Toward Practical Useful Polymers for Highly Efficient Solar Cells via a Random Copolymer Approach

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1 Supporting Information Toward Practical Useful Polymers for Highly Efficient Solar Cells via a Random Copolymer Approach Chunhui Duan, a Ke Gao, b Jacobus J. van Franeker, ac Feng Liu,*,b Martijn M. Wienk, ad René A. J. Janssen*,ad a. Molecular Materials and Nanosystems, Institute for Complex Molecular Systems, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands. r.a.j.janssen@tue.nl b. Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai , China. fengliu82@sjtu.edu.cn. c. Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands. d. Dutch Institute for Fundamental Energy Research, De Zaale 20, 5612 AJ Eindhoven, The Netherlands. S1

2 1. Synthesis All synthetic procedures were performed under argon atmosphere. All reactants and reagents are commercially available and used as received, unless otherwise specified. [60]PCBM (purity of 99%) and [70]PCBM (purity of 99%) were purchased from Solenne BV. 2,6-Bis(trimethyltin)-4,8-di(2,3-dioctylthiophen-5-yl)-benzo[1,2-b:4,5-b']dithiophene 2,6-bis(trimethyltin)-4,8-di(2,3-didecylthiophen-5-yl)-benzo[1,2-b:4,5-b']dithiophene (M1), (M2), 5,6-difluoro-4,7-dibromobenzo[2,1,3]thiadiazole (M4) were purchased from SunaTech Inc. 4,7-bis(5-bromothiophen-2-yl)-5,6-difluorobenzo[2,1,3]thiadiazole (M3) was synthesized according to our previous method. 1 Scheme S1. Chemical structures and synthesis of the polymers. Synthesis of Th00. The synthesis procedures of Th00 were described in our previous paper, 2 where Th00 is denoted as BDT-FBT-C32. Mn = 37.8 kda, PDI = 3.2. Synthesis of Th25. To a degassed solution of M1 (9 mmol, mg), M3 (225 mmol, mg), and M4 (675 mmol, mg) in anhydrous o-xylene (1.6 ml) and anhydrous DMF ( ml), Pd2(dba)3 CHCl3 (1.863 mg, 018 mmol) and tri(o-tolyl)phosphine (4.383 mg, 144 mmol) were added. The mixture was stirred at 120 C for 18 hours, after which 2-(tributylstannyl)thiophene and 2-bromothiophene were sequentially added to the reaction with 2 hours interval. After another 2 hours, the reaction S2

3 mixture was diluted with 1,2-dichlorobenzene (o-dcb), and refluxed with EDTA (100 mg) for 2 hours. Upon cooling, the reaction mixture was precipitated in methanol and filtered through a Soxhlet thimble. The polymer was subjected to sequential Soxhlet extraction with acetone, hexane, dichloromethane and chloroform under argon protection. The chloroform fraction was concentrated under reduced pressure and precipitated in methanol to obtain the resulting polymer (65 mg, yield = 71%). Mn = 45.2 kda, PDI = 3.0. Synthesis of Th35. To a degassed solution of M1 (9 mmol, mg), M3 (315 mmol, mg), and M4 (585 mmol, mg) in anhydrous o-xylene (1.6 ml) and anhydrous DMF ( ml), Pd2(dba)3 CHCl3 (1.863 mg, 018 mmol) and tri(o-tolyl)phosphine (4.383 mg, 144 mmol) were added. The mixture was stirred at 120 C for 18 hours, after which 2-(tributylstannyl)thiophene and 2-bromothiophene were sequentially added to the reaction with 2 hours interval. After another 2 hours, the reaction mixture was diluted with o-dcb, and refluxed with EDTA (100 mg) for 2 hours. Upon cooling, the reaction mixture was precipitated in methanol and filtered through a Soxhlet thimble. The polymer was subjected to sequential Soxhlet extraction with acetone, hexane, dichloromethane and chloroform under argon protection. The chloroform fraction was concentrated under reduced pressure and precipitated in methanol to obtain the resulting polymer (78 mg, yield = 84%). Mn = 46.8 kda, PDI = 3.0. Synthesis of Th100. The synthesis procedures of Th100 were described in our previous paper, 1 where Th100 is denoted as BDT-FBT-2T. Mn = 45.5 kda, PDI = 2.4. S3

