Orientation Control of Solution-Processed Organic Semiconductor

Aug 16, 2017 - The crystallization of a series of triisopropylsilylethynyl (TIPS)-derivatized acene-based organic semiconductors drop cast from soluti...
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Orientation Control of Solution-Processed Organic Semiconductor Crystals To Improve Out-of-Plane Charge Mobility Xiaoshen Bai,†,¶ Kai Zong,†,¶ Jack Ly,‡ Jeremy S. Mehta,§ Megan Hand,† Kaitlyn Molnar,† Sangchul Lee,† Bart Kahr,# Jeffrey M. Mativetsky,§ Alejandro Briseno,‡ and Stephanie S. Lee*,† †

Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States ‡ Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States § Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 13902, United States # Department of Chemistry, New York University, New York, New York 10002, United States S Supporting Information *

ABSTRACT: The crystallization of a series of triisopropylsilylethynyl (TIPS)-derivatized acene-based organic semiconductors drop cast from solution onto substrates was investigated as a function of the size of their conjugated cores. When drop cast onto a substrate, the molecules in TIPSpentacene crystals adopt a “horizontal” orientation, with the long axis of the pentacene core parallel to the substrate surface. For crystals comprising molecules with dibenzopyrene, anthanthrene, and pyranthrene cores, two-dimensional X-ray diffraction patterns revealed the existence of a second population of crystals adopting a “vertical” molecular orientation with the long axis of the acene core perpendicular to the substrate surface. The ratio of the population of TIPS-pyranthrene crystals with molecules adopting horizontal versus vertical orientations was controlled by varying the surface energy of the underlying substrate. These crystals displayed orientationdependent linear birefringence and linear dichroism, as observed by differential polarizing optical microscopy. Conductive atomic force microscopy (C-AFM) revealed a 42-fold improvement in out-of-plane hole mobility through crystals adopting the vertical molecular orientation compared to those adopting the horizontal molecular orientation.

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pentacene molecules, for example, can either “stand up” or “lie down” on surfaces. On SiO2 substrates, pentacene molecules typically stand up, such that the long axis of pentacene is vertically oriented relative to the substrate surface. When deposited on graphene, on the other hand, pentacene molecules lie down on the substrate with the π-plane parallel to the surface.6−8 A 5-fold increase in light conversion efficiency was achieved for solar cells comprising pentacene crystals with a “lying-down” molecular orientation compared to those comprising pentacene crystals with a standing up, vertical molecular orientation.6 The improvement in efficiency can be attributed to aligning the π-stack direction of pentacene crystals with the charge transport direction in solar cells. Similar control of the out-of-plane orientation of conjugated molecules deposited on graphene has been observed in thermally evaporated copper phthalocyanine films, leading to an improvement in out-of-plane hole mobility.5,8

INTRODUCTION Over the past several decades, small-molecule organic semiconductors have emerged as promising materials for electronic devices, including organic field-effect transistors and solar cells.1,2 Organic semiconductor crystals exhibit anisotropic charge transport along different crystallographic directions, with charge transport occurring fastest along the π-stack direction in conjugated systems.3,4 The orientation of conjugated organic molecules with respect to the device architecture is thus a significant factor in determining performance.5 In organic fieldeffect transistors, for example, charge transport through the organic semiconductor thin film occurs parallel to the substrate surface. In organic solar cells, on the other hand, charge transport occurs perpendicular to the substrate surface. Significant research efforts in the field of organic electronics have therefore focused on controlling out-of-plane molecular orientations in order to improve the performance of organic semiconductor crystals.3,4 Controlling the out-of-plane molecular orientation of organic semiconductor small-molecule crystals has been successfully achieved in systems in which the organic semiconductor is thermally evaporated onto a substrate. Thermally deposited © 2017 American Chemical Society

