Supporting Information for: A wide-bandgap donor polymer for highly efficient non-fullerene. organic solar cells with a small voltage loss

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1 Supporting Information for: A wide-bandgap donor polymer for highly efficient non-fullerene organic solar cells with a small voltage loss Shangshang Chen, Yuhang Liu, Lin Zhang, Philip C. Y. Chow, Zheng Wang, Guangye Zhang, Wei Ma, He Yan.,, Department of Chemistry, Energy Institute and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Hong Kong University of Science and Technology, Clear Water Bay Kowloon, Hong Kong. State Key Laboratory for Mechanical Behavior of Materials, Xi an Jiaotong University, Xi an , P. R. China. Hong Kong University of Science and Technology-Shenzhen Research Institute, No. 9 Yuexing 1st RD, Hi-tech Park, Nanshan Shenzhen , P. R. China. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou , P. R. China. Table of Contents Methods... S2 Supporting Figures... S8 Supporting Discussions... S11 Synthetic Details... S14 NMR Spectra... S16 References... S20 S1

2 Methods Solar cell fabrication and testing. Diethylzinc (15 % wt in toluene) and vanadium (V) oxide (V 2 O 5 ) were purchased from Sigma-Aldrich and used as received without further treatment. O-IDTBR was purchased from Solarmer Materials Inc. All thicknesses of the layers involved were determined by variable angle spectroscopic ellipsometry (J. A. Woollam Co. α-se) in the transparent wavelength range of the films. Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (~23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm. Active layer solutions (D:A ratio 1:1.5 w/w) were prepared in 1,2,4-trimethylbenzene (polymer concentration: 12 mg ml -1 ). To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 100 C for at least 1 hour. Before spin-coating, the polymer solution was cooled down to room temperature and active layers were spin-coated onto the glass/ito/zno substrates in a N 2 glovebox at rpm. The optimized active layer thickness was 90.1 ± 2.3 nm. The active layers were then treated with vacuum to remove the solvent. Subsequently, the blend films were thermally annealed at 80 C for 5 min before being transferred to the vacuum chamber of a thermal evaporator inside the same glovebox, and a thin layer (7 nm) of V 2 O 5 was deposited as the anode S2

3 interlayer, followed by the deposition of 100 nm of Al as the top electrode at a vacuum level of ~ Pa. All devices were encapsulated using epoxy and thin glass slides inside the glovebox. Device J-V characteristics were measured under AM 1.5G (100 mw cm -2 ) using a Newport solar simulator in ambient atmosphere. The light intensity was calibrated using a standard Si diode (with KG5 filter, purchased from PV Measurement) to bring spectral mismatch to unity. J-V characteristics were recorded using a Keithley 2400 source meter unit. Typical cells have devices area of 5.9 mm 2, defined by a metal mask with an aperture aligned with the device area. EQEs were measured using an Enlitech QE-S EQE system equipped with a standard Si diode. Monochromatic light was generated from a Newport 300W lamp source. These test protocols are exactly the same as that we used in previously certified OPVs 1. GIWAXS characterization. GIWAXS measurements were performed at beamline at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory 2. Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10 kev X-ray beam was incident at a grazing angle of 0.13, which maximized the scattering intensity from the samples. The scattered X-rays were detected using a Dectris Pilatus 2 M photon counting detector. The coherence length was calculated by: L C = 2πK Δq (1) S3

4 Where Δq is the full-width at half-maximum of the peak and K is a shape factor (0.94 was used here). R-SoXS characterization. R-SoXS transmission measurements were performed at beamline at the ALS 3. Samples for R-SoXS measurements were prepared on a polystyrene sulfonate (PSS)-modified glass/ito substrate under the same conditions as those used for device fabrication, and then transferred by floating in water to a mm, 100-nm-thick Si 3 N 4 membrane supported by a 5 5 mm, 200-mm-thick Si frame (Norcada). Samples were investigated under high vacuum ( torr) in order to reduce the absorption of soft X-rays in air. Two-dimensional scattering patterns were collected on an in-vacuum charge-coupled device (CCD) camera (Princeton Instrument PI-MTE). The beam size at the sample is ~100 mm by 200 mm. The composition variation (or relative domain purity) over the length scales probed can be extracted by integrating scattering profiles to yield the total scattering intensity. The median domain spacing is calculated from 2π/q, where q here corresponds to the peak position of the intensity distribution. The purer the average domains are, the higher the total scattering intensity. Owing to a lack of absolute flux normalization, the absolute composition cannot be obtained only by R-SoXS. Optical characterization. UV-Vis absorption spectra were acquired on a Perkin Elmer Lambda 20 UV/VIS Spectrophotometer. All film samples were spin-casted on S4

