Three-dimensional tetra(oligothienyl)silanes as donor material for organic solar cells

Article abstract of DOI:10.1039/b604261a

Tetrahedral conjugated systems involving four conjugated oligothiophene chains fixed onto a central silicon node (1, 2) have been synthesized and used as donor materials in hetero-junction solar cells. Bilayer solar cells have been realized by thermal eva

Full text of DOI:10.1039/b604261a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Three-dimensional tetra(oligothienyl)silanes as donor material for organic
solar cells
Sophie Roquet, Re´mi de Bettignies, Philippe Leriche, Antonio Cravino and Jean Roncali*
Received 22nd March 2006, Accepted 16th May 2006
First published as an Advance Article on the web 9th June 2006
DOI: 10.1039/b604261a
Tetrahedral conjugated systems involving four conjugated oligothiophene chains fixed onto a
central silicon node (1, 2) have been synthesized and used as donor materials in hetero-junction
solar cells. Bilayer solar cells have been realized by thermal evaporation of compounds 1 and 2 as
donors and N,N9-bis-tridecylperylenedicarboxyimide as an acceptor. Comparison of the
performances of these devices to those of a reference system based on dihexylterthienyl (H3T)
shows that despite comparable effective conjugation lengths, the 3D compounds 1 and 2 lead to a
power conversion efficiency four–five times higher, suggesting better absorption of the incident
light and better hole transport properties. Whereas fabrication of bulk hetero-junction with H3T
was prevented by the lack of film forming properties, a prototype bulk hetero-junction based on
compound 2 as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor
has been realized. A short-circuit current density of 1.13 mA cm22 and a power conversion
efficiency of 0.30% has been measured under AM 1.5 simulated solar irradiation at 80 mW cm22
.
Introduction
Thiophene-based linear p-conjugated oligomers and polymers
are intensively investigated as active materials in organic field-
effect transistors, (OFETs),1 or solar cells.2,3 Because of the
specificity of charge-transport in these materials by inter-
molecular charge-hopping and aromaticity transfer, achieve-
ment of high charge-carrier mobility requires a high degree of
intermolecular order. The optimization of packing order in
materials based on conjugated oligomers or polymers implies
an increase of the crystallinity of the materials which is
generally pursued through a precise control of the conditions
of thermal evaporation for molecular materials1,3,4 or by
thermal post-treatment for polymer films.2
A consequence of the low dimensionality of organic
semiconductors based on linear p-conjugated systems is the
anisotropy of their charge-transport and optical properties
which poses specific problems for devices fabrication. Thus,
whereas a vertical orientation of the conjugated chains on
the substrate improves mobility in organic field-effect
transistors,1,4 such an orientation is detrimental for solar cells
as it strongly reduces the absorption of the incident light as
well as charge transport to the electrodes.3
In an attempt to solve these problems, we have recently
undertaken the development of three-dimensional conjugated
architectures in order to reach new classes of organic semi-
conductors with isotropic charge-transport and optical
properties.5 Thus, we have shown that hybrid systems
combining triphenylamine with oligothienylenevinylenes or
oligothiophenes branches lead to interesting results for the
fabrication of OFETs or heterojunction solar cells.5
As a further step in the development of organic semi-
conductors based on 3D conjugated systems, we report here
preliminary results on the use of tetrahedral molecules 1 and 2
as donor material in hetero-junction solar cells. These
Groupe Syste`mes Conjugue´s Line´aires, CIMMA, UMR CNRS 6200,
Universite´ d’Angers, 2 Bd Lavoisier F-49045 Angers
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compounds consist in four conjugated oligothiophene chains
fixed onto a central silicon node. The four branches are
end-capped with n-hexyl or hexylsulfanyl chains in order to
increase the solubility of the final 3D system and to stabilize
the cation radical by inductive and resonance effects6 and by
preventing follow-up chemical reaction during the process of
hole-transport.
