Improving Ambipolar Semiconducting Properties of Thiazole-Flanked

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Cite This: Macromolecules XXXX, XXX, XXX−XXX

Improving Ambipolar Semiconducting Properties of ThiazoleFlanked Diketopyrrolopyrrole-Based Terpolymers by Incorporating Urea Groups in the Side-Chains Jing Ma, Zitong Liu,* Jingjing Yao, Zhijie Wang, Guanxin Zhang, Xisha Zhang, and Deqing Zhang*

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Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: Two new ambipolar thiazole-flanked diketopyrrolopyrrole-based polymers pDPPTz2T-1 and pDPPTz2T2 with urea-containing linear side-chains were prepared. The formation of hydrogen bonding enhanced the ambipolar semiconducting properties, including mobilities and on/off ratios. The average mobilities (hole and electron) of pDPPTz2T-2 were 25 and 3 times higher than those of pDPPTz2T without urea groups, whereas the average on/off ratios (Ion/Ioff) for hole and electron were 100 and 4 times higher than those obtained for pDPPTz2T. Thin-film microstructure studies reveal that incorporating urea groups into polymer side-chains can enhance interchain packings, including the alkyl chain lamellar and π−π stackings. Our results clearly show how incorporating urea groups in side-chains significantly influence semiconducting properties, which could be extended to other conjugated systems toward ambipolar and even n-type FETs.

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INTRODUCTION Solution-processable conjugated donor−acceptor (D−A) polymers have been extensively investigated due to their potential applications in field-effect transistors (FETs), photovoltaic cells, and other flexible electronic devices.1−19 It is known that semiconducting properties of these polymers are dependent on the electronic structures of donor and acceptor units in conjugated backbones, which not only affect the charge transporting along the backbones but also influence the interchain π−π interactions and interchain carrier hopping.20−22 Therefore, various conjugated skeletons have been designed and combined to generate the respective polymers with high charge mobilities for the past decades.23−30 Recent studies have demonstrated that the side-chains of conjugated polymers can influence interchain stacking, solidstate microstructure, and charge transporting behavior, other than just endowing solubilities.31−52 Therefore, side-chain engineering has become effective tool to improve the semiconducting performance of conjugated polymers. Sidechains with different structures, including branching alkyl chains,37,38 oligo(ethylene glycol) chains,39−42 semifluorinated chains,43−45 and siloxane-terminated chains,46−51 have been introduced into conjugated polymers for improving the semiconducting properties. For instance, some of us reported diketopyrrolopyrrole (DPP)-based polymers containing urea groups in the side-chains, which boosted hole mobilities significantly.34 Rondeau-Gagné, Chiu, and their co-workers © XXXX American Chemical Society

also observed the hole mobility enhancement by introducing amide groups in the side-chain of conjugated polymers.52 These results show that the incorporation of H-bonding groups such as urea and amide ones in the side-chains is efficient for improving interchain packing order and charge mobilities for conjugated polymers. Although incorporating hydrogen bonding in small molecules53−57 and conjugated polymers34,52,58,59 has been reported, the enhancement of ambipolar mobilities (both hole and electron) are seldom reported. In this paper, we reported two DPP-based D−A terpolymers pDPPTz2T-1 and pDPPTz2T-2 (Scheme 1), which contain thiazole-flanked DPP units and thiophene-flanked DPP units. The thiazole-flanked DPP unit shows a lower LUMO level, which has been employed in ambipolar semiconducting polymers by varying the conjugated backbones.60,61 The thiophene-flanked DPP unit entails urea groups in the sidechains, whereas the thiazole-flanked DPP unit entails branched alkyl chains. pDPPTz2T-1 and pDPPTz2T-2 entail different thiophene-flanked DPP units and thus different contents of urea groups. Thin-film FET studies reveal that thin-film pDPPTz2T-1 and pDPPTz2T-2 exhibit ambipolar semiconducting behavior. In comparison with pDPPTz2T without Received: May 15, 2018 Revised: July 21, 2018

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DOI: 10.1021/acs.macromol.8b01020 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Chemical Structures and Synthetic Approaches for pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T

