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Supporting Information Controlling Side‐Chain Density of Electron Donating Polymers for Improving Their Packing Structure and Photovoltaic Performance Chul‐Hee Cho,1 Hyunbum Kang,1 Tae Eui Kang,1 Han‐Hee Cho,1 Sung‐Cheol Yoon,2 Moon‐Kook Jeon,3 and Bumjoon J. Kim1,* 1
Department of Chemical and Biomolecular Engineering, Korea Advanced Instituted Science and Technology (KAIST), Daejeon 305‐701, Korea 2Advanced Materials Division and 3Center for Drug Discovery Platform Technology, Korea Research Institute of Chemical Technology, Daejeon 305‐600, Korea ◈ Synthesis and Characterization of Polymers All commercially available reagents were used without further purification unless otherwise indicated. The organic solvents (toluene, chlorobenzene, CHCl3, DMF, ethanol, etc.) were used as anhydrous solvents. THF was distilled from sodium/benzophenone under an atmosphere of nitrogen prior to use. All 1H and 13C NMR spectra were recorded at 500 and 125 MHz respectively, using CDCl3 as a solvent, unless otherwise stated. The chemical shifts of all 1H and 13C NMR spectra are referenced to the residual signal of CDCl3 (δ 7.26 ppm for the 1H NMR spectra and δ 77.23 ppm for the 13C NMR spectra) by Bruker 500 MHz NMR instrument. All coupling constants, J, are reported in Hertz (Hz). The progress of monomer reaction was checked by thin‐layer chromatography (TLC) analysis using Merck silica gel 60 F254 precoated plates (0.25 mm) with a fluorescent indicator and visualized with UV light (254 and 365 nm) or by iodine vapor staining. Column chromatography was carried out on Merck silica gel 60 (230‐400 mesh) or Acros aluminium oxide (neutral, 50‐200 μm). * Corresponding Author: phone: 82‐42‐350‐3935 E‐mail: [email protected] ‐ S1 ‐ Supplementary Material (ESI) for Chemical Communications
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Scheme S1. Synthetic approach to POPT and multithiophene‐containing POPT derivatives (POPTT, POPQT). o
Reaction Conditions: (a) 5 mol % Pd(PPh3)4, Na2CO3 (2.5 equiv.), toluene/EtOH/H2O = 10/3/1, 80 C, 12 h, 82% (b) NBS (2.4 equiv.), AcOH:CHCl3 (1:1, v/v), 60 oC, 6 h, 85% (c) i. BuLi (2.4 equiv.), THF, ‐78 oC to r.t., 2 h. ii. Me3SnCl (2.2 equiv.), THF, ‐30 oC to r.t., overnight, (3a: 96%, 3b: 94%) (d) monomer 2 (1.0 equiv.), 5 mol % Pd(PPh3)4, toluene:DMF (4:1, v/v), 135 oC, 24 h (e) NBS (1.0 equiv.), AcOH:CHCl3 (1:1, v/v), r.t., 2 h, 96% (f) NIS (1.2 equiv.), AcOH:CHCl3 (1:1, v/v), 60 oC, 10 h, 84% (g) i. t‐BuMgCl (1.0 equiv.), THF, ‐78 oC to r.t., 2 h. ii. 0.35 mol % Ni(dppp)Cl2, 65 oC, 12 h. ‐ S2 ‐ Supplementary Material (ESI) for Chemical Communications
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◈ Detail Synthetic Procedure Scheme S2 3‐(4‐n‐Octyl)phenylthiophene (1). This compound was prepared according to modified literature procedure.1 To a solution of 4‐octylbromobenzene (8.86 mmol, 2.39 g), 5 mol % tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] (512 mg) and Na2CO3 (22.2 mmol, 2.35 g) in 20:5:1 toluene/ethanol/H2O (45 mL), 3‐thiophene boronic acid (10.6 mmol, 1.36 g) was added under an Ar atmosphere. The reaction mixture was heated at reflux temperature for 12 h with vigorous stirring. Upon cooling to room temperature, the reaction mixture was extracted with EtOAc (150 mL). The organic layer was washed with water (2 × 100 mL) and brine; dried over MgSO4; and concentrated in vacuo. The crude compound was purified by flash column chromatography using hexane as the eluent to afford the corresponding product (1.97 g, 82%) as a white solid: 1H NMR (500 MHz, CDCl3) δ 7.56 (d, J = 8.2 Hz, 2H), 7.38‐7.46 (m, 3H), 7.25 (d, J = 8.2 Hz, 2H), 2.67 (t, J = 7.7 Hz, 2H), 1.66‐1.73 (m, 2H), 1.27‐1.45 (m, 10H), 0.95 (t, J = 6.8 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 142.6, 142.1, 133.5, 129.0 (×2), 126.5 (×2), 126.4, 126.2, 119.8, 35.9, 32.1, 31.7, 29.7, 29.6, 29.5, 22.9, 14.3. 2,5‐Dibromo‐3‐(4‐n‐octyl)phenylthiophene (2).1 To a stirred solution of 3‐(4‐n‐
