Highly Crystalline Films of Organic Small Molecules with Alkyl Chains

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Highly Crystalline Films of Organic Small Molecules with Alkyl Chains Fabricated by Weak Epitaxy Growth Yangjie Zhu,†,‡ Weiping Chen,§ Tong Wang,† Haibo Wang,*,† Yue Wang,§ and Donghang Yan† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 10039, P. R. China § State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: Because side-chain engineering of organic conjugated molecules has been widely utilized to tune organic solid-state optoelectronic properties, the achievement of their high-quality films is important for realizing high-performance devices. Here, highly crystalline films of an organic molecule with short alkyl chains, 5,8,15,18-tetrabutyl5,8,15,18-tetrahydroindolo[3,2-a]indole[30,20:5,6]quinacridone (C4-IDQA), are fabricated by weak epitaxy growth, and highly oriented, large-area, and continuous films are obtained. Because of the soft matter properties, the C4-IDQA molecules can adjust themselves to realize commensurate epitaxy growth on the inducing layers and exhibited good lattice matching in the thin film phase. The crystalline phase is also observed in thicker C4-IDQA films. The growth behavior of C4-IDQA on the inducing layer is further investigated, including the strong dependence of film morphologies on substrate temperatures and deposition rates due to the poor diffusion ability of C4-IDQA molecules. Moreover, highly crystalline films and high electron field-effect mobility are also obtained for the small molecule N,N′-dioctyl-3,4:9,10-perylene tetracarboxylic diimide (C8-PTCDI), which demonstrate that the weak epitaxy growth method could be an effective way to fabricate highly crystalline films of organic small molecules with flexible side chains. of highly crystalline and high-orientation organic films of organic rigid molecules without flexible substituents, for example, disklike molecules like zinc phthalocyanine (ZnPc) and rodlike molecules like N,N′-diphenyl perylene tetracarboxylic diimide (PTCDI-Ph) and rubrene, and all of them exhibited high mobilities.9−11 The method has also been used to construct organic crystalline quantum wells.12 Because of the weak intermolecular interaction of van der Waals force and the anisotropy of organic molecules, the weak epitaxy growth behaviors of organic molecules were influenced by many factors, including lattice matching, the effect of channels on the inducing layer, and the interaction between organic molecules and substrate.13 However, the introduction of the flexible side chains also can change the intermolecular interactions, which may show different epitaxy growth behavior. Thus far, the question of whether the organic small molecules with flexible side chains can be grown by the epitaxy growth method and the epitaxy growth behavior have never been investigated. Furthermore, organic small molecules with side chains also showed their potential applications in organic thin film transistors (OTFTs), organic photovoltaics, photodetectors,

1. INTRODUCTION The fabrication technology of semiconductor devices is of essential importance for the production of high-performance electronic and optoelectronic devices. In the field of inorganic semiconductors, epitaxy growth technology has played a significant role in the fabrication of heterojunction devices, such as resonant tunneling diodes, and quantum wells.1,2 Lattice matching is a key factor in the epitaxy growth of inorganic semiconductors because they are covalently bonded. For organic semiconductors, most organic epitaxial thin films are fabricated on crystalline substrates, for example, inorganic single-crystal substrates such as KCl single-crystal substrate and oxygen-passivated Cu(110) single-crystal substrate3,4 or organic single-crystal substrates such as potassium hydrogen phthalate (KAP) single-crystal substrate and tetracene single-crystal substrate.5−7 The surface corrugations and surface symmetry of the substrates have been shown to be important for driving the epitaxial growth of the overlayer in both organic−organic and organic−inorganic heterostructures.5,6,8 In addition, a method named weak epitaxy growth (WEG) was proposed to fabricate large-area, highly ordered, and continuous films, in which an ultrathin layer of small rodlike molecules was introduced as an inducing layer on an amorphous substrate to realize the epitaxy growth of the following organic semiconductors.9 The WEG method has led to the fabrication © 2016 American Chemical Society

Received: February 20, 2016 Revised: April 23, 2016 Published: April 26, 2016 4310

DOI: 10.1021/acs.jpcb.6b01579 J. Phys. Chem. B 2016, 120, 4310−4318

Article

The Journal of Physical Chemistry B

Figure 1. Molecular information about epitaxy growth. (a) Molecular structures of BP2T and C4-IDQA. (b) DSC second heating and cooling traces of C4-IDQA.

