Article Cite This: J. Phys. Chem. C 2018, 122, 16242−16248
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Polymorphic Behavior of Perylene and Its Influences on OFET Performances Chou-Ting Hsieh, Chin-Yi Chen, Heng-Yi Lin, Cheng-Jui Yang, Tzu-Jung Chen, Kuan-Yi Wu, and Chien-Lung Wang* Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan
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S Supporting Information *
ABSTRACT: Among the polycyclic aromatic hydrocarbons, although perylene is commercially available and possesses higher solubility and stability than the others, its thin-film structures and organic field-effect transistor (OFET) performances have been rarely explored. To understand its potential as an active material in OFETs, the polymorphic behaviors, packing structures, and OFET characteristics of perylene were carefully examined. The well-oriented crystal arrays of perylene prepared via droplet-pinned crystallization delivered the highest hole mobility among the reported perylene OFETs. Fluorescence microscope, electron diffraction, and lattice modeling results confirm the polymorphic behavior of perylene in the solution-processed crystal arrays and its influences on the OFET performances. The concentration-sensitive and temperature-sensitive polymorphic behavior of perylene make processing conditions crucial in the preparation of pure-phase crystal arrays. The results show the great potential of perylene as an active material in low-cost and high-performance OFETs. Moreover, the knowledge regarding the polymorphic behavior of perylene provides opportunity for the further optimization of perylene-based OFETs.
1. INTRODUCTION Molecular packing is a determinative factor to the properties of solid-state materials such as dye, drugs, organic semiconductors (OSCs), etc. Differences in the molecular packing alter the way that molecules deliver their properties in applications.1 In organic field effect transistors (OFETs), charges are transported through π-stacks of conjugated molecules. Since packing structures of conjugated molecules affect the transfer integral among molecules,2 to reach high charge mobility (μ), optimization of the packing structure of conjugated molecules becomes necessary. Packing structures are sensitive not only to molecular structures3−8 but also to processing conditions.9 Variations in solvents, crystallization temperature, applied shearing stress, substrate properties, crystal thickness, and so on have been reported to induce different crystalline polymorphs for conjugated molecules such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pen), αsexithiophene (6T), pentacene, C60, etc.10−15 Although the structural differences in the distinct polymorphs of a conjugated molecule are normally subtle, the slight variations in the molecular arrangements showed significant impact on the μ.16−18 Among all conjugated molecules, acenes, which are polyaromatic hydrocarbons constructed with linearly fused benzene rings,19 are the earliest examples that deliver OFET μ comparable to that of amorphous silicon. Although the high μ stimulated study of acene-based OFET materials, the low © 2018 American Chemical Society
stability and low processability of acenes restrict their practical uses. Like pentacene, perylene is a polyaromatic hydrocarbon (Scheme 1) made with five fused benzene rings. Nevertheless, Scheme 1. Chemical Structures of (a) Pentacene and (b) Perylene
it has much better solubility and stability because of its slightly twisted conjugated plane20 and the absence of reactive sites like carbons at the 6 and 13 positions of pentacene. To date, two crystalline phases of perylene, the α-phase and the βphase, have been found. In the crystal structures of both phases, the herringbone-like molecular arrangements have the ability to act as charge-transport channels in OFETs. These characteristics give perylene high potential as the active material on large-scale, solution-processed, and low-cost OFETs. Nevertheless, studies about the solution process OFETs of perylene remain very limited.21−26 In particular, the Received: March 5, 2018 Revised: June 15, 2018 Published: June 25, 2018 16242
DOI: 10.1021/acs.jpcc.8b02199 J. Phys. Chem. C 2018, 122, 16242−16248
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The Journal of Physical Chemistry C
electrode and dielectric layer. By the spin-coating or DPC method, crystal thin films of perylene were deposited on PETS-treated SiO2/Si substrates from THF solution with different concentration (from 1 to 4 mg mL−1) at ambient conditions (from 0 to 30 °C). Gold source and drain electrodes (40 nm in thickness) were deposited by vacuum evaporation on the long axes of perylene crystal arrays through a shadow mask, affording a bottom-gate, top-contact device configuration. The channel length and width were 50 μm and 1 mm, respectively. Electrical measurements of the OFET devices were carried out at room temperature under N2 (g) using a 4156C semiconductor parameter analyzer (Agilent Technologies). The field-effect mobility was calculated in the saturation regime by using the equation IDS = (μWCi/2L)(VG − VT)2, where IDS is the drain-source current, μ is the fieldeffect mobility, W is the channel width, L is the channel length, Ci is the capacitance per unit area of the gate dielectric layer, and VG is the gate voltage. Crystal Structure Modeling. Cerius2 software package of Accelrys was used to build up the lattice models of the α and β crystals of perylene. The lattice models of two crystal structures were determined by crystallographic information files (CIFs). The simulated electron diffraction patterns are based on α and β crystal structures, generated by the Crystal Maker software.
