Molecular Template Growth and Its Applications in Organic Electronics

Review pubs.acs.org/CR

Molecular Template Growth and Its Applications in Organic Electronics and Optoelectronics Junliang Yang,*,†,‡ Donghang Yan,§ and Tim S. Jones∥ †

Institute of Super-microstructure and Ultrafast Process in Advanced Materials and ‡Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, People’s Republic of China § State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China ∥ Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom 6. Graphene/Graphene Oxide Template Growth and Its Applications 6.1. Graphene/Graphene Oxide Template Growth Behaviors 6.2. Applications in Organic Electronics and Optoelectronics 7. Other Molecular Template Growths and Their Applications in Organic Electronics and Optoelectronics 8. Concluding Remarks and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Organic Semiconductor Thin-Film Growth for Organic Electronics and Optoelectronics 2.1. Organic Semiconductor Thin-Film Growth Behaviors 2.2. Organic Electronic and Optoelectronic Devices 2.3. Molecular Template Growth 3. Multiphenyl- and Multithiophene-Based Molecular Template Growth and Its Applications 3.1. Multiphenyl- and Multithiophene-Based Molecular Template Growth Behaviors 3.2. Extension of Weak Epitaxy Growth 3.2.1. New Template Layer Materials 3.2.2. New Overlayer Materials 3.3. Applications in Organic Electronics and Optoelectronics 4. Perylene-Derivative Molecular Template Growth and Its Applications 4.1. Perylene-Derivative Molecular Template Growth Behaviors 4.2. Applications in Organic Electronics and Optoelectronics 5. Acene Molecular Template Growth and Its Applications 5.1. Acene Molecular Template Growth Behaviors 5.2. Applications in Organic Electronics and Optoelectronics

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1. INTRODUCTION Organic electronics and optoelectronics are continuing to attract huge amounts of interest for their great potential in lowcost, large-area, and flexible consumer products, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and so on.1−9 In particular, OLED displays and lighting, OFET-based active-matrix displays, and organic-based radio frequency identification (RFID) tags have been commercially realized.10,11 In these electronic devices, the organic semiconductor thin film acting as the active layer is one of the key components, and its quality directly determines the device performance. Normally, amorphous thin films are preferable for high-performance OLED devices because the grain boundaries in crystalline thin films serve as efficient trapping and recombination sites that dramatically degrade the device performance,12 and the hopping-type conductivity in amorphous thin films is generally adequate for OLED devices.13 In contrast, highly ordered crystalline organic semiconductor thin films are particularly required to obtain high-performance OFETs and OPVs.8,14−21 Accordingly, many techniques have been developed to fabricate high-quality, ordered organic semiconductor thin films, including micro- and nanopatterning, Langmuir−Blodgett (LB) methods, self-assembled monolayer (SAM) methods,

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Received: March 1, 2014

© XXXX American Chemical Society

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DOI: 10.1021/acs.chemrev.5b00142 Chem. Rev. XXXX, XXX, XXX−XXX

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external field-induced alignment, and so on. These techniques have been reviewed in refs 20−26. Molecular template growth (MTG) is a newly developed method for fabricating high-quality organic semiconductor thin films with controllable morphologies, molecular orientations, electronic structures, interface properties, etc., to produce highperformance organic electronic and optoelectronic devices. There are several MTG methods with different molecular template materials and growth behaviors, including multiphenyl- and multithiophene-based MTG, perylene-derivative MTG, acene MTG, and graphene/graphene oxide template growth. For example, much research progress into the weak epitaxy growth (WEG) method based on multiphenyl and multithiophene molecular templates has been achieved during the past several years, including extensions to new template layer materials and overlayer materials and new applications in organic electronics and optoelectronics.27−51 Here, we summarize all research progress in this area to provide a better understanding of MTG and its applications. This review first provides an overview of the growth behavior of organic semiconductor thin films based on typical rodlike molecules and disklike molecules, which is very important for the development of MTG methods (section 2). Sections 3−7 describe several template growth methods and their applications in organic electronics and optoelectronics in detail. Multiphenyl- and multithiophene-based MTG is described in section 3, perylene-derivative MTG is described in section 4, acene MTG is described in section 5, graphene/graphene oxide template growth is described in section 6, and other MTG methods are described in section 7. Section 8 provides concluding remarks and summarizes the outlook in this area.

process as well. This is different from physical vapor transport deposition (PVTD), in which an inert gas carrier such as N2 or Ar is used to transport organic molecules.54,55 PVTD is useful for growing large-size organic single crystals.55 In this review, we focus on the MTG of highly ordered organic semiconductor thin films fabricated by OMBD on amorphous or polycrystalline substrates for organic electronic and optoelectronic devices such as OFETs and OPVs. Organic semiconductor thin-film growth has been one of the most important research topics over the past decade.56−69 As compared with conventional inorganic semiconductor thin-film growth, organic semiconductor thin-film growth is much more complicated because of the intrinsic anisotropy of the organic molecules, as well as the weak van der Waals forces and other complicated interactions between organic molecules and substrates. Molecules with different shapes can grow in completely different ways, i.e., molecular-shape-dependent growth. The growth behaviors of rodlike and disklike molecules are discussed briefly below using the models of pentacene, psexiphenyl (p-6P), 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), and copper phthalocyanine (CuPc). The typical molecular structures are shown in Figure 1.

2. ORGANIC SEMICONDUCTOR THIN-FILM GROWTH FOR ORGANIC ELECTRONICS AND OPTOELECTRONICS 2.1. Organic Semiconductor Thin-Film Growth Behaviors

Solution deposition and vacuum deposition are two basic methods for fabricating organic semiconductor thin films. The former is suitable for processing soluble organic small-molecule or polymer materials, while the latter is suitable for processing insoluble organic small-molecule materials. Vacuum deposition is originally used to fabricate inorganic semiconductor thin films. In the early development of this field, many studies examined the growth of organic semiconductor thin films and the fabrication of ordered organic semiconductor thin films on inorganic single-crystalline surfaces.52,53 Because of the strong interaction between organic molecules and single-crystalline substrates, organic molecules normally arrange themselves lying down epitaxially on the substrate. Like molecular beam epitaxy (MBE) for conventional inorganic semiconductor thin films, organic molecular beam epitaxy (OMBE) is used to fabricate organic semiconductor thin films on inorganic single-crystalline substrates. However, OMBE is unsuitable for the fabrication of organic electronic and optoelectronic devices on amorphous or polycrystalline substrates, such as silicon oxide (SiO2) or indium tin oxide (ITO), because organic semiconductor thin films do not have an epitaxial relationship with these substrates. Therefore, organic molecular beam deposition (OMBD) was accepted as a standard term to describe the fabrication of organic semiconductor thin films regardless of the kind of substrate. Sometimes, organic vapor deposition (OVD) or physical vapor deposition (PVD) is used to describe this

Figure 1. Molecular structures of representative rodlike molecules αhexathiophene (α-6T), pentacene, p-sexiphenyl (p-6P), PTCDA, planar disklike phthalocyanine compounds (MPcs; M = H2, Zn, Cu, Co, Ni, Fe, etc.), hexadecafluorinated metal phthalocyanines (F16MPcs), and nonplanar disklike vanadylphthalocyanine (VOPc).

Rodlike molecules such as α-sexithiophene (α-6T), pentacene, and p-6P exhibit similar growth behaviors on amorphous or polycrystalline substrates. The lateral molecule−molecule π−π interactions are stronger than either molecule−substrate or molecule−molecule interlayer interactions. Thus, the molecules arrange themselves standing up, and the dominant charge transport direction is parallel to the substrate. In the early growth stages, fractal islands are formed with a low nucleation density, and the monolayer is composed of many large-size domains that coalesce to form a continuous thin film, as shown in Figure 2.57 Interestingly, there is only one molecular orientation in a single fractal island (early growth) or compact islands (late growth). Selected area electron diffraction (SAED) experiments showed that these islands have a singlecrystalline structure (Figure 3).65 For pentacene and p-6P thin films, a second layer nucleates when the coverage reaches a critical value of about 0.90 monolayer (ML). This suggests that layer-by-layer growth occurs for these rodlike molecules, which B


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Figure 2. Evolution of the dendritic shape of pentacene islands with coverages at 0.50 ML (a) and 1.0 ML (b) and their layer coverage during deposition (c). The scale bar applies to (a) and (b). Reprinted with permission from ref 57. Copyright 2001 Nature Publishing Group. Morphology of the p-6P ultrathin film with coverages at 0.63 ML (d) and 1.10 MLs (e) and its layer coverage during deposition (f). Reprinted from ref 65. Copyright 2008 American Chemical Society.

Figure 4. Morphology of a 0.3 nm PTCDA thin film grown on Si(001) (a) and the distribution of the island sizes (b). Reprinted with permission from ref 72. Copyright 2006 Elsevier B.V. Morphology of a 1.0 nm PTCDA thin film grown on SiO2 (c) and X-ray diffraction pattern of 50 nm PTCDA grown on SiO2 (d).

Figure 3. Morphology and corresponding selected area electron diffraction (SAED) patterns for p-6P submonolayer films grown in different stages: (a, c) 0.28 ML, (b, d) 0.90 ML. Each domain is a single-crystalline structure with only one molecular orientation during film growth from a fractal island to a compact island. Reprinted from ref 65. Copyright 2008 American Chemical Society.

The disklike planar CuPc molecules show a film morphology similar to that of PTCDA grown on SiO2 at a low substrate temperature. However, the CuPc molecules are arranged standing up. The size of the crystals increases dramatically with increasing substrate temperature, and large-size needle-like crystals form instead of spherical crystals at a high substrate temperature.73 Other planar phthalocyanine compounds show similar growth behaviors.74−76 In addition to the typical growth behaviors of organic semiconductor thin films discussed briefly above, several more have been observed in different systems. Specific growth behaviors are discussed below when they are relevant.

is helpful for forming high-quality thin films with large-size domains, resulting in good charge transport properties. The perylene derivative PTCDA shows different growth behavior.70−72 No matter what substrate is used (singlecrystalline, polycrystalline, or amorphous), PTCDA thin films are normally composed of small-size spherical crystals or truncated pyramids in which PTCDA molecules are oriented in a lying-down arrangement, i.e., the (102) plane is parallel to the substrate surface, because of the strong binding of the perylene center as well as the strong interaction between O atoms and the substrate (Figure 4). This molecular arrangement is advantageous for charge transport along the direction vertical to the substrate. However, the grain boundaries between the small-size spherical crystals constrain the charge transport in the films.

2.2. Organic Electronic and Optoelectronic Devices

Among the many organic electronic and optoelectronic devices being developed, OFETs and OPVs are described here as typical devices in terms of their structures and the required properties of their organic semiconductor thin films. C


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Figure 5. Typical structural schematics of conventional OFETs (a) and VOFETs (b). The insets in (a) and (b) suggest that lateral and vertical π−π stacking directions could improve the charge transport in conventional OFETs and VOFETs, respectively.

shown in Figure 6. An OPV is an optoelectronic device that converts light energy directly into electricity through the

OFETs and OPVs both require highly ordered, crystalline organic semiconductor thin films for high performance, but they have some different requirements in terms of, for example, the molecular orientation in the thin film. Among OFETs, there are conventional planar-structure OFETs and vertical-structure OFETs (VOFETs),77,78 as shown in Figure 5. Conventional OFETs are composed of three basic elements: an organic semiconductor thin film, an insulating layer, and three electrodes (Figure 5a). The source and the drain electrodes are in contact with the semiconductor film at a short distance from one another, while the gate electrode is separated from the semiconductor film by an insulating layer. When a voltage is applied between the source and gate, charges are induced at the insulator−semiconductor interface and form a conducting channel parallel to the substrate. Under an applied voltage between the source and drain, charges are injected from the source and a current is established. Organic semiconductor molecules in a standing-up molecular orientation could provide high-mobility devices, because the dominant charge transport direction, i.e., the π−π stacking direction, is parallel to the substrate. On the other hand, VOFETs have a different vertically stacked structure with a layer active cell (drain/ organic layer/source) on top of a capacitor cell (source/ dielectric/gate), as shown in Figure 5b. The middle source electrode is shared by the capacitor cell and active cell. VOFETs have three unique characteristics: (i) the middle source electrode is very thin and rough, (ii) the capacitor cell has a high charge-storage capability and influences the active cell when the gate is biased, (iii) there is a large cross-sectional area and a small distance between the source and the drain, allowing current to flow between the source and drain electrodes. In this case, the charge transport direction is perpendicular to the substrate, and lying-down organic semiconductor molecules could provide high mobility because the π−π stacking direction is perpendicular to the substrate. For both conventional OFETs and VOFETs, large crystal domains in highly ordered films could dramatically improve the device performance by reducing the number of grain boundaries and traps. The structure and working principles of OPVs are completely different from those of OFETs and VOFETs, as

Figure 6. Structural schematics of OPVs with a bilayer planar heterojunction (a), a bulk heterojunction (b), and an ordered bulk heterojunction (c). The inset in (c) suggests that the vertical π−π conjugated direction can improve the charge transport in OPVs.

photovoltaic effect. Organic semiconductors act as the active layers in OPVs. Early OPVs were based on a single-active-layer structure, called an organic Schottky solar cell, whose power conversion efficiency (PCE) was typically below 0.10%.79 A major breakthrough came in 1986, when Tang discovered that bringing a donor and an acceptor together to form a planar heterojunction in one cell could dramatically increase the PCE to 1.0% (Figure 6a).80 Then a conventional bulk heterojunction and an ordered bulk heterojunction were developed to further improve the PCE (Figure 6b,c), which could potentially reach 15.0%.81,82 In these structures, the organic light-absorbing layer is sandwiched between two different electrodes. One of the electrodes must be (semi)transparent, often indium−tin oxide (ITO). The other electrode is very often aluminum or another metal. The principle of heterojunction OPV operation includes D


