Molecular Engineering of Potent Sensitizers for Very Efficient Light Harvesting in Thin-Film Solid-State Dye-Sensitized Solar Cells

Article abstract of DOI:10.1021/jacs.6b05281

Dye-sensitized solar cells (DSSCs) have shown significant potential for indoor and building-integrated photovoltaic applications. Herein we present three new D-A-A organic sensitizers, XY1, XY2, and XY3, that exhibit high molar extinction coefficients and a broad absorption range. Molecular modifications of these dyes, featuring a benzothiadiazole (BTZ) auxiliary acceptor, were achieved by introducing a thiophene heterocycle as well as by shifting the position of BTZ on the conjugated bridge. The ensuing high molar absorption coefficients enabled the fabrication of highly efficient thin-film solid-state DSSCs with only 1.3 μm mesoporous TiO₂ layer. XY2 with a molar extinction coefficient of 6.66 × 10⁴ M^-1 cm^-1 at 578 nm led to the best photovoltaic performance of 7.51%.

Full text of DOI:10.1021/jacs.6b05281

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Communication
Molecular engineering of potent sensitizers for very efficient
light harvesting in thin film solid state dye sensitized solar cells
Xiaoyu Zhang, Yaoyao Xu, Fabrizio Giordano, Marcel R. Schreier, Norman Pellet, Yue Hu, Chenyi
Yi, Neil Robertson, Jianli Hua, Shaik Mohammed Zakeeruddin, He Tian, and Michael Grätzel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05281 • Publication Date (Web): 04 Aug 2016
Downloaded from http://pubs.acs.org on August 4, 2016
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Molecular engineering of potent sensitizers for very efficient
light harvesting in thin film solid state dye sensitized solar
cells
a,b,
a,
b,
b,
b
c
Xiaoyu Zhang ‡, Yaoyao Xu ‡, Fabrizio Giordano *, Marcel Schreier *, Norman Pellet , Yue Hu ,
b
c
a,
b,
a
b,
Chenyi Yi , Neil Robertson , Jianli Hua *, Shaik M. Zakeeruddin *, He Tian , and Michael Grꢀtzel *
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering,
East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
b
Laboratoire de Photoniques et Interfaces, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de
Lausanne, Station 6, 1015 Lausanne, Switzerland
c
School of Chemistry, University of Edinburgh, King’s Buildings, Edinburgh, EH9 3FJ, UK.
Supporting Information Placeholder
version efficiencies on thin TiO
forming ssDSSCs,
2
layers in ssDSSCs. Highly per-
exibiting
ABSTRACT: Dye-sensitized solar cells (DSSCs) have shown
significant potential for indoor and building integrated photovoltaic
applications. Herein we present three new D-A-π-A organic sensi-
tizers, XY1, XY2 and XY3, exhibiting high molar extinction coef-
ficients and a broad absorption range. Molecular modifications of
these dyes, featuring a benzothiadiazole (BTZ) auxillary acceptor,
were achieved by introducing a thiophene heterocycle, as well as
by shifting the position of BTZ on the conjugated bridge. The en-
suing high molar absorption coefficients enable highly efficient
thin-film solid-state dye-sensitized solar cells with only 1.3 μm
2
mesoporous TiO layer. XY2 with a molar extinction coefficient of
4
-1
-1
6
.66×10 M cm at 578 nm led to the best photovoltaic perfor-
mance of 7.51%.
After 25 years of thorough study and development, dye-sensi-
tized solar cells (DSSCs) are nowadays moving towards industrial
production, aided by their simple and low-cost fabrication process,
large range of colors, transparency and high photovoltaic conver-
sion efficiency (PCE), particularly under low illumination condi-
Figure 1. Molecular structures of XY1, XY2, XY3 and Y123.
thin TiO layers and strongly absorbing dyes do also not require
2
scattering layers for optimal light harvesting. It is for this reason
that they are very promising candidates for future application in
building-integrated photovoltaic windows and other areas, where
aesthetics play an important role. Organic sensitizers are ideal can-
didates for thin film cells due to their significantly higher molar
extinction coefficients and lower cost than ruthenium-based sensi-
1,2
tions. Solid-state DSSCs (ssDSSCs) employing hole-transporting
materials (HTMs) instead of redox shuttles in solution, have at-
tracted significant interest as a practical solution for the problems
3
-5
posed by traditional liquid electrolytes. In 1998, Spiro-OMeTAD
2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spiro-
(
tizers. Furthermore, their structural flexibility allows accessing a
bifluorene) was reported as the first HTM for ssDSSCs. This mate-
rial has since been widely used, not only in ssDSSCs but also in
11-13
larger parameter space for fine tuning of dye properties.
