Effects of π-Conjugated Bridges on Photovoltaic Properties of Donor-π

Article pubs.acs.org/Macromolecules

Effects of π-Conjugated Bridges on Photovoltaic Properties of Donor-π-Acceptor Conjugated Copolymers Xiaochen Wang,†,‡ Yeping Sun,†,‡ Song Chen,† Xia Guo,‡ Maojie Zhang,‡ Xiaoyu Li,*,† Yongfang Li,*,‡ and Haiqiao Wang*,† †

State Key Laboratory of Organic−Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A series of conjugated donor (D)-π-acceptor (A) copolymers, P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT), based on benzodithiophene (BDT) donor unit and benzothiadiazole (BT) acceptor unit with different π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The πbridges between the BDT donor unit and BT acceptor unit are furan (F) in P(BDT-F-BT), thiophene (T) in P(BDT-T-BT) and thieno[3,2-b]thiophene (TT) in P(BDT-TT-BT). It was found that the π-bridges significantly affect the molecular architecture and optoelectronic properties of the copolymers. With the π-bridge varied from furan to thiophene, then to thieno[3,2-b]thiophene, the shape of the molecular chains changed from z-shaped to almost straight line gradually. Band gaps of P(BDT-F-BT), P(BDT-T-BT) and P(BDT-TT-BT) were tuned from 1.96 to 1.82 to 1.78 eV with HOMO levels up-shifted from −5.44 to −5.35 to −5.21 eV, respectively. Bulk heterojunction solar cells with the polymers as donor and PC71BM as acceptor demonstrated power conversion efficiency varied from 2.81% for P(BDT-F-BT) to 3.72% for P(BDT-T-BT) and to 4.93% for P(BDT-TT-BT). Compared to furan and thiophene, thieno[3,2-b]thiophene πbridge in the copolymers shows superior photovoltaic performance. The results indicate that the photovoltaic performance of some high efficiency D−A copolymers reported in literatures could be improved further by inserting suitable π-bridges.

■

INTRODUCTION During the past decade, bulk heterojunction (BHJ) polymer solar cells (PSCs) have been the focus of both scientific research and industrial application, because of their potential applications in large area, lightweight, flexible photovoltaic devices through low-cost solution-processable techniques. Conjugated polymers with electron donor−acceptor (D−A) architecture are particularly attractive due to the facile tunability of their electronic structure of the main chain. Changing the nature of the D and A moieties combining with different side chains influences numerous properties such as absorption bands, energy levels, charge carrier mobilities, and morphology.1−13 These properties affect the characteristics of PSCs and finally determine the power conversion efficiency (PCE) of the devices. Many efforts have been devoted to the design and synthesis of new donor and acceptor building blocks,14−27 in order to get efficient D−A copolymers for PSCs. But there are only a few studies focus on the bridges between D and A segments.28−30 Recently, we designed and synthesized a furan-containing D-πA copolymer.28a We found that using furan instead of thiophene as the π-bridge in the copolymer is able to significantly affect the properties of the polymer. Conjugated bridges crucially influence the electronic structure of polymer main chain and the interaction between donor and acceptor © 2012 American Chemical Society

units; therefore remarkably affect stereostructure and consequently optical, electrochemical, charge transport and photovoltaic properties of the D-π-A conjugated copolymers. Thus, it is of prime importance to fully understand the effects of the πbridges on the physicochemical and photovoltaic properties in the design of new polymers for efficient PSCs. In this study, we designed and synthesized a series of D-π-A copolymers based on benzodithiophene (BDT) donor unit and benzodiathiazole (BT) acceptor unit bridged by different heteroaromatic rings, as shown in Figure 1. Bisalkoxysubstituted BDT and bis(octyloxy)-substituted BT were selected as donor and acceptor building blocks because they can endow polymers with broad absorption and suitable molecular energy levels28a,31−33 to ensure sunlight harvesting and charge separation and transportation in the polymers. The properties of the polymers changed regularly from P(BDT-FBT) to P(BDT-T-BT) and P(BDT-TT-BT), corresponding to the π-bridge of furan, thiophene and thieno[3,2-b]thiophene. With the π-bridge units varied from furan to thiophene, then to thieno[3,2-b]thiophene, the shape of the molecular chains changed from z-shaped to almost straight line gradually. In this Received: December 7, 2011 Revised: January 13, 2012 Published: January 31, 2012 1208

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

Article

Figure 1. Molecular structures of the D-π-A conjugated copolymers with different π-bridges.

Scheme 1. Synthesis Routes of the Electron Acceptor Monomers

case, the π-orbital overlap between neighboring aromatic units was increased in turn, leading to regular changes in the frontier orbital energy levels. The absorption peak wavelengths redshifted from 534 to 568 to 631 nm; the highest occupied molecular orbital (HOMO) energy levels increased from −5.44 to −5.35 to −5.21 eV and band gaps decreased from 1.96 to 1.82 to 1.78 eV, for P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT), respectively. The photovoltaic power conversion efficiency (PCE) of solar cells based on the polymers as donor and [6,6]-phenyl C71 -butyric acid methyl ester (PC71BM) as acceptor varied from 2.81% for P(BDT-F-BT) to 3.72% for P(BDT-T-BT) to 4.93% for P(BDT-TT-BT), depending on the π-bridge in the polymer backbones.

