COMMUNICATION pubs.acs.org/cm
Highly Soluble Poly(thienylenevinylene) Derivatives with Charge-Carrier Mobility Exceeding 1 cm2V1s1 Juhwan Kim,† Bogyu Lim,‡ Kang-Jun Baeg,§ Yong-Young Noh,|| Dongyoon Khim,† Hyung-Gu Jeong,† Jin-Mun Yun,† and Dong-Yu Kim*,†,^ †
)
Heeger Center for Advanced Materials, School of Materials Science and Engineering, Gwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu, Gwangju 500-712, Republic of Korea ‡ Department of Material Science and Engineering, Center for Advanced Molecular Photovoltaics (CAMP), Stanford University, Stanford, California 94305, United States § Convergence Components and Materials Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), 218 Gajeongno, Yuseong-gu, Daejeon 305-700, Republic of Korea Department of Chemical Engineering, Hanbat National University, 16-1 Dukmyung-dong, Yuseong-gu, Daejeon 305-719, Republic of Korea ^ Research Institute for Solar and Sustainable Energies, Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu, Gwangju 500-712, Republic of Korea
bS Supporting Information KEYWORDS: conjugated polymer, polymer semiconductor, organic electronics, OTFTs, thienylenevinylene
C
onjugated molecules are emerging candidates for the construction of the active layers of organic thin film transistors (OTFTs) because they offer the possibility for various flexible and large-area electronic applications through cost-effective graphic arts printing techniques.1 Many research groups have reported various conjugated polymers as replacements for vacuum-deposited amorphous silicon (a-Si) and printed small molecules. Among them, thiophene-based polymers have been most actively studied as a result of a higher charge-carrier mobility via a strong intermolecular overlap between thiophene units. The prototype conjugated polymer, regioregular poly(3-hexylthiophene) (rrP3HT), and its derivatives (i.e., pBTTT and PQT) showed impressive charge-carrier mobilities as high as 0.6 cm2V1s1.2 However, only a few conjugated polymers currently exhibit such high charge-carrier mobility, and most polymers still suffer from lower performance than amorphous silicon (a-Si) and soluble small molecules. Therefore, the development of high performance conjugated polymers for use as the active layer of OTFTs is worthy of interest in this field. The presence of vinylene linkages as a conjugated spacer in a backbone of aromatic polymers can reduce band gap and increase the degree of coplanarity by the decrease of the overall aromatic character3a,b and the limitation of the rotational disorder inherent between consecutive bulky aromatic units.3cf Thienylenevinylene (TV) based materials can show a high charge-carrier mobility as a result of a high planarity by introduction of vinylene with double bond character between thiophene units and a large intermolecular attractive force by the thiophene. Therefore, the introduction of a vinylene moiety between the thiophene units resulting in a TV is an attractive building block for the active layer of OTFTs or organic photovoltaics (OPVs).4ac Our group has reported an unsubstituted TV based p-type conjugated polymer (PETV12T) as an active layer for OTFTs and OPVs.4c r 2011 American Chemical Society
This polymer showed compact molecular packing and high crystallinity by improved interdigitation through wider side chain space. However, this polymer showed relatively poor solubility in common organic solvents because of highly packed molecular structures by interdigitation and insufficient alkyl side chain density. In similar cases, pBTTT and PQT, both of which have a wide space for side chain interdigitation, showed high crystallinity and charge-carrier mobility. Although those polymers showed high crystallinity by improved interdigitation through the introduction of unsubstituted conjugated spacer units such as fused aromatic groups or an unsubstituted moiety, they suffer from the limitations of solubility to various organic solvents.5 Therefore, the design strategy presented here for further improved processability and device performance is the introduction of an alkyl-substituted TV unit. The alkyl-substituted TV unit plays a role of planarizing and solubilizing conjugated thiophene units. In this communication, we report the synthesis and characterization of alkyl-substituted TV based conjugated polymers (PC6TV12T and PC12TV12T) with charge-carrier mobility of over 1 cm2V1s1 as an active layer for organic thin film transistors (OTFTs). The synthesis of PC6TV12T and PC12TV12T is shown in Scheme 1. The alkyl-substituted TV units were polymerized by Stille polycondensation with a 4,40 -bidodecylthiophene co-monomer. To avoid regioirregularities during polymerization, the synthetic preparation used two symmetrical monomers. The resulting polymers were purified by multiple Soxhlet. Compared with those of PQT-C12 (solubility in chloroform, ∼10 mg/mL, 80 C), P3HT (chloroform, ∼30 mg/mL, RT), and PETV12T Received: July 28, 2011 Revised: September 26, 2011 Published: October 18, 2011 4663
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Scheme 1. Chemical Structures of PC6TV12T and PC12TV12T and the Synthetic Route to Polymers
Figure 2. AFM phase images of (a) PC6TV12T and (b) PC12TV12T after annealing at 200 C. Scan size 1 μm 1 μm.
