A Stable Blue Photosensitizer for Color Palette of Dye-Sensitized Solar Cells Reaching 12.6% Efficiency

Article abstract of DOI:10.1021/jacs.7b12348

We report a blue dye, coded as R6, which features a polycyclic aromatic hydrocarbon, 9,19-dihydrobenzo[1′,10′]phenanthro[3′,4′:4,5]thieno[3,2-b]benzo[1,10]phenanthro[3,4-d]thiophene, coupled with a diarylamine electron donor and 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid acceptor. Dye R6 displays a brilliant sapphire color in a sensitized TiO₂ mesoporous film with a Co(II/III) tris(bipyridyl)-based redox electrolyte. The R6 based dye-sensitized solar cell achieves an impressive power conversion efficiency of 12.6% under standard air mass 1.5 global, 100 mW cm^-2, and shows a remarkable photostability.

Full text of DOI:10.1021/jacs.7b12348

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Communication
A Stable Blue Photosensitizer for Color Palette of Dye-
Sensitized Solar Cells Reaching 12.6% Efficiency
Yameng Ren, Danyang Sun, Yiming## Cao#, Hoi Nok Tsao, Yi
Yuan, Shaik M. Zakeeruddin, Peng Wang, and Michael Grätzel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12348 • Publication Date (Web): 11 Jan 2018
Downloaded from http://pubs.acs.org on January 11, 2018
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A Stable Blue Photosensitizer for Color Palette of Dye-
Sensitized Solar Cells Reaching 12.6% Efficiency
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Yameng Ren,†,‡ Danyang Sun,† Yiming Cao,ǁ Hoi Nok Tsao,ǁ Yi Yuan,† Shaik M. Zakeeruddin,ǁ Peng
Wang,*† and Michael Grätzel*ǁ
†Center for Chemistry of Novel & HighꢀPerformance Materials, Department of Chemistry, Zhejiang University, 310028
Hangzhou, China
‡Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022 Changchun, China
ǁLaboratory of Photonics and Interfaces, Institute of Chemical Sciences & Engineering, École Polytechnique Fédérale de
Lausanne, CHꢀ1015 Lausanne, Switzerland
Supporting Information Placeholder
DSCs merely displayed a moderate PCE.9,24−28 Recently, Wang
ABSTRACT: We report a blue dye, coded as R6, which features
and coꢀworkers reported an organic dye R2 with a polycyclic
aromatic hydrocarbon (PAH) core 2Hꢀdinaphthopentacene
achieving a PCE up to 10.2%.28. However, R2 exhibited a narrow
spectral response and a modest shortꢀcircuit photocurrent density
(Jsc) of 14.00 mA cm−2.
a
polycyclic
dihydrobenzo[1',10']phenanthro[3',4':4,5]thieno[3,2ꢀ
b]benzo[1,10]phenanthro[3,4ꢀd]thiophene coupled with a diaꢀ
rylamine electronꢀdonor and 4ꢀ(7ꢀ
aromatic
hydrocarbon
9,19ꢀ
ethynylbenzo[c][1,2,5]thiadiazolꢀ4ꢀyl)benzoic acid acceptor. Dye
R6 displays a brilliant sapphire color in a sensitized TiO2 mesopoꢀ
rous film with a Co(II/III)tris(bipyridyl)ꢀbased redox electrolyte.
The R6 based DSC achieves an impressive power conversion
efficiency of 12.6% under standard air mass 1.5 global, 100 mW
cm–2 and shows a remarkable photostability.
Dyeꢀsensitized solar cells (DSCs)1−5 can powerfully generate
electricity at lowꢀcost.6 The power conversion efficiency (PCE) of
DSCs has steadily increased from 7% in 1991 to 14.3% in 2015
under the standard air mass 1.5 global (AM1.5G) conditions.1,7
Under artificial indoor lights, they have displayed an outstanding
PCE over 20%.8,9 Benefiting from the flexibility in tailoring dye
molecules, especially donorꢀacceptor photosensitizers, DSCs have
a unique property of offering a palette of colors, which is attracꢀ
tive for various applications such as buildingꢀintegrated photovolꢀ
taic (BIPV) windows.10−13 In 2014, the west façade of the Swiss
Tech Convention Center was equipped with the world’s first solar
window (Figure S1) composed of 1,400 DSC modules (~200 m2
photoactive area, estimated ~2000 kW h of annual solar electricity
generation).10 Those modules contain five different colors except
blue. Such an impressive achievement motivated us to develop
efficient, stable blue dye molecules. Although some blue dyes
were reported14−18 and the best PCE was 10.1% achieved by Grätꢀ
zel and coꢀworkers,14 their performance and high cost impede
practical applications.
