Roles of 3-Ethylrhodanine in Attaining Highly ... - ACS Publications

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Roles of 3‑Ethylrhodanine in Attaining Highly Ordered Crystal Arrays of Ambipolar Diketopyrrolopyrrole Oligomers San-Lien Wu, Yi-Fan Huang, Chou-Ting Hsieh, Bo-Han Lai, Po-Sen Tseng, Jou-Tsen Ou, Ssu-Ting Liao, Szu-Yu Chou, Kuan-Yi Wu, and Chien-Lung Wang* Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan S Supporting Information *

ABSTRACT: Until now, only limited DPP oligomers delivered ambipolar semiconductor characteristics. To develop a facile strategy of preparing ambipolar mono-DPP oligomers, two dithienyl diketopyrrolopyrrole (DPPT) based-conjugated molecules, DPPT-RD and DPPTDCV, which contain 3-ethylrhodanine (RD) and dicyano-2-vinyl (DCV) end substituents were synthesized. The influences of the -RD end substituents on the molecular properties, solid-state morphology, and OFET performances of the DPPT oligomer were investigated. The UV− vis absorption and CV results showed that the RD end substituents provide the DPPT oligomer suitable EHOMO and ELUMO for hole and electron injection from the Au source-drain electrodes. Moreover, the RD end substituents also improve the crystalline nature of the DPPT oligomer. That is, DPPT-RD can form crystal arrays with good lattice orientation, larger crystalline size, and without polymorphism. With those properties, DPPT-RD thus display ambipolar characteristic with μh and μe reaching 2.16 × 10−2 and 7.27 × 10−2 cm2 V−1 s−1, respectively. KEYWORDS: diketopyrrolopyrrole, ambipolar, dye, organic semiconductor, solid-state morphology

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INTRODUCTION Solution-processed organic field-effect transistors (OFETs) have attracted great attention because of their potential applications as low-cost components in large-area flexible electronics.1−8 The OFET performances of solution-processed conjugated molecules has undergone remarkable progress recently as a result of combined efforts in the optimization of molecular structure,9−16 solid-state morphology,17 and processing techniques.18 Among all the conjugated building blocks, diketopyrrolopyrrole (DPP) has been revealed as a useful building block in producing conjugated polymers that deliver high charge mobility. The planar structure and the cross-axis dipoles of DPP promote the intermolecular interaction of DPPbased conjugated polymers, which produce a close π−stacking and lead to efficient charge transport.19,20 Until now, hole mobility (μh) up to 11.0 cm2 V−1 s−1,10 electron mobility (μe) up to 7.0 cm2 V−1 s−1,13 and ambipolar characteristic with μh and μe of 8.84 and 4.34 cm2 V−1 s−1, respectively,15 have been reported for OFET devices of different DPP polymers. Compared to conjugated polymers, conjugated oligomers can be functionalized more easily and have better crystallinity and batch-to-batch stability.21−26 By attaching different flanking groups to a DPP unit, FET characteristics of DPP oligomers can be effectively modified. For instance, dithienyl diketopyrropopyrrole (DPPT) oligomers with hydrogen,27 bithiophene,28 cyano,29 phenyl,30 and benzofuranyl31 end substituents delivered p-type OFET characteristics. The highest μh of 0.7 cm2 V−1 s−1 were given by the vacuum deposited thin film of © XXXX American Chemical Society

5,5′-(2,5-bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(thiophene-2-carbonitrile).29 However, DPPT oligomers with dicyano-2-vinyl,32 dicyanomethylene,33 naphthalimide, and phthalimide34 end substituents delivered n-type OFET characteristics. The highest μe of 0.96 cm2 V−1 s−1 were delivered by the single crystal of 2,2′-((5,5′(2,5-bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4c]pyrrole-1,4-diyl)bis(thiophene-5,2-diyl))bis(methanylylidene)) dimalononitrile (DPPT-DCV).32 In addition to unipolar characteristic, bis-DPP compounds, which contain two DPP units in a conjugated backbone gave balanced ambipolar carrier mobilities up to 10−2 cm2 V−1 s−1.35−37 Besides the bis-DPP oligomers, by using (E)-2-(benzo[b]thiophen-2-yl)-1,2-difluorovinyl as the end substituents, Cai et al. showed that a mono-DPP oligomer can also deliver ambipolar characteristic with μh of 0.42 cm2 V−1 s−1 and μe of 0.80 cm2 V−1 s−1.38 Therefore, the fewer synthetic steps and the better performances make the mono-DPP oligomer attractive in the OFET application. As a result, searching for suitable end substituents becomes an important step to endow DPP oligomers with ambipolar characteristics. In order to have both efficient hole- and electron- injection in an ambipolar OFET, conjugated molecules should have narrow band gap (Eg) (below 1.8 eV), and suitable highest occupied Received: February 5, 2017 Accepted: April 11, 2017 Published: April 11, 2017 A