Received: July 3, 2017 Revised: August 15, 2017 Published: August 16, 2017 7571

DOI: 10.1021/acs.chemmater.7b02771 Chem. Mater. 2017, 29, 7571−7578

Article

Chemistry of Materials In solution-processed organic semiconductor thin films, controlling the orientation of crystals has proven significantly more difficult. These crystals comprise molecules with a conjugated core, such as pentacene, but additionally incorporate solubilizing side groups that affect the molecular packing, crystal morphology, and crystal orientation.2,9−13 The in-plane orientation of molecules in crystals deposited by solution processing has been controlled using a variety of methods, including the application of directional shear forces during film deposition14,15 and suppressing crystallization along specific inplane directions.16 Recently, the out-of-plane orientation of phenyl/phenyl-capped tetraaniline was controlled on graphene surfaces in the presence of an antisolvent such that the molecules adopted a “lying-down” molecular orientation.17 For organic semiconducting small molecules incorporating solubilizing side groups, the out-of-plane molecular orientation tends to be dominated by interactions between the insulating group and the underlying substrate. Organic semiconductors functionalized with bulky silyl groups10,16,18−30 and alkyl chains,31−35 for example, orient with the solubilizing groups in contact with the substrate surface. To the best of our knowledge, controlling the out-of-plane molecular orientation of crystals comprising such functionalized molecules has been achieved only in fluorinated bis(triethylsilylethynyl)anthradithiophene (FTESADT) systems.36,37 While nonfluorinated TES-ADT orients with the silyl substituents in contact with the substrate surface,16,18,19 the out-of-plane molecular orientation of FTES-ADT crystals can be controlled by treating the underlying substrate surface with pentafluorobenzenethiophenol (PFBT).37 On untreated surfaces, populations of crystals with both the (111) plane and (001) plane parallel to the substrate surface are present. On PFBT-treated surfaces, on the other hand, FTES-ADT crystals preferentially adopted an orientation with the (001) plane parallel to the substrate surface. This preferential crystal orientation on PFBT-treated surface was attributed to specific fluorine−fluorine interactions between PFBT and FTES-ADT.37 The ability to dictate the out-of-plane orientation of molecules in organic semiconductor crystals is critical for advancing the use of these materials in optoelectronic devices. Taking advantage of the ability to tune the chemical structure of small-molecule organic semiconductors via synthetic routes, in this paper, we explore how systematic changes in chemical structure affect the orientation of crystals formed during drop casting onto substrates. Specifically, a series of solutionprocessable bis(triisopropylsilylethynyl) (TIPS)-functionalized acene derivatives was selected in which the size of the acene core was systematically increased. Interestingly, two-dimensional X-ray diffraction (2D XRD) analysis revealed that the out-of-plane molecular orientation adopted by crystals formed via drop casting onto substrates depends on the size of the conjugated core, with the long axes of the cores orienting either horizontally or vertically relative to the substrate surface. For crystals comprising bis(triisopropylsilylethynyl)pyranthrene, it was further discovered that the ratio of crystals with molecules adopting horizontal versus vertical orientations could be tuned by varying the surface energy of the underlying substrate prior to solution deposition. Linear dichroism and linear birefringence of these crystals were characterized via differential polarizing optical microscopy. Conductive atomic force microscopy (c-AFM) further revealed a 42-fold difference in out-of-plane charge carrier mobility depending on the orientation of the crystals. Collectively, these findings reveal

the role of molecule−substrate interactions on crystallization during solution processing and provide design rules for enhancing the performance of solution-processable optoelectronic devices via control of the out-of-plane orientation of molecules in organic semiconductor crystals.

