Effect of Branched Conjugation Structure on the Optical

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26062

J. Phys. Chem. B 2006, 110, 26062-26067

Effect of Branched Conjugation Structure on the Optical, Electrochemical, Hole Mobility, and Photovoltaic Properties of Polythiophenes Erjun Zhou,†,‡ Zhan’ao Tan,†,‡ Lijun Huo,† Youjun He,† Chunhe Yang,† and Yongfang Li*,† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Graduate School, Chinese Academy of Sciences, Beijing 100039, China ReceiVed: August 23, 2006; In Final Form: October 20, 2006

Four branched polythiophenes (PTs) with different ratios of conjugated terthiophene-vinylene side chains, PT-TThV10 to PT-TThV40, were synthesized by Stille coupling reaction. The polymers exhibited reversible p-doping/dedoping (oxidation/re-reduction) and n-doping/dedoping (reduction/reoxidation) processes. The absorption spectra, hole mobility, and photovoltaic properties of the polymers were much improved in comparison with the PT derivative without the terthiophene-vinylene side chain and were influenced by the content of the conjugated side chains. With the increase of the content of the conjugated side chains, the absorption peak of the branched PTs enhanced and blue-shifted. The maximum hole mobility reached 6.35 × 10-4 cm2/V‚s (SCLC method) and the maximum power conversion efficiency of the polymer solar cell reached 1.91% under the illumination of AM 1.5, 100 mW/cm2, for the polymer with 20% terthiophenevinylene side chains. The results indicate that the branched PTs with suitable content of the terthiophenevinylene side chains could be promising photovoltaic materials.

1. Introduction Polymer solar cells (PSCs) have attracted strong interest in recent years due to their advantages of low cost, light weight, easy fabrication, and capability to fabricate flexible devices.1 The power conversion efficiency (PCE) of the PSCs acquired a significant improvement due to the introduction of so-called bulk-heterojunction concept,2 where a photoactive layer routinely consists of an interpenetrating network of π-conjugated polymer donors and soluble fullerene acceptors. The most successful system until now is a blend of regioregular poly(3hexylthiophene) (rr-P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).3 At present, the highest PCE of the PSCs based on P3HT and PCBM has reached 5%4 by thermal annealing after the device fabrication to have nanoscale control of the interpenetrating network morphology. To improve the PCE of the PSCs further for future commercial application, design and synthesis of new conjugated polymers with broad absorption in the visible region and high hole mobility are of crucial importance. To make the absorption spectrum of the conjugated polymers match the solar spectrum, some low band gap polymers5 and side-chain conjugated polymers6-10 have been synthesized. Among these polymers, the polythiophene (PT) derivative with conjugated bithienylenevinylene side chains showed broad absorption in the visible region from 350 to 650 nm and high PCE (3.18%).10 Moreover, to increase the hole mobility of the polymers, besides increasing molecular weight and decreasing polydispersity of polymers,11 cross-linking the polymer backbone by conjugated bridges to form network structure was proven to be an effective way.12-14 The cross-linking PTs with conjugated vinylene-phenylene-vinylene bridges and suitable cross-linking degree show increased hole mobility and increased PCE.12 We think the high PCE of PSCs based on the side-chain conjugated PTs is attributed to not only broad absorption but * Corresponding author. E-mail: [email protected]. Fax: 86-10-62559373. † Institute of Chemistry. ‡ Graduate School.

also high hole mobility because the conjugated side chains can also improve the electronic communication between polymer chains, like the cross-linking polymers. To investigate systematically the effect of the conjugated side chains on the optical, electrical, hole mobility, and photovoltaic properties of conjugated polymers, in this paper, we designed and synthesized a class of new branched PTs with terthiophene-vinylene side chains. We think the terthiophene-vinylene side chain should have significant benefits for the hole “hopping” from one polythiophene backbone to another in the polymer/fullerene bulk-heterojunction PSC structure.15 Figure 1 is the sketch diagram of the bulk heterojunction system, where the red bars show the increased hole transportation path from the conjugated side chains. The detailed studies of the branched PTs were presented in this paper. 2. Experimental Section 2.1. Materials. All reagents and chemicals were purchased from Alfa and Aldrich. Toluene and DMF were freshly distilled prior to use. The other materials were used without further purification. The following compounds were synthesized according to the literature procedures: 5-formyl-2,2′:5′,2′′-terthiophene (1),16 (2,5-dibromo-thiophen-3-ylmethyl)-phosphonic acid diethyl ester (3),6,7 2,5-dibromo-3-hexylthiophene (2),17 2,5bis(tributylstannyl)thiophene (4),16 and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).18 2.2. Instruments and Measurements. 1H NMR (400 MHz) spectra were measured on a Bruker spectrometer. Absorption spectra were taken on an Hitachi U-3010 UV-vis spectrophotometer. The molecular weight of polymers was measured by the GPC method, and polystyrene was used as a standard. The electrochemical cyclic voltammetry was conducted on a Zahner IM6e Electrochemical Workstation. A Pt plate coated with a thin polymer film was used as the working electrode. A Pt wire and an Ag/Ag+ electrode were used as the counter electrode and reference electrode, respectively. The current-voltage (IV) measurement of the PSC devices was conducted on a