Figure 1. (a) FT-IR spectra of thin films of pDPPTz2T-1 (red), pDPPTz2T-2 (blue), and pDPPTz2T (blue-green); the spectra were recorded under vacuum. (b) Variable-temperature 1H NMR spectra for pDPPTz2T-2 and after addition of D2O at 373 K.

pDPPTz2T-1 and pDPPTz2T-2, in which the feed ratios of branching and urea-containing chains were 20:1 and 10:1, respectively, were obtained by varying the molar ratios between monomers 1 and 2. For comparison, pDPPTz2T without urea-containing alkyl chains was also prepared. After precipitated with methanol, the resulting polymers were extracted sequentially polar and nonpolar solvents by Soxhlet extractor to remove monomers and oligomers (see the Supporting Information). Finally, the polymers were dried under vacuum. The yields for pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T were 75%, 76%, and 72%, respectively. 1 H NMR, solid state 13C NMR, FT-IR, and elemental analysis were used to verify the chemical structures. The Mws of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T, determined by HT-GPC (see the Supporting Information), were 43, 50, and 57 kg mol−1 with polydispersity values of 2.2, 2.0, and 2.8, respectively. No thermal transitions were detected by differential scanning calorimetry (DSC) (Figure S1) until 250 °C

thiophene-flanked DPP units, pDPPTz2T-1 and pDPPTz2T-2 show higher hole and electron mobilities and Ion/Ioff ratios. The average μh and μe of pDPPTz2T-2 are 25 and 3 times higher than those of pDPPTz2T, whereas the average Ion/Ioff ratios of pDPPTz2T-2 for hole and electron transporting are 100 and 4 times higher than those of pDPPTz2T. The UV−vis absorption, GIWAXS, and AFM data indicate that intermolecular hydrogen bonding of urea groups can induce polymer chains of pDPPTz2T-1 and pDPPTz2T-2 to form ordered microstructures, which are beneficial for charge transporting.

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RESULTS AND DISCUSSION Synthesis and Characterization. pDPPTz2T-1 and pDPPTz2T-2 were synthesized by Stille polycondensation reactions of monomers 1, 2, and 3 as outlined in Scheme 1. The synthesis of compounds 1 and 2 were reported previously.34,62 Compound 3 was commercially available and was used without further purification. The terpolymers B


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Figure 2. Absorption spectra (normalized) of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T.

Table 1. Absorption Maxima, Redox Potentials, HOMO/LUMO Energies, and Bandgaps of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T λmax (nm)a polymer

Mn/Mw

PDI

solution

thin film

b Eonset redl (V)

ELUMO (eV)

b Eonset oxl (V)

EHOMO (eV)

c Eopt g (eV)

d Ecv g (eV)

pDPPTz2T-1 pDPPTz2T-2 pDPPTz2T

18/43 25/50 20/57

2.2 2.0 2.8

708, 774 708, 776 706, 770

694, 767 695, 769 692, 764

−1.02 −1.01 −1.10

−3.78 −3.79 −3.70

0.55 0.53 0.56

−5.35 −5.33 −5.36

1.44 1.43 1.45

1.57 1.54 1.66

a

Absorption maxima. bOnset potentials (V vs Fc/Fc+). cBased on absorption spectral data. dBased on redox potentials.

spectra: pDPPTz2T-2 < pDPPTz2T-1 < pDPPTz2T. It has been reported that a low 0−1/0−0 intensity ratio indicates more ordered interchain packing of polymers.63 In addition, the absorption spectra of three polymers were measured at 120 °C, and their 0−0 and 0−1 absorption bands became almost overlapped (Figure S3). Thus, the absorption differences between pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T at room temperature (see Figure 2a) can be attributed to the preaggregation of pDPPTz2T-1 and pDPPTz2T-2 in solution. Such preaggregation of pDPPTz2T-1 and pDPPTz2T-2 in solutions at room temperature can be induced by interchain Hbonding owing to urea groups. Based on the onset absorptions of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T thin films, the optical bandgaps were estimated to be 1.44, 1.43, and 1.45 eV, respectively. The energy levels of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T were also determined via cyclic voltammetry (CV) (Figure S4 and Table 1). The respective onset oxidation and reduction potentials (vs Fc/Fc+) of pDPPTz2T-1, pDPPTz2T2, and pDPPTz2T were determined, and HOMO/LUMO energies of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T were calculated to be −5.35/−3.78, −5.33/−3.79 and −5.36/−3.70 eV (for details, see the Supporting Information). Compared to pDPPTz2T, LUMO levels of pDPPTz2T-1 and pDPPTz2T-2 are weakly lowered, whereas the HOMO levels are slightly enhanced with increasing the urea-group contents. The bandgaps are slightly reduced from 1.66 eV for pDPPTz2T to 1.57 and 1.54 eV for pDPPTz2T-1 and pDPPTz2T-2, respectively. Polymers pDPPTz2T-1 and pDPPTz2T-2 are expected to show ambipolar semiconducting behavior in consideration of their energy levels. Charge Mobility Enhancement. Thin-film BGBC FETs of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T for comparison were fabricated under the same conditions and measured

