Frank-Julian Kahle1), Christina Saller2), Selina Olthof3), Cheng Li2), Jenny Lebert4), Sebastian Weiß4), Eva M. Herzig5), Sven Hüttner2), Klaus Meerholz3), Peter Strohriegl2), Anna Köhler1,*)
1) Soft Matter Optoelectronics, Department of Physics, University of Bayreuth, 95447 Bayreuth, Germany
2) Macromolecular Chemistry I, Department of Chemistry, University of Bayreuth, 95447 Bayreuth, Germany
3) Department of Chemistry, University of Cologne, 50939 Cologne, Germany
4) Herzig Group, Munich School of Engineering, Technical University Munich, 85748 Garching, Germany
5) Dynamics and Structure Formation – Herzig Group, Department of Physics, University of Bayreuth, 95447 Bayreuth, Germany
S2 Synthesis of the polymer PCDTBTOx:
Materials and methods
All chemicals and anhydrous solvents were purchased from commercials suppliers and used as received.
Solvents needed for extraction and purification were distilled prior to use. The monomer 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole was delivered by SunaTech Inc. and used without further purification. 1H NMR spectra at room temperature were recorded on a Bruker Avance 300 spectrometer in deuterated solvents at 300 MHz. High temperature 1H NMR spectra were measured at 120 °C with a Varian INOVA 300 spectrometer in 1,1,2,2-tetrachloroethane as solvent. As internal references, the residual solvent peaks were used. Mass spectra were recorded on a Finnigan MAT 8500 via electron ionization.
9-Bromononanal
Oxalyl chloride (4.24 mL, 49.29 mmol) was dissolved in anhydrous dichloromethane (100 mL) and cooled to -78 °C under argon. A solution of anhydrous dimethyl sulfoxide (6.99 mL, 98.59 mmol) and anhydrous dichloromethane (20 mL) was added dropwise. After stirring for 5 min, a solution of 9-bromononanol (10.000 g, 44.81 mmol) in anhydrous dichloromethane (45 mL) was added dropwise over a short time and the reaction mixture was stirred for 30 min at -78 °C before triethylamine (31,23 mL, 224.06 mmol) was added dropwise. The reaction mixture was again stirred for 15 min at -78 °C, allowed to warm to room temperature and poured into water. After extraction with dichloromethane, the organic phase was washed twice with HCl solution (2%), twice with deionised water, twice with NaHCO3 solution (5%) and again twice with deionised water. The organic phase was dried over Na2SO4 and the solvent was evaporated. Drying in vacuum overnight yielded 9-bromononanal (9.500 g, 42.96 mmol, 96%) as a colourless oil.
EI-MS: m/z (%) = 221 (M+, 4), 204 (M+ ‒ O, 23), 192 (M+ ‒ HCO, 17), 176 (M+ ‒ C2H3O, 100), 163 (M+ ‒ C3H5O, 5), 149 (M+ ‒ C4H7O, 7), 135 (M+ ‒ C5H9O, 22).
1H NMR (300 MHz, CDCl3): δ (ppm) = 1.18-1.49 (m, 8H, CH2), 1.50-1.70 (m, 2H, HCO-CH2-CH2), 1.75-1.92 (m, 2H, CH2-CH2-Br), 2.33-2.48 (m, 2H, HCO-CH2), 3.40 (t, J = 6.8 Hz, 2H, CH2-Br), 9.67-9.76 (t, J = 1.8 Hz, 1H, HCO).
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S3 1-Bromoheptadecan-9-ol
Bromooctane (8.39 mL, 48.57 mmol) was dissolved in anhydrous THF (24 mL) and added slowly to magnesium chips (1.476 g, 60.71 mmol) under argon atmosphere. When the exothermic reaction has started, the remaining solution is added dropwise under stirring and cooling if necessary. The reaction mixture is heated to reflux and stirred for 1 h. After cooling to room temperature, anhydrous THF (8 mL) was added for dilution of the reaction mixture. A solution of 9-bromononanal (8.950 g, 221.13 mmol) in anhydrous THF (15 mL) was added slowly under intermittent cooling. The reaction mixture was stirred at room temperature overnight, poured into water and extracted with diethyl ether. After washing twice with saturated NaHCO3 solution, twice with deionised water and twice with brine, the organic phase was dried over Na2SO4 and the solvent was evaporated. After purification via column chromatography (hexanes:ethyl acetate = 5:1), 1-bromoheptadecan-9-ol (9.318 g, 27.78 mmol, 69%) was obtained as a colourless solid.
