Enhanced photovoltaic performance and long-term stability of quasi-solid-state dye-sensitized solar cells via molecular engineering

Article abstract of DOI:10.1039/b811401c

Organic dyes with long alkyl chains have been synthesized and demonstrated to be highly efficient sensitizers for liquid and quasi-solid-state solar cells, giving power conversion efficiencies of 8.31-8.39% and 7.03-7.31% under AM 1.5 G irradiation, respe

Full text of DOI:10.1039/b811401c

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Enhanced photovoltaic performance and long-term stability of
quasi-solid-state dye-sensitized solar cells via molecular engineeringw
Sanghoon Kim,a Duckhyun Kim,a Hyunbong Choi,a Moon-Sung Kang,b
Kihyung Song,c Sang Ook Kang*a and Jaejung Ko*a
Received (in Cambridge, UK) 7th July 2008, Accepted 28th July 2008
First published as an Advance Article on the web 30th August 2008
DOI: 10.1039/b811401c
Organic dyes with long alkyl chains have been synthesized and
demonstrated to be highly efficient sensitizers for liquid and
quasi-solid-state solar cells, giving power conversion efficiencies
of 8.31–8.39% and 7.03–7.31% under AM 1.5 G irradiation,
respectively.
Increasing energy demands and global warming urge the
development of cheap and accessible renewable energy
sources. Dye-sensitized solar cells (DSSCs) are one of the
promising candidates for an alternative form of solar cell.1
Several ruthenium polypyridyl complexes have achieved
power conversion efficiencies of 10–11%;2 however, achieving
of long-term stability has become a major challenge. The
introduction of a long alkyl chain to the ruthenium poly-
pyridyl unit has resulted in thermal and long-term stability.3
Some organic dyes have also been utilized as promising
photosensitizers, and several groups have obtained impressive
efficiencies in the range of 7–9%.4 However, the long-term
stability of organic dye-based devices raises a serious problem.
Fig. 1 The structures of JK-2, JK-70 and JK-71.
stability tests of two organic sensitizers, coded as JK-70 and
JK-71 (Fig. 1).
The organic sensitizers, JK-70 and JK-71, were readily
synthesized in eight steps (see the ESIw). The visible absorption
spectrum of JK-70 exhibited two bands centered at 448 (e =
42 520 MÀ1 cmÀ1) and 367 (e = 51 580 MÀ1 cmÀ1) nm, which
were assigned to the p–p* transitions of the conjugated
molecule (Fig. 2). Under similar conditions, the JK-71 sensi-
tizer exhibited absorption bands at 447 (e = 36 630 MÀ1 cmÀ1
)
Another disadvantage of organic dyes in DSSCs is the forma-
tion of aggregates on the TiO2 surface and the facile charge
recombination that is responsible for their low photoconver-
sion efficiency. Therefore, the molecular engineering of
organic dyes with high efficiencies, and which meet the stabi-
lity criteria, is required. Our strategy towards obtaining high
photovoltaic performance from organic DSSCs is based upon
the structural modification of the dye to minimize charge
recombination. It is well documented that the introduction
of long alkyl chains to amphiphilic ruthenium or organic dyes
exerts an immense effect on their photovoltaic performance,
and increases their stability due to the large distance between
the TiO2 and the hole conductors.5
and 368 (e = 45 090 MÀ1 cmÀ1) nm. Excitation of the low
energy p–p* transition of JK-70 and JK-71 resulted in strong
emissions centered at 583 and 608 nm, respectively. The
excitation transition energy (E0À0) of JK-70 and JK-71 was
calculated to be 2.39 and 2.37 eV, respectively. The excited-
state oxidation potentials (E*ox) of the dyes (JK-70: À1.35 V
and JK-71: À1.31 V vs. the normal hydrogen electrode (NHE))
were much more negative with respect to the conduction band
edge of TiO2, providing the thermodynamic driving force for
electron injection.
Molecular orbital calculations illustrated that the HOMO of
both sensitizers was delocalized over the fluorenylamino unit,
Recently, successful molecular engineering was achieved by
incorporating long alkyl units into the organic framework,
which not only increased the photoconversion efficiency but
also exhibited excellent stability under light soaking at 60 1C.
Herein, we report the synthesis, photovoltaic properties and
a Department of New Material Chemistry, Korea University,
Jochiwon, Chungnam 339-700, Korea. E-mail: jko@korea.ac.kr;
E-mail: sangok@korea.ac.kr;
Fax: +82 41 867 5396; Tel: +82 41 860 1337
b Energy & Environment Laboratory, Samsung Advanced Institute of
Technology (SAIT), Yongin 446-712, Korea
c Department of Chemical Education, Korea National University of
Education, Cheongwon, Chungbuk 363-791, Korea
w Electronic supplementary information (ESI) available: Further syn-
thetic details and cyclic voltammogram. See DOI: 10.1039/b811401c
Fig. 2 The absorption and emission spectra of JK-70 (TT) and JK-71
(--) in THF, and the absorption spectra of JK-70 (Á Á Á) and JK-71 (-Á-)
absorbed on a TiO2 film.
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between the TiO2 surface and the oxidizing hole by the
introduction of long alkyl chains to the dyes, resulting in a
long electron lifetime. The alkyl chains can lead to an effective
spatial separation of the charges, which aids the retardation of
charge recombination. The minimization of interfacial charge
recombination losses in the device is also evident from the
dark-current data for the cells.