10.1021/jp065442x CCC: $33.50 © 2006 American Chemical Society Published on Web 12/07/2006

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Figure 1. Sketch diagram of the polymer/fullerene bulk-heterojunction PSC based on the side chain conjugated PTs for increasing the hole transportation.

SCHEME 1: Synthetic Routes of the Monomer and Polymers

computer-controlled Keithley 236 Source Measure Unit. A Xenon lamp with AM 1.5 filter was used as the white light source, and the optical power at the sample was 100 mW/cm2. The thickness of films was determined by a surface profilometer (XP-2, USA). 2.3. Fabrication of Polymer Solar Cells. The PSCs were constructed in the traditional sandwich structure through the following steps. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, from Bayer AG) was spin-coated from its aqueous solution on a pre-cleaned indium tin oxide (ITO)/ glass substrate and dried at 150 °C for 10 min, giving a thickness of ca. 30 nm. The photosensitive blend layer was prepared by spin-coating a chlorobenzene solution of the polymers and PCBM (1:1. w/w) with the polymer concentration of 15 mg/ mL on the ITO/PEDOT:PSS electrode and dried at 80 °C for 30 min in a nitrogen-filled glovebox. The cathode of the devices, consisting of magnesium (10 nm) and aluminum (150 nm), was thermally deposited on top of the polymer film at 5 × 10-5 Pa. The active area of the devices is 4 mm2. 2.4. Synthesis. The synthetic routes of monomer and polymers are shown in Scheme 1. 5-[(E)-2-(2,5-Dibromo-thiophen-3-yl)-Vinyl]-2,2′:5′,2′′-terthiophene (A). Under an ice-water bath, compound 1 (1.38 g, 5 mmol) and compound 3 (1.96 g, 5 mmol) were dissolved in 30 mL of DMF, and then CH3ONa (0.6 g, 11 mmol) in 10 mL of DMF was added dropwise to the solution. After reaction for 2 h at room temperature, the solution was poured into cold CH3OH, filtered, and purified by column chromatography (silica gel, PE/CH2Cl2, 15:1) to give 1.16 g (45%) of product as a yellow powder. GC-MS: m/z ) 512. Anal. Calcd for C18H10 Br2S4: C, 42.03; H, 1.96; Br, 31.07; S, 29.94. Found: C, 41.89; H, 1.98; Br, 31.12; S, 29.83.

