Engineering of osmium(II)-based light absorbers for dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.201200071

Panchromatic Os^II sensitizers for dye-sensitized solar cells (DSCs) were prepared. A DSC based on TF-52 (see picture) showed promising performance characteristics: short-circuit photocurrent density J _SC=23.3 mA cm^-2, open-circuit photovoltage V _OC=600 mV, fill factor (FF)=0.633, and power conversion efficiency =8.85 % under AM 1.5G simulated one-sun irradiation. Copyright

Full text of DOI:10.1002/anie.201200071

Angewandte
Chemie
DOI: 10.1002/anie.201200071
Solar Cells
Engineering of Osmium(II)-Based Light Absorbers for Dye-Sensitized
Solar Cells**
Kuan-Lin Wu, Shu-Te Ho, Chun-Cheng Chou, Yuh-Chia Chang, Hsiao-An Pan, Yun Chi,* and
Pi-Tai Chou*
The light absorber or sensitizer is one of the most important
components of dye-sensitized solar cells (DSCs). Adequate
engineering of this material allows DSCs to achieve a fine
balance among higher solar energy-to-electricity conversion
efficiency, lower manufacturing costs, and better long-term
stability. The most efficient DSCs to date are fabricated with
transition metal based sensitizers.[1] For example, Grꢀtzel and
co-workers recently demonstrated that a ZnII porphyrin with
donor–acceptor substituent shows a remarkable power con-
version efficiency of h ꢀ 13% under illumination with stan-
dard AM 1.5G simulated sunlight.[2] Furthermore, many RuII
sensitizers were also known to attain efficiencies greater than
10%,[3] long before the discovery of the above ZnII dye.
Besides these successes, a few quaterpyridine RuII sensitizers
showed notable absorption in the far-red to near-infrared
(NIR) region,[4] with the potential to harvest lower energy
protons needed for higher current density.
In this study, the design of OsII sensitizers conceptually
takes advantage of our previously reported RuII sensitizer TF-
1, which contains 4,4’,4’’-tricarboxy-2,2’:6,2’’-terpyridine
(H3tctpy) and dianionic 2,6-bis(1,2-pyrazol-5-yl)pyridine che-
lating ligands (Scheme 1).[9] This RuII-based sensitizer showed
panchromatic absorption extending to 830 nm and an oxida-
With a view to harvesting lower energy photons, OsII-
based sensitizers seem to be an excellent option for expanding
the spectral response well into the NIR region.[5] First, OsII
polypyridine complexes tend to show lower energy metal-to-
ligand charge-transfer (MLCT) transition, as a consequence
of the lower oxidation potential compared to their RuII
counterparts.[6] In addition, larger spin–orbit coupling for
the heavier OsII cation, in theory, induces nontrivial absorp-
Scheme 1. RuII sensitizers TF-1 and TF-5.
tion potential of 0.94 V versus the normal hydrogen electrode
(NHE) that ensures efficient regeneration of the oxidized
sensitizers. However, if the identical architecture were
adopted, the oxidation potential of the corresponding OsII
sensitizer is predicted to be much less positive.[6,10] This hurdle
can be circumvented by replacing pyrazolate with triazolate
with aim of decreasing the electron density at the central OsII
ion. This hypothesis is supported by the prior preparation of
a relevant triazolate-based RuII sensitizer, namely, TF-5 (see
Scheme 1). The oxidation potential of TF-5 is shifted to
1.19 V (vs. NHE), which is 0.25 V higher in energy than that
of TF-1.
Encouraged by this preliminary result, we focused on the
synthesis and characterization of the respective OsII triazo-
lates (see Scheme 2) and DSCs based thereon, which show
unprecedented JSC values and the highest overall conversion
efficiency among all current OsII-based DSCs.
The required 2,6-bis(3-trifluoromethyl-1H-1,2,4-triazol-5-
yl)pyridine ligand was prepared from commercially available
pyridine-2,6-dicarbonitrile, followed by triazole cyclization by
known procedures.[11] The 4-thiophene-substituted ligand was
synthesized from 4-chloropyridine-2,6-dicarbonitrile by
Suzuki coupling, followed by triazole cyclization. The OsII
complexes TF-51 and TF-52 were obtained by addition of the
corresponding 2,6-bis(1,2,4-triazol-5-yl)pyridine to [Os-
(tcetpy)Cl3] in xylenes (tcetpy = 4,4’,4’’-tricarboethoxyl-
2,2’:6’,2’’-terpyridine). The carboethoxyl groups were then
hydrolyzed in basified acetone, followed by acidification to
pH 3 to precipitate the products, which were isolated in yields
3
tion of the MLCT states extended to even lower energy.
