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J. Phys. Chem. C 2008, 112, 7866–7871
Control in Energy Levels of Conjugated Polymers for Photovoltaic Application† Yongye Liang, Shengqiang Xiao, Danqin Feng, and Luping Yu* Department of Chemistry and The James Franck Institute, The UniVersity of Chicago, 929 E. 57th Street, Chicago, Illinois 60637 ReceiVed: December 10, 2007; In Final Form: January 28, 2008
A series of copolymers based on thieno [3,4-b]thiophene and thiophene unit have been synthesized. By controlling the ratio of thieno [3,4-b]thiophene to alkyl thiophene in the copolymer composition, the electrooptic properties of the copolymers can be fine tuned. It was shown that the energy gap of copolymers narrowed when the content of thieno [3,4-b]thiophene increased, brought by the decrease in lowest unoccupied molecular orbital and increase in highest occupied molecular orbital energy levels. When these copolymers were blended with [6,6]-phenyl-C61-butyric acid methyl ester to form solar cell’s active layer, an optimized copolymer composition was found that gave the best photovoltaic performance. Introduction New photovoltaic (PV) systems exhibiting low cost and high efficiency in solar energy conversion are strategically important and actively pursued worldwide.1 Recent surge in exploration of alternative PV materials based on organic or organic/inorganic hybrid materials illustrates the importance of these research activities.2 Impressive progress has been made in engineering organic devices with high solar conversion efficiency. To further enhance the efficiency of light conversion to electricity, several unique fundamental processes in organic photovoltaic devices, including light absorption, exciton generation/migration/dissociation and charge transport, need to be optimized. Most of the disclosed polymers only harvest photons with a wavelength below about 650 nm, which is a small portion of the solar spectrum.3 Therefore, polymers with low band gaps, which can efficiently harvest the energy in whole solar spectrum, are attractive to improve the efficiency of organic photovoltaics (OPVs). However, several reported low band gap polymer PV systems showed efficiencies much lower than the wide band gap counterparts.4 The energy level mismatch between electron donating polymer and electron acceptor, like [6,6]-phenyl-C61butyric acid methyl ester (PCBM), small light absorption coefficient and low hole mobility are several reasons for the low efficiency.4Most recently, several low band gap polymer systems with PCBM as the acceptor have achieved better efficiency by tuning the energy level of the polymers through modifying the monomer structures based on the known thienopyrazine or benzothiadiazole systems.5 To further increase the photovoltaic performances for practical application, it is important to synthesize polymers with the possibility to tune their band gap, energy levels, and other physical properties. In this paper, we report a strategy to control the bandgap and redox potential of new polymers for optimized PV applications. The strategy is to introduce thieno[3,4-b]thiophene structure to the polythiophene backbone to lower the band gap of the polymers (Scheme 1). It was shown previously that thieno[3,4-b]thiophene structure can stabilize the quinoidal structure of the backbone, which reduces the band gap of the conjugated system.6–8 We reasoned that incorporation of the † Part of the “Larry Dalton Festschrift”. * To whom correspondence should be addressed. E-mail: lupingyu@ uchicago.edu.
thienothiophene moiety via copolymerization at different ratio will bring different degree of qunoidal component to the conjugated system and thus provide a means to tune the energy levels and energy gap. Indeed, we found that the energy level and absorption spectra of copolymers containing alkyl thiophene and thieno[3,4-b]thiophene can be fine tuned by changing the ratio between these two components. There is an optimized composition that gives impressive light conversion efficiency under ambient condition. Experimental Section General. Unless otherwise specified, all chemicals and reagents were purchased from commercial suppliers and used without further purification. Toluene was distilled over sodium prior to use. 2,5-Dibromo-3-hexylthiophene,9 2,5- bis(tributylstannyl)thiophene10 and 4,6-dibromo-thieno[3,4-b]thiophene-2carboxylic acid11 were synthesized according to the procedure reported in literatures. The regioregular poly(3-hexylthiophene) (P3HT) was synthesized and purified according to the method reported by McCullough and co-workers,12 and the obtained P3HT has Mn ) 90 K and PDI ) 1.31 with regioregularity >95%. PCBM was synthesized using the method reported by Wudl and co-workers.13 1H NMR spectra were recorded at 400 or 500 MHz (13C spectra at 100 or 125 MHz) on Bruker DRX400 or DRX-500 spectrometers, respectively. Molecular weights and distributions of polymers were determined by using gel permeation chromatography (GPC) with Waters Associates liquid chromatograph equipped with a Waters 510 HPLC pump, a Waters 410 differential refractometer, and a Waters 486 tunable absorbance detector. Tetrahydrofuran was used as the eluent and polystyrene as the standard. The optical absorption spectra were taken by a Hewlett-Packard 8453 UV-vis spectrometer. Thin films of the polymers were spin-coated from their solutions in chlorobenzene, and the film absorption spectra were measured. The electrochemical properties of the polymers were investigated by using cyclic voltammetry (CV). The polymer thin films coated on glass carbon electrode were studied in a 0.10 M Bu4NPF6 acetonitrile solution with scan rate at 100 mV/s. The Ag/Ag+ reference electrode was calibrated with ferrocene.14 Synthesis. The synthetic routes of monomers and polymers are shown in Scheme 1.