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EXPERIMENTAL SECTION

Synthesis of Chemical Compounds. Bis(TIPS)pentacene (TIPS-PEN) was purchased from Sigma-Aldrich company, ≥99% purity. Bis(TIPS)dibenzopyrene (TIPS-DBP), bis(TIPS) anthanthrene (TIPS-AT), and bis(TIPS)pyranthrene (TIPS-PY) were synthesized according to previously published procedures.38 Preparation of Octadecyltrichlorosilane (OTS)-Treated SiO2/ Si Substrates. SiO2/Si substrates were immersed in a 0.5 vol % solution of OTS (Sigma-Aldrich, 90% purity) for 30 min. They were then rinsed with neat toluene and allowed to dry for 15 min at room temperature in a nitrogen-filled glovebox. The substrates were then removed from the glovebox and sonicated in acetone for 5 min to remove excess OTS. Preparation of Monolayer Graphene on SiO2/Si Substrates. Monolayer graphene on Cu foil sheets (Graphenea) were attached to glass cover slides. Poly(methyl methacrylate) (PMMA) was spun cast on top of the sheets at 3000 rpm for 30 s. After spin coating, the PMMA-coated samples were placed in ferric chloride solution (Transene) at 50 °C for 7 min. The detached PMMA-coated graphene monolayers were transferred to clean SiO2/Si substrates and immersed three times in deionized water for 5 min each, followed by removal of residual copper with 35 vol % HCl solution. The samples were allowed to dry overnight and then thermally annealed at 150 °C for 5 min and immersed in acetone at 100 °C for 30 min to remove the PMMA coating. Preparation of Pentafluorobenzenethiophenol (PFBT)-Treated Au Substrates. 4 nm Cr and 40 nm Au were thermally evaporated onto clean SiO2/Si substrates successively. The samples were then immersed in a 1 wt % solution of PFBT (Sigma-Aldrich, 97% purity) in anhydrous ethyl alcohol (Sigma-Aldrich, 99.5% purity) in a nitrogen-filled glovebox for 60 min. The samples were then rinsed with neat ethyl alcohol and allowed to dry in the glovebox for 30 min. Preparation of Hexamethyldisilazane (HMDS)-Treated SiO2/ Si Substrates. HMDS (Sigma-Aldrich, 99.9% purity) was spun cast onto SiO2/Si substrates at 2000 rpm for 30 s, and samples were allowed to dry for 30 min. Preparation of UV−Ozone (UVO)-Treated SiO2/Si Substrates. Cleaned SiO2/Si substrates were exposed to UV−ozone for 10 min. Solution Deposition of Organic Semiconductors onto Substrates. After appropriate surface treatment of the underlying substrate, organic semiconductor compounds were drop cast onto substrates from 0.25 wt % solutions in chlorobenzene at 110 °C. After drop casting, the samples were covered by a glass Petri dish and allowed to dry. 2D X-ray Diffraction Measurements. 2D X-ray diffraction patterns (XRD) were collected using a Bruker AXS D8 DISCOVER GADDS diffractometer with VANTEC 2000 detector. The diffractometer was operated in reflection mode at 40 kV × 40 mA. Data was collected using an incident X-ray wavelength of λ = 1.5405 Å at an incident angle of 3° and for a collection time of 300 s in air at room temperature. Crystal morphology predictions were performed in Mercury v3.01 using the Bravais, Friedel, Donnay and Harker (BFDH) model. Differential Polarization Mapping. Quantitative polarized light imaging was achieved by the rotating polarizer method. Measurements were performed using an LED light source (λ = 680 nm) and mechanically rotating polarizer based on the Metripol rotating polarizer technique.39−41 Intensity measurements at increments of 7.2° of rotation of the polarizer were collected for a full 360° rotation. A false color map of the sine of the retardance, δ, was extracted from the angle-dependent intensity measurements. Atomic Force Microscope Measurements. Atomic force microscopy (AFM) measurements were performed under ambient 7572

