A study of oligothiophene-acceptor dyes in p-type dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra26310g

Two new dyes, E1 and E2, equipped with triphenylamine as the electron donor, oligothiophene as the linker and different electron acceptor groups, have been designed and synthesized as photosensitizers for p-type dye-sensitized solar cells (p-DSCs). A systematic study of the effect of molecular structures on the observed photophysical properties, the electron/hole recombination process, the overall performance and the interfacial charge separation was carried out. Transient absorption spectroscopy (TAS) shows that the E1 dye with a napthoilene-1,2-benzimidazole (NBI) unit as the acceptor has a longer lifetime in the reduced state than the E2 dye with a malononitrile subunit on the NiO surface.

Full text of DOI:10.1039/c5ra26310g

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A Study of OligothiopheneꢀAcceptor Dyes in Pꢀ
Type DyeꢀSensitized Solar Cells
Esmaeil Sheibani a,¶, †, Lei Zhang b, ¶, Peng Liu c, Bo Xu a, Edgar Mijangos b, Gerrit Boschloo b,
Anders Hagfeldt b, ‡, Leif Hammarström b, Lars Kloo c, Haining Tian a,b,*
a Organic Chemistry, Department of Chemistry, Chemical Science and Engineering, KTH Royal
Institute of Technology, SEꢀ10044, Stockholm, Sweden. Eꢀmail: hainingt@kth.se
b Physical Chemistry, Department of ChemistryꢀÅngström Laboratory, Uppsala University, Box
523, SEꢀ751 20 Uppsala, Sweden. Eꢀmail: haining.tian@kemi.uu.se
c Applied Physical Chemistry, School of Chemical Science and Engineering, Department of
Chemistry, KTH Royal Institute of Technology, Teknikringen 30, SEꢀ10044, Stockholm ,
Sweden
† Present Address: Department of Chemistry, University of Isfahan, Isfahan 81746ꢀ73441, Iran
‡ Present Address: Institute of Chemical Sciences and Engineering, École Polytechnique de
Fédérale de Lausanne, EPFL SB ISIC LSPM, CH G1 523, Chemin des Alambics, Station 6, CHꢀ
1015 Lausanne, Switzerland
¶ these authors contribute equally
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Keywords: Oligothiophene, Photosensitizer, pꢀType DyeꢀSensitized Solar Cell, Nickel Oxide,
Transient Absorption Spectroscopy
Abstract: Two new dyes, E1 and E2, equipped with triphenylamine as electron donor, an
oligothiophene as linker and different electron acceptor groups, have been designed and
synthesized as photosensitizers for pꢀtype dyeꢀsensitized solar cells (pꢀDSCs). A systematic
study of the effect of molecular structures on the observed photophysical properties, the
electron/hole recombination process, the overall performance and the interfacial charge
separation was carried out. Transient absorption spectroscopy (TAS) shows that E1 dye with
napthoileneꢀ1,2ꢀbenzimidazole (NBI) unit as acceptor has longer lifetime of reduced state that
E2 dye with malononitrile subunit on NiO surface.
Introduction
Representing an inverted device structure with respect to the conventional dyeꢀsensitized solar
cells (nꢀDSCs)1, pꢀtype dyeꢀsensitized solar cells (pꢀDSCs) were first reported in 1999 by
Lindquist and coꢀworkers 2. The pꢀtype device demonstrated cathodic photocurrent through an
erythrosineꢀsensitized porous nickel oxide electrode. Recently, this type of solar cells has
attracted intense research interest from the research community, since it can be used in tandem
dyeꢀsensitized solar cells3, 4 (tꢀDSCs) as well as in photoelectrocatalytic solar fuel production
devices 3, 5ꢀ10 11. The configuration of pꢀDSCs quite similar as for nꢀDSCs, where pꢀDSCs consist
of a pꢀtype semiconductor substrate 12ꢀ15, a photosensitizer2, 3, 16ꢀ26, a redoxꢀactive electrolyte 27ꢀ29
and a counter electrode. As one of the crucial components, the photosensitizer undertakes the
task of light harvesting and extracts electrons from pꢀtype semiconductor substrate (or,
alternatively the process can be regarded as hole injection). The first breakthrough result of pꢀ
2

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30
DSCs by Sun and coꢀworkers in 2008 was achieved from employing a triphenylamineꢀbased
photosensitizer with anchorꢀdonorꢀlinkerꢀacceptor (anchꢀDꢀπꢀA) configuration. After this work,
much effort has been devoted to the optimization of the dye structure using different subunits.3,
23, 26, 31ꢀ33 An ideal photosensitizer is also expected to boost the efficiency of corresponding solid
state devices.34 Furthemore, in analogous solar fuel devices, the photosensitizer also represents
the heart of the device to generate the energyꢀrich electrons to be converted into an energyꢀrich
substance, such as hydrogen gas. Therefore, the development and detailed investigation of new
photosensitizers for pꢀDSSCs is very central for the overall improvement of the ability to convert
solar light to electricity or fuels. DesignꢀSynthesisꢀStudyꢀOptimization of new photosensitizers is
a classic and efficient strategy to seek new and efficient dyes for molecular solar cells and solar
fuels. In this work, we have designed and synthesized two new organic dyes (E1 and E2) with
two ꢀCOOH anchoring groups, triphenylamine as donor unit, oligothiophene as linker entity and
different electronꢀwithdrawing groups as electron acceptors. Taking the organic P1 dye as
reference photosensitizer, we have systematically studied the effects of different dye structures
on the solar cell performance based on photophysical properties, electrochemical and
photoelectrochemical performance and transient absorption characteristics to guide the future
dye design. The dye structures of E1, E2 and the reference dye P1 are shown in Figure 1.
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Figure1. Molecular structure of the E1, E2 and P1 dyes
Experiments
1
General: All commercial chemicals were used as received without further purification. H and
13C NMR spectra were recorded on a Bruker AVANCE 400 NMR and a Bruker AVANCE 500
MHz spectrometers in CDCl3, at ambient temperature, using TMS as internal standard. Chemical
shifts are reported in ppm relative to an internal standard of residual chloroform peak (δ ppm =
1
13
7.27 ppm for H NMR and 77.00 ppm for C NMR spectra, respectively). UVꢀvis spectra were
recorded using an Ocean Optics HR2000 spectrophotometer. Electrochemical experiments were
performed using a CH Instruments electrochemical workstation (model 660A). Dry solvents
were obtained from an MB SPSꢀ800 dry solvent dispenser system. Chromatographic separations
were carried out on silica gel 60Å (35ꢀ63 ꢁm). Cobalt(II/III) tris(4,4′ꢀdiꢀtertꢀbutylꢀ2,2′ꢀdipyridyl)
hexafluoridephorsphate (herein referred to as Co2+/3+(dtbpy)3) were provided by Dyenamo AB
(Sweden).
Electrochemistry: The measurements were carried out at room temperature with a conventional
threeꢀelectrode configuration consisting of a platinum working electrode, a platinum wire
4

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counter electrode and a nonꢀaqueous Ag/AgNO3 reference electrode. The potentials were
referenced using an internal ferrocene standard.
