Benzothiadiazole-containing donor-acceptor-acceptor type organic sensitizers for solar cells with ZnO photoanodes

Article abstract of DOI:10.1039/c2cc37184g

Dye-sensitized solar cells using nanocrystalline ZnO as the photoanode and a metal-free sensitizer with a benzothiadiazole entity directly connected to the 2-cyanoacrylic acid acceptor exhibited an efficiency (5.18%) higher than those using the TiO₂ photoanode. Use of a hierarchical ZnO back scattering layer further improved the efficiency to 5.82%.

Full text of DOI:10.1039/c2cc37184g

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COMMUNICATION
Benzothiadiazole-containing donor–acceptor–acceptor type organic
sensitizers for solar cells with ZnO photoanodesw
Ryan Yeh-Yung Lin,ab Chuan-Pei Lee,b Yung-Chung Chen,a Jia-De Peng,b Te-Chun Chu,b
Hsien-Hsin Chou,a Hung-Ming Yang,a Jiann T. Lin*a and Kuo-Chuan Ho*bc
Received 2nd October 2012, Accepted 30th October 2012
DOI: 10.1039/c2cc37184g
Dye-sensitized solar cells using nanocrystalline ZnO as the
photoanode and a metal-free sensitizer with a benzothiadiazole
entity directly connected to the 2-cyanoacrylic acid acceptor
exhibited an efficiency (5.18%) higher than those using the TiO2
photoanode. Use of a hierarchical ZnO back scattering layer
further improved the efficiency to 5.82%.
The benzothiadiazole (BT) unit is used as the building block of
low-band gap polymers for organic photovoltaic (OPV) cells.9 We
also found that incorporation of a BT unit in the conjugated
space of metal-free sensitizers effectively red shifted the absorption
spectra of the dyes.10 DSSCs based on several BT-based metal-
free sensitizers have been reported to have good performance
since then.11 Although incorporation of the BT entity in the
spacer results in a red shift of the absorption, partial charge-
trapping at the BT entity may hamper electron injection from the
dye to the TiO2 conduction band and counteracts the perfor-
mance of the DSSCs.11e By tethering two electron-donating
alkoxy chains at the BT entity, the cell efficiency reached
B92% of the N719-based standard DSSC.11g However, charge
trapping was not eased. We therefore decided to synthesize
BT-containing sensitizers with the BT entity directly connected
to 2-cyanoacrylic acid (also as the anchoring group) for DSSCs
using ZnO as the photoanode based on the following reasons: (1)
the charge accumulated at the BT entity may have better
opportunity to inject into ZnO due to a shorter distance between
BT and the anchoring group; (2) the electrons injected into ZnO
may be quickly transported to avoid charge recombination. In
this communication, new BT-based sensitizers comprising an
arylamine donor, a conjugated spacer containing a BT entity,
and 2-cyanoacrylic acid acceptor are reported. DSSCs using these
sensitizers and the ZnO photoanode will be also discussed.
New dyes, RL1 to RL3, are shown in Fig. 1, and the synthetic
details are described in Scheme S1 (ESIw). The absorption spectra
of the dyes (in THF) have two bands in the range of 330–540 nm
(Fig. 2). The band below 400 nm is due to the p–p* transition of
the conjugated molecules, and that in the longer wavelength region
(480–540 nm) is ascribed to the intramolecular charge transfer
transition with p–p* transition character. The labs values are in the
order of RL3 (540 nm) > RL2 (513 nm) > RL1 (486 nm).
Incorporation of two alkoxy substituents increases the electron
donating power of the triarylamine, consequently, RL2 displays a
red-shift of the absorption compared to RL1. Insertion of a
thiophene ring between BT and the arylamine of RL2 increases
the conjugation length and further red shifts the absorption
spectrum: RL2, 513 nm; RL3, 540 nm. Similarly, insertion of a
thiophene ring between BT and the 2-cyanovinyl entity leads to a
red shift of the absorption: RL1, 486 nm; S1, 492 nm (Fig. 2).
A blue shift of the absorption spectra (Fig. S1, ESIw) is noticed
when the dyes are adsorbed on ZnO; this is attributed to the
ZnO is attractive as the photoanode of dye-sensitized solar cells
(DSSCs) because of its wide band gap (3.37 eV) characteristic and
comparable conduction band energy with TiO2.1a,b Compared to
TiO2, ZnO has even higher electron mobility and electron-
lifetime.1c,d Many studies on ZnO were focused on modified
nanostructures such as nanoparticles,2 nanosheets,3 nanowires,4
and nanoflowers.5 DSSCs using the ZnO photoanode exhibit
improved efficiencies through optimization of the pore surface,
treatment temperature, synthetic procedure or amalgamating
different materials (TiO2 and Sn).1c,d,6
ZnO was found to be incompatible with some Ru dyes because
of two factors: (1) ZnO is more basic than TiO2 and is vulnerable
under lower pH conditions; (2) Ru dyes contain complexing agents
capable of removing Zn2+ ions from the ZnO lattice.7 Therefore,
higher dye uptake by increasing the immersion time of ZnO in dye
solution is problematic. In comparison, ZnO seems to be more
robust towards metal-free dyes, and there have been several
successful examples. In 2010, Cheng and Hsieh developed self-
assembled hierarchical ZnO secondary nanoparticles which
provided high light scattering and harvesting. Under AM 1.5 one
sun condition, efficiencies of 5.34% and 4.95% were achieved
for indoline dyes, D205 and D149, respectively.8a A conversion
efficiency of 1.23% was reported by Funabiki et al. by using a near-
infrared absorbing heptamethine–cyanine dye (KFH-3) and porous
ZnO photoanode prepared at low temperature.8b A cell efficiency
of 2.53% was demonstrated by Bendall et al. using a metal-free dye
(C220) and core–shell nanowires of ZnO/TiO2.4b
a Institute of Chemistry, Academia Sinica, Nankang 11529, Taipei,
Taiwan. E-mail: jtlin@gate.sinica.edu.tw
b Department of Chemical Engineering, National Taiwan University,
Taipei 10617, Taiwan. E-mail: kcho@ntu.edu.tw
c Institute of Polymer Science and Engineering, National Taiwan
University, Taipei 10617, Taiwan
w Electronic supplementary information (ESI) available: Detailed
experimental information, synthetic procedure and characterization,
photophysics, electrochemistry, device performance data. See DOI:
10.1039/c2cc37184g
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12071–12073 12071