10.1021/jp711623w CCC: $40.75 2008 American Chemical Society Published on Web 04/11/2008
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SCHEME 1: Synthesis of Polymers by the Stille Coupling Reaction
2-Methyl-tetradecan-2-ol (ROH). To dodecylmagnesium bromide solution (25 mL, 2.0 M) in ether was dropwise added acetone (2.32 g, 40 mmol) in anhydrous ether (40 mL) in an ice water bath. The reaction mixture was stirred in an ice water bath for 1 h and then at room temperature for 8 h. The mixture was quenched with 1 N HCl solution and extracted with ether. The organic phase was washed with brine and dried with sodium sulfate. After removal of the solvent, the residue purified by column chromatography (silica gel, ether/methylene chloride ) 1:5). The pure product (compound 1) obtained is a colorless liquid 5.37 g (78%). 1HNMR (CDCl3): δ 0.87-0.89 (3H, t, J ) 7 Hz), 1.21 (7H, s), 1.23-1.36 (20H, m), 1.44-1.47 (2H, m). MTTD. To the mixture of 4,6-dibromo-thieno[3,4-b]thiophene2-carboxylic acid (0.68 g, 2.0 mmol), DCC (0.50 g, 2.4 mmol), DMAP (84 mg, 0.69 mmol) in a 10 mL round-bottom flask with 5 mL CH2Cl2 were added ROH (2.84 g, 10.0 mmol). The mixture was stirred for 20 h under N2 atmosphere, was poured into 30 mL water, and then extracted with CH2Cl2. The organic phase was dried with sodium sulfate and the solvent was removed. The product was purified with column chromatography on silica gel using hexane/CH2Cl2 ) 4:1, yielding the pure compound as a light orange solid 0.63 g (75%). 1HNMR (CDCl3): δ 0.86-0.89 (3H, t, J ) 7 Hz), 1.22-1.38 (20H, m), 1.56 (6H, s), 1.82-1.86 (2H, m), 7.44 (1H, s). General Procedure of Stille Coupling Polymerization. The monomer (276 mg, 0.50 mmol for MTTD) was weighed into a 25 mL round-bottom flask. 2,5-Bis(tributylstannyl)thiophene (331 mg, 0.50 mmol) and Pd(PPh3)4 (25 mg) were added. The flask was subjected to three successive cycles of vacuum followed by refilling with argon. Then, anhydrous dimethylformamide (DMF) (2 mL) and anhydrous toluene (8 mL) were added via a syringe. The polymerization was carried out at 120 °C for 24 h. The raw product was collected by precipitating in methanol. The precipitate was dissolved in chloroform and filtered with Celite to remove the metal catalyst. The final polymers were obtained by precipitating in hexanes and drying in vacuum for 12 h. Other copolymers are synthesized according to the similar procedure when MTTD and 2,5-dibromo-3hexylthiophene were added at the same time with different ratio. The yield, molecular weight, and polydispersity (PDI) of the polymers were measured as PTTD: Yield, 60%; Mn ) 56 K; PDI ) 1.76. A: Yield, 50%; Mn ) 95 K; PDI ) 2.90. B: Yield, 53%; Mn ) 46 K; PDI ) 2.37. C: Yield, 58%; Mn ) 132 K; PDI ) 2.47. D: Yield, 52%; Mn ) 47 K; PDI ) 2.50.