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tions, π−π interactions, interlayer interactions, intralayer interactions, and H-bonding interactions. This balance results in an energetically favorable and stable thin film. In conventional growth of an organic semiconductor thin film on an inorganic single-crystalline substrate, molecule−substrate interactions and lattice matching are much stronger than other forces. Organic molecules prefer to lie down on inorganic single-crystalline substrates. In these cases, epitaxy and quasiepitaxy are usually used to describe the growth behaviors of organic semiconductor thin films,52 and they are normally regarded as the energetically most favorable with respect to the overlayer−substrate interaction. Epitaxy refers to the commensurate relationship between the overlayer and the substrate, which was also described as point-on-point (POP) coincidence.53 In epitaxial growth, organic molecules are chemisorbed or physisorbed onto the surface of the substrate, and these processes are regarded as conventional epitaxy and van der Waals epitaxy, respectively. As implied by the name, van der Waals epitaxy is dominated by the van der Waals bonding interactions. A slight mismatch between the lattices of the overlayer and the substrate results in strained van der Waals epitaxy. Quasi-epitaxy is related to an incommensurate relationship between the overlayer and the substrate over any meaningful lattice length scale. However, a well-defined orientational relationship between the lattices of the overlayer and the substrate could still be observed, resulting in an azimuthal order. Quasi-epitaxy normally includes point-on-line (POL) coincidence and geometrical coincidence, which describe the cases in which the lattice point of the overlayer lies on at least one primitive lattice line of the substrate and in which the geometrical channels coincide between the overlayer and the substrate, respectively. In some cases of quasi-epitaxy, residual stress develops in the overlayer film as it attempts to conform to the substrate lattice; these situations are called strained quasi-epitaxy. In quasi-epitaxial growth, the azimuthal rotation is adjusted to achieve a minimum interfacial energy.53 As mentioned above, the driving force of MTG also results from competition among many kinds of interactions. In MTG methods, organic molecules are physisorbed onto the surface of the substrate. On one hand, the overlayer and the template layer can form a commensurate epitaxial relationship. In this case the interaction between the organic molecules and the substrate is usually weaker than the π−π stacking interaction between the organic molecules themselves, resulting in a standing-up configuration. Therefore, for a commensurate epitaxial relationship, the driving force comes from the matching of lattices between the overlayer and the template layer, and the molecules arrange to minimize the surface potentials with an energetically most favorable mode. On the other hand, the surface geometric channels of the template layer can dominate the oriented nucleation and growth of the overlayer crystals in incommensurate epitaxy. In this case, the minimum interfacial energy can be obtained by adjusting the azimuthal rotation. Both of these growth behaviors are different from the growth of organic molecules on an inorganic singlecrystalline substrate, in which a very strong interaction between the organic molecules and inorganic single-crystalline substrate results in a lying-down molecular arrangement. To emphasize this distinction, WEG has been used to describe both commensurate and incommensurate epitaxial growth in MTG.26 In contrast, some molecular template layers, such as PTCDA or graphene layers, can interact so strongly with the overlayer that this interaction dominates the molecular

four steps: (i) optical absorption and formation of excitons, (ii) exciton diffusion to the donor−acceptor interface, (iii) exciton dissociation at the donor−acceptor interface, and (iv) charge carrier transport and collection at the electrodes. These four steps are greatly influenced by the morphology, structure, and molecular orientation of the organic/polymer active layer. Like VOFETs, OPVs have vertical charge transport direction relative to the substrate. Hence, a lying-down molecular arrangement could improve the charge transport in the organic layer, resulting in an improvement in the PCE. 2.3. Molecular Template Growth

Individualized methods for fabricating high-quality organic semiconductor thin films with controllable thin-film properties can be developed on the basis of specific understandings of the growth behaviors of organic semiconductor molecules and the desired device structures and working principles of OFETs and OPVs. For example, rodlike molecules of p-6P normally form large-size, highly ordered domains with standing-up molecular orientations on amorphous substrates (Figure 2d,e), whereas disklike molecules of MPc compounds easily form small-size, random, needle-like crystals with a standing-up molecular orientation. Therefore, the use of rodlike molecular domains to induce the growth of disklike molecules would likely lead to high-quality films and high-performance OFETs and OPVs. On the other hand, molecules that prefer to lie down on the substrate such as PTCDA or graphene could act as template layers to induce lying-down arrangements of other organic molecules because of the strong face-to-face interactions between molecules. This behavior would improve the OPV and VOFET performance. As discussed above, the term MTG describes all such fabrication methods. In MTG, an ultrathin molecular layer (normally a monolayer or bilayer) is first deposited on the amorphous or polycrystalline substrate, and then organic semiconductor thin films are grown on it. The ultrathin molecular template layer can induce the growth of organic semiconductor thin films with controllable properties, including their molecular orientation, crystal size, film morphology, electronic structure, and so on, which are helpful for improving the performance of organic electronic and optoelectronic devices. A schematic of MTG is shown in Figure 7. Many kinds of interactions are balanced during the growth of the organic semiconductor thin film, including lattice matching, molecule−substrate interactions, molecule−molecule interac-

Figure 7. Schematic of MTG. A highly ordered ultrathin template layer (normally a monolayer or bilayer) could induce the growth of the organic semiconductor thin films with controllable properties, including morphology, structure, molecular orientation, electronic structure, and so on, which are helpful for producing high-performance organic electronic and optoelectronic devices. E


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Figure 8. Molecular structures of new template materials used for WEG.

semiconductor thin films with both high out-of-plane order and regular in-plane orientation on amorphous substrates.

arrangement, nucleation, and growth, leading to a lying-down molecular orientation. In this case, it is difficult to observe any epitaxial relationship (commensurate or incommensurate) between the template layer and the overlayer, and the direct driving force is the molecule−substrate interactions and π−π interactions.

3.1. Multiphenyl- and Multithiophene-Based Molecular Template Growth Behaviors

The WEG behavior and growth mechanism of both planar and nonplanar metal phthalocyanine compounds on a multiphenylbased molecular template, i.e., on a p-6P ultrathin film, have been discussed in detail before.26 In particular, the p-type planar compounds metal-free phthalocyanine (H2Pc), zinc phthalocyanine (ZnPc), nickel phthalocyanine (NiPc), CuPc, iron phthalocyanine (FePc), and cobalt phthalocyanine (CoPc), the p-type nonplanar compound VOPc, and the ntype compounds F16CuPc and phthalocyanine tin dichloride (SnCl2Pc) have all been extensively examined. These compounds can exhibit either a commensurate epitaxial relationship or an incommensurate epitaxial relationship. On the basis of these results, significant research progress into new materials for WEG and its applications in organic electronics and optoelectronics has been made. Here, we provide an overview of recent WEG research progress related to its extension and applications in organic electronics and optoelectronics.

3. MULTIPHENYL- AND MULTITHIOPHENE-BASED MOLECULAR TEMPLATE GROWTH AND ITS APPLICATIONS WEG is a typical technique for multiphenyl- and multithiophene-based molecular template growth. Here we focus on WEG using all kinds of multiphenyl- and multithiophene-based ultrathin molecular templates. On the basis of the observed growth mechanisms of organic semiconductor thin films consisting of rodlike and disklike molecules, WEG was developed specifically to fabricate highly ordered organic semiconductor thin films.26,83,84 In WEG, an ordered ultrathin film of rodlike molecules (usually a monolayer or bilayer film), for example, a p-6P ultrathin film, is introduced as a template layer to produce large-size, highly ordered domains and ultimately smooth organic semiconductor thin films (named single-crystal-like thin films) on amorphous substrates. These highly ordered films have a standing-up molecular orientation because the interaction between the organic molecules and the substrate is weaker than the π−π stacking interaction between the organic molecules themselves compared with that for organic molecules grown on an inorganic single-crystalline substrate. This leads to a significant improvement of the inplane charge transport in organic electronic devices. Furthermore, the overlayer molecular crystals are epitaxially oriented relative to the template layer, accelerating the formation of regular in-plane molecular orientations. Hence, the WEG method can be used to fabricate high-quality organic

3.2. Extension of Weak Epitaxy Growth

Originally, research into the WEG method focused on p-6P ultrathin films as template layers and phthalocyanine compounds as overlayer materials. As the development of WEG progressed, new template layers and overlayer materials were introduced for different applications. It is well-known that the 2-D lattice parameters and geometrical channels are the two key attributes for growing highly ordered organic semiconductor thin films. Hence, materials with 2-D lattice parameters and geometrical channels similar to those of p-6P F


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the first-layer domains. The domain size is temperaturedependent. Normally, large-size domains approaching 100 μm2 can be obtained under optimized conditions. The 2-D structure of most ultrathin films is thicknessdependent. Figure 10 shows the 2-D lattice parameters of two typical materials, BP2T and m-F2BP3T, as functions of coverage based on grazing incidence X-ray diffraction (GIXD) measurements using synchrotron radiation. Changes in the 2-D lattice parameters would result in changes in the epitaxial growth mechanism, as discussed below. Table 1 lists the structural parameters of bulk materials and ultrathin films of some typical template molecules. The first template material used for WEG is the p-6P molecule.26 Monolayer and bilayer p-6Ps have different 2-D structures and, accordingly, different surface geometric channels. These differences influence the oriented nucleation and growth of epitaxial crystals, and monolayer and bilayer p6Ps exhibit incommensurate and commensurate epitaxy, respectively. Although p-6P could induce the formation of single-crystal-like thin films, its energy levels, i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), do not match those of many metal electrodes, which limits its application to the fabrication of organic devices. BP2T was designed as a template material for WEG with an energy level tuned to match that of the ITO in OPVs.31 Monolayer, bilayer, and trilayer BP2Ts have almost the same d(010) and d(110) values, while the d(100) values are a little different (Table 1). The planar molecule ZnPc exhibits incommensurate epitaxy on the BP2T monolayer and bilayer, with in-plane molecular orientations with angles of 12° and 9°, respectively (Figure 11). Stripelike crystals with a liquidlike behavior are present on the BP2T monolayer. In contrast, on the BP2T bilayer, fiberlike crystals with regular, strongly anisotropic shapes are formed with a length-to-width ratio of over 20. On the trilayer, there is only one in-plane molecular orientation, which is commensurate with the BP2T molecular crystals. Similar growth behavior also happened on an ITO substrate with a BP2T template layer.31 All the obtained morphologies are totally different from those grown directly on SiO2 or ITO, which are composed of randomly ordered crystals with small grain sizes.31,84 On the other hand, BP2T could also be used as a template layer to induce the growth of nonplanar phthalocyanine compounds such as TiOPc.40 Normally, TiOPc thin films grown directly on ITO substrates are composed of small-size and randomly ordered spherical grains with dense grain boundaries at room temperature (Figure 12a). It is difficult to observe any diffraction peaks in their XRD patterns, suggesting that TiOPc thin films grown on ITO substrates are amorphous or poorly crystalline (Figure 12c). TiOPc thin films grow on BP2T/ITO through a layer-by-layer mode with lamellar crystals (Figure 12b). The resulting thin films show good continuity and integrity with a low density of grain boundaries. A strong diffraction peak located at 7.5° could be observed in the XRD curve, corresponding to the (010) lattice plane of phase II, and an epitaxial relationship was observed between TiOPc and BP2T (Figure 12d). Within each BP2T domain, TiOPc crystals show three different in-plane orientations corresponding to incommensurate epitaxy and commensurate epitaxy, respectively. For the incommensurate epitaxy, the angles of (2̅1̅2)TiOPc‑2 and (2̅1̅2)TiOPc‑3 with respect to bBP2T are ±55°, respectively. For commensurate epitaxy, evidenced by the coincidence of (2̅1̅2)TiOPc‑1 and (020)BP2T, the

thin films or phthalocyanines could be used as template layer materials and overlayer materials, respectively, for WEG. 3.2.1. New Template Layer Materials. Figure 8 shows a series of multiphenyl- or multithiophene-based molecules designed as molecular template layer materials, including 2,5bis(1,1′:4′,1″-terphenyl-4-yl)thiophene (3PT), 2,5-bis(biphenyl-4-yl)bithiophene (BP2T), 5,5″-bis(biphenyl-4-yl)2,2′:5′,2″-terthiophene (BP3T), 5,5″-bis(3′,5′-difluorobiphenyl-4-yl)-2,2′:5′,2″-terthiophene (F4BP3T), 5,5″-bis(30-fluorobiphenyl-4-yl)-2,2′:5′,2″-terthiophene (m-F2BP3T), 5,5″-bis(4′-fluorobiphenyl-4-yl)-2,2′:5′,2″-terthiophene (p-F2BP3T), 5,5‴-diphenyl-2,2′:5′,2″:5″,2‴-quaterthiophene (P4T), 5,5‴bis(4-fluorophenyl)-2,2′:5′,2″:5″,2‴-quaterthiophene (F2P4T), 2,7-bis(biphenyl-4-yl)phenanthrene (BPPh), 2,5bis(biphenyl-4-yl)thieno[3,2-b]thiophene (BPTT), 2,6-bis(biphenyl-4-yl)benzo[1,2-b:4,5-b′]bithiophene (BPTBT), and 2,7-bis(biphenyl-4-yl)dibenzothiophene (BPBTB).85 All these molecules show growth behaviors similar to that of p-6P.30,32,34,39,42,64,65,69,85 The growth of these organic semiconductor thin films qualitatively involves four steps: (i) the diffusion of molecules and the formation of a stable nucleus as a critical number of molecules meet together, (ii) the formation of a new nucleus and the growth of existing islands through adsorption of new molecules, (iii) the growth of islands through the diffusion and aggregation of adsorbed molecules, called the aggregation regime, and (iv) island coalescence. Figure 9 shows the typical morphology of monolayer films with

Figure 9. AFM morphology images (20 μm × 20 μm) of template layer films grown on a SiO2/Si substrate: (a) 0.21 ML of 3PT, (b) 0.85 ML of 3PT, (c) 0.86 ML of BP2T, (d) 1.0 of ML m-F2BP3T, (e) 1.0 ML of BP3T, and (f) 1.0 ML of F4BP3T. Panels a and b reprinted from ref 30. Copyright 2010 American Chemical Society. Panel c reprinted from ref 32. Copyright 2010 American Chemical Society. Panels d−f reprinted from ref 39. Copyright 2012 American Chemical Society.

different coverages in the aggregation regime and during island coalescence. In the aggregation regime, the diffusing molecules find certain sites at the island edges as the island grows. In particular, the branches or kinks in fractal islands are the lowest energy sites for new molecules, and they exert larger van der Waals forces on the diffusing molecules than any edges. This accelerates the formation of large and nearly compact islands as the coverage increases. Then large-size compact neighbor islands coalesce together, although the molecular orientations in the neighboring domains are likely different. As shown in Figure 2, the second layer nucleates during the coalescence of G


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Figure 10. Closed symbols show the evolution of lattice parameters as a function of the coverage based on GIXD for a BP2T molecule (a) and mF2BP3T (b). The open symbols indicate that a thin film with a new structure emerges when the coverage is over a critical thickness. Panel a reprinted from ref 32. Copyright 2010 American Chemical Society. Panel b reprinted with permission from ref 42. Copyright 2012 Royal Society of Chemistry.