octyl)phenylthiophene 1 (4.77 mmol, 1.30 g) in chloroform and acetic acid (50 mL, 1:1 v/v), N‐
bromosuccinimide (NBS) (11.45 mmol, 2.04 g) was added, and the mixture was warmed and stirred at 60 °C for 6 h. The reaction was monitored by TLC to establish completion. The excess NBS was removed by washing with saturated aq Na2S2O3. The organic layer was extracted with EtOAc (80 mL), washed with 5.0 N aq NaOH, water (2 × 50 mL) and brine; dried over MgSO4; and concentrated in ‐ S3 ‐ Supplementary Material (ESI) for Chemical Communications
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vacuo. The resulting dark oil was filtered through a short pad of silica gel with hexane as the eluent to afford the corresponding product (1.74 g, 85%) as colorless oil: 1H NMR (500 MHz, CDCl3) δ 7.45 (d, J = 8.2 Hz, 2H), 7.27 (d, J = 7.9 Hz, 2H), 7.04 (s, 1H), 2.69 (t, J = 7.8 Hz, 2H), 1.66‐1.73 (m, 2H), 1.29‐1.46 (m, 10H), 0.96 (t, J = 6.8 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 143.1, 142.2, 131.9, 131.5, 128.7 (×2), 128.5 (×2), 111.2, 107.4, 36.0, 32.1, 31.6, 29.7, 29.6, 29.5, 22.9, 14.4. 2‐Bromo‐3‐(4‐n‐octyl)phenylthiophene (4).1 To a stirred solution of 3‐(4‐n‐octyl)phenylthiophene 1 (4.07 mmol, 1.11 g) in chloroform and acetic acid (40 mL, 1:1 v/v), NBS (4.07 mmol, 725 mg) was added at room temperature for 2 h. The reaction was monitored by TLC to establish completion. The organic layer was extracted with EtOAc (80 mL), washed with 5.0 N aq NaOH, water (2 × 40 mL) and brine; dried over MgSO4; and concentrated in vacuo. The resulting oil was filtered through a short pad of silica gel with hexane as the eluent to afford the corresponding product (1.38 g, 96%) as colorless oil: 1H NMR (500 MHz, CDCl3) δ 7.52 (d, J = 8.0 Hz, 2H), 7.33 (d, J = 5.5 Hz, 1H), 7.29 (d, J = 8.5 Hz, 2H), 7.07 (d, J = 5.5 Hz, 1H), 2.69 (t, J = 7.5 Hz, 2H), 1.66‐1.73 (m, 2H), 1.24‐1.45 (m, 12H), 0.95 (t, J = 6.8 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 142.6, 141.3, 132.4, 129.3, 128.6 (×2), 128.5 (×2), 125.8, 108.3, 35.9, 32.0, 31.5, 29.6, 29.5, 29.4, 22.8, 14.2. 2‐Bromo‐5‐iodo‐3‐(4‐n‐octyl)phenylthiophene (5).1 To a stirred solution of 2‐bromo‐3‐(4‐n‐
octyl)phenylthiophene 4 (1.69 mmol, 595 mg) in chloroform and acetic acid (18 mL, 1:1 v/v), N‐
iodosuccinimide (NIS) (2.03 mmol, 458 mg) was added, and the mixture was warmed and stirred at 60 °C for 10 h. The reaction was monitored by TLC to establish completion. The excess NIS was removed by washing with saturated aq Na2S2O3. The organic layer was extracted with EtOAc (40 mL), washed with 5.0 N aq NaOH, water (2 × 20 mL) and brine; dried over MgSO4; and concentrated in vacuo. The resulting oil was filtered through a short pad of silica gel with hexane as the eluent to afford the corresponding product (675 mg, 84%) as colorless oil: 1H NMR (500 MHz, CDCl3) δ 7.44 (d, J = 8.0 Hz, 2H), 7.26 (d, J = 8.5 Hz, 2H), 7.22 (s, 1H), 2.68 (t, J = 8.0 Hz, 2H), 1.64‐1.72 (m, 2H), 1.25‐1.45 (m, 12H), 0.94 (t, J = 7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 143.5, 143.1, 138.9, 131.2, 128.7 (×2), 128.5 (×2), 111.2, 72.1, 36.0, 32.1, 31.6, 29.7, 29.6, 29.5, 22.9, 14.3. ‐ S4 ‐ Supplementary Material (ESI) for Chemical Communications
