Influences of Conjugation Length on Organic Field-Effect Transistor

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Influences of Conjugation Length on OFET Performances and Thin Film Structures of DPP-Oligomers Yi-Fan Huang, Shu-Ting Chang, Kuan-Yi Wu, San-Lien Wu, GuanTing Ciou, Chin-Yi Chen, Cheng-Liang Liu, and Chien-Lung Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15983 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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ACS Applied Materials & Interfaces

Influences of Conjugation Length on OFET Performances and Thin Film Structures of DPP-Oligomers Yi-Fan Huang†, Shu-Ting Chang†, Kuan-Yi Wu†, San-Lien Wu†, , Guan-Ting Ciou†, Chin-Yi Chen†, Cheng-Liang Liu‡ and Chien-Lung Wang†*

†

Department of Applied Chemistry, National Chiao Tung University,1001 Ta Hsueh

Road, Hsinchu 30010, Taiwan

‡

Department of Chemical and Materials Engineering, National Central University,

Taoyuan, 32001, Taiwan

KEYWORDS: diketopyrrolopyrrole, conjugation length, grain boundary, crystallinity, organic semiconductor

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Abstract: Here, two DPP-based oligomers, DPP-4T and DPP-6T, are studied to reveal the influences of conjugation length on thin-film morphology and OFET performances. PDMS-assisted crystallization in a solvent-annealing chamber is applied to prepare crystal arrays of DPP-4T and DPP-6T to optimize the quality of charge channels for OFET characterizations. To deliver insight into microstructure and morphology of thin-film, a characterization procedure for determining molecular packing in thin film and crystallinity of the crystal arrays is presented via GIWAXS, ED, lattice simulation software package (Cerius2). With the lattice parameters, derived from analyses of GIWAXS and ED, the lattice modeling results indicate that the inferior OFET performances of DPP-6T are attributed to longer π-stacking distance. Also, less order molecular arrangement and lower continuity of crystalline domains, both of which are revealed from crystallinity results, lead to lower mobility of DPP-6T. In this case, longer conjugated backbones with more conformational degree of freedom thus causes inherent crystal defects during the crystal growth process despite the potential to enhance intermolecular π-orbital overlap. Therefore, to achieve better OFET performance, suitable backbone length makes conjugated oligomers give high intermolecular π-orbital overlap and low density of structural disorder, which are the priorities for constructing good


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charge channel. 1. Introduction In 1974, diketopyrrolopyrrole (DPP) was first synthesized by Novartis, a Swiss pharmaceutical company. Since later 1980s, DPP pigment has been being used as red dye in plastics and surface coating, such as paints, printing inks and so on.1 Because of broad optical absorption, strong fluorescence, and photoconduction, DPP unit has been incorporated into conjugated molecules used in organic light-emitting devices,2-3 chemosensors, two-photon absorption, dye lasers, solar cell and organic field-effect transistors (OFETs).4 Particularly, DPP-based conjugated polymers have attained impressive hole mobility (µh) of 12 cm2 V-1 s-1, electron mobility (µe) of 3 cm2 V-1 s-1, and balanced ambipolar mobility (µh / µe : 1.18 / 1.86 cm2 V-1 s-1) in OFETs.5 Because DPP-based oligomers have inherent advantages including well-defined structure, better crystallinity, and greater batch-to-batch quality than DPP-based polymers, their photovoltaic properties,6-7 and single-crystal structures8 have been widely studied. However, DPP oligomers still exhibited lower charge mobility (less than 1 cm2 V-1 s-1)9-11 than their polymer counterparts so far, partly because the intramolecular charge-transporting pathway is cut off for DPP oligomers. To outperform their polymer counterparts, knowing how to perfect the intermolecular charge-transporting pathway becomes critical. 3 ACS Paragon Plus Environment


