Monolayer-Mediated Growth of Organic Semiconductor Films with

Invited Feature Article pubs.acs.org/Langmuir

Monolayer-Mediated Growth of Organic Semiconductor Films with Improved Device Performance Lizhen Huang, Xiaorong Hu, and Lifeng Chi* Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, PR China ABSTRACT: Increased interest in wearable and smart electronics is driving numerous research works on organic electronics. The control of film growth and patterning is of great importance when targeting high-performance organic semiconductor devices. In this Feature Article, we summarize our recent work focusing on the growth, crystallization, and device operation of organic semiconductors intermediated by ultrathin organic films (in most cases, only a monolayer). The site-selective growth, modified crystallization and morphology, and improved device performance of organic semiconductor films are demonstrated with the help of the inducing layers, including patterned and uniform Langmuir−Blodgett monolayers, crystalline ultrathin organic films, and self-assembled polymer brush films. The introduction of the inducing layers could dramatically change the diffusion of the organic semiconductors on the surface and the interactions between the active layer with the inducing layer, leading to improved aggregation/crystallization behavior and device performance. This review will mainly focus on film growth from vacuum evaporation. Compared to the well-understood film growth mechanism of inorganic compounds, the film growth of organic molecules is far from being understood. A diversity in film quality and film structure as well as the polymorphism of the organic molecular films have been widely observed because of the intrinsic multiple bonds and weak interactions in organic systems.19−23 Many research groups have summarized the general growth mechanism of organic semiconductors and their application to organic electronics,6,19−23 which will not be the main focus here. In most cases, organic semiconductor molecules directly deposited on substrates such as Si/SiO2 or glass will result in randomly oriented films or moderate chargetransport abilities.24 For these reasons, a substrate modification with an ultrathin organic layer (here referred to as the inducing layer) is widely employed to improve the quality of the active layers as well as the device performance. This inducing layer can be a self-assembled monolayer through chemical bonding with the substrate25 or through physical absorption on the substrate26 or a vacuum-deposited organic ultrathin film.27 The change in surface energy or the improved interface by such a surface modification can greatly affect the film growth behavior and significantly influence the device performance. For example, the popular octadecyltrichlorosilane (OTS)-modified Si/SiO2 substrate could lead to a mobility improvement of about 1 order of magnitude.28 Moreover, if a prepatterned inducing layer is introduced onto the substrate, the patterning of organic

1. INTRODUCTION Portable, wearable, and smart electronics, as a major future branch of information technology, has attracted much attention in recent years. 1−3 In particular, organic-material-based electronics is a promising candidate.4 The advantages of organic semiconductors, such as the compatibility with flexible fabrication processes and the tunable properties (optical, electrical, and recognition) by controlled chemical composition, have been demonstrated by intensive studies.5−9 Because molecular aggregation, orientation, and crystallization in the active layer play key roles in tailoring properties such as fluorescence, sensing response, and charge carrier mobility at the device level,9−11 significant efforts have been made to develop methods aimed at good control of the growth of molecular films, particularly on the insulator or inert substrates.12−15 The organic films can be prepared by solution processing or vacuum sublimation techniques. In solution processing, the Langmuir−Blodgett (LB), spin-coating, dip-coating, and solution shearing techniques are typically used,16−18 in which the film morphology is determined by the condensation of molecules from the solution or at the three-phase contact line. In vacuum deposition, the molecules evaporating from the heating crucible reach the substrate where aggregation or crystallization takes place.19,20 In general, vacuum deposition is suitable for small nonsoluble molecules, while solution processing is compatible with both polymers and small soluble molecules. Despite the great advantages of low cost and large-scale production, the great challenge for solution processing is the uniformity and the reproducibility. The well-developed vacuum deposition technique, in contrast, is able to provide more uniform and stable films with good control of the film growth and device fabrication. © XXXX American Chemical Society

Received: January 27, 2015 Revised: May 18, 2015

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DOI: 10.1021/acs.langmuir.5b00349 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. (a) Schematic diagram of the DPPC periodic pattern formation through LB transfer and the molecular structure of DPPC. (b) AFM height of the periodic pure DPPC pattern on the mica surface. (c) Composition of the DPPC pattern: the channel is composed of the LE phase whereas the stripe is composed of the LC phase. Reproduced with permission from ref 30. Copyright 2007 American Chemical Society.