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Scheme S3 Preparation of 2,2'‐Bithiophene. A mixture of 10 mol % bis(triphenylphosphine)nickel(II) chloride [(PPh3)2NiCl2] (1.8 g), 20 mol % PPh3 (1.45 g), Zn dust (27.6 mmol, 1.81 g), and dry DMF (55 mL) was stirred at room temperature for 0.5 h under Ar atmosphere, resulting in the change of the color from green blue to reddish brown. And then, 2‐bromothiophene (27.6 mmol, 4.5 g) was added and the mixture was heated at 60 °C for 12 h. The reaction mixture was filtered through celite from EtOAc several times. The organic layer was concentrated in vacuo. The resulting oil was purified by flash column chromatography using hexane as the eluent to afford 2,2'‐bithiophene product (2.12 g, 92%) as white solid. 2,2':5',2''‐Terthiophene was purchased. General procedure for the preparation of bis(trimethylstannyl)thiophenes (3)2 To a solution of appropriate thiophenes (2,2'‐bithiophene and/or 2,2':5',2''‐terthiophene) in dry THF (0.1 M conc.) cooled to ‐78 °C, 2.1 equiv of n‐butyllithium (2.5 M in THF) was slowly added and stirred for 1 h under Ar. The solution was warmed to 0 °C, upon which trimethyltin chloride (2.2 equiv) was added and the reaction was allowed to warm to RT. The mixture was stirred overnight. The reaction mixture was diluted with ethyl acetate. The organic layer was washed with water and brine; dried over MgSO4; and concentrated in vacuo. 2,2'‐Bithiophene‐5,5'‐bis(trimethylstannane) (3a)2 The crude product was filtered through celite from hexane to afford as a light green‐wish solid (96% yield): 1H NMR (500 MHz, CDCl3) δ 7.28 (d, J = 3.0 Hz, 2H), 7.09 (d, J = 3.5 Hz, 2H), 0.39 (s, 18H). 2,2':5',2''‐Terthiophene‐5,5''‐bis(trimethylstannane) (3b) The crude product was purified by column chromatography (alumina, 95:5 hexane:triethyl amine) to give as a yellow solid (94% yield): 1H NMR (500 MHz, CDCl3) δ 0.39 (s, 18H), 7.06 (s, 2H), 7.09 (d, J = 3.5 Hz, 2H), 7.27 (d, J = 3.0 Hz, 2H). ‐ S5 ‐ Supplementary Material (ESI) for Chemical Communications
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Scheme S4 Poly(3‐(4‐n‐octyl)phenylthiophene) (POPT). This compound was prepared according to modified literature procedure.1 2‐Bromo‐5‐iodo‐3‐(4‐n‐octyl)phenylthiophene 5 (354 mg, 0.74 mmol) was dissolved in 15 mL of dry THF. The resulting solution was cooled to ‐78 oC and stirred for 0.5 h. Then t‐
BuMgCl (2.0 M in Et2O, 0.37 mL, 0.74 mmol) was added to the reaction mixture dropwise. After stirring at ‐78 oC for 1.5 h, the reaction was allowed to warm to RT over a 0.5 h period. And then, [[1,3‐bis(diphenylphosphino)propane]dichloronickel(II)] Ni(dppp)Cl2 (1.6 mg, 0.35 mol %) was added. After 12 h at 65 oC, the polymer was precipitated into methanol from the THF reaction solution and filtered through a Soxhlet thimble. The polymer was purified by Soxhlet extraction with methanol, hexanes and chlorobenzene. The polymer was recovered from the chlorobenzene fraction, followed by concentration and finally precipitation into methanol. The polymer was dried under vacuum to give a black solid with metallic luster (92 mg, 46%): 1H NMR (500 MHz, C6D6) δ 7.40 (d, J = 8.0 Hz, 2H), 7.11 (d, J = 8.0 Hz, 2H), 7.09 (s, 1H), 2.53 (t, J = 7.6 Hz, 2H), 1.54‐1.65 (m, 2H), 1.29‐1.42 (m, 10H), 1.02 (t, J = 7.1 Hz, 3H); Mn = 16 K, PDI = 1.1. Scheme S5 Poly(3‐(4‐n‐octyl)phenyl‐5,2',5',2''‐terthiophene) (POPTT). The 2,5‐dibromo‐3‐(4‐n‐