Solar Cell Fabrication: Mesoporous, nanostructured NiO films with a thickness of 3 µm (0.25
cm2) were prepared on FTO by screenꢀprinting forming the pꢀtype semiconductor electrodes.
o
The printed NiO films were sintered at 450 C for 30 min. All films used in the study originated
from the same NiO nanoparticle and paste batch. The NiO electrodes were sensitized in 0.2 mM
dichloromethane (DCM) solution of the dyes for 2 h. All of the sensitized NiO electrodes were
washed with ethanol (EtOH) and dried. The electrodes were then sealed with a platinized FTO
electrode using a 25 ꢁm thick hotꢀmelt film (Surlyn, Solaronix) through heating the system to
120 oC. The electrolyte used contained 0.05 M Co2+(dtbp)3, 0.05 M Co3+(dtbp)3, 0.1 M LiClO4 in
a mixed solvent consisting of propylene carbonate (PC) and acetonitrile (MeCN) (1:1, v:v). The
electrolyte was filled into the devices via preꢀdrilled holes in the counter electrodes using a
vacuum procedure. Finally, the holes were sealed with a Surlyn sheet and a thin glass slide by
heating.
Solar Cell Characterization: JꢀV characteristics were recorded using a Keithley source/meter
under AM 1.5 G simulated sunlight of 100 mW·cmꢀ2 light intensity from a Newport 300 W solar
simulator. Incident photonꢀtoꢀcurrent conversion efficiency (IPCE) was obtained using
monochromatic light from a system consisting of a xenon lamp, a monochromator and
transmittance filters. Light filters were used for calibration of the light spectrum. Both systems
were calibrated using a certified reference solar cell (IRꢀfiltered silicon solar cell, Fraunhofer
ISE, Freiburg, Germany). All solar cells were illuminated from the photocathode side and the
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samples were masked during the measurements with an aperture area of 0.25 cm2 (0.5×0.5 cm2).
Electron lifetime and extracted charge was determined by a customꢀmade ‘‘toolbox setup’’ using
a greenꢀlightꢀemitting diode (Luxeon K2 star 5 W, λmax=530 nm) as light source.
Transient Absorption Spectroscopy: The femtosecond transient spectroscopy setup has been
described in detail elsewhere.35 In brief, the femtosecond transient absorption spectrometer
includes a Coherent Legend Ti:Sapphire amplifier (1 kHz, λ = 800 nm, fwhm 100 fs). The output
is split to form pump and probe beams. The desired pump wavelengths were obtained using a
TOPAS white light source, and by using neutral density filters the energy of each pulse could be
kept between 200 and 400 nJ. The white light continuum probe was obtained by focusing part of
the 800 nm light on a moving CaF2 plate. Polarization of the pump was set at the magic angle,
54.7° relative to the probe. The instrument response time depends on the pump and the probe
wavelengths, but is typically about 150 fs. The NiO samples were translated during the
experiment to avoid photodegradation. For each scan of signal vs. time, the transient absorption
for 1000 laser shots was integrated for each delay time. Between 5 and 10 scans were averaged
depending on signal amplitude. The group velocity dispersion of the probe white light was fitted
to a thirdꢀorder polynomial by use of a homemade MATLAB program.
Nanosecond transient absorption spectroscopy was applied and sample was excited by a Qꢀ
switched YAGꢀlaser/MOPO combination (Spectra Physics) that delivered ca. 10 ns pulses at 10
Hz repetition frequency. The pulses were attenuated to 2 mJ/pulse over ca. 0.3 cm2. The
spectrometer (Edinburgh Instruments) was coupled to a probe light from a pulsed Xeꢀlamp (long
pass filtered at 380 nm) at a 90 degree angle from the laser excitation, while the sample was
placed at 45 degrees with respect to both beams for sensitized NiO films or in 1 cm cuvettes for
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the solution samples. Kinetic traces were recorded using a PMT and a digital oscilloscope.
Transient spectra were detected with a CCD (ANDOR).
Synthesis: tertꢀButyl 4ꢀ(phenyl(4ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)phenyl)methyl)36
(5) , 5,5''''ꢀdibromoꢀ3',3'',4,4'',4''',4''''ꢀhexahexylꢀ2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophene37, 38 (10)
and
4ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ7Hꢀbenzo[de]benzo[4,5]imidazo[2,1ꢀ
a]isoquinolinꢀ7ꢀone (12) were prepared according to a literature procedure39.
Scheme 1. Synthesis of the triphenylamine compound 5. Reagents and conditions: (i) KOtBu,
o
THF, 5 C (ii) Pd2dba3, [HP(tBu)3]BF4, KOtBu, toluene, rt; (iii) Pd2dba3, [HP(tBu)3]BF4, tꢀ
BuOK, toluene, 80 oC; (iv) NBS, DMF; (v) PdCl2(dppf), bis(pinacolato)diboron, KOAc, DMF.
4ꢀBromoꢀbenzoic acid tertꢀbutyl ester (1). Under nitrogen potassium tertꢀbutoxide (8.4 g, 75
mmol) was added to a round flask containing 60 ml dry tetrahydrofuran (THF). The temperature
o
was lowered to 0 C by adding ice; this mixture was added dropꢀwise to a dry THF solution (60
mL) of 4ꢀbromobenzyl chloride (15.0 g, 68 mmol) during about one hour while the temperature
o
was not allowed to exceed 5 C. After the addition was complete, the solution was stirred
continuously at room temperature for 2 h. The mixture was extracted with diethyl ether, and the
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organic phased collected was dried over MgSO4. The liquid product is almost pure with
1
quantitative yield. H NMR (500 MHz, CDCl3) δ ppm 7.87 (d, J = 8.8 Hz, 2H), 7.57 (d, J = 8.8
Hz, 2H), 1.62 (s, 9H).
4ꢀ(NꢀPhenylamino)benzoic acid tertꢀbutyl ester(2). Aniline (4.58 g, 49 mmol) and the ester 1
(8.5 g, 33 mmol) were dissolved in 100 mL dry toluene. After the solution was degassed,
potassium tertꢀbutylate (8.2 g, 73 mmol), Pd2(dba)3 catalyst (381 mg, 0.33 mmol) and [HP(tꢀ
Bu3)]BF4 (153 mg, 0.53 mmol) were added. Subsequently, the resulting suspension was degassed
o
and stirred for 2 hours at 40 C. After the reaction was completed, the solution was poured into
water and the aqueous phase was acidified with hydrochloric acid. The aqueous phase was
extracted with dichloromethane (DCM) and the organic phase was dried with MgSO4. After the
solvent was removed by rotary evaporation the residue was passed through a silica column for
purification to remove the catalytic residues. The pure product compound 2 was obtained by
1
crystallization in hexane with 85% yield (7.57 g). H NMR (500 MHz, CDCl3) δ ppm 7.9 (d, J =
8.8 Hz, 2H), 7.35 (t, J = 8.1 Hz, 2H), 7.18 (d, J = 8.1 Hz, 2H), 7.07 (t, J = 7.5 Hz, 1H), 7.01 (d, J
= 8.8 Hz, 2H), 5.99 (s, b, NH), 1.61 (s, 9H).
