Promising ZnO-based DSSC performance using HMP molecular dyes of high extinction coefficients

Article abstract of DOI:10.1039/c4dt01179a

Employing newly synthesized di-substituted tri-phenyl amine (HMP-9) and carbazole (HMP-11) dyes (with limited acidic carboxyl anchor groups), a power conversion efficiency as high as 7.03% in ZnO nanocrystallite (NC)-based dye-sensitized solar cells (DSSCs) is achieved. The specific molecular designs of HMP-09 and HMP-11 consisting of with and without hexyloxy spacer groups, and added tri-phenyl amine or 9-phenyl-9H-carbazole donor groups, respectively, attached on the ancillary ligands are advantageous, evidenced from electrochemical impedance spectroscopy measurements, for ZnO NC-based DSSCs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01179a

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Promising ZnO-based DSSC performance using HMP
molecular dyes of high extinction coefficients†
T. Ganesh,a Hong-Minh Nguyen,a Rajaram S. Mane,*a,b Nakjoong Kim,a
Dipak V. Shinde,a Sambhaji S. Bhande,b Mu. Naushad,c K. N. Huid and
Sung Hwan Han*a
Cite this: Dalton Trans., 2014, 43,
11305
Received 22nd April 2014,
Accepted 5th June 2014
DOI: 10.1039/c4dt01179a
www.rsc.org/dalton
Employing newly synthesized di-substituted tri-phenyl amine of a YD2-o-C8 sensitizer and a Co(II/III) tris(bipyridyl)-based redox
(HMP-9) and carbazole (HMP-11) dyes (with limited acidic carboxyl electrolyte.4 It seems that DSSCs based on TiO2 reflect the
anchor groups), a power conversion efficiency as high as 7.03% in limits imposed by its low electron mobility due to which there
ZnO nanocrystallite (NC)-based dye-sensitized solar cells (DSSCs) is more chance of electron–hole recombination.5–7 The ZnO
is achieved. The specific molecular designs of HMP-09 and nanostructure-based photoanode with similar band gap and
HMP-11 consisting of with and without hexyloxy spacer groups, conduction band position of the TiO2 photoanode suffered
and added tri-phenyl amine or 9-phenyl-9H-carbazole donor from the problems of chemical and thermal stabilities.8 ZnO
groups, respectively, attached on the ancillary ligands are advan- forms, i.e. morphology, structure and phase, are considered for
tageous, evidenced from electrochemical impedance spectroscopy good electron mobility and electrical conductivity. By Ga doping,
measurements, for ZnO NC-based DSSCs.
5–6% conversion efficiency of the ZnO photoanode has been
reported in the literature,9 which can be further improved
either by surface treatment or by designing the dyes of higher
molar extinction coefficients (ε).10 In one of the studies, the
ZnO photoanode composed of hierarchically assembled nano-
crystallites has been envisaged for record 7.5% power conver-
sion efficiency.11
As promising, low-cost, efficient conversion of visible light into
electricity becomes inevitably important, dye-sensitized solar
cells (DSSCs), a new class of photoelectrochemical cells based
on porous metal oxide semiconductors, Ru-complexes as sen-
−
sitizers and
a
liquid electrolyte containing I−/I3
,
have
attracted significant attention as potential alternatives to con-
ventional Si based solar cells. New molecular sensitizing dyes,
in addition to polypyridyl ruthenium compounds of higher
extinction coefficients that extend the absorption in the near-
infrared region of the solar spectrum are providing better
charge generation and transport properties have attracted signifi-
cant attention.1,2 Few dyes with amphiphilic heteroleptic charac-
teristics, e.g. Z907, C101 and C106, yielded 9–11% solar-to-
electrical conversion efficiencies (η%) under 1 Sun illumination.3
After 20 years of research, the power conversion efficiency of
DSSCs has just been increased from 11% to 12.3% in the presence
It is quiet accepted that the N719 (cis-dithiocyanato-bis(2,2′-
bipyridyl-4,4′-dicarboxylate)-ruthenium(II) and N3 sensitizers
with 2 and 4 carboxylic acid groups incorporated onto bipyri-
dine ligands, respectively, facilitate anchoring to the photo-
anode surface and for the transfer of electrons from the dye to
the metal oxide (MO) conduction band. In ZnO nanostructure
based DSSCs, preventing the formation of the Zn2+/dye
