconducting applications because of their planar configuration, good crystallinity, and tunable energy level.14 Consequently, the resulting complexes adopt tightly packed structures in the solid state, which provide an ideal material system for investigation of the structure−semiconducting property relationship, as the tailored LUMO energy leads to a diverse CT ratio between the donor and acceptor molecules and a systemic tunable molecular interaction and packing in the solid state. Naphthalenediimide (NDI) and perylenediimide (PDI) derivatives have been utilized as components, as their coplanar π-conjugated cores facilitate strong intermolecular π−π interactions. These components have the potential to form hydrogen bonds through their oxygen atoms, which allows for tailoring of the packing modes of these molecular-level heterojunctions through the construction and stabilization of hybrid networks and enables tuning of the electronic structure by charge redistribution.15−17 Nonplanar fullerenes have been utilized to construct many host−guest

2. ORGANIC DONOR−ACCEPTOR COMPLEXES DA complexes are molecular solids composed of two types of organic chemical species (the donor and the acceptor); they are not just collections of molecules. Binary component systems are the simplest and most studied systems because of their welldefined structure and relatively easy to control crystal growth as well as tunable physical properties.12 1655

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Accounts of Chemical Research binary frameworks too. Here donor molecules are attracted to the bent convex surface that enhances the π−π interactions, and hence, the assembly ability of the components is improved.18,19 Donor molecules such as acenes, oligothiophenes, and TTF derivatives are able to easily form complexes with fullerenes because of their extended π-conjugated systems, strong electrondonating ability, and π−π-stacking interactions, which enable them to self-organize with fullerenes. Another strategy to realize complexes for organic semiconductors is to use a donor and acceptor with isometric structure, such as a complex with 1,4-bis(3,5-dimethylstyryl)benzene (4M-DSB) as the donor and isometric (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile) (CN-TFPA) as the acceptor. Isometric DA pairs with rationally designed substituents can give rise to the most tightly packed DA complex with a short π-stacking distance (3.36 Å).20

Figure 3. Schematic representations of DA complexes with different structural modes. The red and blue rectangles represent the donor and acceptor constituents, respectively.

2.2. Synthesis Methods for DA Complexes

The growth methods for joining different elements into a binary system are generally classified into two pathways: (i) The cosublimation method uses physical vapor transport to orderly pack small molecules in a certain ratio from the vapor phase (Figure 2a). The obtained complex is often of high quality and

packings of the single-crystal structures from single-crystal X-ray diffraction measurements. Common binary DA compounds with a 1:1 stoichiometry are primarily divided into mixed-stack (MS) or segregated-stack (SS) systems. In MS systems, the donor and acceptor molecules alternate along the stacking direction (−D−A−D−A−), forming MS with DA-type alternating π columns (1).26 Although the nearest-neighbor D (or A) molecules in the stacking column are too far away from each other to interact directly, superexchange effects result from the hybridization of the frontier orbitals of the two closest donor molecules with the orbitals of the “bridging” acceptor molecule; therefore, electrons promote both hole and electron transport along the alternatingly stacked DA columns.27 Meanwhile, in SS systems, the donor and acceptor molecules pack into columns separately (−A−A−A− and −D−D−D−),28 which further pack alternatingly in one plane to form two-dimensional (2D) layers or form donor and acceptor planes separately, resulting in alternating lamina structures (2). In this case, the individual electron- and hole-transport pathways in the cocrystals provide transport networks and determine effective transfer integrals, respectively. Other stoichiometries of DA compounds (generally 2:1 or 3:1) also exhibit semiconducting properties. For 2:1 compounds, the ADDADDADDA arrangement allows donors and acceptors to pack in close contact (face-to-face interactions) on either side of the acceptors with separations between the two donors (3).29 Another 2:1 arrangement is composed of −DD− DD−DD− and −A−A−A− stacks with intermolecular interactions and two separate adjacent D and A columns (4).15 They prefer to grow along 2D networks, where their effective transfer integrals account for their own packing domains of intermolecular π−π interactions. The third crystal structure of a 2:1 complex consists of MS arrays along the π-stacking orientations, with an additional donor molecule inserted between adjacent stacks (5).23 For the 3:1 complexes reported to date,30 one acceptor interlaces with three donors, and all of the donors have direct interactions with the middle acceptor molecule via face-toface or face-to-edge links, building a highly ordered supramolecular system (6).