5 glass/ito/zno substrates. PL spectra were measured on samples on glass/ito/zno substrates upon excitation of a laser beam using a Renishaw RM 3000 Micro-Raman/Photoluminescence system. All film samples were spin-casted on glass/ito/zno substrates. Electrochemical characterization. Cyclic voltammetry was carried out on a CHI760E electrochemical workstation with three-electrode configuration, using Ag/AgCl as the reference electrode, a Pt plate as the counter electrode, and a glassy carbon as the working electrode. The polymer was drop-cast onto the electrode from a chloroform solution (2 g L -1 ) to form thin films. 0.1 mol L -1 tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile was used as the supporting electrolyte. Potentials were referenced to the ferrocenium/ferrocene couple by using ferrocene as external standards in acetonitrile solutions. The scan rate is 0.05 V s -1. The solution was degassed by bubbling nitrogen for 30 min before measurement. The error bar of CV measurement is 0.05 ev. Hole mobility measurements. The hole mobilities were measured using the space charge limited current (SCLC) method, employing a device architecture of glass/ito/v 2 O 5 /active layer/v 2 O 5 /Al. The mobilities were obtained by taking current-voltage curves and fitting the results to a space charge limited form, where the SCLC is described by: S5

6 J = 9ε 0ε r μ(v appl V bi V s ) 2 8L 3 (2) Where ε 0 is the permittivity of free space, ε r is the relative permittivity of the material (assumed to be 3), μ is the hole mobility, V appl is the applied voltage, V bi is the built-in voltage (0 V), V s is the voltage drop from the substrate s series resistance (V s = IR, R is measured to be 10.8 Ω) and L is the thickness of the film. By linearly fitting J 1/2 with V appl -V bi -V s, the mobilities were extracted from the slope and L: μ = slope2 8L 3 9ε 0 ε r (3) Electron mobility measurements. The electron mobilities were measured using the SCLC method, employing a device architecture of glass/ito/zno/active layer/ca/al. The mobilities were obtained by taking current-voltage curves and fitting the results to a space charge limited form, where the SCLC is described by: J = 9ε 0ε r μ(v appl V bi V s ) 2 8L 3 (4) Where ε 0 is the permittivity of free space, ε r is the relative permittivity of the material (assumed to be 3), μ is the hole mobility, V appl is the applied voltage, V bi is the built-in voltage (0.7 V), V s is the voltage drop from the substrate s series resistance (V s = IR, R S6

7 is measured to be 10.8 Ω) and L is the thickness of the film. By linearly fitting J 1/2 with V appl -V bi -V s, the mobilities were extracted from the slope and L: μ = slope2 8L 3 9ε 0 ε r (5) AFM analysis. AFM measurements were performed by using a Scanning Probe Microscope-Dimension 3100 in tapping mode. All film samples were spin-casted on glass/ito/zno substrates. IQE simulations. Variable angle spectroscopic ellipsometry (VASE) measurements were first performed. Then, by fitting the VASE results using various mathematical models built in the CompleteEASE software (J. A. Woollam Co.), the optical constants (n and k values) were obtained for all materials except Al (literature value). The thickness of each layer, e.g. ITO, ZnO, active layer and V 2 O 5 were also determined by fitting the long wavelength (not absorbing) regions. Next, optical modeling was carried out using the transfer-matrix optical model. 4 Each layer s optical constants and thickness values were used as inputs to calculate the electrical filed distribution, along with the fraction of light absorbed by each layer. To obtain the IQE, the EQE values at each wavelength were divided by the actual absorption by the active layer (PvBDTTAZ:O-IDTBR) that was obtained through optical modeling. Fraction of light absorbed or reflected of each layer involved in the devices are presented in Figure S5, while electrical field intensity of the selected wavelengths in S7