■

UV-3150. Absorption spectra measurements of the polymer solutions were carried out in chloroform (analytical reagent) at 25 °C. Absorption spectra measurements of the polymer films were carried out on the quartz plates with the polymer films spin-coated from the polymer solutions in chloroform (analytical reagent) at 25 °C. The electrochemical cyclic voltammetry was conducted on a Zahner IM6e electrochemical workstation with a Pt plate, Pt wire, and Ag/Ag+ electrode as the working electrode, counter electrode, and reference electrode, respectively, in a 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. Polymer thin films were formed by drop-casting of polymer solutions in CHCl3 (analytical reagent) on the working electrode and then dried in air. Surface morphology of films was characterized on a Digital Instruments Multimode Nanoscope IIIa atomic force microscopy (AFM) by tapping mode in an ambient atmosphere. Fabrication of Photovoltaic Devices. PSCs were fabricated with ITO glass as a positive electrode, Ca/Al as a negative electrode, and the blend film of the polymer/PC71BM between them as a photosensitive layer. The ITO glass was precleaned and modified by a thin layer of PEDOT:PSS, which was spin-cast from a PEDOT:PSS aqueous solution (Clevious P VP AI 4083 H. C. Stark, Germany) on the ITO substrate, and the thickness of the PEDOT:PSS layer was about 60 nm. The photosensitive layer was prepared by spin-coating a blend solution of polymer and PC71BM in o-dichlorobenzene on the ITO/PEDOT:PSS electrode. Then, the Ca/Al cathode was deposited on the polymer layer by vacuum evaporation under 3 × 10−4 Pa. The thickness of the photosensitive layer was ca. 100 nm, measured on an Ambios Tech XP-2 profilometer. The effective area of one cell was ca. 4 mm2. The current−voltage (I versus V) measurement of the devices was conducted on a computer controlled Keithley 236 source measure unit. A xenon lamp with an AM 1.5 filter was used as the white-light source, and the optical power at the sample was 100 mW/cm2. Synthesis of the Monomers. 5,6-Bis(octyloxy)-4,7-bis(thieno[3,2-b]thiophen-2-yl)benzo-[c][1,2,5]-thiadiazole (5). Thieno[3,2-b]thiophene (4.8 g, 34 mmol) was dissolved in 250 mL anhydrous THF in a well-dried round flask under nitrogen atomosphere. n-Butyllithium (2.4 M in hexanes, 13.6 mL, 34 mmol) was added dropwise at 0 °C.

EXPERIMENTAL SECTION

Materials. 4,7-dibromo-5,6-bis(octyloxy)benzo-2,1,3-thiadiazole (1),22,34 4,8-dihydrobenzo[1,2-b:4,5-b′]dithiophen-4,8-dione(6)31 and 2-octyl-1-dodecyl bromide35 were synthesized according to the literatures. Tetrahydrofuran (THF) was dried over Na/benzophenone ketyl and freshly distilled prior to use. Other reagents and solvents were commercial grade and used as received without further purification. All reactions were performed under nitrogen atmosphere. Measurements and Characterization. Molecular weights of the polymers were measured by gel permeation chromatography (GPC) method on Waters 515−2410 with polystyrenes as reference standard and tetrahydrofuran (THF) as an eluent. All new compounds were characterized by nuclear magnetic resonance spectra (NMR) recorded on a Bruker AV 600 spectrometer in CDCl3 at room temperature. Chemical shifts of 1H NMR were reported in ppm. Splitting patterns were designated as s (singlet), d (doublet), t (triplet), m (multiplet), and br (broad). Elemental analyses were performed on a Flash EA 1112 analyzer or Elementar vario EL III. Thermal gravimetric analysis (TGA, Netzsch TG209C) measurements were carried out under a nitrogen atmosphere at a heating rate of 10 °C/min. UV−vis absorption spectra were recorded on a Shimadzu spectrometer model 1209

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

Article

Scheme 2. Synthesis Routes of the Electron Donor Monomers

Scheme 3. Synthetic Route of the Copolymers P(BDT-F-BT), P(BDT-T-BT) and P(BDT-TT-BT)-EH.

After stirring for 1 h, trimethyltinchloride (1 M in THF, 34 mL, 34 mmol) was added dropwise, and the mixture was allowed to warm to room temperature. After stirring for another 6 h, compound 1 (7.5 g, 13.6 mmol) and PdCl2(PPh3)2 (0.42 g, 0.6 mmol) were added to this mixture. After flushing with nitrogen for 10 min, the reactant was heated to reflux for 24 h. After cooled to room temperature, the reaction mixture was poured into water. The product was extracted with ethyl acetate (3 × 150 mL). The extracts were combined and washed with water and brine then dried over anhydrous magnesium sulfate. After filtration, the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica, eluting with petroleum ether/dichloromethane (from 20:0 to 20:1), to give compound 5 as a dark red liquid (7.2 g, 79%). 1H NMR (600 MHz, CDCl3): δ (ppm) 8.80 (s, 2H, Ar−H), 7.46 (d, 2H, Ar− H), 7.33 (d, 2H, Ar−H), 4.16 (t, 4H, CH2), 1.98 (m, 4H, CH2), 1.47 (m, 4H, CH2), 1.38−1.30 (m, 16H, CH2), 0.90 (t, 6H, CH3) 4,7-Bis(5-bromothieno[3,2-b]thiophen-2-yl)-5,6-bis(octyloxy)benzo-[c][1,2,5]-thiadiazole (TTBT). Compound 5 (5.89 g, 8.8 mmol) was added to a mixture of chloroform (150 mL) and acetic acid (150 mL). After N-bromosuccimide (NBS) (3.29 g, 18.5 mmol) was added,

