Effect of the End Group of Regioregular Poly(3-hexylthiophene

May 23, 2007 - Department of Physics, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, United Kingdom, Department of...
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2007, 111, 8137-8141 Published on Web 05/23/2007

Effect of the End Group of Regioregular Poly(3-hexylthiophene) Polymers on the Performance of Polymer/Fullerene Solar Cells Y. Kim,*,†,| S. Cook,‡ J. Kirkpatrick,† J. Nelson,*,† J. R. Durrant,‡ D. D. C. Bradley,*,† M. Giles,§ M. Heeney,§ R. Hamilton,§ and I. McCulloch§ Department of Physics, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, United Kingdom, Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom, Merck Chemicals, Chilworth Science Park, Southampton, United Kingdom, and Organic Nanoelectronics Laboratory, Department of Chemical Engineering, Kyungpook National UniVersity, Daegu 702-701, South Korea ReceiVed: March 22, 2007; In Final Form: May 4, 2007

In this report, we have studied the influence of the nature of the end groups (bromine or hydrogen) of regioregular poly(3-hexylthiophene) (P3HT) polymers on the performance of polymer solar cells made with blend films of P3HT and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Films and devices were studied before and after annealing at 140 °C for 2 h. The effects of the end-group type on the properties of pristine polymers and blend films were examined using optical absorption and emission spectroscopy, transient absorption spectroscopy, and measurements of photovoltaic device performance. It was observed that hydrogen end groups result in slightly higher absorption coefficients, higher photoluminescence intensities, faster and less dispersive charge recombination, and superior solar cell performance (notably a higher fill factor) compared to bromine end groups. The results are attributed to more-ordered polymer-chain packing in blend films made with hydrogen-capped P3HT, on account of the smaller size and weaker electrostatic interactions resulting from hydrogen compared to bromine. Some influence of the C-Br group on exciton quenching may also be present. The effect of the bromine end group on solar-cell performance became more pronounced with reducing incident-light intensity. Comparison of the polymer transport characteristics in organic field-effect transistor configuration indicated that the bromine end group enhances hole trapping.

Organic solar cells based on small molecules and polymers have attracted much interest because of their potential for cheap renewable energy conversion,1-5 since the pioneering studies of photocurrent collection in devices made with polymerpolymer and polymer-molecule “bulk heterojunction” (BHJ) organic films.6,7 One of the most important findings was the discovery of ultrafast photoinduced electron transfer from an electron-donating conjugated polymer to an electron-accepting fullerene,8 which opened the possibility for high-photocurrent quantum efficiency in BHJ polymer solar cells. In the 10 years since the first BHJ devices were reported, significant improvements in power conversion efficiency (PCE is identified as the ratio of the maximum output electrical power to the input sunlight power)2-5 have been made for polymerbased solar cells, first by controlling the morphology of blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)-propyl1-phenyl-(6,6)C61 (PCBM) on the nanoscale level,9 and later by replacing MDMO-PPV with regioregular poly(3-hexylthiophene) (P3HT). P3HT offers the advantages of extended light * Corresponding authors. E-mail: [email protected]; jenny.nelson@ imperial.ac.uk; [email protected]. † Department of Physics, Blackett Laboratory, Imperial College London. ‡ Department of Chemistry, Imperial College London. § Merck Chemicals. | Kyungpook National University.

10.1021/jp072306z CCC: $37.00

absorption up to 650 nm,10-12 higher charge carrier mobilities probably resulting from its highly self-organizing (crystalline) structure,13,14 and a relatively high glass transition temperature (110 °C for the pristine polymer in the bulk).12 The performance of P3HT/PCBM devices is sensitive to processing conditions. In particular, the PCE of P3HT/PCBM solar cells is enhanced by thermal annealing treatment leading to recrystallization of P3HT chains11,12 and further enhanced by control of the composition,15,16 solvent,11 film thickness,17 and thermal annealing time.17 Very recently, it has been reported that the chemical configuration (regioregularity) of P3HT strongly influences the solar cell performance,18 which stresses the importance of chain packing19 in BHJ films for high PCE. We also note that a slightly different result has been reported on achieving comparably high PCE using a copolymer of P3HT, which has a lower regioregularity.20 However, no study has been reported on the effect of the end groups of regioregular P3HT polymer chains on the polymer optoelectronic properties and device performance, even though the end group content is as high as 2 mol % with respect to the repeat unit (3-hexylthiophene) in the P3HT chain (approximately 1019 cm-3) when the molecular weight is low. The nature of the end group may influence device performance in a number of ways, for instance by acting as chemical traps for the direct trapping of charges, by quenching photogenerated excitons, by © 2007 American Chemical Society