Figure 2. Growth methods for DA complex crystals: (a) cosublimation process showing a physical vapor transport technique; (b) solution processes showing the evaporation of a solvent, cooling of a solution, and diffusion of mixed solvents.

clearly distinguished from the starting materials by the shape and color.21,22 However, it is difficult to maintain the stoichiometry of the participating organic compounds when several ratios of binary compounds are produced, which often results in a new phase of the complex compound.23,24 (ii) Solution-processed self-assembly methods usually involve cooling of a solution, evaporation of a solvent, or diffusion of mixed solvents (Figure 2b). Compared with the cosublimation process, solution processes are easier to carry out and control. At the same time, they can sometimes cause doping of solvent molecules into the organic binary system, which may then influence the intrinsic interactions of the organic semiconductor molecules and the performance of their devices.25 By variation of the mixing ratio and growth conditions, the intrinsic supramolecular structure of the complex can be finely tuned.

3. DA COMPLEX DEVICE PERFORMANCE Systematic investigations have been made on organic DA complexes utilizing the corresponding single crystals as powerful tools to reveal the structure−property relationships of this emerging multifunctional system.31 The techniques to grow organic micro/nano-cocrystals for single-crystal field-effect

2.3. DA Complex Stacking Modes

Schematic representations of the DA complex stacking motifs in structural types are depicted in Figure 3, according to molecular 1656

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molecules was observed in the crystal, the band structure indicated that there was a large dispersion of the valence band (VB) and conduction band (CB) along the stacking direction. We used a drop-casting method to prepare microcrystals of the DPTTA−TCNQ complex, which grew along the MS direction into a ribbon-shaped structure. The micro-cocrystal-based device showed air-stable balanced hole- and electron-transport behaviors in ambient atmosphere with mobilities of 0.03 and 0.04 cm2 V−1 s−1 (on/off current ratio of ∼102) for the holes and electrons, respectively (Figure 5b). Quantum simulations were employed for further investigations into the transport properties of this MS DA complex, and a molecular orbital splitting method was applied to address the superexchange effects. The results (Figure 5c) showed that the acceptor (donor) bridge contributed to the electronic coupling between the donor (acceptor) molecules and effective transfer integrals along the stacking direction, which provided both electron- and hole-transport channels. Interestingly, the corresponding characterization revealed that the degree of charge transfer in this complex could be neglected. For DA complexes with their components in neutral states, it is easy to tune the band gap by choosing appropriate molecules to meet the demands, as the highest occupied molecular orbital (HOMO) of the donor determines the top of the VB and the LUMO of the acceptor dominates the bottom of the CB. The transfer integral values are known to be strongly dependent on the degree of orbital overlap and correlated with the effective mass of the charge carrier. As charge transfer between the donor and acceptor molecules acts as a strong driving force to promote intermolecular orbital overlap and to tune the energy levels of the resulting complexes simultaneously, the ubiquity of CT in DA complexes affords great opportunities for the development of organic semiconductors through these bimolecular frameworks. In the following work, we prepared an extended-π-conjugated acceptor (the TCNQ derivative 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo[1,2-b:4,5-b′]-dithiophene, DTTCNQ) to complex with DPTTA.35 In the resulting 1:1 cocrystal, the donor and acceptor stacked alternatingly along the c axis with an intermolecular distance between the adjacent DPTTA and DTTCNQ of approximately 3.34 Å. Compared with DPTTA−TCNQ, weak CT interactions and direct lateral π−π interactions among the donor or acceptor molecules in neighboring columns were found in this cocrystal.

transistor (SCFET) fabrications have mainly focused on solution-processed procedures, including drop-casting assembly, solvent-vapor annealing, and solvent diffusion,32 which provide a facile method for the in situ growth of molecular cocrystals (even with low solubility). For example, drop-casting is performed by pouring a solution containing the donor and acceptor at a specific molar ratio, and the organic components cocrystallize directly onto the substrate (Figure 4). On the other hand, vapor-growth

Figure 4. Schematic representation of drop-casting assembly of microribbons of DPTTA−TCNQ cocrystals.