Figure 2. (a−c)AFM images of various pure DPPC patterns on a mica substrate with a constant pressure of 3 mN/m at (a) 60 mm/min, (b) 40 mm/ min, (c) 10 mm/min. (d−f) Illustration of the process for the formation of different patterns of pure DPPC through vertical LB transfer. Reproduced with permission from ref 31. Copyright 2006 American Chemical Society.

semiconductor films could possibly be achieved by selective growth.15 Here, we will review and discuss our recent research on surface modification by utilizing ultrathin organic layers (in most cases, only one monolayer) and its significance in improving the growth, crystallization, and device performance of organic semiconductor films. We will start with an introduction of the different types of inducing layers (section 2) followed by a section that addresses the effects of different inducing layers on

the growth and device operation of organic semiconductor thin films (section 3). In the summary, future challenges for the growth of organic semiconductors will be discussed.

2. TYPES OF INDUCING LAYERS Intentionally adding an inducing layer between the substrate and the active layer is an effective approach to changing the substrate surface property thus to modulate the growth process of the B


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Figure 3. (a) Schematic diagram and molecular structures and (b) AFM height images of four inducing layers fabricated from vacuum deposition. Reproduced with permission from ref 35. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

subsequent active layer.6 On the other hand, introducing such a modifying layer in an electronic device may bring in a new interface that would significantly affect the device operation,6 especially in field effect transistors (FETs), which are intrinsically a surface or interface charge-transport device. According to numerous publications, the organic inducing layers used to assist the growth of organic semiconductor films could be classified into two types according to the bonding properties: one is modification through chemical bonding with the surface of substrate and the other is through physisorption. We have developed three kinds of inducing layers that cover both modification processes and have applied them for different applications, from assisting the growth and crystallization of the organic semiconductor to improving the device performance. 2.1. Langmuir−Blodgett Monolayers. The Langmuir− Blodgett (LB) technique is a well-established and sophisticated method to control interfacial molecular orientation and packing.29 Through LB transfer, it is easy to obtain a homogeneous Langmuir monolayer on a solid substrate, specifically from a patterned monolayer. One of the typical examples that our group studied is a structured monolayer of L-αdipalmitoyl-phosphatidycholine (DPPC). As a lipid component of biological membranes, DPPC can be fabricated into a monolayer film with a period line pattern through the LB technique under certain conditions.30 This monolayer turned out to be a good inducing layer for controlling organic film growth according to our later studies. This periodic monolayer can be obtained by rapidly withdrawing, e.g., 60 mmin−1, the mica substrate or other solid substrates at a low surface pressure and constant temperature. Under these conditions, the adsorption becomes unstable, leading to periodic interruptions in the molecular deposition during the LB transfer process. The resulting linear lattice structure, composed of channels and stripes, is revealed by atomic force microscopy (AFM), as shown in Figure 1. The stripes are attributed to the liquid condensed phase of DPPC (LC phase) whereas the channels are composed of the liquid expanded phase (LE phase). The difference in height between the stripe and the channel is about 1 nm and originates from the different tilt angles of the DPPC molecules in the films. The

mechanism behind the formation of periodic stripes is based on the oscillation of the meniscus. These periodic patterns can be tuned by the transfer velocity, surface pressure, monolayer composition, and substrate properties.31 Figure 2a−c shows three typical patterns obtained upon adjusting the transfer velocity at a constant surface pressure. Patterns from the lines that are parallel to the three-phase contact line, from the grids, and from the vertical lines that are perpendicular to the three-phase contact line were observed by changing the transfer velocity. Different substrates or substrate treatments will result in the formation of difference stripe patterns.32 For instance, with the same transfer velocity, a plasma-treated silicon substrate gives a line pattern with a surface pressure of 2.5 mN/m, and the RCA (ammonium hydroxide, hydrogen peroxide)-treated substrate requires a surface pressure of 5 mN/m to form a similar pattern. The reason is the surface energy differences between plasma-treated (88 ± 2 mJ/m2) and RCA-treated wafers (106 ± 3 mJ/m2).26,33 In addition, by controlling the transfer condition, a monolayer composed of a pure LC phase or LE phase can be obtained. 2.2. Organic Ultrathin Films from Vacuum Deposition. An inducing layer formed by vacuum sublimation is another effective modification layer for the control of organic semiconductors, as has been clearly demonstrated by a series of work from Yan et al.23,27 They developed a weak epitaxy growth method using a rodlike molecule to prepare an ultrathin film as the inducing layer for growing highly ordered organic semiconductor films.27,34 The important feature is that the inducing layer presents two-dimensional (2D) layer-by-layer growth in the initial stage and forms a smooth crystalline monolayer or bilayer with large domain size (10−20 μm) on the substrate at elevated temperature, guaranteeing the subsequent semiconductor growth on a flat substrate. With the aim of obtaining the desired inducing layer from vacuum deposition, three factors should be considered: (1) The growth mode should be nearly 2D growth in the initial stage. (2) The surface properties should be suitable for the subsequent molecules. (3) The lattice parameter should be suitable because during epitaxy growth a large lattice mismatch is not desirable for growing a high-quality film. Hence, the choice of molecular structure and optimized growth conditions are of great C