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Figure 1. (Top panel) Chemical structure of 95.2% regioregular P3HTs with different end groups (X): P3HT-H and P3HT-Br for hydrogen and bromine end groups, respectively. Optical absorption and PL (λexc ) 520 nm) spectra of pristine (middle panel) and P3HT/PCBM (1:1 by weight) blend (bottom panel) films: unannealed films with P3HT-H (solid red line), annealed films with P3HT-H (dashed pink line), unannealed films with P3HT-Br (dash-dot blue line), and annealed films with P3HT-Br (dash-dot-dot green line). Note that the PL spectrum of annealed pristine P3HT-H film (dashed pink line) is obscured by overlapping with those of pristine P3HT-Br films (blue and green lines).

disrupting chain packing through steric or electrostatic interactions, or by accelerating degradation through enhanced chemical reactivity. In this work, we have studied the effect of end groups in regioregular P3HT on the performance of polymer/fullerene blend solar cells, focusing on the bromine end group (P3HTBr) because it is a typical end group for P3HT polymers that are polymerized through dehalogenation synthesis routes (see Figure 1 for the chemical structure of P3HTs).21-23 As a comparison, the hydrogen end group (P3HT-H) was studied. The two polymers may be expected to differ as a result of the different sizes of the H and Br atoms and of the different electrostatic interactions resulting from their different electronegativities, both of which may affect chain packing and charge trapping. Quantum chemical calculations (see the Experimental section) yielded a dipole moment for thiophene bromide of 1.6 D directed from the bromine toward the carbon atom to which the bromine is bonded,24 compared to 0.6 D directed from the sulfur atom toward the center of the molecule in the case of unsubstituted thiophene. This large difference in dipole moments is due to both polarization of the C-Br bond and the reorganization of partial charges on the various atoms. The same effect is expected to lead to a larger net dipole moment for bromine-capped than hydrogen-capped oligothiophene chains. Dipolar interactions are known to lead to energetic disorder (effectively, more charge trapping) in organic semiconductors.25 In addition to measurements of solar-cell performance, we monitored the effect of end-group type on ground-state optical properties using UV-visible and photoluminescence (PL)

Letters spectroscopy and on charge recombination dynamics using transient absorption spectroscopy (TAS), which can elucidate the role of charge trapping in P3HT/PCBM blend films. Finally, organic field-effect transistor (OFET) measurements were used as an independent probe of charge-trapping effects in the two P3HT polymers. Bromine end-capped P3HT (regioregularity ) 95.2%) (P3HTBr) was synthesized using a Merck synthetic method, which features nickel-catalyzed cross coupling of thiophene Grignard reagents that are prepared by the direct insertion of magnesium metal into 2,5-dibromo-3-hexylthiophene;23 and then for comparative study a fraction of the polymer was converted to hydrogen end-capped P3HT (P3HT-H).23 We note that some different synthetic routes are reported in the literature.26,27 Weight (Mw), number (Mn) average molecular weights, polydispersity index (PDI), and melting point (Tm) of P3HTs were Mw ) 2.1 × 104, Mn ) 1.2 × 104, PDI ) 1.8, and Tm ) 220 °C, respectively. The bromine content of the proton endcapped sample (P3HT-H) was 5 times lower than that of P3HTBr, suggesting a high level of conversion, and the transformation reaction was relatively easy to carry out. The metallic impurities were as low as less than 2 µg/g for Ni and 50 µg/g for Mg because of the extensive purification processes: The purity was assessed by elemental analysis using particle-induced X-ray and γ-ray emission (PIXE/PIGE) measurements, which were carried out at A° bo Akademi University, Finland.28 As an electron acceptor, the same batch of PCBM was used as described in ref 12. Blend solutions (P3HT/PCBM ) 1:1 by weight) were prepared using chlorobenzene with a solid concentration of 60 mg/mL. These solutions were spin-coated at 1500 rpm for 30 s onto quartz substrates (Spectrosil B) and indium-tin oxide (ITO) coated glass substrates with PEDOT/PSS layers12 for optical measurements and device fabrication, respectively, followed by soft-baking at 50 °C for 15 min. The thickness of the as-coated blend films was 160-165 nm determined by a surface profiler (Dektak), whereas that of the annealed blend films was 190195 nm. Polymer solar cells were fabricated in the same way as in ref 12 [see the device structure in the inset of Figure 2 (bottom panel)]. The UV-visible and photoluminescence spectra were measured in a way similar to that of previous work.18,29 The decay dynamics of the photoinduced absorption at 980 nm (1.27 eV), which is assigned primarily to PCBM radical anion absorption.30,31 The dipole moment of thiophene bromide was calculated using the B3LYP hybrid density functional. The basis set used was Pople’s double-split basis set 6-31g*. All calculations were performed on Gaussian03.32 To elucidate the origin of the large dipole moment in thiophene bromide, Bader analysis of the Gaussian03 results was performed using Henkelman’s Bader analysis suite.33 These calculations confirmed that in thiophene bromide the bromine atom is not negatively charged but that the electronic charge in the Bader volume defining the atom is highly polarized. Organic field-effect transistors (OFET) were fabricated, under dry nitrogen, on highly n-doped silicon substrates as a common gate electrode with a 230-nm-thick thermally grown silicon oxide (SiO2) insulating layer as a gate dielectric. Gold source and drain electrodes were patterned by photolithography on the dielectric: Channel length (L) and width (W) were 10 µm and 1 cm, respectively. Prior to coating the P3HT layers, the substrates were treated with hexamethyldisilane (HMDS). Then, P3HT layers were spin-coated using corresponding solutions (0.5-1 wt % P3HT in chloroform). The typical film thickness

