Comparative Studies on the Relations between Composition Ratio

Oct 16, 2014 - Random copolymerization strategy is introduced to increase the solubility of donor–acceptor copolymers so that they can be processed ...
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Comparative Studies on the Relations between Composition Ratio and Charge Transport of Diketopyrrolopyrrole-Based Random Copolymers Hui-Jun Yun,† Jangwhan Cho,§ Dae Sung Chung,*,§ Yun-Hi Kim,*,‡ and Soon-Ki Kwon*,† †

School of Materials Science and Engineering and ERI and ‡Department of Chemistry and ERI, Gyeongsang National University, Jinju 660-701, Korea § School of Chemical Engineering and Material Science, Chung-Ang University, Seoul 156-756, Korea S Supporting Information *

ABSTRACT: Random copolymerization strategy is introduced to increase the solubility of donor−acceptor copolymers so that they can be processed with environmentally benign, halogen-free solvents. Traditionally, it has been believed that the random copolymer with a lower crystalline order should have a significantly lower charge carrier mobility. This report shows that random copolymerization between two highly planar, charge-delocalized repeating units can significantly enhance the solubility while almost preserving the high charge carrier mobility. A comparative study was conducted for a series of random copolymers consisting of diketopyrrolopyrrole−thienothiophene (DPP-TT) and DPP−selenophene− vinylene−selenophene (DPP-SVS) with composition ratios of 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, and 0:10 by using the crystalline order determined from grazing-incident X-ray diffraction measurements and charge carrier mobility determined from field effect transistor measurements. The results showed that the copolymer tends to have a lower degree of intermolecular ordering as the DPP-TT:DPP-SVS composition ratio approaches 5:5 because of the increased segmental randomness in the polymer chain. Nevertheless, the FET mobility did not follow the tendency of crystalline order; instead, it simply followed the order of increasing content of DPP-SVS in the copolymer. This result implies that we can use the random copolymerization strategy to increase the solubility of donor−acceptor copolymers if the two constituent repeating units are structurally planar and the electrons are fully delocalized.

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INTRODUCTION Owing to the tremendous research efforts worldwide for developing high-performance polymeric semiconductors, the field effect mobility of polymer field effect transistors (PFETs) exceed that of benchmark thin film amorphous silicon transistors (∼0.5 cm2/(V s)). Furthermore, some PFETs reported very recently show a field effect mobility of even higher than 5 cm2/(V s),1−4 which is accepted as a minimum requirement of pixel transistors for use in active-matrix organic light-emitting diodes.5 Such success in the field of polymeric semiconductors, especially in terms of mobility, can be mainly attributed to the development of alternating electron donor−acceptor conjugated copolymers.6 This is actually an analogue of n- and p-doped silicon semiconductors with the corresponding n- and p-dopants, respectively. A weakly polarized electron orbital between donor and acceptor moieties in copolymers can facilitate the delocalization of charge carriers, resulting in an apparently higher charge carrier mobility of PFET than that of pristine homopolymers. In the conjugated donor−acceptor copolymers, the majority of carriers are determined by the relative strengths of their donor and acceptor moieties. For © 2014 American Chemical Society

example, copolymers comprising stronger donors with weaker acceptors generally yield hole-dominant charge transport.1 The synthetic strategy of such donor−acceptor copolymers can be divided into two categories: main chain synthesis and side chain synthesis. Among the synthetic strategies of the former, incorporating diketopyrrolopyrrole (DPP) as the acceptor moiety has been found to be the most successful strategy.1−3,7−9 The lactam part of DPP makes the moiety exhibit a strong electron-withdrawing effect and high electron affinity. Furthermore, when the thiophene unit is attached to the DPP unit, it has been shown that the close (2.1 Å) intramolecular hydrogen bonding between the carbonyl oxygen of DPP and the nearest thiophene hydrogen facilitates backbone planarity, leading to further charge delocalization.10 Consequently, the Th-DPP-Th moiety has been copolymerized with many other repeating units to yield very successful donor− acceptor copolymers with exceptionally high mobility.1−3,7−9 For example, DPP-TT copolymers, comprising DPP as an Received: September 24, 2014 Revised: September 30, 2014 Published: October 16, 2014 7030