DOI: 10.1021/acsami.7b01702 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Synthetic Routes of DPPT-DCV and DPPT-RDa

(i) LDA, N-formylpiperidine, THF, 0 °C to r.t., under N2, 16 h, yield: 68%; (ii) malononitrile, triethylamine, chloroform, r.t., under N2, 3 h, yield: 58% or 3-ethylrhodanine, triethylamine, chloroform, reflux, under N2, 3 h, yield: 94%.

a

Figure 1. UV−vis absorption spectra of CHCl3 (10−6 M) solution and thin film of (a) DPPT-RD and (b) DPPT-DCV. (c) The HOMO/LUMO energy levels of DPPT-DCV and DPPT-RD.

because of the less ordered thin film morphology and deeperlying EHOMO. The study shows that -RD is a useful end substituents to introduce ambipolar characteristic for monoDPP oligomers, and provides a facile protocol of producing solution-processable ambipolar DPP oligomers.

molecular orbital energy (EHOMO) and lowest unoccupied molecular orbital energy (ELUMO).39 Although end groups, such as dicyano-2-vinyl and malononitrile, can narrow the Eg of DPP oligomers significantly, the EHOMOs of the DPP oligomers are too low to give p-type characteristics in OFETs with Au electrodes. 3-ethylrhodanine (RD) is a useful end substituent for conjugated oligomers used in organic photovoltaics.40−45 It can effectively reduce the Eg of oligothiophenes without significantly changing the EHOMO and crystalline nature of the oligomers.46−48 Intriguingly, the influences of the RD end substituent on the DPP unit have not been studied. Therefore, in this study, we synthesized DPPT-RD, which contains a DPPT unit and two RD end substituents (Scheme 1), and evaluated the effects of the RD end substituents on the molecular properties, solid-state morphology, and OFET performances of the DPP oligomer. The benchmark of n-type DPP oligomer, DPPT-DCV,32 was chosen as the reference to study the different influences of the -DCV and -RD end substituents. Through the PDMS-assisted crystallization (PAC) method,3 both DPPT-RD and DPPT-DCV were grown into oriented crystal arrays in order to reduce the impact of crystal morphology on the OFET characteristics. The experimental results show that DPPT-RD has suitable EHOMO and ELUMO for both hole and electron injection, and form crystal array with better lattice orientation and no polymorphism. As a result, the crystal array of DPPT-RD delivered ambipolar characteristic μh of 2.16 × 10−2 cm2 V−1 s−1 and μe of 7.27 × 10−2 cm2 V−1 s−1. On the contrary, the crystal array of DPPT-DCV gives only a unipolar characteristic with μe of 1.84 × 10−2 cm2 V−1 s−1,

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RESULTS AND DISCUSSION DPPT-DCV and DPPT-RD were synthesized according to Scheme 1. 2,5-Bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (1) and 5,5′-(2,5-bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4diyl)bis(thiophene-2-carbaldehyde) (2) were prepared according to literature procedures. 49−51 The formylation of compound 1 with lithium diisopropylamide (LDA) and Nformylpiperidine gave 2 in 68% yield. In Figure S1 of the Supporting Information (SI), the observation of the proton signal of the aldehyde group at 10.03 ppm, and the 1:1:1 ratio for the integrations of the proton signals on the aldehyde and the thiophene groups (9.03, 7.87 ppm) confirm the success of the formylation reaction. Reacting compound 2 with malononitrile or 3-ethylrhodanine via Knoevenagel condensation52 resulting in DPPT-DCV or DPPT-RD, respectively. In Figure S2a,b, the absence of the signal for the aldehyde proton at 10.03 ppm, and the observation of the proton signal of the -DCV or -RD groups at 7.89 or 7.91 ppm, indicated that the aldehyde groups of 2 have been converted into the -DCV, or the -RD groups of two final products. The m/z value of DPPTRD measured from the field-desorption mass spectroscopy (FDMS) (Figures S3 and S4) also match the theoretical one. B