Results and discussion
The synthesis of the target compounds 1 and 2 is shown
in Scheme 1. Friedel–Crafts acylation of terthiophene with
hexanoic anhydride in the presence of tin chloride, followed
by reduction of the obtained ketone by lithium aluminium
hydride gave 2-hexyl-5,29:59,20-terthiophene 3 in 80% overall
yield. Reaction of bromohexane on thiophene-2-thiol 9 in
the presence of tetrabutylammonium hydroxide gave 2-hexyl-
sulfanylthiophene 8 in 85% yield. This compound was then
converted into the corresponding stannyl derivative 7 by
reaction with butyllithium and Bu3SnCl. This Stille reagent
was then coupled to 2-bromobithiophene (prepared by
reaction of NBS on bithiophene) in the presence of a
palladium catalyst to give terthienyl 6 in 72% yield. The target
tetrahedral compounds were then prepared using the already
reported procedure by treatment of terthienyls 3 and 6 with
butyllitium at 278 uC followed by reaction with silicon
tetrachloride.7 Column chromatography on alumina gave
compounds 1 and 2 in 34 and 59% yield respectively.
However, all attempts to extend this approach to the synthesis
of analog systems containing longer conjugated branches such
as 2-hexylquaterthiophene remained unsuccessful due to the
insufficient reactivity of the quaterthiophene anion. This
implies that the future synthesis of analog systems containing
longer conjugated branches should resort to a different
synthetic approach.
Fig. 1 Top: cyclic voltammograms of compounds 3 (left) and 1
(right); bottom: right 6 (left) and 2 (right) ca. 1 mmol L21 in 0.10 M
Bu4NPF6–CH2Cl2, scan rate 100 mV s21, ref. Ag.AgCl.
Fig. 1 shows the cyclic voltammograms of compounds 1 and
2 and of the reference linear compounds 3 and 6. The CV of
compounds 3 and 6 shows an irreversible anodic wave process
at a peak potential (Epa) of 1.10 and 0.90 V (Table 1). The less
positive Epa value of compound 6 reflects the stronger
electron-releasing effect of the alkylsufanyl group compared
to the hexyl chain. The weak cathodic peak observed at 0.60 V
in the reverse scan suggests a dimerization of the cation radical
to produce the corresponding sexithiophene. The CV of the
tetrahedral compounds 1 shows an irreversible anodic peak at
1.15 V. The slight increase of Epa compared to the linear
compound 3 can be attributed to a lower diffusion coefficient
of the tetrahedral compound 1.8 The CV of compound 1 shows
a broad cathodic wave in which a weak peak is still discernible
at 0.60 V. This behaviour suggests that the cation radical of
compound 1 is not stable but undergoes a follow-up reaction
with a break of the carbon–silicon bond and formation of a
dimerization product. In fact, it has been shown already that
thiophenes and oligothiophenes bearing silyl group at the
terminal a-positions can be readily electropolymerized.9
As shown by the CV of compound 2, the introduction of
the sulfide group in the substituent leads to a fully reversible
oxidation process with Epa at 0.94 V indicating a stable cation
radical. Compared to compound 1, this stabilization can be
Table 1 Cyclic voltammetric data (in the conditions of Fig. 1), and
optical data in CH2Cl2 for compounds 1, 2 and the corresponding
branches 3 and 6
Compd
Epa/V
lmax/nm
3
1
6
2
1.10
1.15
0.90
0.94
367
384
371
390
Scheme 1 Synthesis of the target compounds 1 and 2.
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attributed to the stronger electron-releasing effect of the
alkylsufanyl group.
thermal evaporation under vacuum and the devices were
˚
completed by deposition of a 5 A thick layer of LiF and of an
80 nm thick aluminium electrode.
UV–vis. absorption data recorded in solution show that
for both linear and 3D compounds, replacement of hexyl
substituents by hexylsulfanyl ones produces a slight red shift of
Fig. 3 shows the UV–vis. absorption and photo-current
action spectra of the cells based on compounds 1 and 2 under
monochromatic irradiation. The absorption spectra show that
the cell based on 1 presents a much higher absorbance in the
400 nm region. While the smaller thickness of the film of 2
(16 nm instead of 20 nm for 1) can account for part of this
difference, other factors probably play a major role such as
molecular packing in the solid state and interactions of the
molecules with the Baytron1 treated ITO surface. Another
possible origin for this phenomenon could involve partial
re-sublimation of the film of compound 2 during the thermal
evaporation of the acceptor layer due to the relatively low
melting point of donor 2 (132 uC). Further work is needed to
clarify this point.