for these three polymers. The 5% weight loss temperatures (Td) of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T were estimated to be 366, 302, and 342 °C, respectively, based on the TGA data (see Figure S2). The respective mass contents of nitrogen element in pDPPTz2T-1 and pDPPTz2T-2 were measured to be 5.62% and 5.69%. Based on elemental analysis data, the molar ratios of urea-containing DPP block vs thiazoleflanked DPP block in pDPPTz2T-1 and pDPPTz2T-2 were determined to be 1:20 and 1:13.5, respectively. As shown in Figure 1a, typical FT-IR absorption signals around 3400 cm−1 owing to the hydrogen-bonding formation among urea groups were observed for solid states of pDPPTz2T-1 and pDPPTz2T-2, which was not observed for that of pDPPTz2T. 1H NMR spectra at different temperatures of pDPPTz2T-2 (1,1,2,2-tetrachloroethane-d2, ∼1 mg/mL) were examined to confirm the interchain H-bonding formation. As shown in Figure 1b, the 1H NMR signal attributed to urea groups in pDPPTz2T-2 was found at 5.20 ppm at 323 K. The signal showed an upfield shift to 5.14 ppm at 353 K and to 5.09 ppm at 373 K, which agrees with interchain hydrogen-bonding formation of urea groups. To further support the existence of hydrogen bonding, we added D2O to the solution of pDPPTz2T-2 at 373 K and found that the signal at 5.09 ppm (due to urea groups) disappeared. These 1H NMR data provide the evidence for the interchain hydrogen-bonding formation of urea groups. Optical and Electrochemical Properties. Figure 2 shows the solution and thin film absorption spectra of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T. Their absorption spectra show distinct 0−0 and 0−1 vibronic features, and the absorption maxima are slightly red-shifted for pDPPTz2T-1 and pDPPTz2T-2 in comparison with those of pDPPTz2T. Interestingly, the 0−1/0−0 intensity ratios increase in the following order for their solution and thin-film absorption C


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Table 2. Ambipolar Mobilities in “Highest/Average” Form, On/Off Ratios, and Threshold Voltages for FETs with Thin Films of pDPPTz2T-1, pDPPTz2T-2, pDPPTz2T-A, pDPPTz2T-B, and pDPPTz2T polymer pDPPTz2T-1

pDPPTz2T-2

pDPPTz2T-A

pDPPTz2T-B

pDPPTz2T

temp (°C) as cast 160 200 as cast 160 200 as cast 160 200 as cast 160 200 as cast 160 200

μh (cm2 V−1 s−1)a 0.13/0.10 0.86/0.75 0.50/0.42 0.26/0.23 1. 10/1.02 0.92/0.87 0.05/0.042 0.15/0.12 0.11/0.09 0.06/0.053 0.25/0.20 0.18/0.10 3 × 10−3/2 × 10−3 0.06/0.04 4 × 10−2/3 × 10−2

Vth,h (V) −30 −30 −23 −22 −20 −25 −30 −25 −25 −25 −20 −25 −25 −24 −20

to to to to to to to to to to to to to to to

μe(cm2 V−1 s−1)a

Ion/Ioff

−14 −10 −18 −10 −5 −7 −15 −10 −18 −15 −10 −10 −20 0 −10

(2−4) × (2−6) × (3−6) × (1−4) × (1−6) × (1−4) × (2−4) × (3−5) × (1−3) × (4−6) × 104−105 104−105 (1−4) × (2−7) × (1−5) ×