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ACS Applied Materials & Interfaces These results thus confirm the successful formation of the final products. The UV/vis spectra of DPPT-RD and DPPT-DCV are shown in Figure 1. DPPT-RD and DPPT-DCV exhibit similar λonsets and optical band gaps (Eg) as summarized in Table 1. Table 1. Absorption and Electrochemical Data of DPPT-RD and DPPT-DCV. λonset(nm)

Eg

EHOMO

Figure 3. POM images of (a) DPPT-RD and (b) DPPT-DCV crystal arrays on Si wafers grown by the PAC method. The DPPT-RD crystal array was prepared on preheated Si wafers (T = 100 °C) from TCB solution of DPPT-RD (concentration: 1 mg mL−1); the DPPT-DCV crystal array was prepared on preheated Si substrate (T = 40 °C) from ODCB solution of DPPT-DCV (concentration: 2 mg mL−1).

ELUMO

compd.

solution

film

(eV)

(eV)

(eV)

DPPT-RD DPPT-DCV

748 742

833 833

1.49a 1.48c

−5.57b −5.93c

−4.08b −4.45c

Deduced from the absorption onset of the thin films. bEHOMO = c − Eonset −(Eonset ox ferrocene + 4.8) eV, and ELUMO = (EHOMO + Eg) eV. From ref 32

a

crystalline size could only be obtained from the 1,2,4trichlorobenzene (TCB) solution of DPPT-RD and the odichlorobenzene (ODCB) solution of DPPT-DCV instead of low Tb solvents including dichloromethane (DCM), chloroform, and tetrahydrofuran (THF), which dried so fast that the crystal growth could not be well controlled. Since the PAC method aligns the in-plane lattice orientation in the thin film,6 it provides a better ground to compare the OFET characteristics of DPPT-RD and DPPT-DCV, and allows the grazing incidence X-ray diffraction (GIXD) and electron diffraction (ED) to provide more detailed morphological information about the quality of the charge transport channel (vide infra). In the Figure 4 and Figure S7, the output and transfer plots of the devices revealed typical n-type OFET characteristics for DPPT-DCV, but ambipolar OFET characteristics for DPPTRD. The μ of the crystal arrays were obtained from the transfer characteristics of the devices in saturation regimes and summarized in Table 2. The as-prepared DPPT-DCV crystal array showed μe of 1.30 × 10−2 cm2 V−1 s−1. Then thermal annealing at 85 °C for 20 min promotes the μe value of the DPPT-DCV crystal array to 1.84 × 10−2 cm2 V−1 s−1. The μh and μe of the DPPT-RD crystal array were observed to be 2.16 × 10−2 and 7.27 × 10−2 cm2 V−1 s−1, respectively. The origins of the different OFET characteristics of DPPTRD and DPPT-DCV were first investigated from the density functional theory (DFT) calculation (Figure 5). The DFT results show that the minimum-energy configurations of both molecules are quite planar, and the HOMO and LUMO of DPPT-DCV and DPPT-RD are delocalized on the whole conjugated backbone of the both molecules. However, compared to the -RD substituent of DPPT-RD, the -DCV substituent resulted in the lower EHOMO and ELUMO of DPPTDCV. The DFT results match the observations in the summary