complex, an insulating layer that eventually blocks the overall
electron injection efficiency by producing inferior power con-
version efficiency is presently a great challenge.10 Dye architec-
ture, addition of a donor, conjugated linker groups and
electronic coupling between the lowest unoccupied molecular
orbital (LUMO) level of the dye and the MO conduction band
can enhance the device performance to a greater extent by
improving the light harvesting efficiency and charge transport
in the photoanode.12
With this perspective, herein, we would like to stress the
importance of newly designed dye molecules, cis-[Ru(H2dcbpy) (L)
(NCS)2], where H2dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine
and L = 4,4′-bis-(4-di-p-hexyloxypheny-lamino)-styryl-2,2′-bipyri-
dine (HMP-9) and 4,4′-bis-(4-(N-carbazolyl)-phenyl-2-vinyl)-2,2′-
bipyridine (HMP-11) for efficient ZnO NC-based DSSCs. The
obtained conversion efficiencies are superior to Ru(H2dcbpy)
aInorganic Nano-materials Laboratory, Department of Chemistry,
Hanyang University, Seongdong-gu, 133791 Seoul, Republic of Korea.
E-mail: shhan@hanyang.ac.kr
bCentre for Nano-materials & Energy Devices, School of Physical Sciences,
SRTM University, 431606 Nanded, India. E-mail: rsmane_2000@yahoo.com
cAdvanced Materials Research Chair, Department of Chemistry, College of Science,
Building#5, King Saud University, Riyadh, Saudi Arabia
dSchool of Materials Science and Engineering Pusan National University,
San 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea
†Electronic supplementary information (ESI) available: Detailed electrode
synthesis, dye synthesis, optical absorption and NMR details. See DOI:
10.1039/c4dt01179a
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Fig. 2 (a) Electronic absorption spectra, and (b) J–V characteristics of
■
▲
)
the devices employing different dyes. N3 (black, ), HMP-9 (green,
▼
and HMP-11 (blue, ) under the illumination of AM 1.5 G sunlight
(100 mW cm−2).
difference in the quantity of dyes adsorbed (due to the four
hexyloxy chains incorporated on ancillary ligands, the HMP-9
dye has relatively low adsorption compared to the HMP-11 dye)
onto ZnO NCs. The replacement of the carboxylic group of the
N3 bipyridine ligand site with strong donor functional groups
has positively shifted the highest occupied molecular orbital
(HOMO) level by 0.22 eV (vs. vacuum level, in HMP-11) which
matches well with the redox potential of I−/I3−. Whereas, the
tri-phenyl amine donor functional group with hexyloxy groups
has shifted the HOMO position negatively to 0.11 eV from the
N3 dye position. Fig. 2a shows the UV-Vis absorption spectra
of the HMP-9 and HMP-11 sensitizers along with the N3 sensi-
tizer (10−5 M). The absorption spectra of the HMP-9 and
HMP-11 sensitizers are dominated by metal-to-ligand charge
transfer transitions (MLCT), which are located at two posi-
tions, i.e. 441 and 526 nm, and 393 and 539 nm, respectively,
and the high-energy bands below 320 nm are due to ligand
π–π* transitions. The peak position of the MLCT band at
441 nm in the HMP-9 sensitizer is red-shifted compared to
HMP-11 and N3. The presence of the carbazole moiety has
increased extinction coefficient significantly. The increased
π-conjugation length of ancillary ligands with the electron
donation functional group in HMP-9 and HMP-11 might be
responsible for the increased molar extinction coefficients in
the visible region as compared to that of N3. The ε value of
the low-energy MLCT absorption band for HMP-9 and HMP-11
are 23.5 × 103 M−1 cm−1 and 21.4 × 103 M−1 cm−1, respectively,
which are significantly higher than the corresponding values
for the N3 (14.5 × 103 M−1 cm−1). Whereas other bands
observed at 441 and 393 nm wavelength regions show signifi-
cant improvement in absorption characteristics peculiar to the
nature of the dye donor molecular characteristics. A blue shift
of 9 nm along with 3.76 times higher ε values of the HMP-11
sensitizer compared to N3 would certainly fascinate its unique-
ness in the DSSC’s performance. Fig. 2b presents the typical
current–voltage (J–V) spectra of the designed HMPs and N3
dyes immobilized on the ZnO NC photoanodes.