8 a film architecture and a device architecture with top V 2 O 5 /Al electrode are shown in Figure S6. UPS measurements. UPS measurements were performed by using Axis Ultra DLD equipment. PvBDTTAZ neat and PvBDTTAZ:O-IDTBR blend fims were spin-casted on glass/ito/zno substrates. The incident photon energy (He I) was ev, and a -10 V bias was applied on the samples in order to enhance the signals of with kinetic energy near zero. Supporting Figures Figure S1. UV-Vis absorption spectra of a PvBDTTAZ film and a PvBDTTAZ solution (0.04 mg ml -1 in chlorobenzene) at temperatures indicated. S8

9 Figure S2. The optimized conformation of a vbdttaz trimer based on the DFT calculations. The alkyl chains were replaced with methyl groups to simplify the calculations. Figure S3. Cyclic voltammograms of FeCp 2 0/+ and PvBDTTAZ. Figure S4. PL spectra of (a) PvBDTTAZ neat and PvBDTTAZ:O-IDTBR blend films excited at 514 nm, and (b) O-IDTBR neat and PvBDTTAZ:O-IDTBR blend films excited at 633 nm. Each spectrum was corrected for the absorption of the film at the excitation wavelength. S9

10 Figure S5. Fraction of light absorbed or reflected of each layer involved in the devices. Figure S6. Electrical field intensity of the selected wavelengths in (a) a film architecture and (b) a device architecture with top V 2 O 5 /Al electrode. Figure S7. IQE as a function of wavelength was calculated based on the optical transfer matrix modeling 4. The absorption coefficients and refractive indexes used in the optical model were determined by variable spectroscopic ellipsometry. S10

11 Figure S8. J 1/2 ~(V appl -V bi -V s ) characteristics. (a) hole-only and (b) electron-only devices. Dashed lines are linear fitting results. Figure S9. AFM (1 μm 1 μm) height (left) and phase (right) images of PvBDTTAZ:O-IDTBR blend film. Supporting Discussions The HOMO levels of PvBDTTAZ neat and PvBDTTAZ:O-IDTBR blend films Ultraviolet photoelectron spectroscopy (UPS) was carried out to investigate the HOMO levels of both PvBDTTAZ neat and PvBDTTAZ:O-IDTBR blend films, and detailed results are presented in Figure S10 below. S11

12 Figure S10. UPS results of PvBDTTAZ neat and PvBDTTAZ:O-IDTBR blend films. As the incident photon energy hv (He I) was ev, the HOMO energy levels were determined according to the followed equation: -E HOMO = hv - (E cutoff -E onset ) Consequently, the calculated HOMO levels for PvBDTTAZ neat and PvBDTTAZ:O-IDTBR blend were and ev, respectively. The HOMO of PvBDTTAZ upshifts by 20 mev blended with O-IDTBR. The upshifted HOMO of PvBDTTAZ slightly increases the HOMO offset between PvBDTTAZ and O-IDTBR to 60 mev, which is still relatively small among the reported non-fullerene organic solar cells. Molecular weight dependence on polymer properties and device performances To investigate the molecular weight (MW) dependence on device performance, the solar cells fabricated from the donor polymers with different MWs were tested, and detailed photovoltaic performances are summarized in Table S1 below. Table S1. Photovoltaic performances of the solar cells based on PvBDTTAZ with different molecular weights. Donor polymer M n (kda) M w (kda) Hole mobility (cm 2 V -1 s -1 ) V oc (V) J sc (ma cm -2 ) FF PCE/PCE max low MW a /10.3 high MW b /11.6 a Average PCE calculated from 12 devices; b average PCE calculated from 32 devices. (%) The high-mw PvBDTTAZ (used in this work) shows superior PCEs to those of low-mw PvBDTTAZ, and the performance differences comes from J sc and FF. The lower J sc of low-mw PvBDTTAZ based solar cells is attributed to the lower S12

