Enediyne as π linker in D-π-A dyes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra25938j

Three Donor-π-Acceptor (D-π-A) type dyes, with triphenylamine (TPA) as electronic donor, enediyne (EDY) moiety as π linker and cyanoacrylic acid as both electronic acceptor and anchoring group, were designed and synthesized. These EDY-bridged dyes with di

Full text of DOI:10.1039/c5ra25938j

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DOI: 10.1039/C5RA25938J
Journal Name
ARTICLE
Enediyne as π linker in D-π-A dyes for dye-sensitized solar cells†
Youfu Wang,a Luhua Dong,a Zhiwei Zheng,b Xing Li,b Rulin Xiong,a Jianli Hua*b and Aiguo Hu*a
Received 00th January 20xx,
Accepted 00th January 20xx
Three Donor-π-Acceptor (D-π-A) type dyes, with triphenylamine (TPA) as electronic donor, enediyne (EDY) moiety as π
linker and cyanoacrylic acid as both electronic acceptor and anchoring group, were designed and synthesized. These EDY-
bridged dyes with different donor position and number are used as sensitizers in dye-sensitized solar cells (DSSCs) to study
the influence of these factors on their performance. The results show that varying the donor position and number shifts
the absorbance peak or enhances the absorption intensity, but does not broaden the absorption range. With two donors
at both meta- and para-positions, the conjugation length in EDY-3 is broadened, meanwhile, the twisty donors with
asymmetric structure endow EDY-3 retarded dye aggregation and charge recombination. Therefore, EDY-3 shows the best
performance in DSSCs among the three kinds of dyes with iodide/triiodide solution as electrolyte (JSC = 12.28 mA•cm-2, VOC
= 730 mV, FF = 60.43% and PCE = 5.41%).
DOI: 10.1039/x0xx00000x
www.rsc.org/
structure-device efficiency relationship” in every aspect of D-π-
A structural dyes in DSSCs.11, 12 The effect of donor moiety,13 π-
linker type,14, 15 acceptor group, and anchoring group16 on the
Introduction
Dye-sensitized solar cells (DSSCs) have got significant
attentions during the last two decades owing to their low-cost
fabrication and high power conversion efficiency (PCE) from
solar energy to electricity.1-3 The performance of DSSC
depends upon its constituent parts, such as the nature of
sensitizer(s), photoanode, counter electrode, and electrolyte
and their combination.4 Among them, the nature of
sensitizer(s) is pivotal to influence the DSSCs performance. The
dyes should possess broad absorption spectra and high molar
extinction coefficients to absorb more than enough photons
from sunlight for efficient energy conversion. At present, state-
of-the-art DSSCs based on ruthenium-containing complexes as
the active material have an overall PCE approaching 11.9%
under standard (Global Air Mass 1.5) illumination.5-7 At the
same time, more and more efforts have been devoted to find
ruthenium-free sensitizers, as ruthenium is a noble and
12, 17
performance of DSSCs have been exploited.11,
Furthermore, the donor size,18 the donor-position,19 the π-
linker length,20 the extra Donor/acceptor group for D-D-π-A21
or D-A-π-A structure,22 the branched alkane23 and the
25
anchoring group24,
are also well-studied. Increasing the
donor size via adding phenyl or electron-donating group has
little influence on the photoresponse range but increase the
absorption intensity,18 while increasing the π-linker length
promotes a bathochromic shift of the absorbance band as well
a decrease in the IPCE due to the dye-aggregation and
photoisomerization.26 The donor in para-position is better
than in meta-position related to the acceptor in similar
structured dyes.19 The introduction of branched alkane or co-
adsorbent retards the dye aggregation.27 Previous work
showed that the donor position and number are important for
D-π-A type dyes.9, 28 However, prototypical D-π-A dyes with
enediyne as π-linker and different donor position and number
were first synthesized and their performance in DSSCs has also
been systematically investigated.