This is
4
,6-8
particularly important in the case of dye regeneration and charge
recombination processes at the TiO /dye/HTM interface, where
perovskite solar cells.
however, remains the fact that spiro-OMeTAD only insufficiently
infiltrates into thick mesoporous TiO films. These films are there-
fore limited to a thickness of about 2 – 5 um.
A challenge for ssDSSC applications,
2
molecular-level modifications of organic sensitizers are employed
as an effective approach to impede undesirable charge recombina-
2
9,10
Therefore, dyes
1
3-15
tion processes.
This is exemplified in the case of Y123 (Figure
with high extinction coefficients and broad absorption range are of
great importance in order to achieve high solar-to-electricity con-
1
) and LEG4, featuring bulky donors and cyclopentadithiophene
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(
CPDT)-based π-bridges, protected by alkoxy and alkyl chains, re-
The absorption spectra of XY1, XY2, XY3 as well as the refer-
ence dye Y123 in CH Cl are shown in Figure 2 whilst the param-
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spectively. These bulky side chains shield the dye from the oxi-
dized HTM and thereby inhibit recombination at the interface of
TiO
2
2
eters of their optical and electrochemical properties are displayed
in Table 1. The main absorption band can be assigned to the intra-
molecular charge transfer (ICT) between the bulky donor and the
2
/dye/HTM, leading to highly efficient ssDSSCs with spiro-
1
6-18
OMeTAD.
For example, Burschka et. al. achieved a PCE of 7.2%
1
3,23
from Y123-sensitized ssDSSCs using tris(2-(1H-pyrazol-1-yl)pyr-
cyanoacetic acid acceptor moiety.
From Figure 2, we can
idine)cobalt(III) (FK102) as p-type HTM dopant (film thickness:
clearly see that XY1 and XY2 not only show bathochromic shifts
of the low energy absorption band but also an increase in molar
extinction coefficients compared to Y123. Among the investigated
structures, XY2 emerges as the most suitable sensitizer for ssDSSC
16
2
.5 μm). Similarly, LEG4-sensitized ssDSSCs obtained a PCE
of 7.7% with 1,1,2,2-tetracholoroethane-doped spiro-OMeTAD
1
8
(film thickness: ~ 2.5 μm) and 8.2% with copper phenanthroline
as hole conductor (film : ~ 6 μm transparent + 3 μm scattering
4
-1
-1
due to its high molar extinction coefficient of 6.66×10 M cm at
its maximum absorption wavelength of 578 nm. Interestingly, by
switching the position of BTZ and CPDT (XY3), the absorption
band was largely extended to the near infrared region, red-shifting
the low energy absorption peak by 40 nm compared to XY2. How-
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layer) .
Previous work on DSSCs from our own, as well as Zhu’s group
demonstrated, that organic sensitizers with D-A-π-A structures out-
perform their D-π-A analogues because the introduction of an aux-
iliary acceptor between the donor and π-bridge facilitates fine-tun-
ing of the molecular energy levels. This leads to increased absorp-
4
-
ever, the molar extinction coefficient of XY3 drops to 2.62×10 M
1
-1
cm . At the same time, the HOMO energy level of XY3 was up-
shifted to 0.75 V vs. NHE, whilst XY2 has its HOMO energy level
at 0.91 V vs. NHE, similar to that of Y123 and XY1. Density func-
tional theory (DFT) calculations revealed better overlap of the
HOMO and LUMO orbitals in XY2, XY1 and Y123 than XY3
tion and to an extension of the spectral response towards the
13,20,21
red.
From these works, benzothiadiazole (BTZ) emerged as a
20,22
promising acceptor functionality.