Thus, appropriately designed OsII sensitizers should display
a much broader absorption profile and faster electron
injection from both nonthermalized 1MLCT and thermalized
3MLCT excited states.[7] We expect that such a photophysical
property should be important to both the DSC community
and groups whose interests are in developing sensitizers for
water splitting with dye-sensitized oxide semiconductors.[8]
[*] K.-L. Wu, Dr. S.-T. Ho, C.-C. Chou, Prof. Y. Chi
Department of Chemistry and Low-Carbon Energy Research Center,
National Tsing Hua University
Hsinchu, Taiwan 30013 (R.O.C.)
E-mail: ychi@mx.nthu.edu.tw
Y.-C. Chang, H.-A. Pan, P.-T. Chou
Department of Chemistry and Center for Emerging Material and
Advanced Devices, National Taiwan University
Taipei, Taiwan 10617 (R.O.C.)
E-mail: chop@ntu.edu.tw
[**] This research was supported by National Science Council of Taiwan
under grants NSC 100-2119-M-002-008 and NSC-98-3114-E-007-
005.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200071.
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and its potential per se as the true panchromatic absorber.
The emission spectra of TF-51 and TF-52, which show peak
wavelengths of 860 and 856 nm, respectively, are shown in
Figure S1 of the Supporting Information. The calculated
energy levels for frontier orbitals of TF-5, TF-51, and TF-52
that are involved in the S0!S1 and S0!T1 transitions, the
associated orbital transition analyses, and detailed assignment
of lower lying electronic transitions based on time-dependent
(TD) DFT calculations of all three sensitizers are provided in
Table S1 and Figure S2 of the Supporting Information. The
lowest lying transition in both singlet and triplet manifolds
mainly involves electron transfer from the metal center and
2,6-bis(1,2,4-triazol-5-yl)pyridine ligand to the tricarboxyter-
pyridine ligand, and hence has substantial MLCT character
(> 35%, see Supporting Information Table S1). The calcu-
lated S0!S1 and S0!T1 energy gap for both TF-51 and TF-52
extending to about 900 nm are significantly lower than that of
TF-5, consistent with the UV/Vis absorption measurements.
Cyclic voltammetric data for TF-51 and TF-52 are listed in
Table 1. The Os2+/Os3+ oxidation potentials (E8’ox) of TF-51
Scheme 2. OsII sensitizers TF-51 and TF-52.
Table 1: Photophysical and electrochemical data of TF sensitizers.
of about 40% based on [Os(tcetpy)Cl3] as limiting reagent,
which are comparable to the best yields reported for OsII
polypyridine complexes.[12]
[b]
[c]
Dye
labs [nm] (e [Lmolꢁ1 cmꢁ1])[a]
E8’ox
E0–0
E8’*[d]
TF-5
340 (22764), 384 (12642),
491 (11955), 667 (2291)
335 (22503), 382 (11799),
436 (13204), 495 (12767),
601 (3804), 778 (2514)
324 (32321), 391 (13305),
444 (20956), 504 (23007),
610 (6708), 766 (3105)
396 (8847), 549 (5813),
601 (7330)
1.19
1.91
1.61
ꢁ0.72
The UV/Vis spectra of OsII sensitizers TF-51 and TF-52 in
DMF solution are depicted in Figure 1, together with those of
RuII references N749 and TF-5. The lowest lying MLCT
absorption bands for TF-51 and TF-52 are located at 778 and
TF-51
TF-52
N749
0.94
0.91
0.89
ꢁ0.67
ꢁ0.70
ꢁ0.81
1.61
1.70
[a] Absorption and emission spectra were measured in DMF solution.
[b] Oxidation potentials of dyes were measured in DMF with 0.1m
[TBA][PF6] and at a scan rate of 50 mVsꢁ1. They were calibrated with Fc/
Fc+ as internal reference and converted to the NHE scale by addition of
0.63 V. [c] E0–0 was determined from the intersection of the absorption
and tangent of the emission peak in DMF. [d] E8’* was calculated as
E8’oxꢁE0–0
.
and TF-52 were measured to be 0.94 and 0.91 V, respectively,
which are comparable to that of hypothetical Iꢁ/I2Cꢁ couple
ꢁ
(ca. 0.79–0.93 V vs. NHE), but are greater than that of Iꢁ/I3
( ꢀ 0.35 V vs. NHE), and hence ensure optimal dye regener-
ation.[13] On the other hand, the excited-state oxidation
potentials (E8’*) of TF-51 and TF-52, derived from both
oxidation potential and optical energy gap, are ꢁ0.67 and
ꢁ0.70 V, which are marginally higher than that of the
conduction band edge of TiO2 (ꢁ0.5 V vs. NHE). With the
lack of sufficient driving voltage for efficient electron
injection, it becomes essential to introduce additives such as
Li+ cation in the electrolyte to further lower the TiO2 band-
edge potential and hence enhancing the photocurrent.