expensive metal.3 Metal-free organic sensitizers have
a
number of advantages, such as high molar extinction
coefficient, infinite flexibility of molecular tailoring, tunable
spectral properties, relatively high efficiency, low cost and
elimination of environmental issues.8 Generally, donor-π-
acceptor (D-π-A) type sensitizers are the most interesting and
promising dyes for DSSCs, due to their efficient intramolecular
charge transfer (ICT) properties.9, 10
Enediyne (EDY) molecules,29 as the active parts in many
natural antibiotics, form diradicals to cleavage DNA or form
polyarylenes when they undergo Bergman cyclization.30-32 EDY
structure and related Bergman cyclization have been
appreciated in a broad of fields, such as cancer therapy,33
Many efforts have been dedicated on the “molecular
37-39
polymer chemistry34-36 and material science.32,
Herein,
three EDY-bridged D-π-A dyes with TPA as electronic donor,
EDY moiety as π linker and cyanoacrylic acid as both electronic
acceptor and anchoring group are designed and synthesized.
The results show that the donor position and number show
strong influence on their performance in DSSCs for D-π-A type
all-organic dyes.
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Journal Name
4-((4-(diphenylamino)phenyl)ethynyl)-3-
DOI: 10.1039/C5RA25938J
((trimethylsilyl)ethynyl)benzaldhyde (7a). Compound
mg, 2 mmol), Pd(PPh3)2Cl2 (42.1 mg, 0.06 mmol), CuI (22.9 mg,
0.12 mmol) and dry TEA (15 mL) were mixed in a 50 mL
Schlenk flask. Under nitrogen bubbling, compound
Experimental
Materials
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6 (560
Triethylamine (TEA) and acetonitrile were distilled over
calcium hydride. Tetrahydrofuran (THF) was pre-dried over
calcium hydride, refluxed with sodium wire and benzophenone
till the solution turned to deep blue and then distilled before
use. Other chemicals were commercially available and used as
received.
Synthesis of EDY-bridged dyes
The synthesis of three EDY-bridged D-π-A dyes with different
donor numbers and donor positions is illustrated in Scheme 1.
2 (595 mg,
2.2 mmol) dissolved in dry THF (15 mL) was added and then
reacted at 90 °C for 12 hours. After cooling to room
temperature, the resulting solution was concentrated under
reduced pressure and extracted with saturated NaCl solution
and DCM. The combined organic layer was dried over
anhydrous MgSO4. After filtration and removal of the solvent
under vacuum, the crude product was purified through column
chromatography (200-300 mesh silica, PE : DCM = 3 : 1) to
yield orange-yellow power 7a (530 mg, yield 56%). 1H-NMR
(DMSO-D6, 400 MHz, ppm): 9.98 (s, 1H); 8.04 (s, 1H); 7.87 (d, J
= 7.9 Hz, 1H); 7.73 (d, J = 8.0 Hz, 1H); 7.42 (d, J = 7.9 Hz, 2H);
7.37 (t, J = 7.3 Hz, 4H); 7.17-7.10 (m, 6H); 6.91 (d, J = 8.0 Hz,
2H); 0.24 (s, 9H). 13C-NMR (DMSO-D6, 100 MHz, ppm): 191.83;
148.53; 146.15; 134.99; 133.74; 132.86; 132.09; 130.63;
129.84; 128.42; 125.41; 124.83; 124.47; 120.45; 113.33;
102.30; 99.96; 97.87; 87.00; -0.23. HR-MS (ESI): m/z calcd. for
C32H28NOSi (M+H)+: 470.1862, found: 470.1937.
N,N-diphenyl-4-((trimethylsilyl)ethynyl)aniline
ethynyl-N,N-diphenylaniline ( ) were synthesized following the
(
1
)
and 4-
2
literature procedures.40 As all the EDY-bridged dyes are
synthesized in a similar way, only the synthesis of EDY-2 is
detailed here as an example. The synthetic processes of EDY-1
and EDY-3 and related structural characterizations are
available in electronic supplementary information (ESI).