EI-MS: m/z (%) = 334 (M+, 1), 318 (M+ ‒ OH, 19), 221 (M+ ‒ C8H17, 73), 143 (M+ ‒ C8H16Br, 59).
1H NMR (300 MHz, CDCl3): δ (ppm) = 0.88 (t, J = 6.5 Hz, 3H, CH3), 1.19-1.52 (m, 26H, CH2), 1.85 (qui, J = 7.3 Hz, 2H, CH2-CH2-Br), 3.41 (t, J = 6.8 Hz, 2H, CH2-Br), 3.52-3.64 (br, CH-OH).
1-((3’-Ethyloxetan-3’-yl)-methoxy)-heptadecan-9-ol
Tetrabutylammonium bromide (0.448 g, 1.39 mmol) was dissolved in aqueous NaOH solution (48.624 g, 45 wt%). A solution of 1-bromoheptadecan-9-ol (9.318 g, 27.79 mmol) and (3-ethyloxetan-3-yl)-methanol (5.54 mL, 48.63 mmol) in distilled hexanes (160 mL) was added. The reaction mixture was stirred overnight under reflux. After cooling to room temperature, the reaction mixture was extracted with deionised water and hexanes. The organic phase was dried over Na2SO4 and the solvent was evaporated. 1-((3’-Ethyloxetan-3’-yl)-methoxy)-heptadecan-9-ol (6.530 g, 17.62 mmol, 63%) was obtained as a colourless oil after column chromatography (hexanes:ethylacetate = 5:1).
EI-MS: m/z (%) = 371 (M+, 1), 353 (M+ ‒ OH, 3), 340 (M+ ‒ CH2O, 10), 322 (M+ ‒ CH2O ‒ OH, 8), 257 (M+ ‒ C6H11O2, 22), 227 (M+ ‒ C8H15O2, 8).
1H NMR (300 MHz, CDCl3): δ (ppm) = 0.88 (t, J = 7.5 Hz, 6H, CH3), 1.16-1.49 (m, 26H, CH2), 1.50-1.62 (m, 2H, CH2-CH2-O), 1.74 (q, J = 7.5 Hz, 2H, oxetane-CH2-CH3), 3.44 (t, J = 6.5 Hz, 2H, CH2-O), 3.52 (s, 2H, O-CH2-oxetane), 3.53-3.63 (br, CH), 4.41 (q, J = 5.8 Hz, 4H, oxetane).
S4
(1’-((3‘’-Ethyloxetan-3’‘-yl)-methoxy)-heptadecan-9’-yl)-4-toluenesulfonate
A solution of 1-((3’-ethyloxetan-3’-yl)-methoxy)-heptadecan-9-ol (3.400 g, 9.17 mmol), triethylamine (2.312 g, 22.84 mmol), and trimethylammonium hydrochloride (0.877 g, 9.17 mmol) in anhydrous dichloromethane (20 mL) was cooled to 0 °C. Tosyl chloride (2.169 g, 11.38 mmol) was dissolved in anhydrous dichloromethane (20 mL) and added to the reaction mixture in a time range of 10 min. After stirring for 90 min at 0 °C, the reaction mixture was allowed to warm to room temperature and stirred overnight. Extraction was carried out with dichloromethane and water. The organic phase was washed with deionised water, dried over Na2SO4 and the solvent was evaporated. Column chromatography (hexanes:ethyl acetate = 5:1) yielded the spacer molecule (1’-((3‘’-ethyloxetan-3’‘-yl)-methoxy)-heptadecan-9’-yl)-4-toluenesulfonate (3.820 g, 7.28 mmol, 79%) as a colourless oil.
EI-MS: m/z (%) = 524 (M+, 1), 494 (M+ ‒ CH2O, 15), 353 (M+ ‒ C7H7O3S, 37), 322 (M+ ‒ C8H9O4S,23), 255 (M+ ‒ C13H18O4S, 78).