Table 1. Lattice Parameters of Typical Template Materials as Well as the Mismatch and Oriented Angles for MPc’s Grown on Template Layers

a (Å)

b (Å)

c (Å)

β (deg)

p-6Pb 8.091

5.565

26.24

98.17

7.604

5.708

52.87

97.15

BP2Tc

BP3Td m-F2BP3Td F4BP3Td p-F2BP3Te

3PTf

7.53 7.66 7.76

5.78 5.85 5.87

60.00 61.04 60.65

92.81 90.0 90.0

7.50

5.89

62.31

98.4

monolayer bilayer bulk monolayer bilayer trilayer bulk

monolayer bilayer bulk

d(100) (Å)

d(010) (Å)

d(110) (Å)

7.84 7.96 8.01 7.70 7.68 7.66 7.54 7.64 7.60 7.80 7.54 7.60 7.50 7.60

5.59 5.58 5.57 5.73 5.73 5.73 5.71 5.76 5.83 5.85 5.83 5.78 5.83 5.76

4.55 4.60 4.57 4.60 4.59 4.59 4.55 4.60 4.65 4.68 4.61 4.60 4.60 4.59

commensurate (mismatch,a %)

incommensurate (angle, deg)

b and bMPc a and cMPc

a* and c*MPc

2.34 2.53

1.91 0.38

13.5

12 9 −1.05

5.35

−0.68 −1.87 −2.21 −2.49 −1.64

4.58 3.77 2.44 5.75 4.91

13 10 13

−0.68

5.11

10

All mismatches were recalculated: mismatch (%) = (dMPc − dtemplate)/ dtemplate. b2-D parameters of the mono- or bilayer were obtained by SAED. The mismatch and angle values are based on H2Pc/p-6P.86 cValues of lattices a and b are switched with each other. 2-D parameters were obtained by GIXD. The mismatch and angle values are based on ZnPc/BP2T.32 d2-D parameters were obtained by GIXD. The mismatch and angle values are based on H2Pc/BP3T, H2Pc/m-F2BP3T, and H2Pc/F4BP3T, respectively.39 2-D parameters of m-F2BP3T are also thickness-dependent.42 eThe angle value 98.4° is α. 2-D parameters were obtained by GIXD. The mismatch and angle values are based on CuPc/p-F2BP3T.39 f2-D parameters were obtained by both SAED and GIXD. The mismatch and angle values are based on H2Pc and ZnPc/3PT, respectively.30 a

symmetrical orientations in each domain of BP3T. The angle of c*H2Pc with respect to a*BP3T is about ±13°. However, H2Pc crystals show three orientations in each domain of m-F2BP3T and F4BP3T, corresponding to incommensurate epitaxy and commensurate epitaxy. For incommensurate epitaxy, the angles between c*H2Pc and a*m‑F2BP3T or a*F4BP3T are about ±10° or ±13°, respectively. For commensurate epitaxy, the relationships between H2Pc and each molecule are (100)H2Pc//(001)BP3Ts, [001]H2Pc//[100]BP3Ts, and [010]H2Pc//[010]BP3Ts. In contrast, the H2Pc film is polycrystalline with multiple orientations on pF2BP3T. The lattice mismatch in commensurate epitaxy and the angle in incommensurate epitaxy are summarized in Table 1. In a word, H2Pc exhibits selective epitaxial growth depending on the position of fluorine. BP3T derivatives with herringbone packing modes provide configurations of the surface channels similar to those of p-6P. The channel effect and lattice matching dominate the nucleation and subsequent growth of phthalocyanines. The weak C−H···F hydrogen bonds play an important role in the formation of molecular thin films.87,88 In particular, C− H···F hydrogen bonds between H2Pc and the fluorinated

relationships are (010)TiOPc//(001)BP2T and [101]TiOPc// [010]BP2T. The BP2T template layer leads to a highly inplane orientation and compact stacking of TiOPc molecules through layer-by-layer growth. Furthermore, a series of BP3T derivative compounds (BP3T, m-F2BP3T, F4BP3T, and p-F2BP3T) were synthesized to act as template layers to induce the growth of phthalocyanines.39 All BP3T compounds could form large domains with smooth surfaces, a standing-up molecular orientation, and a herringbone packing motif on SiO2 or ITO substrates (Figure 9). However, the introduction of fluorine atoms in BP3T significantly changes the thin-film growth behavior and 2-D lattice structure. H2Pc crystals grown on BP3T, m-F2BP3T, F4BP3T, and p-F2BP3T exhibit extraordinary differences in morphology as the number and position of fluorine atoms change (Figure 13). On BP3T, m-F2BP3T, and F4BP3T template layers, H2Pc films are composed of large-size, stripelike crystals, while, on p-F2BP3T, the H2Pc films are composed of small-size, netlike crystals. The (200) plane of H2Pc contacts the (002) plane of BP3T substrates with incommensurate epitaxy, in which H2Pc crystals have two H


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Figure 11. AFM morphology images of 1.0 nm ZnPc grown on BP2T ultrathin films: (a) ZnPc/BP2T monolayer; (b) ZnPc/BP2T bilayer; (c) ZnPc/BP2T trilayer. SAED patterns of (d) ZnPc (5 nm)/BP2T monolayer, (e) ZnPc (5 nm)/BP2T bilayer, and (f) ZnPc (5 nm)/BP2T trilayer. Reprinted from ref 32. Copyright 2010 American Chemical Society.

Figure 13. AFM morphology images of 3 nm H2Pc grown on monolayer BP3T (a), m-F2BP3T (b), F4BP3T (c), and p-F2BP3T (d). Reprinted from ref 39. Copyright 2012 American Chemical Society.

crystals. For example, CuPc shows only one in-plane orientation with commensurate epitaxy evidenced by the coincidence of (006)CuPc and (200)p‑F2BP3T. Although the interactions between CuPc and monolayer p-F2BP3T are the same as those between CuPc and bilayer p-F2BP3T, the lattice parameters of the monolayer and bilayer are different. Obviously, the lattice mismatch is smaller for CuPc on a pF2BP3T bilayer than that on a monolayer (Table 1), promoting the formation of regular fiberlike crystals.39 3PT is another molecule designed to form a template layer for WEG. The growth behavior of phthalocyanines on 3PT is similar to that of phthalocyanines on p-6P.30 For example, the needle-like ZnPc crystals grown on 3PT present two sets of inplane orientations in incommensurate epitaxy with angles of about ±10° and form many parallelograms in a single domain. On the other hand, stripelike H2Pc crystals with a single orientation and long-range order are formed on a 3PT monolayer film. The epitaxial relations between H2Pc and 3PT are (100)H2Pc//(001)3PT, [010]H2Pc//[010]3PT, and [001]H2Pc//[100]3PT. The lattice mismatches are −0.68% and 5.11% along [010]3PT and [100]3PT, respectively.

Figure 12. AFM morphology images (2 μm × 2 μm) of (a) TiOPc (15 nm)/ITO and (b) TiOPc (20 nm)/BP2T (8 nm)/ITO films. (c) XRD patterns of TiOPc/BP2T and TiOPc/ITO films. (d) SAED pattern of TiOPc/BP2T films. Reprinted with permission from ref 40. Copyright 2010 Elsevier B.V.

template layer affect the oriented growth of H2Pc. In molecular crystals, the F atoms are the highest points of the p-F2BP3T surface, whereas the highest points of the m-F2BP3T and F4BP4T surfaces are H atoms, similar to the case for the p-6P surface. This results in only van der Waals (C−H···H) interactions and no C−H···F hydrogen bonds, owing to the longer distance between H2Pc and the F atoms of m-F2BP3T or F4BP4T. In contrast, stronger C−H···F bonds are formed in the H2Pc/p-F2BP3T system, leading to a special morphology. However, increasing the p-F2BP3T thickness from a monolayer to a bilayer not only changes the 2-D lattice structure (Table 1), but also causes fiberlike crystals to form instead of netlike I


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Figure 14. AFM morphology images of (a) a 3 nm BPTT/BPPh (1:1) film (20 μm × 20 μm) and (b) 5 nm VOPc grown on a 5 nm BPTT/BPPh (1:1) layer (4 μm × 4 μm) as well as (c) its SAED pattern. (d) Lattice spacing of (110) of the mixed films as a function of the concentration of one component based on GIXD results. (e) Lattice mismatch between VOPc and the template layer along the [100] and [010] directions as a function of the BPTT concentration in the mixed layer. Reprinted with permission from ref 34. Copyright 2011 John Wiley & Sons, Inc.

Figure 15. Molecular structure of PTCDI-Ph and AFM morphology images of 2 nm PTCDI-Ph on bare SiO2 (a), 2 nm PTCDI-Ph on monolayer p6P (b), and 4 nm PTCDI-Ph on bilayer p-6P (c). SAED patterns of PTCDI-Ph/p-6P monolayer (d) and PTCDI-Ph/p-6P bilayer (e). Out-of-plane XRD patterns for 20 nm PTCDI-Ph grown on p-6P (2 and 5 nm) and bare SiO2 (f). The inset in (f) gives the out-of-plane orientation of the PTCDI-Ph. Reprinted with permission from ref 33. Copyright 2010 Elsevier B.V.

In addition, P4T and F2P4T were synthesized to fabricate highly ordered thin films of both planar and nonplanar phthalocyanines.35,38 More importantly, the binary template layers with tunable energy levels and 2-D lattice parameters

were designed to grow high-quality nonplanar phthalocyanine thin films.34 Figure 14 shows the atomic force microscopy (AFM) topographic images of a 3 nm BPTT/BPPh (1:1) binary film and a 5 nm VOPc film grown on a 5 nm BPTT/ J


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BPPh (1:1) layer. The lattice spacings of the binary films and the lattice mismatch between VOPc and the template layer are also shown as a function of the concentration of one component. Layer-by-layer growth was observed for both pure monolayer films and BPTT/BPPh binary films with different ratios, and smooth films with large grain sizes were formed. The binary films are uniform crystal films or solid solutions, because each grain is a single-crystalline domain with a rectangular unit cell. The lattice spacing of the binary films gradually increases as the BPPh concentration increases (Figure 14d), implying that the binary films are a class of mixed crystals that can present a smooth morphology similar to that of the pure films but possess a unique structure with ratio-controlled lattice parameters. The controllability of the lattice parameter of the template layer is very important, because the lattice parameter greatly influences the subsequent growth of organic semiconductor films. For VOPc grown on mixed-crystal films with various mixing ratios, large lamellar crystals with good coalescence were observed on BPTT-rich films. Meanwhile, smaller and poorly continuous lamellar crystals were formed on pure BPPh and BPPh-rich films. For a 1:1 ratio between BPTT and BPPh, (2̅1̅2)VOPc is parallel to (200)template, resulting in an orientational relationship: (010)VOPc//(001)template, [101]VOPc//[010]template. In addition, the pure BPPh film has a larger mismatch in both directions than the other films. As BPTT is added, the mismatch along the [100] direction decreases. Along [010], all the mixed films show a mismatch smaller than 3%. In particular, the 1:1 and 2:1 mixed films show mismatches close to 0.0%. In other words, BPTT addition promotes orientational growth in the layer-by-layer growth mode instead of the island growth mode, resulting in the formation of smooth and continuous films. Thus, a better morphology of VOPc could be achieved on 1:1 and 2:1 mixed films (BPTT/BPPh) or on pure BPTT. 3.2.2. New Overlayer Materials. The discussions above focused on the different kinds of template materials developed to grow high-quality thin films of phthalocyanine compounds for WEG. This technique could be potentially extended to other overlayer materials. For example, N,N′-diphenylperylenetetracarboxylic diimide (PTCDI-Ph), whose molecular structure is given in Figure 15, has been grown on p-6P ordered template layers.33 PTCDI-Ph films grown on bare SiO2, a p-6P monolayer, and a p-6P bilayer have obviously different morphologies (Figure 15), as follows. On bare SiO2, they are composed of small-size, randomly ordered crystals (Figure 15a). On a p-6P monolayer, large-size ribbonlike PTCDI-Ph crystals with preferred orientations were formed (Figure 15b), demonstrating incommensurate epitaxy. Finally, on a p-6P bilayer, single-crystal-like PTCDI-Ph films grew with only one orientation (Figure 15c), demonstrating commensurate epitaxy with the epitaxial relationships (100)PTCDI‑Ph// (001) p‑6P , [001] PTCDI‑Ph //[100]p‑6P , and [010] PTCDI‑Ph// [010]p‑6P. The strong diffraction peaks in the X-ray diffraction (XRD) pattern suggest that the PTCDI-Ph films are highly ordered with a standing-up arrangement, implying a π−π stacking direction parallel to the film plane. PTCDI-Ph films grown on SiO2 and on a p-6P monolayer present a primary peak with a d spacing of 17.17 Å, while the PTCDI-Ph films on p-6P bilayers mainly present a strong diffraction peak with a d spacing of 16.10 Å and a diffraction shoulder with a d spacing of 17.17 Å, implying different polymorphs with different tilt angles for the standing-up molecules.