N,NꢀDi(4ꢀbenzoic acid tertꢀbutyl ester)phenylamine(3). The diphenylamine compound 2 (5.5g,
20.42 mmol) and ester compound 1 (5.75 g, 22.36 mmol) were dissolved in 60 mL dry toluene.
After the solution was degassed, potassium tertꢀbutylate (4.8 mg, 42.8 mmol), Pd2dba3 catalyst
(231 mg, 230 µmol] and [HP(tꢀBu3)]BF4 (100 mg, 370 µmol) were added. Then, the resulting
o
suspension was degassed and stirred for 2 hours at room temperature and 1 hour at 80 C.
Subsequently, the mixture was poured into water and the aqueous phase was neutralized using
hydrochloric acid. The aqueous phase was extracted with DCM and the organic phase was dried
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using MgSO4. After the solvent was removed by rotary evaporation the crude product was
purified by silica gel column using petroleum ether (PE)/DCM (3:1) to afford 8.2 g (90%) of the
product as a white powder. 1H NMR (500 MHz, CDCl3) δ ppm 7.88 (d, J = 8.8 Hz, 4H), 7.35 (t,
J = 8.1 Hz, 2H), 7.19 (t, J = 8.1 Hz, 1H), 7.15 (d, J = 8.1 Hz, 2H), 7.08 (d, J = 8.8 Hz, 4H), 1.61
(s, 18H).
N,NꢀDi(4ꢀbenzoic acid tertꢀbutyl ester)ꢀ4ꢀbromoꢀphenylamine(4). N,Nꢀdi(4ꢀbenzoic acid tertꢀ
butyl ester)phenylamine (3) (5.75 g, 12.90 mmol) was dissolved in dried dimethyl formamide
(DMF) (50 mL) and cooled on an ice bath. A solution of Nꢀbromosuccinimide (2.75 g, 15.4
mmol) in dried DMF (10 mL) was slowly added to the reaction mixture, then the ice bath was
removed and the mixture was stirred at room temperature overnight. DMF was evaporated under
vacuum and the resulting solid residue was further crystallized from EtOH to afford 5.4 g (80%)
of compound 4 as a white solid. 1H NMR (500 MHz, CDCl3) δ ppm 7.9 (d, J = 8.8 Hz, 4H), 7.44
(t, J = 8.8 Hz, 2H), 7.08 (d, J = 8.8 Hz, 4H), 7.02 (d, J = 8.8 Hz, 2H), 1.61 (s, 18H).
tertꢀButyl 4ꢀ(phenyl(4ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)phenyl)methyl) (5).
A
mixture of compound 4 (3.5 g, 6.7 mmol), PdCl2(dppf) (0.59 g, 0.8 mmol), diborane pinacol
o
ester (2.37 g, 9.3 mmol) and KOAc (2.5 g, 26.8 mmol) in 50 mL DMF was heated to 90 C for
12 h. After cooling, the solvent was removed by vacuum distillation. The reaction mixture was
dissolved in DCM and washed with water and then dried over anhydrous MgSO4. After
removing the solvent, the residue was purified by column chromatography (PE:EtOAc, 8:1, v/v)
to afford 2.48 g (65% yield) of product. 1H NMR (500 MHz, CDCl3) δ ppm 7.88 (d, J = 8.7 Hz,
4H), 7.76 (t, J = 8.3 Hz, 2H), 7.11 (m, 6H), 1.61 (s, 18H), 1.37 (s, 12H). 13C NMR (500 MHz,
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CDCl3) δ ppm 165.36, 150.51, 149.03, 136.24, 130.84, 126.54, 124.44, 123.09, 83.84, 80.78,
28.24, 24.87.
Scheme 2. Synthesis of the oligothiophene building block 10. Reagents and conditions: (i) nꢀ
o
BuLi, THF, pinacol isopropyl borate, ꢀ78 C; (ii) Pd(PPh3)4, Na2CO3, toluene, reflux, (iii) Nꢀ
Bromosuccinimide (NBS), DMF, (iv) Pd(PPh3)4, Na2CO3, toluene, reflux, (v) NBS, DMF
4ꢀhexylthiopheneꢀ2ꢀboronic acid (6). 3ꢀHexylthiophene (10 g, 59.4 mmol) in 100 ml dry THF
o
was cooled to ꢀ78 C in dry ice. Butyl lithium (26.2 mL, 26.2 mmol) was added dropwise by a
o
syringe and the mixture was stirred at ꢀ78 C for 2 h. 2ꢀIsopropoxyꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀ
dioxaborolane (14.6 mL, 71.5 mmol) was added quickly and the mixture was left to heat up to
room temperature and stirred overnight. The solvent was then evaporated under vacuum to afford
a colorless oil which was dissolved in 100 mL DCM, washed with water (3×80 mL), dried over
MgSO4, filtered off and finally the solvent was evaporated under vacuum. Chromatography
(silica, petroleum ether/ethyl acetate 20/1) afforded the desired colorless oily product (13.1 g,
1
75%). H NMR (500 MHz, CDCl3) δ ppm 7.5 (s, 1H), δ ppm 7.23 (s, 1H), 2.65 (d, J = 7.8 Hz,
2H), 1.64 (m, 2H), 1.29ꢀ1.38 (m, 18H), 0.9 (t, J = 6.8 Hz, 3H).
10

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3',4,4'ꢀtrihexylꢀ4''ꢀpentylꢀ2,2':5',2''ꢀterthiophene (7). 2,5ꢀDibromoꢀ3,4ꢀdihexylthiophene (3 g, 7.3
mmol) and 2ꢀ(4ꢀhexylthiophenꢀ2ꢀyl)ꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane (6) (5.1 g, 17.3
mmol) were dissolved in 60 mL THF. The resulting solution was carefully degassed, and
Pd(PPh3)4 catalyst (422 mg, 5 mol%) and Na2CO3 (2 M, 24 ml) were added. Next, the reaction
mixture was carefully degassed and stirred at refluxing condition overnight. After the above
reaction time water was poured into the mixture and the organic layer was separated using
diethyl ether. The combined organic phases were dried over MgSO4 and the solvent was
removed by rotary evaporation. The crude product was purified by column chromatography (nꢀ
hexane) to give product (3.47 g, 83%) as a bright yellow oil. 1H NMR (500 MHz, CDCl3) δ ppm
6.98 (s, 2H), δ ppm 6.90 (s, 2H), 2.7 (t, J = 8.4 Hz, 4H), 2.63 (t, J = 7.7 Hz, 4H), 1.53ꢀ1.7 (m,
10H), 1.29ꢀ1.48 (m, 22H), 0.92 (m, 12H).
5,5''ꢀdibromoꢀ3',4,4'ꢀtrihexylꢀ4''ꢀpentylꢀ2,2':5',2''ꢀterthiophene (8). Bromination of the thiophene
compound 7 was executed using the same procedure as previously described for compound 4. 1H
NMR (500 MHz, CDCl3) δ ppm 6.83 (s, 2H), 2.65 (m, 4H), 2.58 (t, J = 7.7 Hz, 4H), 1.50ꢀ166
(m, 10H), 1.30ꢀ146 (m, 22H), 0.93 (m, 12H).