13 extinction coefficients of low-mw PvBDTTAZ as shown in Figure S11 below. On the other hand, the FF differences result from the hole mobilities, and the low-mw PvBDTTAZ exhibits a lower hole-mobility ( cm2 V-1 s-1) than that of high-mw PvBDTTAZ ( cm2 V-1 s-1). Figure S11. (a) Extinction coefficients as a function of wavelength of high-mw and low-mw PvBDTTAZ thin films. (b) J1/2~(Vappl-Vbi-Vs) characteristics of high-mw and low-mw PvBDTTAZ based hole-only devices. Dashed lines are linear fitting results. Thermal properties of PvBDTTAZ Both thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to investigate the thermal properties of PvBDTTAZ. PvBDTTAZ exhibits good thermal stability with no decomposition below 380 C and 5 % weight loss at 429 C. DSC analysis gave PvBDTTAZ an endothertmic transition at 174 C ( H = 5.81 J g-1) in the heating process, and an exothermic transition at 144 C ( H = 6.86 J g-1). Both DSC and TGA indicate the relatively good thermal stability of PvBDTTAZ. Figure S12. TGA curve of PvBDTTAZ (N2, 10 C min-1). (b) DSC thermogram of PvBDTTAZ (N2, 10 C min-1) Stability test of PvBDTTAZ:O-IDTBR-based solar cells S13

14 Regarding the stability of the solar cells, we monitored the degradation of the 17 devices encapsulated simply with ultraviolet-curable epoxy and thin glass slices. To minimize the degradation factor due to the oxidation of electrode and further explore the stability of the BHJ active layer, the devices were stored in a glove box filled with nitrogen and were monitored over a time span of 14 days. The cells retained 90 % of initial average PCE after 6 days, and 85 % of original PCE after 14 days, indicating a relatively good stability of the devices. Figure S13. Stability test of PvBDTTAZ:O-IDTBR-based solar cells in N 2 filled glovebox for 14 days. Synthetic Details General Information All reagents and chemicals were purchased from commercial sources and used without further purification unless stated otherwise. Tetrahydrofuran (THF) was freshly distilled before use from sodium using benzophenone as indicator. 1 H NMR and 13 C NMR spectra were recorded on a Bruker AV-400 MHZ NMR spectrometer. Chemical shifts are reported in parts per million (ppm, δ). 1 H NMR and 13 C NMR spectra were referenced to tetramethylsilane (0 ppm) for CDCl 3. Synthesis of polymer S14

15 4,8-bis(4-(2-decyltetradecyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (S1). To a solution of 3-(2-decyltetradecyl)thiophene (4.2 g, 10 mmol) in THF (40 ml) was added butyllithium solution (5 ml, 2.0 M in hexane) dropwise at 0 o C. The solution was then allowed to stir at 0 o C for 1 h before benzo[1,2-b:4,5-b']dithiophene-4,8-dione (550 mg, 2.5 mmol) was added in one portion. The resulting yellow solution was allowed to stir at 50 o C for 2 h before SnCl 2 2H 2 O (11 g, 50 mmol) in 10 % HCl solution (40 ml) was added and the solution was allowed to stir for additional 2 h. Hexane was added to the mixture and was followed by washing with water for three times and dried over sodium sulphate. The resulting yellow oil was purified by flash chromatography to get pure product as yellowish oil (1.8 g, 70 %) 1 H NMR: (400 MHz, CDCl 3 ) 7.65 (d, J = 5.6 Hz, 2H), 7.45 (s, 2H), 7.29 (d, J = 1.2 Hz, 2H), 7.08 (d, J = 0.8 Hz, 2H), 2.66 (d, J = 6.8 Hz, 4H), (br, 2H), (m, 80H), 0.88 (t, J = 6.8 Hz, 12H). 13 C NMR: (100 MHz, CDCl 3 ) δ , , , , , , , , , 39.31, 35.24, 33.72, 32.14, 30.29, 29.93, 29.86, 29.59, 26.97, 22.91, ,8-bis(5-bromo-4-(2-decyltetradecyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophe ne (S2). To a solution of S1 (1g, 0.93 mmol) was added NBS (375.9 mg, 2.1 mmol) in one portion at 0 o C and the reaction was allowed to stir overnight. After the reaction finished, the solvent was removed on a rotary evaporator. Then, the residue was purified by flash chromatography over silica gel eluting with hexane to get pure product was yellowish oil (960 mg, 87%). 1 H NMR: (400 MHz, CDCl 3 ) 7.60 (d, J = 6.0 Hz, 2H), 7.47 (d, J = 5.6 Hz, 2H), 7.14 (s, 2H), 2.61 (d, J = 7.2 Hz, 4H), (br, 2H), (m, 80H), 0.89 (t, J = 6.4 Hz, 12H). 13 C NMR: (100 MHz, CDCl 3 ) δ , , , , , , , , , 38.81, 34.55, 33.77, 32.14, 30.27, 29.92, 29.90, 29.88, 28.58, 26.87, 22.91, S15