Another example is 5,6,11,12-tetraphenylnaphthacene (rubrene) grown on p-6P and 1,3-bis(terphenyl-4-yl)benzene (m7P) template layers, respectively.44,51 Normally, it is very difficult to obtain highly ordered rubrene thin films using conventional deposition methods. However, highly ordered and continuous rubrene thin films form on a p-6P or an m-7P template layer. Figure 16 shows the growth of a rubrene thin

Figure 16. AFM morphology image (a) and XRD pattern of highly crystalline 20 nm rubrene grown on an 8 nm p-6P template layer (b). The inset in (a) is the molecular structure of rubrene, and the inset in (b) shows a schematic of the molecular arrangement on the template layer. Reprinted with permission from ref 44. Copyright 2010 Elsevier B.V.

film on a p-6P template layer. The domain size is much larger than that of a film grown without a p-6P template layer, and the XRD peak at 2θ = 6.55° (d = 13.48 Å), indexed as the (002) plane of rubrene, indicates that the rubrene molecules stand up on the template layer, supporting carrier transport in the film plane. The strong high-order diffraction peaks reveal that the templated rubrene thin films possess a high degree of crystallinity. There are two crystal orientations in each p-6P domain with epitaxial relationships of (001)rubrene//(001)p‑6P, [100] rubrene //[100] p‑6P , and [010] rubrene //[010] p‑6P , or [100]rubrene//[010] p‑6P and [010]rubrene//[100] p‑6P. 3.3. Applications in Organic Electronics and Optoelectronics

The primary aim of these developments in the WEG technique was to optimize the performance of organic electronic and optoelectronic devices, especially OFETs based on phthalocyanine compounds.84 However, WEG was subsequently extended to the fabrication of other organic devices such as OPVs, sensors, and superlattice devices. Results regarding the applications of WEG in the fabrication of organic electronic and optoelectronic devices are summarized below. As discussed above, the WEG technique can produce films with molecules arranged in a standing-up configuration and with a regular in-plane orientation. Furthermore, the coalescence of crystal domains during WEG is helpful to form large-size, continuous thin films, and reduce grain boundaries. These high-quality thin films improve the charge transport and the overall performance of OFETs. The performance parameters of OFETs fabricated by WEG and traditional vacuum deposition are summarized in Table 2, and more work is still ongoing. The charge mobility of OFET devices fabricated by WEG is dramatically improved to the level obtained in the corresponding single-crystalline devices. The electrical properties of the model VOPc/p-6P system have been studied in detail.98,99 Metal−insulator−semiconductor (MIS) diodes with a VOPc active layer fabricated by WEG on p-6P had a low hysteresis effect and a high transition K


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Table 2. Charge Mobilities of OFETs Fabricated by the WEG Method and Traditional Vacuum Deposition, Respectively, as Well as Their Single-Crystal OFETs mobility (cm2/(V s)) template material

overlayer material

WEG method

traditional vacuum deposition

p-6P p-6P p-6P p-6P p-6P p-6P p-6P p-6P m-7P BPTT/BPPh(1:1) F2P4T

ZnPc84 CuPc73,84,89 VOPc90,91 F16CuPc84,92,93 SnCl2Pc94 SnOPc95 PTCDI-Ph 33 a rubrene44,47,96,97 b rubrene51,96,97 b VOPc34 VOPc35 c

0.32 0.15 0.6−1.5 0.11 0.25−0.30 0.44 0.10−0.20 1.00−5.10 5.10−11.60 2.20−3.20 1.20−2.60

0.022 0.02 5 × 10 −3 0.03 10−3 10−3 10−3 10−2−10 −5 10−2−10−5 5 × 10−3 5 × 10−3

single crystal 0.1−1.0 0.17−0.35

1−20 1−20

a OFETs show ambipolar transport behavior with a hole mobility of 0.09 cm2/(V s) and electron mobility of 0.17 cm2/(V s).33 bOFETs using the p− p heterojunction of rubrene/VOPc as a buffer layer result in hole mobilities as high as 5.10 and 11.6 cm2/(V s), respectively.47,51 cA hole mobility of 4.08 cm2/(V s) could be obtained if using an F16CuPc/CuPc heterojunction unit as a buffer layer to reduce contact resistance between the metal electrodes and VOPc.35,38

Figure 17. (a) Schematic of band bending of CuPc and CuPc/p-6P thin films. The difference between the local valence band (VL) and the VL of the tip is the value of the surface potential. (b) Current−voltage curves of a 6 nm BP2T layer, 15 nm CuPc layer, and 15 nm CuPc/6 nm BP2T heterojunction film. Panel a reprinted with permission from ref 26. Copyright 2010 IOP Publishing Ltd. Panel b reprinted with permission from ref 37. Copyright 2011 American Institute of Physics.

of 105, and a low leakage current of 10−9 A.101 VOPc/p-6P OFETs demonstrate good durability against flexing and possess great potential in flexible electronics. WEG was also used to fabricate some high-quality organic heterojunctions that have been successfully used in organic electronic devices over the past two decades.102 An organic heterojunction is the interface that occurs between two organic semiconductor layers or between two regions of dissimilar organic semiconductors. Actually, an ultrathin template layer could form heterojunctions with the overlayer films. The heterojunction effect in CuPc/p-6P films was studied by Kelvin probe force microscopy (KPFM), in which the surface electrical potential distribution was directly mapped for organic semiconductor thin films grown both conventionally and by WEG.29 The band bending and hole accumulation are more obvious in the films grown by WEG than in conventional polycrystalline films, which explains the crystal-like charge transport in the highly ordered thin films grown by WEG. For this reason, the thin films grown by WEG show both the heterojunction effect and a high charge mobility (Figure 17a). Meanwhile, a BP2T template layer could also form a single-crystal-like organic− organic heterojunction with a CuPc overlayer.37 The energy level bending was observed using photoemission spectroscopy

frequency of about 10 kHz in their accumulation mode because of the low interface trap density and high carrier mobility. The electrical instability measurements of VOPc/p-6P OFETs at various temperatures showed a slow threshold voltage shift in the bias stress process and a rapid recovery after the removal of bias stress. This demonstrates that a slower degradation process occurs in the on state, whereas a faster removal occurs in the off state. A relaxation time of 107 s could be obtained at room temperature, and VOPc/p-6P could afford long operation times of 10 years as switching elements. VOPc/p-6P OFETs show performance comparable to that of a-Si:H thin-film transistors, and could be applied in active-matrix liquid crystal displays and OLED displays as well as organic logic circuits. As a replacement for SiO2 insulators, high-dielectric-constant materials such as tantalum pentoxide (Ta2O5) and polymer benzocyclobutenone (BCBO) derivatives could be used as double-layer insulators to obtain similar performance for VOPc/p-6P devices.100 With a silicon nitride (SiNx) insulator layer, mobilities of over 1.0 cm2/(V s) were obtained as well.43,46 Flexible VOPc/p-6P OFETs fabricated by WEG on a poly(ethylene terephthalate) (PET) substrate using a poly(vinyl alcohol) (PVA) and BCBO as a double-layer insulator could show a mobility of about 0.5 cm2/(V s), an on/off ratio L


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Figure 18. EQE of ZnPc/C60 cells (a) and TiOPc/C60 cells (b) with and without the WEG method, respectively. Panel a reprinted with permission from ref 28. Copyright 2010 John Wiley & Sons, Inc. Panel b reprinted with permission from ref 40. Copyright 2012 Elsevier B.V.

Table 3. Device Parameters for OPVs Based on ZnPc/C60 and TiOPc/C60 Heterojunctions Fabricated with and without a BP2T Template Layer cella b

ITO/PEDOT:PSS/ZnPc/C60/Alq3/Al (PHJ) ITO/PEDOT:PSS/BP2T/ZnPc/C60/Alq3/Alb (PHJ) ITO/PEDOT:PSS/ZnPc/ZnPc/C60/C60/Alq3/Alb (PM-HJ) ITO/PEDOT:PSS/BP2T/ZnPc/ZnPc/C60/C60/Alq3/Alb (PM-HJ) ITO/PEDOT:PSS/BP2T/ZnPc/ZnPc/C60/C60/Alq3/Alc (OBHJ) ITO/PEDOT:PSS/TiOPc/C60/Alq3/Al (PHJ)d ITO/PEDOT:PSS/BP2T/TiOPc/C60/Alq3/Ald (PHJ)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0.52 0.56 0.52 0.56 0.56 0.26 0.48

4.16 5.76 8.19 9.97 10.00 4.18 9.26

0.55 0.65 0.41 0.55 0.55 0.35 0.60

1.19 2.10 1.75 3.07 3.10 0.38 2.67

a Abbreviations: PHJ: planar heterojunction, PM-HJ: planar mixed heterojucntion, OBHJ: ordered bulk heterojunction. bReference 31. cReference 45. dReference 40.

thin film with the same in-plane orientations. In this case, the H2Pc and F16CuPc thin films exhibit heteroepitaxy, and a layerby-layer structure is guaranteed when more periods are formed. High-density mobile electrons are present in the F16CuPc layer, and holes are distributed periodically in the H2Pc layers, which significantly changes the intrinsic properties of these organic semiconductors. An obvious Ohmic conductance of about 10−3 S/cm was observed in five-period H2Pc/F16CuPc superlattices, which is 4−5 orders of magnitude higher than that of singleperiod films grown by WEG. A series of highly ordered organic heterostructure films based on ZnPc and PTCDI-Ph with well-defined alternate heteroepitaxy were grown on a p-6P template layer.49 The alternating crystalline heterostructures showed good ambipolar transport behaviors because of the high quality of the films and balanced transport abilities of ZnPc and PTCDI-Ph. More importantly, organic quantum wells (OQWs) could be structured on the basis of alternate PTCDI-Ph and ZnPc heterostructure films.50 As discussed above, high-quality crystalline ZnPc and PTCDIPh films fabricated by WEG exhibit excellent transport performance.33,84 The offset of their HOMO energy levels forms a 0.6 eV deep quantum well, where ZnPc acts as the well and PTCDI-Ph acts as the barrier. The accumulation-type heterojunction formed between ZnPc and PTCDI-Ph reduces the interface trap density and increases the effective well depth. Here, the BP2T template layer instead of p-6P deposited on the Si/SiO2 substrate also acts as a quantum well emitter, because of its energy matching with ZnPc. As mentioned above, films grown by WEG exhibit superior charge transport and exciton diffusion lengths, making them suitable for OPVs. For these devices, BP2T is the optimal template layer to match the energy level of ITO.31,40 In ZnPc/ C60 planar heterojunction (PHJ) and planar mixed heterojunction (PM-HJ) OPVs, the ZnPc donor layer was first deposited by WEG on the BP2T template layer, and then a

(PES) for the CuPc/BP2T heterojunction. A conduction channel is formed at this interface, resulting in Ohmic behavior with a conductivity of 2.4 × 10 −4 S/cm, 3 orders of magnitude higher than those of the individual layers (Figure 17b). Besides the large enhancement in charge transport for WEG films, there is a dramatic increase in the exciton diffusion length as well.41 Thus, WEG films have a potential application in OPVs. In single-crystal-like WEG films, the charge transport is controlled by the shallow-trap-filled process with a low deep trap density, whereas the charge transport in conventional phthalocyanine films is controlled by the deep-trap-filled process with a high deep trap density. Although singlecrystal-like films could be formed on different template layers, the properties of the crystalline heterojunction are influenced by the type of rodlike molecular layer underneath and the interfacial electronic structure.36 For example, in ZnPc/p-6P and ZnPc/BP2T systems, the ZnPc crystals have similar qualities and morphologies. However, p-6P acts as a dielectric layer that blocks the holes coming from ZnPc, whereas BP2T enables good hole transport across the interface with ZnPc. This demonstrates that the charge transport characteristics in crystalline organic heterojunctions can be tuned by selecting proper template layer molecules. Controlling the quality of crystalline films is an effective method for adjusting the thickness of the accumulation layers of organic heterojunctions. When single-crystal-like CuPc/ F16CuPc heterojunction films are grown by WEG, the accumulation layer can reach about 40 nm thick because of the high-quality crystalline film and the low density of deep bulk traps.27 Furthermore, periodically alternating singlecrystal-like H2Pc/F16CuPc heterojunctions on a single device can be fabricated by WEG to form organic superlattices.26,28,103 In this structure, the template p-6P monolayer induces the growth of a H2Pc thin film with regular in-plane orientations, and the H2Pc thin film further induces the growth of a F16CuPc M


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Figure 19. (a) I−V curves of a 7 nm polycrystalline film and 1.8, 3.1, 4.4, and 5.7 nm WEG films before the injection of NO2 gas. (b) Relative response of a 7 nm polycrystalline VOPc film, a 1.8 nm WEG film, and a 6 nm p-6P film to NO2 pulses. The relative response is plotted as a function of time as the devices are exposed to various NO2 concentrations. Reprinted with permission from ref 105. Copyright 2011 Elsevier B.V.

Figure 20. (a) XRD patterns for a PTCDA single layer, a H2Pc single layer, and H2Pc grown on a PTCDA layer. Schematic of the bulk structures of (b) α-PTCDA and (c) α-H2Pc projected onto the (102) and (100) planes, respectively. These projection planes correspond to the planes parallel to the substrate in thin films deposited on glass. (d) Schematic of the template growth of H2Pc on PTCDA. Reprinted with permission from ref 108. Copyright 2000 American Institute of Physics.

penetration is also an important factor in room-temperature device recovery, especially in the films fabricated by WEG. In addition, room-temperature NO2 sensors with a fast response and recovery were achieved utilizing ultrathin highly ordered heterojunction films based on PTCDI-Ph grown on a p-6P template layer.48 The use of a highly sensitive VOPc layer on top of the PTCDI-Ph layer to form a double heterojunction dramatically improved the relative-response intensity to nearly 5 times larger than that of a single-heterojunction device, without affecting the fast response and recovery.

single C60 layer or a mixed ZnPc/C60 layer was deposited. The large improvements in short-circuit current (Jsc), external quantum efficiency (EQE) (Figure 18a), and fill factor (FF) are attributed to the single-crystal-like thin film with few bulk traps and a high carrier mobility, resulting in a PCE of 3.07% (Table 3). Furthermore, vertically ordered bulk heterojunction OPVs based on ZnPc and C60 could be fabricated by WEG under certain deposition conditions, and a PCE of 3.10% was achieved.45 In TiOPc/C60 OPVs, a near-infrared (NIR) response was obtained when highly ordered TiOPc films were fabricated by WEG as the donor layer. A high EQE was obtained in the NIR with a peak value over 38%, and the EQE was over 18% in the entire response range (Figure 18b), resulting in a PCE of 2.67%. This excellent response could result from the long exciton diffusion length and high carrier mobility of the highly ordered films. WEG can also be used to fabricate highly sensitive gas sensors.48,104,105 For example, an organic heterojunction gas sensor based on TiOPc/F16CuPc fabricated by WEG showed a strong response to nitrogen dioxide (NO2) below 5 ppm and a detection limit of 250 ppm at room temperature. Furthermore, an ultrathin VOPc film sensor device was successfully fabricated by WEG for room-temperature NO2 sensing. It showed a stronger response and better recovery character than the corresponding device with a polycrystalline film (Figure 19). This could be the reason that the crystal grains show faster desorption behavior than grain boundaries. The bulk

4. PERYLENE-DERIVATIVE MOLECULAR TEMPLATE GROWTH AND ITS APPLICATIONS 4.1. Perylene-Derivative Molecular Template Growth Behaviors

PTCDA is a typical perylene derivative. As introduced in section 2, PTCDA grows with a lying-down orientation on most substrates (single-crystalline, polycrystalline, or amorphous substrates) because of the strong binding of the perylene center as well as the strong interaction between O atoms and the substrate.70,71 This lying-down molecular arrangement improves the charge transport along the vertical direction. However, most other molecular materials such as phthalocyanine compounds grow with a standing-up arrangement on polycrystalline or amorphous substrates such as ITO and SiO2. To tune the molecular orientation, a PTCDA molecular layer N