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Table 1 Performance parameters of DSSCsa
JSC
VOC (V) (mA cmÀ2
Rct2
Dye loading
Dye
)
FF
Z (%) (ohm) (mol cmÀ2
)
RL1
RL2
RL3
S1
0.59
0.54
0.51
0.61
13.50
13.09
12.03
12.06
13.20
13.17
0.65 5.18
0.61 4.29
0.66 4.02
0.65 4.79
0.61 4.43
0.60 4.74
16.69 2.41 Â 10À7
32.20 1.58 Â 10À7
37.07 1.09 Â 10À7
35.42 1.78 Â 10À7
2.84 Â 10À7
RL1b 0.55
D149 0.61
a
Experiments were conducted using ZnO photoelectrodes with
approximately 15 mm thickness (transparent layer only) and 0.16 cm2
b
working area on the FTO (15 O per sq.) substrates. RL1 was
conducted using TiO2 photoelectrodes with approximately 15 mm
thickness (transparent layer only) and 0.16 cm2 working area.
Fig. 1 Structure of the dyes.
Fig. 3 (a) J–V curves of DSSCs based on the dyes. The lower half
shows the dark current densities of DSSCs. These results were collected
based on TL only. (b) IPCE spectra of DSSCs based on the dyes.
Fig. 2 Absorption spectra of the dyes in THF.
more compact packing of the dye molecules on the ZnO surface.
DSSC based on RL1 has only a slightly higher conversion
efficiency than S1-based cells despite the higher dye density of
RL1 than S1 by B1.3 times. This can be rationalized by the
following reasons: (1) S1 has a significantly higher absorption
intensity compared to RL1; (2) the cell of S1 has lower dark current
than that of RL1. In contrast, the cell of RL1 exhibits a smaller
charge transport resistance, that is, more efficient electron transport
(vide infra, see EIS discussions). RL2 (or RL3) has a lower efficiency
than RL1, possibly because of the lower dye density which results
in larger dark current (vide infra) and therefore lower VOC values.
Slower regeneration of the oxidized dye by the electrolyte due to
the higher HOMO level of RL2 (or RL3) than RL1 leading to less
efficient electron injection cannot be excluded, however.
weaker electron-withdrawing power of the carboxylate than
the carboxylic acid.12
Cyclic voltammograms for the compounds are shown in
Fig. S2 (ESIw) and the relevant electrochemical data are pre-
sented in Table S1 (ESIw). The reversible wave due to the
oxidation of arylamine decreases in the order of RL1 > RL2 >
RL3. Apparently, the presence of electron-donating alkoxy
substituents in RL2 and RL3 significantly increases the electron
density of arylamine. The lower oxidation potential of RL3
than RL2 is attributable to the nearby electron excessive
thiophene in the former. Being closer to the electron deficient
BT entity, the arylamine in RL1 is expected to have a lower
electron density and is more difficult to be oxidized. More
negative excited state oxidation potential of the RL dyes than
the conduction-band-edge of ZnO (À0.41 V vs. NHE) ensures
electron injection into the conduction band of ZnO.
The dark current density and the electrochemical impedance
spectra (EIS) of DSSCs in the dark are shown in Fig. 3a and
Fig. S3a (ESIw), respectively. The intermediate frequency semicircle
in the Nyquist plot represents the charge transfer phenomena
between the TiO2 surface and the electrolyte, i.e., dark current.
The resistance towards the dark current (the size of semicircle)
decreases in the order of S1 > RL1 > RL2 > RL3, which is
consistent with the dark current (Fig. 3a) and the VOC values
(Table 1). Electrochemical impedance upon illumination of