influence of the polymorphic behaviors on the OFET performances of perylene have never been systematically studied, to the best of our knowledge. Since perylene can pack into two crystal structures, it is anticipated that the OFET performances of perylene will be strongly influenced by its polymorphic behaviors. Knowing how the process conditions affect the polymorphic behaviors and OFET performances of perylene is thus important for the purpose of applying this lowcost and commercially available conjugated molecule into practical uses. Knowing that well-aligned crystal arrays of conjugated molecules deliver higher μ than the polycrystalline thin films, in this study, crystal arrays of perylene were first prepared by the droplet-pinned crystallization (DPC) method.27−30 The exact molecular packing and polymorphic behaviors of perylene in the crystal arrays were characterized by fluorescence microscope (FLM) and electron diffraction (ED). Results show that both α and β phases of perylene formed during the solution processes, suggesting the similar nucleation barrier of the two phases. Nevertheless, formation of the β phase is more sensitive to the process conditions because of its lower thermodynamic stability shown by the differential scanning calorimetry (DSC) data. Formation of the β phase crystal arrays thus needs low solution concentration and low crystallization temperature. Finally, by correlating the molecular packing with the OFET performances of the crystal arrays, it was found that the β-phase crystal arrays can deliver 5−10 times higher hole mobility (μh) than the mixed-phase crystal arrays and the α-phase crystal arrays.
3. RESULTS AND DISCUSSION 3.1. Polymorphic Behaviors of Perylene. Before investigating the crystal arrays of perylene, the polymorphic behaviors and thermal stability of perylene were first studied in bulk crystals. Both rectangular and rhombic crystals were obtained when perylene slowly crystallized from a THF solution with concentration of 2 mg mL−1, as shown in Figure 1. The rectangular crystal and the rhombic crystal are the αcrystal (α-phase of perylene) and β-crystal (β-phase of perylene) according to the study of Tanaka et al.31 In Figure 1b, both crystals show birefringence under a polarized optical microscope, indicating that perylene are orderly packed in the two types of crystals. Upon heating, although the shape of the two crystals remain unchanged, birefringence of the rhombic crystal starts to disappear above temperatures of 110 °C, as can be seen in Figure 1d,f, whereas the rectangular crystal keeps its birefringence even at higher temperatures. The loss of birefringence suggests that the β-crystal undergoes the phase transition during the heating process. To identify the phase transition, DSC thermograms of the rectangular and rhombic crystals were recorded. In Figure S1, the rectangular crystal (αcrystal) exhibits only one endothermic phase transition at 277 °C during the heating. This phase transition represents the melting of the α-crystal.20 The high melting temperature of the α-crystal explains why the α-crystal can retain its birefringence at 180 °C (Figure 1b). As for the DSC thermogram of the rhombic crystal (β-crystal), Figure S1b shows two endothermic transition during heating. Besides the sharp endothermic melting of α-crystal at 277 °C, the other one is the broad endothermic transition between 100−140 °C. Correlated with POM results (Figure 1d), the endothermic transition at 100− 140 °C in the DSC thermogram confirms that the loss of birefringence of the β-crystal involves a phase transition to a more stable phase. Since the melting peak of the α-crystal at 277 °C can be observed after the broad transition around 100−140 °C, the broad transition is a solid-to-solid transition that transforms the β-crystal into the α-crystal. In literature, the X-ray diffraction study of the β-to-α phase transition by
2. METHODS Material. Perylene with purity of 99.0% was purchased from Sigma-Aldrich Corp. and used without further purification. Preparation of Crystal Array. The crystal arrays of perylene was prepared on the cleaned substrates (glass slides and SiO2/Si substrates) via the droplet pinned crystallization (DPC) method. The substrates were soaked in piranha solution (H2SO4 and H2O2 with volume ratio of 3:1) at 80 °C for 40 min and then sonicated in deionized water for 40 min. Finally, the substrates were cleaned with isopropyl alcohol and then exposed under an ultraviolet−ozone environment for 40 min. Next, through the DPC method, at ambient conditions (from 0 to 30 °C), the THF solutions of perylene (from 1 to 4 mg mL−1) were added onto substrates that have a capillary tube in the middle. During solvent evaporation, crystal arrays of perylene were found to grow along the solvent receding direction. General Characterization. The