octyl)phenylthiophene 2 (503 mg, 1.17 mmol), bis(trimethylstannyl)bithiophene 3a (575 mg, 1.17 mmol) and 5 mol % (PPh3)4Pd (68 mg) were mixed into a 100 mL round‐bottom flask. The flask was ‐ S6 ‐ Supplementary Material (ESI) for Chemical Communications
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subjected to 3 successive cycles of vacuum followed by refilling with argon. Then, toluene (16 mL) and DMF (4 mL) were added via a syringe. The polymerization was carried out at 130 °C for 24 h under argon atmosphere. At the completion of the reaction, the mixture was concentrated under reduced pressure; the residue was precipitated with methanol. A dark colored powdery solid was filtered through an extraction thimble for subsequent Soxhlet extraction. The solid was washed with methanol, acetone, hexane and chloroform. The desired polymer was then isolated by extraction with chlorobenzene, followed by concentration and finally precipitation into methanol to obtain khaki solid with metallic luster 203 mg (40%): 1H NMR (500 MHz, C6D6) δ 7.25‐7.58 (m, 4H), 7.09 (s, 1H), 6.70‐7.02 (m, 4H), 2.61 (br s, 2H), 1.60‐1.72 (m, 2H), 1.21‐1.48 (m, 10H), 1.01 (br s, 3H); Mn = 14 K, PDI = 1.44. Poly(3‐(4‐n‐octyl)phenyl‐5,2',5',2'',5'',2'''‐quarterthiophene) (POPQT). The 2,5‐dibromo‐3‐(4‐n‐
octyl)phenylthiophene 2 (602 mg, 1.40 mmol), Bis(trimethylstannyl)terthiophene 3b (803 mg, 1.40 mmol) and 5 mol % (PPh3)4Pd (81 mg) were mixed into a 100 mL round‐bottom flask. The flask was subjected to 3 successive cycles of vacuum followed by refilling with argon. Then, toluene (24 mL) and DMF (6 mL) were added via a syringe. The polymerization was carried out at 130 °C for 24 h under argon atmosphere. At the completion of the reaction, the reaction mixture was concentrated under reduced pressure; the residue was precipitated with methanol. A dark colored powdery solid separated and was filtered through an extraction thimble for subsequent Soxhlet extraction. The solid was washed thoroughly with methanol, acetone, hexane and chloroform. The desired polymer was then isolated by extraction with chlorobenzene, followed by concentration under a stream of nitrogen and finally precipitation into methanol to obtain khaki solid with metallic luster 208 mg (29%): 1H NMR (500 MHz, C6D6) δ 7.25‐7.60 (m, 5H), 7.09 (s, 1H), 6.68‐7.05 (m, 5H), 2.63 (br s, 2H), 1.59‐1.72 (m, 2H), 1.20‐1.48 (m, 10H), 1.01 (br s, 3H); Mn = 6.2 K, PDI = 1.48. ‐ S7 ‐ Supplementary Material (ESI) for Chemical Communications
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◈ Solar Cell Fabrication To investigate the photovoltaic properties of the copolymers, BHJ photovoltaic cells were fabricated with an ITO/PEDOT:PSS/polymer:PCBM/LiF/Al structure, where the polymers were used as donors and PCBM as the acceptor. ITO‐coated glass substrates were subjected to ultrasonication for 20 min in acetone and 2% Helmanex soap in water followed by extensive rinsing with deionized water and ultrasonication in deionized water and then isopropyl alcohol. A filtered dispersion of PEDOT:PSS in water (Baytron P VP AI 4083) was applied by spin‐coating at 3,000 rpm for 40 sec and followed by baking for 15 min at 140 °C in air. All procedures after applying the PEDOT:PSS layer were performed in a glove box under N2 atmosphere. Each solution of POPT, POPTT, POPQT, and PCBM was prepared in 1,2‐dichlorobenzene and stirred at 120 °C overnight to ensure complete dissolution. All solutions were passed through a 0.2‐μm polytetrafluoroethylene syringe