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Figure 4. Schematic diagram of the polymer brush formation process: (a) synthesis of the initiator and (b) in situ polymerization after the assembly of the initiator on the substrates.

Figure 5. AFM height images and fluorescence micrograph of (a, b) 2 nm ANP deposited on the DPPC pattern, (c, d) 10 nm ANP deposited on the DPPC pattern, (e) molecular structure of ANP, and (f) kinetic process of molecular transport between the LE phase and LC phase and the relative energy barrier. Reproduced with permission from ref 41. Copyright 2008 American Chemical Society.

importance. The substrate temperature and film thickness need to be optimized to achieve smooth inducing layers with a large grain size. Figure 3a displays the schematic diagram of an inducing layer on a substrate and the structures of four molecules: para-sexiphenyl (p-6P), 2,7-bis(4-biphenylyl)-phenanthrene (BPPh), 2,5-bis (4-biphenylyl)bithiophene (BP2T), and 2,5-bis(4-biphenylyl)-thieno[3,2-b]thiophene (BPTT). AFM height images, with a thickness of around 0.9−1.1 ML, are shown in Figure 3b.35 All of the films present layer-by-layer growth in the initial layer and a similar morphology, but their lattice parameters vary owing to their different chemical structures, which will induce a distinct morphology in the subsequent films. Besides, the electrical properties of the inducing layer should be considered when the films are engineered into devices. 2.3. Polymer Brush Films. A self-assembled monolayer through chemical bonding to the substrate is the most popular approach for fabricating high-performance organic semiconduc-

tor devices, especially the alkyl/aryl phosphoric acid SAMs and organosilane-based SAMs.6,36,37 A polymer brush layer as a surface-modifying layer is not broadly applied in organic electronics38,39 but possesses great potential in flexible electronics. The formation of a polymer brush starts from selfassembling a polymerization initiator on a substrate, similar to silane materials assembled on SiO2/Si, and is followed by in situ surface-initiated polymerization as shown in Figure 4.38,39 This method, named surface-initiated atomic transfer radical polymerization (SIATRP), is usually applied to generate structural homopolymers and block copolymers. Moreover, the polymer brush could act as a dielectric layer or electrodes in terms of the properties of the polymer.

3. EFFECTS OF THE INDUCING LAYER ON THE GROWTH OF ORGANIC SEMICONDUCTOR FILMS The high performance of organic devices, especially transistors, requires well-controlled, high-quality crystalline films in terms of D


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Figure 6. (a−c) AFM of perylene with various thicknesses deposited on the DPPC pattern: (a) 0, (b) 24, and (c) 48 nm perylene. (d) 70 nm perylene deposited on the DPPC pattern and corresponding high-resolution AFM images. (e) Out-of plane XRD of perylene films on the DPPC pattern. (f) Illustration of the anisotropy OFETs measurement and (g) corresponding transfer characteristics. Reproduced with permission from ref 42. Copyright 2010 Royal Society of Chemistry.