Synthesis of the Polymers. Pd(PPh3)4 (50 mg), compound A (x mmol), compound 2 ((1 - x) mmol), and compound 4 (0.67 g; 1.0 mmol) were put into a three-neck flask. The mixture was flushed with argon for 10 min, and then 10 mL of toluene was added. At the protection of argon, the reactant was heated to reflux for 12 h. The mixture was cooled to room temperature, poured into 30 mL of methanol, and then filtered into a Soxhlet thimble. Soxhlet extractions were performed with methanol, hexane, and CHCl3. The polymer was recovered from the CHCl3 fraction by rotary evaporation. The solid was dried under vacuum overnight. The yield, elemental analysis, and molecular weight of the four polymers are as follows. PT-TThV10: Yield: 55%. Anal. Calcd for [(C14H16S2)0.9 + (C22H12S5)0.1]: C, 66.52; H, 5.88; S, 27.60. Found: C, 65.30; H, 5.75; S, 26.97. Mn ) 34 K; polydispersity ) 1.48. PT-TThV20: Yield: 52%. Anal. Calcd for [(C14H16S2)0.8 + (C22H12S5)0.2]: C, 65.50; H, 5.36; S, 29.14. Found: C, 64.53; H, 5.50; S, 28.27. Mn ) 36 K; polydispersity ) 1.56. PT-TThV30: Yield: 45%. Anal. Calcd for [(C14H16S2)0.7 + (C22H12S5)0.3]: C, 64.61; H, 4.89; S, 30.50. Found: C, 64.09; H, 5.07; S, 29.32. Mn ) 40 K; polydispersity ) 1.74. PT-TThV40: Yield: 42%. Anal. Calcd for [(C14H16S2)0.6 +(C22H12S5)0.4]: C, 63.82; H, 4.48; S, 31.70. Found: C, 63.64; H, 4.87; S, 30.53. Mn ) 43 K; polydispersity ) 1.51. 3. Results and Discussions 3.1. Synthesis of the Branched PTs. The synthesis routes of monomer A and the branched PTs are shown in Scheme 1. Monomer A was synthesized by Wittig-Hornor reaction.19,6-10 Figure 2 shows the 1H NMR spectrum of monomer A, where all the peaks were assigned to their corresponding hydrogen atoms according to the integral area, coupling constant, peak shape, and experience. Moreover, it can be seen there is no cis isomer of the compound.

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Figure 2. 1H NMR spectrum of monomer A.

Figure 3. 1H NMR spectra of the four branched polythiophenes.

The branched PTs were synthesized by Stille coupling reaction20 by adding different ratios of monomer A and 2,5dibromine-3-hexylthiophene. The products are all dark-red powder with the number-average molecular weight of 34-43 kg/mol and polydispersity of ca. 1.48-1.74. The 1H NMR spectra of the four polymers are shown in Figure 3. The peaks between 2.5-2.8 ppm are corresponding to the hydrogen atoms at position 1 of the hexyl substituent (see Figure 3). Since there is only one kind of hydrogen at that position, the two peaks (peak 1a and peak 1b) should be attributed to the difference of regioregularity of the polymers. The regioregularity of the four branched PTs with the terphiophene-vinylene side chain ratio of 10-40% is 65%, 65%, 67%, and 59%, respectively, which were calculated from the ratio of the integral area of peak 1a to the sum of the integral area of peaks 1a and 1b.21

3.2. Optical Properties. Figure 4 shows the UV-visible absorption spectra of the four branched PTs together with that of monomer A. From Figure 4a, we can see that, with the increase of the content of terthiophene-vinylene side chains, the absorption peaks of the branched PTs enhanced and shifted toward the absorption of monomer A (with the same concentration of the polymer solutions). A similar trend is found for the polymer films. The peak positions of the absorption spectra are listed in Table 1. The absorption spectra exhibit a pronounced red shift from solutions to solid films, which should result from the interchain interaction of the polymers in the solid state. The shoulder in the absorption spectrum in the dilute solution around 600 nm (see Figure 4a) indicates that aggregation of the polymer molecules occurs in the chloroform solution. The effect of the content of the terthiophene-vinylene side chains on the absorp-

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Figure 5. Cyclic voltammograms of the four branched PTs films on platinum plates in an acetonitrile solution of 0.1 mol/L [Bu4N]PF6 (Bu ) butyl) with a scan rate of 0.1 V/s.

re-reduction) and n-doping/dedoping (reduction/reoxidation) processes. From PT-TThV10 to PT-TThV40, the onset oxidation potentials and onset reduction potentials shifted positively a little. The HOMO energy level, LUMO energy level, and the electrochemical band gaps (EEC g ) can be calculated from the onset oxidation and reduction potentials according to the following equations.22

HOMO ) -e(Eox on + 4.71) (eV) LUMO ) -e(Ered on + 4.71) (eV) Figure 4. UV-vis absorption spectra of (a) the four branched PTs and monomer A in chloroform with the concentration of ca. 5.0 × 10-5 M (repeat unit) and (b) the polymer films on a quartz plate spincoated from chloroform solutions.