polarized optical microscope (POM) images were recorded on the Leica DM2700 optical microscopy. The FLM images were recorded by using Olympus FV300 fluorescence microscope. The differential scanning calorimetry (DSC) thermograms were recorded by TA DSC Q20 under nitrogen atmosphere. The scan rate was 10 °C min−1. Transmission Electron Microscopy (TEM) characterization. The TEM images and the electron-diffraction patterns of the α and β crystals of perylene were recorded on a JEOL JEM-2010 transmission electron microscope with an accelerating voltage of 200 kV. OFET Device Fabrication and Characterization. An ntype heavily doped Si wafer with a SiO2 layer of 300 nm and a capacitance per unit area of 11 nF cm−2 was used as the gate 16243
DOI: 10.1021/acs.jpcc.8b02199 J. Phys. Chem. C 2018, 122, 16242−16248
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the β-crystals appear green under FLM. The FLM images in Figure 3 clearly show that the crystal phase of perylene in the crystal arrays highly depends on the process conditions. The yellow α-crystals are predominate in the crystal arrays grown at a higher solution concentration (4 mg mL−1) and high temperature (T = 30 °C), as can be seen in Figure 3a−e. In contrast, parts k an dl of Figure 3 show the green β-crystals are the overwhelming majority in the crystal arrays grown at a lower solution concentration (1−2 mg mL−1) and lower temperature (T = 0 °C). In addition to the pure-phase crystal arrays, mixed-phase crystal arrays, which contain crystals of both colors, were also obtained under the process conditions with intermediate concentration and temperature, as shown in Figure 3f−j. After the FLM images differentiate the crystal types of perylene in the crystal array, the ED pattern provides detailed information about the lattice structure and molecular orientation in the crystals. To examine the exact molecular arrangements of perylene along the charge-transport pathway, the yellow and green crystals were studied with transmission electron microscope (TEM) and the ED techniqued. ED patterns were collected according to the setup illustrated in Figure S2. For the sample preparation, the studied crystals were first peeled from the crystal arrays on a Si substrate by a poly(acrylic acid) sheet. Second, the peeled crystals along with the poly(acrylic acid) sheet were float onto water surface to allow the dissolution of the poly(acrylic acid) sheet. After poly(acrylic acid) dissolved in water, the crystals were floated on the water surface and then picked up by a copper grid for the TEM and ED measurements. Parts a and b of Figure 4 show the TEM micrographs of α- and β-crystals, respectively. The long-axis of the crystals were placed horizontally. The corresponding ED patterns of the two crystals are shown in Figure 4c,d. To confirm the packing structure and molecular orientation in the two crystals, CIF files of the α-crystal and the β-crystal of perylene were first downloaded.20 The packing models of the two crystals were then processed with the Cerius2 software package to generate simulated ED patterns from all possible crystallographic zone axes. After the simulated ED patterns were screened, the simulated ED patterns (Figure 4e,f) matched well with the experimental ones in Figure 4c,d. Both of the simulated ED patterns in Figure 4e,f are generated from the [100] zone of the packing models shown in Figure 4c,d. Therefore, the results confirm that the yellow crystal is in the α phase, whereas the green crystal is in the β phase. The packing models indicate that perylene molecules form dimers first and pack into the so-called “sandwich-herringbone” packing in the α phase (Figure 4g). On the other hand, in the β phase, perylene unimers directly stack into the herringbone packing (Figure 4h). The dimer structure in the α phase further helps us to understand why the α phase crystals are the majority in the crystal arrays prepared at the higher solution concentrations, since high solution concentration can promote the formation of perylene dimers. In contrast, low solution concentration and low temperature facilitate perylene molecules to crystallize directly through the unimers and eventually resulted in the β phase crystals. Since molecular crystals of conjugated molecules have anisotropic electronic properties,33 identifying the molecular orientation of perylene in the α- and β-crystal arrays is important. Moreover, by correlating the lattice models (Figure 4e,f) with the TEM micrographs in Figure 4a,b, it is also confirmed that perylene molecules adopt an end-on orientation
Figure 1. Optical microscope images (left panel) and polarized optical microscope images (right panel) of the rectangular crystals (αphase) and the rhombic crystals (β-phase) at different temperatures: (a, b) T = 25 °C, (c, d) T = 110 °C, and (e, f) T = 180 °C.