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perpendicular to the Caryl−Cethynyl bonds. The π-surface area of TIPS-PEN is 0.26 nm2. TIPS-DBP and TIPS-AT molecules comprise six benzene rings in two rows, with π-surface areas of 0.31 nm2. TIPS-PY molecules comprise eight benzene rings in three rows, with π-surface areas of 0.41 nm2. Highlighted in Figure 1 are the widths of the acene cores, which range from 2.838 Å for TIPS-PEN to 7.053 Å for TIPS-PY. To examine the effect of the chemical structure on the outof-plane orientation of molecules in crystals comprising these compounds, we collected 2D X-ray diffraction (2D XRD) patterns on crystals drop cast onto untreated SiO2/Si substrates. Figure 2A displays a 2D XRD pattern collected on TIPS-PEN crystals drop cast onto SiO2/Si. The appearance of diffraction spots, as opposed to isotropic rings, indicates that TIPS-PEN crystals adopt a preferred out-of-plane molecular orientation with respect to the substrate surface. Reflections along qxy = 0 Å−1 were identified as (00l).22,23 On the basis of this analysis, TIPS-PEN crystals were found to comprise molecules in a standing up orientation with the long axis of the pentacene core horizontal relative to the substrate surface (Figure 2B, top), consistent with previous reports in the literature.22,24−30 Figure 2A also displays 2D XRD patterns collected on TIPSAT, TIPS-DBP, and TIPS-PY crystals deposited via drop casting onto untreated SiO2/Si substrates. Interestingly, two distinct out-of-plane molecular orientations were observed for crystals of all three compounds. In the diffraction pattern collected on TIPS-AT crystals, reflections associated with both the (00l) and (0kl ̅) family of planes were observed along qxy = 0 Å−1. The (00l) family of reflections corresponds to crystals adopting a horizontal molecular orientation with the conjugated π-plane tilted 86.3° with respect to the substrate surface. The (0kl ̅) family of reflections, on the other hand, corresponds to crystals with molecules adopting a vertical orientation (Figure 2B, bottom) with an angle of 82.2° between the conjugated π-plane and the substrate surface. These results indicate that TIPS-AT crystals, unlike TIPS-PEN crystals, adopt two distinct out-of-plane molecular orientations with respect to the substrate surface. Compared to TIPS-AT, TIPS-DBP also comprises six aromatic rings organized in two rows but with the rows offset from one another by one aromatic ring. Reflections along qxy = 0 Å−1 were identified to belong to the (00l) and (0l0) family of reflections. The (00l) family of reflections corresponds to

conditions using an AIST-NT Combiscope 1000SPM AFM. A 0.25 wt % TIPS-PY solution in chlorobenzene solvent was drop cast onto UVO-treated indium tin oxide-coated glass at a temperature of 110 °C covered with a Petri dish. Topography and film thickness were measured using intermittent-contact (tapping) mode with BudgetSensors Tap300Al-G silicon probes. Conductive atomic force microscopy (C-AFM) current maps were acquired by imaging in contact mode with an applied force of 6.5 nN and a 9 V bias applied to the ITO substrate. Out-of-plane hole mobility was determined by recording current−voltage curves of at least 900 sample positions for each crystal type and then using a modified space-charge limited current (SCLC) model that takes the probe−sample geometry into account.42 SCLC curve fitting was performed with a field-independent treatment and correction factor (δ) of 1.43 Budget-Sensors ContE-G probes, coated with chromium and then platinum, were used for all CAFM measurements.

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RESULTS AND DISCUSSION To examine how the molecular structure of solutionprocessable organic semiconductors affects their assembly on surfaces, we studied a series of TIPS-derivatized conjugated acene compounds displayed in Figure 1: (A) bis-

Figure 1. Chemical structures of (A) bis(TIPS)pentacene (TIPSPEN),21 (B) bis(TIPS)anthanthrene (TIPS-AT),38 (C) bis(TIPS)dibenzopyrene (TIPS-DBP),38 and (D) bis(TIPS)pyranthrene (TIPSPY). Electrically active cores are highlighted in orange, and insulating TIPS side groups are highlighted in blue.