4-bromo-3-iodobenzaldehyde (3).41 Potassium iodate (1.28 g,
6 mmol) and potassium iodide (2.99 g, 18 mmol) were added
slowly to concentrated sulphuric acid (30 mL) with vigorous
stirring at 0 °C in dark. After stirring for 0.5 h, the above
solution was added to 4-bromobenzaldehyde (3.7 g, 20 mmol)
in concentrated sulphuric acid (50 mL) and stirred strongly for
another 1 h. The reaction mixture was diluted with ice water
and neutralized with sodium bicarbonate, the aqueous
solution was extracted with ethyl acetate (EtOAc) and the
combined organic phase was further washed with saturated
sodium sulphite solution. After drying and recrystallizing in
4-((4-(diphenylamino)phenyl)ethynyl)-3-ethynylbenzaldhyde
(7b). Compound 7a (470 mg, 1 mmol), p-toluenesulfonic acid
(PTSA, 172 mg, 1 mmol) and dry THF (10 mL) were mixed in 50
mL Schlenk flask under nitrogen at 0 °C. Tetrabutylammonium
fluoride (TBAF•3H2O, 0.63 g, 2 mmol) dissolved in dry THF (10
mL) was added dropwise. After reacting at 0 °C for 0.5 h, the
resulting solution was concentrated under reduced pressure
and extracted with 1 M hydrochloric acid solution and EtOAc.
The organic layer was dried over anhydrous MgSO4. After
filtration and removal of the solvent under vacuum, the crude
product was purified through column chromatography (200-
300 mesh silica, PE : DCM = 3 : 1) to yield orange-yellow power
THF/petroleum ether (PE), white needle-like crystal
3 (4.32 g,
70% yield) was obtained. 1H-NMR (CDCl3, 400 MHz, ppm): 9.90
(s, 1H); 8.32 (d, J = 1.8 Hz, 1H); 7.80 (d, J = 8.2 Hz, 1H); 7.70 (dd,
J = 8.2, 1.9 Hz, 1H). 13C-NMR (CDCl3, 100 MHz, ppm): 189.82;
141.40; 137.15; 136.22; 133.56; 129.84; 102.15. HR-MS (ESI):
m/z calcd. For C7H4BrIO (M)+: 309.8490, found: 309.8491.
1
7b (360 mg, yield 92%). H-NMR (DMSO-D6, 400 MHz, ppm):
9.99 (s, 1H); 8.06 (s, 1H); 7.90 (d, J = 8.1 Hz, 1H); 7.75 (d, J = 8.0
Hz, 1H); 7.53-7.42 (m, 2H); 7.37 (dd, J = 7.4, 7.5 Hz, 4H); 7.18-
7.07 (m, 6H); 6.97-6.86 (m, 2H); 4.64 (s, 1H). 13C-NMR (DMSO-
D6, 100 MHz, ppm): 191.78; 148.51; 146.17; 135.02; 133.61;
132.96; 132.05; 130.86; 129.82; 128.75; 125.34; 124.55;
124.41; 120.62; 113.38; 97.81; 86.90; 86.04; 80.98. HR-MS
(ESI): m/z calcd. for C29H20NO (M+H)+: 398.1467, found:
398.1543.
4-bromo-3-((trimethylsily)ethynyl)benzaldhyde
(6).
Compound (1.55 g, 5 mmol), CuI (57.1 mg, 0.3 mmol),
3
Pd(PPh3)2Cl2 (105.3 mg, 0.15 mmol) and dry TEA (30 mL) were
mixed in a 100 mL Schlenk flask. Then trimethylsilylacetylene
(TMSA, 3.6 mL, 25 mmol) was added followed by free-pump-
thaw degassing of the mixture. After reacting at 30 °C for 5 h,
the dark brown solution was concentrated under reduced
pressure and extracted with saturated sodium chloride (NaCl)
solution and dichloromethane (DCM). The combined organic
layer was dried over anhydrous MgSO4. After filtration and
removal of the solvent under vacuum, the crude product was
purified through column chromatography (200-300 mesh silica,
2-cyano-3-(4-((4-(diphenylamino)phenyl)ethynyl)-3-
ethynylphenyl)acrylic acid (EDY-2).