4 2. Measurements and characterization Molecular weights and polydispersity index (PDI) were determined with Gel permeation chromatography(gpc) at 140 C on a PL-GPC 120 system using a PL-GEL 10 μm MIXED-B column and o-dcb as the eluent against polystyrene standards. The polymers were dissolved in o-dcb at 140 C overnight and the solutions were filtered through PTFE filters ( μm) prior to injection. UV-visible spectroscopy was recorded on a Pekin Elmer Lambda 900 UV/vis/near IR spectrophotometer at room temperature. All solution UV-vis experiments were performed in o-dcb with sample concentration of 5 mg ml 1. Films were prepared by spin coating o-dcb solutions on glass substrates. Cyclic voltammetry (CV) studies were performed with a scan rate of V s 1 under an inert atmosphere with 1 M tetrabutylammonium hexafluorophosphate in acetonitrile as the electrolyte. The working electrode was indium tin oxide (ITO) bar and the counter electrode was a silver electrode. A silver wire coated with silver chloride (Ag/AgCl) was used as quasi-reference electrode in combination with Fc/Fc + as an internal standard. The samples were spin coated on top of ITO work electrode to form ~10 nm thick films. Transmission electron microscopy (TEM) was performed on a Tecnai G 2 Sphera transmission electron microscope (FEI) operated at 200 kv. Grazing incidence X-ray diffraction characterization of the thin films was performed at the Advanced Light Source on beamline 7.3.3, Lawrence Berkeley National Lab (LBNL). Thin film samples were prepared on wafer substrates. The scattering signal was recorded on a 2D detector (Pilatus 2M) with a pixel size of 72 mm by 72 mm. The samples were 15 mm long in the direction of the beam path, and the detector was located at a distance of 300 mm from the sample center (distance calibrated using a silver behenate standard). The incidence S4

5 angle of 6 was chosen which gave the optimized signal-to-background ratio. The beam energy was 10 kev, operating at top-off mode. Typically, 10 seconds exposure time was used to collect diffraction signals. All GIXD experiments were done in helium atmosphere. The data was processed and analyzed using Nika software package. Resonant soft X-ray scattering was performed at beamline Advanced Light Source, LBNL. Thin film samples were spin-casted on top of the PEDOT:PSS covered Si wafers under exactly the same condition as those for the fabrication of solar cell devices. Then BHJ thin films were floated and transferred onto silicon nitride membrane windows. The scattering was done in transmission mode and signals were collected in vacuum using Princeton Instrument PI-MTE CCD camera. 3. Fabrication and characterization of solar cells Photovoltaic devices were made by spin coating poly(3,4-ethylenedioxythiophene):poly- (styrenesulfonate) (PEDOT:PSS) (Clevios P, VP AI 4083) onto pre-cleaned, patterned indium tin oxide (ITO) substrates in air (14 Ω per square) (Naranjo Substrates). The polymer-fullerene photoactive layers were deposited by spin coating in air from the solutions containing corresponding polymers and [60]PCBM or [70]PCBM at room temperature. Unless indicated otherwise, no thermal annealing was applied to the blend films. The concentration of the solutions used for spin-coating are 4 mg ml -1 of Th00 in CF, 7 mg ml -1 of Th25 in CB, 6.5 mg ml -1 of Th35 in CB, 10 mg ml -1 of Th100 in o-dcb, 8.5 mg ml -1 of Th25 in TMB, and 8 mg ml -1 in TMB. The amount of solvent additives are specified in the following tables. The thickness of active layer films was controlled by spin speed to be 100 ± 10 nm. LiF (1 nm) and Al (100 nm) were deposited by vacuum evaporation at ~ mbar S5

6 as the back electrode. The active area of the cells was 9 or 6 cm 2, which provided similar results. Current density voltage (J V) curves were measured under simulated solar light (100 mw cm -2 ) from a tungsten halogen lamp filtered by a Hoya LB100 daylight using a Keithley 2400 source meter. No mismatch correction was done. All measurements were conducted in nitrogen-filled glove box. The accurate short-circuit current density (Jsc) was determined from the EQE by convolution with the AM1.5G solar spectrum. External quantum efficiency (EQE) measurements were performed in a homebuilt set-up, with the devices kept in a nitrogen filled box with a quartz window and illuminated through an aperture of 2 mm. Mechanically modulated (Stanford Research, SR 540) monochromatic (Oriel, Cornerstone 130) light from a 50 W tungsten halogen lamp (Osram 64610) was used as probe light, in combination with continuous bias light from a solid state laser (B&W Tek Inc. 532 nm, 30 mw). The intensity of the bias laser light was adjusted using a variable-neutral density filter. The response was recorded as the voltage over a 50 Ω resistance, using a lock-in amplifier (Stanford Research Systems SR 830). For all the single junction devices, the measurement was carried out under representative illumination intensity (AM1.5G equivalent, provided by the 532 nm laser). S6