periodic pattern of DPPC.41 Site-selective behavior was clearly observed in the early stage, with a thickness of 2 nm. The ANP molecules tend to grow on the channel of the pattern, forming a nearly one-dimensional (1D) particle line along the channel (Figure 5a,b). Obviously, this growth behavior indirectly reflects the different interfacial energy between the LE and LC phases. Upon further increasing the coverage of ANP, nuclei were also found on the stripes with an LC phase (Figure 5c,d), although with a smaller grain size compared to that of the particles on the channel. Importantly, the selective aggregation of ANP is dependent on the width of the stripe (a narrow stripe results in the preferred nucleation on the channel) as well as on the evaporation rate (slow evaporation leads to better selectivity). Such a phenomenon is analogous to the classic growth behavior in which the diffusion length of molecules on substrates is limited.12 However, the smaller grain size on the LC phase indicated a smaller diffusion length than on the LE phase. Why

crystallization, anisotropy, orientation, and so on. Besides the intrinsic packing governed by the molecular structure, the film quality mainly depends on the parameters for film growth, e.g., the surface properties of the substrate and the substrate temperature, with respect to growing highly ordered films with preferred orientation and crystallization. The inducing layer on the substrate certainly changes the surface properties to different extents and affects the growth of the subsequent films as well as the device performance in different ways. 3.1. Organic Semiconductor Film Growth Mediated by the DPPC LB Monolayer. The DPPC monolayer formed from LB transfer consists of periodic patterns over large areas. The variable surface properties between the stripe and the channel are expected to induce different growth behaviors of subsequent molecules deposited from the liquid or gas phase.40 3(5)-(9-Anthryl) pyrazole (ANP), one of the fluorescent molecules, was deposited through vacuum deposition on the E


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Figure 7. (a) Average MRSD of DPPC in different phases. (b) Schematic illustration of the difference between the LE and LC phases and the resulting differences in diffusion and transition. (c) Simulation of site-selective deposition behavior of perylene on mixed SAM. Reproduced with permission from refs 43 and 44. Copyright 2011 Elsevier B.V. and copyright 2012 Elsevier Inc.

then do the molecules still selectively nucleate on the LE phase? This is explained by the different diffusion barriers between molecules diffused from the LE to the LC phase and diffused from the LC to the LE phase as shown in Figure 5f. A relevant molecular dynamics study will be discussed later. The patterned DPPC monolayer was also employed to induce the growth of organic semiconductors with highly anisotropic morphology that is due to the difference in wettability between the LE and LC phases.42 Because anisotropic molecular packing and anisotropic charge carrier mobility along different crystal directions are very common for organic semiconductors, the growth of crystal films with a preferred orientation is highly desirable. Figure 6a−c shows a series of AFM images of perylene molecules grown on the periodic line pattern of a DPPC monolayer. The organic semiconductors tend to start nucleating at the channel, similar to the ANP molecules. However, perylene differs from the ANP molecules in that it is crystalline with a preferred orientation in the channel region. Accompanied by the anisotropic packing of perylene molecules, quasi-1D crystals were formed along the channel. Upon increasing the thickness of the perylene films, continuous 1D crystalline films could be obtained along the channel. Because of the small height difference (about 1 nm) between the channel and stripe, the

1D structure will extend to the stripe area as the thickness further increases. This film shows good crystalline features and wellordered packing as confirmed by XRD and high-resolution AFM (Figure 6d,e). A transistor based on the highly anisotropic films exhibit considerable anisotropy in mobility, as observed in two directions: one is parallel to the 1D structure, and the other is perpendicular to the 1D structure (Figure 6f,g). Similarly, anisotropic growth has also been observed on para-sexiphenyl (p-6P) molecules. A molecular dynamics study has been conducted to elucidate the mechanism of those selective deposition behaviors.43,44 Distinct diffusion abilities of molecules in the LE and LC phases were confirmed, which is consistent with our experimental results (Figure 7a,b). In addition, the results suggested that organic molecules tend to submerge into the alkyl chain of the DPPC layer, both in the LC and the LE phases. The molecular dynamics study indicates that structural plasticity, which is directly connected to the different packing densities in the LC and LE phases, yields a higher root-mean-square deviation (rmsd) for the LE phase than for the LC phase. Hence, dense packing will hinder the diffusion of organic molecules owing to their submerging. This difference in packing was also accounted for the asymmetric energy barrier between the LC and LE phases. F


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Figure 8. AFM image of (a) a rubrene film grown on the LC phase of DPPC, (b) after annealing for 8 h, and (c) a closeup view of the image in b. (d) Rubrene molecular structure. (e) XRD pattern of crystalline rubrene films on the DPPC layer (incident wavelength = 1.5406 Å). (f, g) Rubrene film on a gold-patterned substrate with different spacings covered by the DPPC monolayer. Reproduced with permission from refs 50 and 51. Copyright 2013 Elsevier B.V. and copyright 2014 Royal Society of Chemistry.