1H NMR (300 MHz, CDCl3): δ (ppm) = 0.80-0.93 (m, 6H, CH3), 1.06-1.37 (m, 26H, CH2), 1.44-1.65 (m, 2H, CH2-CH2-O), 1.74 (q, J = 7.5 Hz, 2H, oxetane-CH2-CH3), 2.44 (s, 3H, tosylate-CH3), 3.44 (t, J = 6.5 Hz, 2H, CH2-O), 3.52 (s, 2H, O-CH2-oxetane), 4.41 (q, J = 5.8 Hz, 4H, oxetane), 4.53 (qui, J = 6.0 Hz, 1H, CH), 7.19 (d, J = 8.0 Hz, 2H, tosylate), 7.79 (d, J = 8.3 Hz, 2H, tosylate).
2,7-Dibromo-N-(1’-((3’’-ethyloxetan-3’’-yl)-methoxy)-heptadecan-9’-yl)-carbazole
In an argon atmosphere, 2,7-dibromocarbazole (0.991 g, 3.05 mmol) and KOH (0.855 g, 15.24 mmol) were stirred in dimethyl sulfoxide (8 mL) at room temperature. A solution of (1’-((3‘’-ethyloxetan-3’‘-yl)-methoxy)-heptadecan-9’-yl)-4-toluenesulfonate (2.400 g, 4.57 mmol) in dimethyl sulfoxide (6 mL) was added slowly over a time range of 1 h. The reaction mixture was stirred at room temperature overnight and extracted with water and diethyl ether. After the organic phase was washed twice with deionised water and dried over Na2SO4, the solvent was evaporated. Purification was carried out via column chromatography (hexanes:toluene = 1:2). 2,7-dibromo-N-(1’-((3’’-ethyloxetan-3’’-yl)-methoxy)-heptadecan-9’-yl)-carbazole (1.400 g, 2.07 mmol, 68%) was obtained as a colourless oil.
EI-MS: m/z (%) = 677 (M+, 100), 647 (M+ ‒ CH2O, 9), 450 (M+ ‒ C14H27O2, 42), 322 (M+ ‒ C22H43O2, 12).
1H NMR (300 MHz, CDCl3): δ (ppm) = 0.77-0.91 (m, 6H, CH3), 0.92-1.37 (m, 22H, CH2), 1.42-1.56 (m, 2H, CH2-CH2-O), 1.72 (q, J = 7.5 Hz, 2H, oxetane-CH2-CH3), 1.77-1.97 (br, 2H, carbazole-CH-CH2), 2.10-2.28 (br, 2H, carbazole-CH-CH2), 3.39 (t, J = 6.6 Hz, 2H, CH2-O), 3.49 (s, 2H, O-CH2-oxetane), 4.40 (q, J = 5.8 Hz, 4H,
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S5
oxetane), 4.33-4.47 (br, 1H, CH), 7.28-7.37 (br, 2H, carbazole), 7.49-7.57 (br, 1H, carbazole), 7.64-7.73 (br, 1H, carbazole), 7.84-7.96 (br, 2H, carbazole). Broadened and multiple signals are due to atropisomerism.
2,7-Di(thiophen-2’-yl)-N-(1’’-((3’’’-ethyloxetan-3’’’-yl)-methoxy)-heptadecan-9’’-yl)-carbazole
2,7-dibromo-N-(1’-((3’’-ethyloxetan-3’’-yl)-methoxy)-heptadecan-9’-yl)-carbazole (0.500 g, 0.74 mmol) and 2-(4’,4’,5’,5’-tetramethyl-1’,3’,2’-dioxaborolan-2’-yl)-thiophene (0.465 g, 2.21 mmol) were dissolved in toluene (20 mL). After addition of four drops of Aliquat 336 and aqueous Na2CO3 solution (24.95 mL, 2M), the reaction mixture was degassed by three freeze-thaw cycles.
Tetrakis(triphenylphosphine)palladium(0) (0.028 g, 0.02 mmol) was added and the reaction mixture was again degassed by three freeze-thaw cycles before stirred under reflux for 90 h. The reaction mixture was poured into water and extracted with dichloromethane. The organic phase was washed twice with deionised water and dried over Na2SO4. After evaporation of the solvent, column chromatography (hexanes:THF = 10:1) was performed to remove the catalyst. 2,7-di(thiophen-2’-yl)-N-(1’’-((3’’’-ethyloxetan-3’’’-yl)-methoxy)-heptadecan-9’’-yl)-carbazole (0.485 g, 0.71 mmol, 96%) was yielded as a slightly yellowish oil.