The evolution of the thin film phase and crystalline phase and the good lattice matching between thin film phase C4-IDQA and BP2T are also discussed.

and nano- or micro-optoelectronics based on one-dimensional nano- or microstructures,14−22 and side-chain engineering has begun to play an increasing role in tuning the solid-state optoelectronic properties. The introduction of flexible side chains not only improves the solubility of organic conjugated semiconductors23 but also changes the molecular arrangement in the solid state and other properties, such as absorption, fluorescence, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO).19,24−26 Recently, the impact of side chains with different lengths, positions, orientations, and structures, the odd−even effect, the isomeric effect, and alkyl branching on the molecular packing in the solid state, film quality, and device performance were thoroughly investigated.15,25−29 There are usually two methods for fabricating active layers of organic small molecules with flexible side chains, solution processing and vacuum deposition. Thermal annealing and solvent vapor annealing are very effective methods for improving the crystallinity and morphology for the solution processing small molecules with sufficiently long alkyl substituents or a liquid crystalline phase or other mesophase or molecules moving easily in the film during the annealing process.20,26,28,30 However, it is difficult to attain high-quality films of organic small molecules with shorter alkyl chains or molecules that cannot move easily in the films.30,31 Vacuum deposition is another method for fabricating films of highly thermally stable organic small molecules with flexible side chains. However, this method always led to many grain boundaries in the active layers.32−34 As we know, highly crystalline thin films with good morphology and well-aligned molecular orientations are crucial for excellent OTFT device performance. Therefore, WEG could be a new method for fabricating highly ordered and crystalline films of organic small molecules with flexible side chains. In this study, WEG was utilized to fabricate a quinacridone derivative with short alkyl side chains, 5,8,15,18-tetrabutyl5,8,15,18-tetrahydroindolo[3,2-a]indole[30,20:5,6]quinacridone (C4-IDQA), and 2,5-bis(4-biphenylyl)bithiophene (BP2T) was used as the inducing layer. Continuous and highly oriented films with a large grain size and smooth surface were formed under optimized growth conditions, which were very different from those for films fabricated by solution processing. The influences of substrate temperature and deposition rates on film morphologies were also studied because of the weak diffusion ability of C4-IDQA.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Organic Thin Films. C4-IDQA and BP2T were synthesized according to the methods described in refs 51 and 52, respectively. C8-PTCDI (98%) was purchased from Aldrich and used without purification. Heavily doped ntype silicon wafers with thermal oxidation SiO2 layers [300 nm, capacitance per unit area (Ci) of 10 nF/cm2] were used as the substrate. In the WEG process, ∼5.0 nm of BP2T was first deposited on the precleaned substrate with the substrate temperature kept at 145 °C, and then different thicknesses of C4-IDQA were deposited on the BP2T thin film at varied substrate temperatures. The deposition was performed under a pressure of 10−4−10−5 Pa at a rate of 0.1 or 0.3 nm/min. For comparison, we fabricated C4-IDQA films by a solution process. The SiO2/Si substrates were modified by the selfassembly monolayers (SAMs) of OTS. C4-IDQA was deposited by spin-casting a 4 mg/mL solution in chloroform at 1000 rpm for 30 s under an ambient atmosphere. After spincasting, the samples were annealed at different temperatures in the same chamber in which vacuum deposition was conducted. 2.2. Characterization. The DSC tests were conducted by a PerkinElmer DSC-7 instrument under a nitrogen atmosphere, and the heating and cooling rates were 10 °C/min. Film morphologies were imaged by an SPI 3800/SPA 300HV instrument (Seiko Instruments Inc.) in tapping mode. A 150 μm scanner and a commercially available SiN4 cantilever with a spring constant of 2 N/m were used. The out-of-plane XRD patterns were taken from a D8 discovery thin film diffractometer with Cu Kα radiation (λ = 1.54056 Å), and the selected voltage and current were 40 kV and 40 mA, respectively. The in-plane GIXRD diffraction patterns were obtained at the Shanghai Synchrotron Radiation Facility (SSRF) on beamline BL14B1 (λ = 1.24 Å). Selected area electron diffraction (SAED) was performed with a JEOL JEM1011 transmission electron microscope operated at 100 kV. For SAED measurement, organic films were deposited on the SiO2/ Si substrate first; subsequently, a carbon thin film was deposited on the films to be used as a support layer. Then the films were separated from the SiO2 surface by floating from 10% HF solution and then transferred to a copper grid for measurement. Au was directly deposited on the copper grid for demarcating if 4311

DOI: 10.1021/acs.jpcb.6b01579 J. Phys. Chem. B 2016, 120, 4310−4318

Article

The Journal of Physical Chemistry B

Figure 2. Film morphologies and structural properties of C4-IDQA grown on bilayer BP2T. (a) Morphologies of various thick C4-IDQA films. (b) Out-of-plane X-ray diffraction patterns. (c) In-plane X-ray diffraction patterns. (d) Fine scanning of the first diffraction angle in panel c. (e) SAED of 6 nm C4-IDQA. (f) SAED of 10 nm C4-IDQA. The inset at the top right is an enlarged view of (0−11)C4‑IDQA splitting. (g) Morphologies of the C4IDQA films prepared by spin-casting and annealed at 145 °C for 1 h. (h) Morphology of 30 nm C4-IDQA directly deposited on the SiO2 substrate at a substrate temperature of 145 °C.