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could act as a template to induce a lying-down arrangement of overlayer molecules. Forrest et al. first carried out a study on the growth of ultrathin CuPc (2 nm) on an ultrathin PTCDA layer (2 nm) by reflection high-energy electron diffraction (RHEED), in which a quasi-epitaxial growth was found.106 Nanai et al. then studied the bilayer structure of nonplanar VOPc and PTCDA by XRD and optical absorption spectroscopy. They found that a predeposited PTCDA layer could induce the growth of phase I VOPc films with a molecular layer thickness of 0.34 nm, which is a typical π−π interaction distance between two VOPc molecules. In contrast, phase II films with a molecular layer thickness of 1.17 nm are formed by being deposited directly onto the substrate, corresponding to (010)phase II.107 The Jones group has performed detailed studies on both planar and nonplanar phthalocyanine compounds grown on PTCDA template layers.108−118 One typical example is H2Pc grown on a PTCDA template layer. The XRD results showed a new diffraction peak at 26.8° with an interplanar spacing of 3.33 Å for H2Pc films grown on a PTCDA layer, suggesting that H2Pc molecules adopt a different crystal structure with the molecular plane oriented parallel to the substrate, as shown in Figure 20.108 Normally, an α-herringbone H2Pc structure with a standing-up molecular arrangement is most stable for H2Pc grown on noninteracting substrates. In contrast, the lying-down molecular arrangement is more stable on a PTCDA layer. The interplanar stacking distance calculated on the basis of the minimum energy is 3.29 Å, which is very close to the experimentally determined value of 3.33 Å. This obviously indicates that the structural templating effect observed in the H2 Pc/PTCDA heterostructure results from the strong intermolecular interactions between the two different materials at the heterointerface.111 Furthermore, the H2Pc layer deposited on the PTCDA template layer is very stable and could tolerate annealing temperatures up to 330 °C. No transition to any other phase happens during annealing, and the film crystallinity increases (H2Pc normally has a transition from the α-phase to the β-phase at about 325 °C).112 Meanwhile, Sakurai et al. also researched the growth of H2Pc on a PTCDA template layer.119−121 They found that a PTCDA template with a thickness of 0.5 nm almost covers the substrate surface and could induce all H2Pc molecules to arrange themselves lying down.119 The optical absorption intensity of the H2Pc film is significantly influenced by its molecular orientation, which changes gently as its thickness increases during growth on a PTCDA template layer.120 The H2Pc molecular plane is almost parallel to the substrate surface when the H2Pc layer thickness is less than 10 nm. Then the molecular orientation tilts as the thickness of the H2Pc layer increases. When the film is thicker than 300 nm, the tilt angle of the H2Pc molecular plane approaches 26.5°, and the b-axis of the H2Pc crystals becomes perpendicular to the substrate surface. The thin H2Pc layer has a columnar structure, while the thick H2Pc layer has a tangled fiber structure (Figure 21). Similarly, a PTCDA template layer could be used to control the molecular orientation of H2Pc/PTCDA mixed layers and p−i−n triplelayer structures. When a PTCDA template layer is inserted between a H2Pc/PTCDA (1:1) mixed layer and a glass substrate, the orientation of the molecular planes of both H2Pc and PTCDA in the mixed layer becomes parallel to the substrate surface.121 Besides H2Pc, other phthalocyanine compounds such as CuPc, MnPc, F16CuPc, and ClAlPc were also grown on

Figure 21. SEM morphology images of the H2Pc/SiO2 structure with 120 nm thickness H2Pc (a, e) and H2Pc/PTCDA (20 nm)/SiO2 structures with H2Pc thicknesses of 120 nm (b, f), 300 nm (c, g), and 600 nm (d, h). The images in (a)−(d) are SEM surface morphologies, and the images in (e)−(h) are SEM cross-section morphologies. Reprinted with permission from ref 119. Copyright 2006 The Japan Society of Applied Physics.

PTCDA template layers.113−118,122−124 CuPc showed growth behavior similar to that of H2Pc on a PTCDA template layer. In particular, the strong diffraction peak in the XRD patterns at 6.8° corresponding to diffraction from the (200) plane of αCuPc and consistent with a standing-up molecular arrangement disappeared when a PTCDA template layer was added. Instead, a new peak at 26.6° emerged gradually as the PTCDA thickness increased, corresponding to an interplanar separation of 3.33 Å and a lying-down molecular arrangement.114,124 The diffraction peak at 27.6° from the (102) plane of the PTCDA layer appeared as well. Even when the PTCDA was deposited in the (110) orientation instead of the (102) orientation, meaning that the PTCDA was tilted at an angle of 25° relative to the substrate surface, the CuPc molecules were induced to arrange parallel to the PTCDA molecules.123 This suggests that there was strong intermolecular π−π interaction between the CuPc and PTCDA molecules. When CuPc was fully fluorinated to produce F16CuPc, the film morphology changed, although the molecular orientation was similar to those of H2Pc and CuPc layers grown on PTCDA template layers (Figure 22).116 The templated

Figure 22. SEM morphology images (a−c) and corresponding crosssectional images (d−f) of F16CuPc films grown on a PTCDA template layer: (a, d) 20 nm F16CuPc/5 nm PTCDA; (b, e) 80 nm F16CuPc/5 nm PTCDA; (c, f) 80 nm F16CuPc/50 nm PTCDA. The crosssectional images were tilted at 45°. Reprinted with permission from ref 116. Copyright 2011 Royal Society of Chemistry. O


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Figure 23. (a) XRD patterns for films grown on ITO substrates: (i) pristine 70 nm ClAlPc, (ii) 70 nm ClAlPc/1 nm PTCDA, (iii) 70 nm ClAlPc/70 nm PTCDA, and (iv) pristine 70 nm PTCDA. The arrow illustrates an increase in film crystallinity on the templated ClAlPc with PTCDA. (b) Schematic of the proposed structural changes for the ClAlPc film with and without a PTCDA template layer. (c) Representation of the molecular orientation of the 014̅ plane with respect to the substrate. Reprinted from ref 115. Copyright 2010 American Chemical Society.

tilted molecular orientation of DIP and ZnPc thin films than do bare amorphous or polycrystalline substrates (SiO2, ITO, quartz, etc.), and this orientation significantly enhances the absorption of the thin films. Interestingly, Schünemann et al. also observed the coexistence of standing-up and lying-down molecular orientations in some ZnPc and DIP crystal domains deposited on PTCDA template layers.127 As another perylene derivative, DIP can also act as a template material, but its growth behavior is totally different from that of PTCDA. Normally, DIP molecules are arranged standing up on amorphous or polycrystalline substrates,59,66 and it is difficult to induce disklike or rodlike organic molecules to assemble with a lying-down orientation. Interestingly, the structural order of isotropic C60 molecular thin films could be significantly improved by using a DIP template layer on a SiO2 substrate.128 C60 grown on a DIP template layer exhibits an alignment of face-centered cubic (fcc) domains with the (111) plane parallel to the substrate and a significant increase in the coherent inplane island size by a factor of ∼4, leading to a change in the structural order in the C60 top layer (Figure 24). The crystal quality of the C60 thin film is enhanced strongly when 2−3 monolayers of DIP are used.

F16CuPc films were composed of arrays of standing-up nanowire-shaped crystals instead of the needle-like crystals in nontemplated films, and the nanowire arrays were preserved as the F16CuPc thickness increased. The standing-up crystalline nanowire array mainly results from one specific consequence of the F functionalization of CuPc: the highly electronegative F lowers the electron density in the phthalocyanine macrocycle and allows a closer molecular face-to-face stacking. Thus, the F substituents could control the distance, direction, and strength of the π-stacking. In F16CuPc nanowire crystals, the face-to-face intermolecular distance is 3.14 Å, which is smaller than the 3.33 Å distance between CuPc and H2Pc molecules. The enhanced π−π interaction between the diagonally stacked F16CuPc molecules is advantageous for the formation of 1-D nanowire crystals. Although VOPc was reported to grow on a PTCDA template layer, its molecular orientation behavior is not clear because of its complex nonplanar structure. Another nonplanar molecule grown on a PTCDA template layer is ClAlPc.115,117 On a PTCDA layer, the ClAlPc molecular stacking in the films is improved, and the film crystallinity increases (Figure 23). In particular, ClAlPc molecules stack nearly parallel in a lyingdown orientation with respect to the substrate, in which the angle between the two planes is ∼14°. By combining PTCDA template growth and glancing angle deposition (GLAD), ordered nanocolumn-array phthalocyanine semiconductor thin films could be fabricated with controllable molecular orientation.125 As discussed above, the PTCDA molecular template layer induces phthalocyanine molecules to arrange lying down, while the GLAD technique supports the formation of nanocolumn-array thin films. With these thin films, ordered bulk heterojunctions could be fabricated to improve the performance of OPVs. A PTCDA template layer was used to fabricate ZnPc and diindenoperylene (DIP) thin films with controllable molecular orientations as well.126,127 Schünemann et al. performed a detailed study on the growth of ZnPc and DIP on PTCDA template layers using variable-angle spectroscopic ellipsometry (VASE) and GIXD.127 They found that ZnPc and DIP films grow similarly on both PTCDA and on metal substrates such as Au and Ag. The PTCDA template layer induces a much more

4.2. Applications in Organic Electronics and Optoelectronics

One of the aims for developing PTCDA template growth is to obtain a lying-down molecular orientation. In this case, the π−π conjugated direction is vertical to the substrate, and the charge transport along this direction can be improved. Thus, PTCDA template growth could improve the performance of vertically structured devices such as OPVs and VOFETs. Another aim is to form nanowire or columnar arrays for the fabrication of bulk heterojunctions and potentially improve the OPV performance. Sullivan et al. first introduced PTCDA template growth to improve CuPc/C 60 OPV performance.114 They clearly demonstrated that the charge collection could be improved by aligning the stacking axis of the molecular columns with the desired direction of charge transport. A 60% improvement in Jsc was obtained using an optimized PTCDA thickness of 1 nm. However, because of the mismatch between the energy levels between PTCDA, ITO, and CuPc, the PTCDA layer causes a P


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Figure 24. (a, b) Schematic of the domain orientations of C60 thin films grown on SiO2 and DIP/SiO2, respectively. (c) Correlation between the in-plane coherent island size (ls) of C60 thin films and the DIP film thickness, which is determined from the averaged values of Bragg reflections on the basis of GIXD experiments. Reprinted from ref 128. Copyright 2010 American Chemical Society.

Figure 25. (a) UPS data for 1.5 nm thick PTCDA, 5.0 nm thick CuPc, and 5.0 nm thick templated films of DIP and CuPc on ITO. The highenergy cutoff of CuPc shifts by ∼0.2 eV when CuPc is templated on PTCDA compared to films on ITO. Dashed lines indicate the intercepts of the data to the energy axis. (b) Energy level diagrams inferred from the measured HOMO energies of CuPc and PTCDA. The schematic should be corrected as the work function of ITO is normally 4.7−5.1 eV, and the Ef of ITO should match the HOMO of CuPc; i.e., the HOMO of CuPc is below the Ef of ITO. Symbols and colors in (a) correspond to those in (b). Reprinted with permission from ref 122. Copyright 2010 The Optical Society of America.

20% reduction in Voc (Table 4). To improve the interface properties, a DIP layer was inserted between the PTCDA and CuPc.122 The inserted DIP layer could propagate the templating effect of PTCDA, act as an exciton blocking layer, and influence the surface morphology of the subsequently deposited films. Increases in both Jsc and Voc were obtained, leading to a concomitant increase in PCEs. On the other hand, inserting a molybdenum oxide (MoO3) layer before the deposition of the PTCDA template layer also increased the Jsc but did not sacrifice the Voc, resulting in an obvious improvement in PCEs as well.129 As discussed above, this is a universal method to improve the Jsc of MPc-based OPVs using the PTCDA MTG (Table 4). The HOMO and LUMO energy levels of organic semiconductor thin films could be tuned by molecular orientation.130,131 The ultraviolet photoelectron spectroscopy (UPS) data in Figure 25a for CuPc films grown with and without

PTCDA template layers suggest that the PTCDA template layer induces a shift in the highest energy cutoff with a difference of about 0.23 eV below the Fermi level upon templating, while the shift in the vacuum level is about 0.15 eV. Hence, the HOMO energy of PTCDA-templated CuPc increased by about 0.08 ± 0.02 eV. The PTCDA/CuPc interface could act as a type II (staggered) heterojunction, while

Table 4. Device Parameters for OPVs Based on CuPc/C60 and CuPc/PCBM as Well as ClAlPc/C60 Heterojunctions Fabricated with and without a PTCDA Template Layer cell

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

ITO/CuPc/C60/BCP/Al (PHJ) ITO/PTCDA(1 nm)/CuPc/C60/BCP/Ala (PHJ) ITO/PTCDA/DIP/CuPc/C60/BCP/Alb (PHJ) ITO/CuPc/CuPc/C60/C60/BCP/Alb (PM-HJ) ITO/PTCDA/CuPc/CuPc/C60/C60/BCP/Alb (PM-HJ) ITO/PTCDA/DIP/CuPc/CuPc/C60/C60/BCP/Alb (PM-HJ) ITO/CuPc/C60/Alq3/Alc (PHJ) ITO/MoO3/CuPc/C60/Alq3/Alc (PHJ) ITO/MoO3/PTCDA/CuPc/C60/Alq3/Alc (PHJ) ITO/CuPc/PCBM/Alq3/Alc (PHJ) ITO/MoO3/CuPc/PCBM/Alq3/Alc (PHJ) ITO/MoO3/PTCDA/CuPc/PCBM/Alq3/Alc (PHJ) ITO/ClAlPc/C60/BCP/Ald (PHJ) ITO/PTCDA/ClAlPc/C60/BCP/Ald (PHJ) ITO/MoO3/ClAlPc/C60/BCP/Ald (PHJ) ITO/MoO3/PTCDA/ClAlPc/C60/BCP/Ald (PHJ)

0.50 0.40 0.54 0.50 0.48 0.50 0.39 0.56 0.53 0.44 0.61 0.60 0.59 0.46 0.81 0.79

2.56 4.06 6.60 6.20 7.10 8.10 3.54 3.76 5.31 3.16 3.15 4.29 5.33 6.13 5.30 6.53

0.57 0.50 0.62 0.61 0.59 0.63 0.55 0.50 0.54 0.53 0.55 0.56 0.58 0.50 0.58 0.58

0.73 0.81 2.19 1.89 1.99 2.49 0.76 1.05 1.48 0.73 1.05 1.44 1.80 1.40 2.60 3.00

a

a

Reference 114. bReference 122. cReference 129. dReference 115. Q


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Figure 26. Energy level alignments of (a) C60/ClAlPc/MoO3 and (b) C60/ClAlPc/PTCDA/MoO3 derived from PES data. Reprinted with permission from ref 117. Copyright 2012 American Institute of Physics.