Botoshansky et al. also showed that the β-to-α phase transition occurs at 100−140 °C.20 Moreover, in the subsequent cooling process, no exothermic transition at 100−140 °C was observed, indicating that the α-crystal does not change back to the β-crystal in the cooling process. Our Attempt for the formation of the β-crystal from the α-crystal via annealing the α-crystal at 90 °C for 3 days was not successful. Thus, we only obtained the β-crystal only from solution, but not from the bulk phase. The difficulty in obtaining the β-crystal can be attributed to the either that the β-phase is a metastable phase32 or that the β-to-α transition has a high kinetic barrier.20 3.2. Preparation and Structural Characterization of the Crystal Arrays of Perylene. To prepare crystal arrays of perylene, a DPC method illustrated in Figure 2 was used. The
Figure 2. Schematic illustration of the droplet-pinned crystallization.
DPC method is effective in generating highly oriented crystal arrays of perylene, as shown in Figure 3. To confirm whether or not the polymorphic behavior of perylene is involved in the crystallization, FLM and ED were used to identify the packing structure of perylene in the crystal arrays. According to the study of Tanaka et al.,31 the α-crystals appear yellow, whereas 16244
DOI: 10.1021/acs.jpcc.8b02199 J. Phys. Chem. C 2018, 122, 16242−16248
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Figure 3. Fluorescence microscope images of the perylene crystal arrays grown from THF solutions at different temperatures and concentrations by the DPC method. The preparation temperatures are (a−d) T = 30 °C, (e−h) T = 20 °C, and (i−l) T = 0 °C. The α-crystals show yellow color, whereas the β-crystals show green color under FLM.
Figure 4. TEM micrographs and ED patterns of the yellow crystal (a, b) and the green crystal (c, d). Simulated ED patterns of the α-crystal (e) and the β-crystal (f) generated from [100] zone. The corresponding molecular arrangement in the α-crystal and the β-crystal are shown in (g) and (h).
in the crystal arrays. The π-stacking direction of perylene molecules points toward the long axis of the crystals in the crystal arrays of both the α- and β-crystals. Since in the OFET device architecture, the source, and the drain electrodes are deposited on the two ends of the crystals, the π-stacking direction of the perylene molecules aligns well with the chargetransport direction in the OFET devices. Therefore, it is anticipated that the crystal arrays will deliver high μ.
3.3. Influences of Polymorphism on the OFET Characteristics of Perylene. The OFET characteristics of the perylene crystals were evaluated in the bottom-gate, topcontact (BG/TC) OFET devices. Besides the crystal arrays, the OFET characteristics of the spin-coated polycrystalline films were included to show the performance differences between the random oriented and well aligned crystalline films. Figure 5 is the bright-field optical microscopy images of the BG/TC OFETs of perylene. The spin-coated film contains 16245
DOI: 10.1021/acs.jpcc.8b02199 J. Phys. Chem. C 2018, 122, 16242−16248
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Figure 5. Bright-field optical microscopy images (top panel) and morphological illustrations (bottom panel) of the perylene BG/TC OFETs of (a) mix-crystals (spin coating), (b) α-crystals (DPC), (c) mix-crystals (DPC), (d) β-crystals (DPC).
Figure 6. Output (up) and transfer (down) characteristics of the perylene BG/TC OFETs prepared from THF solutions of perylene by (a) spin coating and (b) DPC at conc = 2 mg mL−1, T = 20 °C, (c) DPC at conc = 4 mg mL−1, T = 30 °C, and (d) DPC at conc = 1 mg mL−1, T = 0 °C.