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Scheme 1. Synthetic Scheme of Copolymers

and to apply them in organic solar cells.14−18 Although very few studies showed the field effect mobility of random copolymers, the resulting mobility value was not high enough to attract interest; moreover, the reports did not focus on solubility tuning.19,20 Only Li et al. reported a reasonable mobility of 0.4 cm2/(V s) by random copolymerization;21 however, this report also did not focus on solubility tuning. One method of enhancing the solubility of copolymers while not compromising their mobility is the random copolymerization strategy using two highly conducting units: DPP− selenophene−vinylene−selenophene (DPP-SVS) and DPP− thienothiophene (DPP-TT). We have shown in an earlier report22 that random copolymers with more DPP-SVS unit than the DPP-BTT unit showed a high mobility >5 cm2/(V s), while still maintaining a reasonably high solubility in nonchlorinated solvents. This research report describes a comparative study of the same copolymer systems but with various composition ratios: pristine P-DPP-TT, P-DPP-TT(9)SVS(1), P-DPP-TT(7)-SVS(3), P-DPP-TT(5)-SVS(5), PDPP-TT(3)-SVS(7), P-DPP-TT(1)-SVS(9), and P-DPP-SVS. (The sequence follows the direction of increase in the contents of the DPP-SVS unit: from 0%, 10%, 30%, 50%, 70%, and 90% to 100%.) The results show that the charge mobility tends to increase as the content of the DPP-SVS unit increases in the copolymer while the crystalline order tends to decrease as the content of DPP-SVS approaches 50%, implying that the charge transport is more influenced by the electronic characteristics of the chemical structure itself, rather than by the degree of long-

acceptor and thienothiophene (TT) as a relatively stronger donor, showed an unprecedentedly high mobility exceeding 10 cm2/(V s).1 After this discovery, similar studies were followed by using various donor moieties, resulting in similar charge carrier mobilities. In another important synthetic strategy developed by our group and by Lei et al.11 and Zhang et al.,12 the branching position of the branched alkyl side chains exerts a dramatic effect on the intermolecular assembly of polymers and thus on the charge transport characteristics. This was mainly due to the decreased intermolecular π−π stacking distance between adjacent polymer chains owing to moving the branching position from the polymer backbone to the outer position. This strategy was also applied to many other polymer backbone systems, resulting in much improved charge carrier mobilities.2 While the above-mentioned synthetic strategy greatly contributed to enhancing the charge transport behavior of conjugated polymers, not much effort has been made to make these semiconducting polymers soluble in environmentally benign solvents for commercial use.13 This should be an important criterion for next-generation polymeric semiconductors, considering the increasing environmental cost of harmful halogenated solvents. An efficient synthetic strategy would be incorporating the concept of random copolymers so that the irregularity of polymer chains is increased, making the resulting polymers more soluble in nonhalogenated solvents. So far, such random copolymer strategies have been used only to tune the absorption range or energy level of polymer semiconductors 7031

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tetralin, as opposed to the results obtained with the above two homopolymers. Table 1 summarizes the UV−vis absorption maximum (λmax) of all the copolymers, in both solution and the thin film state.

range ordering. The detailed analysis and discussion are fully described in conjunction with morphological/structural analysis.