To a solution of
compound 7b (100 mg, 0.25 mmol) in dry acetonitrile (30 mL),
cyanoacetic acid (43 mg, 0.50 mmol) and morpholine (0.2 mL)
were added under nitrogen and refluxed at 85 ºC for 8 h. After
cooling to room temperature, the resulting solution was
concentrated under reduced pressure and purified through
column chromatography (200-300 mesh silica, DCM : MeOH =
10 : 1) to yield orange-yellow power EDY-2 (68.9 mg, yield
58%). 1H-NMR (DMSO-D6, 400 MHz, ppm): 8.06 (s, 2H); 7.92 (d,
J = 8.4 Hz, 1H); 7.66 (d, J = 8.2 Hz, 1H); 7.42 (d, J = 8.7 Hz, 2H);
PE : EtOAc = 15 : 1) to yield a white power
6 (1.01 g, 72% yield).
1H-NMR (CDCl3, 400 MHz, ppm): 9.94 (s, 1H); 7.96 (d, J = 1.9 Hz,
1H); 7.75 (d, J = 8.3 Hz, 1H); 7.65 (dd, J = 8.3, 1.9 Hz, 1H); 0.29
(s, 9H). 13C-NMR (CDCl3, 100 MHz, ppm): 190.58; 135.19;
134.95; 133.45; 132.75; 129.37; 126.64; 101.95; 101.72; -0.14.
HR-MS (ESI): m/z calcd. for C12H13BrOSi (M)+: 279.9919, found:
279.9900.
2 | J. Name., 2012, 00, 1-3
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7.35 (t, J = 7.9 Hz, 7.6 Hz, 4H); 7.13 (t, J = 7.4 Hz, 2H); 7.08 (d, J mW•cm-2). Monochromatic IPCE was measuVrieewdArtwicleitOhnline
a
16
= 7.6 Hz, 4H); 6.90 (d, J = 8.7 Hz, 2H); 4.57 (s, 1H). 13C-NMR monochromatic incident light of 1 × 10 photons per
DOI: 10.1039/C5RA25938J
(DMSO-D6, 100 MHz, ppm): 163.86; 148.37; 146.85; 146.28; centimetre under 100 mW•cm-2 in the director current mode
133.04; 132.90; 132.60; 131.95; 129.87; 129.71; 127.67; measurements, which were made on a CEP-2000 system
125.30; 124.53; 124.39; 120.85; 118.37; 113.81; 113.05; 96.58; (Bunkoh-Keiki Co. Ltd.).
87.14; 85.65; 81.30. HR-MS (ESI): m/z calcd. for C32H20N2O2 Electrochemical impedance spectroscopy (EIS) experiments
(M)+: 464.1525, found: 464.1618.
Characterization
were carried out in the dark using a ZAHNER ZENNIUM
electrochemical workstation with frequency ranging from 50
1H-NMR (400 MHz, ppm) and 13C-NMR (100 MHz, ppm) spectra mHz to 100 kHz and a potential amplitude of 5 mV. The
were recorded on an Ultra Shield 400 spectrometer (BRUKER obtained impedance spectra were fitted with the ZSimpWin
BIOSPIN AG, Magnet System 400 MHz/54 mm) using software (v3.10) in terms of appropriate equivalent circuits.
tetramethylsilane as an internal standard. High resolution
mass spectra (HR-MS) were obtained using a Waters LCT
Results and discussion
Premier
XE
spectrometer.
Matrix
assisted
laser
deionization/time of flight (MALDI-TOF) mass spectroscopy
were performed on mass spectrometer 4800 plus MALDI-
TOF/TOF MS Analyzer (AB Sciex, USA) equipped with a Nd: YAG
EDY-bridged dyes were synthesized according to the protocol
shown in Scheme 1. Sonogashira coupling between aryl halide
and alkyne and subsequent Knoevenagel condensation
laser emitting at 355 nm, operating at an accelerating voltage between arylaldehyde and cyanoacetic acid were frequently
used in this protocols. The detail of synthetic processes of EDY-
and EDY-3 and related structural characterizations are shown
in ESI. Generally, these reactions produce the desired products
in a facial and high-yielding manner.