3',3'',4,4'',4''',4''''ꢀhexahexylꢀ2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophene (9). Suzuki coupling was
used to couple 2ꢀ(4ꢀhexylthiophenꢀ2ꢀyl)ꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane (6) and
compound 8 by a procedure similar to the one used to synthesize compound 7 to give compound
1
9 with 76% yield as a bright yellow oil. H NMR (500 MHz, CDCl3) δ ppm 6.99 (m, 4H), δ ppm
6.92 (s, 2H), 2.75 (m, 6H), 2.64 (m, 6H), 1.57ꢀ1.72 (m, 12H), 1.28ꢀ1.45 (m, 36H), 0.93 (m,
18H).
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5,5''''ꢀdibromoꢀ3',3'',4,4'',4''',4''''ꢀhexahexylꢀ2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophene
(10).
Bromination of thiophene 9 was performed using the same procedure as previously described for
compounds 4 and 8. 1H NMR (500 MHz, CDCl3) δ ppm 6.98 (m, 2H), 6.84 (s, 2H), 2.73 (t, 7.8,
13
6H), 2.59 (t, 7.7, 6H), 1.53ꢀ1.71 (m, 12H), 1.30ꢀ1.50 (m, 36H), 0.85ꢀ 0.92 (m, 18H). C NMR
(500 MHz, CDCl3) δ ppm142.48, 140.30, 140.09, 135.42, 134.35, 129.88, 129.68, 128.42,
126.59,108.72, 31.71, 31.67, 31.53, 30.57, 30.44, 29.69, 29.58, 29.31, 29.24, 29.06, 28.95, 28.28,
22.71, 22.67, 14.14.
Scheme 3. Synthesis of the acceptor building block 12. Reagents and conditions: (i) acetic acid,
reflux; (ii) nꢀBuLi, THF, pinacol isopropyl borate, ꢀ78 0C.
Synthesis of 4ꢀbromoꢀ1,8ꢀnapthoileneꢀ1,2ꢀbenzimidazole (11). To a 100 mL roundꢀbottom flask
containing of 4ꢀbromoꢀ1,8ꢀnaphthalic anhydride (5 g, 18 mmol) in 60 mL of acetic acid, (2.4 g,
22.2 mmol) of ortho phenylenediamine was added. The mixture was refluxed overnight. After
the reaction time, the mixture was poured into ice water. The resulting precipitate was filtered
and purified by recrystallization from acetone. A bright yellow powder of the product was
obtained (6.3 g, quantitative). NMR spectra show an exact 50:50 mixture of two isomers. The
polarity of these two isomers is very similar and is hard to be purified; therefore, the mixture was
directly used in the next reaction. 1H NMR (500 MHz, CDCl3) δ ppm 8.87 (d, J = 7.3 Hz, 1H),
8.8 (d, J = 7.3 Hz, 1H), 8.63 (m, 2H), 8.56 (d, J = 8.1 Hz, 1H), 8.46ꢀ8.53 (m, 3H), 8.08ꢀ8.12 (m,
2H), 7.88ꢀ7.93 (m, 4H), 7.5 (m, 4H).
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4ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ7Hꢀbenzo[de]benzo[4,5]imidazo[2,1ꢀ
a]isoquinolinꢀ7ꢀone (12). The isomeric mixture of 11 (2 g, 5.72 mmol), PdCl2(dppf) (1 g, 1.4
mmol), diborane pinacol ester (2 g, 8 mmol) and KOAc (2.25 g, 22.8 mmol) in 50 mL DMF was
o
heated to 90 C and was kept at that temperature for 12 h. After cooling, the solvent was
removed by vacuum distillation. The reaction mixture was dissolved in DCM, washed with water
and then dried over anhydrous MgSO4. After removing the solvent, the residue was purified by
long column chromatography (PE:EtOAc; 30:1 to 10:1; v/v) to afford 0.59 g (52% yield) of pure
product. 1H NMR (500 MHz, CDCl3) δ ppm 9.08 (d, J = 8.5 Hz, 1H), 8.86 (d, J = 7.3 Hz, 1H),
8.78 (d, J = 7.3 Hz, 1H), 8.53ꢀ8.59 (m, 1H), 8.39 (t, J = 7.3 Hz, 1H), 7.87ꢀ7.93 (m, 1H), 7.84 (t, J
13
= 8 Hz, 1H), 7.46ꢀ7.53 (m, 2H), 1.5 (s, 12H). C NMR (500 MHz, CDCl3) δ ppm160.79,
149.60, 143.89, 135.95, 135.67, 132.84, 131.81, 130.14, 127.46, 126.86, 126.78, 125.77, 125.28,
125.13, 120.53, 119.91, 115.86, 84.67, 25.02.
diꢀtertꢀbutyl4,4'ꢀ((4ꢀ(3',3'',4,4'',4''',4''''ꢀhexahexylꢀ5''''ꢀ(7ꢀoxoꢀ7Hꢀ
benzo[de]benzo[4,5]imidazo[2,1ꢀa]isoquinolinꢀ4ꢀyl)ꢀ[2,2':5',2'':5'',2''':5''',2''''ꢀ
quinquethiophen]ꢀ5ꢀyl)phenyl)azanediyl)dibenzoate (13). A double Suzuki coupling was
performed in one step, in which compound 5 (58 mg, 0.1 µmol), compound 12 (40 mg, 0.1
µmol) and compound 10 (110 mg, 0.1 µmol) were added to 10 mL THF. The resulting solution
was carefully degassed, and Pd(PPh3)4 catalyst (32 mg, 10 mol%) and 2 M aqueous sodium
carbonate solution (1 ml) were added. Then, the reaction mixture was carefully degassed and
stirred at refluxing condition for 24 h. After the reaction water was poured into the mixture and
the organic layer was separated using diethyl ether. The combined organic phases were dried
with anhydrous MgSO4, and the solvent was removed by rotary evaporation. The crude product
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was purified using chromatography (PE:EtOAc 3:1) to afford 103 mg (68% yield) of the red
product. 1H NMR (500 MHz, CDCl3) δ ppm 8.95 (d, J = 7.3 Hz, 1H), 8.77 (d, J = 7.3 Hz, 1H),
8.60ꢀ8.64 (m, 1H), 8.27 (d, J = 8.6 Hz, 1H), 7.90ꢀ7.96 (m, 6H), 7.84 (dd, J = 7.9 Hz, 1H), 7.51ꢀ
7.55 (m, 2H), 7.42 (m, 2H), 7.12ꢀ7.21 (m, 7H), 7.05 (s, 2H), 7.02 (s, 1H), 2.67ꢀ2.90 (m, 10H),
2.46 (m, 2H), 1.08ꢀ1.79 (m, 57H), 0.87ꢀ 0.99 (m, 24H), 0.8 (m, 3H). 13C NMR (500 MHz,
CDCl3) δ ppm165.36, 165.28, 160.58, 150.55, 149.51, 145.46, 143.92, 142.54, 140.37, 140.22,
140.13, 139.62, 139.16, 137.01, 136.79, 134.43, 133.96, 133.76, 132.66, 131.99, 131.90, 130.73,
130.26, 130.08, 129.97, 129.93, 129.62, 128.61, 127.62, 127.58, 127.39, 127.22, 126.43, 125.88,
125.67, 125.54, 125.48, 122.99, 122.83, 122.60, 120.97, 120,02, 115.96, 80.82, 31.73, 31.67,
31.54, 31.44, 30.92, 30.66, 30.54, 30.48, 29.63, 29.55, 29.46, 29.28, 29.19, 29.08, 28.87, 28.30,
23.86, 22.69, 22.64, 222.60, 22.47, 20.82, 14.13, 13.99.