Figure 1. Color palette of photosensitizers achieved by molecular
tailoring. (a) Chemical structures of R4 and R6 dye molecules. (b)
Spectra of dye molecules in tetrahydrofuran (10 ꢀM). (c) The
image of dyeꢀsensitized TiO2 films (size: 8.5 cm × 1.3 cm). (d)
The image of a DSC based on the R6ꢀsensitized nanocrystalline
TiO2 film with a Co(II/III)tris(bipyridyl)ꢀbased electrolyte.
Herein, we report a narrow energyꢀgap, blue dye R6 (Figure 1a)
featuring
a
stronger
electronꢀdonating
PAH,
9,19ꢀ
Anthraceneꢀbased conjugated materials have been developed
successfully for blue emitting organic lightꢀemitting diodes,19−21
thin film transistors,22 and bulk heterojunction solar cells.23 Howꢀ
ever, photosensitizers with anthracene as the kernel segment in
dihydrobenzo[1',10']phenanthro[3',4':4,5]thieno[3,2ꢀ
b]benzo[1,10]phenanthro[3,4ꢀd]thiophene (BPT2), compared with
4H,4'Hꢀ2,2'ꢀbibenzo[1,10]phenanthro[4,3ꢀb]thiophene (2BPT) in
a counterpart dye R4. BPT2 and 2BPT are tethered with an auxilꢀ
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iary diarylamine electronꢀdonor and an electron acceptor of 4ꢀ(7ꢀ
ethynylbenzo[c][1,2,5]thiadiazolꢀ4ꢀyl)benzoic acid (EBTBA). R6
exhibits a brilliant sapphire color when adsorbed on the TiO2 film
in the presence of a Co(II/III)tris(bipyridyl)ꢀbased redox electroꢀ
lyte. The DSC with R6 achieves an impressive PCE of 12.6%
under the standard AM1.5G sunlight and remarkable photostabilꢀ
ity.
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Synthesis of R4 and R6. As illustrated in Scheme S1, we startꢀ
ed the synthesis of R6 from the easily available diethyl 2,5ꢀ
dichlorothieno[3,2ꢀb]thiopheneꢀ3,6ꢀdicarboxylate (2).29 First,
compound diethyl 2,5ꢀdi(anthracenꢀ9ꢀyl)thieno[3,2ꢀb]thiopheneꢀ
3,6ꢀdicarboxylate (3) was prepared by twoꢀfold Suzuki coupling
at 84% yield from 2. Subsequent double Grignard nucleophilic
addition of 3 with (4ꢀhexylphenyl)magnesium bromide30 generatꢀ
ed a bisꢀtertiary alcohol intermediate in square brackets, which
was further subjected to an acidꢀcatalyzed intramolecular Friedelꢀ
Crafts cyclization to afford the key product 9,9,19,19ꢀtetrakis(4ꢀ
hexylphenyl)ꢀ9,19ꢀ
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Figure 3. Photoꢀinduced charge transfer of sensitizers. Upꢀ
converted PL traces of R4 (a) and R6 (b) grafted mesoporous
Al2O3 and TiO2 films. The grey lines are multiexponential fittings.
Probe wavelength: 970 nm; pump wavelength: 490 nm; pulse
fluence: 28 µJ cm–2. Transient absorption spectra of R4 (c) and R6
dihydrobenzo[1',10']phenanthro[3',4':4,5]thieno[3,2ꢀ
b]benzo[1,10]phenanthro[3,4ꢀd]thiophene (4) at 83% yield. Afꢀ
terwards, the electronꢀdonor N,Nꢀbis(4ꢀ(hexyloxy)phenyl)ꢀ
(d) grafted TiO2 films with an inert electrolyte and
a
Co(II/III)tris(bipyridyl)ꢀbased redox electrolyte. The inert electroꢀ
lyte contains 0.1 M LiTFSI and 0.1 M TBP in acetonitrile. The
grey lines are multiexponential fittings. Pump wavelength: 680
nm for R4 and 692 nm for R6; probe wavelength: 850 nm.