substrates.47−49 We tried to adopt a DPPC monolayer as an inducing layer to assist the crystallization of rubrene thin films.50 When rubrene molecules are deposited on the DPPC monolayer at room temperature, they form typical dotlike amorphous grains, whether on LE or LC phases, as shown in Figure 8. However, after annealing the film at 80 °C for approximately 8 h, the crystalline-featured rubrene film first appeared on the surface of the LC phase. In the case of the LE phase, longer annealing times will be required to obtain the crystalline rubrene films. A netlike morphology with small holes was observed on this crystalline film. Inspired by the morphology, we speculated a crystallization process by which the rubrene molecules crystallize from the edge of the dot grain and consume the molecules in the dot grain until all of the dotlike grains transfer to terracelike crystalline films. As a result, holes should appear in the position originally occupied by the dot grains. Thin-film transistors were fabricated on the basis of the crystalline films of rubrene, and an

Meanwhile, another simulation using a series of self-assembled monolayers (SAMs) with structures similar to that of DPPC showed44 that the interaction between the organic molecules and the SAM varied with the packing density of the SAM.44 The less dense the packing in the SAM, the stronger the interaction between the organic molecules and the SAM and the more deeply the organic molecules can submerge into the alkyl chain of the SAM. Figure 7c illustrates the site-selective deposition behavior of perylene on the mixed SAM according to the simulations. Further studies revealed that the DPPC monolayer could also extend its effect in assisting the crystallization of organic films. Rubrene molecules, typical representatives of high-mobility organic semiconductors,45,46 usually prefer to form amorphous films on inert substrates such as glass or SiO2/Si. Carefully modified substrates could generate crystalline films by improving the interaction between the rubrene molecules and the G


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average mobility of 0.98 cm2 V−1 s−1 was achieved, which is greatly improved compared to that of amorphous rubrene films (10−4 cm2 V−1 s−1). In addition, the morphology of crystalline films can be further improved with the help of gold patterns51 (Figure 8f,g). Taking advantage of this gold-pattern-modified morphology, we developed a simple way to fabricate rubrene transistor arrays with suppressed cross-talk between adjacent devices by finely controlling the difference in morphology on the channel and space in the transistor arrays. Rubrene crystallization on the DPPC LC phase exhibits an abnormal phenomenon, and the mechanism behind such crystallization is still vague. A molecular dynamics simulation was performed to study the effect of the inducing layer on the crystallization of rubrene.52 Evidence from the aggregation of rubrene on different SAMs and from the interaction between rubrene and the substrate suggested a favorable crystallization behavior on the LC DPPC layer. Nevertheless, this is not adequate for understanding the transition from an amorphous to a crystalline state, and further studies need to be carried out. 3.2. Highly Ordered Crystalline Film Growth on Crystalline Organic Ultrathin Inducing Layers. The significant effect of the DPPC monolayer mainly depends on its surface properties and anisotropic pattern structures. Unlike the DPPC monolayer, the ultrathin films from vacuum deposition are uniform crystalline films with highly ordered packing, which will have different effects on the film growth of organic semiconductors. Figure 9 depicts the molecular structures of several organic semiconductors and their film morphologies on the molecular inducing layer.53−55 One of the general effects is that the film morphology of all semiconductors on the inducing layers is greatly improved, such as a larger grain size, nearly 2D growth, and oriented crystals. Additionally, the morphology exhibits a significant dependence on the parameters of the inducing layer, such as its thickness. Taking ZnPc/p-6P as an example, oriented stripe crystals of zinc phthalocyanine (ZnPc) formed in each domain of the p-6P layer. The crystal orientation of ZnPc is dependent on the packing of the p-6P underneath, as suggested from the morphology between two adjacent domains, indicating the similarity to epitaxy growth in inorganic systems. Furthermore, this oriented growth can maintain a very broad range of thicknesses (from submonolayer to tens of molecular layers). During the growth process, the film phase and morphology will gradually change without generating obvious defects. Figure 10 shows the AFM images of a series of copper hexadecafluorophthalocyanine (F16CuPc) films with different coverages grown on a p-6P monolayer.56 The darker areas in Figure 10a depict F16CuPc on bare SiO2/Si. The initially formed gridlike morphology composed of two oriented stripe crystals will be retained in thicker films, albeit with a gradual change in the angle. AFM images of the detailed packing in Figure 10e−h indicate highly ordered packing in the films. Meanwhile, grain boundary defects are revealed in these films, suggesting a polycrystalline feature. The inducing layer plays an important role in controlling the film morphology and device performance. As revealed in Figure 9b, the films on monolayer and bilayer domains present obvious differences in grain shape and orientation. Systematic investigations of these differences were carried out using a BP2T inducing layer. Figure 11 illustrates the morphology of a 1 nm ZnPc film grown on a BP2T layer with various thicknesses, where ZnPc on BP2T domains ranging from monolayers to trilayers can be observed.57 The morphology of ZnPc changes from stripelike

