Article pubs.acs.org/cm
Benzodithiophene and Imide-Based Copolymers for Photovoltaic Applications Wade A. Braunecker,*,† Zbyslaw R. Owczarczyk,† Andres Garcia,† Nikos Kopidakis,† Ross E. Larsen,† Scott R. Hammond,† David S. Ginley,† and Dana C. Olson† †
National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: Conjugated alternating copolymers were designed with low optical band gaps for organic photovoltaic (OPV) applications by considering quinoid resonance stabilization. Copolymers of thienoisoindoledione (TID) and benzodithiophene (BDT) had appreciably lower band gaps (by ∼0.4 eV) than copolymers of thienopyrroledione (TPD) and BDT. In addition to intramolecular charge transfer stabilization (i.e., the “push-pull” effect), the former copolymer’s quinoid resonance structure is stabilized by a gain in aromatic resonance energy in the isoindole unit. Additionally, the HOMO levels of the copolymers could be tuned with chemical modifications to the BDT monomer, resulting in open circuit voltages of greater than 1 V in photovoltaic devices. Despite the optimized band gap, TID containing polymers displayed lower photoconductance, as determined by time-resolved microwave conductivity, and decreased device efficiency (2.1% vs 4.8%) as compared with TPD analogues. These results were partially attributed to morphology, as computational modeling suggests TID copolymers have a twisted backbone, and X-ray diffraction data indicate the polymer films do not form ordered domains, whereas TPD copolymers are considerably more planar and are shown to form partially ordered domains. KEYWORDS: pyrroledione, isoindoledione, benzodithiophene, solar cell, low band gap polymer
■
INTRODUCTION The ongoing development of polymer solar cells as low cost, lightweight, flexible alternatives to silicon-based inorganic devices is advancing the field of organic photovoltaics (OPV) and helping address the ever-increasing global energy demand.1−3 The most successful OPV devices to date are based on the bulk heterojunction (BHJ) concept, which typically have an interpenetrating network of photoactive electron-donating polymers and fullerene-based acceptors.4−6 Indeed, the field has witnessed explosive growth and development in just the past few years, with rational design efforts playing a key role in fine-tuning the electronic properties of the polymer donors that in turn enhance power conversion efficiencies (PCEs).7−10 Low band gap polymers, whose absorption better overlaps with the solar spectrum, are now being targeted (thereby maximizing photon absorption and the short-circuit current, JSC), in conjunction with polymers having lower highest occupied molecular orbital (HOMO) energy levels (thereby maximizing the open-circuit voltage, VOC).11−13 While certain parameters that clearly and dramatically affect device performance are more difficult to predict and control, including polymer morphology14 and polymer-fullerene intercalation,15 rational design efforts are beginning to fuel clear progress in the field. Reports of device efficiencies now approaching 10%16 confirm that OPV is becoming a viable technology. Designing a semiconducting polymer with a narrow band gap requires consideration of the conjugated polymer’s quinoid resonance structures or delocalization along its backbone (see Figure 1).17,18 One strategy that has proven particularly © 2012 American Chemical Society
efficient at stabilizing this delocalization is the use of donor− acceptor alternating copolymers. Intramolecular charge transfer between alternating units of electron-donating and electronaccepting moieties effectively promotes conjugation throughout the polymer backbone by quinoid resonance stabilization, thereby appreciably narrowing the band gap through the socalled “push-pull” effect.13,19 Another approach to achieve this stabilization employs fused aromatic units, as in polyisothianaphthene (PITN), wherein the dearomatization of the thiophene ring to assume a quinoid structure is accompanied by a gain in aromatic resonance energy in the fused benzene ring, resulting in a band gap as much as one full electronvolt lower than in polythiophene.20 Since the discovery of PITN, several other reports have appeared on this class of materials, describing polymers with structural variations of the isothianaphthene unit,21,22 as well as polymers based on thienopyrazine23 and thienothiophene.24 Figure 1 illustrates a new copolymer investigated in this report for which the quinoid structure is stabilized both by intramolecular charge transfer as well as aromatic resonance. Recently, the photovoltaic performances of alternating copolymers containing benzodithiophene (BDT) as the donating moiety have attracted much attention.9,25,26 Among these systems, polymers containing derivatives of thieno[3,4b]thiophene-2-carboxylates (TT) as the electron-accepting unit have shown great promise, with PCEs as high as 7.4%.27 Received: December 23, 2011 Revised: March 5, 2012 Published: March 6, 2012 1346
dx.doi.org/10.1021/cm2038427 | Chem. Mater. 2012, 24, 1346−1356
Chemistry of Materials
Article
Figure 1. Thienoisoindoledione-benzodithiophene alternating copolymer whose quinoid structures can be stabilized through intramolecular charge transfer and aromatic resonance.