(triisopropylsilylethynyl)pentacene (TIPS-PEN),10 (B) bis(triisopropylsilylethynyl)anthanthrene (TIPS-AT),38 (C) bis(triisopropylsilylethynyl)dibenzo-pyrene (TIPS-DBP),38 and (D) bis(triisopropylsilylethynyl)pyranthrene (TIPS-PY).38 These compounds all incorporate triisopropylsilylethynyl (TIPS) substituents to render them soluble in organic solvents. The difference among these compounds lies in the number and configuration of benzene units in the acene core. In TIPS-PEN molecules, five benzene rings are connected in a single row

Figure 2. 2-D XRD patterns of (A) TIPS-PEN, TIPS-AT, TIPS-DBP, and TIPS-PY crystals, with major reflections labeled. Insets display predicted BFDH crystal morphologies, with planes exposing continuous sheets of silyl groups in blue and planes exposing the ends of the acene cores in orange. (B) Molecular orientations associated with those observed in the 2D XRD patterns, one in which the long axis of the conjugated core is horizontal with respect to the substrate surface and one in which it is vertical with respect to the substrate surface. Silyl groups are represented by blue circles, and the conjugated core is represented by an orange rectangle. Crystallographic details (lattice constants, space group, and CSD reference codes) are provided in Table S1. 7573

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Figure 3. (A) Graph of the ratio of the intensity of the (200) reflection to the (002) reflection in the 2D XRD patterns of TIPS-PY crystals versus the surface energy16 of the underlying substrate. (B, C) Optical micrographs of TIPS-PY crystals drop cast on UVO-treated glass and HMDS-treated glass, respectively. (D) False color map of sin(δ) collected via differential polarization imaging collected on TIPS-PY crystals drop cast on HMDStreated glass. Red and green outlines indicate crystals adopting vertical and horizontal molecular orientations, respectively.

molecular orientations indicates the existence of competing factors governing the solution-phase crystallization of these compounds. To examine the role of molecule−substrate interactions in determining the orientation of TIPS-derivatized organic semiconductor crystals, we deposited these compounds onto SiO2/Si substrates treated by a variety of methods16 to alter the substrate surface energy from 20 to 78 ergs/cm2 (see Experimental Section) and examined the resulting crystals via 2D XRD. Unsurprisingly, TIPS-PEN crystals comprise horizontally oriented molecules regardless of the surface energy of the underlying substrate (Figure S1). This observation is consistent with previous reports in the literature. Since the synthesis of TIPS-PEN was first published in 2002,10 hundreds of papers have been published on solution-processed TIPS-PEN crystals for organic electronic applications. Invariably, solutionprocessed TIPS-PEN crystals orient with the TIPS substituents in contact with the substrate surface such that the long axis of the acene core is horizontally oriented with respect to the substrate surface.26−53 The orientations of TIPS-AT and TIPS-DBP crystals were also found to be insensitive to the surface energy of the underlying substrate (Figures S2 and S3), indicating that competition between TIPS−substrate and core−substrate interactions is weak. For these compounds and TIPS-PEN, the BFDH model predicts the areas of the faces exposing the ends of the conjugated cores to be 0.2−0.4 times smaller than the faces exposing the silyl groups, indicating that the former faces are less stable. In contrast, the areal ratio of the (100) face exposing the conjugated core ends compared to the (001) face exposing the silyl groups of TIPS-PY molecules is predicted to be ∼0.6, indicating that the (100) face of TIPS-PY is more stable compared to the other compounds examined. Consistent with this prediction, the molecular orientation in TIPS-PY crystals was found to depend strongly on the surface energy of the underlying substrate. Figure 3A displays a graph of the ratio of the intensities of the (200) and (002) reflections along qxy = 0 Å−1 in the 2D XRD patterns of TIPS-PY crystals formed via drop casting onto substrates with varying surface energies. The film thicknesses were confirmed via atomic force microscopy to be between 350 and 450 nm irrespective of the underlying substrate surface energy in the range examined (Figure S4). On low surface energy substrates, the formation of TIPS-PY crystals with horizontally oriented molecules is promoted, while on high surface energy substrates, the formation of TIPS-PY crystals with vertically oriented molecules dominates (Figure S5). These results suggest that judicious design of the molecular structure of solution-processable organic semiconductors and the surface energies of the substrates onto