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A good charge-transporting channel in an OFET device requires conjugated molecules to form an active layer that are low in the density of grain boundary and structural disorder.12-14 Previous studies showed that extending conjugation length of acenes serves to the increase of charge mobility, resulting from the increasing transfer integral (t) among the neighboring molecules.15 However, in the cases of oligo(thiophene) homologues,16-17 and DPP derivatives,18-20 charge mobility did not always have correlation with the length of conjugated backbone. The unpredictable charge mobility was vaguely attributed to film morphology although the exact morphological factors, which could be grain boundaries, structural disorder, crystallinity, etc., and the origin of the morphological defects remain largely unclear to the best of our knowledge. In this study, to identify the main causes of the structural defects and to reveal the influences of the morphological factors stemming from conjugation length on the charge mobility of DPP oligomers, we synthesized two DPP oligomers, DPP-4T and DPP-6T shown in Figure 1, and investigated the µhs of the oriented crystal arrays of the two DPP oligomers. Since molecular packing in thin film is often different from it is in single crystal,21 we also developed a technique that combines the grazing incidence wide-angle X-ray scattering (GIWAXS), electron diffraction (ED) and lattice modeling of the DPP crystal array to reveal actual molecular packing in the


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charge transporting channel. The roles of crystallinity and structural disorder in the mobility of the DPP oligomers were identified from the structural characterization data.

Figure 1. Molecular structures of DPP-4T and DPP-6T

2. Materials and Methods

Materials and Instrumentation: Unless otherwise noted, reagents and solvents were purchased as reagent grade and used without further purification. Tetrahydrofuran (THF) was dried over a mixture of sodium/benzophenone under nitrogen. It was freshly distilled prior to use. All reactions were carried out with standard glassware under an inert nitrogen atmosphere. Evaporation and concentration were carried out with water aspirator pressure and drying in vacuum at 10-2 Torr. Column chromatography was carried out with SiO2 60



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(230-400 mesh, 0.040-0.063 mm) which was purchased from E. Merck. Thin Layer Chromatography (TLC) was performed on aluminum sheets coated with SiO2 60 F254 purchased from E. Merck, visualization by UV light. All 1H NMR spectra which were referenced to the residual proton impurities in the CDCl3 at δ 7.26 ppm were recorded on Agilent 400-MR DD2 at 400 MHz. Field Desorption mass spectroscopy measurements were carried out on AccuTOF GCX spectrometer (JEOL). All spectra were measured in the positive reflector mode.

Cyclic Voltammetry (CV) and UV-vis absorption characterization: Cyclic voltammetry measurements were carried out using a CH Instruments Model 611D equipped with a standard three electrode configuration. (a glass carbon working electrode, a Ag/AgCl (0.01M in anhydrous acetonitrile) reference electrode, and a Pt wire counter electrode). DPP-4T and DPP-6T were cast onto the glassy carbon working electrode from THF solution and dried under nitrogen. The measurements were done in anhydrous acetonitrile with 0.1 M tetra-1-butylammonium hexafluorophosphate as the supporting electrolyte under nitrogen atmosphere at a scan rate of 20 mV s-1. CV curves were calibrated using ferrocene/ferrocenium redox couple (Fc/Fc+) as the standard, whose oxidation potential is set at −4.8 eV with respect to the zero vacuum level. The highest occupied molecular orbital energy level (EHOMO) of DPP-4T and DPP-6T were calculated from the onset oxidation potentials 6 ACS Paragon Plus Environment

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), according to equations: EHOMO = − (E − E

+ 4.8) eV. UV-vis (E

absorption

spectra

were

recorded

on

a

Hitachi

UV-400

UV-Visible

Spectrophotometers. The bandgap was calculated according to the onset absorption of UV-vis spectrum (Egopt = 1240/λonset eV). The lowest unoccupied molecular orbital energy level (ELUMO) of DPP-4T and DPP-6T were obtained from the EHOMO and the optical band gap (Egopt), according to equations: ELUMO = EHOMO + Egopt eV.