7 4. Additional Figures and Tables Normalized voltage Th00 Th25 Th35 Th35_batch 2 Th Time (min) Figure S1. GPC traces of the polymers measured with o-dcb as eluent at 140 C. (a) Normalized absorbance Th00 Th25 Th35 Th100 (b) Normalized absorbance Th00 Th25 Th35 Th Figure S2. The optical absorption spectra of the polymers in o-dcb solutions (a) and in thin films (b). (a) Current Th00 Th25 Th35 Th100 Ferrocene Potential applied (V) (b) E (V vs Fc/Fc + ) LUMO HOMO Th00 Th25 Th35 Th100 [70]PCBM E (ev vs vacuum) Figure S3. (a) Cyclic voltammograms of the polymers and ferrocene recorded in acetonitrile. (b) Energy levels of the polymers and [70]PCBM. S7

8 Table S1. Molecular weights, optical properties, and energy levels of the polymers. Polymer Mn (kda) PDI λmax (nm) Solution Film Eg opt (ev) HOMO a (ev) LUMO a (ev) Eg CV (ev) ΔELUMO b (ev) Th Th Th Th b ΔELUMO = ELUMO (polymer) ELUMO ([70]PCBM). (b) Current density J [ma/cm 2 ] Th00-C10 Th25-C10 Th50-C10 Th75-C10 Th100 (c) EQE (bias) Th00-C10 Th25-C10 Th50-C10 Th75-C10 Th Voltage (V) Figure S4. (a) Chemical structures of the copolymers with different monomer composition. (b) J V curves of the polymer:[70]pcbm solar cells based on these polymers. (c) Corresponding EQE spectra. S8

9 Table S2. Solar cell characteristics of the copolymers with different monomer compositions. Mn / PDI Polymer Solvent Jsc (ma cm (kda) / - -2 ) Voc (V) FF PCE (%) Th00-C /3.1 CF (3% DIO) Th25-C /3.2 CF (3% DIO) Th50-C /3.7 CB/CN (7/3) Th75-C /3.0 CB/CN (7/3) Th /2.4 o-dcb (a) Current density J [ma/cm 2 ] Th35_1, 110 nm ITO Th35_2, 150 nm ITO (b) EQE (bias) Th35_1, 110 nm ITO Th35_2, 150 nm ITO Voltage (V) Figure S5. (a) J V curves of the Th35:[70]PCBM solar cells with different polymer batches. The active layers were processed from CB (3% DIO). (b) The corresponding EQE spectra. It is worth pointing out that the EQE spectrum shape of Th35_1 solar cell is different from Th35_2. This is caused by the light interference effect due to different ITO substrates were used for Th35_1 (110 nm ITO) and Th35_2 (150 nm ITO). 3 S9

10 Table S3. Solar cell characteristics of Th35:[70]PCBM devices with different polymer batches. The active layers were processed from CB (3% DIO). Polymer Mn / PDI (kda) / - Jsc a (ma cm -2 ) Jsc b (ma cm -2 ) Voc a (V) FF a Pmax a (mw cm -2 ) PCE c (%) Th35_1 46.8/ (11.8±9) (0.89±0) 9 (9±1) 7.4 (7.3±) 7.9 Th35_2 5/ (11.8±9) (0.89±0) 2 (1±1) 7.6 (7.5±) 7.9 a Acquired from J V curves, and the data in parenthesis are average values from 6 devices; b Determined by integrating the EQE spectrum with the AM1.5G spectrum; c Estimated using Jsc determined from EQE integration. It is worth pointing out that the deviation of the data from J V measurement and EQE integration are different for different polymers. This is caused by the intensity and spectral variations of illuminated light, while J V measurements were not performed at the same day. (a) Current density J [ma/cm 2 ] Th25, CB (2% DIO) Th25, CB (3% DIO) Th25, CB (4% DIO) (b) EQE (bias) Th25, CB (2% DIO) Th25, CB (3% DIO) Th25, CB (4% DIO) Voltage (V) Figure S6. (a) J V curves of the Th25:[70]PCBM solar cells processed from CB solutions with different content of DIO. (b) The corresponding EQE spectra. It is worth pointing out that the EQE spectrum shape of the CB (3% DIO) solar cells is different from the other two. This is caused by the light interference effect due to different ITO substrates were used for CB (3% DIO) devices (150 nm ITO) and the others (110 nm ITO). 3 S10