Figure 9. (a) Molecular structure of some semiconductors. (b) AFM images of typical semiconductor films on the inducing layer. Reproduced with permission from refs 35 and 53−55. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2008 American Chemical Society and copyright 2010 Elsevier B.V.

to fiberlike crystals upon simply adjusting the thickness of the inducing layer. Meanwhile, the crystal orientations also change slightly. Two preferential in-plane orientations are observed in each domain, along with the fact that the angle between two orientations gradually decreases as the number of layers increases, until only one orientation exists. Structural evolution, in terms of the thickness of the inducing layer, is accounted for by the changeable morphologies, as evidenced from grazing incidence X-ray diffraction (GIXRD) results (Figure 12a). Along with the layer number increment, the in-plane lattice spacing of the inducing layer shows an obvious decrease in the a axis but no obvious change in the b axis, indicating more dense packing in the bilayer and trilayer films (Figure 12b). Therefore, the diffusion of molecules on the inducing layer will vary with the number of molecular layers. The thicker the inducing layer, the longer the diffusion length of the semiconductor on the inducing layer. The surface energy maybe different too, but such a difference has not yet been confirmed at present. H


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Figure 10. (a−d) AFM images of 0.5−2.2 ML F16CuPc on the p-6P monolayer and (e−h) high-resolution AFM of F16CuPc films on a p-6P monolayer. Reproduced with permission from ref 56. Copyright 2009 American Chemical Society.

Figure 11. AFM topography of 1.0 nm ZnPc grown on the first, second, and third layers of BP2T: (a) ZnPc/BP2T monolayer, (b) ZnPc/BP2T bilayer, and (c) ZnPc/BP2T trilayer. (d−f) Corresponding close-up images of (a−c). Reproduced with permission from ref 57. Copyright 2010 American Chemical Society.

during film deposition. The use of a molecular inducing layer (p6P) could greatly improve the film crystallization and especially increase the grain size.58 However, the 3D growth of VOPc is retentive, and thus the size increment is limited, with the existence of deep grain boundaries. One approach to avoiding 3D growth based on classical inorganic epitaxy growth theory is to introduce an inducing layer that possesses a small lattice mismatch with the semiconductor. This can be solved by modifying the chemical structure of the inducing layer to allow for generating different lattice parameters without losing their 2D growth behavior. Through intentionally changing the midgroup

Owing to the strong influence of the inducing layer, the finetuning of its properties allows for control of the morphology and charge-transport properties of the following organic semiconductors. Vanadyl phthalocyanine (VOPc), one of the nonplanar phthalocyanines, is a potential high-mobility material owing to its two-dimensional π−π stacking and short π−π distance. However, a high-quality film of VOPc is hard to achieve because of the permanent polarity of the molecule. Indeed, the strong intermolecular interaction resulting from the polarity in turn drives the formation of convex and concave dimers of VOPc molecules and leads to typical island growths with small grains I