Figure 1. Out of plane XRD scans for spin-coated (a) PC6TV12T and (b) PC12TV12T with various annealing temperatures.
(chloroform, ∼10 mg/mL, 80 C), these polymers are much more soluble in common organic solvents such as chloroform (∼50 mg/mL, RT), chlorobenzene (∼50 mg/mL, RT), and tetrahydrofuran (∼40 mg/mL, RT). The number-average molecular weights of PC6TV12T and PC12TV12T were 14 700 and 24 900 g/mol, respectively, as determined by gel permeation chromatography (GPC) versus the polystyrene standard. Both polymers revealed two discrete endotherms upon heating, whereas only PC12TV12T exhibited two exotherms following cooling (see the Supporting Information). PC12TV12T showed two lower discrete endotherms than PC6TV12T as a result of the increased flexibility resulting from elongation of the alkyl side chains in these polymers. The highest occupied molecular orbital (HOMO) levels for both polymers were measured by cyclic voltammetry, using 0.1 M tetrabutylammonium perchlorate (Bu4NCl4). The HOMO levels of PC6TV12T and PC12TV12T were 4.99 and 5.02 eV, respectively. The HOMO levels of both polymers were ∼0.1 eV lower than rr-P3HT (see the Supporting Information). The details of the properties for both polymers are summarized in Table 1 of the Supporting Information. The thin film microstructural order was investigated by an outof plane X-ray diffraction (XRD) scan. Thin films of PC6TV12T and PC12TV12T were annealed at 110, 150, and 200 C. As shown in Figure 1, the out-of-plane scattering for annealed thin films of PC6TV12T and PC12TV12T showed high crystallinity with lamellar d-spacing of 19.6 and 24.2 Å, respectively, in the aaxis (a00) after the annealing process (30 min). In the case of PC6TV12T, d-spacing was observed similar to that previously reported for pBTTT-C12 (19.6 Å) and PQT-C12 (17.2 Å), indicating that side chains of PC6TV12T also had enough space for interdigitation or closely packed lamellar ordering by side chain tilting out of molecular plane despite the presence of a hexyl-substituted TV unit between dodecyl thiophene units.2b,c On the other hand, PC12TV12T had higher density of side chain attachment than typical interdigitated conjugated polymers and, therefore, showed end-to-end packing instead of interdigitation of the side chains. The d-spacing of annealed PC12TV12T
film (24.2 Å) was similar to that of poly(3-dodecylthiophene) (26.2 Å) with end-to-end packing.6 It was noted that PC12TV12T showed a very strong end-to-end lamellar ordering up to fifth (500) order as shown in Figure 1b. This was mainly due to a strong reorganization of the polymer chains by the annealing process. In addition, this result was consistent with the appearance of nanoribbons in the atomic force microscopy (AFM) images and means that closed packing and strong interdigitation of the side chain might not be essential factors for high lamellar ordering.7 The surface morphology of the polymer thin films was investigated by AFM. Figure 2 shows the phase mode AFM images of spin-coated PC6TV12T and PC12TV12T films after annealed at 200 C. Annealed PC12TV12T film showed a morphology with a more disorderly nanoribbon pattern, as compared with PC6TV12T. The disordered pattern in the nanoribbons of PC12TV12T could be due to a molecular weight that was higher than that of PC6TV12T.8 Similar to the nanoribbon structures of P3HT film, these images could be explained using two models such as the (i) chain folding9a and (ii) tie-molecules models.9b As shown in Figure 2a and b, nanoribbons of both polymers exhibited a similar diameter of about 19 nm. According to the chain folding model, a rough estimate would suggest that a nanoribbon of PC12TV12T (∼25 kg/mol, a chain length ∼37 nm) consists of almost one polymer chain. However, a PC6TV12T polymer with a relatively small molecular weight (∼15 kg/mol, a chain length ∼26 nm) could have more than one chain-end defect inside the nanoribbon. This means that PC6TV12T may have more chainend defects than