9,9,19,19ꢀtetrakis(4ꢀhexylphenyl)ꢀ9,19ꢀdihydrobenzo
[1',10']phenanthro[3',4':4,5]thieno[3,2ꢀ
ꢀ
b]benzo[1,10]phenanthro[3,4ꢀd]thiophenꢀ5ꢀamine (6) was obꢀ
tained at 62% yield via the BuchwaldꢀHartwig coupling of bis(4ꢀ
(hexyloxy)phenyl)amine (5)31 with a bromide gained by the broꢀ
mination of 4 at room temperature. We further performed monoꢀ
bromination of 6 to obtain a bromide, which was further coupled
with butyl 4ꢀ(7ꢀethynylbenzo[c][1,2,5]thiadiazolꢀ4ꢀyl)benzoate32
using the Sonogashira−Hagihara reaction to afford an esterified
dye precursor at 86% yield. Eventually, the desired dye R6 was
obtained via hydrolysis and acidification at 98% yield over two
steps. Following a similar synthetic strategy, dye R4 was obtained
from diethyl 5,5'ꢀdibromoꢀ[2,2'ꢀbithiophene]ꢀ4,4'ꢀdicarboxylate
(10),33 as shown in Scheme S2.
To unveil the influences of conjugatedꢀbackbone (2BPT vs
BPT2) on the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of R4 and R6, we
measured their cyclic voltammograms (CV). Their vacuum enerꢀ
gy levels were estimated via the equation E = − 4.88 − e Eonset
,
where Eonset is the onset potential of reduction and oxidation of
the groundꢀstate molecule.34 Figure S3 shows that the alternation
of conjugatedꢀbackbones hardly affects the HOMO and LUMO
energy levels of R4 and R6, both with the HOMO energy level
being −4.90 eV and the LUMO being −3.35 eV. The
CV
L/H
LUMO/HOMO energy gap ( ꢁE ) is 1.55 eV. This is in conꢀ
sistent with the results of Density functional theory (DFT) calcuꢀ
lations (Figure S4). However, R6 red shifts the maximum absorpꢀ
tion by 18 nm and has a lower maximum
ε compared to R4
(Figure 1b). This result motivated us to explore their electronic
transitions in detail.
In general, the S1←S0 vertical electronic transitions to LUMO
for these two dyes mainly stem from HOMO, displaying a feature
of intramolecular charge transfer as perceived from the contour
plots of molecular orbitals (Figure S5). The superficial inconsistꢀ
ence between LUMO/HOMO energy gap ( ꢁELB/3HLYP , Figure S4)
and maximum absorption wavelength calculated at the TDꢀ
Figure 2. Photocurrent action spectra and charge transport of
solar cells. (a) IPCE (dot line) of DSCs and φLH (solid line) of a
dyeꢀsensitized TiO2 film (8.0ꢀꢀmꢀthick transparent TiO2 layer). (b)
Ln as a function of DOS of DSCs. The dotted line represents the
thickness of nanocrystalline TiO2 layer in the DSCs.
λTDꢀMPW1K
MPW1K/6ꢀ311G(d,p) level of theory (
, Table S1) for
ABS,MAX
Spectroscopic and electronic properties. Figure 1b shows the
R4 and R6 can be better assessed by resorting to the detailed tranꢀ
sition assignments tabulated in Table S2. The proportion of exciꢀ
tation from HOMO to LUMO is 80% in R6, compared to 71% in
R4. Thus, it is reasonable to detect the redꢀshifted absorption
maxima for R6. Figure S8 shows that the lifetimes of R4 and R6
in THF at the equilibrium excited state are respective 231 and 289
ps (see detailed discussions in Figure S8). This result renders a
clue for the judicious design of organic photosensitizers with a
rigid PAH.
wavelengthꢀdependent molar extinction coefficients (
ε ) of R4
and R6 dissolved in tetrahydrofuran (THF). Both dyes have a
strong lightꢀharvesting capability in the red region and a weak
absorption in the blue part of the visible spectrum. Their maxiꢀ
mum
ε
are 81.8×103 M−1 cm−1 at 631 nm for R6 and 85.5×103
M−1 cm−1 at 613 nm for R4. Figure 1c shows an image of dyeꢀ
sensitized TiO2 films (30 nm sized nanocrystals, 4 ꢁm thick). The
R6ꢀsensitized panel displays a shiny blue tint compared with R4ꢀ
sensitized one. A cell fabricated with 4 ꢁm thick R6ꢀsensitized
film in conjunction with a Co(II/III)tris(bipyridyl)ꢀbased redox
electrolyte also displays attractive sapphire (Figure 1d), enriching
the family of colorful DSCs modules. The image of a DSC with
R4 was shown in Figure S2.
