An achievement of over 12 percent efficiency in an organic dye-sensitized solar cell

Article abstract of DOI:10.1039/c4cc02192d

Dye-sensitized solar cells fabricated by using a novel metal-free alkoxysilyl carbazole as a sensitizing dye and a Co^3+/2+-complex redox electrolyte exhibited light-to-electric energy conversion efficiencies of over 12% with open-circuit photovoltages higher than 1 V by applying a hierarchical multi-capping treatment to the photoanode. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02192d

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An achievement of over 12 percent efficiency in
an organic dye-sensitized solar cell†
Cite this: Chem. Commun., 2014,
50, 6379
Kenji Kakiage,ab Yohei Aoyama,a Toru Yano,*a Takahiro Otsuka,a Toru Kyomen,c
Masafumi Unnoc and Minoru Hanaya*c
Received 25th March 2014,
Accepted 23rd April 2014
DOI: 10.1039/c4cc02192d
www.rsc.org/chemcomm
Dye-sensitized solar cells fabricated by using a novel metal-free
alkoxysilyl carbazole as a sensitizing dye and a Co3+/2+-complex
redox electrolyte exhibited light-to-electric energy conversion
efficiencies of over 12% with open-circuit photovoltages higher
than 1 V by applying a hierarchical multi-capping treatment to the
photoanode.
Dye-sensitized solar cells (DSSCs), composed of mesoporous
nanocrystalline-TiO2 films modified via sensitizing dyes as
photoelectrodes, redox electrolytes and counter electrodes,
have become a promising alternative photovoltaic technology
of the next generation to conventional inorganic solar cells
because of their ease of fabrication and potential low costs.1–3
Towards the realization of higher photovoltaic performance
and durability in DSSCs, the development of the sensitizing
dyes is one of the most important approaches. However, the
Fig. 1 Molecular structures of carbazole dyes with a trimethoxysilyl group
(ADEKA-1) and with a carboxy group (ADEKA-2 and MK-2) as the anchor
moiety for chemical bonding to the surface of the TiO2 electrodes.
investigation of the sensitizing dyes has been mostly limited to to possess high photostability and showed relatively high light-
those including carboxy groups as the anchor moiety for to-electric energy conversion efficiencies (Z) of about 8%,11,12 and
chemical bonding to the surface of the TiO2 electrodes.1–5
9–10% efficiencies have been obtained by using a cobalt(III/II)
Organosilicon compounds such as alkoxysilanes and sila- tris(2,20-bipyridine) complex ([Co(bpy)3]3+/2+) as the redox electrolyte
nols have high bonding ability to metal-oxide surfaces by in the cells.13,14 By the application of anchoring with titanosiloxane
forming firm Si–O–metal bonds,6–9 and alkoxysilyl dyes have bonds to this type of dyes and by employing Co3+/2+-complex redox
been demonstrated as potential materials for sensitizing dyes electrolytes, it would be possible to produce efficient DSSCs. In this
by a higher electron-transfer efficiency from the light-excited work, therefore, we designed and synthesized an alkoxysilyl deri-
dye to the TiO2 electrode through Si–O–Ti (titanosiloxane) vative of MK-2, ADEKA-1 (Fig. 1), and succeeded in achieving the
bonds and a higher photovoltage of the cell than that using Z value of 12.5% in a DSSC sensitized by the dye.
the carboxy analog.10 Recently, metal-free MK dyes of carbazole/
ADEKA-1 and ADEKA-2, which is a benzoic-acid derivative of
alkyl-functionalized oligothiophene/cyanoacrylic acid type ADEKA-1 synthesized as a reference dye, exhibited similar UV-visible
compounds were reported by Koumura and Hara.11–13 The absorption spectra to MK-2 in toluene solutions; their major
DSSCs sensitized with those organic dyes were demonstrated absorption bands assignable to the p–p* transition were observed
in the visible region between 400 and 600 nm (Fig. S1, ESI†). The
a Environmental & Energy Materials Laboratory, ADEKA CORPORATION,
7-2-35 Higashiogu, Arakawa, Tokyo 116-8554, Japan. E-mail: yanotoru@adeka.co.jp
b Human Resource Cultivation Center (HRCC), Gunma University, 1-5-1 Tenjin-cho,
Kiryu, Gunma 376-8515, Japan
c Division of Molecular Science, Faculty of Science and Technology, Gunma University,
1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan. E-mail: mhanaya@gunma-u.ac.jp
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc02192d
maximum molar absorption coefficients (emax) at lmax were evalu-
ated to be 43 200 (498 nm), 42 700 (504 nm) and 37 400 (489 nm) for
ADEKA-1, ADEKA-2 and MK-2, respectively. Slight red shifts of lmax
and increments of emax in ADEKA-1 and ADEKA-2 compared with
MK-2 are considered to be due to the introduction of the phenyl-
amide moieties. The oxidation potentials (Eox: approximately the
energy levels of HOMO) of these dyes were determined by means of
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The photovoltaic properties of the cells fabricated using the three
carbazole dyes as sensitizers are listed in Table 1. The photovoltaic
parameters, short-circuit photocurrent density ( Jsc), open-circuit
photovoltage (Voc), fill factor (FF) and Z, were assessed from the
photocurrent–voltage (I–V) curves under the simulated AM-1.5G solar
irradiation with the intensity of 100 mW cmꢀ2 (Fig. S6, ESI†). Entries
1–3 are the cells using the [Co(bpy)3]3+/2+ redox electrolyte. Among the
cells, the ADEKA-1-sensitized cell exhibited slightly higher Jsc and Voc
values than the cells sensitized by the carboxy dyes of ADEKA-2 and
MK-2, and a higher Z over 10% was evaluated than those for the cells
with ADEKA-2 and MK-2. The higher Z is considered to be mainly due
to the higher Voc caused by the linkage of the dye by the titano-
siloxane bonds on the surface of the TiO2 electrode (Fig. S3, ESI†).10
The maximum photovoltage (Vmax) obtained in the DSSC is
attributed to the energy gap between the quasi-Fermi level [approxi-
mately the energy level of the conduction-band edge (EC.B.)] of the
Fig. 2 Changes of the absorbances due to the carbazole dyes (ADEKA-1,
ADEKA-2 and MK-2) adsorbed on the TiO2 electrodes with the soaked time
into 3-methoxypropionitrile–water (1 : 1 in volume) at 85 1C in the dark.