Fabrication and Characterization of Polymer Solar Cells. The ITO-coated glass substrate was cleaned stepwise in water, acetone, and isopropyl alcohol under ultrasonication for 30 min and subsequently dried by using Argon gas. The substrate was further cleaned with ultraviolet ozone for 10 min. A layer of highly conducting poly (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT-PSS) (Baytron PHCV4 from H. C. Starck) was spin-cast with a thickness of ∼100 nm (4000 rpm) from aqueous solution on ITO surface. The substrate was dried for 1 h at 60 °C under vacuum and then cooled by Argon steam. The chlorobenzene solution composed of polymer and PCBM in 1:1 mass ratio (10 mg/mL for polymer) was then spin-casted on top of the PEDOT-PSS layer. The thickness of the polymer film was controlled to about 100 nm by varying the rotation speed of spin-coating. Subsequently, the device was pumped down in vacuum (10-6 torr), and an 80 nm thick Al electrode was deposited on the top. The area of deposited Al electrode was defined as 3.14 mm2 by a shadow mask. Current-voltage measurements were recorded with a selfmade source unit under ambient atmosphere. A 150 W Oriel Xenon lamp with an AM1.5G filter was used as the light source without calibrating the mismatch of the simulated spectrum with the solar spectrum. Hole mobility was measured by using a diode configuration of ITO/PEDOT-PSS/Polymer/Al by taking current-voltage current in the range of 0-6 V and fitting the results to a space charge limited form, the SCLC is described by:
J ) 90rµV2 ⁄ 8L3
(1)
In the equation, 0 is the permittivity of free space, r is the dielectric constant of the polymer, µ is the hole mobility, V is the voltage drop across the device, and L is the polymer thickness. V ) Vappl - Vr - Vbi, where Vappl is the applied voltage to the device, Vr is the voltage drop due to contact resistance and series resistance across the electrodes, and Vbi is the built-in voltage due to the difference in work function of the two electrodes. The resistance of the device was measured using a blank configuration ITO-PEDOT-Al and was found to be about 10-20 Ω. The Vbi was deduced from the best fit of the J0.5 versus Vappl plot at voltages above 2.5 V and is found to be about 1.5 V. The dielectric constant, r is assumed to be 3 in our analysis, which is a typical value for conjugated polymers. The thickness of the polymer films is measured by AFM.
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Figure 1.
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Liang et al.
HNMR spectra of copolymers A-D for determination of m/n ratio.
TABLE 1: Copolymers’ Composition (m/n Ratio) and Molecular Weight polymer
PTTD
A
B
C
D
feed m/n ratio final m/n ratio Mn (K) PDI
1:0 1:0 56 1.76
1:1 1:1.0 95 2.90
1:2 1:1.9 46 2.37
1:4 1:3.7 132 2.47
1:8 1:6.3 47 2.50
Results and Discussion Synthesis of the Polymers. The thieno[3,4-b]thiophene monomer was synthesized according to a modified method based on the reported approaches.15 A long tertiary alkyl side chain alcohol is used to form ester group to increase the solubility of polymers. The polymers were synthesized according to scheme 1 via the palladium-catalyzed Stille polycondensation at 120 °C for 24 h, using Pd(PPh3)4 as the catalyst and toluene/DMF mixture as the solvent. The polymer PTTD is an alternating copolymer of thieno [3,4-b]thiophene and thiophene, and the copolymers A-D were random copolymers in which the composition was controlled by adjusting the addition ratio m/n between thieno[3,4-b]thiophene (MTTD) and 2,5-dibromo-3hexylthiophene. The actual m/n values in the copolymers were determined from the 1HNMR spectra. In the spectra, there are peaks ranging from 2.7-3.1 ppm, which are attributed to the hydrogens at position 1 from hexyl thiophene, while there is a peak at 2.15 ppm, which corresponds to the hydrogens at position 2 from the thieno[3,4-b]thiophene (Figure 1). By comparing the integral areas of these peaks, the m/n ratio is obtained. The comparison of the feed m/n ratio and final m/n ratio determined by 1HNMR spectra of the polymers is listed in Table 1. The molecular weights of the copolymers were determined by using gel permeation chromatography (GPC). The Mw values of the copolymers PTTD and A-D all exceed 9 × 104 g/mol. PTTD and copolymers A-D are readily soluble in organic solvents such as chloroform and chlorobenzene, while 5,5′-poly(3-hexyl2,2′-bithiophene) synthesized in this method exhibits limited solubility in chloroform. Optical Properties. The absorption spectra of polymer films were shown in Figure 2. For comparison, the absorption spectrum of thin film of regioregular P3HT, synthesized according to McCullough’s method,12 was shown in the same figure. It is clear that the copolymers exhibit the maximum absorption at longer wavelength than P3HT. The copolymer PTTD has a absorption range between 500 and 1000 nm. When
Figure 2. Film absorption spectra of the polymers and P3HT.