processes, such as physical vapor transport, are good methods to produce high-quality cocrystals. This method has been employed in the growth of TCNQ- and FnTCNQ-based cocrystals. In addition, vacuum coevaporation has been conducted to achieve large-area thin-film growth. Oriented polycrystalline thin films of the complexes have been prepared by coevaporation of the component donor and acceptor molecules.33,34 3.1. 1:1 Mixed-Stack Complexes

Most 1:1 MS complexes tend to organize into one-dimensional (1D) single crystals because of their strong 1D intermolecular π−π interactions. The macrocyclic compound meso-diphenyltetrathia[22]annulene[2,1,2,1] (DPTTA) with four thiophene groups forming a coplanar aromatic core (Figure 5a) was utilized in the design of DA complexes. DPTTA and TCNQ stack alternatingly into a 1D column structure along the a axis. The intermolecular distance is approximately 3.4 Å, showing a strong nonbonding interaction between DPTTA and TCNQ. Although no continuous, separate π stacking of donor or acceptor

Figure 5. (a) Chemical structure of DPTTA and the alternating stacking pattern of the DPTTA−TCNQ complex. (b) Transfer and output characteristics of the device. (c, d) Transfer integrals along the stacking direction based on an energy-splitting method: teff 1 = (ELUMO+1 − ELUMO)/2 = 63 meV; teff 2 = (EHOMO − EHOMO−1)/2 = 62 meV. 1657

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reorganization energy. Meanwhile, the molecular-scale p−n junction formed by self-assembly was reported to have a drastically enhanced power conversion efficiency in solution-processed solar cells.39

Devices based on the self-assembled microcrystals, prepared by a simple drop-casting method, showed remarkable airstable ambipolar behavior with a superior hole mobility of 0.77 cm2 V−1 s−1 and electron mobility of 0.24 cm2 V−1 s−1 in ambient atmosphere and Ion/Ioff ratios of 1.5 × 103 and 5 × 102, respectively. This improvement is due to the quasi-2D ambipolar transport network formed by the intermolecular electronic coupling introduced by the superexchange effect along the stacking direction and by the direct coupling along the perpendicular direction (Figure 6). In addition to the extension of the

3.2. 1:1 Segregated Stacking Complexes

SS cocrystals, where donors and acceptors form molecular-levelordered heterojunctions with perfect interlocked arrangement, can serve as an ideal model system for studies of charge separation and transport, which will help us understand the physical processes involved in OPVs and further expand the suite of properties resulting from this unique architecture. We reported the first ideal 2D SS fullerene-based cocrystals using DPTTA as the donor molecule.40 In the complex crystals (C60−DPTTA and C70−DPTTA), the DPTTA conformation slightly deviated from planarity with half of the molecule bent up and the other half bent down to fit to the curved surface of the fullerenes. The alternating stacking of the DPTTAs and fullerenes formed a linear column structure, as shown in Figure 7a, and nonbonding interactions were observed between the donor and acceptor molecules along these columns. In the fullerene or TPTTA layer, non-covalent intermolecular interactions were observed, which showed efficient π−π interactions. These features are unambiguously different from other fullerenebased supermolecular architectures ever reported.18 Plateshaped microcrystals of fullerene−TPTTA complexes were grown via a drop-casting method, in which the donor and acceptor molecules alternatingly stacked layer-by-layer over the substrates (Figure 7b). The cocrystals exhibited typical ambipolar transport behavior according to the corresponding SCFET characterization, especially for C70−TPTTA, which showed balanced ambipolar transport properties of μe = 0.05 cm2 V−1 s−1 and μh = 0.07 cm2 V−1 s−1 in vacuum and on/off current ratios of ∼102 (Figure 7c). Theoretical calculations illustrated that no noticeable superexchange interactions existed in this SS system and illustrated the CT capability of the holes and electrons derived from direct electronic couplings in the corresponding donor or acceptor layers. The photovoltaic properties of these crystals were then studied (Figure 8),41 and an overall power conversion efficiency (PCE) of 0.27% for the C60−DPTTA cocrystal solar cell was observed. This was the first bulk-heterojunction solar cell with a well-defined molecular-scale structure, which provides an ideal model system for quantum simulations of OPVs. Studies have shown that intermolecular hydrogen bonding can enhance π−π interactions in the solid state and tune the dual channel of donor−acceptor organic semiconductors. For example, when diphenyldipyrollopyridine (DP-P2P) was used as a p-type semiconductor with an integrated hydrogenbonding moiety combined with NDIs, n-type semiconductors were formed.15 DP-P2P−NDI-81 obtained from a vapor-growth method showed an SS structure with alternating dimerized (DD···AA···DD···AA) columns defined as the p−n junction. Adjacent donor columns stacked in an edge-to-face herringbone mode, while the coparallel NDI columns formed isolated 1D transport pathways due to the separation of the alkyl chains. The corresponding cocrystal transistors showed relatively balanced ambipolar transport properties with hole and electron mobilities of 0.043 and 0.089 cm2 V−1 s−1, respectively. When the N-substituent on the NDI was changed to a bulkier secondary alkyl group (i.e., cyclohexyl, CyHex), the DP-P2P−NDI-CyHex cocrystals adopted an MS arrangement from an increased steric effect. However, the distortion of DP-P2P caused the hole mobility to be much lower than the electron mobility. These