Scheme 1. Synthesis of Monomer Derivatives of TPD (2), TID (7), and BDT (9, 14, 15)
fullerene acceptor,11 the shallow HOMO contributes to a relatively low VOC. Another successful copolymer of BDT with deeper HOMO levels (but a wider band gap) than that with TT, which was independently reported by three groups,28−30 employs 5-alkyl-thieno[3,4-c]pyrrole-4,6-dione (TPD) with an alkoxy-chain derivative of BDT, 4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithiophene. The optical band gap for these
Interestingly, these TT units stabilize the quinoid resonance structure both through intramolecular charge transfer as well as aromatic resonance. This stabilization is ultimately responsible for the narrow polymer band gap (∼1.6 eV). However, these polymers generally possess a relatively shallow HOMO energy level (typically around −5.0 eV). As the VOC of these BHJ devices are directly related to the energy difference between the HOMO level of the donor polymer and the LUMO level of the 1347
dx.doi.org/10.1021/cm2038427 | Chem. Mater. 2012, 24, 1346−1356
Chemistry of Materials
Article
systems was reported as ∼1.8 eV, with a HOMO approximately −5.4 eV, and optimized efficiencies varying from 4.2% to 6.8%. The reported band gaps and HOMO levels for these TPDBDT systems suggest room for further optimization. We report herein on our efforts to further narrow the band gap of this system by employing an electron-accepting moiety of similar strength to TPD, but whose quinoid structure can also be stabilized by aromatic resonance, 5-octyl-thieno[3,4-f ]isoindole-5,7-dione (TID). This monomer has been used to synthesize polymers with band gaps as low as 1.10 eV for certain electrochromic devices22,31 but, to our knowledge, has not been used specifically for any OPV applications. Additionally, we attempt to push the HOMOs of our donor polymers a little deeper by employing alkyl-chain derivatives of BDT (Scheme 1). Literature reports have demonstrated this strategy can improve the VOC of similar copolymers by more than 100 mV as compared to alkoxy-chain derivatives of BDT.9 Note, this synthetic effort is not a serendipitous approach to combining known donor monomers with known acceptors in different combinations to achieve “new” OPV polymers. Rather, we primarily seek to contribute to the ongoing design effort aiming to fine-tune band gaps and HOMO levels of OPV polymers. Some additional results for the two basic polymer structures (TID vs TPD containing polymers) are provided to help understand their performance in solar cell devices, including the photoconductance of polymer films, as determined by time-resolved microwave conductivity; the presence or absence of ordered domains in the films, as determined by X-ray diffraction; and the relative planarity of the polymer backbones, as determined by computational modeling.