lmax. Comparison of the lmax of the 3D systems to those of the
corresponding linear chain reveals a red shift of 17 and 19 nm
for compounds 1 and 2 respectively. This phenomenon can be
attributed to the inductive effect of the silane substituent. Fig. 2
shows the UV–vis. spectra of compounds 1 and 2 in methylene
chloride solution and as a thin film on glass prepared by
thermal evaporation under vacuum. The spectrum of the film
of compound 1 shows a red shift of lmax from 384 to 406 nm
and the development of a fine structure with a second
maximum at 427 nm. This phenomenon can be attributed to
intermolecular interactions between the conjugated branches
in the solid state. Surprisingly, this effect is not observed for
compound 2 for which the spectrum of the film is practically
The action spectrum of the cell based on compound 1 shows
a first broad maximum of the external quantum efficiency
(EQE) of ca. 18% at 400 nm corresponding to the absorption
band of the donor 1 followed by a tail extending to 650 nm
corresponding to the absorption region of DP13. The EQE
spectrum of the cell based on compound 2 shows a maximum
EQE of 12% at 400 nm followed by shoulders of 8% and
5% around 480 and 580 nm corresponding to the absorption
features of DP13. The higher EQE observed at these
wavelengths compared to the cell based on 1 is probably the
consequence of the lower absorbance of the film of 2 which
allows a direct photo-excitation of the DP13 layer.
identical to the solution spectrum except for
broadening of the absorption band.
a slight
This different behaviour suggests that the sulfide groups in
the substituents do not allow tight interactions between the
conjugated branches in the solid. Further work and in
particular X-ray analysis is needed to clarify this point. The
spectra of the two films present an absorption onset at around
470 nm which correspond to a band gap of y2.65 eV.
Compounds 1 and 2 have been used as the donor material in
bilayer hetero-junction solar cells with N,N9-bis-tridecyl-
perylenedicarboxyimide (DP13) as the acceptor and electron
transport layer. Cells of 6 mm diameter have been realized on
ITO substrates spin-coated with a 30 nm film of Baytron P1.
The donor and acceptor layers were deposited successively by
Fig. 4 shows the current-density voltage curves of the two
cells in the dark and under simulated AM 1.5 solar irradiation
at 80 mW cm22 power intensity. The curves recorded in the
dark show a rectification behaviour with a current onset at 0.90
and 0.60 V for the cells based on donor 1 and 2 respectively.
Fig. 3 Photo-current action spectra of the hetero-junction solar cells.
Top: donor 1 (20 nm) and DP13 (20 nm), bottom donor 2 (16 nm)
and DP 13 20 nm. The dotted lines correspond to the optical spectra of
the bilayer.
Fig. 2 Normalized UV–vis absorption spectra of compound 1 (top)
and 2 (bottom). Dotted line: in CH2Cl2, solid line: thin films prepared
by thermal evaporation on glass.
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Fig. 5 Current density curves of a bulk hetero-junction based on
donor 2 and PCBM (ratio 1 : 3) in the dark (&) and under white light
irradiation at 80 mW cm22 light intensity (#).
evaluation of the potentialities of this compound as donor in
bulk hetero-junctions solar cells has been carried out. To this
end, a prototype cell has been realized by spin-casting a
composite film of donor 2 and [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM) as the acceptor in a 1 : 3 ratio. A mixture
of a donor 2 and PCBM dissolved in o-dichlorobenzene
was spun cast on a Baytron modified ITO electrode and
the device was completed by evaporation of a 80 nm thick
aluminium electrode.
Fig. 4 Current density curves of the bilayer cells based on donor 1
(top) and 2 (bottom) in the dark (&) and under white light irradiation
at 80 mW cm22 light intensity (#).
Under irradiation the cells deliver short-circuit current
densities (Jsc) of 0.36 and 0.44 mA cm22 respectively and
open-circuit voltage (Voc) of 0.99 and 0.74 V. The Voc values
which are tightly correlated with the oxidation potentials of
the two donors confirm the major influence of the oxidation
potential of the donor on Voc.10 Combined with filling factors
(FF) of 0.45 and 0.49 these data lead to power conversion
efficiencies of 0.20 and 0.17% for the cells based on donor 1
and 2 respectively (Table 2).