Polarizing Optical Microscope (POM): Optical microscopy (OM) images were recorded by a Leica DM2700 optical microscopy. Via the PDMS assisted crystallization (PAC) method, crystal arrays of DPP-4T or DPP-6T were deposited on PTS-treated or PETS-treated SiO2/Si substrates or glasses from their solutions (2 mg mL-1 in bromobenzene for DPP-4T; 2 mg mL-1 in 1, 2, 4-trichlorobenzene for DPP-6T) in a solvent annealing (SA) chamber at ambient conditions. The annealing vapor was THF vapor for DPP-4T, and bromobenzene vapor for DPP-6T.

Transmission Electron Microscopy (TEM): TEM observations were performed in bright-field, high-resolution mode, and ED configuration on a JEOL JEM-2010 transmission electron microscope with an accelerating voltage of 200 kV. The samples were dried under vacuum overnight before the TEM observation. 7 ACS Paragon Plus Environment


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Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) characterization: The GIWAXS experiments for DPP-4T and DPP-6T were acquired at BL17A1 beamline in the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The wavelength of the incident X-rays is 1.33 Å. The diffraction pattern was recorded with a Mar345 image plate area detector. The angle between the film surface and the incident beam was fixed at 0.2°. The GIWAXS patterns were obtained from the crystal arrays of DPP-4T and DPP-6T on Si wafer which were prepared by the same procedure for the OFET device fabrications. The details are described in following part.

OFET Device Fabrication and Characterization: A 300 nm thick silicon oxide gate dielectric with capacitance Ci=11.3 nF cm–2 was thermally grown on an n-type highly doped silicon wafer, which was used as the gate electrode and dielectric layer. First, the Si wafers were soaked in Piranha solution (H2SO4:H2O2= 7:3) for 1 hr, rinsed by deionized water and dried under house nitrogen stream, subsequently followed by UV-ozone treatment for 40 min. In this paper, (2-phenylethyl)trichlorosilane (PETS) and phenyltrichlorosilane (PTS) are used as self-assembled monolayer (SAM) to improve the charge mobility. For PETS-modified surface, the gate dielectric was treated with PETS in anhydrous toluene at 60°C to form the self-assembled monolayer under N2. As for phenyltrichlorosilane 8 ACS Paragon Plus Environment

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(PTS)-modified surface, the gate dielectric was placed into 20 mL vial with PTS solution for 3 hr. After formation of SAM, the Si wafer was cleaned by isopropanol and dried under house nitrogen stream. The crystal arrays were deposited on PETS-treated or PTS-treated SiO2 / Si substrates from solutions and annealed by solvent vapors in solvent chamber at ambient conditions. The PDMS sheet was removed after the crystal array was completely dried, and then sent into the vacuum chamber for the following vacuum deposition of the Au electrodes. Then, the Au source and drain electrodes (40 nm in thickness) were deposited by vacuum evaporation on the crystal arrays through a shadow mask, affording a bottom-gate, top-contact device configuration. Characterization of the OFET devices was carried out at room temperature under N2 atmosphere using a Keithley 4156C Semiconductor Parameter Analyzers, Agilent Technologies. The field-effect mobility was calculated in the saturation regime by using the equation IDS = (µWCi /2L)(VG – Vth)2 , where IDS is the drain–source current, µ is the field-effect mobility, W is the channel width (1000 µm), L is the channel length (100 µm), Ci is the capacitance per unit area of the gate dielectric layer, VG is the gate voltage and Vth is threshold voltage.