Table 1. Summation of Perylene BG/TC OFET Performancesa crystal morphology
method
temp (°C)
conc (mg mL−1)
mixed-phase (polycrystals) mixed-phase (crystal arrays) α-phase (crystal arrays) β-phase (crystal arrays)
spin coating DPC DPC DPC
20 20 30 0
3.0 2.0 4.0 1.0
μh,max (μh,avg)b (cm2 V−1 s−1) 6.2 2.4 1.6 1.1
× × × ×
10−4 10−2 10−2 10−1
(4.8 (1.1 (1.0 (6.0
× × × ×
10−4) 10−2) 10−2) 10−2)
standard deviation 1.5 2.3 2.5 1.9
× × × ×
10−4 10−3 10−3 10−2
Ion/Ioff
Vth (V)
∼103 ∼105 ∼105 ∼105
11.3 29.1 21.7 24.3
PETS was used as the SAM layer. bμh is provided in “highest [average]” form, and the performance data were obtained based on more than 20 OFET devices.
a
transport pathway in the perylene OFETs. Furthermore, the crystal arrays of perylene give the highest μh among the perylene OFETs reported so far.22,23,35,36 In addition, the influences of the polymorphic behaviors of perylene on the OFET performances can be clearly revealed by comparing the μh of the mixed-phase, α-phase, and β-phase crystal arrays. The β-crystal array delivered μh of 1.1 × 10−1 cm2 V−1 s−1, which is nearly 5−10 times higher than that of the mixed-phase crystal array (2.40 × 10−2 cm2 V−1 s−1) and α-phase crystal arrays (1.6 × 10−2 cm2 V−1 s−1). Since charges are delivered through the π-stacks of perylene, it is evident that the packing model of perylene profoundly influences the OFET performances. The
randomly oriented polycrystalline domains of perylene, whereas the crystal arrays contain ribbon-like crystals with their long-axes point to the source and drain electrodes. In Figure 6, the output and transfer plots of the devices indicate that the perylene active layers exhibited typical p-channel OFET characteristics. The μh, on−off ratio (Ion/Ioff) and threshold voltage (Vth) of the devices deduced from the transfer plots were summarized in Table 1. The spin-coated thins delivered μh that is about 40 to 180 times lower than the μh of the crystal arrays. The difference thus demonstrates that aligning the crystalline domains is essential in eliminating inplane grain boundaries34 and improving the quality of charge16246
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higher mobility of the β-crystal arrays suggests that the herringbone packing shown in Figure 3g provides a more effective charge-transport channel than the “sandwichherringbone” packing shown in Figure 3h.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02199.
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REFERENCES
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4. CONCLUSIONS In this study, the polymorphic behaviors, packing structures, and OFET characteristics of a readily available and low-cost polyaromatic hydrocarbon, perylene, were carefully examined. The mixed-phase, α-phase, and β-phase crystal arrays of perylene were successfully prepared via the selection of processing conditions. In droplet-pinned crystallization, our results show that the higher solution concentration (4 mg mL−1) and crystallization temperature (T = 30 °C) promote the formation of the α-phase crystal array, whereas the lower solution concentration (1−2 mg mL−1) and crystallization temperature (T = 0 °C) facilitate the growth of the β-phase crystal array. Furthermore, the mixed-phase crystal arrays were produced under the intermediate concentrations and crystallization temperatures. The analysis of ED patterns of the crystal arrays resulted in valuable information regarding the packing model and molecular orientation of perylene in the crystal arrays. Perylene molecules were found to adopt end-on orientation in both the α-phase and β-phase crystal arrays, which enable the π-stacking direction to align with the charge transport pathway in OFET devices. The well-aligned π-stacks of perylene thus delivered the recorded high μhs among the perylene-based OFETs. Moreover, the characterization data clearly show that the “herringbone packing” in the β-phase crystal arrays transport charges more effective than the “sandwich-herringbone” packing in the α-phase crystal arrays. These results show not only the importance of knowing the polymorphic behaviors of conjugated molecules but also the great potential of perylene for the applications of low-cost and high-performance OFETs.
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Characterization data for the compounds, including differential scanning calorimetry plots and the illustration of the electron diffraction experiment (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chien-Lung Wang: 0000-0002-5977-2836 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology (MOST 104-2628-E-009-007-MY3, MOST 1062221-E-009-130-MY3). We thank Prof. Masuahara at National Chaio-Tung University for his kind helps on the FLM characterizations. We also thank the Science Class and Miss Ming-Chuan Chang at Wu-Ling Senior High School, Taiwan, for actively participating this work. 16247
DOI: 10.1021/acs.jpcc.8b02199 J. Phys. Chem. C 2018, 122, 16242−16248
Article
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DOI: 10.1021/acs.jpcc.8b02199 J. Phys. Chem. C 2018, 122, 16242−16248