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EXPERIMENTAL SECTION

Table 1. Summary of λmax Measured from UV−Vis Absorption Spectra of Copolymersa

Methods. Synthesis and characterization of polymers, NMR spectra, cyclic voltammetry plots, differential scanning calorimetry plots, thermogravimetric analysis data, and elemental analysis data can be found in the Supporting Information. Device Preparation. Top-contact OFETs were fabricated on a common gate of highly n-doped silicon with a 300 nm thick thermally grown SiO2 dielectric layer. The dielectric layer was further modified by thin (∼10 nm) Cytop layer by spin-coating. Solutions containing the polymer semiconductors were spin-coated at 2000 rpm from 0.5 to 1 wt % solutions to form thin films with a nominal thickness of 30 nm regardless the choice of processing solvents, as confirmed using a surface profiler (Alpha Step 500, Tencor). The films were annealed at 200 °C for 10 min under a nitrogen atmosphere. Gold source and drain electrodes were evaporated on top of the semiconductor layers (80 nm). For all measurements, typical channel widths (W) and lengths (L) were 1000 and 100 μm, respectively. Characterization. The film morphologies were characterized by AFM (Digital Instruments Multimode) under ambient condition. The electrical characteristics of the OFETs were measured under ambient condition using parameter analyzer. Field-effect mobilities were extracted in the saturation regime from the slope of the source− drain current. GIXD measurements were performed using the 3C and 3D beamline at the Pohang Accelerator Laboratory (PAL).

SVS contents in the random copolymer (%) solution (nm) film (nm)

0

10

30

50

70

90

100

822 824

821 820

820 817

811 802

811 815

812 818

814 825

Film absorption data were obtained after thermal annealing at 200 °C for 10 min.

a

(also see Supporting Information) It is quite clear that λmax tends to red-shift as the content of the copolymer approaches that of a homopolymer. For example, in the thin film state, λmax decreases from 824 to 802 nm when the contents of DPP-SVS is increased from 0% to 50%, and it increases from 802 to 825 nm when the content of DPP-SVS is further increased from 50% to 100%. These results imply that charge delocalization within the polymer chain is quite disturbed as the content of the DPP-TT unit becomes similar to that of the DPP-SVS unit in the copolymer. This result sounds reasonable because the segmental randomness of the copolymer should be highest when equal amounts of two different components exist. At the same time, it is notable that the degree of blue-shift is not that significant even in the case of P-DPP-TT(5)-SVS(5). This can be attributed to the fact that both the repeating units, DPP-TT and DPP-SVS, already have a highly planar structure. Therefore, we may expect reasonably good charge transport even in the case of P-DPP-TT(5)-SVS(5), which is believed to have the largest segmental randomness. To further study the effect of random copolymerization with various copolymer composition ratios on the thin film crystalline behavior, 2-D grazing-incident X-ray diffraction (GIXD) studies were conducted on all the copolymer films. Both out-of-plane (OOP) and in-plane (IP) diffraction spectra are summarized in Figure 1. From OOP analysis, it is interesting to note that the peak position gradually shifts to a higher angle as the content of DPP-SVS increases, implying that the lamellar distance decreases. This result was not expected because both the DPP-TT and DPP-SVS units have the same side chain length and structure. As summarized in Figure 1b, the lamellar d-spacing between adjacent polymer chains is 28.2 Å in the case of P-DPP-TT; it gradually decreases as the content of DPP-SVS increases and finally becomes 26.3 Å in the case of P-DPP-SVS. This result can be partially attributed to the difference in spacing between each side chain. Because the S−V−S moiety is longer than the TT moiety along the direction of the polymer chain, it is believed that the side chain can be further intercalated in the case of P-DPP-SVS rather than P-DPP-TT. The results obtained with polyhexylthiophene and polyquaterthiophene can be similarly explained.23 The IP diffraction results show that all the copolymers, except P-DPP-TT(5)-SVS(5), give very clear evidence of π−π stacking at 2θ ∼ 17.6°, corresponding to the π−π distance of 3.65 Å (Figure 1c). Interestingly, P-DPP-TT(5)-SVS(5), which was expected to have the highest segmental randomness from both the initial intuitive and UV−vis absorption results, actually did not show any strong π−π stacking behavior. Even from OOP diffraction