of 20 kV in reflection mode. 4000 series Explorer and DATA
Explorer (AB Sciex, USA) were used for data acquisition and
processing. The extraction delay time used was 450 ns. All
mass spectra were collected by averaging the signals of 500
individual laser shots. Dried-droplet samples were prepared by
spotting 1 μL aliquot of a mixture of 5 μL of matrix (dithranol,
10 mg/ml in THF) and 1 μL of sample (10 mg/ml in THF) on the
target plate. The UV-vis absorption of EDY-bridged dyes in
solution and on TiO2 films were measured with a Varian Cary
500 spectrophotometer. Cyclic voltammograms (CV) were
1
obtained on
a Versastat II electrochemical workstation
(Princeton Applied Research) using a three-electrode cell with
a glassy carbon working electrode, a Pt wire auxiliary electrode,
and a saturated calomel reference electrode (SCE) in saturated
potassium chloride solution with a scan rate of 100 mV•s-1.
Ferrocene (Fc) was used as an internal reference and 0.1 M
tetra-n-butylammonium hexafluorophosphate (TBAPF6) was
used as the supporting electrolyte in chloroform (CHCl3).
DSSCs device fabrication and photovoltaic performance tests
A 17 μm thick double-layer TiO2 photoelectrode, composing of
a 12 μm-thick nanoporous layer and a 5 μm-thick scattering
layer (area: 0.5 × 0.5 cm2), was prepared by screen printing on
a conducting glass substrate, fluorine-doped tin oxide (FTO).
The TiO2 films were immersed in the dye solution (3 × 10-4 M in
CHCl3) for 16 h. Photovoltaic measurements were performed
in a sandwich-type solar cell in conjunction with an electrolyte
Scheme
1 The synthetic protocol of enediyne-bridged donor-π-
acceptor dyes with different donor numbers and positions.
Three EDY-bridged dyes with different donor position and
number were synthesized. EDY-1 has one donor at meta-
position related to acceptor, while the donor in EDY-2 is at the
para-position, EDY-3 has two donors at both meta- and para-
position. It is hope to find out if the donor position and
number in EDY-bridged dyes effects on their performance in
DSSCs.
consisting of
a solution of I2 (0.05 M), LiI (0.1 M),
dimethylpropylimidazolium iodide (0.6 M), and 4-tert-butyl
pyridine (0.5 M) in acetonitrile. The dye-absorbed TiO2 film
and a piece of platinum-coated conducting glass were
separated by a Surlyn spacer (40 μm thick) and sealed by
heating the polymer frame. The photocurrent density-voltage
(J-V) of the sealed solar cell device was measured with a black
metal mask of 0.25 cm2 by illuminating the cell through the
UV-vis spectra of EDY-bridged dyes and N719 in chloroform
and on the surface of TiO2 film are shown in Fig. 1. There are
two obvious absorbance peaks for EDY-bridged dyes in
conducting glass from the anode side with a solar simulator chloroform. The lower energy bands at 350–500 nm with
variable intensities are assign to intramolecular charge transfer
(WXS-155S-10) at AM 1.5 illumination (light intensities: 100
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(ICT) transitions from the triphenylamine donors to the
Journal Name
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DOI: 10.1039/C5RA25938J
cyanoacrylic acid acceptors. Comparing these dyes with the Table 1 Photophysical and electrochemical properties of EDY-bridged
same units but different donor position or number, the dyes.
a
c
EDY
λmax
ɛa
HOMOb
(V/vs. NHE)
0.80
0.74
0.81
E0-0
LUMOc
(V/vs. NHE)
-2.25
-1.78
-1.67
absorption bands of para-conjugated dyes (EDY-2) were more
red-shifted when compared with the corresponding meta-
conjugated dyes (EDY-1), as a result of better resonance in the
former. The onset absorbance of these EDY-bridged dyes are
around 550 nm and the EDY-2 and EDY-3 show almost the
same absorbance peaks with different intensity. This indicates
that the donor position affects significantly on their
photophysical properties, while the number of electron-donor
does not lead to significant changes in the position of the
energy absorbance bands but has a strong influence on the
related absorption intensity.18 For EDY-2 and EDY-3, the lowest
energy absorbance peak is around 418−425 nm, stronger than
corresponding absorption of N719. The absorbance band
positions and their relative molar extinction coefficients are
tabulated in Table 1. The broadened absorption spectra of
these dyes adsorbed on the surface of TiO2 film compared to
the absorption spectra in solution might be ascribed to the
slight aggregation of the dyes.