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of p-6P with different chemical units, a nearly layer-by-layer growth behavior was realized for the VOPc molecules on two inducing layers.35 On four different inducing layers, the VOPc molecules present small grains with 3D island growth on p-6P and BPPh, whereas continuous films with nearly 2D growth were obtained on the BP2T and BPTT surfaces. According to the calculation results, the latter two inducing layers have a smaller lattice mismatch with VOPc films in the in-plane unit cell compared to p-6P and BPPh. Besides using one molecule as the inducing layer, a mixed inducing layer, obtained through the codeposition of two molecules, was applied to tune the morphology and device performance of transistors (Figure 13).59 This was aimed to solve the problem of some rodlike oligomers of the inducing layer being p-type semiconductors but with relatively low chargecarrier mobility. In this case, besides working as an inducing layer, they might be involved in charge transport in the OFFTs, which will reduce the charge mobility of the whole transistor. The HOMO level of the inducing layer molecules could determine whether the inducing layer acts as an active layer or a dielectric layer by tailoring the charge-carrier injection barrier between the organic semiconductors and the inducing layer. The relatively poor performance of BPTT/VOPc is a typical example even though a high-quality VOPc film was obtained. The HOMO energy level of BPTT is very close to that of the HOMO of VOPc, allowing holes to transfer freely from the VOPc layer into the inducing layer of BPTT and leading to reduced field-effect mobility. A mixed inducing layer could realize ratio-dependent lattice parameters and electronic structures, allowing for artificial control of the properties of the inducing layer. Another molecule of BPPh with a lower HOMO level that could block charge-carrier injection was added to tune the

Figure 12. (a) In-plane GIXRD patterns of BP2T films with different coverages. (b) Evolution of the lattice parameter with the coverage. Reproduced with permission from ref 57. Copyright 2010 American Chemical Society.

Figure 13. (a) Schematic diagram of a mixed inducing layer and transistor configuration. (b) Lattice spacing and HOMO level of the mixed inducing layer with different ratios. (c) Transfer characteristics of the transistors. (d) Dependence of the field-effect mobility of a VOPc transistor on the mixing ratio. Reproduced with permission from ref 59. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. J


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Figure 14. (a) Molecular structure of PTCDI-C8 and (b, c) AFM images of its monolayer films on the SiO2/Si substrate. (d, e) AFM images of (d) 30 nm pentacene on PTCDI-C8/SiO2/Si and (e) 5 nm pentacene on PTCDI-C8/SiO2/Si. (f) 5 nm pentacene on SiO2/Si. (g) 5 nm pentacene on ODTS/ SiO2/Si. Reproduced from ref 60.

Figure 15. (a) Illustration of the formation of the patterned PPY polymer electrode. (b) Transistor configuration based on the patterned PPY electrode. (c, d) Output and transfer characteristics of pentacene transistors. (e, f) Output and transfer characteristics of PDI-8CN2. Reproduced with permission from ref 61. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

electronic structure of the inducing layer of BPTT/VOPc. A series of mixed BPTT and BPPh inducing layers were obtained with corresponding ratio-dependent lattice spacing and HOMO levels, as shown in Figure 13b. These ratio-dependent lattice and electronic structure properties were utilized to tune the performance of the VOPc film transistor. As the ratio of BPPh increased, the HOMO level gradually decreased, and an increased charge-carrier mobility was observed. The BPPh-rich films possess a HOMO level of approximately −6.0 eV that is

sufficiently different from the HOMO level of VOPc (−5.5 eV) and is thus sufficient to block the charge carrier transferring from VOPc to the inducing layer without losing the high-quality film feature. Thus, an accordingly higher mobility of the VOPc is expected. A high mobility exceeding 3.0 cm2 V−1 s−1 was achieved by controlling the ratio of the mixed inducing layer as shown in Figure 13c,d. Besides rodlike molecules, other molecules have also been used as the inducing layer. For instance, N,N′-dioctyl-3,4,9,10K


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perylenedicarboximide (PTCDI-C8),60 an n-type semiconductor, could also present layer-by-layer growth after finely tailoring the deposition conditions. Under certain conditions, a perfect monolayer of PTCDI-C8 on a SiO2 substrate can be achieved, as shown in Figure 14a−c. Using such a monolayer as the inducing layer, we deposited a pentacene film on top of it. An enlarged domain size (Figure 14d−g) and an average mobility of 1.5 cm2 V−1 s−1 in air were achieved for the pentacene transistors. The ntype field effect of PTCDI-C8 did not appear because of the unstable performance of PTCDI-C8 films in the ambient atmosphere. 3.3. Organic Semiconductors Growth on Self-Assembled Polymer Brushes. The effect of a polymer brush layer on the growth of organic semiconductors is not obvious. However, acting as a dielectric layer or electrodes, they can improve a device’s performance. For example, using the PMMA brush layer on SiO2/Si as a dielectric layer,39 a high-performance transistor was achieved. Organic semiconductor films of pentacene on the PMMA brush layer present highly ordered features that ensure good mobility. A more special characteristic is the low threshold voltage based on this polymer/SiO2 dielectric layer. In addition, the polymer brush could be patterned to form electrodes when assembling the conducting polymer. Figure 15a shows a pattern procedure of polypyrrole (PPY) electrodes on a SiO2 substrate.61 A detailed comparison of the polymer electrode with a gold contact shows that the polymer electrode provides a better interface with a particularly small contact resistance, whether on p- or n-type transistors (Figure 15b−f). One reason is that the polymer materials can improve the interface morphology and structure through the electrode/channel. This polymer brush electrode will be more applicable to the flexible electronics in which the mechanical bending stability is essential.