the thieno-thiophene ratio (m/n value) increased from polymer D to PTTD, the absorption peak of the corresponding copolymer shifted to longer wavelength. The band gaps of the polymers were calculated from the onset point of the absorption spectra (Table 2). The results showed that copolymers have smaller band gaps than P3HT, and the band gap of the polymer becomes smaller when m/n value increases. Electrochemical Properties. The electrochemical properties of the polymers were investigated by using cyclic voltammetry (CV) with polymer thin films coated on glass carbon electrode. The redox potential of Fc/Fc+ in the same condition is located at 0.08 V, which is assumed to have an absolute energy level of -4.80 eV to vacuum for calibration.14 So the energy levels of highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), as well as the band gaps were calculated according to the following equations:
EHOMO ) -(φox + 4.72)(eV) ELUMO ) -(φred ) 4.72)(eV)
(2) (3)
Egec ) ELUMO - EHOMO
(4)
where φox is the onset oxidation potential versus Ag/Ag+, and φred is the onset reduction potential versus Ag/Ag+. The HOMO and LUMO results of the polymes were summarized in Figure 3. It is clear that the HOMO level of the copolymers rises when the m/n ratio increases. HOMO of PTTD is about 0.3 eV higher than that of P3HT. The LUMO level of the polymers lowers when the m/n ratio increases. LUMO of PTTD is about 0.8 eV lower than P3HT. The band gap values calculated from CV are
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TABLE 2: Optical and Electrochemical Band Gap of the Polymer Films polymer
PTTD
A
B
C
D
P3HT
Egopt (eV) Egec (eV)
1.22 1.11
1.27 1.26
1.31 1.67
1.45 1.88
1.78 2.04
1.91 2.22
consistently larger than the optical band gap, but with the same trend that the band gap of the polymer becomes smaller when the corresponding m/n ration increases. These results demonstrate that the electrochemical properties of polymers can indeed be tuned by changing the polymer composition. Hole Mobility. Hole mobility in the polymer is an important factor for the performance of polymer solar cells. The high hole mobility of the polymer allows thicker solar cell devices and can improve the efficiency of the polymer solar cells.16 We used space charge limited current (SCLC) model to determine the hole mobility in these polymers.17 The results are plotted in Figure 4 as J0.5 versus (Vappl - Vbi - Vr). The hole mobility of 2.7 × 10-4, 1.5 × 10-4, 8.4 × 10-5, 8.4 × 10-5, 7.8 × 10-5, and 4.8 × 10-5 cm2/V.s are found for P3HT, PTTD, A, B, C, and D, respectively. Even though the thienothiophene unit contains a tertiary ester side group that brings steric effect to prevent polymer chains from effective π-π stacking, the mobility of PTTD is still comparable to well-packing P3HT. It can be suggested that because the fused thiophene ring can stabilize the quinoidal structure in the polymer chain, the planarity along the aromatic backbone is enhanced, which improves the delocalization of the π electrons. When the thienothiophene ratio decreases in the copolymers from A to D, the hexyl groups in the copolymer are irregularly attached to the backbone, which causes twist in backbone and leads to the decreases in the hole mobility. The hole mobility in poly(3hexylthiophene-co-thophene) also obtained from the SCLC model is reported to be 5.2 × 10-6 cm2/V.s,18 much smaller than those in copolymers A-D, which shows that the thienothiophene structure may increase the hole mobility. Film Topography. In the polymer solar cells, a heterojunction structure is formed by the phase separation in the blend of polymers and fullerene derivative. Large