EI-MS: m/z (%) = 684 (M+, 98), 654 (M+ ‒ CH2O, 10), 568 (M+ ‒ C6H11O2, 4), 457 (M+ ‒ C14H27O2, 40), 345 (M+ ‒ C22H44O2, 38), 332 (M+ ‒ C23H45O2, 28).
1H NMR (300 MHz, CDCl3): δ (ppm) = 0.74-0.89 (m, 6H, CH3), 0.97-1.36 (m, 22H, CH2), 1.38-1.51 (m, 2H, CH2-CH2-O), 1.70 (q, J = 7.5 Hz, 2H, oxetane-CH2-CH3), 1.88-2.04 (br, 2H, carbazole-CH-CH2), 2.24-2.42 (br, 2H, carbazole-CH-CH2), 3.34 (t, J = 6.6 Hz, 2H, CH2-O), 3.46 (s, 2H, O-CH2-oxetane), 4.38 (q, J = 5.8 Hz, 4H, oxetane), 4.54-4.67 (br, 1H, CH), 7.13 (dd, J = 5.1 Hz, J = 3.7 Hz, 2H, thiophene), 7.31 (dd, J = 5.1 Hz, J = 1.1 Hz, 2H, thiophene), 7.37-7.44 (br, 2H, carbazole), 7.50 (d, J = 8.0 Hz, 2H, thiophene), 7.56-7.62 (br, 1H, carbazole), 7.74-7.81 (br, 1H, carbazole), 8.01-8.11 (br, 2H, carbazole). Broadened and multiple signals are due to atropisomerism.
2,7-Bis(5’-bromothien-2’-yl)-N-(1’’-((3’’’-ethyloxetan-3’’’-yl)-methoxy)-heptadecan-9’’-yl)-carbazole
A solution of 2,7-di(thiophen-2’-yl)-N-(1’’-((3’’’-ethyloxetan-3’’’-yl)-methoxy)-heptadecan-9’’-yl)-car-bazole (0.280 g, 0.41 mmol) in anhydrous chloroform (10 mL) was cooled to 0 °C. In the dark, N-bromosuccinimide (0.146 g, 0.82 mmol) was added in portions. The reaction mixture was stirred at room
S6
temperature for 1 h in the dark, allowed to cool to room temperature and stirred overnight in the dark.
NMR spectroscopy was used for reaction control and if required NBS is added to the reaction mixture.
After the reaction was completed, the reaction mixture was extracted with water and dichloromethane and the organic phase was washed twice with deionised water. The organic phase was dried over Na2SO4
before the solvent was evaporated. After column chromatography (hexanes:THF = 20:1), 2,7-bis(5’-bromothien-2’-yl)-N-(1’’-((3’’’-ethyloxetan-3’’’-yl)methoxy)-heptadecan-9’’-yl)-carbazole (0.230 g, 0.27 mmol, 67%) was obtained as a yellowish oil.
EI-MS: m/z (%) = 841 (M+, 100), 811 (M+ ‒ CH2O, 9), 763 (M+ ‒ Br, 13), 725 (M+ ‒ C6H11O2, 4), 647 (M+ C6H11O2Br, 11), 614 (M+ ‒ C14H27O2, 23), 502 (M+ ‒ C22H44O2, 19), 488 (M+ ‒ C23H45O2, 12), 422 (M+ ‒ C22H44O2Br, 6), 408 (M+ ‒ C23H45O2Br, 5).