4,4'ꢀ((4ꢀ(3',3'',4,4'',4''',4''''ꢀhexahexylꢀ5''''ꢀ(7ꢀoxoꢀ7Hꢀbenzo[de]benzo[4,5]imidazo[2,1ꢀ
a]isoquinolinꢀ4ꢀyl)ꢀ[2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophen]ꢀ5ꢀyl)phenyl)ꢀ azanediyl)dibenzoic
acid (E1). To a DCM solution of 13 (100 mg, 1.40 mmol), trifluoroacetic acid (243 µL, 3.1
µmol) was added (5 mL). The mixture was stirred at room temperature for 12 h. The reaction
mixture was neutralized using triethylamine. After being extracted with DCM, the organic phase
was dried over anhydrous MgSO4. The crude product was purified by column chromatography
(petroleum ether/ethyl acetate as gradient eluent) to gain a lightꢀred product (67 mg, 72%).1H
NMR (500 MHz, CDCl3) δ ppm 8.96 (d, J = 7.3 Hz, 1H), 8.85 (d, J = 7.3 Hz, 1H), 8.61 (m, 1H),
8.27 (d, J = 8.5 Hz, 1H), 8.03ꢀ8.16 (m, 4H), 7.96 (m, 1H), 7.90 (d, J = 7.6 Hz, 1H), 7.83 (dd, J =
7.6 Hz, 1H), 7.46ꢀ7.54 (m, 4H), 7.25 (m, 7H), 7.01ꢀ7.09 (m, 3H), 2.68ꢀ2.93 (m, 10H), 2.46 (m,
13
2H), 1.26ꢀ1.83 (m, 42H), 1.19 (m, 6H), 0.92 (m, 15H), 0.8 (m, 3H),. C NMR (500 MHz,
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CDCl3) δ ppm171.43, 160.53, 151.48, 149.46, 144.96, 143.64, 142.57, 140.37, 140.28, 140.23,
140.13, 139.67, 139.33, 136.81, 136.77, 134.43, 134.18, 133.83, 132.64, 131.94, 131.86, 131.80,
131.63, 130.95, 130.84, 130.66, 130.47, 130.10, 129.96, 129.94, 129.56, 128.61, 128.29, 127.61,
127.23, 126.36, 125.95, 125.55, 123.78, 122.89, 122.78, 120.76, 119.95, 115.95, 31.76, 31.73,
31.69, 31.56, 31.46, 30.94, 30.67, 30.55, 30.50, 29.66, 26.58, 29.49, 29.30, 29.22, 29.11, 28.91,
28.33, 22.71, 22.68, 22.66, 22.64, 22.49, 14.16. HRꢀMS ES (m/z): calcd. for
[C94H105N3O5S5], 1515.6658; found, 1515.6735
3',3'',4,4'',4''',4''''ꢀhexahexylꢀ[2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophene]ꢀ5ꢀcarbaldehyde (14).
3',3'',4,4'',4''',4''''ꢀhexahexylꢀ2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophene (9) (1.3 g, 1.42 mmol) and
DMF (126 µL, 1.63 mmol) was cooled in 1,2ꢀdichloroethane (10 mL) on an ice bath.
Phosphorous oxychloride (152 µL, 1.63 mmol) was added dropwise to the solution and was
refluxed overnight. The resulting suspension was poured into a cold, saturated aqueous sodium
acetate solution (20 mL) and allowed to stir for 2ꢀ3 hours. Extraction was completed with
dichloromethane (3 ×20 mL), the extracted solution was dried with MgSO4 and concentrated by
rotary evaporation. Silica column chromatography purification of the residue (hexane:ethyl
1
acetate 5:1) yielded 14 as a yellow solid (780 mg, 58%). H NMR (500 MHz, CDCl3) δ ppm
10.05 (s, CHO, 1H), 7.07 (s, 1H), 6.99ꢀ7.05 (m, 3H), 6.93 (s, 1H), 2.98 (t, 7.8, 2H), 2.85 (t, 7.8,
2H), 2.72ꢀ2.81 (m, 6H), 2.65 (t, 7.7, 2H), 1.57ꢀ1.78 (m, 12H), 1.31ꢀ1.54 (m, 36H), 0.84ꢀ1 (m,
18H). 13C NMR (500 MHz, CDCl3) δ ppm181.46, 153.31, 145.19, 143.68, 142.42, 140.82,
140.18, 139.52, 136.29, 136.21, 135.52, 133.52, 131.20, 130.62, 129.37, 129.01, 128.77, 128.57,
127.95, 127.09, 120.07, 31.75, 31.64, 31.56, 31.46, 30.58, 30.55, 30.45, 30.23, 29.89, 29.67,
29.36, 29.32, 29.30, 29.08, 28.51, 28.37, 28.29, 22.72, 22.69, 22.63. 14.14.
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5''''ꢀbromoꢀ3',3'',4,4'',4''',4''''ꢀhexahexylꢀ[2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophene]ꢀ5ꢀ
carbaldehyde (15). Bromination of 14 was performed as previously described for the compounds
4 with a yield of 87%.1H NMR (500 MHz, CDCl3) δ ppm 10.04 (s, CHO, 1H), 7.06 (s, 1H), 7.03
(s, 1H), 6.98 (s, 1H), 6.85 (s, 1H), 2.95ꢀ2.98 (m, 4H), 2. 85 (t, 7.8, 2H), 2.69ꢀ2.78 (m, 6H), 2.56ꢀ
2.63 (m, 2H), 1.56ꢀ1.77 (m, 12H), 1.24ꢀ1.51 (m, 36H), 0.84ꢀ0.97 (m, 18H).
diꢀtertꢀbutyl
4,4'ꢀ((4ꢀ(5''''ꢀformylꢀ3',3'',4,4'',4''',4''''ꢀhexahexylꢀ[2,2':5',2'':5'',2''':5''',2''''ꢀ
quinquethiophen]ꢀ5ꢀyl)phenyl)azanediyl)dibenzoate (16). The A Suzuki coupling reaction was
repeated under the same conditions as described above for compound 13 with a yield of 78%. 1H
NMR (500 MHz, CDCl3) δ ppm 10.04 (s, CHO, 1H), 7.94 (d, 7.8, 4H), 7.39ꢀ7.45 (m, 2H), 7.13ꢀ
7.23 (m, 6H), 7.01ꢀ7.09 (m, 4H), 2.98 (t, 7.8, 2H), 2.57ꢀ2.88 (m, 10H), 1.57ꢀ1.78 (m, 30H), 1.26ꢀ
1.52 (m, 36H), 0.84ꢀ 0.99 (m, 18H). 13C NMR (500 MHz, CDCl3) δ ppm181.46, 165.34, 153.33,
150.57, 145.46, 145.14, 143.40, 142.42, 140.85, 140.22, 139.58, 139.15, 137.06, 136.21, 133.94,
133.55, 130.93, 130.25, 129.38, 129.05, 128.78, 128.66, 128.25, 127.97, 126.65, 126.44, 125.68,
123.00, 122.83, 80.77, 31.76, 31.73, 31.69, 31.62, 31.54, 31.44, 30.94, 30.56, 30.23, 29.87,
29.65, 29.31, 29.21, 29.04, 28.87, 28.50, 28.25, 22.71, 22.66, 22.60, 14.14.