Fig. 5 shows the current-density–voltage curves of the cell.
In the dark the J–V curve is symmetrical indicating an
absence of rectification. The curve recorded under white light
illumination shows an increasing slope at negative voltage
which may reflect either important charge recombination or
current leakage. Under white light irradiation the cell delivers
a Jsc of 1.13 mA cm22 and Voc of 0.85 V. With a low FF
value of 0.24 these data lead to a power conversion efficiency
of 0.29%. An attempt to realize a reference cell with H3T
failed because of the lack of film forming properties of this
compound.
A reference cell realized using dihexylterthienyl (H3T) as the
donor gave the following results: Jsc = 0.14 mA cm22, Voc
=
0.76 V and FF = 0.32. Comparison of these results to those
obtained with the tetrahedral systems 1 and 2 clearly show
that the 3D donors 1 and 2 lead to much better results. Since
dihexylterthienyl and the 3D compounds 1 and 2 have
comparable absorption spectra, the better results obtained
with the 3D donors can be attributed to the combined effect of
a better absorption of the incident light and better hole
transport (as suggested by the better FF values). Since
thermally evaporated dialkyl-oligothiophenes are known to
orient vertically on surface such an orientation limits both the
absorption of the incident light and hole transport through the
cell thickness.1,3,4
Conclusion
Tetrahedral conjugated systems have been synthesized by
grafting terthienyl chains end capped with hexyl and hexyl-
sulfanyl chains on a silicon node. Electrochemical and UV–vis
results underline the major role of the end-substituents on the
stability of the cation-radical and on the intermolecular
interactions in the solid state.
A preliminary evaluation of the potentialities of these 3D
conjugated systems as donor material in organic solar cells has
been carried out. The results obtained with bilayer and bulk
hetero-junction solar cells show that compared to a reference
1D terthienyl donor, the 3D architecture leads to a significant
improvement of the cells’ performances attributed to the
isotropic absorption and hole transport associated with the
3D geometry.
Based on a larger solubility of compound 2 due among
other things, to weaker interactions in the solid state, a first
Table 2 Photovoltaic characteristics of the bilayera and bulkb hetero-
junctions based on donor 1 and 2 under while light irradiation at
80 mW cm22
Device
Voc/V
Jsc/mA cm22
FF
g(%)
Although the cell performances are drastically limited by the
poor absorption of the visible light by the donors, the first
results obtained with these short chain model compounds
confirm the interest of the 3D approach and they provide a
strong incitement to synthesize other 3D conjugated systems
H3T-DP13
1-DP13a
0.76
0.99
0.76
0.85
0.14
0.36
0.37
1.13
0.32
0.46
0.49
0.24
0.04
0.20
0.17
0.29
2- DP13a
2/PCBMb
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with longer effective conjugation length and hence extended
absorption in the visible spectral region. Work in these
directions is now underway and will be reported in future
publications.
30 min at 0 uC then 2 h at room temperature. Silicon
tetrachloride (0.11 mL, 0.17 eq.) in 10 mL of dry ether is added
at 0 uC. The mixture is stirred for 10 min at 0 uC and overnight
at room temperature. After addition of 450 mL of dichloro-
methane the organic phase is washed with water and dried over
MgSO4. Solvent evaporation and column chromatography
(alumina, 5 : 1 petroleum ether/methylene chloride) gave 0.86 g
Experimental
1
5-hexanoyl-2, 29:59, 20-terthiophene
(59%) of a yellow solid. M.p. 160 uC. H NMR (CDCl3) 7.43
3
3
(d, 1H, J = 3.58Hz), 7.29 (d, 1H, J = 3.57Hz), 7.13 (d, 1H,
3J = 3.82Hz), 6.99 (d, 1H, 3J = 3.78Hz), 6.98 (d, 1H, 3J =
To a solution of terthiophene (4.5 g, 16 mmol) and hexanoic
anhydride (4.23 mL, 1.1 eq.) in anhydrous methylene chloride
are added dropwise 2.08 mL (1.1 eq.) of tin tetrachloride. The
dark red solution is stirred at room temperature for 5 h. The
mixture is poured onto a mixture of ice and acetic acid,
methylene chloride is added and the organic phase is washed
with 10% aqueous NaOH and water and dried over MgSO4.