the mixture was stirred at room temperature in dark overnight. Then the reaction mixture was poured into water (200 mL) and extracted with chloroform (3 × 50 mL). The extracts were combined and washed with water and sat. sodium bicarbonate solution then dried over anhydrous magnesium sulfate. After filtration, the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica, eluting with petroleum ether to give TTBT as a dark red crystal (5.9 g, 81%). Mp 109−111 °C. 1H NMR (600 MHz, CDCl3): δ (ppm) 8.75 (s, 2H, Ar−H), 7.33 (s, 2H, Ar−H), 4.15 (t, 4H, CH2), 1.96 (m, 4H, CH2), 1.46 (m, 4H, CH2), 1.36−1.25 (m, 16H, CH2), 0.90 (t, 6H, CH3). 13C NMR (600 MHz, CDCl3): δ(ppm) 151.87, 150.65, 140.22, 139.49, 135.61, 122.47, 122.20, 117.77, 114.66,77.20, 76.99, 76.78, 74.79, 31.81, 30.33, 29.49, 29.28, 25.95, 22.66, 14.09. 4,8-Di(2-octyldodecyloxy)benzo[1,2-b;3,4-b]dithiophene (8). Compound 6 (4.4 g, 20 mmol), zinc powder (3.9 g, 60 mmol), and NaOH (24 g, 0.6 mol) were added to 100 mL of water. The mixture was well stirred and heated to reflux for 1 h. Then, 2-octyl-1-dodecyl bromide (29 g, 80 mmol) and tetrabutylammonium bromide (0.65 g, 2 mmol) were added into the flask. After being refluxed for 6 h, the 1210

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

Article

Scheme 4. Synthetic Route of the Polymer P(BDT-TT-BT).

reactant was cooled to room temperature and added to 200 mL dichloromethane. Water was separated and extracted with dichloromethane (3 × 50 mL). The combined dichloromethane layer was dried over anhydrous magnesium sulfate. After filtration, the solvent was removed under reduced pressure. The crude product was purified by column chromatography on silica, eluting with petroleum ether to give Compound 8 as a white waxy solid (11.6 g, 75%). Mp 33−35 °C. 1 H NMR (600 MHz, CDCl3): δ (ppm) 7.48 (d, 2H), 7.36 (d, 2H), 4.18 (d, 4H), 1.86 (m, 2H), 1.64(m, 4H), 1.51 (m, 4H), 1.43 (m, 8H),1.29 (m, 48H), 0.90 (m, 12H). 13C NMR (600 MHz, CDCl3): δ (ppm) 144.65, 131.47, 129.91, 125.85, 120.25, 76.38, 39.20, 31.92, 31.32, 30.06, 29.66, 29.35, 26.98, 22.69, 14.11. 2,6-Bis(trimethyltin)-4,8-di(2-octyldodecyloxy)benzo[1,2-b;3,4-b]dithiophene (ODBDT). Compound 8 (3.9 g, 5 mmol) was dissolved in 80 mL of anhydrous THF and cooled down to −78 °C by a liquid nitrogen-acetone bath under nitrogen protection. Butyllithium solution (2.4 M, 5 mL, 12 mmol) was added dropwise with stirring. After being stirred at −78 °C for 1 h, the mixture was stirred for another 1 h without liquid nitrogen-acetone bath. When the mixture was cooled down to −78 °C again, trimethyltin chloride solution (1 M in THF, 12 mL, 12 mmol) was added and the reaction mixture was warmed to room temperature. After stirred overnight, the reactant was quenched with 50 mL of water and extracted with petroleum ether. The organic extraction was dried with anhydrous sodium sulfate and evaporated in vacuo. The crude product was purified by recrystallization in ethanol to yield the titled compound ODBDT as a white needle crystal (4.65 g, 84%). Mp 36−38 °C. 1H NMR (600 MHz, CDCl3): δ (ppm) 7.51 (s, 2H), 4.18 (d, 4H), 1.86 (m, 2H), 1.64(m, 4H), 1.54−1.42 (m, 12H),1.27 (m, 48H), 0.88 (m, 12H), 0.44(s, 18H). 13C NMR (600 MHz, CDCl3): δ (ppm) 143.24; 140.31, 133.84; 132.89, 127.96, 75.92, 39.20, 31.93, 31.41, 30.18, 29.76, 29.72, 29.69, 29.40, 29.37, 27.07, 22.69, 14.11. Synthesis of the Polymer. P(BDT-T-BT). TBT monomer (357 mg, 0.5 mmol) and EHBDT monomer (386 mg, 0.5 mmol) were put into a 25 mL two-neck flask, and 10 mL of toluene was added. The mixture was stirred and purged with nitrogen for 10 min, and then Pd(PPh3)4 (30 mg, 0.025 mmol) was added. After being purged for 15 min, the mixture was heated at 110 °C for 48 h. After cooled to room temperature, the reaction mixture was added dropwise to 200 mL acidic methanol (HCl + CH3OH), then collected by filtration and washed with methanol. The black solid was filtered into a Soxhlet funnel and extracted by methanol, hexane, and chloroform successively. The polymer recovered from chloroform was purified by preparative gel permeation chromatography. Then, the product was dried under vacuum for 1 day to recover the target polymer P(BDT-FBT) as a dark red solid (yield 66%, Mn= 10.4 kDa, Mw = 23.2 kDa, PDI = 2.2). 1H NMR (CDCl3, 600 MHz): δ (ppm) 8.55 (br,2H), 7.71−6.95 (br, 4H), 4.23 (br, 8H), 2.06(br, 6H), 1.91−1.25 (m, 36H), 1.25−0.90 (m, 18H). Anal. Calcd for (C56H74N2O4S5)n: C, 67.23; H, 7.40; N, 2.80. Found: C, 66.85; H, 7.44; N, 2.87. P(BDT-TT-BT)-EH. TTBT monomer (165 mg, 0.2 mmol) and EHBDT monomer (154 mg, 0.2 mmol) were put into a 25 mL twoneck flask, and 10 mL of toluene was added. The mixture was stirred and purged with nitrogen for 10 min, and then Pd(PPh3)4 (30 mg, 0.025 mmol) was added. When the mixture was heated at 110 °C for 6 h after being purged for 15 min, a great deal of dark solid precipitate appeared in the flask. After cooled to room temperature, the