The spectra of the thin films exhibit an additional absorption band at ca. 800 nm, likely arising from the aggregations in solid states. The cyclic voltammetry (CV) result of DPPT-RD is shown in Figure S5. The energy level of EHOMO and ELUMO of DPPT-RD are −5.57 eV and −4.08 eV, respectively. From the summary of UV−vis absorption and CV results, Figure 1c reveals that DPPT-RD and DPPT-DCV have similar Eg, whereas DPPT-RD has higher-lying HOMO and LUMO energy levels. As for the work function of Au electrode (−5.1 eV), it is closer to the ELUMO of DPPT-DCV, but in the middle of the ELUMO and EHOMO of DPPT-RD. The charge transport properties of the DPPT-RD and DPPT-DCV were investigated with a bottom-gate, top-contact OFET devices. In addition to the EHOMO and ELUMO, molecular packing53 and solid-state morphology54 in thin film are essential for the ambipolar characteristics the DPP oligomers. To clearly identify the morphological influences caused by the end substituents (-DCV and -RD), we first used PAC method (illustrated in Figure 2)3 to prepare the crystal arrays of DPPTRD and DPPT-DCV. The PDMS slab acts as a solvent sponge to absorb the solvents of the DPPT-RD and DPPT-DCV solutions. Supersaturation and nucleation of the DPPT-RD and DPPT-DCV solutions thus occurred near the PDMS/Si substrate contact line, and the following crystal growth resulted in the well-aligned crystal arrays of DPPT-RD and DPPT-DCV as shown in Figure 3. Notably, as shown in Figure S6, owing to the highly crystalline nature of DPPT-RD and DPPT-DCV, random nucleation also occurred during the crystal growth. Consequently, the well-aligned crystal arrays with large

Figure 2. Schematic illustration of PDMS-assisted crystallization method (PAC). C


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Figure 4. Representative output (up) and transfer (down) characteristics of the DPPT-RD OFET devices prepared by PAC technique for (a) pchannel and (b) n-channel operation. Octadecyltrichlorosilane (ODTS) was used as the SAM layer.

Table 2. OFET Device Characteristics of the DPPT-RD and DPPT-DCV Crystal Arraysa compd. DPPT-RD DPPT-DCV

solvent TCB ODCB ODCB

μhb (cm2V−1s−1) −2

2.16 × 10

Ion/Ioff −3

[(9.05 ± 7.59) × 10 ]

∼10

2

Vth (V) −38.4 ± 11.5

μeb (cm2V−1s−1) −2

7.27 × 10 [(4.04 ± 1.88) × 10−2] 1.84 × 10−2 [(1.04 ± 0.36) × 10−2 ]c 1.30 × 10−2 [(1.14 ± 0.23) × 10−2]

Ion/Ioff

Vth (V)

∼102 ∼103 ∼104

27.4 ± 10.6 17.1 ± 4.7 19.8 ± 8.5

ODTS was used as the SAM layer. bμh and μe were provided in “highest [average]” form, and the performance data were obtained based on more than 8 OFET devices. cAfter thermal annealing at 85 °C for 20 min.

a

To study the morphological influences of the end substituentse, ED patterns (Figure 6) and GIXD patterns (Figure 7) of the DPPT-RD and DPPT-DCV crystal arrays were collected. As illustrated in Figure 6a, in the ED measurement, the electron beam penetrates the crystal array from the top. Considering the long axis of the crystal as the b* axis and the direction close to the short axis of the crystal as c* axis, Figure 6b thus indicates that DPPT-RD packs into a twodimensional (2D) lattice with b = 5.03 Å, c = 13.42 Å, and α = 93°. In Figure 6c, it can be found that DPPT-DCV packs into a 2D lattice with b = 7.60 Å, c = 7.77 Å, and α = 105°. The parallel GIXD patterns in Figure 7 were obtained when the incident X-ray is along the b axis (long axis) of the DPPT-RD and DPPT-DCV crystal arrays. By comparing the two GIXD patterns, it can be found that, in Figure 7b, the crystal array of DPPT-RD gives sharp and concentrated diffraction spots, whereas DPPT-DCV (Figure 7c) produces broad diffraction arcs along the azimuthal angle. The difference in the GIXD patterns indicates that DPPT-RD forms crystal arrays with better lattice orientation and larger averaged crystalline domains than DPPT-DCV does.6 Moreover, because the incident X-ray is along the b-axis of the two crystal arrays, the GIXD patterns reveal the 2D a*c* reciprocal lattices of DPPTRD and DPPT-DCV. Using the relationships that (1) dhkl = 2π/qhkl and (2) the angles of lattice and reciprocal lattice are supplementary angles, the lattice parameters of DPPT-RD were calculated as a = 30.44 Å, c = 13.42 Å, and β = 90°.

