Using chemical modifications to tune molecular packing

The use of chemical modifications to vary molecular packing of OSCs has been studied extensively.Reference Jung, Tremblay, Yeh and Katz^37–Reference Takimiya, Shinamura, Osaka and Miyazaki⁴¹ Chemical modifications through the use of synthesizing OSCs with different side chains or synthesizing OSCs with different conjugated backbones can affect the molecular packing to a large extent.Reference Payne, Parkin, Anthony, Kuo and Jackson⁴⁰ Additionally, fine-tuning of molecular packing is also possible.Reference Takimiya, Shinamura, Osaka and Miyazaki^41, Reference Anthony, Eaton and Parkin⁴² Ideally, the chemical modifications change the molecular packing and increase the electronic overlap, which results in a higher charge-carrier mobility.

As an example, pentacene has a high hole transport mobility, and recent studies have demonstrated that single crystalline and highly pure pentacene crystals have mobilities as high as 35 cm² V^–1 s^–1 at room temperature.Reference Klauk, Halik, Zschieschang, Schmid, Radlik and Weber^43, Reference Jurchescu, Baas and Palstra⁴⁴ Depending on the bulkiness of the 6 and 13 position (Figure 1a, arrows) substituents in pentacene, its resulting packing structure can be either a 1D slipped-stack packing motif, characterized by strong intermolecular interactions along one axis, or a 2D brick-wall packing motif, characterized by strong intermolecular interactions along two axesReference Anthony, Eaton and Parkin^42, Reference Anthony, Brooks, Eaton and Parkin^45, Reference Liu, Song, Zeng, Zhou, Shi, Ma, Yang, Zhang and Gong⁴⁶ (Figure 1). These bulky substituent pentacene derivatives are stable and solution soluble.Reference Anthony, Eaton and Parkin⁴² The brick-wall packing structure may increase the intrinsic charge-transport properties because of good electronic overlap along the OSC π-conjugated backbones and a 2D film morphology.

Figure 1.

Using chemical modifications to tune molecular packing. (a) Chemical structure of 6,13-substituted pentacene. The 6 and 13 positions (arrows) can be modified with a variety of substituents, R, such as those listed here. (b) The herringbone packing motif in a thin film of pentacene.Reference Yoshida and Sato⁴⁷ (c) Single-crystal packing structure of R = SiMe₃ pentacene derivative, showing 1D slipped-stack packing.Reference Anthony, Eaton and Parkin⁴² (d) Single-crystal structure of R = Sii Pr₃ pentacene derivative, showing 2D brick-wall packing.Reference Mannsfeld, Tang and Bao⁴⁸ In (b) and (d), the rectangle marks the view of the unit cell looking down from the c-axis, with the red and green lines representing the a- and b-axes. The atoms present in the organic semiconductors are represented as spheres (white = hydrogen, black = carbon, yellow = sulfur).

Several reviews have already discussed the chemical structure-molecular packing relationships for a variety of small molecule OSCs.Reference Takimiya, Shinamura, Osaka and Miyazaki^41, Reference Tang, Oh, Reichardt and Bao^49–Reference Anthony, Facchetti, Heeney, Marder and Zhan⁵¹ Briefly, Moon et al. have shown that chlorination of tetracene changes the molecular packing from a herringbone structure to a slipped-stack structure.Reference Moon, Zeis, Borkent, Besnard, Lovinger, Siegrist, Kloc and Bao⁵² Increasing the number of chlorination sites in a perylene-3,4:9,10-tetracarboxylic diimide backbone also changes the molecular packing from a herringbone arrangement to either a planar or a 2D contorted brick-wall packing.⁵³

Additionally, Reese et al. have shown that attaching bulky trimethylsilane side groups to an aromatic oligothiophene core can significantly affect the tilt and packing compared to the unsubstituted oligothiophene core.Reference Reese, Roberts, Parkin and Bao⁵⁴ The tilt of the molecular packing can cause improved electronic overlap between OSC molecules in a crystal, thus improving charge carrier mobility. The use of OSCs that can pack together stoichiometrically as a co-crystal can be used as an alternative route to modify molecular packing by making OSC dimers.Reference Sokolov⁵⁵ Finally, Che et al. have incorporated hydrogen-bonding pyrazolyl and imidazolyl units into organoplatinum (II) complexes to reduce the π–π stacking distances and increase charge transport mobilities.Reference Che, Chow, Yuen, Roy, Lu, Chen, Chui and Zhu⁵⁶ Chemical modifications are widely used to change the molecular packing of OSCs and subsequently affect charge transport properties in these materials.