1H NMR (300 MHz, CDCl3): δ (ppm) = 0.76-0.93 (m, 6H, CH3), 0.94-1.37 (m, 22H, CH2), 1.39-1.53 (m, 2H, CH2-CH2-O), 1.71 (q, J = 7.4 Hz, 2H, oxetane-CH2-CH3), 1.89-2.03 (br, 2H, carbazole-CH-CH2), 2.21-2.40 (br, 2H, carbazole-CH-CH2), 3.35 (t, J = 6.6 Hz, 2H, CH2-O), 3.47 (s, 2H, O-CH2-oxetane), 4.39 (q, J = 5.8 Hz, 4H, oxetane), 4.50-4.64 (br, 1H, CH), 7.08 (d, J = 3.8 Hz, 2H, thiophene), 7.11-7.19 (br, 2H, carbazole), 7.39 (d, J = 8.0 Hz, 2H, thiophene), 7.45-7.55 (br, 1H, carbazole), 7.62-7.72 (br, 1H, carbazole), 7.98-8.11 (br, 2H, carbazole). Broadened and multiple signals are due to atropisomerism.
Poly-[(N-1’-((3’’-ethyloxetan-3’’-yl)-methoxy)-heptadecan-9’-yl)-2,7-carbazole-alt -5,5-(4‘,7‘-bis(thien-2-yl)-2‘,1‘,3‘-benzothiadiazole)] PCDTBTOx
The monomers 2,7-bis(5’-bromothien-2’-yl)-N-(1’’-((3’’’-ethyloxetan-3’’’-yl)methoxy)-heptadecan-9’’-yl)carbazole (0.137 g, 0.16 mmol) and 4,7-bis(4’,4’,5’,5’-tetramethyl-1’,3’,2’-dioxaborolan-2’-yl)-2,1,3-benzothiadiazole (0.063 g, 0.16 mmol) were dissolved in toluene (7 mL) under argon. Four drops of Aliquat 336 and aqueous Na2CO3 solution (7,5 mL, 2 M) were added before degassing the reaction mixture by three freeze-thaw cycles. After adding tetrakis(triphenylphosphine)palladium(0) (0.003 g, 0.002 mmol), again three freeze-thaw cycles were conducted. The reaction mixture was stirred under reflux in an argon atmosphere for 90 h. Bromobenzene (0.017 g, 0.16 mmol) was added and the reaction mixture was stirred under reflux for 1 h. Subsequently, phenylboronic acid (0.020 g, 0.16 mmol) was added and the endcapping reaction was completed by stirring the reaction mixture under reflux overnight. After cooling to room temperature, the polymer was extracted with toluene and washed with water. The organic phase was reduced and the polymer was precipitated into cold methanol. Soxhlet extraction was carried out with acetone, hexanes and toluene as solvents. The toluene fraction was
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evaporated to dryness, the polymer was dissolved in chlorobenzene and precipitated into cold methanol.
Drying in vacuum overnight yielded PCDTBTOx (0.084 g, 0.10 mmol, 60%) as a dark-red powder.
1H NMR (300 MHz, C2D2Cl4, 120 °C): δ (ppm) = 0.72-0.93 (m, 6H, CH3), 1.09-1.56 (m, 26H, CH2, CH2-CH2 -O), 1.57-1.79 (m, 2H, oxetane-CH2-CH3), 1.96-2.22 (br, 2H, carbazole-CH-CH2), 2.25-2.56 (br, 2H, carbazole-CH-CH2), 3.24-3.54 (m, 4H, CH2-O, -O-CH2-oxetane), 4.17-4.45 (m, 4H, oxetane), 4.54-4.77 (br, 1H, CH), 7.04 - 8.61 (m, 12H, ar-CH). Broadened and multiple signals are due to atropisomerism.
(Figure S1)
Figure S1: 1H NMR spectrum of PCDTBTOx (300 MHz) in C2D2Cl4 at 120 °C.
S8 Remarks on energy levels and work function:
The high binding energy cutoff (HBEC) and valence band spectra for UPS as well as the IPES spectra are displayed in Figure S2. The layer thicknesses of the acceptor were in the range of 10-13 nm. Valence spectra of bilayer samples consistently show the typical features of the molecular levels for the respective fullerenes.1-2 These spectra were used to determine IE and
Figure S2: (a) UPS measured high binding energy cutoff (HBEC) and (b) low binding energy region of a pristine 14nm spin-coated and crosslinked PCDTBTOx film on an ITO/MoO3-substrate (purple), as well as bilayer samples with an additional layer of C60 (black), PCBM (red) or ICBA (blue) on top. Layer thicknesses are in the range of 10-13 nm. (c) IPES spectra of 14 nm thick pristine PCDTBTOx (purple) and samples with an additional layer of C60 (black), PCBM (red) or ICBA (blue) on top. The layer thicknesses ranged from 10 to 13nm. Light grey open dots indicate raw data of the measurements.
