SUPPLEMENTAL MATERIAL
Controlled Energy Shuttling in Terpolymers Enabling
Independent Optimization of Absorption and Transport
Properties in Organic Solar Cell Materials
Hannah Mangold1, Ian A. Howard1*, Silvia Janietz2, Frédéric Laquai1
*ian.howard@mpip-mainz.mpg.de, silvia.janietz@iap.fraunhofer.de, laquai@mpip-mainz.mpg.de
1
Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research Mainz,
Ackermannweg 10, D-55128 Mainz, Germany
2
Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, D-14476 Potsdam, Germany
Figure S1: Transient absorption of pristine P2A showing characteristic exciton spectrum and
simultaneous decay of stimulated emission and absorption bleach.
Figure S2: Charge extraction experiment with pulsed LED for (a) P1A:PCBM and (b)
P2A:PCBM, the lower image is an enlargement of the extraction current after switching off the
LED. The illumination intensities were varied from 0.1 to 0.7 suns.
Materials and Methods
Synthesis and characterization of ter- and copolymers
The terpolymers P1A and P2A were synthesized via Suzuki C-C-cross-coupling reaction,
starting from the diboronic ester of 9,9-dioctylfluorene (M1) and the dibrominated 4``methyl-triphenylamine (M2) and 5,8-di(4,4`dibromothienyl)-2,3-bis-[4-octyloxy-phenyl]quinoxaline
(M5)
or
5,7-bis(5-bromothiophen-2-yl)-2,3-diphenyl-(4-octyloxy)thieno[3,4]pyrazine (M6).
Scheme S1: Synthesis of the terpolymers P1A and P2A
The dithienylquinoxaline and the dithienylthienopyrazine skeletal structures of the
monomers M5 and M6 were selected as donor acceptor donor parts which contribute to the
absorption in the visible range. The exchange of benzene (P1 and P1A) against thiophene
(P2A) in the condensed ring system is the only difference in the structure and leads to a
significant red shift in the absorption for P2A. Additional alkyloxy side chains were
introduced into the polymer backbone to enable a better solubility of the synthesized rigid rod
polymers. Triphenylamine units M2 were additionally introduced into the polymer backbone
as they are well known hole transporters with mobilities in the range of 10-3 cm2/Vs.1 All
polymerizations proceeded in good yields. Traces of the Pd-catalyst were removed through
the treatment of the polymer chloroform solution with ammonia solution. The polymers were
purified via Soxhlet extraction with methanol, hexane and chloroform. The polymer structures
were characterized by 1H-NMR spectroscopy and elemental analysis. The molecular weights
Mw and Mn were determined by GPC versus polystyrene.
Table I: Composition and molecular weights of the co- and terpolymers
Name
Part
fluorene
unit
Part
triphenylamine unit
Part
dithienylbenzopyrazine unit
Molecular weight
Mw and Mn
(g/mol)
P1
1
0
1
22,300
10,400
P1A
1
0.33
0.67
38,200
16,800
P2A
1
0.33
0.67
91,800
20,500
The number average molecular weight is in all cases above the 10 kDa range. The molecular
weight and also the polydispersity are not identical for all polymers as they depend strongly
on the purity of the donor-acceptor monomer (M5 or M6). These monomers were
recrystallized several times from isopropanol. All the polymers show good film forming
properties and are soluble in organic solvents such as chloroform, THF and chlorobenzene as
well as in more printing friendly solvents like toluene or anisole. No first order thermal
transitions could be observed by differential scanning calorimetry in the temperature range
from -20°C up to 270°C.
Chemicals
9,9-Dioctylfluorene-2,7-bis(trimethylene borate) (M1) was purchased from Aldrich and
used without further purification. 4,4`-Dibromo-4´´-methyl-triphenylamine (M2) was
purchased from Sensient Imaging Technologies GmbH, SynTec divisions. 5,8-Dibromo-2,3bis-[4-octyloxy)-phenyl]-quinoxaline (M3) was synthesized according to the literature.2 5,8Dithienyl-2,3-bis-[4-octyloxy-phenyl]-quinoxaline (M4) was synthesized via Stille coupling
reaction.