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Figure 2. (Top panel) Light and dark (inset in semilogarithmic scale) J-V characteristics under AM1.5 simulated solar light illumination (85 mW/cm2) of polymer solar cells: For P3HT-H, unannealed device (solid red line): JSC )7.26 mA/cm2, VOC ) 0.56 V, FF ) 43.9%, PCE ) 2.1%, RS ) 676 Ω; annealed device (dashed pink line): JSC ) 7.28 mA/cm2, VOC ) 0.57 V, FF ) 49.7%, PCE ) 2.4%, RS ) 572 Ω; For P3HT-Br, unannealed device (dash-dot blue line): JSC ) 7.02 mA/cm2, VOC ) 0.48 V, FF ) 34.9%, PCE ) 1.4%, RS ) 1009 Ω; annealed device (dash-dot-dot green line): JSC ) 6.97 mA/cm2, VOC ) 0.56 V, FF ) 39.3%, PCE ) 1.8%, RS ) 926 Ω. (Bottom panel) Short circuit current density (JSC) as a function of intensity (PIN) for annealed devices: the inset shows the cross-sectional device structure in which “BL” and “BHJ” denote the PEDOT/PSS layer and the P3HT/ PCBM layer, respectively.

was 30-60 nm (measured by KLA Tencor Alpha-Step 500 profilometer). After spin-coating, the transistor devices were then annealed at 100 °C for 10 min under nitrogen. The I-V characteristics of OFET devices were measured in a nitrogen environment using a semiconductor parameter analyzer (Agilent 4155C). As shown in Figure 1, the optical absorption coefficient of pristine P3HT-Br films (both unannealed and annealed) is slightly lower than that of P3HT-H. This could result from the effect of the large -Br end group on the P3HT chain-packing density, leading to a looser chain packing and weaker light absorption. A slightly larger absorption coefficient for -H- than -Br-capped P3HT is also found in blend films (lower panel, Figure 1). Thermal annealing enhances the absorption coefficient for both blends, as seen previously,18 presumably through the restoration of chain packing after disruption through addition of PCBM. For both blend and pristine polymer films, the positions of the characteristic absorption bands were the same for both P3HTs, indicating that the different end groups do not significantly alter the electronic structure in the ground state. For unannealed pristine films, the PL intensity of P3HT-Br is lower than that of P3HT-H. The difference can be explained largely by the lower absorption coefficient of P3HT-Br than that of P3HT-H. A lower PL intensity for the P3HT-Br- than for the P3HT-H-based films is also observed for the P3HT/ PCBM blend films, both before and after annealing. In the case of the blend films, the effect is too large to be explained by the reduced absorption coefficient and therefore suggests that the