■
3,4-Bis(hydroxymethyl)-thiophene (3). A slurry of thiophene3,4-dicarboxylic acid (10.0 g, 58.0 mmol) in 100 mL of anhydrous 1,2dichloroethane was first converted to the acid chloride by following the procedure described above for compound (2) using oxalyl chloride. After the conversion and removal of solvent, 50 mL of dry THF was added. In a separate flask, 2.0 eq. of lithium aluminum hydride (4.40 g, 116 mmol) was added to 500 mL of dry THF and stirred in an ice bath. The acid chloride solution was transferred dropwise via cannula to the LAH slurry. The mixture was stirred at r.t. for three additional hrs. The THF was evaporated, and a saturated solution (100 mL) of NaCl was then added dropwise. The solution was acidified (pH ∼ 2) and then washed 5× with 200 mL of ether. The combined organic extracts were washed with 20 mL of water and then dried over MgSO4. The ether was removed to give 7.1 g (85%) of the title compound as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.10 (s, 2H), 4.49 (s, 4H), 3.75 (br, 2H). 13C NMR (100 MHz, CDCl3): δ 140.7, 125.1, 59.5. Thiophene-3,4-dicarbaldehyde (4). Compound (3) (6.50 g, 45.1 mmol) was stirred in 400 mL of anhydrous methylene chloride at r.t. for 3 h with 3.0 eq. of pyridinium chlorochromate (29.2 g, 135 mmol) and 40 g of crushed molecular sieves. The reaction mixture was then filtered through a pad of silica gel and washed with an additional 1 L of methylene chloride. The solvent was evaporated to give 4.42 g (70%) of the title compound as an off-white solid, mp 76 °C. 1H NMR (400 MHz, CDCl3): δ 10.28 (s, 2H), 8.20 (s, 2H). 13C NMR (100 MHz, CDCl3): δ 186.1, 140.5, 137.9. 1-Octyl-pyrrole-2,5-dione (5). This compound was synthesized according to a literature procedure34 for N-alkyl-pyrrole-2,5-dione derivatives in 85% yield as a light tan solid, mp 36 °C. 1H NMR (400 MHz, CDCl3): δ 6.65 (s, 2H), 3.48 (m, 2H), 1.58 (br m, 2H), 1.27 (br m, 10H), 0.87 (t, 3H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3): δ 171.1, 134.2, 38.1, 32.0, 29.3, 29.3, 28.7, 26.9, 22.8, 14.3. 6-Octyl-thieno[3,4-f ]isoindole-5−7-dione (6). This compound was synthesized based on a literature procedure35 for the elongation of acene derivatives. Dialdehyde (4) (1.50 g, 10.7 mmol) and maleimide (5) (2.07 g, 10.7 mmol) were dissolved in 100 mL of anhydrous methylene chloride and stirred in an ice bath. To this cooled solution, a 10 mL solution of methylene chloride containing 0.1 eq. DBU (160 μL, 1.07 mmol) and 1.5 eq. tri-n-butylphosphine (4.00 mL, 16.1 mmol) was added dropwise. The reaction was stirred for 1 h at r.t., after which it was concentrated in vacuo and the residue was purified via flash chromatography in hexane:ethyl acetate (4:1). After removing the solvent, the red solid was recrystallized twice from methanol to give 2.26 g (67%) of off-white crystals. 1H NMR (400 MHz, CDCl3): δ 8.13 (s, 2H), 8.02 (s, 2H), δ 3.70 (t, 2H, J = 7.4 Hz), 1.68 (m, 2H), 1.38−1.2 (m, 10H), 0.85 (t, 3H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3): δ 168.2, 138.4, 126.0, 122.8, 119.5, 38.9, 32.0, 29.4, 28.6, 27.1, 22.8, 14.3. 