Figure 6. Most important charge-transport pathways and electronic couplings for holes and electrons. (a) Hole: tdirect = 41.4 meV and tsuper = 23.4 meV. (b) Electron: tdirect = 26.4 meV and tsuper = 53.6 meV, indicating a quasi-2D ambipolar transport network.

π-conjugated system of the corresponding donor and acceptor molecules, tuning the CT ratio by adjusting the IP or EA value is another efficient technique to modulate the superexchange effect and hence the electrical transport properties. Therefore, TCNQ derivatives with different electron-deficient substituents, including 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), and 1,2,5-thiadiazolotetracyanoquinodimethane (SNTCNQ), were investigated to construct 1:1 complexes with DPTTA. The corresponding complexes all cocrystallized in an MS mode along the a axis. As the CT increased, distortion in the conformation became more significant. Accordingly, the superexchange coupling increased and reached a value of 85−91 meV in the mixed stack of F4TCNQ−DPTTA and F2TCNQ−DPTTA. These cocrystal devices exhibited good stability in air even after months of storage in air. Experimentally, the hole and electron field-effect mobilities were calculated to be as high as 1.57 and 0.47 cm2 V−1 s−1, respectively, for the F2TCNQ−DPTTA cocrystal. A clear tendency was observed where the superexchange-induced electronic coupling increased with the degree of intermolecular CT of the DA pairs. Unipolar transport properties were also expected in the bicomponent materials. Alkyl-substituted benzothienobenzothiophene (BTBT) derivatives (diCmBTBT) can form CT complexes with TCNQ and F nTCNQ.36 The single-crystal transistors showed n-type behavior for diC8BTBT−F2TCNQ and diC8BTBT−F4TCNQ, whereas ambipolar operation was observed for diC8BTBT−TCNQ, indicating that p- and n-type behavior can be tuned by altering the fluorine substitution in TCNQ (energy level alignment). In addition to the ambipolar transport and intense emission, MS CT complexes have also been regarded as model systems for studies of photocarrier generation and transport phenomena, which are key steps in DA-copolymer-based organic photovoltaic (OPV) devices.37,38 Tsutsumi and co-workers found that the diffusion length of the photocarriers strongly depended on the CT energy gap (larger than 10 μm when the CT energy gap was larger than 0.7 eV) and that electron−hole recombination dominated when the CT energy gap was as small as the molecular 1658

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Figure 7. (a) ORTEP diagrams and stacking patterns viewed along the b axis for C70−DPTTA. (b) Optical micrograph and schematic diagram of microcrystals of a self-assembled C70−DPTTA cocrystal with alternating single n-channel and p-channel layers. (c) Transfer characteristics of a C70−DPTTA nanosheet-based device. (d) Charge-hopping pathways in the fullerene and DPTTA layers.