cyclic voltammetry (Fig. S2, ESI†). ADEKA-1 and ADEKA-2 intro- TiO2 [ꢀ0.5 V vs. NHE for anatase2,3] and the redox potential of the
duced phenyl-amide moieties showed more positive Eox of 0.99 V electrolyte. To obtain an even higher photovoltage and to improve
and 1.01 V vs. NHE, respectively, compared to that of MK-2 (0.95 V). the Z value further in the cells, therefore, we employed a cobalt(III/II)
The values of Eox are more positive than that of the redox potential tris(5-chloro-1,10-phenanthroline) complex ([Co(Cl-phen)3]3+/2+) as the
of 0.56 V vs. NHE in [Co(bpy)3]3+/2+ 15
driving forces for the dye regeneration reactions by the electron than [Co(bpy)3]3+/2+ 15
transfer from the [Co(bpy)3]3+/2+ redox.
in the ADEKA-1-sensitized cell (entry 4), the Voc value was increased to
,
thus providing thermodynamic redox electrolyte, which has a higher redox potential of 0.72 V vs. NHE
By using the [Co(Cl-phen)3]3+/2+ redox electrolyte
.
The adsorptions of carboxy dyes such as ADEKA-2 and MK-2 on 0.897 V from that in the cell with the [Co(bpy)3]3+/2+ redox electrolyte
the TiO2 electrodes are understood to proceed from the formation of 0.848 V. The Z was actually improved by an increment of the Voc
of ester-like and/or bridging linkages between the carboxy groups value, although the Jsc value decreased slightly by a sluggish diffusion
and the hydroxy groups on the surface of the TiO2 electrode.3,11 On of [Co(Cl-phen)3]3+/2+ compared to [Co(bpy)3]3+/2+ due to the bulki-
the other hand, ADEKA-1, which has a trimethoxysilyl group as the ness.15–17 On the other hand, a significant lowering of Z was observed
anchor moiety to the TiO2 electrode, is considered to adsorb on the in the MK-2-sensitized cell using the [Co(Cl-phen)3]3+/2+ redox electro-
electrode with the formation of three titanosiloxane (Si–O–Ti) bonds lyte (entry 5). An energy gap between the Eox of MK-2 and the redox
(Fig. S3, ESI†).7,9,10 To examine the durability of the linkages potential of [Co(Cl-phen)3]3+/2+ was estimated to be 0.23 eV, which is
between the dyes and the TiO2 electrodes, we soaked the dye- smaller than 0.27 eV for ADEKA-1. The low Z value observed is
adsorbed TiO2 electrodes into a mixed solvent of 3-methoxy- considered to be the result of the lack of the thermodynamic driving
propionitrile and water (1 : 1 in volume) at 85 1C in the dark and force for the dye regeneration reaction, which proceeds through the
traced the time dependence of the light absorption due to the dyes electron transfer from the Co2+ complex to the dye in the oxidized
on the TiO2 electrodes (Fig. 2, Fig. S4 and S5, ESI†). As shown in state, by the small energy gap.15–18
Fig. 2, although 70–80% of adsorbed dye molecules were observed
The Voc value in the ADEKA-1-sensitized cell was increased by
to eliminate from the electrodes after 120 min soaking in the cases using [Co(Cl-phen)3]3+/2+ redox instead of [Co(bpy)3]3+/2+. However
of ADEKA-2 and MK-2, more than 95% of ADEKA-1 molecules were the increment and the observed Voc were much lower than the
confirmed to be on the TiO2 electrode. These results evidence the expected value from the estimated Vmax of the cells, which are 1.22 V
robustness of the linkage by the titanosiloxane bonds.