Following these results, we
here investigated three novel BTZ-based D-A-π-A sensitizers,
XY1, XY2 and XY3 (Figure 1). XY1 showed a high molar extinc-
tion coefficient, which could further be increased in XY2 by intro-
ducing a heterocyclic thiophene unit onto the conjugated chain,
also leading to an extension of the absorption towards the red. This
effect was further enhanced upon exchanging the position of BTZ
and CPDT, XY3, yielding a dye that absorbed up to the near infra-
red region. The high molar extinction coefficients obtained from
XY1 and XY2 enabled the fabrication of devices using very thin
(
Figure S1), which provides a likely explanation for the higher mo-
lar extinction coefficients of XY2, XY1 and Y123 compared to
2
4
XY3. From these data, it could also be noticed that CPDT con-
tributes to electron donation in XY3, Figure S1, thereby shifting
the HOMO energy level upwards.
In order to test the performance of these dyes in an ssDSSC ap-
plication, devices were prepared using spin coated thin transparent
mesoporous TiO layers. The resulting XY2-sensitized ssDSSC de-
2
vices showed a reddish-purple color and substantial transparency
of the light absorbing layer as shown in Figure 3 (a). The cross-
sectional SEM micrograph of the same device, Figure 3 (b), shows
that spiro-OMeTAD penetrates the entire mesoporous structure,
while exhibiting a 100 nm capping layer on the surface, isolating
mesoporous TiO
vice performance.
2
layers (~ 1.3 μm), without compromising on de-
2
the TiO film from the Au counter electrode.
Figure 2. Absorption spectra of Y123 (black) and XY1(red), XY2
blue), and XY3 (green) in CH Cl
(
2
2
.
Figure 3. Photograph (a) and cross section SEM image (b) of
ssDSSC device based on XY2 with a ~1.3 μm-thick transparent
mesoporous TiO layer.
2
Table 1. Optical and electrochemical properties of XY1, XY2,
XY3 and Y123.
maxa/nm
b
c
d
The photovoltaic performance of ssDSSCs based on Y123, XY1,
XY2 and XY3 under different light intensities are tabulated in Ta-
ble 2 and their corresponding current densitiy-voltage (J-V) curves,
dark current plots and incident photon-to-electron conversion effi-
ciency (IPCE) spectra are displayed in Figure 4. Solid-state DSSC
Dye
λ
HOMO /V
E
0–0
LUMO /V
4
-1
-1
(
ε×10 M cm ) (vs. NHE)
/eV
(vs. NHE)
Y123
XY1
XY2
543 (4.90)
552 (5.65)
578 (6.66)
618 (2.62)
0.92
0.99
0.91
0.75
2.01 -1.09
1.97 -0.98
1.92 -1.01
1.80 -1.05
2
with 1.3 μm TiO films showed good reproducibility (Figure S3).
XY1 and XY2 perform significantly better than Y123 in ssDSSC
devices due to their high molar extinction coefficients. The short
circuit current densities (Jsc) of ssDSSC devices with XY1 and
XY3
a
b
Absorption maximum in CH
tetra-n-butylammonium hexafluorophosphate
) as electrolyte (working electrode: Pt; counter electrode:
Pt wire; reference electrode: SCE; calibrated with ferrocene/ fer-
2
Cl
2
solution. measured in CH
2
Cl
2
-2
-2
XY2 reached over 10 mAcm , which exceeds the 8.45 mAcm ob-
served from Y123. For devices based on the green dye XY3, a Jsc
with 0.1
TBAPF
M
(
6
-2
of 11.06 mAcm was obtained but with a considerably lower Voc
value, losing over 100 mV compared to the other dyes. A Voc of
+
c
rocenium (Fc/Fc ) as an external reference). E0-0 was estimated
from the absorption onset wavelength from UV-Vis absorption
spectra of the dyes. d LUMO was estimated by subtracting E0-0
0
.798 V, 0.929 V, 0.942 V and 0.904 V was obtained with XY3,
XY2, XY1 and Y123, respectively. Devices based on XY2 and
XY1 led to good PCEs of 6.89% and 6.69%, respectively under 95%
Sun illumination, considerably higher than devices based on the
reference dye Y123 (5.77%). A PCE of 5.50% was obtained for
XY3, which is remarkable since its HOMO energy level is very
from EHOMO
.