Solvent evaporation and column chromatography (silica gel,
1 : 1 petroleum ether/methylene chloride, leads to 3.64 g (66%)
of a brown solid. M.p. 133 uC
3
3
3.58Hz), 6.68 (d, 1H, J = 3.57Hz), 2.79 (t, 2H, J = 7.55Hz),
1.66 (quint, 2H, 3J = 7.52Hz), 1.36 (m, 2H), 1.30 (m, 4H), 0.87
(m, 3H). 13C NMR (CDCl3): 145.8, 145.4, 139.4, 137.6, 134.8,
134.3, 131.4, 125.2, 124.9, 124.9, 123.6, 123.5, 31.5, 31.5, 30.2,
28.7, 22.6, 14.1. MS MALDI-TOF calcd for C72H76S12Si 1352,
found 1353 (M + H)+.
Tetrakis(50-hexylsulfanyl-5-terthienyl)silane (2)
To a solution of 1 g of 5-hexylsulfanyl-2,29:59,20-terthiophene
in 15 mL of dry ether is added dropwise at 278 uC, 1.7 mL,
(0.97 eq.) of butyllithium 1.6 M in hexanes. The solution is
stirred for 30 min at 0 uC then 2 h at room temperature. Silicon
tetrachloride (0.05 mL, 0.17 eq.) in 5 mL of dry ether is added
at 0 uC. The mixture is stirred 10 min at 0 uC and 4 h at room
temperature. After addition of 60 mL of dichloromethane the
organic phase is washed with water and with a saturated
aqueous NaCl and dried over MgSO4. Solvent evaporation
and column chromatography (alumina, 4 : 1 petroleum
ether/methylene chloride) gave 0.23 g (34%) of a yellow solid.
Mp 132 uC. 1H NMR (CDCl3): 7.43 (d, 1H, 3J = 3.58Hz), 7.31
(d, 1H, J = 3.57Hz), 7.14 (d, 1H, J = 3.86Hz), 7.04 (d, 1H,
3J = 3.77Hz), 7.02 (d, 1H, 3J = 3.67Hz), 6.99 (d, 1H, 3J =
3.79Hz), 2.81 (t, 2H, 3J = 7.37Hz), 1.62 (quint, 2H, 3J =
7.50Hz), 1.40 (quint, 2H, 3J = 7.53Hz), 1,30 (m, 4HH), 0.88 (t,
3H, 3J = 6.91Hz). 13C NMR (CDCl3): 145.1, 139.8, 139.4,
136.6, 135.6, 134.6, 133.9, 131.6, 125.2, 125.1, 124.4, 123.8,
38.9, 31.3, 29.3, 28.1, 22.5, 13.9. M.S. MALDI-TOF SM calcd
for C72H76S16Si 1480 found 1481 (M + H)+ 1480. Anal (calcd)
C 58.84 (58.33) H 5.54 (5.17) Si 1.87 (1.89).
5-hexyl-2, 29:59,20-terthiophene
A mixture of 5-hexanoylterthiophene (3.61 g, 10.4 mmol),
aluminium trichloride (2.76 g) and LiAlH4 (3.15 g, 8 eq) in
100 mL of anhydrous ether is stirred under inert atmosphere at
ambient temperature for 3 h 30 min. Ethyl acetate (25 mL) and
6 M aqueous HCl (30 mL) are added and the solution is stirred
for 1 h 30 min. After addition of water, the aqueous phase
is extracted with ether, the organic phase is washed with
saturated aqueous Na2CO3 and water and dried over MgSO4.
Solvent evaporation and column chromatography (silica gel,
petroleum ether) gave 2.78 g (80%) of a yellow solid. Mp 66 uC.
3
3
1H NMR (CDCl3) 7.20 (dd, 1H, J = 5.10Hz; J = 1.10Hz),
7.15 (dd, 1H, 3J = 3.60Hz, 4J = 1.10Hz), 7.05 (d, 1H, 3J =
3.80Hz), 7.01 (dd, 1H, 3J = 5.10Hz, 3J = 3.60Hz), 6.99 (d, 1H,
3J = 3.80Hz), 6.98 (d, 1H, 3J = 3.50Hz), 6.68 (d, 1H, 3J =
3.50Hz), 2.79 (t, 2H, 3J = 7.70Hz), 1.38 (quint, 2H, 3J =
3
4
3
7.60Hz), 1.31 (m, 6H), 0.89 (t, 3H, J = 6.90Hz, CH3).