Fig. 1 Molecular structures and respective energy levels of N3, HMP-9
and HMP-11 sensitizers (for experimental scheme, synthesis procedure
and structural characterization, see ESI†).
(4-(4-(N,N-di-(p-hexyloxyphenyl)-amino)styryl)-4′-methyl-2,2′-
bipyridine) (NCS)2, (HMP-2), recently reported previously10
and N3 dyes.
Synthetic procedures, structural characteristics, cyclic vol-
tammograms, UV-Vis absorption spectra of HMP-9 and
HMP-11 sensitizers can be obtained from the ESI.† Synthesis
of ZnO NC photoanodes and DSSC device fabrication details,
as well as efficiency measurements are also provided in ESI.†
Final molecular structures of these newly in-lab designed dyes
are shown in Fig. 1. Our previous results on the HMP-2 dye
yielded a photo-conversion efficiency of 4.01% where the
soaking time for ZnO NC electrodes was as long as 24 h.13
While designing these new dyes with electron-rich moieties
consisting of a planar heterocyclic fused-ring, the carbazole
molecule is explored.14
The carbazole unit acts as an electron donor to pump the
electron to the ruthenium metal in the excited state and
improve the power conversion efficiency of DSSCs. Recently,
carbazole has been used as an electron donating functional
group in organic dyes,15 where power conversion efficiency is
jumped from 6 to 8.3% for TiO2-based DSSCs. Here, HMP-9
and HMP-11 dyes with and without hexyloxy spacer groups,
and added tri-phenyl amine or 9-phenyl-9H-carbazole donor
groups, respectively, attached on the ancillary ligands were
engineered for optimizing acidity as well as visible light
absorption to achieve high efficiency in ZnO-based DSSCs. The
extinction coefficients (ε) and electrochemical characteristics
of HMP dyes are shown in Table 1. There is no noticeable
Table 1 Photophysical and electrochemical properties of N3 and HMP
dyes (vs. vacuum level)
Devices sensitized with HMP-11 exhibited a highest short
circuit current density (Jsc) of 23.4 mA cm−2 and an overall
power conversion efficiency (η%) of 7.09%, whereas those sen-
sitized with HMP-9 and N3 exhibited efficiencies of 5.34 and
4.94% (Table 2). All measurements were performed without
mask. Open circuit voltage (Voc) is almost same in all the cases.
HOMO HOMO–LUMO LUMO λmax
Dye
(eV)
(eV)
(eV)
(nm) ε (M−1 cm−1
)
N3
HMP 9
HMP 11 −5.38
−5.52
−5.41
1.68
2.06
2.05
−3.84
−3.35
−3.33
530
526
539
14 500
23 576
21 435
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based DSSCs with dyes, considered in the present study, under
1 Sun illumination and open circuit voltage conditions. It is
clear from the figure that photoanodes utilizing N3 and
HMP-11 dyes demonstrate higher recombination resistance
values confirming high electron lifetimes and obviously more
photoconversion efficiencies. On the other hand, the photo-
anode utilizing the HMP-9 dye has lower recombination resist-
ance and thus electron recombination will be higher and
lifetime will be lower.19,20 The HMP-11 dye with an electron
rich carbazole moiety aids in superior electron transport
within the device, and thus is expected to produce higher
power conversion efficiency.