interfacial interaction is needed for the efficient charge separation, and at the same time, bicontinuous network is necessary for charge transport and extraction from the device before recombination. As a result, the morphology of the heterojunction plays an important role on the performance of polymer solar cells. Recent reports showed that the control of morphology can increase the power conversion efficiency20 and stability of solar cells.21 We studied the topography of blend films of polymers and PCBM (1:1, w/w) by atomic force microscopy (AFM). The images of film surfaces topography are showed in Figure 5. The roughness of the blend films decreases from PTTD, A to D, and P3HT has the most smooth surface. The root-mean-square roughness for PTTD blend is 1.43 nm, and P3HT blend is only 0.36 nm. It has been reported that the presence of the long alkyl chains may decrease the miscibility of PCBM into conjugated polymer domain.22 The more rough surface observed in PTTD may be due to the existence of the long tertiary alkyl chain from the thienothiophene unit, which reduces the miscibility with PCBM. When the thieno-thiophene ratio decreases in the polymers, the film becomes smoother. As the sizes of the feature domains are compared, it was found that polymer C blend film has the largest feature size, which is a signal for better photovoltaic performance.20 It has been proposed that certain degree of disorder in polythiophenes can improve the miscibility with fullerene and prefer the formation of bicontinuous interpenetrating
Figure 3. Electrochemical properties of the polymer films.
Figure 4. J0.5 vs V plots for the polymer films. The thickness of the films is indicated in the plots. The solid lines are fits of the data points.
network in nanoscale phase separation.23 The moderate disorder in C may favor the formation of bicontinuous network. Photovoltaic properties. The polymer solar cells were fabricated with the structure of ITO/PEDT-PSS/polymer:PCBM/ Al, and the measurement of the performance was conducted at ambient atmosphere. Figure 6 shows the I-V curves of the polymer solar cells under AM 1.5 condition with 100 mW/cm2. Representative characteristics of the solar cells are listed in Table 4. It should be pointed out that the efficiencies reported in Table 3 are the best values for the corresponding devices. There are six devices on one substrate, and the average efficiencies for six devices are 0.68% for PTTD, 1.19% for A, 1.36% for B, 1.88% for C, 1.73% for D, and 1.34% for P3HT. It was found that when m/n ratio increases, HOMO level of the corresponding polymer rises, and the Voc of the polymer solar cell decreases. It is known that the driving force for charge separation is related to ∆E00 - (∆φDonor - ∆φAcceptor); here, ∆E00 is the first excitedstate energy of either donor or acceptor block, ∆φDonor, oxidation potential of donor and ∆φAcceptor, the reduction potential of acceptor. The energy level of the charge-separated state and Voc are related to (∆φDonor - ∆φAccep).24 The Voc results listed in Table 3 illustrate that higher HOMO level provides smaller Voc. Although PTTD and polymer A have the widest absorption range, the largest short circuit current density (Jsc) was observed in polymer C. There are many factors that influence the current
7870 J. Phys. Chem. C, Vol. 112, No. 21, 2008
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Figure 5. AFM topography images (2 µm × 2 µm) of polymer/PCBM blend films (1:1, w/w) cast from chlorobenzene solutions.
Figure 6. Current-voltage characteristics of polymer solar cells under AM 1.5 condition (100 mW/cm2) at ambient atmosphere.