Controlling packing through co-deposition

A second, but less frequently used, method to control molecular packing is through co-deposition of two similar OSCs that possess slightly different crystal packing structures. This method of tuning crystal lattice parameters is reminiscent of methods used in the inorganic semiconductor industry, where epitaxial deposition of silicon and germanium is used to strain the silicon lattice.Reference Thompson, Armstrong, Auth, Cea, Chau, Glass, Hoffman, Klaus, Ma, McIntyre, Murthy, Obradovic, Shifren, Sivakumar, Tyagi, Ghani, Mistry, Bohr and El-Mansy⁵⁷ Unlike synthetic methods, this method usually only affords small changes to the OSC crystal structure. For example, the OSCs copper phthalocyanine (H₁₆CuPc) and copper-hexadecafluoro-phthalocyanine (F₁₆CuPc) have been co-deposited by Opitz et al., and the lattice spacing was found to be dependent on the H₁₆CuPc concentration.Reference Opitz, Ecker, Wagner, Hinderhofer, Schreiber, Manara, Pflaum and Brutting⁵⁸ The interlayer spacing linearly changed as a function of composition, from 1.3 nm for pure H₁₆CuPc to 1.45 nm for pure F₁₆CuPc. While the H₁₆CuPc/F₁₆CuPc bilayer has reciprocal lattice peaks similar to those of the pure phases, the co-deposited blend showed reciprocal lattice peaks between the two pure phases.Reference Opitz, Ecker, Wagner, Hinderhofer, Schreiber, Manara, Pflaum and Brutting⁵⁸

Co-deposition has also been performed with pentacene and perfluoropentacene to tune the ionization energy of the resulting thin film.Reference Salzmann, Duhm, Heimel, Oehzelt, Kniprath, Johnson, Rabe and Koch⁵⁹ Similarly, by varying the relative concentrations of α-sexithiophene (6T) or the alkylated version, α,ω-dihexylsexithiophene (DH6T), with the phenyl analogue sexiphenyl (6P), intercalation and subnanometer control of the lattice spacing was achieved.Reference Vogel, Salzmann, Opitz, Duhm, Nickel, Rabe and Koch⁶⁰ These intercalated 6T/DH6T thin films were then used to tune the performance of organic transistors from enhancement to depletion modes.Reference Cosseddu, Vogel, Fraboni, Rabe, Koch and Bonfiglio⁶¹

More recently, Huang et al. have shown that two similar molecules, 2,5-bis(4-biphenylyl)-thieno[3,2-b]thiophene (BPTT) and 2,7-bis(4-biphenylyl)-phenanthrene (BPPh), can be co-deposited epitaxially to form an induction layer for another OSC, vanadyl phthalocyanine (VOPc). Tuning the lattice parameter of this induction layer varied the morphology of VOPc and the injection barrier. VOPc mobilities as high as 3.0 cm² V^–1 s^–1 were obtained using this method.Reference Huang, Liu, Qiao, Tian, Geng and Yan⁶² The use of co-deposited OSCs finely tunes the crystal packing, but the degree of tuning is restricted to the range between the original packing structures of the pure OSCs.

Changing molecular packing through the use of an interfacial layer

Many polymorphs of the same OSC molecule can be present because of the weak intermolecular bonding in OSC crystals. One method of introducing polymorphism is to use the “symmetry breaking effect” of different interfaces to prevent crystal formation from equilibrium growth. In pentacene, at least four polymorphs are known and are characterized by their different interlayer d(001) spacings.Reference Mattheus, de Wijs, de Groot and Palstra⁶³ Recently, Mannsfeld et al. showed that the first layer of pentacene on amorphous SiO₂ grows in a herringbone fashion, with the pentacene molecules upright on the surface; this structure differs from other known pentacene polymorphs.Reference Mannsfeld, Virkar, Reese, Toney and Bao⁶⁴ This interfacial structure has a higher electronic overlap compared to the other pentacene polymorphs, resulting in a higher mobility in pentacene thin films than the reported bulk single-crystal mobility.