The DSC performance parameters are summarized in
Table 2. The corresponding cells were prepared by using
a double-layered TiO2 film, which consisted of a transparent
15 mm absorption layer (composed of 20 nm particles) and
a 7 mm scattering layer (composed of 400 nm particles),
Figure 1. Absorption spectra of TF-5, TF-51, TF-52,and N749 in DMF.
Inset: absorption spectra for all samples adsorbed on 6 mm mesopo-
rous TiO2 thin film.
766 nm with extinction coefficients of 2514 and
3105 Lmolꢁ1 cmꢁ1, respectively; both are substantially red-
shifted and more intense than those of N749 and TF-5.
Moreover, dodecylthienyl-substituted sensitizer TF-52
showed even better absorptivity of those peaks across the
whole spectral region. Of particular importance is that the
peak at 610 nm showed an equal intensity to the 601 nm signal
of N749, which demonstrates the pivotal contribution of
thienyl pendant group to enhancing the electronic transitions
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excitation, may eject an electron from either 1MLCT or
Table 2: Performance of DSCs under AM 1.5G one-sun irradiation.
3MLCT state. Thermodynamically, the 1MLCT state lies
substantially higher in energy than the TiO2 conduction
band, and thus the difference is much less affected by in situ
deprotonation of carboxyl anchors.
Dye
J
SC [mAcmꢁ2
]
VOC [V]
FF
h [%]
Notes
TF-5
17.2
18.0
17.9
20.1
19.7
23.3
20.7
0.66
0.64
0.57
0.56
0.62
0.60
0.60
0.715
0.716
0.648
0.664
0.622
0.633
0.661
8.12
8.25
6.61
7.47
7.60
8.85
8.21
[a]
[b]
[a]
[b]
[a]
[b]
[a]
TF-51
TF-52
N749
The J–V characteristics and IPCE action spectra of
devices employing the TBA+-exchanged sensitizers are
shown in Figure 2a and b. In the IPCE spectrum, TF-52
obviously covers a wider spectral response from 380 to
960 nm and reaches a maximum of 78% at 480 nm, whereas
[a] All devices were fabricated by using a 15+7 mm TiO2 anode with
4ꢀ4 mm working area. The dye (0.3 mm) was dissolved in absolute
ethanol with 20 vol% DMSO and 0.6 mm of deoxycholic acid. The
electrolyte consisted of 0.6m DMPII, 0.05m I2, 0.5m TBP, and 0.6m LiI in
acetonitrile. Device performance was measured with a 6ꢀ6 mm shadow
mask. [b] The dye solution was alternatively prepared with addition of
2 equiv of [TBA][DOC] to initiate H+/TBA+ metathesis of sensitizers.