Scheme S2: Synthesis of monomer M4 via Stille coupling
5,8-Dibromo-2,3-bis-[4-octyloxy)-phenyl]-quinoxaline (M3) (1.86 g, 0.00267 mol) and Pd
catalyst (PdCl2(pph)4 25.54 mg) were presented in a flask, evacuated and filled with argon. 30
ml degassed THF was added. After that followed the addition of trisbutylstannylthiophene
(2.3 g, 0.00617 mol). The reaction mixture was stirred over night at 80°C and cooled down on
the next day to room temperature and quenched with NH4Cl, then it was extracted with
CHCl3/water. The organic phase was dried over MgSO4 and the solvent was removed. The
crude product was solved again in CHCl3 (100 ml) and stirred with a 0.5 mol/l KF-solution in
water (200 ml) over 3 days. The phases were separated. The organic phase was dried again
over MgSO4, separated and evaporated. The crude product was recrystallized from methanol
and dried to produce 1.41 g (yield: 75%). Anal. cald for C44H50N2 O2S2 (MW 702.33 g mol-1):
C 75.17, H 7.17, N 3.98, S 9.12; found: C 72.45, H 7.58, N 3.17, S 7.96; 1H NMR (500 MHz,
CDCl3): [ppm]: 0.8 – 1.9 (m, CH2 and CH3; 30 H); 4.0 (OCH2,, t, 4H, J = 6.6); 6.9 (Ar, d,
4H, J = 8.5.); 7.2 (thianthren, t, 2H, J = 4.2); 7.5 (thianthren, d, 2H, J = 5.1); 7.9 (thianthren, d,
2H,J = 3.7); 7.7 (Ar, d, 4H, J = 8.5.); 8.1 (chinoxaline, s, 2H).
Scheme S3: Bromination of 5,8-Dithienyl-2,3-bis-[4-octyloxy-phenyl]-quinoxaline (M4)
5,8-Dithienyl-2,3-bis-[4-octyloxy-phenyl]-quinoxaline (M4) (2.0 g, 2.84 mmol) was
dissolved in THF (400 ml) and cooled down to 5°C. NBS (1.01 g, 5.68 mmol) was dissolved
in THF (30 ml) and this solution was added dropwise to the reaction mixture at 5°C. The
mixture was stirred for 30 min at 5°C and afterwards directly rotated on Celite and
chromatographed over silica gel using hexane/CHCl3 (1:1). The product was recrystallized
from n-butanol: 1.41 g (yield: 75%). Anal. Cald for C44H48Br2N2O2S2(MW 858.15 g mol-1):
C 61.39, H 5.62, Br 18.57, N 3.25, S 7.45; found: C 61.39, H 5.64, N 3.28, S 7.45; 1H NMR
(500 MHz, CDCl3): [ppm]: 0.8 – 1.9 (m, CH2 and CH3; 30 H); 4.0 (OCH2,, t, 4 H, J = 6.6);
6.9 (Ar, d, 4H, J = 8.5.); 7.1 (thianthren, d, 2H, J = 4.1); 7.5 (thianthren, d, 2H, J = 4.1); 7.7
(Ar, d, 4H, J = 8.5.); 8.0 (chinoxaline, s, 2H).
5,7-Bis(5-bromothiophen-2-yl)-2,3-diphenyl-(4-octyloxy)-thieno[3,4]pyrazine (M6) was
also synthesized according to the literature.2
Polymerization of P1A
The monomers 9,9-Dioctylfluorene-2,7-bis(trimethylene borate) (M1) (558.4 mg, 1 mmol),
4,4`-Dibromo-4´´-methyl-triphenylamine (M2) (139.05 mg, 0.33 mmol) and 5,8Di(4,4`dibromothienyl-2,3-bis-[4-octyloxy-phenyl]-quinoxaline (M5) (573.88 mg, 0.67
mmol) were put in a flask, which was evacuated and refilled with argon three times. After
this, 18 ml degassed THF and 2 ml toluene were added as a solvent mixture, followed by the
addition of 25 ml degassed 2M Ca2CO3 solution in water. The catalyst solution was prepared
as follows: Pd(II)acetate (11.2 mg) and Tris(o-tolyl)phosphine (91.32) mg were solved in 7.5
ml THF and 2.5 ml ethanol . 1 ml of this catalyst solution was added to the reaction mixture.