Figure 27. (a) UPS data from 13 nm C60 grown on SiO2 and 4 nm DIP/SiO2, respectively. The inset shows the normalized HOMO regions of both data sets with the C60/DIP data shifted by 150 meV. (b) Energy level alignments derived from UPS data in (a). Reprinted from ref 128. Copyright 2010 American Chemical Society.

molecules is dependent on their relative orientation. For example, the usual orientation of the molecules in the MPc thin film results in the vanishing of the interaction between magnetic moments, and thus, MPc nanomagnets point in random directions and do not create a net magnetic moment. However, when the molecular orientation is slightly rotated on different substrates, the magnetic interaction between the molecules could be switched on.133 Serri et al. found that nanostructured CoPc thin films fabricated by PTCDA templating could produce high-temperature antiferromagnetism.132 As compared with nontemplated CoPc thin films, PTCDA-templated ones have longer antiferromagnetic chains and lower exchange coupling values (J/kB = 80 and 107 K for templated and nontemplated CoPc thin films, respectively). For DIP-templated C60 thin films, UPS spectra only show a shift of 150 meV resulting from the different energy level alignments of the C60 thin film with the DIP layer and the SiO2 substrate (Figure 27). The other characteristics of the UPS spectra from the templated and nontemplated C60 films are essentially identical. Normally, large changes in the structure and domain orientation of organic semiconductor thin films would lead to a significant change in the spectral width of the HOMO.130,134 The absence of any difference in the spectral width of the C60 HOMO suggests that the size of the small coherent islands in C60 films has almost no significant impact

DIP could act as an exciton blocking layer in a type I (nested) heterojunction with CuPc. In ClAlPc/C60 planar heterojunction OPVs, the Jsc could also be improved using only a PTCDA template layer, but the Voc decreased as well (Table 4). Both Jsc and Voc could be optimized by inserting both MoO3 and PTCDA template layers. The effect of the MoO3 interlayer is to minimize the losses in Voc and FF caused by significant band bending and pinning of the adjacent organic layer HOMO levels to nonstoichiometric defect states in the near-Fermi-level region of MoO3. The energy level difference between the HOMO of the donor and the LUMO of the acceptor, i.e., EDHOMO − EALUMO, is an important factor affecting the magnitude of the Voc in heterojunction OPVs. On the basis of the angledependent X-ray absorption spectra (XAS) and PES, level diagrams could be derived (Figure 26). The measured EDHOMO − EALUMO values were 1.05 and 1.15 eV for PTCDA-templated and nontemplated heterojunctions on the MoO3/ITO substrate. This small difference has little effect on the Voc values, which are 0.79 and 0.81 eV, respectively. In addition, PTCDA-templated growth could also be used to tune the magnetic properties of the thin films.113,117,132 In particular, the crystal form of MPc thin films can be controlled by growing them on different substrates or template layers, and the interaction between the magnetic moments of neighboring R


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template-based growth behavior was also found on flexible PET or poly(methyl methacrylate) (PMMA) substrates, which can potentially be used as flexible mechanical supports for completely flexible electronic devices.139,140 Another typical material for organic semiconductor thin-film growth using a pentacene template layer is rubrene, which has one of the highest mobilities among organic semiconductor materials.97,141 A highly oriented crystalline rubrene thin film could be fabricated on pentacene, whereas normally rubrene thin films grown without a pentacene template layer are amorphous.142−146 Pentacene has better affinity/wettability for most amorphous substrates than rubrene but is compatible with rubrene as well, and thus, it can potentially improve the crystallinity of the rubrene film. Meanwhile, pentacene crystals could also act as crystal seeds to induce the crystallization of the rubrene thin film. On the pentacene template layer, rubrene grows in the layer-by-layer mode instead of by amorphous aggregation as well. The crystalline rubrene film is orthorhombic, and its XRD diffraction peaks could be indexed with crystal lattice parameters of a = 1.444 nm, b = 0.718 nm, and c = 2.697 nm.147 Thus, c-axis-oriented rubrene thin films form on pentacene-templated substrates. The oriented growth of a polycrystalline rubrene thin film on a pentacene template layer grown on a rubbed PVA surface was also reported.145 Highly oriented rubrene crystallites with the c-axis (called the a-axis in ref 145, because the a-axis and c-axis were switched) aligned along the surface normal and the (200) plane preferentially oriented 45° away from the rubbing direction were found. In contrast, rubrene thin films deposited on a PVA or rubbed-PVA substrate without a pentacene template layer were amorphous. The third typical type of material used for organic semiconductor thin-film growth on a pentacene template layer is phthalocyanines. Several phthalocyanine compounds grown on pentacene template layers were reported.148−153 For example, F16CuPc grown on pentacene exhibits two competitive structures: a standing-up structure (s-structure) on top of the pentacene terraces and a lying-down structure (lstructure) along the pentacene step edges (Figure 29).148 Normally, pentacene terraces expose the (001) facets, corresponding to the lowest energy pentacene planes. Similarly, F16CuPc also adopts its energetically preferred standing configuration on the pentacene (001) plane, like F16CuPc grown on other weakly interacting substrates such as SiO2 or polymers.154,155 On the other hand, the F16CuPc molecules at pentacene steps adopt an orientation with the molecular plane parallel to the substrate surface because of the different energetic and electronic environments. This orientation results in a rather strong π−π interaction between π orbitals favoring cofacial vertical stacking. Hence, after the initial nucleus with the l-structure is formed, a rapid vertical growth of this lyingdown molecular orientation proceeds. The effect of a pentacene template layer on the H2Pc molecular orientation in thin films was studied as well.149−151 It was found that the pentacene buffer layer caused the H2Pc molecules to have only one orientation with the molecular plane parallel to the substrate surface, even though the H2Pc molecular plane is perpendicular to the substrates without a pentacene template layer. In contrast, a pentacene template layer did not cause planar CuPc molecules to have a lying-down orientation; they remained standing up as they are on other substrates such as SiO2 and ITO. Nevertheless, the pentacene template layer could facilitate the formation of highly crystalline CuPc films with larger grain sizes and fewer grain

on the polarization energy or intermolecular interactions because of the highly symmetric shape of the C60 molecule and its rotation at room temperature.135 Hence, it shows much smaller variations in local polarization or interactions due to crystal defects, unlike anisotropic rodlike molecules. Besides PTCDA and DIP, another perylene derivative, N,Ndioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), was also used as a template layer to induce the growth of crystalline pentacene thin films with enlarged grain sizes and laterally wellordered crystallinity. This approach yielded an enhanced performance of pentacene-based OFETs, with a high mobility up to 1.71 cm2/(V s) and a low threshold voltage of −4 V.136

5. ACENE MOLECULAR TEMPLATE GROWTH AND ITS APPLICATIONS 5.1. Acene Molecular Template Growth Behaviors

Pentacene, as a typical acene molecule, can easily form films with large-size domains.57 Because of its high field-effect mobility, pentacene is widely used in OFETs and OPVs. Meanwhile, pentacene is also a good candidate as a template layer to induce the growth of other organic semiconductor thin films with highly ordered domains. Normally it has a triclinic structure, which is different from the monoclinic structure of p6P.137 Thus, the growth behavior of pentacene-templated films is not the same as that of p-6P-templated layers. Itaka et al. studied the growth of C60 using a pentacene template layer.138 Highly oriented crystalline C60 thin films could be fabricated on molecular substrates covered with an atomically flat pentacene monolayer for wetting control. The pentacene monolayer supports the crystallization of C60 molecules, which forms hexagonal crystalline grains with very smooth surfaces (Figure 28). Normally the C60 film is amorphous, but the pentacene-templated C60 film exhibited the strong diffraction peaks of a (001) oriented crystal. In these studies, Al2O3 or SiO2 was used as the growth substrate. Similar

Figure 28. (a) and (b) are AFM morphology images of 20 nm C60 thin films grown on a sapphire substrate without and with an atomically flat pentacene monolayer. (c) and (d) are their XRD patterns, respectively. Reprinted with permission from ref 138. Copyright 2006 John Wiley & Sons, Inc. S


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Figure 29. (a) Evolution of XRD data of a 13 nm thick pentacene film upon subsequent deposition of F16CuPc. The inset represents the evolution of the spacing (open symbols) and integrated intensity (filled symbols) obtained from fits to the (002) Bragg reflection. (b) AFM morphology image of 8 Å F16CuPc grown on 3.5 nm pentacene. (c) Topographic profile of the marked line in (b) and a schematic representation of the different molecular orientations forming on the pentacene template layer. Reprinted from ref 148. Copyright 2006 American Chemical Society.

Table 5. Charge Mobilities of OFETs Fabricated with and without Pentacene MTG as Well as Those of Single-Crystal OFETs mobility (cm2/(V s)) template material

overlayer material

with template layer

without template layer

single crystal

pentacene pentacene pentacene

C60138−140,158 a rubrene97,142−144,159 CuPc89,152

2.0−5.17 0.07−0.6 0.20

0.25−1.0 10−2−10−5 0.03

3.0−11 1.0−20 0.1−1.0

Using PET as a flexible substrate, the electron mobility could also be improved with a pentacene template layer. Increasing pentacene layer thickness could lead to ambipolar device behavior and further increases in the electron mobility.139 An electron mobility as high as 5.17 cm2/(V s) could be obtained using a pentacene template layer on a PMMA insulator and a bathophenanthroline (Bphen)/Ag bilayer electrode.140

a

boundaries.152 Pentacene can also serve as a template layer for the growth of nonplanar PbPc thin films. It considerably changes the morphology of the PbPc layer, resulting in an improved crystallinity in the early stages of PbPc film growth and a higher amount of the triclinic phase.153 Besides pentacene, another acene, tetracene, can also act as a template layer to induce the growth of rubrene thin films.156 Highly oriented crystalline rubrene nanostructures were successfully grown on a tetracene template layer with an atomic-scale corrugation matching the prominent corrugation of the close-packed planes of rubrene crystals. The natural corrugation of the (001)tetracene surface, with 4.96 Å periodicity, allows sufficient adhesion to induce solidification of rubrene in its orthorhombic phase. The low symmetry and chirality of (001)tetracene promote the selective nucleation of rubrene crystalline domains with a unique orientation. The high crystallinity of the rubrene film and its unique orientation were achieved through a line-on-line epitaxial relation. Meanwhile, tetracene could also template the growth of phthalocyanine compounds such as ZnPc.157 The tetracene template layer improves the film crystallinity and induces

molecular arrangement with both standing-up and lying-down molecular orientations. 5.2. Applications in Organic Electronics and Optoelectronics

The aim of acene MTG is to improve the OFET performance. Because acenes have better wettability on most amorphous substrates than other organic molecules such as C60 and rubrene, they can facilitate the formation of these thin films and increase their crystallinity. Highly crystalline thin films are desired to improve the charge transport in OFET devices. The performance parameters of OFETs fabricated with and without pentacene-templated growth are summarized in Table 5. The charge mobility of OFET devices is obviously improved by pentacene-templated growth. For C60 and CuPc, the pentacenetemplated OFET mobilities reach their single-crystal levels. Although the mobilities of the pentacene-templated rubrene OFETs are much smaller than those of single-crystal rubrene OFETs, they are over 1 order of magnitude higher than those of the amorphous rubrene OFETs fabricated without a pentacene template layer. T


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Figure 30. (a) UPS spectra in the high-binging-energy cutoff region of rubrene with different thicknesses on pentacene (left) and the HOMO region for the rubrene layers (right). (b) XPS spectra of the C 1s core levels as a function of the rubrene coverage on the pentacene layer. (c) Energy level diagrams of rubrene grown on a pentacene template layer. Reprinted from ref 144. Copyright 2007 American Chemical Society.

interface electronic structure totally changes if pentacene is grown on the rubrene layer. In other words, the electronic structure of the pentacene−rubrene interface has a strong dependence on the interface characteristics and, in particular, on the layer order. Inserting a pentacene layer between layers of rubrene and SiO2 can increase the hole-transition probabilities, which results in an enhancement of the charge transport. In contrast, inserting a rubrene layer between pentacene and SiO2 layers decreases the hole-transition probability. A pentacene template layer can cause H2Pc and F16CuPc molecules to adopt the lying-down molecular orientation, which can also enhance their optical absorption.148−151 As discussed in section 4.2, pentacene template layers could potentially improve the OPV performance, especially the Jsc. However, the OPV performance is even worse when pentacene template layers are used because they produce a large hole injection barrier between H2Pc and pentacene and increase the interface resistance.150 When pentacene was inserted between the ITO and PbPc as a template layer, it optimized the PbPc morphology and improved the crystallinity of the PbPc film with the triclinic phase, resulting in a stronger absorption in the

The pentacene template layer not only induces the growth of overlayers to form highly ordered thin films and improve the charge transport, but also improves the interface electronic structure. For example, the high-binding-energy cutoff position in UPS spectra is 0.4 eV higher when a 25.6 nm rubrene layer is deposited than for the bare pentacene surface (Figure 28a, left), suggesting that rubrene has a lower vacuum level than pentacene.144 This shift results from an interface dipole layer between rubrene and pentacene. The electrons at the interface between rubrene and pentacene redistribute as rubrene is deposited on pentacene. The HOMO onset is 1.16 eV below the Au Fermi level (Figure 30a, right). The C 1s XPS spectra for rubrene grown on pentacene showed that the C 1s peak for pentacene is shifted toward lower binding energy, indicating charge redistribution from pentacene to rubrene (Figure 30b). On the other hand, the C 1s peak of rubrene emerges after only 0.2 nm of rubrene is deposited, and the peak shift reaches a maximum of 0.27 eV toward the low-binding-energy side for the 25.6 nm rubrene layer grown on pentacene. These results obviously suggest that band bending occurs on both sides of the pentacene−rubrene interface (Figure 30c). However, the U


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Table 6. Device Parameters for OPVs Based on Pb/C60 and ZnPc/C60 Heterojunctions Fabricated with and without a Template Layer of Pentacene or Tetracene cell

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

ITO/PbPc/C60/BCP/Al (PHJ) ITO/pentacene (10 nm)/PbPc/C60/BCP/Ala (PHJ) ITO/PEDOT:PSS/ZnPc/C60/PTCDI/BCP/Alb (PHJ) ITO/PEDOT:PSS/tetracene/ZnPc/C60/PTCDI/BCP/Alb (PHJ) ITO/PEDOT:PSS/tetracene/ZnPc/ZnPc/C60/C60/PTCDI/BCP/Alb (PM-HJ)

0.50 0.50 0.64 0.67 0.66

6.00 9.60 6.20 10.30 13.90

0.53 0.51 0.68 0.69 0.63

1.59 2.24 2.80 4.70 5.80

a

a Reference 153. bReference 157. The modified ZnPc was synthesized by a solvent-free reaction using inexpensive reagents 1,2-dicyanobenzene and zinc chloride to produce partially chlorinated ZnPc.160

the substrates,177,178 although these topics are not addressed here. Unlike most other template materials, graphene forms single atomic layers, is transparent, and is highly conductive. Thus, aside from templating, it can potentially serve as a transparent electrode in organic electronic and optoelectronic devices. Graphene is a good candidate as a template layer to induce the thin-film growth of phthalocyanine compounds such as FePc, CuPc, F16CuPc, or ClAlPc.179−186 Similar to the PTCDA template, the graphene template layer causes the phthalocyanine molecules to orient lying down instead of standing up as they would normally on bare amorphous or polycrystalline substrates (Figure 31). Furthermore, CuPc grown on a

near-infrared and an enhanced EQE over the entire spectrum where PbPc absorbs. Bilayer PbPc (20 nm)/C60 (40 nm) OPVs with a pentacene template layer exhibited a 48% enhancement in Jsc with no obvious change in Voc and FF, and the PCE was improved from 1.59% to 2.45% (Table 6).153 When tetracene was inserted as a template layer and anode interfacial layer, the EQEs of the efficient small-molecule OPVs based on modified ZnPc and C60 were enhanced by 150% in the spectral region of ZnPc absorption.157 This enhancement results from the increased crystallinity and more uniform molecular stacking achieved when the tetracene template layer was used, which can potentially increase the exciton diffusion length in the ZnPc layer. Meanwhile, tetracene could also act as an anode interfacial layer, providing exciton blocking at the anode/donor layer. PCEs of 4.7% and 5.8% were achieved for planar heterojunction and planar mixed heterojunction OPVs, respectively (Table 6). In particular, a peak EQE of nearly 70% was obtained in planar mixed heterojunction structural OPVs, which is almost the maximum EQE achieved for phthalocyanine-based OPVs.