crystals adopting a horizontal molecular orientation, as displayed in the diagram to the top right of the 2D diffraction pattern. In this orientation, the conjugated π-plane is tilted 56.2° with respect to the substrate surface. The (0l0) family of reflections, on the other hand, corresponds to crystals adopting a vertical molecular orientation, with a tilt angle of 87.3° between the conjugated π-plane and the substrate surface. It is interesting to note that two diffraction spots that appear between the (001) and (002) reflections cannot be identified with the known crystal structure of TIPS-DBP. It is likely that TIPS-DBP forms a previously unobserved thin film phase and will be the subject of future investigations. The molecule with the largest acene core in this study is TIPS-PY, with eight aromatic rings organized in three rows and a π-surface area of 0.41 nm2. As displayed in the 2D XRD pattern in Figure 2A, TIPS-PY crystals are highly oriented in the out-of-plane direction. Again, reflections associated with two families of planes, the (00l) and (l00) planes, were identified along qxy = 0 Å−1. The (00l) family of reflections corresponds to crystals adopting a horizontal molecular orientation with the conjugated π-plane tilted 63.7° with respect to the substrate surface. The (l00) family of reflections corresponds to crystals adopting a vertical orientation, with a tilt angle of 89.0° between the conjugated π-plane and the substrate surface. In all four compounds tested, the dominant orientation adopted, i.e., crystals with horizontally oriented molecules, is different than that adopted by pentacene molecules thermally deposited on SiO2/Si substrates. Pentacene molecules typically adopt a vertical orientation on untreated SiO2/Si substrates.6,44−46 It has been hypothesized that, in the absence of strong molecule−substrate interactions, this orientation is preferred because it exposes the lowest surface energy crystal surface to the crystal−air and crystal−substrate interfaces.47,48 For TIPS-derivatized compounds, the Bravais, Friedel, Donnay and Harker (BFDH) model predicts that the (001) face, comprising a continuous layer of silyl substituents, is the lowest surface energy surface. As displayed in the insets in Figure 2A, this face (highlighted in blue) occupies the largest surface area of the crystals for all four compounds. The horizontal orientation adopted by TIPS-derivatized crystals on substrates is thus a direct consequence of the presence of the solubilizing TIPS substituents. As revealed by 2D-XRD, TIPS-AT, TIPS-DBP, and TIPS-PY crystals adopt a second molecular orientation that exposes the ends of the conjugated cores to the air/crystal and crystal/ substrate interfaces (highlighted in orange). This molecular orientation is similar to that adopted by thermally evaporated pentacene molecules on SiO2.6,44,49,50 The presence of two distinct populations of crystals with different out-of-plane 7574

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Figure 4. (A) AFM height map and (B) corresponding current map at the boundary between TIPS-PY crystals (L = 0.32 um) with horizontally oriented molecules (top) and vertically oriented molecules (bottom). Insets in B illustrate the orientation of TIPS-PY molecules in the two regions with respect to the AFM tip, with the crystallographic axis corresponding to the out-of-plane direction labeled. Electrically active cores are highlighted in orange, and insulating TIPS side groups are highlighted in blue. (C) Histograms of hole mobilities measured through crystals with vertically and horizontally oriented molecules. The thicknesses of these two crystals are 74 and 54 nm, respectively. Inset displays the average of all collected I−V curves.