precipitate was collected by filtration and washed with methanol. The black solid can not be dissolved even in hot THF or chlorobenzene. This polymer can not be further purified and characterized due to its poor solubility in common organic solvents. P(BDT-TT-BT). TTBT monomer (124 mg, 0.15 mmol) and ODBDT monomer (166 mg, 0.15 mmol) were put into a 25 mL two-neck flask, then 6 mL of chlorobenzene was added. The mixture was stirred and purged with nitrogen for 10 min, and then Pd2(dba)3 (10 mg, 0.01 mmol), (o-tol)3P (25 mg, 0.08 mmol) was added. After being purged for 15 min, the mixture was heated at 140 °C for 48 h. After cooled to room temperature, the reaction mixture was added dropwise to 200 mL of methanol. The black solid was filtered into a Soxhlet funnel and extracted by methanol, hexane, and chloroform successively. The polymer recovered from chloroform solution was purified by preparative gel permeation chromatography. Then, the product was dried under vacuum for 1 day to yield the titled polymer P(BDT-TT-BT) as a black solid (yield 68%, Mn= 32.8 kDa, Mw = 65.0 kDa, PDI = 2.0). 1H NMR (CDCl3, 600 MHz): δ (ppm) 8.45 (br,2H), 6.88 (br, 4H), 4.20 (br, 8H), 1.45−1.19 (br, 90H), 0.88 (br, 18H). Anal. Calcd for (C84H126N2O4S7)n: C, 69.40; H, 8.67; N, 1.93. Found: C, 69.40; H, 8.59; N, 1.96.

■

RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis routes of monomers and corresponding polymers are outlined in Schemes 1−4, respectively. Monomer FBT, TBT, EHBDT, and polymer P(BDT-F-BT) were synthesized according to our previously proposed procedure.28a,34 Polymers P(BDT-F-BT), P(BDT-T-BT), P(BDT-TT-BT)-EH and P(BDT-TT-BT) were synthesized by palladium-catalyzed Stille-coupling36 polymerization. It should be mentioned that in the polymerization for the synthesis of P(BDT-TT-BT), the catalyst of Pd2(dba)3/(o-tol)3P was used instead of Pd(PPh3)4 used in the synthesis of P(BDT-F-BT) and P(BDT-T-BT). Higher yield of P(BDT-TT-BT) was achieved with the catalyst of Pd2(dba)3 /(o-tol)3P. All starting materials, reagents, and solvents were carefully purified, and all procedures were performed under an air-free environment. P(BDT-TT-BT)-EH was insoluble, and can not be further characterized. To resolve this problem, P(BDT-TT-BT) with longer branched 2-octyldodecyloxy groups on BDT segment was designed and synthesized. All the polymers except P(BDT-TT-BT)-EH can be readily dissolved in common solvents, such as chloroform, toluene, THF and chlorobenzene, and processed to form smooth and pinhole-free films upon spin-coating. The structure of P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT) were confirmed by 1H NMR spectroscopy and elemental analysis. Thermal properties of the polymers were determined by thermogravimetric analysis (TGA) under nitrogen atmosphere at a heating rate of 10 °C/min. The three polymers have good thermal stability with onset decomposition temperatures corresponding to 5% weight loss at 332, 327, and 320 °C, respectively, as shown in Figure 2. Obviously, the 1211

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

Article

Electrochemical Properties. Cyclic voltammetry has been employed and considered as an effective tool in investigating electrochemical properties of conjugated oligomers and polymers.37 From the onset oxidation and reduction potentials in the cyclic voltammogram, energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels can be readily estimated, which correspond to ionization potential (IP) and electron affinity (EA), respectively.26,38 Cyclic voltammograms of the polymer films are shown in Figure 4. The onset oxidation potential (Eox)/onset reduction

Figure 2. TGA plots of the polymers with a heating rate of 10 °C/min under an inert atmosphere.

thermal stability of these polymers is adequate for their applications in PSCs and other optoelectronic devices. Optical Properties. Figure 3 shows absorption spectra of the polymers in chloroform solution (1 mg/100 mL) and as

Figure 4. Cyclic voltammograms of the polymer films on Pt electrode in 0.1 mol/L Bu4NPF6, CH3CN solution with a scan rate of 100 mV/s.

potential (Ered) of P(BDT-F-BT), P(BDT-T-BT) and P(BDTTT-BT) are 0.73/−1.33, 0.64/−1.27 and 0.50/−1.17 V vs. Ag/ Ag+ respectively. The HOMO and LUMO energy levels of the polymers were calculated from the onset oxidation potential and the onset reduction potential according to the equations:39

Figure 3. Absorption spectra of P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT) in chloroform solution (a, concentration of 1 mg/ 100 mL) and solid film on a quartz plate (b).

spin-coated films on quartz substrates. Table 1 summarizes the optical data, including the absorption peak wavelengths (λabs), absorption edge wavelengths (λonset), and the optical band gap (Egopt). All of the absorption spectra recorded from dilute chloroform solutions feature three absorption bands: the first one around 345 nm, the second one at 360−480 nm, which can be assigned to localized π−π* transitions, and the third broader band from 480 to 700 nm in the long wavelength region, corresponding to intramolecular charge transfer (ICT) between donor and acceptor units. The difference in position and intensity of the absorptions are mainly duo to the different structures of the π-bridges in the polymers. The absorption spectra of the three polymers in the solid film were similar to their corresponding solution spectra, with obviously red-shifts (ca. 12−19 nm) of their absorption maxima, indicating intermolecular interactions existing in the solid state. In addition, these polymers all displayed a vibronic shoulders in films at 578, 616, and 631 nm, respectively, implying that strong π−π stacking between the polymeric backbones have formed ordered arrangement in their solid films. The absorption edges for solid films of P(BDT-F-BT), P(BDT-TBT), and P(BDT-TT-BT) increase from 632 to 680 nm and to 696 nm, corresponding to optical band gaps (Egopt) decreasing from 1.96 to1.82 to 1.78 eV, respectively, as shown in Table 1.