Crystal Density Measurement: A small amount of crystals was peeled from crystal arrays on Si substrate for density measurement. The sample was placed in a Petri dish with methanol to remove 9 ACS Paragon Plus Environment


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the air bubbles which are embedded within the sample. Then, the sample was transferred into a sample vial with water. After that, the sample sank to the bottom of the vial due to high density. Saturated sodium chloride solution was then added into the solution until the sample suspend in the middle of the solution at least 20 min to ensure the equilibrium within the solution. The density of the sample is identical to that of the solution.22

Crystal Structure Modeling: Lattice parameters deduced by GIWAXS and ED pattern were used to build the unit cell and calculate the theoretical density also. The number of molecules in a unit cell were determined based on the measured and theoretical density, which was deduced from unit cell dimensions. The basic structures and conformations of the DPP-4T and DPP-6T repeat unit were first constructed.

During the model

simulation via Cerius2 software package, the assumption of π-π molecular stacking (i.e., b-axis) along crystal growth direction was set first. Then, by tuning the variables, such as molecular orientation, conformation of branched alkyal side chain, the lattice models whose simulated diffraction pattern in best agreement with experimental one were determined. Results and discussion 2.1 Synthesis of DPP-4T and DPP-6T


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DPP-4T and DPP-6T were synthesized according to the literature.23-24 The 1H NMR,

13

C NMR and mass spectra shown in Figure S1-S3 confirm the chemical

identities of the two molecules. According to the thermogravimetric analysis (Figure S4), the 5% weight loss temperatures of DPP-4T and DPP-6T are at 383.2 oC and 391.0 oC, respectively, indicating the good thermal stability of the two molecules. 2.2 Optical and Electrical Properties

Figure 2. UV-vis absorption spectra of (a) DPP-4T and (b) DPP-6T in THF (10-6 M) solution and in thin film state. Table 1. Absorption and electrochemical data of DPP-4T and DPP-6T.

λmax (nm) Solution

λonset (nm)

Film

Egopt

EHOMO

ELUMO

(eV)

(eV)

(eV)

Solution

Film

DPP-4T 581, 621 591, 655

667

722

1.72

-5.46

-3.74

DPP-6T 610, 647 663, 728

702

798

1.55

-5.34

-3.79

Figure 2a-2b show the normalized UV–vis absorption of the DPP derivatives in



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tetrahydrofuran (THF) solution and thin film. Optical properties of DPP-4T and DPP-6T were summarized in Table 1. In solution state, the maximum of absorption wavelength (λmax) of DPP-6T is at 647 nm, which is more red shift than DPP-4T’s, because of two additional thiophenes on the conjugation backbone resulting in more resonance. Compared the spectra in thin film state to solution state, DPP-4T and DPP-6T thin films both exhibit a bathochromic shift, likely resulting from aggregations and the decreasing intermolecular distance in solid state. Molecular packing in thin film would be further discussed in the GIWAXS and ED parts with simulated molecular arrangement. The estimated optical band gap, Egopt, of DPP-4T and DPP-6T are 1.72 eV and 1.55 eV, respectively. Cyclic voltammetry (CV) measurements were conducted to calculate the energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular orbital

(ELUMO). Figure S5 shows the oxidative curve of DPP-4T and DPP-6T. The EHOMO was calculated from the onset oxidation potential (E ), according to equations: EHOMO = − (E − E

+ 4.8) eV, where E

is -0.08 V. The ELUMO

was deduced from EHOMO and Egopt, according to this equation: ELUMO = EHOMO + Egopt. As a result, the HOMO energy levels were calculated to be -5.46 and -5.34 eV for DPP-4T and DPP-6T, respectively. The enhanced HOMO energy level of DPP-6T resulted from more extending conjugation backbone, increasing the number of π


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electrons.25 The LUMO energy levels are -3.74 and -3.79 eV, respectively. 2.3 OFET device performance of DPP-4T and DPP-6T The charge transport properties of the DPP-4T and DPP-6T were investigated in OFET devices with a bottom-gate, top-contact configuration. The active layers were prepared by the PDMS-assisted crystallization (PAC) method.26 As shown in Figure 3, solvent annealing (SA) chamber was applied in the PAC procedure to improve the quality of crystal array.27 In Figure 4a and 4b, the crystal arrays prepared only with the PAC method contain cracks and are not well-oriented. On the contrary, the crystal arrays grown in the SA chamber are continuous and well-aligned, as can be clearly seen in Figure 4c and 4d.