Another well-studied effect of changing molecular packing of OSCs is through the use of single crystalline metallic or graphitic interfaces. Single crystalline metal surfaces and the interface dipole moments formed between the metal and OSC molecules can be used as an ordering layer for OSCs. This effect has been studied for a variety of OSCs, such as C₆₀, phthalocyanines, and porphyrins on different metal surfaces such as Cu(111), Ag(111), and Au(111).Reference Yoshimoto and Kobayashi^65–Reference Chen, Pan, Yan, Wan, Matsuo and Nakamura⁶⁸ Co-deposition of different adsorbates have also been studied, forming mono- and multilayers where the adsorbates pack as co-crystals.Reference Fendrich, Wagner, Stohr and Moller^69–Reference Wei and Reutt-Robey⁷² Wakayama et al.Reference Wakayama, de Oteyza, Garcia-Lastra and Mowbray⁷⁰ have deposited mixed F₁₆CuPc:pentacene blends at different relative concentrations on Au(111). It was observed that phase segregation occurred at >50% F₁₆CuPc. However, different crystalline phases and packing were observed at 3:1, 2:1, and 1:1 ratios of F₁₆CuPc:pentacene (Figure 2).

Figure 2.

Different packing structures in pentacene: F₁₆CuPc blends with a ratio of (a) 1:1, (b) 2:1, and (c) 3:1. The lattice structure changes significantly.Reference Wakayama, de Oteyza, Garcia-Lastra and Mowbray⁷⁰ For an increasing ratio, the crystal unit cell accommodates the increasing density of pentacene molecules. The molecules were assembled in rows (arrows) in a similar manner to pentacene assembly. Pentacene at the trimer center and surrounding four F₁₆CuPc were labeled c, t, b, l, and r, respectively. Adapted from Reference Reference Wakayama, de Oteyza, Garcia-Lastra and Mowbray70 with permission. © 2011 American Chemical Society.

De Wild et al. observed the formation of a crystalline phase when co-depositing chloro[subphthalocyanianto] boron(III) (SubPc) and C₆₀.Reference de Wild, Berner, Suzuki, Yanagi, Schlettwein, Ivan, Baratoff, Guentherodt and Jung⁷¹ Recently, Wei and Reutt-Robey have shown Kagome lattice packing of C₇₀ molecules on a layer of titanyl phthalocyanine (TiOPc) (Figure 3), showing directed design of OSC molecular packing is possible, which can lead to higher mobilities.Reference Wei and Reutt-Robey⁷² Blunt et al. have also extended self-assembly into the third dimension. They deposited a layer of p-terphenyl-3,5,3,“5”-tetracarboxylic acid (TPTC) onto highly oriented pyrolytic graphite, and then deposited C₆₀ as a guest molecule. The C₆₀ molecules were deposited in the pores formed by the first layer of TPTC, after which a subsequent layer of TPTC was deposited above and parallel to the surface.Reference Blunt, Russell, Gimenez-Lopez, Taleb, Lin, Schroder, Champness and Beton⁷³ Thus, molecular packing and crystallization can be modified in a variety of ways, such as using crystal-metal or crystal-graphite interfaces.

Figure 3.

Kagome lattice of C₇₀ on TiOPc.Reference Wei and Reutt-Robey⁷² Left: a TiOPc monolayer on a metal surface. Right: deposition of C₇₀. Green signifies the TiOPc template layer, red signifies the first layer of deposited C₇₀, and yellow signifies the second layer of C₇₀ that deposits in a Kagome style lattice. Adapted from Reference Reference Wei and Reutt-Robey72 with permission. © 2011 American Chemical Society.