4

10 105 105 105 105 105 104 105 105 104

103 103 103

−3

−3

8 × 10 /6 × 10 0.014/0.01 9 × 10−3/7 × 10−3 1.2 × 10−2/1 × 10−2 0.020/0.018 1 × 10−2/8 × 10−3 9 × 10−4 /8 × 10−4 8 × 10−3/7 × 10−3 5 × 10−3 /4 × 10−3 1.2 × 10−3/1 × 10−3 1 × 10−2/8 × 10−3 7 × 10−3/6.8 × 10−3 2 × 10−4/1 × 10−4 7 × 10−3/6 × 10−3 3 × 10−3/2 × 10−3

Vth,e (V)

Ion/Ioff

30−40 30−58 30−35 30−40 30−55 32−60 50−55 40−50 45−55 45−55 42−50 40−55 45−62 40−55 44−58

6 × 103 (2−6) × 104 (2−6) × 104 4 × 104 2 × 104−1 × 105 4 × 104 103 103−104 103 103 103−104 103 (2−4) × 103 2 × 104 2 × 104

a

Data were collected based on at least 20 devices.

Figure 3. Ambipolar mobilities and Ion/Ioff ratios of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T. All data were based on statistics of at least 20 FET devices with W = 1440 μm and L = 50 μm.

Figure 4. GIWAXS patterns for thin films of three polymers spin-coated on OTS-modified SiO2/Si substrates.

in a N2 glovebox (Supporting Information). As shown in Figure S5, thin films of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T exhibit ambipolar semiconducting behaviors. The semiconducting performance data of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T are listed in Table 2. The ambipolar mobilities (μh and μe) of pDPPTz2T-1, pDPPTz2T2, and pDPPTz2T increase after annealing at 160 °C, which reduce by increasing temperature (200 °C). Interestingly, hole and electron mobilities of pDPPTz2T-1 and pDPPTz2T-2 are

higher than those of pDPPTz2T as shown in Table 2. Moreover, average hole and electron mobilities of pDPPTz2T1/pDPPTz2T-2 after thermal annealing at 160 °C are about 18/25 and 2/3 times higher than those of pDPPTz2T as shown in Figure 3. Incorporating urea groups in side-chains of these two polymers not only induces the enhancement of hole and electron mobilities but also increases current on/off ratios. As depicted in Figure 3, the current on/off ratios for both the hole and electron conducting channels increase in the D


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Figure 5. Thin-film AFM images of three polymers after thermal annealing at 160 and 200 °C.

directions was 22.89 Å for pDPPTz2T-1, 25.47 Å for pDPPTz2T-2, and 20.79 Å for pDPPTz2T. We also calculated the LC attributed to side-chain lamellar stacking in the out-ofplane direction, which was 128.51 Å for pDPPTz2T-1, 157.07 Å for pDPPTz2T-2, and 88.36 Å for pDPPTz2T.These findings indicate that (i) incorporation of urea groups could shorten the interchain π−π stacking distances and d-spacings of the lamellar stacking of alkyl chains, (ii) polymer chains of pDPPTz2T-1 and pDPPTz2T-2 adopted both face-on and edge-on packing modes, whereas pDPPTz2T exhibited predominantly edge-on packing mode, and (iii) the formation of hydrogen-bonding side-chain can enhance lamellar packing order of interchain π−π and alkyl chains stackings. Hence, the incorporation of urea groups in side chains can improve thin film crystallinities and induce polymer chains to form large ordered aggregates, which are beneficial for charge transporting as discussed above. AFM height images are shown in Figure 5 for pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T after thermal annealing at 160 and 200 °C. All three polymers show similar surface morphologies with interconnected nanofibers after thermal annealing at 160 °C. However, the nanofibers became short, and more boundary areas emerged after thermal annealing at 200 °C. Furthermore, the RRMS (root-mean-square roughness) was 1.41 nm for pDPPTz2T-1, 1.60 nm for pDPPTz2T-2, and 1.20 nm for pDPPTz2T. Clearly, RRMS increases by increasing the urea group contents in the terpolymers. This might be due to the interchain H-bonding among urea groups, leading to formation of larger aggregates, which are beneficial for the charge carrier transporting, according to previous reports.66