investigate the mechanisms; however, they are applicable only to a limited number of molecules and not for the precise explanation of film growth and crystallization behaviors. In addition, many challenges exist in this field as well. For example, the sensitivity of organic molecules hinders the application of some in situ techniques, thus limiting in situ characterization. Moreover, although the introduction of a monolayer could improve the morphology and even control the morphology in some cases, it cannot be generalized for different active molecules. This approach is therefore strongly dependent on individual molecular structures, so that general rules are still missing. With the increased interest in wearable and smart electronics based on organic semiconductors, further studies on the above-mentioned issues will undoubtedly gain more attention both from a scientific and a technological point of view.

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*E-mail: [email protected]. Fax: +86-512-65880725. Tel: +86512-65880725. Notes

The authors declare no competing financial interest. Biographies

4. CONCLUSIONS AND PERSPECTIVE Understanding and controlling the thin-film growth of organic molecules is still an ongoing research subject owing to their importance in organic electronics and microelectronics, and many novel results have emerged. In this review, we summarized our recent results, particularly focusing on the monolayerassisted growth and crystallization of organic semiconductors. The positive effects of such ultrathin inducing layers (whether LB monolayers, vapor-deposited inducing layers, or self-assembled monolayers) on the growth of organic semiconductor films are demonstrated. The introduction of an organic layer on an inert substrate can, on one hand, tailor the diffusion of organic molecules by controlling the packing density of the underlying layer, which also leads to the site-selective growth of the active films as demonstrated by DPPC periodic patterns. On the other hand, the introduced layer provides a more favorable interface than SiO 2 or other inert inorganic substrates for the crystallization of organic molecules, partially due to the increased diffusion ability. The increased diffusion could in some cases help to realize film crystallization that is hard to achieve, such as that of rubrene films. In the case of a crystalline inducing layer, the flat crystalline surface allows for 2D layer-by-layer growth or for large grains with good continuity. In addition, the local epitaxy growth between organic−organic films helps in obtaining oriented crystals. The improved morphology is confirmed by the transistor performance. Despite the advantages of an inducing layer in controlling the growth of semiconductor films, the exact mechanism remains unclear. Molecular dynamics simulations are often used to

Lizhen Huang is an associate professor in the Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University. She received her Ph.D. degree from Changchun Institute of Applied Chemistry at 2011 and then joined Professir Frank Wurthner’s group as a postdoctoral researcher. In 2013, she accepted a position at Soochow University. Her research interest is focused on controlling the growth of structured organic thin films and developing the applications in organic electronics.

Xiaorong Hu is a master’s student in the Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University. She received her L


Langmuir


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B.S. degree in physics and electronic information, Hubei University of Education. Her research interest is the growth of highly ordered organic films and the application on organic thin film transistors.

Lifeng Chi received her B.S. degree in physics and M.S. degree in physical chemistry from Jilin University, China. She earned her Ph.D degree in 1989 in Göttingen, Germany, working on electron/energy transfer in thin organic films. She worked at the University of Mainz and BASF from 1993 to 1994 as a postdoctoral researcher and then joined the University of Münster and received a Lise Meitner scholarship in 1997. In 2000, she finished her habilitation in physics in the area of nanostructuring through self-organization. She became a professor in physics at the University of Münster in 2004 and was appointed as chair professor at Soochow University, China, in 2012. Her research is focused on the assembling of structured surfaces and interfaces, onsurface chemistry, and organic ultrathin films.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (91227201 and 11304213), the China Postdoctoral Science Foundation (2014M550304), and Suzhou Industrial Park (SUN-WIN project 2013). We are also grateful for support from the Collaborative Innovation Center of Suzhou Nano Science & Technology.

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Monolayer-Mediated Growth of Organic Semiconductor Films with

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