EA values for the different materials. Their derivation from the experimental data is exemplary given for C60 in the following. For the excitation a side band of the He Iβ line with energy
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23.09 eV was used. The difference between this energy and the high binding energy cutoff of the signal at (18.2 ± 0.05) eV gives the work function Φ = 23.09eV - 18.2eV = 4.89eV. With a binding energy of (1.62 ± 0.1) eV as determined from the HOMO onset position this results in an ionization energy of IE = 4.89 eV + 1.62 eV = 6.51 eV (± 0.1 eV). With the known work function Φ from UPS and the “LUMO onset position” of -0.34 eV from IPES we can accordingly calculate the electron affinity EA = 4.89 eV - 0.34 eV = 4.16 eV. The respective values for PCBM, ICBA and PCDTBT are derived accordingly.
Deeper insight into the actual energy landscape may eventually be gained from our Kelvin-Probe measurements (Figure S3). Due to a well-defined interface and the use of a bilayer structure for our solar cells, we can directly study interface energetics which are identical to the situation in our actual devices. In order to allow for the determination of absolute workfunction values from the measured contact potential difference (CPD) between the measurement tip and the sample surface, the tip was calibrated using a well-characterized batch of PEDOT:PSS (ΦPEDOT = 5.100eV, Φtip = 4.828 eV). The workfunction is then simply calculated according to Φ = Φtip – CPD. Film thicknesses of the acceptors were (4±1) nm. The acceptors were deposited on top of a crosslinked layer of PCDTBTOx. The error in these measurements simply corresponds to the experimentally observed fluctuation of CPD values between different measurements (deviation from average) and is in the order of 10-20 meV.
Figure S3: Measured Contact Potential Difference (CPD) between tip and sample surface (left axis) as well as derived work function values Φ for all the investigated materials (right axis).
0 1 2 3 4 5 6
4.9 5.0 5.1 PCDTBT C60 PCBM ICBA
workfunction (eV)
measurement 0.0
-0.1 -0.2 -0.3
CPD (eV)
S10
Figure S4: (a) Left axis: EQE of a PCDTBTOx-x/PCBM bilayer solar cell (green filled dots) in comparison to the EQE of single layer devices of the pristine materials PCDTBTOx-x (grey filled diamonds) and PCBM (light green open dots). The suffix –x refers to the fact, that the donor layer is crosslinked. EQE was measured under short circuit conditions. Right axis: Simulated
(a) (b)
(c) (d)
(e)
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fraction of absorbed light of the respective layer in a bilayer device using the transfer matrix algorithm (dashed lines). (b) Left axis: EQE of a PCDTBTOx-x/ICBA bilayer solar cell (blue filled triangles) in comparison to the EQE of single layer devices of the pristine materials PCDTBTOx-x (grey filled diamonds) and ICBA (light blue open triangles). EQE was again measured under short circuit conditions. Right axis: Simulated fraction of absorbed light of the respective layer in a bilayer device using the transfer matrix algorithm (dashed lines). (c) Comparison of bilayer EQE with the EQE of single layer devices of the pristine materials for PCDTBTOx-x/C60. The region below about 1.7 eV can be clearly identified to be related to CT-absorption. (d) Same as in (c) but for PCDTBTOx-x/PCBM. (e) Same as in (c) and (d) but for PCDTBTOx-x/ICBA.
S12
Discussion of the internal quantum efficiency (Figure 3c):
To exclude the possibility, that the observed EQE differences between the acceptors are simply caused by differences in absorption or reflection, we calculated the internal quantum efficiency (IQE) using the absorption profiles we obtained via the transfer matrix algorithm (Figure 3c).
The detailed deconvolution of the absorption contributions from different layers as well as the reflectance can be found in Figure S5.