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close to that of Spiro-OMeTAD, suggesting that the dye regenera-
tion takes place despite the small driving force between the HOMO
levels of dye and HTM. The lower Voc potentially indicates in-
creased electron recombination at the TiO /dye/HTM interface.
This effect will be further investigated below. PCEs under 50% and
0% light intensity were also measured, illustrating excellent po-
tential for Building Integrated Photovoltaic (BIPV) applications.
Beyond 500 nm, devices based on XY1 and XY2 showed much
higher IPCE than Y123. The peak IPCE was found to be 71% at
560 nm for XY1 and 70% at 580 nm for XY2, in contrast to 63%
at 480 nm for Y123-based devices. In analogy to its absorption
spectrum, devices based on XY3 showed two absorption peaks
with IPCEs of 60% at 590 nm and 50% at 430 nm, respectively.
The photovoltaic performance of ssDSSCs based on these dyes was
assessed as a function of film thickness, ranging from 0.25 μm to
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Figure 5. (a) I-V curves of champion XY2-sensitized ssDSSC de-
vice under different light intensities (solid: 1 Sun; dash: 0.5 Sun;
dash dot: 0.1 Sun; dot: dark) and (b) the current dynamics (solid:
measured data; dot, integrated results under 1 Sun).
0
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Table 3. Performance parameters of champion ssDSSC based
on XY2 under different light intensities (I ).
0
2
.0 μm (see Supporting Information, section 8 and 9). Amazingly,
with a TiO film as thin as ~250 nm, ssDSSCs based on XY2 and
2
I
0
/Sun
V
oc/V
J
sc/mAcm-2
FF
PCE/%
7.51
XY1 reached PCEs of 4.34% and 4.01%, respectively.
1
5
00.5%
1.7%
0.902
0.870
0.799
10.96
5.80
1.05
0.764
0.785
0.803
7.64
9.6%
7.00
In order to better understand the relationship between the molec-
ular structure of the dye and the resulting device performance, we
performed charge extraction and transient optoelectronic analyses.
Two devices were measured for each dye. The voltage vs. charge
plot (Figure 6a) showed a weak upward shift of the electron quasi
Figure 4. (a) I-V curves and dark currents of ssDSSCs with ~1.3
2
fermi level in TiO for XY1 and XY2 when compared to Y123. No
μm-thick TiO
2
layers based on Y123 (black) and XY1 (red),
shift was found for XY3. The higher open circuit voltage of the
devices employing XY1 and XY2 compared to Y123 is in good
agreement with these charge extraction measurement. The higher
voltage is likely related to the more efficient photo-conversion of
the dyes with higher molar extinction coefficients. Moreover, the
XY2(blue), and XY3(green) and (b) their corresponding IPCE plots
under illumination of 10% Sun.
Table 2. Performance parameters of ssDSSCs based on XY1,
XY2, XY3 and Y123 under different light intensities (I
Dyea
Y123 96.6% 0.904
9.9% 0.875
9.3% 0.804
XY1 95.0% 0.942
9.4% 0.907
9.2% 0.815
XY2 95.5% 0.929
0
).
2
energetics of the TiO conduction band are affected by the dipole
generated by the surface adsorbed dye and a downward shift of the
conduction band is expected to be directly proportional to the num-
I
0
/Sun
V
oc/V
J
sc/mA cm-2 FF
PCE/%
5.77
5.87
5.44
6.69
6.98
6.46
6.89
7.02
6.89
5.50
5.85
5.06
2
5
8.45
4.40
0.80
10.02
5.31
0.95
10.14
5.31
1.00
11.06
5.67
0.98
0.729
0.760
0.784
0.674
0.717
0.767
0.698
0.736
0.774
0.596
0.656
0.683
ber of dye molecules covering the surface. An increase in the mo-
lecular size will result in a decreased dye loading on the TiO sur-
surface to the
2
4
2
6
face, which in turn would expose more bare TiO
2
Spiro-OMeTAD thus increasing the recombination rate. This rea-
soning is in agreement with the data on electron lifetime shown in
in Figure 6b. For XY1 and XY2 the electron lifetime is 3 to 2 times
lower compared to Y123, depending on the illumination condition.