Photocurrent action spectra and charge transport of solar
cells. The monochromatic incident photonꢀtoꢀelectron conversion
efficiencies (IPCEs) of DSCs employing R4 or R6 with the
Co(II/III)tris(bipyridyl)ꢀbased redox electrolyte are displayed in
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Figure 2a. The fabrication of devices is elaborated in the Supportꢀ
trons in TiO2 or with cobalt(II) ions in the electrolyte. The absorpꢀ
tion traces can be well fitted with multiexponential functions for
convenient determination of halfꢀreaction times ( τ1/2 ). For the
dyeꢀsensitized TiO2 films with an inert electrolyte (0.1 M lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.1 M 4ꢀtertꢀ
butylpyridine (TBP) in acetonitrile), the decay signals (Figure 3c,
3d) are related to the back electron transfer from TiO2 to D+. The
τ1/2 for this process are 0.48 ms for R4 and 0.84 ms for R6. For
ing Information. Note that the efficient DSC employs the TiO2
film composed of a nanocrystalline layer (30 nm sized nanocrysꢀ
tals, 4 ꢁm thick) and a light scattering layer (350ꢀ450 nm sized
crystals, 5 ꢁm thick). The IPCE of the R6 cell achieves a maxiꢀ
mum value of 85%, which is remarkably higher than that of R4
(75%). Additionally, its onset wavelength red shifts by 20 nm
compared with that of R4. This result is in accord with the waveꢀ
φ
length dependent lightꢀharvesting yields ( LH ) (Figure 2a).
9
the films with the Co(II/III)tris(bipyridyl)ꢀbased electrolyte, acꢀ
celerated decay signals imply the occurrence of hole injection
from D+ to cobalt (II) ions, the τ1/2 of which are 5.0 ꢀs for R4 and
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Apart from the yield of light harvesting, the yields of charge
generation and collection also determine the value of IPCE. Elecꢀ
trical impedance measurements (EIS) show that the electron difꢀ
fusion length (Ln) as a function of density of states (DOS) in
DSCs of R4 and R6 are comparable (Figure 2b), indicating that
both cells have comparable charge collection yields. Significantly,
the Ln in both cells is larger than the thickness of the transparent
TiO2 layer, suggesting close to quantitative charge collection yield.
14.6 ꢀs for R6. Overall, the hole injection yields (
,
φhi
Coꢀbpy
/τ
1/2
inert
1/2
φ
hi =1−
τ
) for R4 and R6 are 99% and 98%, respectiveꢀ
ly.
Table 1. Photovoltaic Parameters Measured under the
AM1.5G Sunlight
EQE
SC
VOC
J
JSC
FF
PCE
[%]
Dye
[mA cm–2] [mA cm–2]
[mV]
[%]
R4
R6
16.80±0.04 17.25±0.2 852±3 75.4±0.1 11.1±0.1
19.36±0.04 19.69±0.15 850±3 75.4±0.2 12.6±0.2
Efficient and photostable blue dye-sensitized solar cells. The
J V
− ) curves (Figure 4a) of DSCs
photocurrent density−voltage (
with R4 and R6 were measured under the standard AM1.5G, 100
mW cm−2 conditions. The averaged parameters of eight cells
made with each dye are compiled in Table 1. The R4 cell delivers
Figure 4. Efficient and photostable blue dyeꢀsensitized solar cells.
(a) J −V curves of R4 and R6 based DSCs measured under the
a
JSC of 17.25 mA cm−2, an openꢀcircuit photovoltage (
V
OC ) of
100 mW cm−2, AM1.5G sunlight. (b) Histogram of PCE of DSCs
with R6 (eight samples). The Gaussian fit has been added to aid
the eye. (c) Normalized PCE of R6 cell measured under the 53.5
mW cm−2, AM1.5 sunlight during the continuous full sunlight
soaking at 60 oC for 1,000 h.
852 mV, and a fill factor (FF) of 75.4%, yielding a PCE of 11.1%.