TABLE 3: Photovoltaic Properties of the Polymer Solar Cells (AM 1.5, 100 mW/cm2) at Ambient Atmosphere PTTD A B C D P3HT
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
4.80 6.61 6.64 8.66 6.84 5.27
0.408 0.507 0.543 0.595 0.608 0.624
0.37 0.37 0.40 0.38 0.41 0.34
0.73 1.28 1.41 1.93 1.79 1.39
densities. Wider absorption may provide larger photon sources for conversion to electricity. However, the low LUMO levels in PTTD and polymer A makes them too close to the LUMO level of PCBM (-3.7 eV).13 The driving force for photoinduced electron transfer from the polymer to PCBM becomes smaller and photons are converted to charges less efficiently. When the LUMO level of polymer rises up, the driving force for charge transfer to PCBM increases. Jsc is also affected by charge car-
riers’ mobility in polymer film, as the hole mobility of these polymers is within a close range, this effect is not obvious here. Finally, morphology plays an important role for Jsc. The blend film of polymer C shows the largest feature, which means polymer C is more favored to form bicontinuous network for charges transport of both carriers. Taking all of these factors into consideration, solar cells made from polymer C have the best power conversion efficiency among these polymers. It should be pointed out that efficiencies as high as 5% has been reported by several groups on P3HT-PCBM-based solar cells through optimizing the film forming and measurement conditions.20a,b,25 Our experimental condition is far from an optimized one. These results indicate that polymer C is a very promising candidate material for further optimization in solar cell performances. Conclusions A series of low band gap polymers based on thieno [3,4b]thiophene unit have been synthesized and shown to exhibit good light conversion efficiency. The important lesson learned is that the control of the thieno [3,4-b]thiophene unit ratio to alkyl thiophene in the polymer is a very effective approach to fine tune the absorption spectra and electrochemical properties of the polymer. Low band gap materials are good for solar cell application only when their energy levels match with those of acceptor molecules and still have enough power output in Voc. The results indicate that polymers with better performances can be prepared by further enhancing π-π stacking and adjusting energy matching. Research on effects of film thickness, combination ratio, and other optimization methods for device performance of the polymer solar cells are underway. Acknowledgment. We gratefully acknowledge the financial support of the National Science Foundation and the NSF
Control in Energy Levels of Conjugated Polymers MRSEC program at the University of Chicago. UC-Argonne collaborative seed grant provided partial support for this research. References and Notes (1) Gratzel, M. Nature 2001, 414, 338. (2) For some represented papers, see: (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (b) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (c) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (d) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (3) Brabec, C. J. Sol. Energy Mater. Sol. Cells 2004, 83, 273–292. (4) (a) Campos, L. M.; Tontcheva, A.; Gunes, S.; Sonmez, G.; Neugebauer, H.; Sariciftci, N. S.; Wudl, F. Chem. Mater. 2005, 17, 4031. (b) Wienk, M. M.; Turbiez, M. G. R.; Struijk, M. P.; Fonrodona, M.; Janssen, R. A. J. Appl. Phys. Lett. 2006, 88. (c) Wienk, M. M.; Struijk, M. P.; Janssen, R. A. J. Chem. Phys. Lett. 2006, 422, 488. (d) Shi, C. J.; Yao, Y.; Yang, Y.; Pei, Q. B. J. Am. Chem. Soc. 2006, 128, 8980. (5) (a) Zhang, F. L.; Mammo, W.; Andersson, L. M.; Admassie, S.; Andersson, M. R.; Inganas, L.; Admassie, S.; Andersson, M. R.; Ingands, O. AdV. Mater. 2006, 18, 2169. (b) Ashraf, R. S.; Shahid, M.; Klemm, E.; Al-Ibrahim, M.; Sensfuss, S. Macromol. Rapid Commun. 2006, 27, 1454. (c) Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z. G.; Waller, D.; Gaudiana, R.; Brabec, C. AdV. Mater. 2006, 18, 2884. (d) Blouin, N.; Michaud, A.; Leclerc, M. AdV. Mater. 2007, 19, 2295. (e) Wong, W. Y.; Wang, X. Z.; He, Z.; Djurisic, A. B.; Yip, C. T.; Cheung, K. Y.; Wang, H.; Mak, C. S. K.; Chan, W. K. Nat. Mater. 2007, 6, 521. (6) Pomerantz, M.; Gu, X. M. Synth. Met. 1997, 84, 243. (7) Neef, C. J.; Brotherston, I. D.; Ferraris, J. P Chem. Mater. 1999, 11, 1957. (8) Lee, K.; Sotzing, G. A. Macromolecules 2001, 34, 5746. (9) Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233. (10) You, W.; Cao, S. K.; Hou, Z. J.; Yu, L. P. Macromolecules 2003, 36, 7014.
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