16 5,6-difluoro-2-propyl-4,7-bis(5-(trimethylstannyl)thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (TAZ). To a solution of 5,6-difluoro-2-propyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole ( g, 2.84 mmol) in tetrahydrofuran (120 ml), lithium diisopropylamide (3.4 ml, 2.0 M in THF, 6.8 mmol) was added dropwise under N 2. After ther reaction mixture was stirred for 2 h at -78 C, trimethyltin chloride (7.7 ml, 1.0 M in hexane, 7.7 mmol) was added dropwise. The reaction mixture was stirred for 12 h at room temperature. Then, aqueous potassium fluoride was added and the mixture was extracted with diethyl ether for three time. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na 2 SO 4 and concentrated under reduced pressure. The crude product was recrystallized from chloroform/isopropyl alcohol to get the product as yellow green needle (535 mg, 27 %). 1 H NMR: (400 MHz, CDCl 3 ) 8.38 (t, J = 3.2 Hz, 2H), 7.32 (m, 2H), 4.77 (t, J = 6.8 Hz, 2H), 2.22 (m, 2H), 1.05 (t, J = 7.2 Hz, 3H), 0.44 (s, 18H). 13 C NMR: (100 MHz, CDCl 3 ) δ , , , , 58.66, 23.68, F NMR: (376.5 MHz, CDCl 3 ) δ Synthesis of PvBDTTAZ. The polymer can be synthesized by microwave assisted polymerization. To a mixture of monomer S2 (60.6 mg, mmol), 5,6-difluoro-2-propyl-4,7-bis(5-(trimethylstannyl)thiophen-2-yl)-2H-benzo[d][1,2,3]tr iazole (TAZ monomer), Pd 2 (dba) 3 (0.5 mg, mmol) and P(o-tol) 3 (1 mg, mmol) were added 300 μl chlorobenzene in a glove box protected with N 2. The reaction mixture was then sealed and heated at 140 C for 2 hours. The mixture was cooled to r.t. and 10 ml toluene was added before precipitated with methanol. The solid was collected by filtration, and loaded into an extraction thimble and washed with hexane then dichloromethane. The polymer was finally collected from chloroform. The chloroform solution was then concentrated by evaporation, precipitated into methanol. The solid was collected by filtration and dried in vacuo to get the polymer as orange red solid (65 mg, 92 %). HT-GPC: M n = 68.8 kda; M w = 138 kda; PDI = NMR Spectra 1 H NMR Spectra S16

17 S17

18 13 C NMR Spectra S18

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20 19 F NMR Spectra References (1) (a) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Nat. Commun. 2014, 5, (b) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Nat. Energy 2016, 1, (2) Hexemer, A.; Bras, W.; Glossinger, J.; Schaible, E.; Gann, E.; Kirian, R.; MacDowell, A.; Church, M.; Rude, B.; Padmore, H. Xiv International Conference on Small-Angle Scattering (Sas09) 2010, 247. (3) Gann, E.; Young, A. T.; Collins, B. A.; Yan, H.; Nasiatka, J.; Padmore, H. A.; Ade, H.; Hexemer, A.; Wang, C. Rev. Sci. Instrum. 2012, 83, (4) Burkhard, G. F.; Hoke, E. T.; McGehee, M. D. Adv. Mater. 2010, 22, S20