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RESULTS AND DISCUSSION The synthetic scheme of copolymers with various composition ratios is described in Scheme 1. The random copolymers with different ratios were obtained by using the Stille coupling reaction and controlling the composition of TT and SVS units. The obtained copolymer composition was confirmed by elemental analysis (Supporting Information, Table S2). The number-average molecular weight (Mn) and polydispersity of the obtained polymers, as determined by using GPC with polystyrene as the standards, are described in the Supporting Information. The number-average molecular weights of the polymers are in the range 31 000−43 800 g mol−1, with polydispersity index values between 1.33 and 1.68. The thermogravimetric analysis (TGA) results show that all the obtained polymers have good thermal stability, with degradation temperatures between 387 and 439 °C; no phase transition was observed within 50−250 °C. The electrochemical properties of the random copolymers were measured by cyclic voltammetry (CV) (Figure S7 and Table S3). The HOMO levels of the random copolymers gradually increased from −5.36 to −5.27 eV on increasing the content of SVS. The solubility of a series of random copolymers was tested in various organic solvents. The results showed that two pristine homopolymers, P-DPP-TT and P-DPP-SVS, were reasonably soluble only with chlorinated solvents and almost insoluble with other nonhalogenated solvents such as toluene, xylene, and tetralin. However, copolymers containing both DPP-TT and DPP-SVS showed greatly enhanced solubilities in almost all types of organic solvents regardless of halogenation. In particular, when the content of one unit, whether DPP-TT or DPP-SVS, is greater than that of another, the solubility tended to increase. As summarized in Table S4, the overall sequence of solubility is as follows: P-DPP-TT(1)-SVS(9) and P-DPPTT(9)-SVS(1) > P-DPP-TT(3)-SVS(7) and P-DPP-TT(7)SVS(3) > P-DPP-TT(5)-SVS(5) ≫ P-DPP-TT and P-DPPSVS. Regardless, all the random copolymers were soluble in 7032

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Figure 1. GIXD patterns of copolymers with various composition ratios. In the figure, percentage implies the contents of the DPP-SVS unit; (a) and (b) are from OOP scans and (c) is from an IP scan.

phene.1,2 The representative transfer characteristics of the copolymers with various composition ratios are summarized in Figure 2. The corresponding output curves are also summarized in the Supporting Information. All the curves represent very sharp switching behavior with near-zero hysteresis and very high mobility ranging from 3 to 9 cm2/(V s). The saturation regime mobility was extracted from 10 randomly selected devices for each random copolymer and is summarized in Figure 3. As a processing solvent, both chloroform and tetralin were applied, and the corresponding results have been separately summarized (Figure 3b). Figure 3a shows the results from chloroform-based PFETs and depicts a clear trend of increase in the mobility as the content of SVS is increased in the copolymer. Compared to P-DPP-TT, P-DPP-SVS shows more than twice the FET mobility on average, reaching up to 9 cm2/(V s). The overall ascendency of P-DPP-SVS over P-DPPTT in terms of mobility can be partially attributed to its better interdigitated crystalline structure, as discussed in previous sessions. Surprisingly, P-DPP-TT(5)-SVS(5), which has the maximum segmental randomness owing to the presence of equal amounts of the two repeating units, also followed the above-mentioned trend. This result is clearly mismatched with the GIXD analysis results because P-DPP-TT(5)-SVS(5) revealed apparently lower crystalline characteristics than all other copolymers, even without any π−π stacking features. Similar results were obtained in the case of tetralin processing

spectra, the Bragg peak intensity was lowest in this random copolymer with equal amount of two repeating units. However, all other random copolymers with composition ratios of 1:9, 3:7, 7:3, and 9:1 showed well-ordered crystalline features, together with clear evidence of π−π stacking. Consequently, we can conclude that within this series of copolymers random copolymerization did not seriously interrupt the long-range ordering of polymer chains, possibly owing to strong aggregating properties of both the repeating units, DPP-TT and DPP-SVS. Nonetheless, P-DPP-TT(5)-SVS(5), which has maximum segmental randomness, appeared to show lower crystalline ordering behavior. The PFET studies were followed to correlate the aforementioned crystalline structure of various copolymers with charge transport behavior. Before starting these studies, we should address that the surface morphology of thin films of all these copolymers were almost identical, regardless of the composition ratios, as summarized in the Figure S12. Therefore, the charge transport behavior should be explained on the basis of the structural or chemical characteristics of the copolymers. When the bottom-gate/top-contact device geometry was used, all the devices yielded well-defined p-type transfer and output characteristics. Note that these series of copolymers have p-type-dominant charge transport behavior because of the presence of relatively strong electron donors: thiophene, selenophene, vinylene group, and thienothio7033

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Figure 2. Summary of PFET transfer curves based on copolymers with various composition ratios: (a) P-DPP-TT, (b) P-DPP-TT(9)-SVS(1), (c) PDPP-TT(7)-SVS(3), (d) P-DPP-TT(5)-SVS(5), (e) P-DPP-TT(3)-SVS(7), (f) P-DPP-TT(1)-SVS(9), and (g) P-DPP-SVS.