HOMO = − IP = − e(Eox + 4.71) (eV)

(1)

LUMO = − EA = − e(Ered + 4.71) (eV)

(2)

From the value of Eox and Ered of the polymers, the HOMO and the LUMO as well as the electrochemical bandgaps (Egec) were calculated and also listed in Table 1. The LUMO energy levels of P(BDT-F-BT), P(BDT-T-BT) and P(BDT-TT-BT) are very close. This phenomenon indicates that different πbridges do not significantly affect the LUMO levels of the D-πA polymers. The LUMO energy levels of the polymers are all located within a suitable range (from −3.44 to −3.54 eV, Table 1) and are significantly higher-lying than that of PC71BM (ca. −3.91 eV);40 thus, efficient charge transfer (excitons dissociation) could be expected to occur in their corresponding devices.1,2,5 From the comparison with P(BDT-F-BT), it was noticed that the π-bridges of thiophene and thieno[3,2b]thiophene lowered band gap of P(BDT-T-BT) and P(BDT-TT-BT) by increasing the HOMO energy level of the polymer from −5.44 eV (P(BDT-F-BT)) to −5.35 eV (P(BDT-T-BT)), and to −5.21 eV (P(BDT-TT-BT)). The electrochemical band gaps of the polymers are well matched with their optical band gaps within the experimental error. Theoretical Calculations. To evaluate the impact of the different π-bridges on molecular architecture and consequently

Table 1. Optical and Electrochemical Properties of P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT). polymer

λabs in CHCl3 (nm)

λabs in films (nm)

λonset in films (nm)

Egopt (eV)

HOMO (eV)

LUMO (eV)

EgCV (eV)

P(BDT-F-BT) P(BDT-T-BT) P(BDT-TT-BT)

343, 391, 522 346, 405, 554 343, 438, 612

344, 398, 534 350, 415, 568 346, 440, 631

632 680 696

1.96 1.82 1.78

−5.44 −5.35 −5.21

−3.48 −3.44 −3.54

1.96 1.91 1.66

1212

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

Article

Figure 5. Top view (top) and side view (bottom) of optimized geometries of the three copolymers backbone units with a chain length n = 1. Color code: gray (C), white (H), red (O), blue (N), and yellow (S).

Figure 6. The frontier molecular orbital (LUMO, top; HOMO, bottom) obtained from DFT calculations on the polymers with a chain length n = 1. Color code: gray (C), white (H), red (O), blue (N), and yellow (S).

Table 2. Calculated Dihedral Angles, Bond Angle and Corresponding HOMO, LUMO Levels and Bandgaps (Egcal) of the Polymers polymer

π-bridge

θ1 (deg)

θ2 (deg)

θ3 (deg)

HOMO (eV)

LUMO (eV)

Egcal (eV)

P(BDT-F-BT) P(BDT-T-BT) P(BDT-TT-BT)

furan thiophene thieno[3,2-b]thiophene

−1.02 8.67 −6.16

4.88 8.51 −7.70

127.50 151.61 181.52

−5.26 −5.21 −5.20

−2.78 −2.86 −2.92

2.48 2.35 2.28

bridge, θ2 is dihedral angles between π-bridge and BDT unit, θ3 is the angle between the bonds across the π-bridge drawn in red. Different π-bridge units significantly affect the molecular architecture, as shown in Figure 5 and Table 2. The dihedral angles between the acceptor unit and π-bridge and between πbridge and donor unit are all less than 10 o in all the three polymers, indicating planar conformation of the copolymer chains. Interestingly, θ1 and θ2 is alternating negative and positive in furan bridged polymer, while always positive in thiophene bridged polymer and negative in thieno[3,2b]thiophene bridged polymer. In other words, taking benzothiadiazole unit as reference plane, furan and BDT unit by turns deflected counterclockwise and clockwise in P(BDTF-BT); while thiophene and BDT unit deflected clockwise in turn in PBTBDT and thieno[3,2-b]thiophene and BDT unit deflected counterclockwise in turn in P(BDT-TT-BT). With the π-bridge varied from furan to thiophene, then to thieno[3,2b]thiophene, the shape of the molecular chains changed from zshaped to almost straight line with increasing of θ3 from 127.50° in P(BDT-F-BT) to 151.61 o in P(BDT-T-BT), then to 181.52 o in P(BDT-TT-BT). In this cases, the π-orbital overlap between neighboring aromatic units are increased, leading to changes in the frontier orbital energy levels and, more specifically, to decrease in band-gaps of thiophene and thieno[3,2-b]thiophene bridged polymers in turn. Note that due to the small number of aromatic cycles taken into account by the calculation, the absolute energy values are not reliable.