Figure 5. DFT-calculated (B3LYP/6-311G) molecular orbitals of DPPT-RD and DPPT-DCV.

of UV−vis absorption and CV measurement (Figure 1c). The lack of p-type OFET characteristics of DPPT-DCV can thus be attributed to the deeper-lying EHOMO of DPPT-DCV, which causes higher energy barrier for the hole-injection from Au electrode. On the contrary, the -RD substituents result in suitable EHOMO and ELUMO for DPPT-RD that render DPPTRD an ambipolar semiconducting characteristic. D


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Figure 6. (a) The schematic illustration of the ED experiment. The ED patterns of (b) DPPT-RD and (c) DPPT-DCV crystal arrays prepared by PAC.

Figure 7. (a) Schematic illustration of the parallel GIXD setup. The GIXD patterns of the crystal arrays of (b) DPPT-RD and (c) DPPT-DCV. The incident X-ray is parallel to the crystal growth direction. Two reciprocal lattice of DPPT-DCV were found in the parallel GIXD pattern of DPPTDCV. They are marked with white and yellow dash boxes in (c).

DPPT-DCV shows two endothermic signals at 84 °C and at 239 °C in the first heating scan, but no exothermic signal in the subsequent cooling scan. In the second heating−cooling cycle (Figure S10d), the exothermic signal at 84 °C is no longer observed, and the exothermic signal at 239 °C in the first scan shifts to a lower temperature (234 °C), and the transition enthalpy of the signal decreases from 40.52 kJ mol−1 to 29.78 kJ mol−1. Since the first heating−cooling cycle shows the phase behavior of the as-precipitate DPPT-DCV sample, and the second cycle shows that of the melt DPPT-DCV sample, the DSC thermograms of DPPT-DCV provide the following information. First, the two endothermic signal in the first heating indicate that DPPT-DCV formed two kinds of ordered phases, a low-Tm phase and a high-Tm phase, when it was precipitated from solution. Second, the absence of the exothermic signal in the first cooling indicates that DPPTDCV has low crystallization rate. Finally, the absence of the endothermic peak of the low-Tm phase suggests that the low-Tm phase is a metatstable phase, which forms only from the DPPTDCV solution, but not from its melt. Thus, by revealing the phase behaviors of DPPT-DCV, DSC results help to explain why the less orderly arranged lattices and polymorphism were observed in the DPPT-DCVcrystal array. As far as the production of efficient charge transport channel in OFET devices is concerned, DPPT-RD is more advantangeous than DPPT-DCV, because it has stronger crystalline nature, and forms crystal arrays with unified lattice structure. The polymorphism in the crystal array of DPPT-DCV can hamper the charge transport, because it creates grain boundaries among the crystalline grains of different lattice structures. The better OFET performances of DPPT-RD can thus be attributed to the fact that it forms crystal arrays with more oriented crystalline domains and no polymorphism.

Noteworthy, in Figure 7c, two reciprocal lattices were observed from the crystal array of DPPT-DCV. The lattice parameters deduced from the reciprocal lattice marked with white box in Figure 7c are a = 12.13 Å, c = 7.77 Å, and β = 90°, whereas those calculated from the reciprocal lattice marked with yellow box are a = 12.18 Å, c = 7.07 Å, and β = 95°. The two reciprocal lattices indicate that DPPT-DCV has polymorphism, which indicates that two types of crystal lattices coexist in the charge transport channel of DPPT-DCV OFETs. Next, to know the molecular packing of DPPT-RD and DPPT-DCV, the lattice models were built in the Cerius2 software package by using the lattice parameter obtained from the ED and GIXD results. Placing the edge of DPPT-RD and DPPT-DCV on the bc plane and tilting the long axis of the molecules away from the growth axis (b axis) for ca. 45° result in the tentative lattice models of DPPT-RD and DPPT-DCV shown in Figure S8. To confirm the validity of the lattice models, simulated ED patterns (Figure S8) were generated from the models, and those resemble the experimental ones in Figure 6. Furthermore, the bc lattice plane of DPPT-RD and DPPT-DCV are parallel to the surface of the Si substrate. Therefore, from the model, it can be found that DPPT-RD and DPPT-DCV take the edge-on orientation on the Si substrate, and have the π-stacking direction point along the growth direction. Moreover, the alternating 2D layers of the flexible -EH (2-ethylhexyl) chains and the rigid conjugated backbones form the lamellar structure along the out-of-plane direction, as illustrated in Figure S9. The edge-on stacks of DPPT-RD and DPPT-DCV in the crystal arrays thus provide the charge transport channels to deliver the OFET characteristics, which were discussed above. Differential scanning calorimetry (DSC) thermograms (Figure S10) were used to understand the different phase behaviors of DPPT-RD and DPPT-DCV. In Figure S10ab, DPPT-RD exhibits a typical enantiotropic phase behavior with a melting temperature (Tm) at 309 °C and a crystallization temperature (Tc) at 269 °C. On the contrary, in Figure S10c, E