TABLE 1: Optical and Electrochemical Properties of the Branched PTs UV-vis absorption spectra

cyclic voltammetry

p-doping n-doping solution film Eox Ered EEC on (V)/ on (V)/ g branched PTs λmax (nm) λmax (nm) HOMO (eV) LUMO (eV) (eV) PT-TThV10 PT-TThV20 PT-TThV30 PT-TThV40

466 459 444 441

512 521 454 456

0.36/-5.07 0.36/-5.07 0.38/-5.09 0.38/-5.09

-1.99/-2.72 -1.96/-2.75 -1.92/-2.79 -1.87/-2.84

2.35 2.32 2.30 2.25

tion spectra provides a simple method to modulate the absorption spectra of the photovoltaic conjugated polymers. The polythiophene derivatives with conjugated side chains of phenylene-vinylene or thienylene-vinylene show two absorption peaks in the wavelength range of 300-700 nm,6,9,10 but the absorption spectra of the branched PTs with the conjugated side chains of terthiophene-vinylene possess only one broad peak in the wavelength range. Probably, the spectral overlap of the absorption of the terthiophene-vinylene side chain with that of the PT main chain (distorted by steric hindrance of the bulky tertiophene side chain) resulted in the one broad absorption band since the absorption peak of the terthiophene-vinylene side chain is red-shifted to ca. 410 nm in comparison with the UV absorption peaks of the bi(phenylene-vinylene) and the bi(thienylene-vinylene) side chains. 3.3. Electrochemical Properties. The electrochemical properties of the branched PT films on Pt electrode were investigated by cyclic voltammetry (CV). As shown in Figure 5, the four polymers show similar reversible p-doping/dedoping (oxidation/

ox red EEC g ) (Eon - Eon ) (eV)

The results of the electrochemical measurements are also listed in Table 1. With the increase of the content of terthiophene-vinylene side chains, the electrochemical band gap of the branched PTs decreased a little. The electrochemical band gap of the branched PTs is higher than that of the polythiophene derivatives with the conjugated bithienylenevinylene side chains,10 which indicates that bithienylenevinylene side chains are more effective at decreasing the band gap of the side-chain conjugated PTs than terthiophene-vinylene side chains. 3.4. Hole Mobility. Hole mobility is an important parameter for photovoltaic application of the conjugated polymers. Here, the effect of the terthiophene-vinylene side chain on the charge transport of the polymers was investigated, and the hole mobility was measured with the space-charge-limited current (SCLC) model23 with a device structure of ITO/PEDOT:PSS/polymer/ Au. The results are plotted as ln(JL3/V2) vs (V/L)0.5, as shown in Figure 6. Hole mobility was calculated from the intercept of the corresponding lines on the axis of ln(JL3/V2);23 it is 1.16 × 10-4, 6.35 × 10-4, 3.48 × 10-4, and 4.27 × 10-5 cm2/V‚s for PT-TThV10 to PT-TThV40, respectively. The hole mobility of the branched PTs are much higher than that of poly(3hexylthiophene-co-thiophene) (5.23 × 10-6 cm2/V‚s)12 without the terthiophene-vinylene side chains. These data indicate that the branched conjugation structure can improve the hole mobility largely. In addition, the content of the terthiophene-vinylene side chains influenced the hole mobility of the branched PTs greatly. With the increase of the conjugated side chain content from PT-TThV10 to PT-TThV20, the hole mobility increased from 1.16 × 10-4 to 6.35 × 10-4 cm2/V‚s, but as the content of the terthiophene-vinylene side chains further increased over 20%,

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Zhou et al. 3.5. Photovoltaic Properties. The higher hole mobility and broader absorption band of the branched PTs suggest that they might be good photovoltaic polymers. The bulk heterojunctiontyped polymer solar cells (PSCs) were fabricated with a structure of ITO/PEDOT:PSS (30 nm)/polymer:PCBM (1:1 wt %)/Mg (10 nm)/Al (150 nm), for studying the photovoltaic properties of the polymers. The photovoltaic data of poly(3-hexylthiophene-co-thiophene) (PT-TThV0) were also measured for comparison. The film thickness of the five photosensitive blend layers of PT-TThV0-40 and PCBM was 91, 73, 83, 72, and 82 nm, respectively. Figure 7 shows I-V curves of the devices and Table 2 lists the corresponding open-circuit voltage (Voc), short-circuit current

Figure 6. Current-voltage data from the devices of ITO/PEDOT: PSS/polymer/Au, plotted in the format ln(JL3/V2) vs (V/L)0.5, where J is the current density and L is the thickness of the polymer layer. The lines are the fit to the respective experimental points.