After 2 hours 20 ml toluene were added to dilute the high viscous solution. The reaction
mixture was refluxed and stirred over night. The next day 100 µl of the catalyst solution and
bromobenzene (0.11 ml, 1 mmol) were added to react for 2 hours, whereupon phenylboronic
acid (121.93 mg, 1 mmol) was added. After two hours the reaction mixture was allowed to
cool to room temperature and was poured into methanol/ 2M HCl solution (3:1). The
precipitated polymer was collected and dried. The polymer was dissolved in 75 ml chloroform
and this solution was stirred for 3 days with ammonia (80 ml, 25% aqueous solution) to
remove catalyst residues. The phases were separated and the organic layer was washed three
times with water. Then it was concentrated, precipitated in methanol and Soxhlet extracted
with methanol and hexane to separate the low molecular weight fraction. The polymer was
solved again in chloroform and precipitated in methanol, dried in vacuum to afford 798 mg of
the polymer P1 (86%). Anal. calcd from chosen monomer ratio C64.67H77 O1.33 N1.67S1.33 (MW
931,7 g mole-1): C 82.74, H 7.94, N 2.49, S 4,55; found: C 81.04, H 8.14, N 2.51, S 4.66; 1H
NMR (500 MHz, CDCl3): [ppm]: 0.6 – 1.6 (m, CH2 and CH3; 50 H); 2.1 (t, 4H; fluorene);
2.4 (s, CH3 -phenylamine , 0.99 H ); 4.1 OCH2,, t, 2.68 H), 7.0 -8.2 (m; Ar-H, 19.34) ;Mw:
38,200 g/mol; Mn: 16,800 g/mol against polystyrene
Polymerization of P1 analog P1A
9,9-Dioctylfluorene-2,7-bis(trimethylene borate) (M1) (558.4 mg, 1 mmol), 5,8-Di
(4,4`dibromothienyl)- 2,3-bis-[4-octyloxy-phenyl]-quinoxaline (M5) (860.82 mg, 1 mmol);
yield: 845 mg; 78%; Anal. calcd from chosen monomer ratio C73H88O2N2 S2 (MW 1089
g/mol): C 80.47, H 8.14, N 2.57, S 5.89; found: C 78.69, H 8.22, N 2.52, S 5.61;1H NMR
(500 MHz, CDCl3): [ppm]: 0.6 – 1.6 (m, CH2 and CH3; 60 H); 2.1 (t, 4H; fluorene);
4.1(OCH2,, t, 4 H), 7.0 -8.2 (m; Ar-H, 20); Mw: 22,300 g/mol; Mn: 10,400 g/mol against
polystyrene
Polymerization of P2A analog P1A
9,9-Dioctylfluorene-2,7-bis(trimethylene borate) (M1) (558.4 mg, 1 mmol), 4,4`-Dibromo4´´-methyl-triphenylamine (M2) (139.05 mg, 0.33 mmol) and 5,7-Bis (5-bromothiophen-2yl)-2,3-diphenyl-(4-octyloxy)-thieno[3,4]pyrazine (M6) (583.73 mg, 0.67 mmol); yield: 788
mg (83%). Anal. Calcd. from chosen monomer ratio C63.3H75.67O1.3N1.67S2 (MW 945.76
g/mol): C 80.43, H 8.06, N 2.47, S 6.78; found: C 79.55, H 8.20, N 2.54, S 7.15; 1H NMR
(500 MHz, CDCl3): [ppm]: 0.6 – 1.6 (m, CH2 and CH3; 50 H); 2.1 (t, 4H; fluorene); 2.4 (s,
CH3 -phenylamine, 0.99 H); 4.1 (OCH2,, t, 2.68 H), 6.9 -7.8 (m, Ar-H, 24.14 H); Mw: 91,800
g/mol; Mn: 20,500 g/mol against polystyrene
Characterization
1
H NMR data were obtained using an INOVA 500 spectrometer. Gel permeation
chromatographic analysis (GPC) utilized Polystyrene (PS)-columns (three columns, 5 µm gel,
effective molecular weight range 500-4x106) connected with UV-vis (= 254 nm) and infrared
(IR) detection. All GPC analyses were performed on solutions of the polymers in
tetrahydrofurane (THF) at 20°C (concentration of the polymer: ca. 2 g/l). The calibration was
based on polystyrene standards with a narrow molecular weight distribution. Elemental
analysis was performed on EA 1110 CHNS-O equipment. Thermal analysis was performed
with differential scanning calorimetry (DSC) using a Netzsch DSC 204 under nitrogen
atmosphere at heating/cooling rate of 10 K/min.
