Dye sensitized solar cells: TiO2 sensitization with a bodipy-porphyrin antenna system

Article abstract of DOI:10.1021/la9031927

A zinc porphyrin derivative (2) and zinc porphyrin-bodipy dyad (3) have been prepared and applied to dye-sensitized solar cells (DSSCs). On the basis of absorption and fluorescence excitation spectra, dyad 3 efficiently transfers energy from the bodipy to zinc porphyrin constituent. The 3-sensitized solar cell demonstrates higher solar spectral coverage, based on incident photon to current efficiency (IPCE) spectra, and an improved power conversion efficiency (η = 1.55%) compared to that of the 2-sensitized cell (η = 0.84%). The better performance of the 3-sensitized cell is attributed largely to the gain in spectral absorbance provided by the bodipy constituent of 3. Also evident, however, are secondary effects reflecting (a) fill-factor improvement and (b) a slight gain in porphyrin red-edge absorbance due to bodipy-conjugate formation.

Full text of DOI:10.1021/la9031927

pubs.acs.org/Langmuir
© 2009 American Chemical Society
Dye Sensitized Solar Cells: TiO2 Sensitization with a Bodipy-Porphyrin
Antenna System
Chang Yeon Lee and Joseph T. Hupp*
Department of Chemistry, 2145 Sheridan Road, Northwestern University, Evanston, Illinois 60208
Received August 26, 2009. Revised Manuscript Received October 19, 2009
A zinc porphyrin derivative (2) and zinc porphyrin-bodipy dyad (3) have been prepared and applied to dye-sensitized
solar cells (DSSCs). On the basis of absorption and fluorescence excitation spectra, dyad 3 efficiently transfers energy from
the bodipy to zinc porphyrin constituent. The 3-sensitized solar cell demonstrates higher solar spectral coverage, based on
incident photon to current efficiency (IPCE) spectra, and an improved power conversion efficiency (η = 1.55%) compared
to that of the 2-sensitized cell (η = 0.84%). The better performance of the 3-sensitized cell is attributed largely to the gain in
spectral absorbance provided by the bodipy constituent of 3. Also evident, however, are secondary effects reflecting (a) fill-
factor improvement and (b) a slight gain in porphyrin red-edge absorbance due to bodipy-conjugate formation.
Introduction
kinetics10 of the two systems. The inferior performance of
porphyrin dyes is attributed, in part, to I3--porphyrin complex
formation that increases the concentration of redox shuttle at the
semiconductor surface, accelerating charge interception of in-
jected electrons by the redox shuttle (with consequent lowering of
photovoltages), and quenching of a fraction of the photoexcited
dyes before injection can occur (with consequent lowering of
photocurrent).11 Additionally, porphyrin absorption spectra,
though strong in the Q and B band regions, are typically poor
matches to the overall visible-light distribution of the solar
spectrum.
To address the poor solar spectral coverage, extended porphyr-
in π systems have been investigated by several groups. This
approach can lead to significantly broadened B band and red-
shifted Q-band absorptions that fill out the solar spectrum. For
instance, elongation of π systems in meso- and β-naphthalene
fused porphyrins improves power conversion efficiencies by 50%
relative to the nonfused systems.9b Similarly, attaching a con-
jugated anchoring group at the β position of the porphyrin ring
has been successfully applied to give the best performing por-
phyrin based cell to date (η = 7.1%).9c,9d
Dye-sensitized solar cells (DSSCs) have received considerable
attention as a low cost alternative to conventional silicon based
solar cells.1 These photoelectrochemical cells employ molecular
dyes to sensitize high-surface area, mesoporous photoelectrodes
consisting of transparent semiconducting materials such as TiO2,
SnO2,2 ZnO,3 and Nb2O3.4 The most efficient solar cells to date
are ca. 11% efficient.5 Efforts to improve cell efficiencies have
focused on the three major components of DSSCs: the dye, the
redox shuttle and the semiconducting electrode.6 In particular,
efforts to develop new, improved dyes have investigated modified
ruthenium polypyridyl complexes7 and numerous organic dyes8
including porphyrins.9 Porphyrin derivatives are particularly
attractive as sensitizers in DSSCs due to their structural similarity
to chlorophylls in natural photosynthetic systems as well as their
strong and tunable visible absorption properties. However, most
porphyrin based cells display lower performance than ruthenium
polypyridyl complex based cells despite the similar charge transfer
*Corresponding author. E-mail: j-hupp@northwestern.edu.
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antenna light harvesting molecule, as it has numerous favorable
properties, such as high fluorescence quantum yield, low rates of
intersystem crossing, large molar absorption coefficients, rela-
tively long excited state lifetimes, and excellent photostability.12 It
can also be tailored by a variety of structural substitutions to tune
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3760 DOI: 10.1021/la9031927
Published on Web 11/03/2009
Langmuir 2010, 26(5), 3760–3765