4,4'ꢀ((4ꢀ(5''''ꢀformylꢀ3',3'',4,4'',4''',4''''ꢀhexahexylꢀ[2,2':5',2'':5'',2''':5''',2''''ꢀquinquethiophen]ꢀ5ꢀ
yl)phenyl)azanediyl)dibenzoic acid (17). The compound was synthesized using the same
1
procedure as described above for E1. H NMR (500 MHz, CDCl3) δ ppm 11.38 (s, b, 2COOH),
10.02 (s, CHO, 1H), 8.01.ꢀ8.09 (m, 4H), 7.42ꢀ7.49 (m, 2H), 7.16ꢀ7.25 (m, 6H), 6.98ꢀ7.08 (m,
4H), 2.96 (t, 7.8, 2H), 2.67ꢀ2.87 (m, 10H), 1.56ꢀ1.77 (m, 12H), 1.23ꢀ1.47 (m, 36H), 0.83ꢀ 0.98
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(m, 18H). 13C NMR (400 MHz, CDCl3) δ ppm 180.73 (s), 170.28 (s), 169.89, 152.56, 150.20,
144.26, 143.99, 141.45, 139.88, 139.26, 138.64, 138.27, 135.83, 135.19, 135.07, 132.98, 132.53,
130.70, 129.37, 128.32, 127.84, 127.69, 127.25, 127.01, 125.19, 122.97, 121.89, 114.73, 76.30,
30.66, 30.62, 30.47, 30.40, 30.37, 29.88, 29.53, 29.39, 29.16, 28.57, 28.41, 28.22, 28.15, 28.02,
27.98, 27.46, 27.28, 21.63, 21.58, 21.53, 13.16, 13.09.
4,4'ꢀ((4ꢀ(5''''ꢀ(2,2ꢀdicyanovinyl)ꢀ3',3'',4,4'',4''',4''''ꢀhexahexylꢀ[2,2':5',2'':5'',2''':5''',2''''ꢀ
quinquethiophen]ꢀ5ꢀyl)phenyl)azanediyl)dibenzoic acid (E2). A mixture of compound 17 (100
mg, 78.3 µmol) and an excess of malononitrile (10 mg, 152 µmol) was poured into a 10 ml
mixture solvent of chloroform/ethanol (2:8, v/v). Then, a few drops of triethylamine were added
and refluxed overnight. The crude sample was purified by silica gel chromatography with
(EtOAc:MeOH; 100:1 to 10:1) to afford 57 mg (55% yield) of product. 13C NMR (400 MHz,
CDCl3): δ ppm 8.06 (d, J = 8.7 Hz, 4H), 7.84 (s, 1H), 7.45ꢀ7.52 (m, 2H), 7.20ꢀ7.26 (m, 6H),
7.12 (s, 1H), 7.04ꢀ7.09 (m, 2H), 2.57ꢀ2.94 (m, 12H), 1.56ꢀ1.78 (m, 12H), 1.26ꢀ1.53 (m, 36H),
13
0.84ꢀ 0.98 (m, 18H). C NMR (400 MHz, CDCl3) δ ppm 171.47, 156.39, 151.53, 147.33,
147.13, 144.27, 143.51, 141.44, 140.52, 139.74, 139.38, 138.20, 136.87, 134.03, 133.44, 131.89,
131.05, 130.49, 129.21, 129.15, 128.88, 128.72, 127.64, 127.09, 126.39, 123.48, 122.78, 115.38,
114.01, 73.34, 31.73,, 31.69, 31.55, 31.50, 31.42, 31.35, 30.93, 30.54, 30.27, 30.18, 29.61, 29.46,
19.28, 29.22, 29.14, 29.08, 28.88, 28.37, 22.68, 22.66, 22.61, 22.57, 14.15, 14.08. HRꢀMS ES
(m/z): calcd. for [C80H97N3O4S5], 1323.6083; found, 1323.6072
Results and Discussions
An efficient synthetic strategy was applied to render the two new dyes and the corresponding
synthetic routes are illustrated in the Schemes 1ꢀ4. The synthesis of 13 required double Suzuki
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crossꢀcouplings (5) between the boronic ester of the arylamine, the boronic ester of the
naphthalic anhydride (NPA) (12) and the polythiophene (10). The subsequent hydrolysis with
trifluoroacetic acid (TFA) provided the target E1 dye. In order to make the corresponding dye
E2, compound 15 was first synthesized by the formylation reaction of 9 and then by bromination
of 14. Thereafter, a Suzuki coupling between 5 and the polythiophene bromide 15 was
performed, followed by hydrolysis with TFA. Finally, the aldehyde condensation with
malononitrile using the Knoevenagel reaction gave the E2 product. The overall syntheses gave
high yields of two new dyes E1 and E2 (Scheme 4 and 5). The syntheses of the intermediates
and building blocks are explained in more detail in the synthesis section.
Scheme 4. Synthetic routes of the dyes E1
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Scheme 5. Synthetic routes of the dyes E2
The absorption spectra and photophysical data are displayed in Figure 2 and Table 1,
respectively. All the dyes exhibit two absorption bands, at 300ꢀ400 and 400ꢀ550 nm. The
absorption at 400ꢀ550 nm corresponds to a πꢀπ* transition (see computational results below). E1
shows a hypochromic shift in contrast to P1 and E2 caused by the weaker electron withdrawing
ability of the NPA component in E1 with respect to that of the malononitrile subunits in P1 and
E2. Notably, a bathochromic absorption shift and a broader absorption band of E2 are observed
as compared to the absorption spectrum of P1, which can be assigned to the larger conjugated
system in E2, although both dyes contain the malononitrile entity as acceptor unit. From the
intersection of the normalized absorption spectra and emission spectra, E0ꢀ0 energies of 2.25,
2.69 and 2.26 eV were obtained for the P1, E1 and E2 dyes in DCM solution, respectively. A
comparison of the solution UVꢀvis absorption and emission profiles of P1, E1, and E2 is
provided in Figure 2 and Figure S3. Figure 2b shows the absorption spectra of the dyes adsorbed
on a NiO film. Unfortunately, it is difficult to accurately quantify the amount of adsorbed dye
because the desorption process of the dyes (used for quantification) from the NiO films totally
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destroys all these dyes. But taking a rough estimate from the absorption spectra of the dyes in
solution and on film, and the small difference in extinction coefficients (Table 1), we can
conclude that E1 and E2 show lower dye loadings than P1 on a NiO film, probably due to their
more bulky molecular structure The lower dye loading for E1 and E2 has to be taken into
account in the following research and dye design.