TABLE 2: Photovoltaic Performance of the Polymer Solar Cells Based on the Branched PTs polymers

Voc (V)

Isc (mA/cm2)

F.F. (%)

PCE (%)

PT-TThV0 PT-TThV10 PT-TThV20 PT-TThV30 PT-TThV40

0.68 0.58 0.73 0.74 0.63

3.80 6.58 6.85 5.69 6.42

34 39 38 39 36

0.87 1.47 1.91 1.65 1.47

the hole mobility decreased, to the value of 3.48 × 10-4 cm2/ V‚s for PT-TThV30 and 4.27 × 10-5 cm2/V‚s for PT-TThV40. Probably, the steric hindrance of the big terthiophene-vinylene side chains cause the PT main chains to be distorted and result in the decrease of the hole mobility when the content of the conjugated side chains exceeded 20%.

Figure 7. I-V curves of the polymer solar cells based on the branched PTs under the illumination of AM 1.5, 100 mW/cm2.

Figure 8. AFM images showing the surface morphology of the blend films of (a) PT-TThV10/PCBM, (b) PT-TThV20/PCBM, (c) PT-TThV30/ PCBM, and (d) PT-TThV40/PCBM; the blend films were spin-coated from their chlorobenzene solutions.

Branched Conjugation Structure and Polythiophenes (Isc), fill factor (F.F.), and power conversion efficiency (PCE) of the devices under the illumination of AM 1.5, 100 mW/cm2. The PCE of the PSCs based on PT-TThV0 to PT-TThV40 are 0.87%, 1.47%, 1.91%, 1.65%, and 1.47%, respectively. (The photovoltaic properties of PT-TThV0 were optimized here and improved in comparison with the results reported before.12) The highest Isc of PT-TThV20 (6.85 mA/cm2) should be attributed to the highest hole mobility of PT-TThV20. The higher PCE of PT-TThV20 (1.91%) is almost twice that of PT-TThV0 (0.87%). However, the fill factor of the PSCs based on the branched PTs is still low (lower than 40%), even though the hole mobility of the polymers is improved, which indicates that the fill factor of the PSCs may be limited by the interface resistance between the photosensitive layer and the electrodes. In addition, Voc values of the PSCs based on PT-TThV10 and PT-TThV40 are obviously lower than those of other devices, which is not well understood at present. We are doing more experiments and hope to give a reasonable explanation before long. It should be pointed out that the efficiencies reported in Table 2 are the best values for the corresponding devices. We fabricated six devices for each polymer in the investigation of the photovoltaic properties. The average efficiencies of the six devices were 0.84% for PT-TThV0, 1.45% for PT-TThV10, 1.86% for PT-TThV20, 1.55% for PH-TThV30, and 1.41% for PT-TThV40. The average values deviate from the best values within 10% and they follow the same changing tendency in considering the effect of the content of the terthiophenevinylene side chains on the photovoltaic properties of the polymers. As we know, the morphology of the thin solid film can also affect the photovoltaic properties of PSCs.4 Here, we investigated the morphology of the blend films of the polymers and PCBM by atomic force microscopy (AFM). Figure 8 shows the AFM photographs of the four blend films. It can be seen that all the samples show relatively small roughness and no significant aggregation is formed. These observations suggest that the branched PTs with different content of terthiophenevinylene side chains are all highly compatible with PCBM molecules. The results also indicate that the different photovoltaic properties of the polymers were not caused by the morphology difference, but resulted from the nature of the polymer structures. 4. Conclusion We synthesized four branched polythiophenes (PTs) with different ratios of conjugated terthiophene-vinylene side chains. The polymers show reversible p-doping/dedoping (oxidation/ re-reduction) and n-doping/dedoping (reduction/reoxidation) processes. The absorption spectra, hole mobililty, and photovoltaic properties of the polymers PT-TThV10-40 were much improved in comparison with those of PT-TThV0 without the terthiophene-vinylene side chain and are influenced by the content of the conjugated side chains. With the increase of the content of the conjugated side chains, the absorption peak of the branched PTs enhanced and blue-shifted. The hole mobility of the polymers increased with the increased of the content of the conjugated side chains from 1.16 × 10-4 cm2/V‚s for PTTThV10 to 6.35 × 10-4 cm2/V‚s for PT-TThV20, but it decreased as the content of the terthiophene-vinylene side chains further increased over 20%. The photovoltaic properties of the