and 1.06 V in the cases of [Co(Cl-phen)3]3+/2+ and [Co(bpy)3]3+/2+
,
Table 1 Photovoltaic parameters of the cells sensitized by the carbazole dyes (ADEKA-1, ADEKA-2 and MK-2) using the cobalt complexes ([Co(bpy)3]3+/2+
and [Co(Cl-phen)3]3+/2+) as the redox electrolytes under the illumination of the simulated sunlight (AM-1.5G, 100 mW cmꢀ2
)
Entry
Dye
Redox/electrolytea
Cappingb
Jsc (mA cmꢀ2
)
Voc (V)
FF
Z (%)
1
2
3
4
5
6
7
8c
ADEKA-1
ADEKA-2
MK-2
ADEKA-1
MK-2
ADEKA-1
ADEKA-1
ADEKA-1
[Co(bpy)3]3+/2+/A
Single
Single
Single
Single
Single
Multi
Multi
Multi
16.1
15.1
15.8
15.6
8.8
15.8
14.9
15.6d
0.848
0.821
0.814
0.897
0.811
0.958
1.034
1.036d
0.762
0.752
0.753
0.764
0.752
0.748
0.777
0.774d
10.4
9.32
9.68
10.7
5.37
11.3
12.0
12.5d
[Co(bpy)3]3+/2+/A
[Co(bpy)3]3+/2+/A
[Co(Cl-phen)3]3+/2+/A
[Co(Cl-phen)3]3+/2+/A
[Co(Cl-phen)3]3+/2+/A
[Co(Cl-phen)3]3+/2+/B
[Co(Cl-phen)3]3+/2+/B
a
Electrolyte A: 0.25 M Co2+ + 0.035 M Co3+ + 0.10 M LiClO4 + 0.50 M TBP in acetonitrile, electrolyte B: 0.25 M [Co(Cl-phen)3]2+ + 0.035 M [Co(Cl-phen)3]3+
+
b
0.07 M LiClO4 + 0.02 M NaClO4 + 0.03 M TBAPF + 0.01 M TBPPF + 0.01 M HMImPF + 0.30 M TBP + 0.10 M TMSP + 0.10 M MP in acetonitrile. Single: the
single-capping with heptanoic acid, multi: the hierarchical multi-capping. The results for the cell attached with an antireflection film on the surface of
the photoanode. The value is the average of the results of the three cells examined (Table S2, ESI).
c
d
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respectively (Fig. S7, ESI†). The reason for the lower Voc in compar- the ‘alkyl-thicket’ structure on the TiO2 electrode by using the
ison with the Vmax is believed to be because the cells employing hierarchical multi-capping treatment.
Co3+/2+-complex redox electrolytes with more positive redox poten-
The achievement of the Z value of 12.5% under the one sun
tials are exposed to the faster recombination reaction, that is, the condition in the cell with the metal-free dye of ADEKA-1 as a single
undesirable electron transfer from the TiO2 electrode to the electro- sensitizer indicates the high potential of organic dyes with organo-
lyte, and the reaction can be suppressed by restricting the contact silicon tethers for binding the metal oxides as the photosensitizers
of the Co3+ complex to the naked surface of the TiO2 electrode for DSSCs, and the present results provide a fertile base for further
between the adsorbed sensitizing dye molecules.16–22 Therefore, we improving the photovoltaic performance of DSSCs. The combi-
examined a hierarchical multi-capping treatment of the ADEKA-1- nation of the co-sensitization using plural sensitizing dyes with
adsorbed TiO2 electrode using eight molecules with various alkyl- organosilyl anchor moieties and the hierarchical multi-capping in
chain lengths and with three kinds of anchor moieties, i.e. carboxy, the cells with Co3+/2+-complex redox electrolytes is one of the
phosphonic and silyl groups (Fig. S8, ESI†). In the treatment, we approaches for achieving a higher efficiency of over 13%, and such
performed the capping by larger agents, which need larger space for a challenge is currently underway in our group.
the adsorption, to smaller ones, which need only small space, and
This work was partly supported by the ‘‘Element Innovation’’
tried to form an ‘alkyl-thicket’ structure on the surface of the TiO2 Project by Ministry of Education, Culture, Sports, Science &
electrode between the adsorbed sensitizing dye molecules for an Technology in Japan. We also thank the Human Resource
effective passivation of the electrode (Fig. S9, ESI†). The hierarchical Cultivation Center, Gunma University for a research fund.
multi-capping treatment worked properly in the ADEKA-1-sensitized
cell with the [Co(Cl-phen)3]3+/2+ redox electrolyte, and the Voc value
was improved from 0.897 V to 0.958 V resulting in a higher
Notes and references
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¨
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experimentally optimized electrolyte (entry 7; Fig. S10, ESI†). In
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Fig. 3 Typical I–V properties of the ADEKA-1-sensitized solar cell with the
highest efficiency of 12.5% (entry 8 in Table 1) under the simulated one sun
irradiation (AM-1.5G, 100 mW cmꢀ2). Inset shows the IPCE spectrum of the cell.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 6379--6381 | 6381