Tuning molecular packing through varying processing conditions

Controlling OSC polymorph formation and the crystal packing through processing conditions is also being actively explored. Cheng and Lin showed that solvent annealing of pentacene using p-xylene and other solvents changes the crystal structure from a metastable “thin-film” polymorph to a more stable “bulk” polymorph.Reference Cheng and Lin⁷⁴

Solution shearing is another method that can be used to tune the crystal packing. In this method, an OSC solution is sandwiched in between a nonstick, moving blade and a heated device substrate. As the blade is translated, an evaporation front is created where the solvent evaporates and a uniform OSC thin film is cast on the device substrate. Giri et al. showed that by controlling the shearing speed, the molecular packing of 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) changes from the thermodynamically stable single-crystal structure to metastable crystal structures.Reference Giri, Verploegen, Mannsfeld, Atahan-Evrenk, Kim, Lee, Becerril, Aspuru-Guzik, Toney and Bao⁷⁵ Because of the rapid solvent evaporation during solution shearing, metastable, “lattice strained” crystal packing structures are obtained (Figure 4). Interestingly, in the case of TIPS-pentacene, the lattice strained structure leads to a shorter π–π stacking distance, which is the distance between the π-conjugated faces of the OSC, leading to a higher charge-carrier mobility. This method is applicable to electronic device fabrication because it can be readily adapted for large-area deposition.

Figure 4.

Tuning the crystal packing by solution shearing. (a) Schematic of 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) solution shearing on phenyl-trichlorosilane (PTS) treated silicon oxide/silicon substrates. (b) Lattice strain in TIPS-pentacene at different shearing speeds. (c) Thin-film molecular packing structure of evaporated TIPS-pentacene. (d) Thin-film molecular packing structure of solution sheared and lattice strained TIPS-pentacene. The green and yellow spheres indicate the TIPS side group. The blue arrow represents the fast charge transport direction. T1 and T2 denote the unique molecular pairs. Adapted from Reference Reference Giri, Verploegen, Mannsfeld, Atahan-Evrenk, Kim, Lee, Becerril, Aspuru-Guzik, Toney and Bao75 with permission. © 2011 Nature Publishing Group.

The control of molecular crystal packing is an exciting area of research in the OSC community because of its potential to achieve higher device charge-carrier mobilities. The control of organic small molecule polymorphs has also been extensively explored in the pharmaceutical industry.Reference Bag, Patni and Reddy^76, Reference Lopez-Mejias, Knight, Brooks and Matzger⁷⁷ Bag et al.Reference Bag, Patni and Reddy⁷⁶ have demonstrated the concept of growing metastable polymorphs using kinetically controlled conditions (through rapid solvent evaporation). Also, different polymorphs can be selected using nano-confinement, as well as directed epitaxial growth on molecular crystal substrates.Reference Beiner, Rengarajan, Pankaj, Enke and Steinhart^78, Reference Mitchell, Yu and Ward⁷⁹ These methods may also be useful to create OSC polymorphs and to explore the metastable states that improve charge transport.

Patterning single crystals

Patterning of OSCs achieves large arrays in devices.Reference Menard, Meitl, Sun, Park, Shir, Nam, Jeon and Rogers⁸¹ Patterning precisely controls the position between semiconductors and electrodes and also minimizes cross-talk between adjacent devices. Typically, OSCs do not grow into large single crystals (e.g., smaller than millimeter-scale), and the fabrication of large arrays of single-crystal FETs relies on bottom-up patterning techniques.

Patterning nucleation sites

OSC crystal patterning has been achieved on substrates modified by self-assembled monolayers (SAMs).Reference Briseno, Aizenberg, Han, Penkala, Moon, Lovinger, Kloc and Bao⁸² Briseno et al. examined the growth of anthracene crystals on SAM-modified gold-coated substrates. They found that crystals nucleated from oligophenylene thiol SAMs in an epitaxial fashion, and the selective nucleation is associated with the interaction between the terminal group of SAMs and the OSC (Figures 5a–b). Oligophenylene-thiols were favored for OSC nucleation rather than alkanethiols. When SAMs with different nucleation activities were micropatterned, crystals were selectively nucleated and formed patterns as governed by the SAMs (Figure 5b).

Figure 5.