on the optoelectronic properties of the copolymers, density functional theory (DFT) calculations were performed to verify stationary points as stable states for the optimized conformations and single point energies, with a molecular main chain length n = 1, at B3LYP/6-311++G(3df, 3pd) level of theory in vacuum using the Gaussian 03 program package.41 The final energies were calculated as the sum of single point and zero point energies. In particular, the HOMO and LUMO level positions and related electron distributions were calculated. Moreover, all the alkyl chains were replaced by methyl groups in the calculation to avoid excessive computation demand. Optimized geometries of the three copolymers in ground state are shown in Figure 5. Calculated dihedral angles, bond angle and corresponding HOMO, LUMO levels and bandgaps (Egcal) are summarized in Table 2. The general molecules structure sketch used for the computational data is shown in Figure 7, where θ1 is dihedral angle between BT unit and π-

Figure 7. Molecular geometry sketch used for the computational data. 1213

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

Article

Only relative changes of the HOMO and LUMO values with respect to the reference molecules are meaningful. The wave functions of the frontier molecular orbital are depicted in Figure 6. As can be observed, the HOMO is delocalized along the whole π-conjugated backbone while the LUMO is mostly concentrated on the benzothiadiazole-based acceptor groups. These images provide further evidence of the formation of well-defined D-π-A structure and the intramolecular charge transfer behavior of the material (i.e., the HOMO to LUMO transition is a donor to acceptor intramolecular charge transfer).42 The calculated HOMO and LUMO level positions and band gaps of the three polymers are listed in Table 2. As can be seen, although discrepancies exist between the calculation and experimental results, the trends of variation in the HOMO and energy gaps are similar. Compared with furan-bridged P(BDT-F-BT), it was noticed that the πbridge of thiophene and thieno[3,2-b]thiophene increased the HOMO energy level of the relative polymer P(BDT-T-BT) and P(BDT-TT-BT), and as a result, lowered band gaps of the two polymers. Photovoltaic Properties. To investigate the effects of πconjugated bridges on the photovoltaic properties of the copolymers, bulk heterojunction PSC devices with a configuration of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al were fabricated. Figure 8 shows the current density-potential

Figure 9. ln(JL3/V2) versus (V/L)0.5 plots for the measurement of the hole mobility in polymer:PC71BM blends by the SCLC method.

Table 3. Photovoltaic Properties of the PSCs Based on P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT) as Donor and PC71BM as Acceptor under the Illumination of AM1.5G, 100 mW/cm2 polymer P(BDTF-BT) P(BDTT-BT) P(BDTTT-BT)

characteristic of PSCs based on the blends of polymer:PC71BM under illumination of AM1.5G, 100 mW/cm2. To further study the performance of the PSCs devices, the hole mobilities in the photosensitive layers were measured by the space charge limited current (SCLC) method using devices with structure of ITO/PEDOT:PSS/polymer:PC71BM/Au. For unipolar transport in a trap-free semiconductor with an ohmic injecting contact, the SCLC can be approximated by the Mott-Gurney equation:43

⎛ 9 V ⎞ V2 εr ε0μ0 exp⎜0.891γ ⎟ 8 L ⎠ L3 ⎝

Voc (V)

Jsc (mA cm−2)

FF

PCE (%)

mobility (cm2 V−1 s−1)

1:2

0.94

6.50

0.46

2.81

2.1 × 10−3

1:1.5

0.82

9.45

0.48

3.72

2.9 × 10−3

1:1.5

0.69

11.34

0.63

4.93

8.6 × 10−3

calculated using eq 3 are 2.1 × 10−3 cm2 V−1 s−1, 2.9 × 10−3 cm2 V−1 s−1 and 8.6 × 10−3 cm2 V−1 s−1 for P(BDT-F-BT), P(BDTT-BT), and P(BDT-TT-BT), respectively. All the three polymers exhibited high hole mobilities in their respective devices (order of magnitude of 10−3 cm2 V−1 s−1). The open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), power conversion efficiency (PCE) and hole mobility of the PSCs are summarized in Table 3. The Voc of PSCs based on P(BDT-F-BT), P(BDT-T-BT), and P(BDTTT-BT) decreased from 0.94 to 0.82 to 0.69 V, which is the result of the increase of the HOMO level of the polymers. Compared to P(BDT-F-BT) and P(BDT-T-BT), the thieno[3,2-b]thiophene bridged polymer showed dramatically high fill factor of 0.63, which is about one-third higher than furan and thiophene bridged polymers. The increased FF of P(BDT-TTBT)-based PSCs could be ascribed to its higher hole mobility (8.6 × 10−3 cm2 V−1 s−1). The short-circuit currents of PSCs based on the polymers increased from 6.50 to 9.45 to 11.34 mA cm−2, which is consistent with the lowered band gaps (from 1.96 to 1.82 to 1.78 eV) and enhanced extinction coefficients (from 31 to 44 to 51 cm−1g−1 L). Although the relative low open-circuit voltage, P(BDT-TT-BT) exhibited the highest PCE of 4.93% in all the three aromatic rings bridged polymers. The high photovoltaic performance of thieno[3,2-b]thiophene bridged polymer should be benefited from its high hole mobility and broad absorption. We also checked the effect of the morphology of the active layers on the photovoltaic performance of the devices. Figure S1 in Supporting Information shows the AFM images of the active layer surfaces. The average surface roughness (Ra) of the AFM topographic images were 0.32 nm, 0.25 and 0.42 nm for the blends of P(BDT-F-BT):PC71BM, P(BDT-T-BT):PC71BM, and P(BDT-TT-BT):PC71BM, respectively, indicating smooth surface of the active layers. The uniformity of the phase image for the film of P(BDT-TT-BT):PC71BM is better than those of the P(BDT-F-BT):PC71BM and P(BDT-T-BT):PC71BM films. Probably, the uniform interpenetrating network in the

Figure 8. Current density−voltage characteristics of PSCs based on polymer:PC71BM blends under illumination of AM1.5G, 100 mW/ cm2.