The calculations were performed using the code provided free of charge by McGehee and coworkers.3 For n and k of glass, ITO, PCBM and Al we used the values already present in the library provided by McGehee. For MoO3 we used the values from the free online library RefractiveIndex.INFO.4 Data for n and k of PCDTBTOx we took values from the work of Schmiedova et al.5 ICBA permittivity data were taken from the work of Leman et al. and converted to n and k values.6 Data for C60 and BCP were extracted from the work of Wynands et al. and Liu et al., respectively.7-8
We find the same trend in the IQE as in the EQE indicating that differences in the absorption profile and strength cannot account for the differences in quantum efficiency. The fact that the IQE decreases for energies below 2.0 eV can be explained by the bandgap of the materials.
Below 2.0 eV (PCDTBTOx) and above 1.7-1.85 eV (fullerenes)9-10 mostly tightly bound Frenkel excitons are created, from which only a fraction reaches the interface due to an acceptor layer thickness larger than the exciton diffusion length of less than 10nm.11 The rest of them will mostly recombine resulting in a low IQE. It is expected to rise again for direct CT excitation as then all the excited states already reside at the interface.12
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Figure S5: (a) Fraction of the incident light absorbed of each individual layer of a glass/ITO/MoO3/PCDTBTOx-x/C60/BCP/Al plotted together with the total reflectance (black). The calculation was performed using the transfer matrix algorithm. (b) Same as in (a) but with PCBM instead of C60. (c) Same as in (a) but with ICBA instead of C60.
S14
Figure S6: Comparison of monochromatic current-voltage (IV) characteristics of bilayer solar cells all with varying acceptors on top of the crosslinked PCDTBTOx layer. The excitation energies were (a) 3.54 eV, (b) 2.76 eV and (c) 1.91 eV. The observed trend in VOC and ISC is discussed in the main text. (d) Dark IV characteristics of the devices shown in (a). A difference in built-in potential is clearly visible from different onset voltages. This is also evidenced by voltage dependent electroabsorption measurements (Figure S10b)
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
S15
Figure S7: Fill Factor (FF) as function of illumination intensity for the same devices as in Figure 3b and c. The excitation energies were 3.54 eV (top), 2.76 eV (middle) and 1.91 eV (bottom).
Note:
Also for intensity dependent measurements of the FF above the autoionization threshold of fullerenes we find no intensity dependence of FF over three decades of illumination intensity.
This again indicates that there is no bimolecular recombination present in our devices. Yet, the FF is a little bit lower than for excitation energies below 2.25 eV (see Figure 4a). This may be attributed to increased geminate recombination within the fullerene bulk due to the presence
20 30 40 50 60 70 80
20 30 40 50 60 70 80
0.01 0.1 1 10
10 20 30 40 50 60 70 80
FF (%)
@3.54 eV
@2.76 eV
FF (%)
@1.91 eV C60
PCBM ICBA
FF (%)
intensity (mW/cm2)
S16
of electrons and holes, that are the result of the splitting of bulk CT states of the neat fullerene (e.g. at 2.4 eV).9, 13
The overall smaller FF for ICBA indicates higher losses due to geminate recombination. This might in part be related to significantly more localized CT states that recombine with a higher rate krec. A major reason will surely be inefficient transport of excitons to the interface as well as charges away from it. The former is due to weak inter-fullerene coupling and the latter because of low electron mobility. As the mobilities of donor and acceptor are rather balanced in the case of ICBA we do not expect and also not observe an s-shape as discussed above.14 Nevertheless, geminate recombination probability will be enhanced for low mobilities14-15(even if they are balanced). This is reflected in a lower FF for ICBA, a low JSC and a considerable reduction of EQE and IQE in comparison to PCBM and C60. With this interpretation, our results are in line with the work of Larson et al. who found that that low mobility in fullerenes also affects the charge generation at the interface.15
Another possibile factor would be exciton recycling to the polymer due to the smaller EA offset for ICBA compared to PCDTBTOx, but as evidenced from voltage dependent electroluminescence measurements, this is not the case (see below).
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S17 Comment on exciton recycling:
Looking at the offsets of IE values between donor and acceptors, we find a difference of at least 0.63 eV, which should be sufficient to suppress large exciton recycling to the acceptor also in the case of ICBA. In the case of EA, ICBA only features a difference of 0.48eV. We therefore investigated by measuring electrolumienscence (EL) whether there is any backtransfer of CT-excitons to the polymer.16 If there was considerable exciton recycling in the PCDTBTOx/ICBA system, it should take place back to the polymer according to the energy levels obtained from UPS and IPES, and thus should emerge as a function of increasing drive voltage in forward direction. A corresponding behavior is not observed even for higher voltages (Figure S8). We rather see EL from ICBA, most likely due to recombination of excitons in the bulk of ICBA that do not reach the interface. Furthermore, we see dominant EL of a CT state at 1.4 eV and only a small contribution of ICBA itself, when biasing the device with only 1.5V. This also indicates that exciton recycling does not play a significant role in our devices.