6. GRAPHENE/GRAPHENE OXIDE TEMPLATE GROWTH AND ITS APPLICATIONS 6.1. Graphene/Graphene Oxide Template Growth Behaviors

Since the first report on graphene by Geim and Novoselov in 2004,161 it has rapidly become a major topic of interest in the areas of materials science, condensed-matter physics, energy science, and engineering, among others.162−168 Graphene is a flat monolayer of carbon atoms tightly packed into a strictly 2D honeycomb lattice, and it exhibits exceptionally good crystalline and electronic qualities. It has already revealed a variety of new physics, and many potential applications have been proposed. Graphene oxide (GO) is one of the precursors for graphene synthesis by either chemical or thermal reduction. It consists of a single layer of graphite oxide in which various oxygen-containing functional groups, mostly hydroxyl and epoxy groups, are present on the basal plane and smaller amounts of carboxy, carbonyl, phenol, lactone, and quinone groups are present at the sheet edges. Several reviews have been focused on GO material, including its fabrication, functionalization, and applications.169−173 In this section, we focus only on the use of graphene or GO as a molecular template layer inducing the growth of organic semiconductor thin films. In particular, we examine the use of graphene to optimize the film quality and the device performance by controlling the film morphology, molecular orientation, and interface electronic structure. However, we note that graphene template layers can also be used to fabricate high-quality inorganic nanostructures174−176 or to study water and organic molecule adlayers on

Figure 31. (a) SEM morphology images of CuPc thermally evaporated on glass and graphene/glass as well as the schematics of CuPc molecular orientation, with a standing-up orientation on glass and a lying-down orientation on graphene/glass. Reprinted from ref 180. Copyright 2012 American Chemical Society. (b) Schematics of the molecular orientations for CuPc/F16CuPc grown on ITO and graphene/ITO, respectively. Reprinted from ref 181. Copyright 2012 American Chemical Society.

graphene template layer had a much larger domain size than that obtained without a graphene layer. In fact, highly ordered and continuous CuPc thin films with thicknesses of several hundred nanometers could be formed.180 A graphene template layer can also control the molecular orientation in organic− organic heterostructures such as CuPc/F16CuPc junctions.181 F16CuPc molecules first orient with their π−π stacking direction almost vertical to the substrate, i.e., lying down, because of the interfacial π−π interaction between F16CuPc and graphene. The exposed molecular π-plane of the lying-down V


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Figure 32. (a) Schematic diagrams illustrating the nucleation and growth of graphene-coated PDI wires (PDI-G wires), (b) SEM image of PDI-G wires, and (c) XRD patterns of as-grown PDI wires and conventional PDI wires obtained from the two-phase solvent method. Reprinted from ref 210. Copyright 2010 American Chemical Society.

commensurate match between PTCDI molecules and the moiré structure. The intermolecular hydrogen bonds (NH···O) between imide groups on neighboring molecules would further stabilize this configuration. However, this interaction could be systematically modified by introducing alkane chains into the PTCDI molecules, and ultimately a triangular molecular junction could be stabilized instead of a linear one.194,195 More interestingly, the stabilization of the trimer vertex could be progressively enhanced by increasing the alkyl chain length from zero for PTCDI to the long chain length in 1,7dibutylcoronene-3,4,9,10-tetracarboxylic acid bisimide (DBCTCDI), which changed the monolayer morphology from rows to a honeycomb. In addition, the van der Waals interactions between alkyl chains also stabilized the trimer vertex. Some organic molecules could form strong chemical bonds with graphene, as summarized by Hong et al. in a recent review.196 These organic molecules include 7,7,8,8-tetracyanoquinodimethane (TCNQ) and fluorinated TCNQ (F4TCNQ), 1 9 7 − 2 0 0 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 4-amino-TEMPO,201−203 benzene and its derivatives, 2 0 4 − 2 0 7 and 4-nitrophenyldiazonium (4NPD).208,209 Their growth behaviors and mechanisms have been discussed in detail.196 In addition, Wang’s research showed that graphene could act as a template layer and structural scaffold for the synthesis of graphene−organic hybrid wires with controllable photovoltaic properties. In these materials, graphene promotes the nucleation and assembly of organic nanostructures.210 The π−π stacking interactions between graphene and aromatic organic molecules lead to synergistic binding interactions. After organic wires are seeded on the graphene template, the outgrown organic wires in turn act as 1-D scaffolds coated with graphene sheets to form a unique graphene−organic hybrid structure. For example, graphene−N,N-dioctyl-3,4,9,10-perylenedicarboximide (PDI) organic wires have been fabricated using this graphene templating approach (Figure 32). The graphene-template-assisted synthesis yields better textural characteristics than the conventional two-phase solvent method, extremely long wires, and a uniform diameter.

F16CuPc molecules further acts as a structural template that influences the molecular orientation of the subsequently deposited CuPc through intermolecular π−π interactions. In other words, the effect of the graphene template can be transferred from one molecule to another molecule in organic− organic heterostructures if the interfacial π−π interactions are strong enough. The growth behavior and molecular orientation of nonplanar phthalocyanine ClAlPc on a graphene template layer are similar to those of the planar CuPc and F16CuPc, and ClAlPc also has a lying-down orientation.182 Graphene is a good template layer for inducing the growth of rodlike molecules. High-quality p-6P thin films with a lyingdown molecular orientation and smooth interfaces are obtained on graphene because of the layer-by-layer growth mode.68 For pentacene grown on epitaxial graphene on Ru(0001), the corrugated moiré structure of the graphene/Ru(0001) interface can significantly modulate the growth behavior.187 The configuration with pentacene adsorbed on fcc regions is the most stable, which causes selective adsorption at low coverage. At high coverage, an amorphous structure, local ordering, or long-range order of the pentacene molecules can be selected by optimizing the deposition rate and substrate temperature. A graphene template layer grown epitaxially on a SiC substrate, which is the normal procedure, or grown by chemical vapor deposition on a Ru(0001) surface can induce the thinfilm growth of perylene derivatives such as PTCDA,188−192 PTCDI,193 or PTCDI-alkyl molecules194,195 as well. The selfassembled PTCDA monolayer on the graphene surface was well-ordered and uniform, with a typical domain size of hundreds of nanometers. It is in a herringbone arrangement and is similar to the (102) plane of the PTCDA bulk crystal structure.191 Because of the weak molecule−substrate interactions and strong intermolecular interactions, the orientation of the PTCDA monolayer on epitaxial graphene is not correlated with the symmetry of the underlying substrate, and the PTCDA domains exhibit relative ordering at arbitrary angles. For PTCDI grown on graphene/Rh(111), a strong influence of the moiré structure on the ordering of the templated PTCDI layer was observed.193 The alignment of PTCDI molecules with extended 1-D chains results from a W


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GO can act as a template layer as well. However, because of its various oxygen-containing functional groups on the basal plane and at the sheet edges, including randomly distributed hydroxyl and epoxy groups, the interfacial π−π interactions can be obscured. On the other hand, hydrogen bonds can form easily between oxygen-containing functional groups and overlayer molecules. Such hydrogen bonding is stronger than the interfacial π−π interactions and leads to a standing-up molecular orientation instead of the lying-down molecular orientation. A typical model of this system is F16CuPc grown on GO.211 Hydrogen bonding between hydroxyl groups on the GO and fluorine on the F16CuPc is thus more important than the π-stacking. The arrangement of F16CuPc molecules with their edges aligned with the substrate maximizes the number of fluorine atoms in close proximity to hydroxyl groups on the GO surface, and this orientation templates the subsequent growth. Meanwhile, the randomly distributed hydroxyl and epoxy groups, like polymer residues, on the GO surface could change the molecular orientation from the lying-down to the standingup configuration.212 Besides templating small organic molecules, GO could also template polymer chains in bulk heterojunctions. The polymer chain arrangement in the bulk heterojunction based on the electron-donating polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and the fullerene electron acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) could be tuned using a GO template layer.213 The grazing incidence X-ray scattering (GIXS) results showed that the PTB7 stacks have an enhanced face-on order and undergo π−π interactions with the GO template, which both favor photogenerated hole extraction. In contrast, on the PEDOT:PSS (poly(3,4-(ethylenedioxy)thiophene)−poly(styrenesulfonate)) layer, there is a higher percentage of PTB7 chains with their edges oriented perpendicular to the substrate surface, which leads to reduced charge extraction by the anode (Figure 33). Moreover, inkjet-printed GO nanosheets could act as a template layer for the fabrication of transparent single-walled carbon nanotube (SWCNT) patterns on hydrophilic substrates.214

Figure 33. (a) Chemical structures of the PTB7 donor polymer and PC71BM acceptor. (b) Schematic of raw 2D GIXS data and processing procedure for comparing signal intensities from different substrates. (c) Structural model from GIXS data showing preferential face-on PTB7 π-stacking on GO/ITO substrates. (d) Horizontal linecuts from the 2D GIXS data collected for PTB7/PC 71 BM films on PEDOT:PSS/ITO and GO/ITO substrates. (e) Vertical linecuts from the same 2D GIXS scans shown in panel d. Reprinted from ref 213. Copyright 2011 American Chemical Society.

direction, which shows the potential application in OPV and VOFET devices. Normally, the standing-up molecular orientation improves OFET performance because the charge transport in OFETs is parallel to the π−π stacking direction. Thus, graphene inhibits charge transport along the substrate plane by causing the molecules to arrange themselves lying down. However, the polymer residues remaining on the graphene surfaces could induce a standing-up orientation for pentacene overlayers.212 Hence, pentacene OFETs using source/drain monolayer graphene electrodes with polymer residues show a high fieldeffect mobility of 1.2 cm2/(V s). In contrast, pentacene adopts the lying-down molecular orientation on clean graphene because of the π−π interaction between the pentacene and the clean graphene electrode without polymer residues. This adversely affects the lateral charge transport at the interface between the electrode and channel (Figure 34). Using patterned graphene is a simple way to modify metal electrodes for high-performance organic devices. Both Ag and Cu were covered with patterned graphene and used as the bottom source/drain electrodes in pentacene-based OFETs.215,216 The strong interaction between graphene and pentacene causes the pentacene molecules to lie down on the electrode and form a buffer surface for subsequent molecule packing. A large grain size was observed in the pentacene domains on graphene, and pentacene crystals can cross the graphene/channel interface, leading to improved electrode/ pentacene contact. Meanwhile, the graphene layer lowers the hole-injection barrier at the electrode/pentacene surface, which

6.2. Applications in Organic Electronics and Optoelectronics

As discussed above, a graphene template layer can control the molecular orientation in an organic−organic heterostructure and influence its interfacial electronic properties.181,184,196 The energy level diagrams derived from UPS and inverse photoemission spectroscopy (IPES) measurements show that the F16CuPc/CuPc heterojunction interface has a much larger vacuum level shift in its work function for the standing-up molecular orientation on ITO than for the lying-down molecular orientation on graphene/ITO. This larger vacuum level shift leads to a larger HOMO onset shift and more interfacial charge transfer.181 Because of the strong charge transfer at the CuPc/F16CuPc interface obtained for the standing-up molecular orientation on ITO, an accumulationtype organic−organic heterojunction is formed, which is disadvantageous for light-induced charge separation at the donor−acceptor interface. While the lying-down molecular orientation on graphene/ITO leads to weaker charge transfer at the CuPc/F16CuPc interface, it does improve the light-induced charge separation and the charge transport along the vertical X


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Figure 34. AFM morphology images of 50 nm pentacene films near the interface between SiO2 and untreated graphene electrodes (a) and thermally treated graphene electrodes (b). Schematics of molecular packing orientations near the interface between SiO2 and untreated graphene electrodes (c) and thermally treated graphene electrodes (d). Reprinted from ref 212. Copyright 2011 American Chemical Society.