Figure 3. The illustration of PAC method with solvent annealing (SA). Sample solution is first injected into the gap between the either PETS-treated or PTS-treated Si substrate and PDMS slab. Then, the whole complex is put into solution annealing



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chamber. The DPP-4T crystal array was prepared from bromobenzene solution of DPP-4T (concentration: 2 mg mL-1) and annealed by THF vapors. The DPP-6T crystal array was prepared from 1, 2, 4-trichlorobenzene solution of DPP-6T (concentration: 2 mg mL-1) and annealed by bromobenzene vapors. Both crystal arrays are prepared under ambient temperature.

Figure 4. The bright-ﬁeld optical microscopy images of (a) DPP-4T and (b) DPP-6T crystal arrays prepared by the PAC method. (c) DPP-4T and (d) DPP-6T crystal arrays prepared by the PAC method in a solvent annealing (SA) chamber. In Figure 5, the output and transfer plots of the DPP-4T crystal arrays and the DPP-6T crystal arrays exhibited typical p-channel OFET characteristics in the bottom-gate, top-contact OFET devices. When the given applied gate voltage (VG ) is lower than -60 V, DPP-4T gives lower current from drain to source, IDS, than DPP-6T. However, with higher VG than -60 V, the value of IDS delivered by DPP-4T rise and


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surpass that of DPP-6T, indicating that even though the charge transporting channel of DPP-4T is harder to turn on, however, the charge transporting channel of DPP-4T afforded higher current from source to drain. The µh of DPP-4T and DPP-6T were obtained from the transfer characteristics of the devices in saturation regimes and summarized in Table 2, also including On/Off current ratio and threshold voltage. Without the use of SA chamber, the crystal arrays of DPP-4T and DPP-6T deliver µh of 3.6×10-2 cm2 V-1 s-1 and 2.9×10-3 cm2 V-1 s-1, respectively, due to poor quality of crystal array, as shown in Figure 4a and 4b. Crystal arrays prepared in the SA chamber deliver higher µhs, which are 5.3×10-2 cm2 V-1 s-1 for DPP-4T and 3.7×10-2 cm2 V-1 s-1 for DPP-6T, because of higher channel coverage for DPP-4T, as shown in Figure 4b. Thus, the SA process indeed improved the quality of the charge transport channel via improving the in-plane orientation and decreasing the density of defects, which leads to higher mobility of DPP-4T and DPP-6T OFET device. Furthermore, to improve the quality of charge transporting channel,28-29 by replacing the self-assembled monolayer (SAM) layer from phenyltrichlorosilane (PTS) to (2-phenylethyl)trichlorosilane (PETS), µh of DPP-4T and DPP-6T further increase to 0.18 cm2 V-1 s-1 and 4.4×10-2 cm2 V-1 s-1, respectively. Besides, derived from transfer curve of DPP-4T and DPP-6T, the OFET devices of DPP-6T exhibits lower threshold voltage, Vth, than their counterparts, which are DPP-4T’s devices, in all



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device treatment. Threshold voltage originates from trapped charges, which could be detrapped by applying voltage bias on gate electrode. Lower Vth represents the charges are easier to be detrapped from localized state, which may results from donors or acceptors. So, in Table 2, the result of Vth suggested that it is easier to conduct current for DPP-6T because high-lying EHOMO of DPP-6T, which means lower ionized energy and more compatible with the work function of Au, would facilitate the trapped charges to be driven and more efficient hole injection from source electrode into semiconducting layer.30

Figure 5. The output (left) and transfer (VDS: -100 V) (right) characteristics of the (a)


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DPP-4T and (b) DPP-6T OFET devices prepared by PAC with SA process. PETS was used as the SAM layer.