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(4) Fan, C.; Zoombelt, A. P.; Jiang, H.; Fu, W.; Wu, J.; Yuan, W.; Wang, Y.; Li, H.; Chen, H.; Bao, Z. Solution-Grown Organic SingleCrystalline p-n Junctions with Ambipolar Charge Transport. Adv. Mater. 2013, 25, 5762−5766. (5) Huang, J.; Fan, R.; Connor, S.; Yang, P. One-Step Patterning of Aligned Nanowire Arrays by Programmed Dip Coating. Angew. Chem. 2007, 119, 2466−2469. (6) Wu, K.-Y.; Hsieh, C.-T.; Wang, L.-H.; Hsu, C.-H.; Chang, S.-T.; Lan, S.-T.; Huang, Y.-F.; Chen, Y.-M.; Wang, C.-L. Influences of OutOf-Plane Lattice Alignment on the OFET Performance of TIPS-PEN Crystal Arrays. Cryst. Growth Des. 2016, 16, 6160−6166. (7) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J. y.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364−367. (8) Huang, C.-F.; Huang, S.-H.; Hsieh, C.-T.; Chao, Y.-H.; Li, C.-H.; Wu, S.-L.; Huang, Y.-F.; Hong, C.-Y.; Hsu, C.-S.; Chuang, W.-T.; Wang, C.-L. Stepwise Structural Evolution of a DTS-F2BT Oligomer and Influence of Structural Disorder on Organic Field Effect Transistors and Organic Photovoltaic Performance. Chem. Mater. 2016, 28, 8980−8987. (9) Chen, H.; Guo, Y.; Yu, G.; Zhao, Y.; Zhang, J.; Gao, D.; Liu, H.; Liu, Y. Highly π-Extended Copolymers with Diketopyrrolopyrrole Moieties for High-Performance Field-Effect Transistors. Adv. Mater. 2012, 24, 4618−4622. (10) Liu, X.; Guo, Y.; Ma, Y.; Chen, H.; Mao, Z.; Wang, H.; Yu, G.; Liu, Y. Flexible, Low-Voltage and High-Performance Polymer ThinFilm Transistors and Their Application in Photo/Thermal Detectors. Adv. Mater. 2014, 26, 3631−3636. (11) Sun, B.; Hong, W.; Yan, Z.; Aziz, H.; Li, Y. Record High Electron Mobility of 6.3 cm2V−1s−1 Achieved for Polymer Semiconductors Using a New Building Block. Adv. Mater. 2014, 26, 2636− 2642. (12) Yi, Z.; Wang, S.; Liu, Y. Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors. Adv. Mater. 2015, 27, 3589−3606. (13) Yun, H. J.; Kang, S. J.; Xu, Y.; Kim, S. O.; Kim, Y. H.; Noh, Y. Y.; Kwon, S. K. Dramatic Inversion of Charge Polarity in Diketopyrrolopyrrole-Based Organic Field-Effect Transistors via a Simple Nitrile Group Substitution. Adv. Mater. 2014, 26, 7300−7307. (14) Tsai, C.-E.; Yu, R.-H.; Lin, F.-J.; Lai, Y.-Y.; Hsu, J.-Y.; Cheng, S.W.; Hsu, C.-S.; Cheng, Y.-J. Synthesis of a 4,9-Didodecyl AngularShaped Naphthodiselenophene Building Block To Achieve HighMobility Transistors. Chem. Mater. 2016, 28, 5121−5130. (15) Lee, J.; Han, A. R.; Yu, H.; Shin, T. J.; Yang, C.; Oh, J. H. Boosting the Ambipolar Performance of Solution-Processable Polymer Semiconductors via Hybrid Side-Chain Eengineering. J. Am. Chem. Soc. 2013, 135, 9540−9547. (16) Lin, H.-W.; Lee, W.-Y.; Chen, W.-C. Selenophene-DPP Donor− Acceptor Conjugated Polymer for High Performance Ambipolar Field Effect Transistor and Nonvolatile Memory Applications. J. Mater. Chem. 2012, 22, 2120−2128. (17) Chiang, C.-J.; Chen, J.-C.; Tsao, H.-Y.; Wu, K.-Y.; Wang, C.-L. Ambient-Stable, Annealing-Free, and Ambipolar Organic Field-Effect Transistors Based on Solution-Processable Poly(2,2′-bis(trifluoromethyl)biphenyl-alt-2,5-divinylthiophene) without Long Alkyl Side Chains. Adv. Funct. Mater. 2015, 25, 606−614. (18) Tseng, H. R.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S. N.; Ying, L.; Kramer, E. J.; Nguyen, T. Q.; Bazan, G. C.; Heeger, A. J. High-Mobility Field-Effect Transistors Fabricated with Macroscopic Aligned Semiconducting Polymers. Adv. Mater. 2014, 26, 2993−2998. (19) Chen, Z.; Lee, M. J.; Shahid Ashraf, R.; Gu, Y.; Albert-Seifried, S.; Meedom Nielsen, M.; Schroeder, B.; Anthopoulos, T. D.; Heeney, M.; McCulloch, I.; Sirringhaus, H. High-Performance Ambipolar Diketopyrrolopyrrole-Thieno[3,2-b]thiophene Copolymer Field-Effect Transistors with Balanced Hole and Electron Mobilities. Adv. Mater. 2012, 24, 647−652. (20) Park, J. H.; Jung, E. H.; Jung, J. W.; Jo, W. H. A Fluorinated Phenylene Unit as a Building Block for High-Performance n-Type Semiconducting Polymer. Adv. Mater. 2013, 25, 2583−2588.