PC12TV12T. In case of tie-molecules model, PC12TV12T with a relatively longer chain length will be connected by a sufficient interlamellar covalent bridge between adjacent crystalline lamella, whereas PC6TV12T will not be connected as such. A lack of interlamellar bridge between crystalline lamella and lamella could probably increase defects such as grain boundaries. Pristine PC6TV12T and PC12TV12T films showed the similar charge-carrier mobilities of as high as 0.29 and 0.24 cm2V1s1, respectively. The charge-carrier mobilities of both films were gradually improved after the annealing process. Maximum charge-carrier mobility of annealed PC6TV12T and PC12TV12T films measured in OTFTs showed 0.47 and 1.05 cm2V1s1, respectively, (Figure 3), and other device characteristics are summarized in Table 2 of the Supporting Information. The highly improved mobility in annealed devices can be attributed to a more orderly nature of the thin film microstructure than of the pristine sample. This result implies that polymer molecules formed a well-ordered crystalline phase in the channel by the annealing process, which is consistent with the aforementioned XRD and AFM data. The reason for the mobility difference between the two polymers could be considered as a difference in the chain-end defect density of one nanoribbon, as previously 4664
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’ REFERENCES
Figure 3. (a) Schematic of the top-gate/bottom contact transistor structure. (b) Saturation hole mobility and threshold voltage depends on thermal annealing temperature.
discussed in the AFM part using two possible models. To clarify this effect, further study on the correlation between mobility versus molecular weight is underway. PC6TV12T and P3HT devices were fabricated in bottom gate/top contact geometry to evaluate the oxidative stability. As a result, PC6TV12T devices are relatively stable, compared to P3HT devices, as a result of the lower HOMO level. The details of the device fabrication and stability results are described in the Supporting Information. Finally, complementary inverters were demonstrated by the inkjet-printing of PC12TV12T (p-type) and N2200 (n-type) polymers, respectively. DC voltage gains were obtained ∼16 at various VDD ranging from 20 to 100 V. The details of the fabrication conditions and characterization of the complementary inverter are noted in the Supporting Information. In conclusion, two polymers containing an alkyl-substituted TV unit between 3-dodecylthiophenes, PC6TV12T and PC12TV12T, were synthesized. Both polymers showed a high solubility to common organic solvents as well as thin film crystallinity due to a coplanar alkyl-substituted TV unit. These polymers showed a strong lamellar ordering and nanoribbon morphology after the annealing process. Their charge-carrier mobilities were measured as 0.47 and 1.05 cm2V1s1, respectively. Even though these polymers contain a high density of insulating alkyl side chains compared with similarly structured high mobility polymers such as pBTTT and PQT, comparable or even higher charge-carrier mobility could be observed in OTFTs. The high solubility of PC12TV12T enables the realization of inkjetprinted complementary inverter circuits through a highly viscous formulation. The inkjet-printed complementary inverters consisting of PC12TV12T and N2200 exhibited a gain of ∼16.
’ ASSOCIATED CONTENT
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bS
Supporting Information. Synthetic procedures and materials and devices characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was financially supported by the National Research Foundation of Korea (NRF), grant funded by the Korea government (MEST) (Grant No. 2011-0029858), NRF-2009-C1AAA0012009-0092950, an NCRC Grant No. R15-2008-006-02001-0, and WCU (World Class University) (R31-10026). We thank the Korea Basic Science Institute (KBSI) for AFM and EA measurements. 4665
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