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Figure 3. EQE spectra (∼40 µW/cm2 at 520 nm) (top panel) of polymer solar cells and transient absorption (∆OD) decay (bottom panel) of PCBM radical anion: unannealed blend film with P3HT-H (a; solid red line), annealed blend film with P3HT-H (c; dashed pink line), unannealed blend film with P3HT-Br (b; dash-dot blue line), and annealed blend film with P3HT-Br (d; dash-dot-dot green line).

presence of the C-Br end groups may increase the rate of nonradiative exciton quenching. (The increased PL intensity of blend films upon thermal annealing has been reported previously.)18 As shown in Figure 2, the current density-voltage (J-V) characteristics under simulated solar illumination (AM1.5, 85mW/cm2) were significantly poorer for unannealed devices made with P3HT-Br/PCBM blend films than for those made with P3HT-H/PCBM blend films. In particular the P3HT-Br/ PCBM device suffers from a poor fill factor. This may be due to poor charge transport: a higher series resistance (RS) for P3HT-Br/PCBM than for P3HT-H/PCBM devices is suggested by the non-exponential dependence of dark current on voltage [shown in the inset of Figure 2 (top panel)]. After device annealing, the fill factors of both devices improved, but that of the P3HT-H/PCBM-based device remained higher, leading to a PCE of 2.4% for the P3HT-H/PCBM-based device compared to 1.8% for the P3HT-Br/PCBM device. To probe the effect of end groups on photocurrent collection in more detail, we have measured the short-circuit current density, JSC, as a function of incident light intensity for the two annealed devices (bottom panel, Figure 2). Although JSC was smaller for devices made with P3HT-Br/PCBM than with P3HT-H/PCBM blend films over the entire range of light intensities, the JSC difference increases with decreasing incidentlight intensity. This trend was maintained even after taking the film thicknesses and slightly different absorption coefficients into account, clearly indicating that replacing the bromine with the hydrogen end group improves photocurrent collection. The measured JSC values for the lowest light intensities used (0.85 mW/cm2) are in good agreement with the values expected from the device external quantum efficiency (EQE) (see Figure 3), which was measured at a similarly low light intensity and is larger by a factor of 1.2-1.4 for the hydrogen than the bromine end groups. However, the dependence of JSC on incident light intensity (PIN), characterized by the exponent γ where JSC ∝ PINγ, is weaker for the hydrogen end group (γ ) 0.83) than the

8140 J. Phys. Chem. C, Vol. 111, No. 23, 2007 bromine end group (γ ) 0.92), with the result that the JSC values at AM1.5 intensity are quite similar for the two polymers. Such sublinear JSC - PIN behavior usually indicates that bimolecular recombination competes more successfully with charge collection at high carrier densities. Similar sublinear behavior is observed in other P3HT/PCBM devices and is generally more pronounced (more sublinear) after annealing,17 which suggests a relationship with polymer morphology that is, in general, more crystalline after annealing of blend films. To study the influence of end groups on bimolecular charge recombination, we have measured the photoinduced absorption (∆OD) of polaron species18 following low-intensity pulsed laser excitation using microsecond (∼0.1 µs) to millisecond transient absorption34,35 measurements, as shown in Figure 3 (bottom panel). Such power-law decays are typical for polymerfullerene blend films34,35 and have been interpreted in terms of the diffusion-limited bimolecular recombination of positive polarons (radical cations) on the polymer and negative polarons (radical anions) on the fullerene. Here, as previously,34 we attribute the slow decay dynamics to the release of positive polarons from deep traps in the P3HT polymer, this being the rate-limiting step for recombination. The dynamics can be characterized by fitting the slow decay to a power law (∆OD ∼ t-R) over the slow (10-6 ∼ 10-2 s) phase, where R ) T/T0 and T0 is the characteristic temperature of an exponential distribution of charge traps. The resulting exponents (R) obtained from the data in Figure 3 were similar for unannealed films (R ) -0.378 for bromine and R ) -0.342 for hydrogen end group), whereas annealed films showed a large difference (R ) -0.364 for bromine and R ) -0.461 for the hydrogen end group). We note that the data were collected at sufficiently high laser intensities where traps are expected to be saturated and the amplitude reflects trap density rather than charge-separation yield. The faster kinetics for the hydrogen end group (after annealing) show that bimolecular recombination is more effective after annealing, possibly through the removal of charge traps by the closer packing and self-organization of the polymer chains. Previous studies have also shown that the microsecondmillisecond decay dynamics are sensitive to the P3HT/PCBM blend film morphology,18 for example, as a result of the influence of polymer-chain packing on charge mobility. The comparison yields the following conclusions: first, that in unannealed blend films the bromine end group does not by itself lead to increased charge trapping compared to the hydrogen end group (R values are similar); rather, a similar trapping mechanism, possibly due to small crystallites within the disordered active layer, is present in both. Second, for the annealed films charge trapping is less pronounced for the hydrogen than the bromine end group (larger R), which is consistent with a higher charge mobility in the annealed P3HT-H/PCBM blend and with the higher fill factor and higher dark current of the photovoltaic devices.18 This change may result from more pronounced changes in morphology of the P3HT-H/PCBM blend than of the P3HT-Br/PCBM blend upon annealing, possibly enabled by the less-disruptive influence of the hydrogen than the bromine group on close chain packing. We may now explain the differences in the performance of the solar cell (annealed devices) as follows: At low light intensities, charge collection is inhibited in the P3HT-Br/PCBM device by charge traps that result from poor chain packing, which in turn results from the large size of bromine compared to hydrogen or to the larger electrostatic interactions associated with the bromine than the hydrogen end group. Bimolecular recombination is largely trap-limited in both cases, but this