Figure 3. Summary of FET mobility values of copolymers processed from (a) chloroform and (b) tetralin. In the case of (b), data of P-DPP-TT and P-DPP-SVS cannot be obtained because of solubility limitation.

therefore requires only occasional intermolecular charge hopping through π−π stacking.25 Noriega and co-workers reported the important finding that low crystalline nature of conjugated polymers does not necessarily lead to low charge carrier mobility. From a literature survey of data reported over 10 years, it was concluded that the presence of short-range intermolecular aggregations are enough for efficient long-range charge transport.26 Therefore, it is expected that the intermolecular charge transport still occurs efficiently in PDPP-TT(5)-SVS(5) at a small aggregate level, although the polymer’s overall crystallinity is relatively low as long as there is still a chance of π hopping. In other words, random copolymerization does not necessarily disrupt charge transport

solvents, while the overall mobility values were slightly decreased. For a long time in the field of OFET, it was believed that a strong intermolecular interaction, especially along the in-plane direction, is the key for efficient charge transport.24 This logic has been quite successful in explaining the discrepancy of charge transport behavior between two semiconducting polymers with high and low mobilities. However, very recently researchers have started to report that this traditional belief may not be necessarily true in the case of highly conjugated donor− acceptor copolymers.25,26 Zhang and co-workers also argued similar logic that charge transport in high-mobility donor− acceptor copolymer occurs along the polymer backbone and 7034

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(3) Chen, H.; Guo, Y.; Yu, G.; Zhao, Y.; Zhang, J.; Gao, D.; Liu, H.; Liu, Y. Adv. Mater. 2012, 24, 4618. (4) Kim, G.; Kang, S.-J.; Dutta, G. K.; Han, Y.-K.; Shin, T. J.; Noh, Y.Y.; Yang, C. J. Am. Chem. Soc. 2014, 136, 9477. (5) Sirringhaus, H. Adv. Mater. 2014, 26, 1319. (6) Biniek, L.; Schroede, B. C.; Nielsen, C. B.; McCulloch, L. J. Mater. Chem. 2012, 22, 14803. (7) Armao, J. J., IV; Maaloum, M.; Ellis, T.; Fuks, G.; Rawiso, M.; Moulin, E.; Giuseppone, N. J. Am. Chem. Soc. 2012, 134, 16532. (8) Li, Y.; Singh, S. P.; Sonar, P. Adv. Mater. 2010, 22, 4862. (9) Li, Y.; Sonar, P.; Singh, S. P.; Soh, M. S.; van Meurs, M.; Tan, J. J. Am. Chem. Soc. 2011, 133, 2198. (10) Wu, P.-T.; Kim, F. S.; Jenekhe, S. A. Chem. Mater. 2011, 23, 4618. (11) Lei, T.; Dou, J.-H.; Pei, J. Adv. Mater. 2012, 24, 6457. (12) Zhang, F.; Hu, Y.; Schuettfort, T.; Di, C.-A.; Gao, X.; Mcneill, C. R.; Thomsen, L.; Mannsfeld, S. C. B.; Yuan, W.; Sirringhaus, H.; Zhu, D. J. Am. Chem. Soc. 2013, 135, 2338. (13) Henson, Z. B.; Zalar, P.; Chen, X.; Welch, G. C.; Nquyen, T.-Q.; Bazan, G. C. J. Mater. Chem. A 2013, 1, 11117. (14) Burkhart, B.; Khlyabich, Petr. P.; Canak, T. C.; Lajoie, T. W.; Thompson, B. C. Macromolecules 2011, 44, 1242. (15) Zhou, J.; Xie, S.; Amond, E. F.; Becker, M. L. Macromolecules 2013, 46, 3391. (16) Chen, C.-H.; Cheng, Y.-J.; Chang, C.-Y.; Hsu, C.-S. Macromolecules 2011, 44, 8415. (17) Zhang, G.; Fu, Y.; Qiu, L.; Xie, Z. Polymer 2012, 53, 4407. (18) Li, J.; Ong, K.-H.; Sonar, P.; Lim, S.-L.; Ng, G.-M.; Wong, H.-K.; Tan, H.-S.; Chen, Z.-K. Polym. Chem. 2013, 4, 804. (19) Li, J.; Ong, K.-H.; Lim, S.-L.; Ng, G.-M.; Tan, H.-S.; Chen, Z.-K. Chem. Commun. 2011, 47, 9480. (20) Wu, P.-T.; Kim, F. S.; Jenekhe, S. A. Chem. Mater. 2011, 23, 4618. (21) Li, H.; Liu, F.; Wang, X.; Gu, C.; Wang, P.; Fu, H. Macromolecules 2013, 46, 9211. (22) Yun, H.-J.; Lee, G. B.; Chung, D. S.; Kim, Y.-H.; Kwon, S.-K. Adv. Mater. 2014, in press. (23) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378. (24) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; Mcgehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328. (25) Zhang, X.; Bronstein, H.; Kronemeijer, A. J.; Smith, J.; Kim, Y.; Kline, R. J.; Richter, L. J.; Anthopoulos, T. D.; Sirringhaus, H.; Song, K.; Heeney, M.; Zhang, W.; McCulloch, I.; Delongchamp, D. M. Nat. Commun. 2013, 4, 2238. (26) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. Nat. Mater. 2013, 12, 1038.