1,3-Dibromo-6-octyl-thieno[3,4-f ]isoindole-5−7-dione (7). This compound was synthesized according to a modified literature procedure.36 NBS (2.05 g, 11.5 mmol) was added to a stirred solution of compound (6) (1.77 g, 5.61 mmol) in a mixture of 75 mL of chloroform and 75 mL of acetic acid. The mixture was stirred at r.t. overnight, and then stirred at 60 °C for 2 h. The mixture was quenched with water, the organic layer was separated, and the water layer was extracted 3× with 10 mL of chloroform. The combined chloroform extracts were washed with a sodium bicarbonate solution. After drying over MgSO4, the solvent was evaporated, and the residue was purified via flash chromatography using methylene chloride:hexane (2:1) as the eluent to afford 2.0 g (76%) of a bright yellow solid. 1 H NMR (400 MHz, CDCl3): δ 7.98 (s, 2H), δ 3.70 (t, 2H, J = 7.4 Hz), 1.68 (m, 2H), 1.38−1.2 (m, 10H), 0.85 (t, 3H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3): δ 167.2, 138.1, 127.5, 118.6, 110.1, 38.9, 32.0, 29.4, 28.6, 27.1, 22.8, 14.3. 4,8-Bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (8). The precursor, benzo[1,2-b:4,5-b′]dithiophene, was synthesized according to a literature procedure.37 Compound (8) was obtained according to a modified literature procedure9 by mixing benzo[1,2b:4,5-b′]dithiophene (1.0 g, 4.5 mmol), Na2S2O4 (1.74 g, 9.97 mmol), and tetrabutylammonium bromide (1.61 g, 5.0 mmol) in 50 mL of
EXPERIMENTAL SECTION
General. All reagents employed in this study were obtained from commercial sources at the highest available purity and used without further purification, unless otherwise noted. All reactions were performed under dry N2. Methylene chloride, toluene, and THF were purified by passing through alumina in an MBraun solvent purification system. Column chromatography was performed with Fluka Silica Gel 60 (220−440 mesh). All small molecules were characterized by 1H NMR (400 MHz) and 13C NMR (100 MHz) on a Varian Unity Inova. Monomers were >99% pure as determined by 1H NMR. UV−vis absorption measurements were performed using a Hewlett-Packard 8453 UV−vis spectrophotometer. Synthesis. 1,3-Dibromo-5-octyl-4H-thieno[3,4-c]pyrrole4,6(5H)-dione (2). This compound was synthesized according to a modified literature procedure.32 A slurry of 2,5-dibromothiophene-3,4dicarboxylic acid (1)33 (2.25 g, 6.86 mmol) was stirred in 30 mL of anhydrous 1,2-dichloroethane. One drop of DMF was added, followed by the dropwise addition of oxalyl chloride (1.83 mL, 20.6 mmol). After 20 min at r.t. and much vigorous bubbling, the solution became homogeneous. It was then heated at 60 °C for one hour, after which the solvent was removed by boiling off under N2. The crude sample was dried under vacuum to give ∼2.5 g of the acid chloride, which was used without further purification. 1.0 eq. of N-octylamine (1.14 mL, 6.86 mmol) was added dropwise at r.t. to the solid acid chloride under N2 (fuming HCl evolves), after which the reaction flask was heated to 130 °C for 30 min. After cooling to r.t., the sticky solid was dissolved in 20 mL of ethyl acetate, washed with a saturated solution of sodium bicarbonate, and dried over MgSO4. The product was then purified by flash chromatography on silica gel with hexane:ethyl acetate (10:1). After removal of the solvent, the solid was twice recrystallized from MeOH to afford 1.7 g (59%) of the title compound, mp 104 °C. 1H NMR (400 MHz, CDCl3): δ 3.59 (t, 2H, J = 7.4 Hz), 1.62 (m, 2H), 1.25 (m, 10H), 0.87 (t, 3H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3): δ 160.6, 135.0, 113.1, 39.1, 32.0, 29.3, 28.5, 27.0, 22.8, 14.3. 1348
dx.doi.org/10.1021/cm2038427 | Chem. Mater. 2012, 24, 1346−1356
Chemistry of Materials
Article