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Figure 4. Source-drain current (Isd) - gate voltage (Vg) transfer characteristics of OFETs made from pristine P3HT-H (top) and P3HTBr (bottom) as a function of the number of scans (scan number given on legend). The schematic device structure is shown in the top panel inset, and the bottom panel inset shows the normalized source-drain current at Vg ) 0 V.

recombination is faster in the annealed P3HT-H/PCBM than the P3HT-Br/PCBM blend on account of its higher crystallinity reducing the average trap density. Therefore, as light intensity increases, JSC increases more slowly for the P3HT-H/PCBM than the P3HT-Br/PCBM blend because bimolecular recombination accelerates and competes more effectively with charge collection in that case. To find further evidence for charge trapping in the presence of the bromine end group, OFET devices were fabricated using pristine P3HT-H and P3HT-Br as the semiconductor (see the schematic device structure for OFET in Figure 4a). OFET measurements were chosen because of the high sensitivity of charge transport to the purity and morphology of the material in the thin (several nanometer) accumulation layer across the channel between source and drain electrodes.36 The OFET transfer characteristics (Figure 4) show that although the P3HT-H OFET source-drain current on repeated gate scans remains identical even after 25 scans, the P3HT-Br OFET shows a rise in OFF-current, as well as a positive shift in threshold voltage, on repeated scans. Such behavior is typically attributed to the presence of additional positive charge in the polymer. These results suggest that the bromine end group introduces hole traps into the polymer, increasing the intrinsic charge density. In addition, the average OFET mobility was lower by 20-30% for P3HT-Br than for P3HT-H (data not shown), which is compatible with hole trapping in P3HT-Br. In conclusion, the end group of regioregular P3HT was found to affect the performance of polymer solar cells made with blend films of P3HT and PCBM, with a pronounced influence at low incident light intensity. The effect is relatively modest given the high density of end groups (1019 cm-3). The fill factor, JSC, and EQE of devices, whether annealed or not, were lower for the P3HT-Br- than for the P3HT-H-based film. This is attributed to increased charge trapping in the P3HT-Br- relative to the