J≅

polymer/ PC71BM (w/w)

(3)

where J is the current density, εr is the dielectric constant of the polymer, ε0 is the free-space permittivity (8.85 × 10−12 F/m), μ0 is the charge mobility at zero field, γ is a constant, L is the thickness of the blended film layer, V = Vappl − Vbi, Vappl is the applied potential, and Vbi is the built-in potential which results from the difference in the work function of the anode and the cathode (in this device structure, Vbi = 0.2 V). Figure 9 displays ln(JL3/V2) versus (V/L)0.5 curve for the measurement of the hole mobility of the copolymers by the SCLC method. As summarized in Table 3, the hole mobilities of the polymers 1214

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

■

P(BDT-TT-BT):PC71BM layer benefited the better photovoltaic performance of the corresponding PSCs. Figure 10 shows the external quantum efficiency (EQE) curves of the PSCs based on the polymer:PC71BM blends.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.W.); [email protected]. cn (Y.L.); [email protected] (X.L.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by NSFC (Nos. 20874106, 20821120293 and 21021091), The Ministry of Science and Technology of China and Chinese Academy of Sciences.

■

Figure 10. EQE spectra of the PSCs based on polymer:PC71BM blends.

These devices exhibited broad EQE responses with maximum EQE values of 40% at 472 nm, 52% at 465 nm, and 60% at 500 nm for the PSCs based on P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT), respectively. Compared to the absorption spectra of pristine polymers, the significantly broadened EQE responses in the visible region can be attributed to both the intrinsic absorptions of the polymers and the response of PC71BM. Compared with that of P(BDT-F-BT), P(BDT-TBT)-based device demonstrated much higher EQE, especially in the range of 500−600 nm, and P(BDT-TT-BT)-based device showed the highest EQE value, especially from 500 to 700 nm. The EQE results are consistent with the Jsc values of the corresponding PSCs.

■

CONCLUSION

■

ASSOCIATED CONTENT

REFERENCES

(1) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324−1338. (2) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58−77. (3) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497−500. (4) Facchetti, A. Chem. Mater. 2011, 23, 733−758. (5) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789−794. (6) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Chem. Soc. Rev. 2011, 40, 1185−1199. (7) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1−16. (8) Boudreault, P.-L. T.; Najari, A.; Leclerc, M. Chem. Mater. 2011, 23, 456−469. (9) Hoppe, H.; Sariciftci, N. S. Adv. Polym. Sci. 2008, 214, 1−86. (10) Li, Y. F.; Zou, Y. P. Adv. Mater. 2008, 20, 2952−2958. (11) Chen, J. W.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709−1718. (12) Po, R.; Maggini, M.; Camaioni, N. J. Phys. Chem. C 2010, 114, 695−706. (13) Chochos, C. L.; Choulis, S. A. Prog. Polym. Sci. 2011, 36, 1326− 1414. (14) (a) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649−653. (b) Wang, M.; Hu, X. W.; Liu, P.; Li, W.; Gong, X.; Huang, F.; Cao, Y. J. Am. Chem. Soc. 2011, 13, 9638−9641. (15) (a) Zhou, E. J.; Nakamura, M.; Nishizawa, T.; Zhang, Y.; Wei, Q. S.; Tajima, K.; Yang, C. H.; Hashimoto, K. Macromolecules 2008, 41, 8302−8305. (b) Zhou, E. J.; Yamakawa, S.; Tajima, K.; Yang, C. H.; Hashimoto, K. Chem. Mater. 2009, 21, 4055−4061. (c) Zhou, E. J.; Cong, J. Z.; Tajima, K.; Hashimoto, K. Chem. Mater. 2010, 22, 4890− 4895. (16) (a) Huo, L. J.; Guo, X.; Zhang, S. Q.; Li, Y. F.; Hou, J. H. Macromolecules 2011, 44, 4035−4037. (b) Zhang, M. J.; Sun, Y. P.; Guo, X.; Cui, C. H.; He, Y. J.; Li, Y. F. Macromolecules 2011, 44, 7625−7631. (17) (a) Liang, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L. P. J. Am. Chem. Soc. 2009, 131, 7792−7799. (b) Zhang, M. J.; Fan, H. J.; Guo, X.; He, Y. J.; Zhang, Z. G.; Min, J.; Zhang, J.; Zhan, X. W.; Li, Y. F. Macromolecules 2010, 43, 5706−5712. (18) Cui, C. H.; Fan, X.; Zhang, M. J.; Zhang, J.; Min, J.; Li, Y. F. Chem. Commun. 2011, 47, 11345−11347. (19) Huo, L. J.; Hou, J. H.; Zhang, S. Q.; Chen, H. Y.; Yang, Y. Angew. Chem., Int. Ed. 2010, 122, 1542−1545. (20) Wang, C. L.; Zhang, W. B.; Van Horn, R. M.; Tu, Y. F.; Gong, X.; Cheng, S. Z. D.; Sun, Y. M.; Tong, M. H.; Seo, J.; Hsu, B. B. Y.; Heeger, A. J. Adv. Mater. 2011, 23, 2951−2956. (21) Zhou, H. X.; Yang, L. Q.; Stuart, A. C.; Price, S. C.; Liu, S. B.; You, W. Angew. Chem., Int. Ed. 2011, 50, 2995−2998. (22) Qin, R. P.; Li, W. W.; Li, C. H.; Du, C.; Veit, C.; Schleiermacher, H.−F.; Andersson, M.; Bo, Z. S.; Liu, Z. P.; Inganäs, O.; Wuerfel, U.; Zhang, F. L. J. Am. Chem. Soc. 2009, 131, 14612−14613. (23) Bronstein, H.; Chen, Z. Y.; Ashraf, R. S.; Zhang, W. M.; Du, J. P.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.;