11 Table S4. Solar cell characteristics of polymer:[70]pcbm devices processed from CB solutions with different content of DIO Polymer Solvent Jsc a (ma cm -2 ) Jsc b (ma cm -2 ) Voc a (V) FF a Pmax a (mw cm -2 ) PCE c (%) Th25 CB (2% DIO) 11.8 (11.8±4) (0.90±0) 0 (9±1) 7.4 (7.3±) 7.9 CB (3% DIO) 12.0 (11.9±9) (0.90±0) 0 (0±1) 7.5 (7.6±) 8.0 CB (4% DIO) 11.9 (11.9±2) (0.90±1) 0 (8±1) 7.5 (7.3±) 7.9 Th35 CB (2% DIO) 11.8±5-0.88±0 8±1 - (7.2±) - CB (3% DIO) 12.0 (11.8±9) (0.89±0) 9 (9±1) 7.4 (7.3±) 7.9 CB (4% DIO) 11.7±0-0.89±0 8±1 - (7.1±) - a Acquired from J V curves, and the data in parenthesis are average values from 6 devices; b Determined by integrating the EQE spectrum with the AM1.5G spectrum; c Estimated using Jsc determined from EQE integration. It is worth pointing out that the deviation of the data from J V measurement and EQE integration are different for different polymers. This is caused by the intensity and spectral variations of illuminated light, while J V measurements were not performed at the same day. S11

12 Table S5. Solar cell characteristics of polymer:[70]pcbm devices processed from TMB/DPE. Polymer Solvent Jsc a (ma cm -2 ) Jsc b (ma cm -2 ) Voc a (V) FF a Pmax a (mw cm -2 ) PCE c (%) Th25 TMB (3% DPE) 11.0 (10.9±) (0.91±0) 0 (9±1) 7.0 (7.0±) 7.9 Th35 TMB (2% DPE) 11.1 (10.9±) (0.90±0) 1 (0±1) 7.0 (6.9±) 7.9 a Acquired from J V curves, and the data in parenthesis are average values from 6 devices; b Determined by integrating the EQE spectrum with the AM1.5G spectrum; c Estimated using Jsc determined from EQE integration. It is worth pointing out that the deviation of the data from J V measurement and EQE integration are different for different polymers. This is caused by the intensity and spectral variations of illuminated light, while J V measurements were not performed at the same day Biased EQE/unbiased EQE Th00 Th25 Th35 Th100 Figure S7. Average EQEbias/EQEno bias values in the wavelength range of nm of polymer:[70]pcbm solar cells based on Th00, Th25, Th35, and Th100. The filled boxes are acquired from halogenated solvents processed PSCs, and the open boxes are acquired from non-halogenated solvents processed PSCs. S12

13 (a) (b) EQE no bias bias EQE no bias bias (c) (d) EQE no bias bias EQE no bias bias (e) (f) EQE no bias bias EQE no bias bias Figure S8. EQEs measured with and without light bias of the polymer:[70]pcbm solar cells processed from halogenated solvents with (a)th00, (b) Th25, (c) Th35, and (d) Th100; and the solar cells processed from non-halogenated solvents with (e) Th25, and (f) T35. S13

14 Figure S9. GIXD of the neat polymer films and BHJ blends. Figure S10. Bright field TEM images of the polymer:[70]pcbm blend films: (a) Th00:[70]PCBM processed from CF (3% DIO), (b) Th25:[70]PCBM processed from CB (3% DIO), (c) Th35:[70]PCBM processed from CB (3% DIO), (d) Th100:[70]PCBM processed from o-dcb, (e) Th25:[70]PCBM processed from TMB (3% DPE), (f) Th35:[70]PCBM processed from TMB (2% DPE). Image size: μm2; scale bar: 200 nm. S14

15 References (1) Duan, C.; Furlan, A.; van Franeker, J. J.; Willems, R. E. M.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2015, 27, (2) Duan, C.; Willems, R. E. M.; van Franeker, J. J.; Bruijnaers, B. J.; Wienk, M. M.; Janssen, R. A. J. J. Mater. Chem. A 2016, 4, (3) Doggart, P.; Bristow, N.; Kettle, J. J. Appl. Phys. 2014, 116, S15