Since long-term stability is a vital parameter for sustained
cell operation, we substituted the liquid electrolyte with
a
quasi-solid-state one, because DSSCs that employ
liquid electrolytes have drawbacks, such as leakage, evapora-
tion of the solÀvent and corrosion of the Pt counter-electrode
4 shows the photovoltaic
by the IÀ/I3 couple. Fig.
Fig.
3
A photocurrent–voltage curve obtained with a DSSC-
based JK-70 (--), JK-71 (TT) and JK-2 (Á Á Á) under AM 1.5 radiation
performance of JK-2-, JK-70- and JK-71-sensitized solar
cells during long-term light soaking using a polymer gel
electrolyte composed of 5 wt% poly(vinylidenefluoride-co-
hexafluoropropylene) (PVDF-HFP), 0.6 M 1,2-dimethyl-3-
propyl-imidazolium iodide (DMPII), 0.1 M I2 and 0.5 M
N-methyl-benzimidazole (NMBI) in 3-methoxypropionitrile
(MPN). The JK-71-sensitized cell yielded a remarkably high
conversion efficiency of 7.31%. To the best of our knowledge,
this is the highest efficiency ever reported for DSSCs based on
organic sensitizers. After 1000 h of light soaking at 60 1C, the
initial efficiency of 7.31% in JK-71 decreased to 6.21%. On the
other hand, the efficiency of JK-2 decreased from 6.31 to
(100 mW cmÀ2).
arising from the nitrogen lone pair, and that the LUMO was
delocalized over the thiophene and cyanoacrylic groups
(see the ESIw). Examination of the HOMO and LUMO of
these dyes indicated that HOMO–LUMO excitation moved
the electron distribution from the triphenylamine moiety to
the cyanoacrylic acid moiety, and that photoinduced electron
transfer from the dye to the TiO2 electrode could efficiently
occur through the HOMO–LUMO transition.
The inset in Fig. 3 shows the photocurrent action spectra for
DSSCs based on JK-70 and JK-71, using an acetonitrile-based
electrolyte composed of 0.6
M
3-hexyl-1,2-dimethyl-
imidazolium iodide, 0.04 M I2, 0.025 M LiI, 0.05 M guanidium
thiocyanate and 0.28 M 4-tert-butylpyridine. The incident
photon-to-current conversion efficiency (IPCE) of JK-71
showed a plateau of over 80% from 430 to 560 nm, reaching
a maximum of 83% at 483 nm. The photovoltaic perfor-
mances of the JK-70- and JK-71-sensitized cells are shown
in Table 1. Under standard global AM 1.5 solar conditions,
the JK-71-sensitized cell gave a short circuit photocurrent
density (Jsc) of 15.43 mA cmÀ2, an open circuit voltage (Voc)
of 0.74 V and a fill factor (ff) of 0.74, corresponding to an
overall conversion efficiency (Z) of 8.39%. Under the same
conditions, the JK-24a-sensitized cell gave a Jsc value of
14.54 mA cmÀ2, a Voc of 0.70 V and an ff of 0.74, correspond-
ing to an Z value of 7.63%. From these results (Table 1), we
have observed that the Z values of the JK-70- and JK-71-based
cells are higher than that of the JK-2-based cell. Of particular
importance is the 40–50 mV increase in Voc of the JK-70- and
JK-71-based cells relative to the JK-2-based cell. The result
can be interpreted as decreasing the electronic coupling
Fig. 4 Evolution of the solar cell parameters with JK-2 (’),
JK-70 (K) and JK-71 (m) during visible light soaking (AM 1.5G,
100 mW cmÀ2) at 60 1C. A 420 nm cut-off filter was placed on the cell
surface during illumination. Quasi-solid gel electrolyte: 5 wt% PVDF-
HFP, 0.6 M DMPII, 0.1 M I2 and 0.5 M NMBI in MPN.
Table 1 The optical, redox and DSSC performance parameters of JK-2, JK-70 and JK-71
l
abs/nm
(e/MÀ1 cmÀ1 a
Eox/V
(DEp/V)b
Dye
)
l
em/nma
E0–0/Vc
ELUMO/Vd
À1.35
Jsc/mA cmÀ2
Voc/V
ff
Z (%)e
JK-70
367 (51 580)
448 (42 520)
368 (45 090)
447 (37 630)
364 (44 000)
452 (39 000)
583
608
670
1.04 (0.17)
1.06 (0.15)
1.04 (0.15)
2.39
15.29
0.73
0.74
8.31
JK-71
JK-2
2.37
2.34
À1.31
À1.30
15.43
14.54
0.74
0.70
0.74
0.74
8.39
7.63
a
b
The absorption spectra were measured in ethanol solution. The oxidation potential of dyes on TiO2 were measured in CH3CN with 0.1 M
(n-C4H9)4NPF6, with a scan rate of 50 mV sÀ1 (vs. NHE). E0–0 was determined from the intersection of the absorption and emission spectra in
c
d
e
ethanol. ELUMO was calculated as Eox À E0–0
.
The performances of the DSSCs were measured with a 0.18 cm2 working area, employing a mask.
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identical Voc conditions, demonstrating that the electron re-
combination process was effectively controlled by the molecular
structure of the dye. The results of the electron lifetime are also
consistent with those of Voc shown in Table 1.
In summary, we have developed novel organic dyes incor-
porating long alkyl chains that achieve over 7.31% power
conversion efficiency and long-term stability using a polymer
gel electrolyte; such a high efficiency is very impressive. We
believe that the development of highly efficient organic dyes
as alternatives to ruthenium complexes is possible through
sophisticated structural modifications.
This work was supported by the Korea Science and
Engineering Foundation (KOSEF) through the NRL pro-
gram, funded by Ministry of Science and Technology (no.
R0A-2005-000-1034-0), the MKE (The Ministry of Knowl-
edge Economy), Korea, under the ITRC (Information
Technology Research Center) Program (IITA-2008-C1090-
0804-0013) and BK-21 (2006).
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solid-state electrolyte have passed such a light-soaking test.
The enhanced long-term stability of JK-70 and JK-71 can
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