Fabrication of solar cells
For solar-cell preparation ITO-coated glass substrates (Praezisions Glas & Optik GmbH,
Germany) were patterned by wet etching and successively cleaned in an ultrasonic bath with
different solvents. Prior to the deposition of the organic layers the substrates were plasmaetched in argon plasma for 15 min. Poly(3,4-ethylene-dioxythiopene):poly-(styrenesulfonate)
(PEDOT:PSS) (Clevios P VP Al 4083, H.C. Stark) solution was then spin-coated onto the
cleaned ITO substrates with a thickness of approximately 40 nm as determined with a Dektak
surface profilometer. The samples were transferred into a nitrogen filled glovebox and the
PEDOT:PSS layer was annealed for 20 min at 150°C. For preparation of the active layer the
donor polymer and PCBM were dissolved in chlorobenzene in a 1:2 ratio at a total
concentration of 25 mg/ml and stirred overnight at 80°C. The active layer was spincoated at
1000 rpm resulting in a film thickness of 100 nm. Aluminum counter electrodes were
evaporated through a mask on top of the active layer with a thickness of 100 nm. The active
area of the pixels as defined by the overlap of anode and cathode area was 14 mm 2. The
photovoltaic performance was determined under simulated sunlight using a commercial solar
simulator (K.H. Steuernagel Lichttechnik GmbH, Germany) at an area power density of 80
mW/cm2.
For spectroscopy the active layer was spin-cast directly onto cleaned and plasma-etched
quartz substrates under the same conditions as those for solar cells.
Transient absorption spectroscopy
Transient absorption (TA) measurements were performed with a home-built pump-probe
setup. To measure in the time range of 1 ps to 4 ns with a resolution of ∼100 fs, the output of
a commercial titanium:sapphire amplifier (Coherent LIBRA HE, 3.5 mJ, 1 kHz, 100 fs) was
split with one portion used to generate an excitation pulse (510 nm for P1 and P1A, 400 nm
for P2A) using a home-built two-stage narrowband non-collinear optical parametric amplifier
(NOPA) and another used to generate a 1300 nm seed pulse (output of an optical parametric
amplifier (Coherent OPerA Solo)) for white-light generation in the visible. The variable delay
of up to 4 ns between pump and probe was introduced by a broadband retroreflector mounted
on a mechanical delay stage. Only reflective elements were used to guide the probe beam to
the sample to minimize chirp. The excitation pulse was chopped at 500 Hz, while the white
light pulses were dispersed onto a linear photodiode array which was read out at 1 kHz.
Adjacent diode readings corresponding to the transmission of the sample after an excitation
pulse and without an excitation pulse were used to calculate ΔT/T. For measuring in the time
range of 1 ns to 1 ms with a resolution of 600 ps, the excitation pulse was provided by an
actively Q-switched Nd:YVO4 laser (AOT Ltd. MOPA) at 532 nm. The delay between pump
and probe in this case was controlled by an electronic delay generator (Stanford Research
Systems DG535). TA measurements were performed at room temperature under dynamic
vacuum at pressures <10-5 mbar.
References
1.
2.
W. Zhang, J. Smith, R. Hamilton, M. Heeney, J. Kirkpatrick, K. Song, S. E. Watkins, T.
Anthopoulos and I. McCulloch, Journal of the American Chemical Society 131 (31), 1081410815 (2009).
W. Mammo, S. Admassie, A. Gadisa, F. Zhang, O. Inganäs and M. R. Andersson, Solar Energy
Materials and Solar Cells 91 (11), 1010 - 1018 (2007).