Table 2. OFET characteristics of the DPP-4T and DPP-6T crystal arrays.

DPP-4T

DPP-6T

Device treatment

SAMa

µ2 max. -1 -1 (cm V s )

PAC+SA

PETS

1.8×10

PAC+SA

PTS

5.3×10

PAC

PTS

3.6×10

PAC+SA

PETS

4.4×10

PAC+SA

PTS

3.7×10

PAC

PTS

2.9×10

-1

-2

-2

-2

-2

-3

*

µ2ave.-1 -1 (cm V s )

Ion/Ioff

-1

-2

-2

-3

-2

-3

-2

-3

-2

-3

-3

-4

1.0×10 ± 3.0×10 3.8×10 ± 9.5×10 2.9×10 ± 8.3×10 3.4×10 ± 9.1×10 3.1×10 ± 5.0×10 3.0×10 ± 4.8×10

a

SAM shows the modified layer on SiO2/Si substrate, either PETS or PTS.

b

Each µave is obtained from 12 devices.

5.6×10 1.3×10 1.0×10 6.3×10 4.8×10 4.0×10

5

5

5

4

3

2

Vth(V) -36.9 -18.6 -37.5 -0.93 -2.43 -2.64

2.4 Morphology of the DPP-4T and DPP-6T crystal arrays



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Figure 6. (a, b) GIWAXS patterns and (c) ED pattern of the DPP-4T crystal array. (d, e) GIWAXS patterns and (f) ED pattern of the DPP-6T crystal array. In (a) and (d), the incident X-ray is perpendicular to crystal growth direction, while it is nearly parallel to crystal growth direction in (b) and (e). Since GIWAXS is common to reveal the structural information in nano-scale,31 the molecular packings of DPP-4T and DPP-6T in the crystal arrays were studied with two GIWAXS patterns and one electron diffraction (ED) pattern (Figure 6). As illustrated in Figure S6, to reveal the 3D reciprocal lattice, the GIWAXS patterns were taken from both perpendicular and parallel to the crystal growth direction, whereas the ED pattern was taken with the incident electron beam penetrate through the crystal array. Comparing the diffraction patterns of DPP-4T (Figure 6a-6c) with


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those of DPP-6T (Figure 6f-6h), the crystal array of DPP-4T gives sharp and concentrated diffraction spots whereas the crystal array of DPP-6T produces blurred and unclear spots. In the Scherrer equation, the mean size of an ordered domain is inversely proportional to the broadness of a diffraction signal; thus, the blurred diffraction signals of DPP-6T suggest that DPP-6T formed smaller and less order crystalline domains in the crystal array. To construct the lattice models of the two DPP molecules, we first assumed that the out-of-plane direction and the crystal growth direction of a crystal array are close to a and b axes of the crystal lattices of DPP-4T and DPP-6T. In this way, the two GIWAXS patterns reveal the a*b* and a*c* reciprocal lattices while the ED pattern shows the b*c* reciprocal lattice of the observed crystal array, as illustrated in Figure S6. Lattice parameters (a, b, c, α, β, and γ) can then be deduced from the GIWAXS and ED patterns in Figure 6 by using the following relationships that (1) =

, and (2) a lattice angle (α, β, and γ) is

supplementary to the corresponding angle in the reciprocal lattice (α*, β*, and γ*). Based on this procedure, the lattice parameters of DPP-4T were calculated as a = 19.95 Å, b = 6.79 Å, c = 10.51 Å, α = 68 o, β = 106o, and γ = 90o. For DPP-6T, a = 13.82 Å, b = 5.44 Å, c = 19.14 Å, α = 85o, β = 90o, γ = 90o. With these parameters, the diffractions observed in Figure 6a-6c for DPP-4T and in Figure 6f-6g for DPP-6T can be indexed, and dhkls of the diffractions matches well with the calculated ones as