100 mW cmÀ2 was also measured under open-circuit conditions,
and the Nyquist plots of DSSCs with different dyes are shown in
Fig. S3b (ESIw). The radius of the intermediate frequency semi-
circle (10À2 to 105 Hz) in the Nyquist plot reflects the electron
transport resistance, and the smaller radius means the lower
electron transport resistance. The electron transport resistance
decreases in the order of RL3 > S1 > RL2 > RL1. We suspect
that the larger dipole (Table S2, ESIw) in RL3 and RL2 results in
up-shift of the Fermi level of ZnO and decreases the electron
The performance data of DSSCs based on ZnO photo-
anodes (film with a thickness of 15 mm from the nanoparticles of
20 nm in diameter) under AM 1.5 illumination are collected in
Table 1. The photocurrent density–voltage (J–V) curves and the
incident photon-to-current conversion efficiencies (IPCE) of the
cells are plotted in Fig. 3a and b, respectively. All the devices
exhibited good power conversion efficiencies ranging from 4.02 to
5.18%. It is noteworthy that the efficiency of RL1 on ZnO
photoanodes is higher than that of RL1 by 0.75% fabricated on
TiO2 and measured under similar conditions even though the
adsorbed dye density of the former is slightly lower (Table 1). This
witnesses the potential of using ZnO as an alternate photoanode
for TiO2. The adsorbed dye density (Table 1) of the sensitizer with
two hexyloxy substituents (RL2 or RL3) is >30% smaller than
that of RL1, indicating that the two flexible long chains impede
c
12072 Chem. Commun., 2012, 48, 12071–12073
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Table 2 Performance parameters of DSSCsa
active layer and hierarchical branching ZnO nanomaterials as
the SL.
Dye
VOC (V) JSC (mA cmÀ2) FF Z (%)
In summary, DSSCs using benzothiadiazole-based dyes and
a ZnO photoanode exhibited high conversion efficiencies
(4.02% to 5.18%). The cell efficiency can be further improved
to 5.82% when a novel hierarchical ZnO was used as the light
back scattering layer of the cell.
RL1 (15 mm TL)
0.59
13.50
14.70
13.66
0.65 5.18
0.65 5.82
0.64 5.20
RL1 (15 mm TL + 3.5 mm SL) 0.61
0.59
RL1b
a
Experiments were conducted using ZnO photoelectrodes with
approximately 18 mm thickness and 0.16 cm2 working area on the FTO
We acknowledge the support of the Academia Sinica
(including Nanoscience and Nanotechnology Program), the
National Science Council of Taiwan, and National Taiwan
University.
b
(15 O per sq.) substrates. RL1 was conducted using TiO2 photoelectrodes
with approximately 15 mm TL + 4 mm SL and 0.16 cm2 working area.
injection efficiency. Quantum chemistry computation results
(Tables S2 and S3, Fig. S4–S7, ESIw) support the charge
transfer character of the S0 - S1 transition. The product of
the oscillator strength and the Mulliken charge changes
(Fig. S7, ESIw) in the 2-cyanoacryc acid group is slightly higher
in the RL dyes (RL1 À0.09: RL2: À0.10; RL3: À0.11) than S1
(À0.07), suggesting that electron injection is slightly favoured for
the RL dyes. The stabilities of the morphology of N3-, RL- and
D149-adsorbed ZnO were also examined briefly by SEM (Fig. S8,
ESIw). There was no noticeable change of morphology for
RL-adsorbed ZnO after 24 h. In comparison, morphology
degradation of N3-adsorbed ZnO was very evident.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12071–12073 12073