sin(δ) ∼ 0.4, corresponding to green crystals. The latter number is likely resonance enhanced by virtue of the broad visible absorbance. A more complete polarimetric analysis with a more complex polarization modulation scheme is requisite. However, it is clear from these measurements that the red crystals have a very small optical anisotropy in the direction presented, compared with the green crystals. From the false color map, the birefringence of the crystals, Δn, was calculated using the following equation:

which they are deposited is critical for controlling the out-ofplane orientation of molecules. Figure 3B,C displays optical micrographs of TIPS-PY crystals deposited on HMDS-treated glass and UVO-treated glass, respectively. On both surfaces, TIPS-PY crystals are approximately 10 μm in length. Two populations of crystals are observed, reddish and greenish-brown, corresponding to the two distinct out-of-plane molecular orientations. These colors are far from pure, and in each orientation, more than one visible transition is being excited. However, the crystals can easily be divided into two orientational subpopulations on the basis of color. On HMDS-treated glass, green crystals account for approximately 60% of the total crystal area. On UVO-treated glass, on the other hand, green crystals account for approximately 33% of the total crystal area. Comparing these findings with the XRD results, we conclude that the green and red crystals correspond to the crystals with horizontally and vertically oriented molecules, respectively. To further characterize the optical properties of two populations of TIPS-PY crystals, we performed differential polarization imaging.54 In these experiments, the intensity of light exiting the sample was measured as a function of the polarization angle of incoming light (λ = 680 nm). Figure 3D displays a false color map of the sin(δ) signal, where δ is the phase difference between the two eigenmodes of light propagating through the crystals.54 Again, two populations of TIPS-PY crystals drop cast on UVO-treated SiO2 were observed in the false color maps, one population characterized by sin(δ) ∼ 0, corresponding to red crystals, and the other by

Δn =

δλ 2πL

where λ is the wavelength of incident light and L is the thickness of the crystals. L was measured using tapping mode AFM (Figure S6) to be 0.7 ± 0.2 μm. For TIPS-PY crystals with horizontally oriented molecules, Δn was calculated to be 0.075. On the other hand, for TIPS-PY crystals with vertically oriented molecules, Δn was calculated to be 0. This difference in birefringence is related to polarizability in orthogonal directions. For crystals with vertically oriented molecules, the polarizabilities in perpendicular directions projected into the (200) plane, that is the in-plane projections of the π-systems, appear to be comparable, consistent with near zero linear birefringence (Figure S6). The presence of two out-of-plane molecular orientations further provides the unique opportunity to measure the out-ofplane charge mobility through solution-processed crystals along different crystallographic directions. As discussed earlier, the out-of-plane molecular orientation of solution-processable 7575

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compounds with larger acene cores, such as TIPS-PY, on the other hand, interactions between the acene core and the substrate become more important, promoting the formation of crystals with vertically oriented molecules. Moreover, the relative strengths of these two competing interactions can be controlled by tuning the surface energy of the underlying substrate. More broadly, our results suggest that controlling the out-of-plane molecular orientation in solution-processable small molecule systems requires judicious design of the conjugated core to promote core−substrate interactions over insulating substituent−substrate interactions. Increasing the width of the conjugated core relative to the insulating substituents is demonstrated here to be a feasible strategy for gaining such control. Taking advantage of this ability to control the out-ofplane molecular orientation in TIPS-PY crystals, the optical and electronic properties of TIPS-PY crystals were analyzed. Most significantly, the hole mobility through TIPS-PY crystals with vertically oriented molecules was measured to be 42 times larger than that through crystals with horizontally oriented molecules. These findings suggest that, through judicious molecular design and treatment of the underlying substrate, organic semiconductor crystals with molecular orientations optimized for both in-plane and out-of-plane charge transport can be achieved, thereby enhancing the efficiency of optoelectronic devices.