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shown in Table S1. After knowing the lattice parameters, DPP-4T and DPP-6T molecules were placed into their own lattices to build the packing models with Cerius2 software package. Using a trial-and-error method, the molecule was rotated in the lattice until the lattice shown in Figure 7a and 7b generated simulated ED pattern (Figure 7c and 7d) that matched with experimental ones (Figure 6c and 6f). Since the simulated ED patterns are in good agreement with the experimental ones, the validity of the packing model of DPP-4T (Figure 7a) and DPP-6T (Figure 7b) were confirmed. The lattice orientation of DPP-4T and DPP-6T on the Si substrate can then be revealed by superimposing the lattice models shown in Figure 7 with the experimental setup shown in Figure S6. It can be found that the bc lattice plane of the DPP molecules contact with the surface of the Si substrate, and the b axes of the DPP-4T and DPP-6T lattices are both along the growth direction of the crystal array. From the top panel of Figure 7a and the bottom panel of Figure 7b, the tentative packing models show that both molecules have their π-stacking points toward the growth direction (i.e. the charge transporting direction), which explained why both crystal arrays delivered reasonable charge mobility. The π-stacking distance of DPP-4T (3.2 Å) is shorter than that of DPP-6T (3.8 Å). Although both molecules form π-stack along the b-axis, their backbone orientations on the substrate are


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different.

As shown in the top panel of Figure 7a, DPP-4T has its backbone points

away from the bc plane (i.e. the substrate surface) whereas in Figure 7b, DPP-6T places its backbone within the bc plane. The results thus suggest that the length of the DPP oligomers also affect the preferable orientation of the molecules on the substrate. The shorter analogue, DPP-4T, takes a tilted end-on orientation, whereas the longer analogue, DPP-6T, takes edge-on orientation on the Si substrate. To understand the differences in the sharpness of the diffractions from the crystal arrays of DPP-4T and DPP-6T, we further calculated the crystallinity of the crystal arrays. In Table 3, crystallinity of the crystal array can be deduced by comparing the theoretical densities of the DPP-4T lattice or the DPP-6T lattice with the actual densities of the DPP-4T or DPP-6T crystal pieces that were peeled from the crystal arrays on the Si substrate. The crystallinity of the DPP-4T crystal array (98.24%) is higher than that of the DPP-6T crystal array (94.59%). The results represent that there are more amorphous domains, which inhibit charge transportation, distributed in the DPP-6T crystal array. The crystallinity result agrees well with the GIWAXS and ED results, in which the less crystalline crystal array of DPP-6T exhibited blurry and unclear spots. From the above analyses, the lower µh of DPP-6T therefore can be attributed to the longer π-stacking distance, less ordered molecular arrangement and lower crystallinity,



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as illustrated in Figure 8. In this case, even though the elongated conjugated backbone offers the potential of higher intermolecular π-orbital overlap, it also gives higher possibility of producing inherent crystal defects in the crystal array during the crystal growth process because of the increasing conformational degree of freedom.24, 32

A

shorter

homologue,

2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dion e, , abbreviated as DPP-2T (Figure S7a), was used to further verify the influence of the conjugation length on the structural order of the DPP crystal arrays. The crystal array of DPP-2T shown in Figure S7b generates sharp ED spots in Figure S7c, confirming that shortening the conjugation length does not broaden the diffraction signals. Thus, since the broadened diffraction spots are only observed in the crystal array of DPP-6T, the concept of extended conjugation length leading to less order crystalline domains in the crystal array is further confirmed. Nevertheless, the OFET characteristic data in Figure S8 and Table S3 show that crystal arrays of DPP-2T -3

2

-1

-1

deliver low µmax of 5.0×10 cm V s , probably because the shorter conjugation length of DPP-2T results in the relatively lower degree of intermolecular π-orbital overlap. Hence, for conjugated oligomers used in solution-processed OFETs, prudent choice of suitable backbone length is crucial to fulfill the demands of both high intermolecular π-orbital overlap and low density of structural disorder in the crystal


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array.