behavior seriously if both the repeating units are highly planar and therefore guarantee successful intramolecular charge delocalization. This result is particularly important because it opens a novel synthetic methodology to preserve high charge carrier mobility while simultaneously enhancing the solubility of polymer semiconductors.

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CONCLUSION This report was a comparative study of a series of random copolymers consisting of DPP-TT and DPP-SVS to develop a synthetic strategy for enhancing the solubility of semiconducting polymers without sacrificing charge transport behavior. It was found that the copolymer tends to have a lower degree of intermolecular ordering as the DPP-TT:DPPSVS composition ratio approaches 5:5 owing to increased segmental randomness in a polymer chain. Nevertheless, the FET mobility did not follow the tendency of crystalline order; instead, it simply followed the order of increasing content of DPP-SVS in the copolymer. This result implies that an efficient intramolecular charge delocalization is enough to maintain high mobility in highly conjugated donor−acceptor copolymers. Therefore, we can use the random copolymerization strategy to increase the solubility of donor−acceptor copolymers if the two constituent repeating units are structurally planar and the electrons are fully delocalized.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S13 and Tables S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.S.C.). *E-mail [email protected] (Y.H.K.). *E-mail [email protected] (S.K.K.). Author Contributions

H.-J.Y. and J.C. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012047047), (NRF2014M1A3A3A02034707) and by a grant (2013073172) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the MEST.

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ABBREVIATIONS CV, cyclic voltammetry; DPP-SVS, diketopyrrolopyrrole− selenophene−vinylene−selenophene; DPP-TT, diketopyrrolopyrrole−thienothiophene; GIXD, grazing-incident X-ray diffraction; IP, in-plane; OOP, out-of-plane; PFET, polymer field effect transistor; TGA, thermogravimetric analysis; TT, thienothiophene.

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REFERENCES

(1) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. Sci. Rep. 2012, 2, 754. (2) Kang, I.; Yun, H. J.; Chung, D. S.; Kwon, S. K.; Kim, Y. H. J. Am. Chem. Soc. 2013, 135, 14896. 7035

dx.doi.org/10.1021/ma501980m | Macromolecules 2014, 47, 7030−7035