(nm)
350
424
418
(M-1cm-1)
11150
5590
(eV)
3.05
2.52
2.48
EDY-1
EDY-2
EDY-3
8450
aAbsorbance peaks (λmax) and molar extinction coefficients (ɛ) in CHCl3.
bHOMO was measured in CHCl3 and calibrated with ferrocene with
cyclic voltammetry (CV) at a scan rate of 100 mV•s-1. E0-0 was
c
estimated from the absorbance thresholds (the intersection of
normalized absorption/emission spectra) in the absorption spectra of
dyes adsorbed on TiO2 film. LUMO was estimated by subtracting E0-0
from the HOMO.
The structural differences in the donor part of EDY-bridged
dyes have little influence on their redox potentials. By
changing the donor position, EDY-2 shows the increased
HOMO level by 60 mV compared to EDY-1, which may due to
the (quasi)planar geometry of EDY-2 promoted by the phenyl-
contained π linker. Estimated from the absorbance thresholds
from absorption spectra of absorbed dyes on TiO2 film, the
bandgap energies (E0-0) were determined to be 3.05, 2.52, and
2.48 V, respectively. The excited-state reduction potentials of
EDY-bridged dyes were obtained by combining the redox
potentials with the energy of the transition (E0-0). The
calculated LUMO level of these dyes (−2.25, −1.78, and −1.67 V,
respectively) are placed sufficiently above the TiO2 conduction
band edge to ensure no energetic barriers for the electron
injection.
Fig. 1 UV-vis spectra of EDY-bridged dyes and N719 in solution
(chloroform, 50 μM) (a) and on the surface of TiO2 film (b).
EDY-3
Fig. 3 The photovoltaic performance of DSSCs sensitized with EDY-
bridged dyes and N719 under AM 1.5G irradiation (100 mW•cm-2). (a)
J-V curves and (b) IPCE plots.
EDY-2
EDY-1
The photovoltaic performance of DSSC devices sensitized by
EDY-bridged dyes and N719 was investigated using double-
layer TiO2 films as photo-anodes (12 μm transparent TiO2 layer
0.0
0.6
1.2
1.8
and
5 μm TiO2 scattering layer) in combination with
iodide/triiodide system as electrolyte and platinum (Pt) coated
FTO as a counter electrode (CE), and the photovoltaic
Potential (V vs. NHE)
Fig. 2 Cyclic voltammetry curves of EDY-bridged dyes.
parameters
are
presented
in
Table
2.
The
The cyclic voltammetry curves (CV) of EYD-bridged dyes are
shown in Fig. 2, and the related redox potential values are
presented in Table 1. The redox potentials of the EDY-1, EDY-2
and EDY-3 are 0.80, 0.74, and 0.81 V vs NHE, respectively,
which are higher than that of the iodide/triiodide (0.40 V vs
NHE) redox electrolyte energy level, guaranteeing ample
driving force for the dye regeneration.
photocurrent−voltage curve as well the incident photon to
current conversion efficiency (IPCE) value of the dye-
containing DSSC devices are shown in Fig. 3. The short-circuit
photocurrent density (JSC) increases significantly with the order
of EDY-1
< EDY-2 < EDY-3, which may due to the red-shift
absorbance and the increased absorption intensity (Fig. 1 and
Fig. 3a). The open-circuit photovoltage (VOC) are increasing in
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the same order may due to the retarded dye aggregation and expanding the conjugation length. The EDY-3 with two donors
View Article Online
DOI: 10.1039/C5RA25938J
reduced charge recombination. The donor position and shows nearly two times efficiency of phenylethylene-bridge
number will influence their configuration on TiO2 film and will dye, indicating that the multi-donor has significantly effect in
be discussed later. It is interesting to note that the VOC of EDY- photovoltaic performance. It is more effective to further
3
(730 mV) are competitive with N719 (731 mV), which may improve the performance of DSSCs by constructing molecular
due to the two twisty donors reduce the tendency of dye dyes or polymeric dyes with multipled donors than the tandem
aggregation and charge recombination between iodide-based donor in D-D-π-A structured dyes.