organic semiconductors is typically dominated by interactions between the substrate and solubilizing side groups, resulting in crystals with horizontally oriented molecules. Here, the insulating side groups arrange in continuous layers parallel to the substrate surface. We expect these layers to act as significant barriers to charge transport in the out-of-plane direction. If crystals comprise vertically oriented molecules, on the other hand, the layer of insulating side groups will arrange perpendicular to the substrate surface and will not act as barriers to vertical charge transport. To test this hypothesis, we performed C-AFM to measure the charge mobility through TIPS-PY crystals along both the c and a axes, corresponding to horizontal and vertical molecular orientations, respectively. Figure 4A displays an AFM height map at the boundary between a crystal with horizontally oriented molecules (top) and a crystal with vertically oriented molecules (bottom). As observed from the height map, the two crystals have approximately the same thickness. Figure 4B displays the corresponding current map in which a voltage was applied across the crystals in the out-of-plane direction, with insets illustrating the orientation of molecules with respect to the conductive AFM probe during measurements. These current maps show clear contrast between the two crystal types, with higher currents through the crystal with vertically oriented molecules. Current−voltage (I−V) curves collected on the two types of crystals exhibit diode-like charge transport behavior (inset of Figure 4C). Figure 4C displays histograms of hole mobilities extracted from at least 900 I−V curves collected on crystals with horizontally and vertically oriented molecules. Consistent with the AFM current map, the hole mobility through crystals with vertically oriented molecules was measured to be 3.9 × 10−5 ± 0.2 × 10−5 cm2/V·s, 42 times larger than the measured mobility of 9.3 × 10−7 ± 0.7 × 10−7 cm2/V·s for crystals with horizontally oriented molecules. These results are consistent with resistivity measurements performed on TIPS-PEN crystals, in which the resistivity along the a axis was measured to be 2 orders of magnitude larger than that along the c axis.55 Collectively, the C-AFM results indicate that the solubilizing silyl groups act as significant barriers to charge transport in these organic semiconducting crystals. Because these groups tend to orient in continuous sheets parallel to the substrate surface, as observed for TIPS-PEN, TIPS-AT, and TIPS-DBP crystals, they hinder the use of this class of organic semiconductors in optoelectronic devices, such as organic solar cells, that require charge transport in the out-of-plane direction. By controlling the molecular orientation within deposited crystals via judicious design of the molecular structure and surface energy of the underlying substrate, the impact of these silyl groups on the crystals’ out-of-plane charge mobility can be mitigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02771. Crystallographic information for the four TIPS-derivatized compounds, 1D line traces of crystals formed during solution drop casting onto substrates with different surface energies, AFM height image of TIPS-PY crystals, and projection of TIPS-PY molecules onto different crystallographic planes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bart Kahr: 0000-0002-7005-4464 Jeffrey M. Mativetsky: 0000-0002-6574-9843 Alejandro Briseno: 0000-0003-2981-9143 Stephanie S. Lee: 0000-0003-0964-6353 Author Contributions ¶

X.B. and K.Z. contributed equally to this work.

Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Increasing the core-to-substituent size ratio in TIPS-functionalized small-molecule organic semiconductors enables the formation of crystals in which the long axis of the acene core orients vertically with respect to the substrate surface, an orientation that has not yet been reported for solutionprocessed TIPS-PEN crystals or other similarly derivatized polyaromatic compounds. For compounds with small acene cores, such as TIPS-PEN, crystals invariably adopt the horizontal orientation, as determined by interactions between the insulating TIPS groups and the substrate surface. For

ACKNOWLEDGMENTS The authors are grateful for the assistance of Dr. Chunhua Hu at the Department of Chemistry of New York University with 2D XRD experiments and acknowledge support by the National Science Foundation under Award Number CRIF/ CHE-0840277 and by the NSF MRSEC Program under Award Number DMR-0820341. J.M.M. and J.S.M. acknowledge support from the National Science Foundation (CAREER award DMR-1555028) for the C-AFM measurements. Research was carried out in part at the Micro Device Laboratory and 7576

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Article

Chemistry of Materials

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used microscopy resources within the Laboratory for Multiscale Imaging at Stevens Institute of Technology, and the authors thank Dr. Tsengming Chou for assistance. J.L. and A.B. acknowledge funding from the National Science Foundation (DMR-1508627).

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