CONCLUSIONS In this study, the influences of the -RD end substituents on the DPPT unit were evaluated. Results show that adding -RD end substituents to a DPPT unit resulting in a DPP oligomer, DPPT-RD, with suitable EHOMO and ELUMO for the hole and electron injection from the Au electrode. For solid-state morphology, compared to the -DCV end substituents, the -RD groups also facilitate the formation of crystal arrays with better lattice orientation, larger crystalline size, and without polymorphism. The suitable EHOMO and ELUMO and unified lattice structure in the crystal arrays thus result in the good ambipolar characteristics of DPPT-RD. The results show the usefulness of the -RD end substituents in introducing ambipolar characteristic to solution-processable mono-DPP oligomers. -RD has been considered as a dye building block by Chen et al. Thus, DPPT-RD can also be categorized as a DPPT-Dye oligomer. Many other dye building blocks such as 1,3dimethylpyrimidine-2,4,6(1H,3H,5H)-trione, (E)-3-ethyl-5-(3octyl-4-oxothiazolidin-2-ylidene)-2-thioxothiazolidin-4-one,1,3indanedione, and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malono-nitrile have properties similar to -RD.21 The influences of these dye units on the performances of mono-DPP oligomer will be further studied.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01702. Detailed synthetic procedures and characterization data for the compounds, and additional figures (cyclic voltammetry plots, POM images, crystal structure modeling, OFET characteristics, and differential scanning calorimetry plots) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-L.W.). 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, Taiwan (MOST 103-2221-E-009-213-MY3 and MOST 104-2628-E-009-007-MY3). The authors thank the National Synchrotron Radiation Center for supporting us to carry out the GIXD experiments at beamlines BL13A1 and BL17A1.

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