5-hexylsulfanyl-2, 29:59, 20-terthiophene
A mixture of 5-bromo-2,29-bithiophene, (2.5 g, 10 mmol),
5-hexylsulfanyl-2-tributylstanniothiophene (12.41 g, 1 eq.),
and Pd(PPh3)4, (1.46 g 5 mol%) and 250 mL of toluene is
refluxed under nitrogen for 17 h. After return to room
temperature the residue is dissolved in 100 mL of dichloro-
methane. The organic phase is washed with water and dried
over MgSO4. Solvent evaporation and column chromatogra-
phy (silica gel, petroleum ether) gave 2.64 g (72%) of a yellow
solid. M.p. 60 uC, 1H NMR (CDCl3) 7.22 (dd, 1H, 3J =
5.08Hz, 4J = 1.02Hz), 7.17 (dd, 1H, 3J = 3.61Hz, 4J = 1.05Hz),
7.07 (d, 1H, 3J = 3.72Hz), 7.04 (d, 1H, 3J = 3.78Hz), 7.02 (dd,
1H, 3J = 5.05Hz, 3J = 3.67Hz), 7.02 (d, 1H, 3J = 4.09Hz), 7.00
(d, 1H, 3J = 3.69Hz), 2.81 (t, 2H, 3J = 7.32Hz), 1.64 (quint, 2H,
3J = 7.50Hz), 1.41 (m, 2H), 1.29 (m, 4H), 0.87 (m, 3H).
Fullerene C60 (99+%) was purchased from Merck and used
without further purification. The indium–tin–oxide (ITO)
coated glass substrates (Solems) with a sheet resistance of
40 ohm square21 were cleaned in an ultrasonic bath with
aqueous detergent, ethanol and acetone dried in an oven and
treated 15 min with an UV–ozone cleaner (Jelight 42–220,
28 W cm22). ITO substrates were then spin-coated with a
60 nm film of poly(3,4-ethylenedioxythiophene)-poly(styrene-
sulfonate) (Baytron P1) and dried at 115 uC for 15 min. Layers
of compound 1 and C60 were thermally evaporated in a Plassys
ME300 chamber at a pressure of about 1026 mbar and at rate
of y1.5 A s21. A glass slide placed near the ITO substrates
˚
was used to obtain samples for UV–vis absorption spectra.
The device were completed by the evaporation of aluminium
films (ca. 60 nm thick) as negative electrodes. A shadow mask
with openings of 6 mm diameter was used to define a device’s
area of 0.30 cm2. After their fabrication, the devices were
stored and characterised in a glove-box (200B, MBraun). The
J–V curves were recorded in the dark and under various light
Tetrakis(50-hexyl-5-terthienyl)silane (1)
To a solution of 5-hexyl-2,29:59,20-terthiophene (2 g) in 30 mL
of dry ether is added dropwise 3.6 mL (0.97 eq.) of
butyllithium (1.6 M in hexanes). The solution is stirred for
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intensities using a Keithley 236 source-measure unit and a
home-made acquisition program.
Power conversion efficiencies were measured with
a
Steuernagel Solar constant 575 simulator. Light intensity was
measured with a calibrated broadband optical power meter
(Melles Griot) and was varied with neutral density filters.
The devices were illuminated through the ITO electrode
side. The efficiency values were neither corrected for a possible
spectral mismatch of the light source with the solar
spectrum nor for the absorption/reflection losses at the various
interfaces.
EQE was measured with a Perkin Elmer 7225 lock-in
amplifier under monochromatic illumination at variable
wavelength at a chopping frequency of 100 Hz. The light
source was a tungsten lamp (Acton SpectraPro150).
Absorption spectra were recorded with a Perkin Elmer
Lambda 19 UV–visible–Near IR spectrophotometer. Cyclic
voltammetry was performed in a three electrode cell with an
EGG 273 potentiostat.
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