Letters P3HT-Br-based film, probably as a result of more disordered morphology rather than through direct charge trapping by the end groups. TAS measurements confirmed that charge recombination dynamics were slower and more dispersive for the P3HT-Br-based film, which is consistent with a higher degree of charge trapping. The charge-trapping effect in P3HT-Br was supported by a comparison of OFET transfer characteristics for the two polymers. Finally, we note that the end-group effect has also been reported for phenylenevinylene oligomers, but the effect was not clear in this case.37 Acknowledgment. We thank Merck Chemicals Ltd. for supplying the P3HT polymer and BP Solar for financial support via the OSCER project. References and Notes (1) Service, R. F. Science 2005, 309, 548. (2) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (3) Nelson, J. Mater. Today 2002, May, 20. (4) Brabec, C. J. Sol. Eng. Mater. Sol. Cell. 2004, 83, 273. (5) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (6) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (7) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (8) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (9) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (10) Schilinsky, P.; Waldauf, C.; Brabec, C. J. Appl. Phys. Lett. 2002, 81, 3885. (11) Padinger, F.; Ritterberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85. (12) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Appl. Phys. Lett. 2005, 86, 063502. (13) Choulis, S. A.; Kim, Y.; Nelson, J.; Bradley, D. D. C.; Giles, M.; Shkunov, M.; McCulloch, I. Appl. Phys. Lett. 2004, 85, 3890. (14) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Macromolecules, to be submitted for publication. (15) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. J. Mater. Sci. 2005, 40, 1371. (16) Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V. Nanotechnology 2004, 15, 1317. (17) Kim, Y.; Nelson, J.; Cook, S.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I., AdV. Funct. Mater., to be submitted for publication. (18) Kim, Y.; Cook, S.; Tuladhar, S. M.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. S.; Ree, M. Nat.Mater. 2006, 5, 193. (19) Kim, Y.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Heo, K.; Park, J.; Kim, H.; McCulloch, I.; Heeney, M.; Ree, M.; Ha, C. S. Soft Matter 2006, 3, 117. (20) Sivula, K.; Luscombe, C. K.; Thompson, B. C.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2006, 128, 13988.

J. Phys. Chem. C, Vol. 111, No. 23, 2007 8141 (21) McCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe, R. D.; Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910. (22) Roewe, R. S.; Khersonsky, S. M.; McCullough, R. D. AdV. Mater. 1999, 11, 250. (23) Koller, G.; Falk, B.; Weller, C.; Giles, M.; McCulloch, I. PCT Int. Appl. WO2005014691, 2005. (24) We note the Textbook values of the dipole moment (µ) of the carbon-bromine and carbon-hydrogen groups are very different: µ ) 0.30.4 D [Debye (D) ) 10-18 statcoulomb‚cm] for C-H compared to µ ) 1.38-1.48 D for C-Br (note µ ) 0 D for C-C). (See the following: (a) Furguson, L. N. In The Modern Structural Theory of Organic Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1963; Chapter 2. (b) Fessenden, R. J.; Fessenden, J. S. In Organic Chemisty, 3rd ed.; Brooks/Cole Publishing Company: Monterey, CA, 1986; Chapter 1. (25) Dunlap, D. H.; Parris, P. E.; Kenkre, V. M. Phys. ReV. Lett. 1996, 77, 542. (26) Yamamoto, T.; Maruyama, T.; Zhou, Z.; Ito, T.; Fukuda, T.; Yoneda, Y.; Begum, F.; Ikeda, T.; Sasaki, S.; Takezoe, H.; Fukuda, A.; Kubota, K. J. Am. Chem. Soc. 1994, 116, 4832. (27) Kreyenschmidt, M.; Uckert, F.; Mu¨llen, K. Macromolecules 1995, 28, 4577. (28) Johansson, S. A. E.; Campbell, J. L.; Malmqvist, K. G. Particle Induced X-ray Emission Spectrometry (PIXE). In Chemical Analysis, A Series of Monographs on Analytical Chemistry and Its Applications; Wiley: New York, 1995; vol. 133. (29) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Chem. Mater. 2004, 16, 4812. (30) Guldi, D. M.; Hungerbu¨hler, H.; Asmus, K.-D. J. Phys. Chem. 1995, 99, 9380. (31) Cook, S. Ph.D. Thesis, Imperial College London (University of London), 2006. (32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (33) Henkelman, G.; Arnaldson, A.; Jonsonn, H. Comp. Mater. Sci. 2005, 36, 354. (34) Nelson, J. Phys. ReV. B 2003, 67, 155209. (35) Montanari, I.; Nogueira, A. F.; Nelson, J.; Durrant, J. R.; Winder, C.; Antonietta, M.; Sariciftchi, N. S.; Brabec, C. J. Appl. Phys. Lett. 2002, 81, 3001. (36) Dodabalapur, A. Mater.Today 2006, 9, 24 and references therein. (37) Jorgensen, M.; Krebs, F. C. J. Org. Chem. 2004, 69, 6688.