filter immediately prior to deposition. For the POPQT/PCBM (1:3, w/w) device, the blend solution was stirred at 120 °C for 1 h and then spun‐cast onto the substrate at 1,200 rpm for 80 sec. The preannealing before the Al deposition was performed at 70 °C for 1 h to dry the solvent. Substrates were then placed in an evaporation chamber and held under high vacuum (10‐6 Torr) for more than 1 h before evaporating ca. 1 nm of LiF/100 nm of Al. The configuration of the shadow mask afforded four independent devices on each substrate. The photovoltaic performances were characterized using a solar simulator (ABET Technologies) with air‐mass AM 1.5 G filters. The intensity of the solar simulator was carefully calibrated using an AIST‐
certified silicon photodiode. Current‐voltage behavior was measured with a Keithly 2400 SMU. The active area of the fabricated devices was 0.10 cm2. Hole mobilities of the polymers were measured by the space‐charge‐limited current (SCLC) method, using a device structure of ITO/PEDOT:PSS/polymer/Au, by taking current‐voltage measurements in the range of 0–8 V and fitting the results to a space‐charge‐limited form. The SCLC is described by: 9
V2
J SCLC = εε 0 μ 3 8
L
where ε0 is the permittivity of free space, ε is the dielectric constant of the polymer, μ is the mobility of the majority charge carriers, V is the potential across the device (V = Vapplied–Vbi–Vr), and L is the polymer layer thickness. The series and contact resistance of the device (~25 Ω) was measured using a ‐ S8 ‐ Supplementary Material (ESI) for Chemical Communications
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blank device (ITO/PEDOT/Al) and the voltage drop due to this resistance (Vr) was subtracted from the applied voltage.3 ◈ Characterization Methods UV‐visible absorption spectra were obtained with a JASCO V‐570 spectrophotometer. And, to probe the electrochemical properties of polymers, cyclic voltammetry (CV) measurements of POPT, POPTT and POPQT polymers were carried out using 0.1 M tetrabutylamminium hexafluorophosphate (n‐
Bu4PF6) solution in nitrogen saturated anhydrous acetonitrile at a potential scan rate of 50 mV/s. The polymers were coated on the platinum‐working electrode. The CV curves were recorded referenced to an Ag quasi‐reference electrode, which was calibrated using a ferrocene/ferrocenium (Fc/Fc+) redox couple (4.80 eV below the vacuum level) as an external standard.4‐5 The blend morphology of the polymer/PCBM active layer was examined by atomic‐force microscopy (AFM) (Veeco Dimension 3100) in tapping mode. Two different samples of POPTT:PCBM and POPQT/PCBM were prepared on a PEDOT:PSS/Si substrate prepared as for the device fabrication (1:3, w/w). GIWAXS measurements. Grazing incidence‐angle wide angle X‐ray scattering (GIWAXS) measurements were performed on beamline 11.3 in the Stanford Synchrotron Radiation Laboratory. X‐rays with a wavelength of 0.9752 Å were used. The incidence angle (~0.1°) was carefully chosen to allow for complete penetration of X‐rays into the polymer film. The scattering spectra were collected as a 2D image map that can be divided into a component in the plane of the substrate (qxy) and a component perpendicular to the substrate (qz) ‐ S9 ‐ Normalized Absorbance
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POPT
1.0
POPTT
POPQT
0.8
0.6
0.4
0.2
0.0
350
400
450
500
550
600
650
Wavelength (nm)
Figure S1. UV‐visible absorption spectra of POPT, POPTT, and POPQT solutions in 1,2‐dichlorobenzene. P3HT
POPTT
POPQT
POPT
Absorbance (a.u.)