Patterning nucleation sites. (a) Molecular modeling of (001)-oriented anthracene crystals on a terphenylthiol (-3P) self-assembled monolayer (SAM). (b) Optical micrographs of the patterned anthracene crystals. Crystal nucleation occurs from the -3P SAM regions (red arrow) and not from the HS(CH₂)₁₁CH₃ SAM regions (white arrow). The patterned domains are 100 × 100 μm². (c) A 3D atomic force microscopy image (20 × 20 μm²) of an OTS layer with polydimethylsiloxane regions (5 × 5 μm²) on a SiO₂ substrate. (d) An optical micrograph of patterned C₆₀ single-crystal arrays. The white dashed square indicates the size and location of one of the octadecyltriethoxysilane-stamped domains. Adapted from References Reference Briseno, Aizenberg, Han, Penkala, Moon, Lovinger, Kloc and Bao82 (© 2005 American Chemical Society) and 83 (© 2006 Nature Publishing Group) with permission.

In addition to SAMs, Briseno et al. used rough surfaces as nucleation sites to pattern crystals. They successfully fabricated large FET arrays based on the patterned single crystals.Reference Briseno, Mannsfeld, Ling, Liu, Tseng, Reese, Roberts, Yang, Wudl and Bao^83, Reference Mannsfeld, Briseno, Liu, Reese, Roberts and Bao⁸⁴ Octadecyltriethoxysilane (OTS, average thickness 13 ± 2 nm) films were micropatterned onto SiO₂/Si surfaces using a polydimethylsiloxane (PDMS) stamp (Figure 5c). Crystals selectively nucleated from the rough OTS-coated regions (Figure 5d). Both p- and n-type crystal patterns, including rubrene, pentacene, tetracene, C₆₀, fluorinated copper phthalocyanine (F₁₆CuPc), and tetracyanoquinodimethane (TCNQ) were demonstrated. Since the OTS-coated domain is small enough to support only one nucleation event, patterning of discrete single crystals was achieved, and device arrays based on individual single crystals became possible (Figure 5d).

Besides OTS-coated surfaces, Liu et al. used spray-coated carbon nanotube (CNT) bundles to introduce rough surfaces as nucleation sites to obtain patterned OSC crystals.Reference Liu, Briseno, Mannsfeld, You, Locklin, Lee, Xia and Bao⁸⁵ They also used an Au film with defined hole patterns (Au mask), to directly pattern α-sexithiophene single crystals that nucleated from the rough edges of the holes.Reference Liu, Mannsfeld, Wang, Sun, Stoltenberg and Bao⁸⁶ One of the advantages of this method is that both the size and shape of the crystals are dictated by the holes in the mask, hence facilitating device fabrication.

In all the methods presented, patterning of OSC single crystals is achieved via nucleation sites. In the OTS/CNT-nucleation method, the crystals nucleate from rough surfaces; therefore the semiconductor/dielectric interfaces are also rough,Reference Mannsfeld, Briseno, Liu, Reese, Roberts and Bao^84, Reference Liu, Briseno, Mannsfeld, You, Locklin, Lee, Xia and Bao⁸⁵ and the orientation and alignment of the crystals are thus random. These features reduce device performance. In contrast, SAM surfaces are smooth, and epitaxial crystal growth is achieved. Crystals grown from SAMs usually exhibit smooth interfaces and controlled crystallographic orientation and alignment. However, it is not possible to pattern only one single crystal in a defined orientation, as the nucleation density is too high.

Patterning solution/dispersion droplets

As OSCs are usually soluble in organic solvents, solution deposition methods, which are cost-effective compared to vacuum deposition methods,Reference Laudise, Kloc, Simpkins and Siegrist⁸⁷ can be used to grow the crystals. Another advantage of solution deposition is that the “precursor” OSC crystal solutions can be easily patterned, hence providing a facile approach to the patterning of solution-grown crystals. In addition to solutions, OSC dispersions in solvents can serve as crystal carriers for patterning.