J. Phys. Chem. B, Vol. 110, No. 51, 2006 26067 polymers show a similar tendency as that of the hole mobility in considering the influence of the content of the terthiophenevinylene side chains. The maximum power conversion efficiency (PCE) of the polymer solar cell (PSC) based on PT-TThV20 reached 1.91% under the illumination of AM 1.5, 100 mW/ cm2. The results indicate that the branched PTs could be promising photovoltaic materials. Acknowledgment. This work was supported by NSFC (Nos. 50373050, 20373078, 20474069, 20421101, 20574078, and 50633050) and the Ministry of Science and Technology of China (973 project, No. 2002CB613404) References and Notes (1) (a) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (b) Brabec, C. J. Sol. Energy. Mater. Sol. Cells 2004, 83, 273. (c) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (d) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005, 30, 10. (2) Yu, G.; Hummelen, J.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (3) (a) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85. (b) Al-Ibrahima, M.; Rotha, H.-K.; Zhokhavetsb, U.; Gobsch, G.; Sensfussa, S. Sol. Energy Mater. Sol. Cells 2005, 85, 13. (c) Reyes-Reyes, M.; Kim, K.; Dewald, J. Lo´pez-Sandoval, R.; Adadhanula, A.; Curran, S.; Carroll, D. L. Org. Lett. 2005, 7, 5749. (4) (a) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (b) Reyes-Reyes, M.; Kim, K.; Carrolla, D. L. Appl. Phys. Lett. 2005, 87, 83506. (5) (a) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077. (b) Smith, A. P.; Smith, R. R.; Taylor, B. E.; Durstock, M. F. Chem. Mater. 2004, 16, 4687. (c) Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan, A.; van Hal, P. A.; Janssen, R. A. J. AdV. Funct. Mater. 2002, 12, 709. (d) Zhang, F. L.; Perzon, E.; Wang, X. J.; Mammo, W.; Andersson, M. R.; Ingana¨s, O. AdV. Funct. Mater. 2005, 15, 745. (e) Svensson, M.; Zhang, F. L.; Veenstra, S. C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Ingana¨s, O.; Andersson, M. R. AdV. Mater. 2003, 15, 988. (f) Zhou, Q. M.; Hou, Q.; Zheng, L. P.; Deng, X. Y.; Yu, G.; Cao, Y. Appl. Phys. Lett. 2004, 84, 1653. (g) Wang, F.; Luo, J.; Yang, K. X.; Chen, J. W.; Huang, F.; Cao, Y. Macromolecules 2005, 38, 244. (6) Hou, J. H.; Huo, L. J.; He, C.; Yang, C. H.; Li, Y. F. Macromolecules 2006, 39, 594. (7) Zhou, E. J.; Hou, J. H.; Yang, C. H.; Li, Y. F. J. Polym. Sci., Part. A: Polym. Chem. 2006, 44, 2206. (8) Zhou, E. J.; He, C.; Tan, Z. A.; Yang, C. H.; Li, Y. F. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4916. (9) Hou, J. H.; Yang, C. H.; Li, Y. F. Chem. Commun. 2006, 871. (10) Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y. F. J. Am. Chem. Soc. 2006, 128, 4911. (11) (a) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macromol. Rapid Commun. 2004, 25, 1229. (b) Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Chem. Mater. 2005, 17, 2175. (12) Zhou, E. J.; Tan, Z. A.; Yang, C. H.; Li, Y. F. Macromol. Rapid Commun. 2006, 27, 793. (13) Hittinger, E.; Kokil, A.; Weder, C. Angew. Chem., Int. Ed. 2004, 43, 1808. (14) Weder, C. Chem. Commun. 2005, 5378. (15) Sylvester-Hvid, K. O.; Rettrup, S. J. Phys. Chem. B 2004, 108, 4296. (16) Wei, Y.; Yang, Y.; Yeh, J. M. Chem. Mater. 1996, 8, 2659. (17) Hou, J. H.; Yang, C. H.; Qiao, J.; Li, Y. F. Synth. Met. 2005, 150, 297. (18) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F. J. Org. Chem. 1995, 60, 532. (19) Collis, G. E.; Burrell, A. K.; Officer, D. L. Tetrahedron Lett. 2001, 42, 8733. (20) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (21) Andersson, M. R.; Mammo, W.; Olinga, T.; Svensson, M.; Theander, M.; Ingana¨s, O. Synth. Met. 1999, 101, 11. (22) Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J. Mater. Chem. 2003, 13, 800. (23) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, C. Phys. ReV. B 1998, 58, 13411.