mmol) was dissolved in 20 mL of dry THF and cooled to −78 °C under N2. A butyllithium solution (2.5 M, 0.594 mL, 1.48 mmol) was added dropwise, and the reaction mixture was stirred for 30 min at −78 °C, warmed to r.t., and then cooled back down to −78 °C. A trimethyltin chloride solution (1 M, 1.7 mL, 1.70 mmol) was added dropwise at −78 °C. The reaction was warmed to r.t. and stirred overnight. The mixture was quenched with 50 mL of water and extracted twice with 50 mL of hexanes. The combined organic phase was dried over MgSO4 and the solvent was evaporated. The residue was twice recrystallized from isopropanol to yield 0.410 g (80%) of the title compound. 1H NMR (400 MHz, CDCl3): δ 7.48 (s, 2H), 3.15 (t, 4H, 8.0 Hz). 1.74 (m, 4H), 1.50−1.30 (m, 18H), 0.95 (m, 12H), 0.44 (s, 18H). 13C NMR (100 MHz, CDCl3): δ 141.6, 140.4, 136.9, 129.9, 128.0, 39.6, 33.0, 32.9, 30.9, 29.2, 26.2, 23.4, 14.5, 11.3, −8.2. 2,6-Bis(trimethyltin)-4,8-(4-butylphenethyl)benzo[1,2-b:4,5b′]dithiophene (15). Compound (15) was synthesized in an analogous manner as compound (14). 1H NMR (400 MHz, CDCl3): δ 7.46 (s, 2H), δ 7.24 (d, 2H, J = 8.0 Hz), 7.14 (d, 4H, J = 8.0 Hz), 3.50 (m, 4H), 3.07 (m, 4H), 2.61 (t, 4H, J = 7.8 Hz), 1.60 (m, 4H), 1.37 (sextet, 4H, J = 7.4 Hz), 0.93 (t, 6H, J = 7.4 Hz), 0.45 (s, 18H). 13C NMR (100 MHz, CDCl3): δ 141.5, 140.6, 140.5, 139.2, 136.9, 129.6, 128.5, 128.3, 126.7, 35.75, 35.3, 33.8, 22.4, 14.0, −8.3. Typical Procedure for Polymer Synthesis. Compound 2 (88.0 mg, 0.21 mmol), compound 9 (161 mg, 0.21 mmol), Pd2(dba)3 (2 mol %), and tri(o-tolyl)phosphine (8 mol %) were placed in a flask, purged with three nitrogen/vacuum cycles, and subsequently dissolved in 5 mL of dry oxygen free chlorobenzene. The mixture was stirred for 36 h at 110 °C, after which 20 μL of 2-bromothiophene was injected as a capping agent. The reaction was stirred for 2 h at 110 °C before 20 μL of 2-(tributyltin)thiophene was injected to complete the endcapping. After an additional 2 h of stirring, a complexing ligand (N,Ndiethylphenylazothioformamide)39 was stirred with the polymer to remove any residual catalyst before being cooled to r.t. and precipitated into methanol (100 mL). The precipitate was purified via Soxhlet extraction overnight with methanol, for 2 h with acetone, and finally was collected with chloroform. The chloroform solution was then concentrated by evaporation and precipitated into methanol (200 mL) and filtered off as a dark purple solid. Typical yields were around 80%. Polymer Molecular Weight Determination. Polymer samples were dissolved in HPLC grade chloroform (∼1 mg/mL), stirred and heated at 50 °C for several hours, stirred overnight at r.t., and then filtered through a 0.45 μm PTFE filter. Size exclusion chromatography was then performed on a PL-Gel 300 × 7.5 mm (5 μm) mixed D column using an Agilent 1200 series autosampler, inline degasser, and refractometer. The column and detector temperatures were 35 °C. HPLC grade chloroform was used as eluent (1 mL/min). Linear polystyrene standards were used for calibration. The same general procedure was performed for larger scale preparatory GPC work. 4.5 mL of a ∼3 mg/mL polymer solution in HPLC grade chloroform was injected on two PL-Gel 300 × 25 mm (10 μm) mixed D columns connected in series. An Agilent 1200 series autosampler, inline degasser, and diode array detector were employed. The column and detector temperatures were 25 °C. HPLC grade chloroform was used as eluent (10 mL/min). Cyclic Voltammetry. All voltammograms were recorded at 25 °C with a CH Instruments Model 600D potentiostat. Unless otherwise specified, measurements were