Figure 7. The simulated lattice model in the ab and the bc lattice projections of (a) DPP-4T and (b) DPP-6T. (c) and (d) show the simulated ED patterns generated from the [100] zone of the DPP-4T and DPP-6T lattice models. Table 3. Crystal density and crystallinity of the DPP-4T and DPP-6T crystal array. Theoretical density -3

Density of the

Crystallinity -3

( g cm )

crystal array (g cm )

(%)

DPP-4T

1.128

1.108

98.24

DPP-6T

1.183

1.119

94.59



Page 24 of 33

Figure 8. Schematic illustration of the presence of amorphous regions in the DPP-4T and DPP-6T crystal arrays.

The lower crystallinity of DPP-6T indicates a higher

density of amorphous regions along the charge transport pathway. 3. Conclusion

In this study, two homologous DPP oligomers, DPP-4T and DPP-6T, which are different in their conjugation length, were studied to reveal the influences of conjugation length on thin-film morphology and OFET performances. In order to clearly identify the key factors affecting the charge mobility of the DPP oligomers, details in thin-film morphology should be revealed. Crystal arrays of DPP-4T and DPP-6T were first prepared via PDMS-assisted crystallization assisted in a solvent-annealing chamber, so that high quality charge transporting channels can be built for OFET characterizations. Furthermore, a characterization procedure based on GIWAXS, ED and lattice modeling software (Cerius2) were established to reveal the


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actual molecular packing and crystallinity of the crystal arrays. GIWAXS and ED analyses of the crystal arrays show that DPP-4T packs into a lattice with lattice parameters, a = 19.95 Å, b = 6.79 Å, c = 10.51 Å, α = 68o, β = 106o, γ = 90o, while DPP-6T packs into a lattice with lattice parameters, a = 13.82 Å, b = 5.44 Å, c = 19.14 Å, α = 85o, β = 90o, γ = 90o. The lattice modeling results show that in the lattices, the π-stacking distance is 3.2 Å for DPP-4T, and 3.8 Å for DPP-6T. The conjugation length thus affects the lattice geometry and π-stacking distance of the DPP oligomers. Crystallinity results further indicate that although both DPP-4T and DPP-6T form well-oriented crystal arrays, DPP-6T contains more amorphous domains in its crystal array, which decrease the averaged correlation length of the crystalline domain, and breaks the continuity of charge transporting channel between the source and drain electrodes. With these detail morphological information, the inferior OFET performances of the DPP-6T crystal array can be attributed to the longer π-stacking distance, less order molecular arrangement and lower continuity of the crystalline domain owing to more amorphous regime disrupting the charge transportation. In this case, longer conjugated backbones with more conformational degree of freedom thus causes inherent crystal defects during the crystal growth process even though giving the potential to enhance intermolecular π-orbital overlap. Therefore, choosing suitable backbone length of conjugated oligomers is crucial to



Page 26 of 33

achieve high intermolecular π-orbital overlap and low density of structural disorder in the crystal array for forming good charge-transporting channel in an OFET device. ASSOCIATED CONTENT

Supporting Information 1

H NMR spectra, mass spectra, TGA, index of XRD and reduction and oxidation

cyclic-voltammetric curves of DPP-4T and DPP-6T are available in supporting information. Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *Prof. Chien-Lung Wang, E-mail: [email protected] ACKNOWLEDGMENT

The authors acknowledge funding support from Ministry of Science and Technology,

Taiwan

(MOST

103-2221-E-009-213-MY3,

MOST

104-2628-E-009-007-MY3) and “ATP” of the National Chiao Tung University and Ministry of Education, Taiwan. The authors thank National Synchrotron Radiation Research Center (NSRRC, Taiwan) for assistance with the GIWAXS measurements. REFERENCES 1.

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Table of Contents


Influences of Conjugation Length on Organic Field-Effect Transistor

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