following order: pDPPTz2T< pDPPTz2T-1< pDPPTz2T-2. For instance, the average current on/off ratio in the hole conducting channel was boosted from 5 × 103 for pDPPTz2T to 5 × 105 for pDPPTz2T-2, whereas the on/off ratio in the electron conducting channel increased to 8 × 104 for pDPPTz2T-2 in comparison with that of pDPPTz2T (2 × 104). Furthermore, we prepared the reference polymers pDPPTz2T-A and pDPPTz2T-B with feed ratios (linear chains vs branching chains) of 1:20 and 1:10 (Scheme S1). The hole and electron mobilities of pDPPTz2T-A and pDPPTz2T-B are enhanced compared to those of pDPPTz2T before and after thermal annealing at 160 °C (Table 2). But, thin-film hole and electron mobilities of pDPPTz2T-1 and pDPPTz2T-2 are higher than those of pDPPTz2T-A and pDPPTz2T-B for both the as-prepared and thermally annealed thin films (at 160 °C). These results demonstrate that the incorporation of urea groups in the side-chains can further boost the mobilities for conjugated D−A polymers. GIWAXS and AFM Studies. Thin-film morphology and interchain packing of pDPPTz2T-1, pDPPTz2T-2, and pDPPTz2T were investigated with grazing-incidence wideangle X-ray scattering (GIWAXS) and atomic force microscope (AFM). Thin-film GIWAXS patterns (out-of-plane and in-plane) of thermally annealed thin films of three polymers are shown in Figure 4. In the out-of-plane direction (Figure 4a), thin films of pDPPTz2T-1 and pDPPTz2T-2 exhibited distinct (x00) scattering signals up to fourth orders and weak (010) scattering signals, whereas thin film of pDPPTz2T only exhibited (100) and (200) scattering signals as well as very broad and weak (300) and (400) ones. The d-spacing owing to the lamellar stacking of alkyl chains was estimated to be 19.96 Å for pDPPTz2T-1, 19.78 Å for pDPPTz2T-2, and 20.33 Å for pDPPTz2T in the out-of-plane direction. The π−π stacking distances were calculated to be 3.53 Å for pDPPTz2T-1 and 3.51 Å for pDPPTz2T-2 based on the (010) scattering signals. As for in-plane direction (Figure 4b), thin films of pDPPTz2T-1 and pDPPTz2T-2 exhibited distinct (010) and (100) scattering signals, whereas thin film of pDPPTz2T only exhibited the (010) scattering signal. The distances of π−π stacking were calculated to be 3.58 Å for pDPPTz2T-1, 3.55 Å for pDPPTz2T-2, and 3.63 Å for pDPPTz2T. On the basis of the fwhm’s of scattering signals (in-plane direction), the correlation length (LC)64,65 attributed to π−π stacking

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CONCLUSION Two new thiazole-flanked diketopyrrolopyrrole-based polymers (pDPPTz2T-1 and pDPPTz2T-2) with different ratios of urea groups in the side-chains were synthesized. The presence of urea groups facilitates the ordered packing of polymer chains, and the resulting thin films show improved crystallinity. Ambipolar semiconducting properties of thin films of pDPPTz2T-1 and pDPPTz2T-2, including the hole and electron mobilities (μh and μe) and on/off ratios are enhanced compared to those of pDPPTz2T without urea groups. Our results indicate that the incorporation of urea groups in sidechains is a promising design strategy for conjugated polymers with improved semiconducting performances. E


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01020. Characterization techniques, synthesis of four terpolymers, DSC, TGA, UV−vis absorption spectra of three terpolymers at 393 K, cyclic voltammograms, FETs, OFET device fabrication and measurements, NMR data and corresponding references (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.Z.). *E-mail: [email protected] (Z.L.). ORCID

Zitong Liu: 0000-0003-1185-9219 Zhijie Wang: 0000-0001-5092-0735 Guanxin Zhang: 0000-0002-1417-6985 Deqing Zhang: 0000-0002-5709-6088 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support of the National Key R&D Program of China (2017YFA0204701), the Strategic Priority Research Program of the CAS (XDB12010300), and NSFC (21661132006). We also gratefully thank the assistance of researchers of 1W1A, Beijing Synchrotron Radiation Facility, for measuring GIWAXS.

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REFERENCES

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