carried out under nitrogen at a scanning rate of 0.1 V s−1 using a platinum wire as the working electrode and a platinum wire as the counter electrode. Potentials were measured vs Ag/Ag+ (and calibrated vs Fc/Fc+) using 0.01 M AgClO4 and a 0.1 M Bu4NBF4 salt bridge to minimize contamination of the analyte with Ag+ ions. Polymer films were drop cast onto a platinum wire working electrode from a 1 mg/mL chloroform solution and dried under a stream of nitrogen prior to measurement in a 0.1 M Bu4NBF4 acetonitrile solution. Theory. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used to predict the properties of the polymers reported in this work for hydrogenterminated oligomers with n = 1,2,3,4. All calculations were performed
water for 10 min. 80 mL of THF was then added, along with 10 g of NaOH. The reaction was stirred for 2 h at r.t. while purging with N2. 2-Ethylhexyl iodide (1.95 mL, 10.9 mmol) was added, and the reaction was stirred at 50 °C overnight. The solution was diluted with 100 mL of water and extracted with 100 mL of ethyl acetate. The organic extract was dried over MgSO4 and the solvent was evaporated. Column chromatography using methylene chloride:hexane (1:2) as the eluent yielded 0.72 g (36%) of the title compound as a light yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.48 (d, 2H, J = 5.6 Hz), 7.36 (d, 2H, J = 5.6 Hz), 4.19 (d, 4H, J = 5.6 Hz), 1.8 (m, 2H), 1.75−1.25 (m, 16H), 1.02 (t, 6H, J = 7.2 Hz), 0.94 (t, 6H, J = 7.2 Hz). 13C NMR (100 MHz, CDCl3): δ 144.9, 131.7, 130.2, 126.1, 120.5, 76.3, 40.9, 30.7, 29.5, 24.1, 23.4, 14.4, 11.5. 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2b:4,5-b′] dithiophene (9). Compound (9) was obtained according to a literature procedure.9 1H NMR (400 MHz, CDCl3): δ 7.51 (s, 2H), 4.18 (d, 4H, J = 5.6 Hz), 1.90−1.35 (m, 18H), 1.02 (t, 6H, J = 7.2 Hz), 0.96 (t, 6H, J = 7.2 Hz). 13C NMR (100 MHz, CDCl3): δ 143.6, 140.7, 134.2, 133.2, 128.3, 75.9, 41.0, 30.9, 29.6, 24.2, 23.5, 14.5, 11.7, −8.1. 4,8-Bis(3-ethylhept-1-ynyl)benzo[1,2-b:4,5-b′]dithiophene (10). 3-Ethylhept-1-yne was synthesized according to a literature procedure,38 and it was used to synthesize compound (10) according to a modified literature procedure.9 Isopropylmagnesium chloride (2M, 1.84 mL, 3.68 mmol) was added dropwise to a solution of 3ethylhept-1-yne (0.5 g, 4.03 mmol) at r.t. in 25 mL of THF. The reaction mixture was stirred at 60 °C for 2 h. After cooling to r.t., benzo[1,2-b:4,5-b′]dithiophene (0.385 g, 1.75 mmol) was added, and the mixture was stirred at 60 °C for an additional 2 h. After cooling to r.t., 2 g of SnCl2 in 4 mL of a 10% HCl solution was added dropwise. The reaction was stirred for 1 more h and then poured into 100 mL of water. It was extracted twice with 50 mL of hexane. The combined organic phase was dried over MgSO4 and the solvent was evaporated. The residue was purified using column chromatography on silica with hexane:methylene chloride (3:1) as the eluent to yield 0.550 g (72%) of the title compound. 1H NMR (400 MHz, CDCl3): δ 7.57 (d, 2H, J = 5.2 Hz), 7.50 (d, 2H, J = 5.6 Hz), 2.70 (m, 2H), 1.75−1.50 (m, 12H), 1.43 (m, 4H), 1.19 (t, 6H, J = 7.4 Hz), 0.97 (t, 6H, J = 7.4 Hz). 13 C NMR (100 MHz, CDCl3): δ 140.5, 138.4, 127.8, 123.4, 112.4, 103.9, 78.6, 34.9, 30.0, 28.5, 22.9, 20.9, 14.4, 12.3. 4,8-Bis((4-butylphenyl)ethynyl)benzo[1,2-b:4,5-b′]dithiophene (11). Compound (11) was synthesized in an analogous manner as compound (10) using commercially available 1-butyl-4ethynlbenzene. 