bonds with acceptor molecules. The donor cores packed in a face-to-face pattern with an interplanar distance of 3.373 Å, indicating the presence of π−π interactions between the donor molecules. However, no effective π−π overlap between the DPNDIs was observed. As expected, only hole-transport activity was observed with mobility (μ) as high as 1.8 cm2 V−1 s−1 with a high on/off ratio above 104 under ambient conditions along the b axis, as revealed by SCFET (Figure 9c). Quantum calculations demonstrated a large dispersion from the stacking direction and neglectable contribution from all other directions, consistent with the experimental results. Additionally, no electron-transport pathway or superexchange contribution to the electronic coupling existed in this complex. Here, the acceptor acted as “a good assistant” to confine the stacking of donor molecules, which led to an enhancement of the hole-transport performance compared to pristine DPPTA, showing an example of “cocrystal engineering.”

Figure 8. (a) Schematic diagram and (b) scanning electron microscopy image of the photovoltaic device based on an individual single cocrystal. (c, d) J−V curves of devices based on (c) C60−DPTTA and (d) C70− DPTTA.

3.3. Nonequal-Ratio Complexes

Reports on functional complexes with different ratios have been much more infrequent than those on 1:1 complexes. One possible reason for this is that an imbalanced proportion of the components may disturb the inherent balanced intermolecular interactions and result in disordered stacking, which could prevent continuous transport channels. Hence, the synthesis and investigation of functional nonequal-ratio cocrystals are highly relevant and challenging for the development of complex-based electronics. The asymmetric donor−acceptor conjugated molecule 2,7-ditert-butyl-10,14-bis(thiophen-2-yl)phenanthro[4,5-abc][1,2,5]thiadiazolo[3,4-i]phenazine (DTPTP) was employed as the p-type semiconductor to generate the trimeric complex DTPTP2−TCNQ, in which TCNQ with a smaller size was found to be sandwiched between dimerized DTPTP molecules (Figure 10a).42 The 2:1 cocrystal adopted a tight DDADDADDA mixed linear column structure with the donors in an antiparallel cofacial stacking mode and exhibited air stable n-type behavior (Figure 10b). Quantum calculations indicated that large transfer integrals in the transport route between the DTPTP−

results demonstrated a practical approach to control the structure of the donor−acceptor blends and hence control their semiconducting performance. The SS complexes are mainly categorized into two groups: coplanar dual channels or alternating-stack donor and acceptor layers. Unipolar transport based on SS modes has also been realized. For example, the weaker electron acceptor N,N′bis(phenyl)naphthalene-1,4,5,8-bis(dicarboximide) (DPNDI) formed a donor−acceptor complex with DPTTA by slow evaporation of a solution of the corresponding mixture.16 X-ray analysis of the cocrystal showed that DPTTA and DPNDI molecules stacked into separate columns along the b axis and that alternating heterojunctions formed by these donor and acceptor columns packed along the ab plane into 2D layers (Figure 9a). Unexpectedly, the solvent molecules remained in the space between the column layers, leading to intermolecular hydrogen 1659

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Figure 9. (a) SS pattern of DPTTA and DPNDI along the b and c axes. (b) Optical micrograph and schematic diagram of microcrystals of DPTTA− DPNDI showing self-assembled alternating n and p channels. (c) Transfer and output characteristics of the device. (d) Band structures of the crystal.

Figure 10. (a) Chemical structure of DTPTP and the mixed linear pattern between 2DTPTP and TCNQ along the a axis. (b) Transfer and output characteristics of the cocrystal device. (c, d) HOMO and LUMO electron density distributions for (c) the DTPTP−TCNQ dimer along the π−πstacking direction and (d) the DTPTP−DTPTP dimer in adjacent columns.