Table 7. Device Parameters for OPVs with and without Graphene or a GO Template Layer cell a

ITO/PEDOT:PSS/P3HT/PDI/LiF/Al (BHJ) ITO/PEDOT:PSS/P3HT/PDI-G mixture/LiF/Ala (BHJ) ITO/PEDOT:PSS/P3HT/PDI-G hybrid wires/LiF/Ala (BHJ) ITO/PEDOT:PSS/PTB7/PC71BM/LiF/Alb (BHJ) ITO/GO/PTB7/PC71BM/LiF/Alb (BHJ) ITO/PEDOT:PSS/P3HT/PCBM/Alc (BHJ) ITO/GO/P3HT/PCBM/Alc (BHJ) ITO/ZnO/C60-SAM/P3HT/PCBM/PEDOT:PSS/Agd (BHJ) ITO/ZnO/C60-SAM/P3HT/PCBM/GO/Agd (BHJ) a

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0.49 0.90 0.78 0.74 0.72 0.58 0.57 0.64 0.64

0.78 0.01 3.85 14.55 15.21 11.15 11.40 8.66 8.70

0.17 0.18 0.36 0.68 0.68 0.57 0.54 0.64 0.65

0.064 0.002 1.04 7.46 7.39 3.60 3.50 3.54 3.61

Reference 210. bReference 213. cReference 219. dReference 220.

devices, severely eroding the OPV performance in hours. In contrast, the GO devices survive for a much longer time period, which is likely associated with the reduced diffusion of water through the active layer. In addition, there are several other reports on the use of GO instead of PEDOT:PSS in conventional and inverted-structure OPVs.218−224 The reported device performance parameters are summarized in Table 7. It is well-known that the electronic states of single-layer graphene are well described as a Dirac core with the valence band and conduction band joining together at the Dirac point. This means that pristine single-layer graphene is semimetallic. To extend the application of graphene in electronic devices, it is necessary to change the energy band of pristine graphene to transform it into a semiconducting material, i.e., to open the band gap of graphene. For this purpose, organic molecule overlayers grown on graphene can influence its energy band and open its band gap.196 For example, TCNQ and F4-TCNQ grown on graphene opened the band gap of graphene by 0.175 and 0.198 eV, respectively.225 On one hand, this results from the strong interaction between the organic molecules and the graphene, which may break the 6-fold symmetry of graphene. On the other hand, the organic molecules can cause p-doping or n-doping of the graphene, which also opens the band gap.

dramatically lowers the contact resistance. In particular, graphene-modified electrodes showed a contact resistance of 0.16−0.18 MΩ, about 1 order of magnitude lower than that of the Cu or Ag electrode. Thus, a mobility up to 1.2 cm2/(V s) was achieved for pentacene-based OFETs with graphene/Au bottom electrodes and channel lengths of 5 μm. This is one of the best results for OFETs with a bottom-contact configuration and a narrow channel length. Graphene and GO are good templates in terms of the improvements obtained in OPV performance. For example, the PDI-G hybrid structure shows enhanced performance as compared with its individual components in donor−acceptortype (PDI−graphene/polythiophene) OPVs. The PDI-G hybrid wires are far superior to pure PDI and to the PDI-G mixture in all performance parameters (Table 7). The vastly improved performance results from the efficient exciton dissociation at the P3HT and PDI-G hybrid interface, as well as the efficient charge transport enabled by the conjugated network of graphene shells surrounding the PDI core.210 GO is both a good template layer and a good hole transport material replacing PEODT:PSS in OPVs.213,217−224 For example, the GO also offers greater light transmission and more efficient face-on stacking in a PTB7/PC71BM heterojunction fabricated on a GO template layer instead of on a PEDOT:PSS layer,213 resulting in increased current generation. Hence, comparable PCEs could be obtained for PTB7/ PC71BM OPVs fabricated on GO and PEDOT:PSS. More importantly, GO-derived devices are significantly more durable than devices using PEDOT:PSS. The hygroscopic nature of PEDOT:PSS facilitates rapid transport of water into such

7. OTHER MOLECULAR TEMPLATE GROWTHS AND THEIR APPLICATIONS IN ORGANIC ELECTRONICS AND OPTOELECTRONICS Multiphenyl- and multithiophene-based MTG, perylene-derivative MTG, acene MTG, and graphene/graphene oxide Y


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based OFET with a mobility of 0.98 cm2/(V s), a threshold voltage of −8 V, and an on/off current ratio higher than 107. A molecular dynamics study showed that DPPC could also template the growth of the organic molecules 3(5)-(9anthryl)pyrazole (ANP), perylene, and p-6P.231 In some OPVs, the electron-transporting material hexaazatriphenylenehexacarbonitrile (HAT-CN) was used as a template material to improve the crystallinity of the CuPc donor films on the ITO electrode. This approach significantly optimized both the FF and Jsc of CuPc/C60 planar OPVs.232 In sections 4 and 5, we summarized PTCDA- and graphenetemplated growth and demonstrated that they cause films to grow with the lying-down molecular orientation, resulting in a performance improvement in OPVs. Rand and Zhou both reported independently that a copper iodide (CuI) template layer can induce the growth of ZnPc thin films with the lyingdown molecular orientation, improving the performance of OPV devices based on ZnPc/C60 planar heterojunctions.233,234 Then CuI-templated growth was extended to fabricate ordered molecular semiconductor thin films of CuPc, FePc, porphyrin derivatives, TiOPc, VOPc, and several other molecules, and some of these films were successfully used in OPV devices.235−241 A mixture of CuI and MoO3 as both the template layer and the anode buffer layer dramatically improved the performance of OPV devices.235,236,242 Meanwhile, C60 thin films grown on the ITO electrode to act as a template layer caused ClAlPc molecules to adopt a lying-down configuration with their molecular π-plane nearly parallel to the substrate surface.243 In fact, inverted OPVs based on C60/ClAlPc could work well. In addition, a C60 monolayer can tune the interfacial electronic properties between pentacene and Au by modifying the interfacial dipole and producing an interface where pentacene molecules adopt a standing-up orientation with their long axis parallel to the surface normal. This arrangement lowers the vertical ionization energy of the pentacene molecules at the interface as compared to that of pentacene directly grown on Au.244 Some thin-film materials have several phases, and one phase can act as a template layer to induce the formation of another phase. Phthalocyanines can normally crystallize as a variety of polymorphs. For example, H2Pc exhibits two predominant phases, a metastable α form and a stable β form.245,246 The α phase is deposited at room temperature and forms small spherical crystallites, while the β phase is formed after further annealing at high temperature and has elongated crystallites, which could be further divided into two subphases, the β1 phase and the β2 phase. The structure and morphology of the subsequently deposited H2Pc layer, i.e., α phase or β phase, were influenced very strongly by the properties of the template layer using the β1 phase or β2 phase, independent of the growth conditions.247 Homoepitaxial MTG can also be used to fabricate high-quality organic semiconductor thin films.248−251 For example, rubrene is normally an amorphous film when grown on Au, SiO2, and Al2O3 substrates by OMBD or ordinary vacuum evaporation. However, if a rubrene single crystal is used as a template substrate, well-ordered rubrene thin films can be obtained by ordinary vacuum evaporation, in which 2-D nucleation, monolayer-by-monolayer growth, and homoepitaxy were demonstrated.249 These MTG techniques also have potential applications in organic electronics and optoelectronics. For example, a microcrystalline rubrene thin film was used as a template layer for subsequent homoepitaxial growth to fabricate an OPV device.251 As the thicknesses of

template growth were discussed above as four typical types of MTG. Furthermore, there are several other kinds of MTG using special template layer materials that have shown potential applications in organic electronics and optoelectronics. Hong et al. reported that a highly oriented CuPc film consisting of many crystal domains could be prepared by introducing TiOPc as a molecular template.226 The standing-up CuPc molecules on top of TiOPc domains exhibited a herringbone structure and high-order arrangement, which improved the performance of OFETs dramatically. In particular, a mobility of 0.12 cm2/(V s) and a 105 on/off ratio were obtained. Li et al. reported that a flat thin template layer of 6,13-pentacenequinone (PQ) could induce the crystallization of rubrene in vacuum-deposited thin films, which yielded an OFET mobility as high as 0.35 cm2/(V s).227 Rubrene forms crystal domains with lengths of several micrometers in the film deposited on a PQ template layer, and a strong (200) diffraction peak was observed in the XRD patterns. In contrast, the rubrene films deposited without a PQ template layer formed irregular domains with smaller sizes, and no peaks were observed in their XRD patterns, indicating an amorphous structure (Figure 35). The PQ template layer could

Figure 35. (a) Molecular structures of PQ and ODPA. (b) XRD pattern of 80 nm rubrene films grown on an ODPA-treated SiO2 substrate with a PQ template layer. The inset is the film without a PQ template layer. (c) and (d) are AFM morphology images of 80 nm rubrene thin films grown on an ODPA-treated SiO2 substrate with and without a PQ template layer, respectively. Reprinted with permisson from ref 227. Copyright 2010 John Wiley & Sons, Inc.

also induce the crystallization of a vacuum-deposited 6,13diphenylpentacene (DPP) thin film and improve the mobility of the OFET accordingly.228 A new report showed that using 6,13-diazapentacene (DAP) as a template instead of PQ could also induce the crystallization of rubrene, resulting in the formation of highly ordered polycrystalline thin films with fieldeffect mobilities as high as 0.68 cm2/(V s).229 A high-quality rubrene thin film was also fabricated using a 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC) monolayer.230 This crystalline rubrene film was interconnected and highly ordered with well-defined molecular orientations, resulting in a rubreneZ


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these films integrated into OPV devices increased to 400 nm, Jsc consistently increased, and unprecedented exciton diffusion lengths of at least 200 nm were obtained.

spintronics, organic superlattices, organic quantum dots, or organic semiconductors with engineered energy bands.

AUTHOR INFORMATION

8. CONCLUDING REMARKS AND OUTLOOK MTG is being developed to fabricate high-quality organic semiconductor thin films with tunable morphologies, molecular orientations, film structures, interface electronics, and so on, which can potentially improve the performance of organic electronic and optoelectronic devices. We summarized the research progress in the MTG field and the corresponding growth mechanisms, mainly focusing on multiphenyl- and multithiophene-based MTG, perylene-derivative MTG, acene MTG, and graphene/graphene oxide template growth, as well as their applications in OFETs, OPVs, and organic sensors. A highly ordered molecular template layer is of great importance in MTG, because it induces the overlayer molecules to nucleate, aggregate, and form high-quality thin films with controllable properties. On the basis of an understanding of the growth behavior of organic semiconductor thin films, more materials could be found to be suitable for MTG, resulting in a wider variety of high-quality thin films and high-performance organic electronic and optoelectronic devices. Although many advances have been made in MTG and its applications in organic electronics and optoelectronics, there are still many challenges: (1) The mechanism of MTG is not uniform and depends on the materials. For example, in WEG, the surface geometric channels and crystal lattice of the template layer dominate the oriented nucleation and growth of the overlayer crystals. This is helpful for forming layers with incommensurate and commensurate epitaxy, which are useful for certain applications. In contrast, the π−π interactions between the template molecules and the overlayer molecules dominate the molecular orientation and nucleation in PTCDAtemplated growth. Therefore, the specific growth mechanisms must still be studied to understand the relationship between the template layer and overlayer for each new type of MTG. (2) Molecular template materials should be multifunctional. They can serve not only as a template layer for the growth of highquality overlayer thin films, but also as another functional layer such as a hole transport layer or electron transport layer in OPVs. Thus, MTG could make the device fabrication process much simpler. The work function of the molecular template materials should match the electrode substrate and overlayer materials to be suitable for the OPV, or else one more layer must be inserted to tune the energy levels, as was the case for PTCDA-templated growth in OPVs. On the other hand, if they are thick enough, organic semiconductor template materials can form heterostructures or heterojunctions with the overlayer thin films, which could allow the fabrication of the bipolar junction transistors and heterojunction OPVs.102,252 (3) In this review, we focused on growth on vacuum-deposited molecular templates. However, low-cost and high-output solutiondeposition techniques such as roll-to-roll printing or coating are very attractive for the processing of large-area organic electronic and optoelectronic devices. MTG is eagerly being introduced into solution-deposition techniques, much like the LB and SAM methods,23,24 and will hopefully be compatible with roll-to-roll printing or coating processes. (4) Finally, at present, MTG is mainly being applied to improve the device performance in OFETs, OPVs, and organic sensors. Its applications should be extended to more organic electronic and optoelectronic devices such as VOFETs, organic

Corresponding Author

*E-mail: [email protected]. Phone: +86-731-88660256. Fax: +86-731-88877805. Notes

The authors declare no competing financial interest. Biographies

Junliang Yang received his Ph.D. in 2008 from the State Key Laboratory of Polymer Physics and Chemistry at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. He then worked for two and a half years as a postdoctoral research fellow in the Department of Chemistry at the University of Warwick. In April 2011, he moved to Australia and worked as a research fellow in the Bio21 Institute at the University of Melbourne and as a visiting scientist in the Flexible Electronics Laboratory at the CSIRO (Commonwealth Scientific and Industrial Research Organisation). In March 2012, he was appointed as a full professor in the School of Physics and Electronics at Central South University. His research interests cover the controllable fabrication of highly ordered organic semiconductor thin films, organic electronic and optoelectronic devices, and printed electronics.

Donghang Yan received his Ph.D. in 1995 from the University of Mainz (Germany). He then joined the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. In 1997, he was appointed as a full professor, and has been leading a research group in the field of organic electronics. His research interests focus on the device physics of organic field-effect transistors and organic photovoltaics, the physics of organic heterojunctions, and the growth of highly ordered organic semiconductor thin films. AA


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Tim S. Jones obtained his Ph.D. in 1988 from the Chemistry Department at Liverpool University. He then worked for two years as a postdoctoral research assistant in the Surface Science Research Centre at Liverpool, and spent a short period working at the FritzHaber Institute in Berlin on a fellowship funded by the DAAD (German Academic Exchange Service). He was appointed Lecturer in the Chemistry Department at Imperial College London in 1991, and was promoted to Reader in 1997 and Professor in 1998, as well as becoming the STS/Sumitomo Professor of Electronic Materials (2000−2005). After spending more than 16 years at Imperial, he moved to the Department of Chemistry at the University of Warwick in 2007. His research interests focus on advanced electronic materials and nanotechnology as well as their applications in electronic and optoelectronic devices.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51203192), the Program for New Century Excellent Talents in University (Grant NCET-130598), and the Hunan Provincial Natural Science Foundation of China (Grant 2015JJ1015). We sincerely express thanks to Prof. Y. L. Gao (University of Rochester) for his kind suggestions and discussion throughout this work. REFERENCES (1) O’Neill, M.; Kelly, S. M. Ordered Materials for Organic Electronics and Photonics. Adv. Mater. 2011, 23, 566−584. (2) Asadi, K.; Li, M. Y.; Blom, P. W. M.; Kemerink, M.; de Leeuw, D. M. Organic Ferroelectric Opto-Electronic Memories. Mater. Today. 2011, 14, 592−599. (3) Figueira-Duarte, T. M.; Müllen, K. Pyrene-Based Materials for Organic Electronics. Chem. Rev. 2011, 111, 7260−7314. (4) Sokolov, A. N.; Tee, B. C. K.; Bettinger, C. J.; Tok, J. B. H.; Bao, Z. N. Chemical and Engineering Approaches To Enable Organic FieldEffect Transistors for Electronic Skin Applications. Acc. Chem. Res. 2012, 45, 361−371. (5) Lin, Y. Z.; Li, Y. F.; Zhan, X. W. Small Molecule Semiconductors for High-Efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41, 4245−4272. (6) Yang, J. L.; Vak, D.; Clark, N.; Subbiah, J.; Wong, W. W. H.; Jones, D. J.; Watkins, S. E.; Wilson, G. Organic Photovoltaic Modules Fabricated by an Industrial Gravure Printing Proofer. Sol. Energy Mater. Sol. Cells 2013, 109, 47−55. (7) Krebs, F. C.; Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. 25th Anniversary Article: Rise to PowerOPV-Based Solar Parks. Adv. Mater. 2014, 26, 29−39. (8) Yuan, Y. B.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C. B.; Chen, J. H.; Nordlund, D.; Toney, M. F.; Huang, J. S.; Bao, Z. N. Ultra-High Mobility Transparent Organic Thin Film Transistors AB


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Molecular Template Growth and Its Applications in Organic Electronics

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