(a)
3,0
2,5
2,0
1,5
1,0
0,5
0,0
0,7
0,6
0,5
0,4
0,3
0,2
0,1
(b)
E1
E2
P1
E1
E2
P1
300
400
500
600
700
400
500
600
700
Wavelength / nm
Wavelength / nm
Figure 2. Absorption spectra of the E1, E2 and P1 dyes in DCM solution (a) and on NiO film (b)
Dye
HOMO
LUMO
E1
E2
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Figure 3. Frontierꢀorbitals density distribution of the E1 and E2 dyes as calculated using the B3LYP/6ꢀ31G*
functional/basis sets for all atoms, without any symmetry constraints (carbon in gray, nitrogen in blue, oxygen in
red, sulfur in yellow and hydrogen in white). In order to accelerate the convergence of the electronic and geometric
optimization, the long hexyl chains were replaced by methyl substituents.
In order to get an insight into the electron distribution of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (HOMO) of the different dyes, density
functional theory (DFT) quantum chemical and timeꢀdependent DFT (TDDFT) calculations were
performed using the B3LYP40/6ꢀ31G* functional/basis sets for all atoms and without any
symmetry constraints. In order to simply the computational process, all hexyl groups were
replaced by methyl units in the E1 and E2 structures. All calculations were carried out by means
of the program system Gaussian09. The TDDFT excited state calculations were carried out in
ethanol solution using the MPW1K41 functional for E1 and E2. The HOMO–LUMO spatial
distribution in the E1 and E2 dyes is shown in Figure 3 and Table S1. The simulated absorption
spectra are shown in Figure S5 and coincide well with the experimentally recorded spectra.
According to the calculations, the HOMO is primarily localized on the oligothiophene groups,
while the LUMO is exclusively located on the acceptor part of the sensitizers. From the
calculation results, we can conclude that the lowest πꢀπ* excited state in E1 and E2 has
substantial charge transfer character, setting up the dyes for efficient interfacial charge separation
with the pꢀtype NiO.
Table 1. Optical and electrochemical parameters of the E1, E2 and P1 dyes
.
b
Dye
λabs
λabs
E0ꢀ0
Eox
Ered
E
E
(1S*)
eVc
(S+/S)
V d
(S/Sꢀ)
V d
(S*/Sꢀ)
Ve
(S+/S*)
Ve
(ε ⨯ 10ꢀ4 Mꢀ1 cmꢀ1) a
nm
nm
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353(3.6), 487(5.7)
366(5.1), 404(4.5)
360(6.7), 500(4.0)
475
440
500
2.25
2.69
2.26
1.35
1.05
1.14
ꢀ0.89
ꢀ
1.36
ꢀ
ꢀ0.9
P1
E1
E2
ꢀ1.64
ꢀ1.12
ꢀ1.05
1.21
a The absorption spectra were recorded from dichloromethane solution. b The absorption spectra were recorded on
c
NiO film (3.5 ꢁm); The transition energy (E0ꢀ0) was estimated from the intersection of the normalized absorption
and emission curves. d All potentials are given vs. NHE and were obtained using a scan rate of 100 mV/s. The
groundꢀstate oxidation potential (Eox) and reduction potential (Ered) of the dyes were determined in THF (E1 and E2)
e
and MeCN (P1) solution containing 0.1 M (TBA)PF6 and using CV with a 100 mV sꢀ1 scan rate. The excited state
potentials were estimated from E0ꢀ0 and the recorded potentials using CV according to the below relations;
E1/2(S+/S*) = E1/2 (S+/S) − E0ꢀ0 (1S*), E1/2(S*/Sꢀ) = E1/2(S/Sꢀ) + E0ꢀ0 (1S*).
a1,5
)
20
E1
E2
P1
b)
E1
E2
P1
15
10
5
1,0
0,5
0,0
0
0,0
0,1
0,2
Voltage / V
0,3
400
500
600
700
Wavelength / nm
Figure 4. (a) JꢀV curves and (b) IPCE spectra of devices based the E1, E2 and P1 dyes using Co2+/3+(dtbpy)3
electrolyte.
The electrochemical data and the cyclic voltammograms (CV) of the dyes are presented in
Table 1 and Figure S4. All compounds exhibit a reversible oneꢀelectron redox process attributed
to the oxidation of the arylamine moiety. The oxidation (Eox) and reduction potentials (Ered) of
the compounds were evaluated using CV and DPV, and an estimate of the exited state energies
(ES*/Sꢀ and ES*/S+) is obtained through the combination of E0ꢀ0 and CV/DPV data. The first
oxidation potential (Eox) of P1, 1.35 V vs. the Normal Hydrogen Electrode (NHE), is obviously
higher than that of E1 and E2, 1.05 V and 1.14 V vs. NHE, respectively. This means that the
presence of the long oligothiophene linker significantly influences the oxidation potential of the
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dyes, making it less positive. Due to weaker electronꢀwithdrawing ability of the NPA unit in the
E1 subunit as compared to that of malononitrile moiety in E2, the Ered value of E1 is much more
negative than that of E2. For E1, we cannot obtain a reliable reduction potential due to the weak
and irreversible reduction behavior. However, from Table 1, we can note that the potentials of
Eox and ES*/Sꢀ for P1 and E2 are quite similar, as are the values of Ered or ES*/S+. Consequently,
we use the ES*/Sꢀ values for P1 and E2 and the Eox value for E1 to get a rough estimate of the
+
driving force of injection. Simlarly, we use either the Ered or ES*/S values to analyze the dye
regeneration process. The Eox or ES*/Sꢀ levels of the dyes range from 1.05 to 1.35 V vs. NHE, and
are much more positive than the energy level of the NiO valence band (∼0.5 V vs. NHE). This
makes the hole injection from the dye to NiO VB thermodynamically favorable. The Ered or
+
ES*/S levels are more negative than the redox potential of the cobalt redox system 42, 43, which
implies an efficient dye regeneration by the electrolyte. Also, from electrochemical experiments,
we can conclude that the electronꢀwithdrawing groups induce a significant change in the
reduction and oxidation potentials of the dyes, thus resulting different absorption spectra.
In order to further evaluate the performance of these dyes in solar cell devices, pꢀDSCs
devices were fabricated. Figure 4a shows the photocurrentꢀvoltage (JꢀV) curves of the solar cells
and the corresponding data have been collected in Table 2. pꢀDSCs based on E1 and E2 show
lower photocurrents as compared to the devices based on P1, which probably can be attributed to
the lower extinction coefficient and also lower dye loading of E1 and E2. The devices based on
the dyes E1 and E2 shows significantly higher openꢀcircuit voltages (VOC). The underlying
reason can probably be attributed to reduced charge recombination of the NiO holes with the
reduced dye and/or the electrolyte, benefiting from the more extended linker unit that may also
block the surface from the electrolyte (see hole lifetime measruements below). As
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aforementioned, one of the performance bottlenecks is the low openꢀcircuit potentials in pꢀDSCs;
in this aspect our new dyes show an improved performance.