We designed and synthesized a series of D-π-A copolymers of BDT donor unit and BT acceptor unit with furan, thiophene and thianothiaphene π-bridges, P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT), by Pd-catalyzed Stille-coupling reaction. These polymers possess good solubility and thermal stability. The π-bridge units significantly influence the molecular architecture and consequently impact on the optoelectronic properties of the copolymers. The absorption peaks of the P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT) films red-shifted from 534 to 568 to 631 nm, with the absorption edges of 632, 680, and 696 nm, respectively. The band gaps of the three polymers were tuned from 1.96 to 1.82 to 1.78 eV with the HOMO levels upshift from −5.44 to −5.35 to −5.21 eV. The PCE of the PSCs based on the polymers as donor and PC71BM as acceptor reached 2.81% for P(BDT-F-BT), 3.72% for P(BDT-T-BT) and 4.93% for P(BDT-TT-BT), respectively. Obviously, the PCE of the PSC based on the thieno[3,2b]thiophene bridged polymer P(BDT-TT-BT) is much higher than that of the furan and thiophene-bridged polymers P(BDTF-BT) and P(BDT-T-BT). These results indicate that thieno[3,2-b]thiophene bridge is propitious to enhance the photovoltaic properties of D-π-A copolymers. Most importantly, these results provide new insights toward the design of high performance photovoltaic materials.

S Supporting Information *

AFM images of the active layers of solar cells. This material is available free of charge via the Internet at http://pubs.acs.org. 1215

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216

Macromolecules

Article

Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272−3275. (24) Zou, Y. P.; Najari, A.; Berrouard, P.; Beaupré, S.; Aïch, B. R.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2010, 132, 5330−5331. (25) Price, S. C.; Stuart, A. C.; Yang, L. Q.; Zhou, H. X.; You, W. J. Am. Chem. Soc. 2011, 133, 4625−4631. (26) Wang, X. C.; Luo, H.; Sun, Y. P.; Zhang, M. J.; Li, X. Y.; Yu, G.; Liu, Y. Q.; Li, Y. F.; Wang, H. Q. J. Polym. Sci., Part A: Polym Chem. 2012, 50, 371−377. (27) Zhang, M. J.; Guo, X.; Wang, X. C.; Wang, H. Q.; Li, Y. F. Chem. Mater. 2011, 23, 4264−4270. (28) (a) Wang, X. C.; Chen, S.; Sun, Y. P.; Zhang, M. J.; Li, Y. F.; Li, X. Y.; Wang, H. Q. Polym. Chem. 2011, 2, 2872−2877. (b) Zhang, M. J.; Guo, X.; Li, Y. F. Macromolecules 2011, 44, 8798−8804. (29) Baek, N. S.; Hau, S. K.; Yip, H.-L.; Acton, O.; Chen, K.-S.; Jen, A. K.-Y. Chem. Mater. 2008, 20, 5734−5736. (30) Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.; Fréchet, J. M. J. J. Am. Chem. Soc. 2010, 132, 15547−15549. (31) Hou, J. H.; Park, M.-H.; Zhang, S. Q.; Yao, Y.; Chen, L. M.; Li, J. H.; Yang, Y. Macromolecules 2008, 41, 6012−6018. (32) Helgesen, M.; Gevorgyan, S. A.; Krebs, F. C.; Janssen, R. A. J. Chem. Mater. 2009, 21, 4669−4675. (33) Ding, P.; Chu, C. C.; Liu, B.; Peng, B.; Zou, Y. P.; He, Y. H.; Zhou, K. C.; Hsu, C. S. Macromol. Chem. Phys. 2010, 211, 2555−2561. (34) Qing, F. Y.; Sun, Y. P.; Wang, X. C.; Li, N.; Li, Y. F.; Li, X. Y.; Wang, H. Q. Polym. Chem. 2011, 2, 2102−2106. (35) Guo, X. G.; Watson, M. D. Org. Lett. 2008, 10, 5333−5336. (36) Bao, Z. N.; Chan, W. K.; Yu., L. P. J. Am. Chem. Soc. 1995, 117, 12426−12435. (37) Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J. Synth. Met. 1999, 99, 243−248. (38) Wang, X. C.; Wang, H. Q.; Yang, Y.; He, Y. J.; Zhang, L.; Li, Y. F.; Li, X. Y. Macromolecules 2010, 43, 709−715. (39) (a) Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J. Mater. Chem. 2003, 13, 800−806. (b) Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y. F. J. Am. Chem. Soc. 2006, 128, 4911−4916. (40) (a) He, Y. J.; Li, Y. F. Phys. Chem. Chem. Phys. 2011, 13, 1970− 1983. (b) He, Y. J.; Zhao, G. J.; Peng, B.; Li, Y. F. Adv. Funct. Mater. 2010, 20, 3383−3389. (41) 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.; 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. (42) Pappenfus, T. M.; Schneiderman, D. K.; Casado, J.; López Navarrete, J. T.; Ruiz Delgado, M. C.; Zotti, G.; Vercelli, B.; Lovander, M. D.; Hinkle, L. M.; Bohnsack, J. N.; Mann, K. R. Chem. Mater. 2011, 23, 823−831. (43) Kippelen, B., Yoo, S., Haddock, J. A., Domercq, B., Barlow, S., Minch, B., Xia, W., Marder, S. R., Armstrong, N. R. In Organic photovoltaics: mechanism, materials, and devices; Sun, S.-S., Sariciftci, N. S., Eds.; Taylor Francis Inc: Boca Raton, FL, 2005; Vol. 11.

1216

dx.doi.org/10.1021/ma202656b | Macromolecules 2012, 45, 1208−1216