TCNQ (DA) dimers along the π−π-stacking column and the DTPTP−DTPTP (DD) dimers in adjacent columns with five-membered rings approaching each other built a quasi-2D electron-transport network (Figure 10c,d) because of the much smaller transfer integral between DD dimers in the same column. Other than the mixed linear columnar structure, a segregated columnar 2:1 architecture was built using unsubstituted NDI-H2 to form a 2:1 donor−acceptor DP-P2P−NDI-H2 system via physical vapor transport (PVT).15 DP-P2P−NDI-H2 segregated donor/acceptor channels were composed of herringbone-packed donor columns and slipped π-stacked acceptor columns. This 2:1 cocrystal displayed unipolar electron-transport properties (μe = 0.00034 cm2 V−1 s−1). In a complex of perylene (P) and TCNQ (T) with a stoichiometry of 2:1 obtained via the PVT method, which is assigned to the third group,23 the interlacing supramolecular structure of P2T1 included MS arrays along the a axis similar to those of a 1:1 complex (P1T1) (Figure 11a), and an additional perylene molecule was inserted between the mixed stacks (Figure 11b). Ambipolar charge-transport characteristics with similar electron and hole mobilities of μe = 2.9 × 10−5 cm2 V−1 s−1 and μh = 7.4 × 10−5 cm2 V−1 s−1 were characterized for the P2T1 cocrystals using Ag electrodes for efficient electronand hole-transport pathways. Through either solution or vapor

processes, the 3:1 stoichiometric cocrystal P3T1 was obtained, which displayed a molecular structure that consisted of donor− acceptor DDA stacks (−DDADDA−) along the c axis and a perylene molecule inserted between the MS columns (Figure 11c). A relatively small effective mass for the holes was calculated, consistent with experimental results. The vapor-processed P3T1 crystal exhibited hole-transport activity and no electron conduction.23 The complexity of the stoichiometry observed in the perylene−TCNQ cocrystals showed the great potential and diversity of this binary approach for the development of organic semiconductors.

4. SUMMARY AND OUTLOOK The design and synthesis of various DA complex heterostructures facilitates the development of organic semiconductors with specific electrical properties from the combination of existing molecules, which avoids complicated and time-consuming organic synthesis procedures. The synergy of DA complex engineering results in (i) expanded π-conjugated DA systems for the realization of molecular-level p−n junction stacking, which favors both high hole and electron transport; (ii) defined molecular topological models for supramolecules for both experimental and theoretical studies; (iii) tunable band 1660

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she became a research fellow at the School of Materials Science and Engineering at Nanyang Technological University (NTU), Singapore in 2014. Currently she is a professor at Nanjing University of Posts & Telecommunications. Her research is focused on organic electronics and functional materials in devices. Wei Xu is a Professor in the Organic Solids Laboratory at ICCAS. He received his Ph.D. from Wuhan University in 1997. After working with Prof. Daoben Zhu as a postdoc for two years, he joined Prof. Zhu’s group in 1999. His research interests include the design, synthesis, and transport properties of organic semiconductors. Peng Sheng is currently an R&D engineer in new electrical materials at the State Grid Smart Grid Research Institute, State Grid Corporation of China. He obtained his M.Sc. in chemistry at the Ocean University of China (2008) and his Ph.D. in organic chemistry from ICCAS, working on organic electronic materials. Guangyao Zhao is currently an engineer at the Global Energy Interconnection Research Institute, working on perovskite solar cells. He obtained his Ph.D. in engineering materialogy from ICCAS (2014) working on single-crystal organic field-effect transistors. Daoben Zhu is a Professor and Director of the Organic Solids Laboratory at ICCAS. He finished his graduate courses at the East China University of Science and Technology in 1968. He was selected as an Academician of the CAS in 1997. His research interests include molecular materials and devices.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (21290191, 21372227, and 21602113).

Figure 11. Molecular packing of perylene−TCNQ complexes with different stoichiometries: (a) P1T1; (b) P2T1; (c) P3T1.

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structures by the combination of diverse donors and acceptors with changeable electron affinities that are capable of switching the charge-transport nature; and (iv) well-defined interdigitated structures to serve as ideal model systems for promoting and understanding photon-to-electron conversion in organic photovoltaics. However, there remain challenges for the application of DA complexes in organic electronics because (a) the performance (mobility, stability) of such supramolecular systems still needs to be improved to compare to their inorganic counterparts such as silicon and (b) further striving toward solutionprocessed, large-area, even printed DA complexes must take place. Overall the unique features of DA complexes provide a promising library of high-performance organic semiconductors.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei Xu: 0000-0003-1950-9037 Daoben Zhu: 0000-0002-6354-940X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Jing Zhang obtained her Ph.D. from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2013. After one year’s work at ICCAS, 1661

DOI: 10.1021/acs.accounts.7b00124 Acc. Chem. Res. 2017, 50, 1654−1662

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