kept at 145 °C. The evolution of the film morphologies and structural properties is shown in Figure 2. At the early stage of growth (0.5 nm), C4-IDQA islands exhibited a large ribbonlike shape, and all ribbonlike islands showed a very smooth edge and the same orientation on each BP2T domain, suggesting a well-defined epitaxy relationship existed between BP2T and C4-IDQA. As the thickness of C4-IDQA increased, the width of C4-IDQA crystals became narrower and eventually turned our to be stripes. The domains composed of stripelike crystals showed a quasi layer-by-layer growth, and the adjacent grains coalesced with each other very well. Consequently, smooth and continuous films with a low density of grain boundaries were formed (Figure 2a and Figure S1). Out-of-plane X-ray diffraction of different thick C4-IDQA films (Figure 2b) indicated the change in the (001) diffraction peak of C4-IDQA. The peak positions moved from 6.7° to 7.02° when the thickness of C4-IDQA increased from 6 to 10 nm and then remained unchanged with the thickness further increasing, corresponding to the d(001)C4‑IDQA of 13.18 Å shifting to 12.58 Å. There was also little change in in-plane parameters among various thick C4-IDQA films. The film of 6 nm C4-IDQA showed only the first diffraction peaks because it was too thin to bring another two diffraction peaks out obviously (Figure 2c,d). The changes in out-of-plane and in-plane X-ray diffractions indicated that there was a phase transition as the

necessary. Dark field was used for experiments to provide a weaker-intensity beam and high contrast. 2.3. OTFT Device Fabrication and Measurements. BP2T/p-6P and 30 nm C4-IDQA/C8-PTCDI bilayers were deposited on the substrate sequentially as the active layer; after that, 50 nm of Au was thermally evaporated on the film surface through shadow masks to form bottom-gate-top-contact transistors with a channel width (W) and a channel length (L) of 6000 and 200 μm, respectively. All current−voltage tests of transistors were performed via two Keithley 236 source measurement units under ambient conditions at room temperature. The field-effect mobilities were extracted from the saturated region of the transfer characteristic curve.

3. RESULTS AND DISCUSSION 3.1. Weak Epitaxy Growth Behavior of C4-IDQA on BP2T. Molecular structures of C4-IDQA and BP2T are shown in Figure 1a, and the C4-IDQA molecule had short alkyl chains, i.e., four tetrane as alkyl substituents. According to the differential scanning calorimetry (DSC) curve of C4-IDQA as shown in Figure 1b, there was no liquid crystalline phase or other mesophase for C4-IDQA below 295 °C. Here, BP2T was used as the inducing layer to realize the WEG of C4-IDQA. Different thicknesses of C4-IDQA were deposited on 5 nm BP2T at a rate of 0.3 nm/min with the substrate temperature 4312

DOI: 10.1021/acs.jpcb.6b01579 J. Phys. Chem. B 2016, 120, 4310−4318

Article

The Journal of Physical Chemistry B

Figure 3. Film morphologies of 6 nm C4-IDQA deposited at different rates and substrate temperatures: (a) 0.1 and (b) 0.3 nm/min. (c) Out-ofplane X-ray diffractions of 6 nm C4-IDQA films corresponding to panel a. (d) Morphologies of 1 and 6 nm C4-IDQA films deposited at a rate of 0.1 nm/min at a substrate temperature of 100 °C.

(POP)] epitaxy relationship. The diffraction patterns of crystalline phase C4-IDQA were the same as that of thin film phase C4-IDQA, except that the (0−11) diffraction of C4IDQA split into two spots and the (0−21) spots appeared more obviously. The splitting of (0−11)C4‑IDQA in a thick C4-IDQA film implied that the thin film phase and crystalline phase coexisted in thick C4-IDQA films grown on BP2T. The new (0−11) spot of C4-IDQA came from the crystalline phase C4IDQA, with a d spacing of 13.06 Å, which had a large lattice mismatch with (020) BP2T, but the orientation relationship with BP2T remained unchanged. For comparison, the C4-IDQA films fabricated by spincasting on OTS-modified SiO2 and vacuum depositing directly on SiO2 substrate are shown in panels g and h of Figure 2. For the solution processing films, the film exhibited only the amorphous morphology and did not exhibit obvious crystalline domains, even after thermal annealing. For the films directly deposited at the SiO2 substrate, only spherical islands were observed. Therefore, their morphologies were totally different from those of highly crystalline films prepared by the WEG method. A high orientation, a flat surface, and good lattice matching with the inducing layer demonstrated that the WEG method is valid for the organic small molecules with short alkyls. The OTFT mobility based on 5 nm BP2T/30 nm C4IDQA achieved a hole mobility of 0.036 cm2 V−1 s−1 and an Ion/Ioff ratio of >105 (Figure S3), while for the other two fabrication methods, no obvious field effect was observed. The introduction of alkyl substituents into rigid π-electron cores brought inevitably the additional van der Waals interactions between alkyl chains, which would change the