stained with a 0.3 mm dye solution in the presence of 0.6 mm
of deoxycholic acid, as this additive is known to improve DSC
performance by blocking surface defects.[14] The electrolyte
solution consisted of 0.6m 1,2-dimethyl-3-propylimidazolium
iodide (DMPII), 0.05m I2, 0.5m tert-butylpyridine (TBP), and
0.6m LiI in acetonitrile. The concentration of Li+ used in the
present study (0.6m) falls between that reported for typical
RuII-based (0.1m)[3b] and OsII-based DSCs (2.0m).[5c,d] Thus,
a less positive TiO2 conduction-band potential can be
achieved, which, in turn, could afford a higher open-circuit
photovoltage VOC, which is also ensured by the addition of
0.5m of TBP to the electrolyte.[15] Moreover, higher Iꢁ
concentration (a total of 1.2m) in the electrolyte is expected
to offset the higher electron concentration in TiO2 and to
impede the undesired recombination between electrons in
TiO2 and oxidized sensitizers.[16]
Under full sunlight irradiation (AM 1.5G, 100 mWcmꢁ2),
the TF-52-sensitized device exhibited a short-circuit photo-
current density JSC of 19.7 mAcmꢁ2, a VOC of 0.62 V, and a fill
factor (FF) of 0.622, corresponding to a power conversion
efficiency h of 7.60%. Since the neutral TF-52 dye has an
excited-state oxidation potential close to the conduction band
of TiO2, electron injection would not be optimal. To
compensate this unfavorable situation, we then added
2 equiv of tetrabutylammonium deoxycholate [TBA][DOC]
to the dye solution, so that in situ metathesis of carboxylic
acid/carboxylate anion of dyes can take place in the dye
solution to raise the virtual oxidation potential of the excited
sensitizers.[17] As expected, this maneuver increased JSC to
23.3 mAcmꢁ2 and FF to 0.633, but slightly decreased VOC to
0.60 V at the same time, giving an enhanced efficiency of h =
8.85%. To provide further support for this approach, DSCs
employing regular RuII sensitizer TF-5 and OsII sensitizer TF-
51 gave conversion efficiencies of 8.25 and 7.47%, respec-
tively. After H+/TBA+ metathesis, OsII sensitizer TF-51 also
showed a much larger enhancement of 0.86% versus that of
the less affected RuII sensitizer TF-5 (0.13%). Despite the
small variation of E8’* values between TF-5 and TF-51, the
invariance of the latter is interesting. Due to the heavier OsII
atom and hence faster rate of intersystem crossing, it is likely
that excited TF-51 ejects an electron mainly from the lowest
lying 3MLCT state. Conversely, the RuII counterpart TF-5, on
Figure 2. a) J–V characteristics measured under AM 1.5 conditions,
b) Incident photon-to-electron conversion efficiency (IPCE) action
spectra, and c) electrochemical impedance spectra measured in the
dark at a forward bias of 0.6 V for the cells employing various TF dyes
and N749.
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the RuII reference TF-5 displays a maximum of 83% at
510 nm and tails off at only 800 nm.
density should pave an alternative route to the summit of
DSC efficiency.
Electrochemical impedance spectroscopy (EIS) was per-
formed in the dark and with a forward bias of 0.6 V. The
Nyquist plots of DSCs based on different sensitizers are
shown in Figure 2c. Two semicircles from left to right in the
Nyquist plot represent the impedances of charge transfer
Received: January 4, 2012
Revised: February 27, 2012
Published online: && &&, &&&&
Keywords: chelates · N ligands · osmium · solar cells ·
(R ) on the Pt counterelectrode and charge recombination
Pt
.
tridentate ligands
(Rr) at the interface of the TiO2/electrolyte.[18] As a result, the
radius of the second semicircle reveals a descending order of
TF-5 > N749 ꢀ TF-52 > TF-51, which is consistent with the
trend of VOC values (see Table 2). A smaller Rr value indicates
faster electron recombination from TiO2 to electron acceptors
in an electrolyte and thus results in lower VOC. The results also
manifest the advantage of molecular engineering from TF-51
to TF-52.
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entire 1000 h testing period at 608C under one-sun light
soaking, the photovoltaic parameters JSC, VOC, FFof the TF-52
based cell varied only slightly from the initial values
(Figure 3). The final h retained 97% of its initial value, that
is, a small drop of VOC by 40 mV is compensated by an
increase in JSC. Such an impressive performance is remarkable
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Figure 3. Evolution of solar-cell parameters of TF-52 under AM 1.5G
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In summary, we have demonstrated that OsII-based
sensitizers can be used to construct highly efficient DSCs,
particularly when a moderate excess of LiI was incorporated
into the electrolyte to promote electron injection into TiO2.
Our experiments give a comprehensive guideline, not only to
achieving panchromatic absorption, but also to controlling
the energy difference of the TiO2 band edge and E8’ox of the
ꢁ
sensitizer and the gap between the redox potential of Iꢁ/I3
couple and E8’ox of the sensitizer. The successful design of
OsII-based sensitizers in combination with their capability to
display both panchromatic absorption and high optical
4
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Communications
Solar Cells
Panchromatic OsII sensitizers for dye-
sensitized solar cells (DSCs) were pre-
pared. A DSC based on TF-52 (see
picture) showed promising performance
characteristics: short-circuit photocur-
rent density JSC =23.3 mAcmꢁ2, open-cir-
cuit photovoltage VOC =600 mV, fill factor
(FF)=0.633 and power conversion effi-
ciency h=8.85% under AM 1.5G simu-
lated one-sun irradiation.
K.-L. Wu, S.-T. Ho, C.-C. Chou,
Y.-C. Chang, H.-A. Pan, Y. Chi,*
P.-T. Chou*
&&&& — &&&&
Engineering of Osmium(II)-Based Light
Absorbers for Dye-Sensitized Solar Cells
6
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