1H NMR (400 MHz, CDCl3): δ 7.70 (d, 2H, J = 5.2 Hz), 7.60 (d, 4H, J = 8.0 Hz), δ 7.57 (d, 2H, J = 5.6 Hz), δ 7.22 (d, 4H, J = 8.0 Hz), 2.65 (t, 4H, J = 7.8 Hz), 1.61 (m, 4H), 1.38 (sextet, 4H, J = 7.4 Hz), 0.94 (t, 6H, J = 7.4 Hz). 13C NMR (100 MHz, CDCl3): δ 144.3, 140.5, 138.4, 131.9, 128.9, 128.2, 123.5, 120.3, 112.3, 99.7, 85.4, 35.9, 33.6, 22.6, 14.2. 4,8-Bis(3-ethylheptyl)benzo[1,2-b:4,5-b′]dithiophene (12). Compound (12) was synthesized according to a modified literature procedure.9 To a solution of 10 (0.55 g, 1.26 mmol) in 20 mL of anhydrous THF was added 0.120 g of 10% Pd/C, and the mixture was stirred vigorously for 20 h at r.t. under 30 atm of H2. The mixture was then filtered through a Celite pad to remove the Pd/C, and the residue was purified by column chromatography on silica with hexane as the eluent to yield 0.300 g (54%) of the title compound as a white solid. 1 H NMR (400 MHz, CDCl3): δ 7.47 (d, 2H, J = 7.6 Hz), 7.46 (d, 2H, J = 7.6 Hz), 3.15 (m, 4H). 1.75 (m, 4H), 1.55−1.20 (m, 18H), 0.95 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 137.4, 136.0, 129.5, 126.1, 121.9, 39.6, 33.1, 33.0, 31.0, 29.2, 26.1, 23.4, 14.4, 11.2. 4,8-Bis(4-butylphenethyl)benzo[1,2-b:4,5-b′]dithiophene (13). Compound (13) was synthesized in an analogous manner as compound (12). 1H NMR (400 MHz, CDCl3): δ 7.45 (d, 2H, J = 7.6 Hz), δ 7.43 (d, 2H, J = 7.6 Hz), 7.20 (d, 4H, J = 8.0 Hz), 7.13 (d, 4H, J = 8.0 Hz), 3.46 (m, 4H), 3.05 (m, 4H), 2.60 (t, 4H, J = 7.8 Hz), 1.60 (m, 4H), 1.37 (sextet, 4H, J = 7.4 Hz), 0.93 (t, 6H, J = 7.4 Hz). 2,6-Bis(trimethyltin)-4,8-(3-ethylheptyl)benzo[1,2-b:4,5-b′]dithiophene (14). Compound (14) was synthesized according to a modified literature procedure.9 Compound (12) (0.300 g, 0.675 1349
dx.doi.org/10.1021/cm2038427 | Chem. Mater. 2012, 24, 1346−1356
Chemistry of Materials
Article
quantum efficiency of free carrier generation per photon absorbed, and Σμ is the sum of the mobilities of electrons and holes.43 X-ray Diffraction. X-ray diffraction measurements were performed with a Rigaku D/MAX-2500H goniometer (185 mm Bragg−Brentano geometry) equipped with a rotating anode X-ray generator (40 kV, 200 mA) using Cu Kα radiation with a wavelength of 0.154 nm and detected with a scintillation counter filtered with a single-bounce graphite monochromator. The scan rate was 1° min−1 in the 2θ range of 2° to 10° using 0.02° steps. Analyses were conducted on polymer films prepared on quartz substrates under the same conditions as their respective optimized devices.
with the default settings in the Gaussian 09 electronic structure package, revision B.01.40 The geometric structure of each oligomer was optimized in vacuum using the Becke-style three-parameter density functional with the Lee−Yang−Parr correlation function (B3LYP) with the 6-31G(d) basis set; subsequently, diffuse functions were included (6-31+G(d)) for calculation of the orbital energies and optical absorption spectra of the optimized structures as described below. Including diffuse functions had little effect on the predicted absorption spectra; the main change was a systematic shift of the molecular orbital energies down by ∼200−300 meV. For a few structures, calculations with larger basis sets were run, including additional polarization functions (6-31+G(d,p)). Little change ( 4.0). Recent studies suggest that optimal solar cell device efficiencies require polymers with optimized molecular weights. Those with low molecular weights (