thickness of C4-IDQA increased from 6 to 10 nm and became even thicker. They were defined as the thin film phase for C4IDQA films thinner than 6 nm and the crystalline phase for C4IDQA films thicker than 10 nm. The phase transition might be caused by the epitaxial strain from the substrate. The epitaxy relationship between C4-IDQA and BP2T was investigated by selected area electron diffraction (SAED). The diffraction patterns of the thin film phase of C4-IDQA (6 nm) and the crystalline phase of C4-IDQA (15 nm) are shown in panels e and f of Figure 2 (Figure S2), respectively. The electron diffraction patterns consisted of one [001] zone of BP2T with strong diffraction of (020) and (110).35 The C4IDQA thin film phase presented only one set of in-plane orientations in a single domain of the BP2T bilayer, (0−11) and (200) with d spacings of 11.46 and 5.68 Å, respectively. According to the epitaxy grammar36,37 and our previous work,38 there was only one orientational relationship and good lattice matching formed between the thin film phase C4-IDQA and BP2T, which was in good agreement with the morphologies. Their epitaxy relationship was as follows: (0−11)C4‑IDQA// (020)BP2T, (200)C4‑IDQA//(200)BP2T, [011]C4‑IDQA//[001]BP2T. The lattice mismatch between (0−11)C4‑IDQA and (020)BP2T was [d(0−11)C4‑IDQA − 3d(020)BP2T]/3d(020)BP2T = 0.5%, and the lattice mismatch between (200)C4‑IDQA and (200)BP2T was [d(200)C4‑IDQA − 2d(200)BP2T]/2d(200)BP2T = 0.3%. The lattice mismatch in the weak epitaxy growth is defined as (doverlayer − dinducing layer)/dinducing layer. As a result, the trans⎡ 3 0 ⎤ 36 formation matrix [C ] = ⎢ , and the thin film phase C4⎣ 0 1 ⎥⎦ IDQA and BP2T exhibited a commensurate [point to point 4313

DOI: 10.1021/acs.jpcb.6b01579 J. Phys. Chem. B 2016, 120, 4310−4318

Article

The Journal of Physical Chemistry B

Figure 4. Film morphologies and structural properties of C4-IDQA grown on bilayer p-6P. (a) Morphologies of 6 and 10 nm C4-IDQA films. (b) Out-of-plane X-ray diffraction patterns of 6 and 10 nm C4-IDQA. (c) SAED of 6 nm C4-IDQA grown on p-6P. (d) SAED of 10 nm C4-IDQA grown on p-6P. The inset is an enlarged view of (0−11)C4‑IDQA splitting. (e) Molecular structure of p-6P.

Table 1. Lattice Parameters of C4-IDQA Films on BP2T and p-6P Obtained by Experiments

BP2T p-6P a

phase

d(200)IDQA (Å)

d(100)BP2T/ d(010)p‑6P (Å)

lattice mismatch (%)

d(001)IDQA (Å)

d(0−11)IDQA (Å)

d(020)BP2T/ d(200)p‑6P (Å)

lattice mismatch (%)

da(010)IDQA (Å)

thin film crystalline thin film crystalline

5.68 5.67 5.67 5.67

5.66

0.3 0.2 1.6 1.6

13.18 12.58 13.18 12.58

11.46 13.06 11.97 13.14

3.8

0.5 14.5 0.2 10.1

15.17 12.46 17.09 12.51

5.58

3.98

d(010)IDQA is calculated by the interplanar spacing formula.

thin film phase of C4-IDQA was thermodynamically metastable at high substrate temperatures. The morphology of C4-IDQA films also was significantly influenced by the deposition rate. At the substrate temperature of 137 °C, when the deposition rate was increased from 0.1 to 0.3 nm/min, small amorphous spherical islands were observed. In addition, at lower temperatures of 130 and 120 °C, more amorphous islands that were larger were observed for the deposition rate of 0.3 nm/min (Figure 3b). These results demonstrated the C4IDQA molecules could not sufficiently diffuse at a higher deposition rate, consequently leading to the formation of amorphous islands. Therefore, the weak epitaxy growth behavior of C4-IDQA was sensitive to substrate temperature and deposition rate because of the poor diffusion ability. 3.2. Reason for Good Lattice Matching. From the previous discussion, we knew that the thin film phase of C4IDQA exhibited good lattice matching with BP2T where the lattice mismatch was