Table 2. Photovoltaic performance of DSCs based on E1, E2 and P1ꢀsensitized NiO and Co2+/3+(dtbpy)3 electrolyte
JSC
VOC
mV
η
%
dye
ff
mA·cmꢀ2
0.93
0.78
1.18
320
320
280
0.44
0.41
0.30
0.130
0.102
0.099
E1
E2
P1
Light intensity: 100 mW·cmꢀ2; solar cell area: 0.25 cm2; Thickness of NiO: 3 ꢁm. Electrolyte: 0.05 M Co2+(dtbpy)3,
0.05 M Co3+(dtbpy)3, 0.1 M LiClO4 in a solvent mixture of PC and MeCN (1:1, v:v).
As electrolyte redox system, we used the CoII/IIIꢀtris(4,4′ꢀdiꢀtertꢀbutylꢀ2,2′ꢀdipyridyl) (herein
referred to as Co2+/3+(dtbpy)3) redox couple. As compared to a standard, iodineꢀbased electrolyte
(Figure S9 and Table S3), the lower solubility of the cobalt polypyridyl complexes in the
electrolyte limits photocurrent; however, the Co2+/3+(dtbpy)3 system can offer a higher voltage
than the Iꢀ/I3 redox couple due to the reduced hole recombination with electrolyte which shifts
ꢀ
the quasiꢀFermi level positively. 5, 24, 44. The overall conversion efficiency of the devices based
on E1 is 0.13% with a shortꢀcircuit photocurrent density (JSC) of 0.93 mA.cmꢀ2, an openꢀcircuit
voltage (VOC) of 320 mV and a fill factor (ff) of 0.44. Under the same conditions, the reference
P1ꢀbased devices rendered an efficiency of 0.099% with a Jsc of 1.18 mA·cmꢀ2, a Voc of 280
mV and an ff of 0.30.
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1
b)
a)
10-4
0,1
E1
E2
P1
E1
E2
P1
10-5
0,01
0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40
Voltage V
0,10
0,15
0,20
0,25
0,30
0,35
Voltage V
Figure 5. (a) Hole lifetime and (b) extracted charge plotted against openꢀcircuit voltage (VOC) for the devices based
on the dyes E1, E2 and P1
Even though the E1ꢀbased devices render better performance as compared to the P1ꢀbased
ones, the Incident PhotonꢀtoꢀCurrent Conversion Efficiency (IPCE) from E1ꢀbased devices are
much lower than that from devices based on P1. The considerably higher IPCE for P1ꢀbased
devices is in agreement with the higher shortꢀcircuit current in the JꢀV studies. However, the
integrated Jsc of the P1ꢀbased device from IPCE measurements, 2.8 mA cmꢀ2, is much higher
than the actual Jsc measured under 100 mW cmꢀ2 light illumination (1 sun), 1.18 mA cmꢀ2. In
contrast, the for the E1ꢀ and E2ꢀbased devices the integrated Jsc values from IPCE match the
Jsc values obtained under 1 sun illumination. Thus, the devices based on the dye P1 experience
much more serious recombination losses at higher light intensities. For the E1 and E2, the long
conjugated systems are designed to prolong the chargeꢀseparation lifetime, making the dye
regeneration process more efficient and inhibiting charge recombination losses between the
reduced dye and the injected hole in the NiO VB.
In order to further investigate and challenge the aforementioned hypothesis,
photoelectrochemical measurements were performed to obtain hole lifetime and extracted charge
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in the NiO in the different devices. Hole lifetime as a function of VOC is shown in Figure 5a. At a
given Voc value, we can note that the devices based on E1 and E2 show much longer hole
lifetimes than the devices based on P1. This means that the recombination loss process in P1ꢀ
based devices is much more pronounced than that in E1ꢀ and E2ꢀbased devices. This result is in
good agreement with the hypothesis formulated above on the basis of photovoltaic and IPCE
results. The dye design, the long and bulky conjugated linker system, in the dyes E1 and E2
really works as expected, inhibiting the recombination between injected holes and redox couple.
From the extracted charge plotted against openꢀcircuit voltage (Figure 5b), we can observe the
influence of the different dyes on the valenceꢀband position. As compared to devices based on
P1, the corresponding devices based on E1 and E2 display a slightly positively shifted valence
band energy, thus positively shifting Fermi level also, which represents another factor that could
give devices based on E1 and E2 a higher photovoltage than observed for devices based on P1.
With the same electrolyte, devices based on E1 and E2 shows a higher VOC than P1 dye,
implying that the VB of E1ꢀ and E2ꢀsensitized NiO also shifted positively as compared to that of
P1ꢀbased NiO electrode.
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Figure 6. Transient absorption spectra of (a) E2 in DCM and (b) P1 in MeCN after excitation at 530 nm. The strong
Rayleigh scattering at the excitation wavelength was omitted.
In order to get insights into the hole injection process and the recombination process between
the injected holes in the NiO VB and the reduced dye, nanoꢀ/femtoꢀsecond transient absorption
spectroscopy (TAS) were performed for the dyes in solution and adsorbed on NiO films (Figure
6 and 7). For sake of clear comparison, P1 and E2 were first selected based on the argument that
the two dyes have the same donor unit, the same acceptor entity and also similar light absorption
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spectra. In solution, the TAS of E2 in DCM shows a ground state bleach around 520 nm, a
stimulated emission signal around 620 nm as well as an absorption at >650 nm immediately after
excitation (Figure 6). Within the first few picoseconds the stimulated emission undergoes a
dynamic Stokes shift to 700 nm, which results in a broad net absorption band centered at 630
nm. The inset shows the kinetic traces of initial excited state relaxation on a few ps time scale
and subsequent decay to the ground state with a lifetime of 940 ps. The TAS of P1 in MeCN has
been reported, and our results are in agreement with those45. The data shows qualitatively the
same behavior as E2, although the excited state absorption and stimulated emission are
somewhat redꢀshifted, and the singlet excited state lifetime is about 34 ps. In ref. 45, the short
excited state lifetime was attributed to photoꢀinduced charge separation in P1 aggregates,
forming P1●ꢀ and P1●+. However, we probe in the region around 400 nm where P1●ꢀ would have
shown strong absorption and the absence of induced absorption in our data around 400 nm
(Figure 6b) excludes significant P1●ꢀ/P1●+ formation. We instead tentatively suggest that twisting
of the dicyanovinyl group in the excited state leads to an increased rate of decay to the ground
state, similar to what has been proposed for related dyes.46
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Figure 7. Transient absorption spectra of (a) NiOꢀE1 (inset is the kinetic trace probed at 560 nm and fitting data),
(b) NiOꢀE2 (inset is the kinetic trace probed at 580 nm and fitting data) and (c) NiOꢀP1 with LiClO4 in PC (λex =
536 nm with laser pulse power about 300 nJ/Pulse).
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