The wetting/dewetting method was first used to pattern OSC solutions. Liu et al. patterned OSC crystals on substrates with patterned wettability.Reference Liu, Wang, Mannsfeld, Locklin, Erk, Gomez, Richter and Bao⁸⁸ Depending on the different solvent–substrate interactions, OSC crystals dispersed in solvents can be selectively deposited or removed, resulting in desired patterns. Mannsfeld et al. developed a simple and efficient method to pattern organic single-crystal FETs from solutions.Reference Mannsfeld, Sharei, Liu, Roberts, McCulloch, Heeney and Bao⁸⁹ A solution of OSCs was first dispersed into tiny droplets in a water host where a liquid vortex was created by rapid stirring before being poured onto a FET substrate pre-patterned with gold pad electrodes. Since the OSC organic solution has better wettability on the gold pads compared to the OTS-modified SiO₂ surface, a clean pattern of droplets is achieved. The droplets evaporate, and single crystals are formed. Although the researchers have patterned only two OSCs [bis(cyclohexyl)-α-quaterthiophene and trimethylsilane-aquarterthiophene],Reference Mannsfeld, Sharei, Liu, Roberts, McCulloch, Heeney and Bao⁸⁹ this method should be applicable to pattern other OSCs as well, as only the wetting characteristics of the solution is important.

One advantage of this method is that OSC crystals adhere easily to the gold electrodes, which facilitates device fabrication. In addition to forming droplets using the liquid vortex technique, micro-liquid inkjet printing techniques are also being used to print droplets directly onto a substrate with pre-patterned wetting/dewetting regions (Figure 6).Reference Minemawari, Yamada, Matsui, Tsutsumi, Haas, Chiba, Kumai and Hasegawa²⁴ Subsequent crystallization from the droplet solutions results in patterned single crystals. Using this method, single-crystal arrays of 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene (C₈-BTBT) were prepared. The observed maximum hole mobility was 31.3 cm² V^–1 s^–1 (average over 54 FETs: 16.4 ± 6.1 cm² V^–1 s^–1), which is one of the highest reported values for OSCs and the highest value reported for solution-processed OSCs.

Figure 6.

(a) Schematic of the inject printing process for organic semiconductor (OSC) crystals. Antisolvent ink is inkjet-printed (step 1), and then OSC solution ink is dropped on top to form intermixed droplets confined to a predefined area (step 2). OSC crystals nucleate at the liquid-air interface (step 3), before the solvents completely evaporate (step 4). (b) Micrographs of a 20 × 7 array of inkjet-printed C₈-BTBT single crystal. (c) Crossed Nicols polarized micrographs of a crystal with the entire crystal extinguishing the light simultaneously, thus showing single crystallinity. Adapted from Reference Reference Minemawari, Yamada, Matsui, Tsutsumi, Haas, Chiba, Kumai and Hasegawa24 with permission. © 2011 Nature Publishing Group. Note: (C₈-BTBT), 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene.

Recently, NakayamaReference Nakayama, Hirose, Soeda, Yoshizumi, Uemura, Uno, Li, Kang, Yamagishi, Okada, Miyazaki, Nakazawa, Nakao, Takimiya and Takeya¹⁹ and Akkerman et al.Reference Akkerman, Li and Bao⁹⁰ reported another interesting approach to pattern OSC solutions by spatially confining the solution droplets using a template (Figure 7). This method is similar to conventional micro-contact printing processes,Reference Qin, Xia and Whitesides⁹¹ except that an inclined gap (Figure 7a) or a restricted channel (Figure 7b) confines the solution volume, allowing the solution to crystallize onto defined regions. Using this method, 2,9-alkyl-dinaphtho[2,3-b:2,‘3’-f]thieno[3,2-b]thiophene (C₁₀-DNTT) single crystals were successfully patterned on both rigid (SiO₂/Si) and flexible polyethylene naphthalate substrates. FETs based on the patterned crystals exhibit hole mobilities as high as 11 cm² V^–1 s^–1 (average over 16 FETs: 7 cm² V^–1 s^–1).

Figure 7.

(a–b) Schematic of the organic semiconductor (OSC) solution patterning droplet process. A stamp is designed with gaps (a) or channels (b) to confine droplets of OSC solution. (c–d) Micrographs of a 4 × 4 array of OSC crystals (green) patterned using the stamp. Adapted from References Reference Nakayama, Hirose, Soeda, Yoshizumi, Uemura, Uno, Li, Kang, Yamagishi, Okada, Miyazaki, Nakazawa, Nakao, Takimiya and Takeya19 (© 2011 Wiley) and 90 (© 2012 Elsevier) with permission. Note: C₁₀-DNTT, 2,9-alkyl-dinaphtho[2,3-b:2,‘3’-f]thieno[3,2-b]thiophene.