Figure S8: Voltage dependent EL spectra of PCDTBT/ICBA solar cells. A distinct shoulder at around 1.4 eV emerges upon decreasing voltage that is not related to emission from pristine ICBA or PCDTBT.
EL measurements: For Electroluminescence (EL) measurements, the solar cells were biased at 1.5 – 3.3V using a Keithley source-measure unit (SMU 237). The luminescence of the sample was recorded by a Si-CCD camera (Andor iDus420) coupled to a monochromator (Oriel MS125) or an IR-CCD camera (Andor iDus InGaAs 1.7 µm) also coupled to a monochromator (Andor
S18
Shamrock). The sample was kept in an appropriate vacuum condition sample holder at room temperature.
Comment on the correction of field dependent EQE data:
For 1.90 eV and 2.14 eV the field dependent EQE was corrected for the donor contribution by subtracting the field dependent EQE of a single layer PCDTBTOx device and taking the actual absorption profile in a bilayer device into account. This is mainly of importance for ICBA due to the low efficiency of the respective devices
For 2.76 eV and 3.54 eV (i.e. above autoionization threshold) saturation field strengths are determined after correcting for the intrinsic contribution in donor and acceptor, which were again corrected for differences in absorption in the actual bilayer. As this is only a simplified correction, the error is a bit higher compared to the data at 1.90 eV and 2.14 eV (i.e. below the threshold), but the qualitative trend is still reliable.
In order to study the dissociation efficiency at the interface quantitatively in terms of the effective mass model, we use data obtained from measurements below the autoionization threshold, because there dissociation at the interface is dominant. At higher energies there will be no distinct saturation regime due to the additional contribution from fullerene bulk CT-states at higher field strength, a factor that is not covered by the model we use. For our analysis, we therefore chose an excitation energy of 2.14 eV.
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S19 Alternative method to determine FSat: 90%-method
An alternative to the tangent method to determine FSat is to define it as the field strength at which EQE reaches 90% of the plateau value in the saturation regime. This method gives the same qualitative trend for FSat, but (slightly) different absolute values.
Figure S9: FSat as function of excitation energy for all three acceptors as obtained via the 90%-method.
S20
Figure S10: (a) Top left: Electroabsorption spectra of bilayer devices using C60 (squares), PCBM (dots) or ICBA (triangles) as acceptor. The experimental conditions were chosen such, that the internal field is comparable among the samples. Top right to bottom right: Detailed view of the spectra shown in (a). The height of the polymer peak at around 2.0 eV is the same for all spectra, while the CT-related peak at around 2.2-2.3 eV (corresponding to the CT absorption peak at 2.4 eV)13 decreases in the order C60 > PCBM > ICBA. (b) Voltage dependent Electroabsorption amplitude recorded at 1.9 eV. The (extrapolated) intersection of the linear
(a)
(b) (c)
-2 -1 0 1 2
0 2 4 6 8
10 C60
PCBM ICBA
∆R/R*10-4
Voltage (V)
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S21
part of the curves at small changes ΔR/R with the abscissa is a measure of the built-in potential of the respective device.17-19 (c) Electroabsorption spectra of pristine crosslinked PCDTBTOx (open circles). It coincides very well with the first derivative of the steady state absorption spectrum (blue line), which is typical for an induced dipole momentum.18-23 Furthermore, it shows a distinct peak at around 2.0 eV that is also present in the electroabsorption spectra of
part of the curves at small changes ΔR/R with the abscissa is a measure of the built-in potential of the respective device.17-19 (c) Electroabsorption spectra of pristine crosslinked PCDTBTOx (open circles). It coincides very well with the first derivative of the steady state absorption spectrum (blue line), which is typical for an induced dipole momentum.18-23 Furthermore, it shows a distinct peak at around 2.0 eV that is also present in the electroabsorption spectra of