Lee and Hupp
Article
Chart 1
trated under reduced pressure. Recrystallization from hexane
yielded the compound 4 as white solid (32 g, 80% yield). H
1
NMR (CDCl3): δ 7.17 (t, J = 8.5 Hz, 1H), 6.49 (m, 3H), 3.95
(t, J = 6.5 Hz, 4H), 1.79 (m, 4H), 1.46 (m, 4H), 1.25-1.38 (br,
32H), 0.91 (t, J = 6.5 Hz, 6H). 13C NMR (CDCl3): δ 160.37,
129.76, 106.60, 101.36, 67.98, 31.96, 29.70, 29.68, 29.64, 29.62,
29.44, 29.39, 29.30, 26.08, 22.73, 14.17.
2,6-Di(dodecyloxy)benzenaldehyde (5). A three-neck flask
was fitted with a pressure-equalizing addition funnel and charged
with 4 (19.2 g, 0.0430 mol) and tetramethylethylenediamine
(TMEDA) (8 mL) in 120 mL of diethyl ether. The solution was
degassedwith a stream of N2 for 15 min and cooledto 0 °C. Under
N2, n-butyllithium (1.6 M solution in hexanes 0.0645 mol)
was added dropwise over 20 min and allowed to stir for 3 h.
After warming to room temperature, dimethylformamide (DMF)
(5.3 mL) was added dropwise, and the reaction was stirred for
an additional 2 h. The reaction was quenched with water, and
the product was extracted with ether (3 ꢀ 150 mL), dried over
MgSO4, and concentrated to leave an oily residue. The product
was recrystallized from hexanes to yield a white solid (16.3 g, 80%
yield). 1H NMR (CDCl3): δ 7.35 (t, J = 10.0 Hz, 1H), 6.51(d, J =
5.0 Hz 2H), 4.00 (t, J = 7.5 Hz, 4H), 1.80 (m, 4H), 1.44 (m, 4H),
1.25-1.38 (br, 32H), 0.87 (t, J = 5.0 Hz, 6H). 13C NMR (CDCl3):
δ 189.20, 161.65, 135.55, 114.65, 104.43, 68.86, 31.94, 29.70, 29.66,
29.64, 29.59, 29.52, 29.47, 29.39, 29.07, 26.00, 22.71, 14.13.
meso-(2,6-Di(dodecyloxy)phenyl)dipyrromethane (6). 5(4.50g,
9.48 mmol) was added to 16.4 mL of pyrrole and degassed for
10 min with a stream of N2. Trifluoroacetic acid (70.0 μL) was
then added in a portion, and the reaction mixture was stirred at
room temperature for 2 h. NaOH (1.32 g, 32.9 mmol) was added
to quench the reaction, followed by additional stirringfor 1 h. The
reaction mixture was filtered and the volatiles were evaporated
from the filtrate using a rotary evaporator. The residue was
subjected to column chromatography over silica gel (hexanes/
CH2Cl2/ethyl acetate 7:2:1 v/v/v) to yield the desired product as a
slightly yellow oil (2.52 g, 45%). Compond 6 was quickly used to
prepare the next porphyrin molecule without further character-
ization due to its instability in the air.
its fluorescence characteristics, allowing development of several
applications in the fields of biological labeling,13 molecular
wires,14 and artificial photosynthetic systems.15
We herein report the details of the preparation and character-
ization of a new zinc porphyrin-bodipy dyad (3) along with
zinc porphyrin derivative (2) and their applications in DSSCs
(Chart 1). In the dyad, the accessory pigment, bodipy, can act as
an antenna light harvesting molecule to fill out the blue-green
region of the spectrum where porphyrin does not absorb. We find
that the dyad indeed does behave as a minimalistic antenna-type
chromophoric system for photoelectrode sensitization. While
there exist a handful of examples of inorganic systems that
functionas antennasensitizer for high-area semiconductor photo-
electrodes,1,16 organic examples thus far are rare.
Experimental Section
Materials and General Procedures. All chemicals were
obtained from commercial sources and used without further
purification. All of the reactions and manipulations were carried
out under N2 with the use of standard inert-atmosphere and
Schlenk techniques. Solvents used in reactions were dried by
standard procedures. Absorbance spectra were obtained using a
Varian Cary 5000 UV-vis-NIR spectrophotometer. Steady-
state fluorescence measurements were performed on a Jobin
Yvon-SPEX Fluorolog-3 spectrofluorimeter. Nuclear magnetic
resonance (NMR) spectra for all the synthesized compounds were
recorded on a Varian INOVA 500 NMR spectrometer (499.773