In summary, dye-sensitized solar cells utilizing ZnO NCs
with newly synthesized di-substituted tri-phenyl amine (HMP-9)
and carbazole (HMP-11) as donor moieties on bipyridine
ligands with limited acidic carboxyl anchoring groups in
addition to the routinely used N3 dye are investigated, where
electron-rich donor moieties increased the extinction coefficient
and improved charge generation and transportation abilities.
The ZnO NC electrode sensitized with the HMP-11 dye demon-
strated as high as 7.05% power conversion efficiency with a
better short current density of ∼23.4 mA cm−2, under simulated
AM 1.5, 100 mW cm−2 illumination. Electrochemical impe-
dance measurements revealed that the devices constructed uti-
lizing newly synthesized dyes, with and without hexyloxy spacer
groups, and added tri-phenyl amine or 9-phenyl-9H-carbazole
donor groups attached on the ancillary ligands have improved
charge generation and transport properties than N3 dye. It
would be interesting to use gel electrolyte21 and compact metal
oxide layers of relatively high band gaps22 for improving both
the stability and performance.
Table 2 The standard device electronic parameters extracted from the
J–V data for N3 and HMP dyes (onto ZnO NCs-based photoanodes)
Dye
Jsc (mA cm−2
)
Voc (V)
ff
η%
N3
HMP-9
HMP-11
13.8
16.7
23.4
0.61
0.58
0.61
0.58
0.55
0.49
4.94
5.34
7.09
The high photocurrent in HMP-11 sensitized devices is attri-
buted to its high extinction coefficient which increases its light
harvesting efficiency and to the superior electron transport
through the device. We believe that further improvement in Voc
and fill factor (ff) values can be achieved using surface treat-
ment such as 4-tert-butylpyridine, Li(CF3SO2)2N and forming
the core–shell structures, etc.16,17
As seen in Fig. 3a, the photocurrent action spectra show the
highest spectral response for the red shifted HMP-11 dye with
a peak incident photon-to-current conversion efficiency (IPCE)
of about 77% at 464 nm. The HMP-9 dye with four hydro-
phobic hexyloxy substituents attached to impede the tri-iodide
ions from reaching the ZnO NC surface and to prevent mole-
cular aggregation due to π–π stacking, disadvantageously, has
reduced the IPCE value to 51%. This is due to an intermolecu-
lar quenching or clouding of long alkyl, non-conjugated, hexyl-
oxy substituents that generally diminish the light absorption
by the filtering effect. Despite the higher ε in HMP-9, it exhi-
bits lower IPCE and η% (discussed later) values suggesting that
there is a need of indepth investigation on the charge gene-
ration, collection and recombination kinetics across electrolyte/
dye/ZnO NC/indium-tin-oxide (ITO) interfaces. The maximum
IPCE value of the HMP-11 dye is supposed to be obtained from
better MLCT and hole conducting carbazole moieties. Electro-
chemical impedance spectroscopic (EIS) analysis is often per-
formed to understand the interfacial charge transfer
resistance, electron transport rate and recombination effect in
DSSCs.18 Impedance spectra of DSSCs generally exhibit three
semicircles. A small semicircle at a high frequency region cor-
responds to the interface between platinum and the electro-
lyte, a big semicircle in the intermediate frequency range
corresponds to photoanode–electrolyte interface whereas one
in the low frequency range is related to Warburg impedance of
the electrolyte. Fig. 3b shows the Nyquist plots of ZnO NC-
The authors wish to thank the Basic Science Research
Program through the National Research Foundation of Korea
(NRF), funded by the Ministry of Education (2013009768).
Authors (R.S. Mane and Mu. Naushad) also extend their grati-
tude to the Visiting Professor (VP) unit of King Saud University
(KSU) for their kind support.
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