Patterning techniques using OSC solution droplets confines the area on which the OSC crystals can grow, leading to the patterning of crystals. To obtain single crystals, the crystallization process inside the droplets (especially the nucleation event) should be controlled. For example, Minemawari et al. showed that the protuberant shape of the droplets seen in Figure 6a is necessary for controlled nucleation of crystals and to avoid polycrystal formation.Reference Minemawari, Yamada, Matsui, Tsutsumi, Haas, Chiba, Kumai and Hasegawa²⁴

Aligning single crystals

Current flows unidirectionally in an FET, while charge-carrier transport is anisotropic in most OSC single crystals.Reference Sundar, Zaumseil, Podzorov, Menard, Willett, Someya, Gershenson and Rogers^16, Reference Li, Jiang, Meng, Gao, Li, Tang, He, Hu, Liu and Zhu^97–Reference He, Zhang, Jia, Li and Tao⁹⁹ Hence, to achieve maximum performance, organic single crystals need to be aligned to allow current to flow along the direction of the highest mobility. Alignment facilitates the relative position between crystals and electrodes, as the source and drain electrodes can be easily deposited perpendicular to the alignment direction. Oh et al. described a filtration and transfer method to align OSC single crystals (Figure 8).Reference Oh, Lee, Mannsfeld, Stoltenberg, Jung, Jin, Kim, Yoo and Bao¹⁵ A dispersion containing single-crystalline microwires (MW) of N,N’-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) was prepared. The suspension was filtered in vacuo through a PDMS template with channels where the MWs could be aligned by fluid flow. The degree of alignment was observed to be associated with the vacuum pressure and the MW length, with lower pressure and longer MW length leading to a better alignment. The aligned MWs were finally transferred onto FET substrates to complete the device fabrication. Although the alignment was demonstrated on a substrate of 2.5 cm in diameter, a larger operational area should also be achievable.

Figure 8.

(a–e) Schematic illustration of the filtration and transfer process. (a) A microwire (MW) dispersion is filtered by vacuum through a polydimethylsiloxane (PDMS) mask placed on a porous anodized aluminum oxide (AAO) membrane. (b) After vacuum filtration, MWs are aligned along PDMS mask channels. (c) Removal of the AAO membrane. (d) Transfer of the MWs onto field-effect transistor substrates (SiO₂/Si wafer covered by n-octadecyl triethoxysilane [OTS]). (e) Removal of the AAO and PDMS masks and transfer of the MWs onto the wafer. (f) Micrographs of the obtained aligned MWs. Adapted from Reference Reference Oh, Lee, Mannsfeld, Stoltenberg, Jung, Jin, Kim, Yoo and Bao15 with permission. © 2009 National Academy of Sciences. Note: BPE-PTCDI, N,N’-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide.

In addition to the filtration and transfer method (in which the crystals are first grown and then aligned), both the growth and alignment processes can be coupled together in one simple step. Dip coating,Reference Rogowski, Dzwilewski, Kemerink and Darhuber^100, Reference Li, Gao, Schuermann, Ostendorp, Wang, Du, Lei, Fuchs, De Cola, Mullen and Chi¹⁰¹ solution-shearing deposition,Reference Becerril, Roberts, Liu, Locklin and Bao^102, Reference Liu, Becerril, Roberts, Nishi and Bao¹⁰³ zone casting,Reference Pisula, Menon, Stepputat, Lieberwirth, Kolb, Tracz, Sirringhaus, Pakula and Mullen^104–Reference Duffy, Andreasen, Breiby, Nielsen, Ando, Minakata and Sirringhaus¹⁰⁶ and confined solution depositionReference Akkerman, Li and Bao⁹⁰ were used to grow aligned OSC crystals from solution. The directional receding of the solution meniscus leads to crystal alignment. It is only when the growth conditions are well-controlled that aligning single crystals becomes possible. For example, the droplet-pinned methodReference Li, Tee, Cha, Cui, Chung, Lee and Bao^31, Reference Uemura, Hirose, Uno, Takimiya and Takeya^107–Reference Li, Tee, Giri, Chung, Lee and Bao¹⁰⁹ was developed recently to provide a steady condition for crystal growth, and aligned single crystals of C₆₀, and TIPS-pentacene have been obtained.