electrolyte and injected electron in the conductive band of
TiO2.28
IPCE curves (Fig. 3b) of EDY-bridged dyes and N719 indicate
that the EDY-bridged dyes have similar photoresponse range
(300-600 nm), which consists with their photophysical
properties as shown in Fig. 1. EDY-2 has a similar curve with
EDY-1 in ultraviolet region except an extra broad peak in range
of 380-600 nm. IPCE values of EDY-3 reach a higher value than
N719 during the range of 350-460 nm, whereas a maximum of
70% is reached at 450 nm. These results consist with the
stronger absorption at related range shown in Fig. 1.
Table 2 Photovoltaic parameters of DSSCs devices absorbed with EDY-
bridged dyes and N719 measured under simulated AM1.5G solar light
Fig. 4 Electrochemical impedance spectroscopy (EIS) spectra for DSSCs
(100 mW•cm-2).
absorbed on EDY-bridged dyes and N719. (a) Nyquist plots; (b) Bode
EDY
JSC (mA•cm-2)
VOC (mV)
FF (%)
PCE (%)
phase plots; (c) Equivalent circuits. The lines of parts (a and b) show
theoretical fits using the equivalent circuits in (c).
EDY-1
EDY-2
EDY-3
N719
2.63
7.46
12.28
17.33
509
607
730
731
60.87
69.10
60.43
65.80
0.82
3.26
5.41
8.34
The Nyquist plot, Bode plot, and equivalent circuit are shown
in Fig. 4. Each of the Nyquist plots in Fig. 4a contains two
semicircles. The left semicircle at the higher frequency region
assigns to the resistance at the interface between the
electrolyte and counter electrode (RCE) and the right semicircle
in the lower frequency region is attributed to the charge
transfer resistance at the dye/TiO2/electrolyte interface
resistance (Rrec). Rrec is related to charge recombination
process from the CB of TiO2 to the oxidized ions in the
electrolyte solution.44, 45 The Rrec value is estimated by the real
component radius of the fitted right semicircle. These
estimated and calculated EIS parameters have been listed in
Table 3. In the equivalent circuit for EIS, C is attributed to the
dye/TiO2/electrolyte interface. Generally, the VOC performance
is closely sensitive to charge transport processes at the
TiO2/dye/electrolyte interface.46, 47 Obviously, with the same
electrode material and the same electrolyte, four DSSCs show
close RS (the TCO sheet resistance) and RCE value (shown in
EDY-2 shows better performance than EDY-1 may due to the
intrinsic better resonance degree for effective ICT transition
when donor moiety is presented on para-position instead of
meta-position as discussed on their photophysical properties.
Moreover, when EDY-bridged dyes are absorbed on the
surface of TiO2, EDY-1 prefers to lie on its side and then its
excited state will recombine with injected electron on ECB of
42
TiO2.25,
And the collapsed EDY-1 reduces the absorbed
amount of dyes on TiO2. While the EDY-2 prefers to vertically
orientate to the surface of TiO2, which results in more
absorbed dyes and a higher electron injection yield. Both the
symmetric structure and the phenyl-contained π linker of EDY-
2
promote the (quasi)planar geometry for efficient electron
transfer, however, the (quasi)planar geometry will induce dye
aggregation, which limit its performance. EDY-3 with two
donors broadens the conjugation length, meanwhile, the
asymmetric structure and twisty donors retard the dye
aggregation and the charge recombination between
electrolyte and injected electrons. Altogether, due to the
enhanced conjugation degree and structure-induced retarded
charge recombination, the photovoltaic performance of EDY-
Table 3), especially for EDY-2, EDY-3 and N719. The strategy to
restrain charge recombination increases the radius of the
second semicircle (in Nyquist plots), moving the phase angle
peak (in Bode plots) to low frequency and increasing the Rrec
value. Apparently, in Fig. 4a and 4b, the smaller width of the
second semicircle of EDY-bridged dyes and the higher
frequency of the phase angle peak indicate a smaller charge-
transfer resistance (compared to the N719 cell) at the
TiO2/dye/electrolyte interface. EDY-3 has a larger value of Rrec
than EDY-1 and EDY-2, showing the two twisted donors will
effectively retard the charge recombination and dye
aggregation. The electron lifetime can be calculated from the
equation 1/2πfp,48 where fp is the peak frequency of lower
bridged dyes are presented as order EDY-1 < EDY-2 < EDY-3.