0.4
0.3
0.2
0.1
0.0
400
500
600
Wavelength (nm)
700
Figure S2. UV‐visible absorption spectra of the P3HT, POPTT, and POPQT thin films (thickness ~ 50 nm).
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Table S1. Light Absorption Properties of Polymers Used in Our Study. Polymer Absorption coefficient (cm‐1) P3HT (RIEKE) 6.88 × 104 Wavelength (λmax) at maximum intensity 524 nm POPT 4.04 × 104 598 nm POPTT 4
537 nm 4
532 nm 9.94 × 10 POPQT 6.53 × 10 POPT
POPTT
POPQT
‐500
0
500
1000
Potential (mV)
Figure S3. Cyclic voltammetry of POPT, POPTT and POPQT. ‐ S11 ‐ 1500
2000
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2
J (A/cm ) 0.3
0.1
0.0
0.8
0
1
2
3
4
5
Vapply (V)
6
0.5
0.5
J (A /cm) 1.2
(a)
0.2
0.4
POPQT (150 nm)
0.0
0
2
4
6
Vapply‐Vbi‐Vr (V)
(b)
J (A/cm ) 0.3
2
0.8
0.2
0.1
0.0
0
1
2
3
4
Vapply (V)
5
6
0.5
0.5
J (A /cm) 1.2
0.4
0.0
POPTT (60 nm)
POPTT (108 nm)
0
2
4
Vapply‐Vbi‐Vr (V)
6
0.5
Figure S4. J vs V plots for (a) POPQT, (b) POPTT at room temperature. The thickness of each polymer film is indicated in the plots. The inset in each figure shows J vs Vapply plot of the thicker film device. ‐ S12 ‐ Supplementary Material (ESI) for Chemical Communications
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POPQT:PC61BM (1:3)
Absorbance (a.u.)
POPQT:PC71BM (1:3)
400
500
600
Wavelength (nm)
700
Figure S5. UV‐visible absorption spectra of POPQT/PC61BM blend film and POPQT/PC71BM film on glass. (a)
500 nm
(b)
500 nm
Figure S6. Tapping‐mode AFM images of (a) POPTT:PC61BM (1:3, w/w) and (b) POPQT:PC61BM (1:3, w/w). The films were prepared under the identical conditions used for device fabrication. The scan size was 2.5 μm × 2.5 μm. ‐ S13 ‐ Supplementary Material (ESI) for Chemical Communications
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Table S2. Device characteristics of bulk‐heterojunction solar cells with different weight ratios of PCBM under AM 1.5 illumination at 100 mW/cm2 Active‐layer system (w/w) Voc (V) Jsc (mA/cm2) FF PCE (%) POPT:PC61BM (1:1) 0.56 5.16 0.60 1.71 POPT:PC61BM (1:2) 0.52 5.07 0.46 1.21 POPT:PC61BM (1:3) 0.52 4.66 0.46 1.10 POPTT:PC61BM (1:3) 0.63 6.83 0.40 1.73 POPTT:PC61BM (1:4) 0.67 5.04 0.40 1.35 POPQT:PC61BM (1:3) 0.75 7.69 0.48 2.77 POPQT:PC61BM (1:4) 0.74 6.64 0.45 2.21 POPQT:PC71BM (1:3) 0.71 8.67 0.56 3.44 POPQT:PC71BM (1:4) 0.71 7.46 0.53 2.81 ◈ References 1. T. W. Holcombe, C. H. Woo, D. F. J. Kavulak, B. C. Thompson, J. M. J. Fréchet, J. Am. Chem. Soc., 2009, 131, 14160. 2. H. Goto, K. Akagi, Angew. Chem. Int. Ed. 2005, 44, 4322. 3. C. H. Woo, T. W. Holcombe, D. A. Unruh, A. Sellinger, J. M. J. Fréchet, Chem. Mater., 2010, 22, 1673. 4. Y. Liang, D. Feng, Y. Wu, S.‐T. Tsai, G. Li, C. Ray, L. Yu, J. Am. Chem. Soc., 2009, 131, 7792. 5. Y.‐T. Chang, S.‐L. Hsu, G.‐Y. Chen, M.‐H. Su, T. A. Singh, E W. Diau, K.‐H. Wei, Adv. Funct. Mater., 2008, 18, 2356. ‐ S14 ‐