4
The lower open circuit voltage of the devices employing XY3 is
directly related to the electron lifetime. From Figure 6b it becomes
clear that the recombination is ten times faster for XY3 compared
to Y123. The higher recombination rate is thus responsible for the
100 mV difference in Voc between these dyes. We suggest that the
5
9
0.1% 0.900
.2% 0.823
2
presence of an auxiliary acceptor close to the TiO surface could
XY3 95.6% 0.798
play a role in accelerating the recombination rate in concomitance
27
4
9
9.3% 0.774
.3% 0.703
with its lower driving force for regeneration. Interestingly, we ob-
served slightly longer electron transport time constants for XY1
and XY2 compared to Y123 and XY3, as shown in Figure 6c.
a
Bath composition in supporting information
The XY2-sensitized champion device achieved a PCE of 7.51%.
This device also showed excellent PCEs under low light intensities
(
Table 3 and Figure 5). Under 50% Sun illumination, a high PCE
of 7.64% was reached, which still amounted to 7.00% below 10%
Sun. Current dynamics measurements of photocurrent vs. light in-
tensity are shown in Figure 5(b). These measurements demonstrate
a significant linearity between photocurrent and light intensity, sug-
gesting that the photogenerated charges are efficiently evacuated in
the investigated device.
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ACKNOWLEDGMENT
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This work was supported by NSFC for Creative Research Groups
21421004), NSFC/China (21372082, 21572062 and 91233207)
(
and the Programme of Introducing Talents of Discipline to Univer-
sities (B16017). Zhang thanks the China Scholarship Council
(CSC) for the financial support of a visiting program at EPFL. MS
thanks Siemens AG for funding. MG acknowledges financial sup-
port from Swiss National Science Foundation and CTI 17622.1
PFNM-NM, glass2energy sa (g2e), Villaz-St-Pierre, Switzerland.
REFERENCES
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Figure 6. Voltage (a) and electron lifetime (b) vs. extracted charge
at Voc and electron transport time constant (c) vs. extracted charge
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In conclusion, through rational molecular structure modifica-
tions, we designed and investigated three novel organic sensitizers
XY1, XY2 and XY3, among which XY1 and XY2 exhibit very
high molar extinction coefficients and XY3 shows a board absorp-
tion range reaching the near infrared region. By these modifications,
we were able to achieve the best efficiency reported to date for D-
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K.; Abate, A.; Giordano, F.; Baena, J.-P. C.; Decoppet, J.-D.; Zakeeruddin,
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(
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A-π-A sensitizers in a ssDSSC application. With a TiO film thick-
Rosenthal, S. J.; Sauvage, F.; Grätzel, M.; McGehee, M. D. Adv. Funct.
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ness of ~1.3 μm, ssDSSCs based on XY1 and XY2 reached impres-
sively high PCEs of 6.69% and 6.89%, much higher than for Y123.
The champion device based on XY2 achieved a PCE of 7.51%. De-
spite the small offset between the HOMO energy levels of XY3 and
spiro-OMeTAD, PCE of 5.50% was achieved in the ssDSSC using
this dye. However, XY3 led to more charge recombination, which
was attributed to the short electron lifetime and low driving force
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(
Details for regents, materials, instruments and characterization.
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AUTHOR INFORMATION
Corresponding Author
E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin M. K.;
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fabrizio.giordano@epfl.ch
marcel.schreier@epfl.ch
jlhua@ecust.edu.cn
shaik.zakeer@epfl.ch
michael.graetzel@epfl.ch
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Energy Environ. Sci. 2013, 6, 183
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Comte, P.; Pei, K.; Holcombe, T. W.; Nazeeruddin, M. K.; Hua, J.;
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Author Contributions
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722
(27) Haid S., Marszalek M., Mishra A., Wielopolski M, Teuscher J,
‡
These authors contributed equally.
Moser J. E., Humphry-Baker R, Zakeeruddin S. M., Grätzel M. , Bäuerle P,
Adv. Funct. Mater. 2012, 22, 1291
Notes
The authors declare no competing financial interests.
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