The photovoltaic performance of EDY-2 bridged with enediyne
is better than reported dyes with almost the same structure,
such as phenylethylene-bridged dye (JSC = 8.56 mA•cm-2, VOC
=
710 mV, FF = 0.47, PCE = 2.89%)14 and phenylethylene-bridged
dye with dimethoxy substituted triphenylamine (JSC = 6.95
mA•cm-2, VOC = 623 mV, FF = 0.742, PCE = 3.21%),43 which
means the enediyne moiety works well for effectively
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ARTICLE
Journal Name
2.
3.
B. E. Hardin, H. J. Snaith and M. D. McGehee, Nature
frequency region in the Bode phase plots. The calculated
electron lifetime decreased in the order of N719 (63.6 ms) >
EDY-3 (27.0 ms) > EDY-2 (22.7 ms) > EDY-1 (4.7 ms), well
View Article Online
Photonics, 2012, 6, 162-169.
DOI: 10.1039/C5RA25938J
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consist with the tendency of VOC together with Rrec
.
Table 3 Parameters obtained by fitting the impedance spectra of the
4.
5.
DSSCs with sensitizers using the equivalent circuit (Fig. 4c).
EDY
RS (Ω)
RCE (Ω)
Rrec (Ω)
τe (ms)
EDY-1
EDY-2
EDY-3
N719
17.6
16.5
17.4
17.1
177.5
50.5
75.7
57.1
529.1
259.5
794.1
1114.7
4.7
22.7
27.0
63.6
6.
7.
Conclusions
In summary, three enediyne bridged donor-π-acceptor dyes
with different donor position and number are designed and
synthesized. Their photophysical and electrochemical
properties show that donor position and number do not
broaden the photoresponse spectra but shift the absorbance
peak or enhance the absorption intensity. Their photophysical
and electrochemical properties are suitable for applying in
DSSCs. Their photovoltaic performance of these dyes in DSSCs
show that EDY-1 with one donor at meta-position may lie on
TiO2 surface which will cause charge recombination and inhibit
the adsorption of the dye molecules, while EDY-2 with one
donor at para-position may vertically orientate to the surface
of TiO2 and results in a good electron injection yield and high
density adsorption of the dye molecules. However, dye-
aggregation is significant in this kind of dye due to its
(quasi)planar geometry. EDY-3 with two donors at both meta-
and para-position broadens the conjugation length,
meanwhile the asymmetric structure and twisty donors helps
to retard the dye aggregation and charge recombination
between electrolyte and injected electrons. The results
showed that by introducing conjugated groups in the para-
position relative to acceptor, the conjugation degree is
enhanced. Moreover, by introducing an extra donor group at
meta-position, the dye aggregation can be effectively retarded
to further help to improve the performance of Donor-π-
Acceptor type organic dyes in DSSCs. The enediyne bridged
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a promise type of dyes for improving the
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Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (91023008,
21474027, 21172073, 91233207), the Key Laboratory of 22.
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Advanced Polymer Materials of Shanghai (WD10140502) and
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thanks the “Eastern Scholar Professorship” support from
Shanghai local government.
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DOI: 10.1039/C5RA25938J
Table of Contents Graphic and Synopsis
Enediyne as π linker in DꢀπꢀA dyes for dyeꢀsensitized solar cells
Youfu Wang,a Luhua Dong,a Zhiwei Zheng,b Xing Li,b Rulin Xiong,a Jianli Hua*b and Aiguo Hu*a
Three enediyneꢀbridged DꢀπꢀA dyes with different donor position and number are synthesized
and their performance on DSSCs are investigated.
1