Impressively, the R6 cell has the PCE of 12.6%, produced from
the JSC of 19.69 mA cm−2, the VOC of 850 mV, and the FF of
75.4%. The comparable VOC of both cells is due to their similar
electron lifetimes (Figure S15a) and profiles of charges ( QCE
)
Photo-induced charge transfer of sensitizers. We further esꢀ
timated the yields of electron injection ( φei ) from excitedꢀstate
stored in TiO2 (Figure S15b). The higher JSC of R6 than that of
R4 benefits from the wider spectrum response and the higher
IPCE peak value. The integrals of IPCE (19.36 mA cm−2) over the
standard AM1.5G emission spectrum (ASTM G173ꢀ03) is in
agreement with the obtained photocurrent density ( JSC = 19.69
mA cm–2). The PCE histogram of the R6 cell is presented in Figꢀ
ure 4b. The normalized PCE of R6 measured at an irradiance of
53.5 mW cm–2 maintains 90% of its initial value after 1,000 h of
continuous full sunlight soaking at 60 oC (Figure 4c). The detailed
photovoltaic parameters are presented in Figure S16. Together
with champion green (11.9%)37 and red dyes (14.3%),7 R6 is the
best candidate so far in the family of blue dyes for the color palꢀ
ette of DSCs.
dye molecules to TiO2 with femtosecond fluorescence upꢀ
conversion measurements.35−36 First, a control cell based on a dyeꢀ
grafted mesoporous Al2O3 film in the Co(II/III)tris(bipyridyl)ꢀ
based electrolyte was excited at 490 nm by the femtosecond pulse.
The photoluminescence (PL) decays due to radiative and nonradiꢀ
ative deactivation of excitedꢀstate dye molecules (blue dots in
τ
(A)
of 117
Figure 3a, 3b) provide amplitudeꢀaveraged lifetimes
and 260 ps for R4 and R6, respectively. The longer PL lifetime of
(T)
τ
R6 may stem from its more rigid electronic skeleton. The
is
significantly shortened in working cells where the dyes are adꢀ
sorbed on mesoporous TiO2 films, both being 26 ps. Note that
τ
(A)
τ (T)
herein are the equilibrium excited state lifetimes
and
(see detailed discussions in Figure S13). The φei can be estimated
from the equation φei = 1−
(T) / (A) , being 78% for R4 and 88%
In summary, we report a novel blue dye R6 characteristic of
electronꢀrich
PAH
of
2Hꢀ
τ
τ
benzophenanthrothienobenzophenanthrothiophene. R6 sensitized
TiO2 film displays attractive blue sapphire color, filling the color
gap of DSCs modules. The DSC with R6 achieves an impressive
PCE of 12.6% under the full sunlight, which is so far the best
performance in the family of blue dyes for DSCs. The R6 cell also
displays a remarkable photostability, promising the color palette
of DSCs to be applied in sunroofs and BIPV.
for R6. The higher φei for R6 can be understood by its longer
equilibrium excited state lifetime on Al2O3. This result gives a
distinct clue for the relative difference in the IPCE values.
We further used the nanosecond laser flash photolysis specꢀ
trometer to monitor the kinetics of dualꢀpath chargeꢀtransfer reacꢀ
tions of photooxidized dye molecules (D+) either with free elecꢀ
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17. Hao, Y.; Saygili, Y.; Cong, J.; Eriksson, A.; Yang, W.; Zhang, J.;
ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Details of the synthesis and characterization of the dyes, theoretiꢀ
cal calculations, electrochemical and photophysical measurements
(CV, EIS, nanosecond laser flash photolysis, femtosecond fluoꢀ
rescence upꢀconversion, transient photovoltage decay, and charge
extraction), and cell fabrication (PDF)
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AUTHOR INFORMATION
Corresponding Author
* pw2015@zju.edu.cn
*michael.graetzel@epfl.ch
Chem.−Eur. J. 2012, 18, 11621.
Notes
26. Lin, R. Y.; Lin, H. W.; Yen, Y. S.; Chang, C. H.; Chou, H. H.;
Chen, P .W.; Hsu, C. Y.; Chen, Y. C.; Lin, J. T.; Ho, K. C. Energy
Environ. Sci. 2013, 6, 2477.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
27. Lin, Y. Z.; Huang, C. H.; Chang, Y. J.; Yeh, C. W.; Chin, T. M.;
Chi, K. M.; Chou, P. T.; Watanabe, M.; Chow, T. J. Tetrahedron
2014, 70, 262.
28. Ren Y.; Liu, J.; Zheng, A.; Dong, X.; Wang, P. Adv. Sci. 2017,
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P.W. acknowledges financial support from the National Science
Foundation of China (No. 51673165 and 91233206), the National
973 Program (2015CB932204), and the Key Technology R&D
Program (BE2014147ꢀ1) of Science and Technology Department
of Jiangsu Province. M.G. acknowledges financial support from
Swiss National Science Foundation, CTI 17622.1 PFNMꢀNM,
and g2e.
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