MHz for 1H NMR, 125.669 MHz for 13C NMR). Atomic layer
deposition was performed with a Savannah 100 instrument
(CambridgeNanotechInc.). 2,6-diethyl-4,4-difluoro-1,3,5,7-tetra-
methyl-8-(4-iodophenyl)-4-bora-3a,4a-diazasinadcene (1) was
synthesized according to a literature procedure.17
[5,15-Bis(ethynyl)-10,20-bis[2,6-di(dodecyloxy)phenyl]por-
phinato]zinc (7). Compound 6 (2.05 g, 3.47 mmol) and trimethyl-
silylpropynal (0.52 mL, 3.47mmol) were added to 547 mLofdried
CH2Cl2 and the solution was degassed for 5 min with a stream of
N2. BF3 2Et2O (87.5 μL) was added slowly, and the reaction
3
mixture wasallowedtostirunderN2 atmospherefor5 minat0 °C.
Then 837 mg of DDQ was added. After stirring for 30 min, 2 mL
of pyridine was added. The precipitates were filtered off and the
volatiles were removed under reduced pressure. The resulting
residue was purified by silica-gel column chromatography
(hexane/dichloromethane (1:1 v/v)) to afford pure 5,15- bis-
[(trimethylsilanyl)ethynyl]-10,20-bis[2,6-di(dodecyloxy)phenyl]por-
phyrin, which was further metalated with zinc(II) acetate accord-
ing to the literature procedure to afford [5,15-bis[(trimethylsila-
nyl)ethynyl]-10,20-bis[2,6-di(dodecyloxy)phenyl]porphinato]zinc.
Then, 400 mg of [5,15-bis[(trimethylsilanyl)ethynyl]-10,20-bis-
[2,6-di(dodecyloxy)phenyl]porphinato]zinc obtained from the
above reaction was treated with TBAF (1.63 mL, 1 M in THF)
in THF (25 mL) for 20 min. The reaction mixture was quenched
with acetic acid (50 μL) and MeOH (100 mL). The resultant deep
green crude product was purified by silica-gel column chroma-
tography (hexane/THF (7:3 v/v)) to afford 315 mg of [5,15-
bis(ethynyl)-10,20-bis[2,6-di(n-hexoxy)phenyl]porphinato]zinc
(15% overall yield).). 1H NMR (CDCl3): δ 9.54 (t, J = 5.0 Hz,
4H), 8.78 (d, J = 5 Hz 4H), 7.62 (t, J = 7.5 Hz, 2H), 6.92 (d, J =
10.0 Hz 4H), 3.99(s, 2H), 3.76 (t, J = 5.0 Hz, 8H) 1.12-1.17 (m,
10H), 1.00-1.10 (m, 16H), 0.91 (m, 8H), 0.75-0.87 (br, 26H),
0.64 (m, 8H), 0.49 (m, 16H), 0.36 (m, 8H). 13C NMR (CDCl3): δ
159.97, 151.83, 150.83, 131.95, 130.68, 129.79, 120.93, 114.86,
105.09, 98.29, 86.87, 82.47, 68.57, 64.46, 31.92, 29.53, 29.44, 29.34,
29.32, 29.13, 28.79, 28.66, 25.27, 23.04, 22.73, 14.18. MS
(MALDI-TOF): m/z 1310 (M þ Hþ).
Synthesis andCharacterization. 1,3-Di(dodecyloxy)ben-
zene. (4). Resorcinol (10 g, 0.0908 mol) and n-bromododecane
(50 mL) were successively added to a stirred suspension of
potassium hydroxide (40 g, 0.713 mol) in 160 mL of dimethyl
sulfoxide (DMSO). The reaction was allowed to stir overnight
at room temperature and was then quenched with 150 mL of
water. The product was extracted with dichloromethane-
(CH2Cl2), dried over magnesium sulfate (MgSO4), and concen-
(13) Martin, H., Rudolf, H., Vlastimil, F., Eds.; Fluorescence Spectroscopy in
Biology: Advanced Methods and their Applications to Membranes, Proteins, DNA,
and Cells; Springer-Verlag: Heidelberg, Germany, 2005.
(14) (a) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759–9760.
(b) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D. F. J. Am. Chem.
Soc. 1996, 118, 3996–3997.
(15) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100–110.
(16) (a) Wolpher, H.; Sinha, S.; Pan, J.; Lomoth, R.; Bergquist, J.; Sun, L.;
€
Sundstrom, V.; Akermark, B.; Polivka, T. Inorg. Chem. 2007, 46, 638–651. (b) Lees,
A. C.; Kleverlaan, C. J.; Bignozzi, C. A.; Vos, J. G. Inorg. Chem. 2001, 40, 5343–5349.
(c) Ardo, S.; Meyer, G. J. Chem. Soc. Rev. 2009, 38, 115–164and references therein. .
(17) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474–14475.
Langmuir 2010, 26(5), 3760–3765
DOI: 10.1021/la9031927 3761

Article
Lee and Hupp
Scheme 1. Reagent and Conditionsa
a Key: (i) n-Bromododecane, KOH, DMSO, room temperature; (ii) n-butyl lithium, tetramethylethylenediamine, DMF, diethyl ether, room
temperature; (iii) pyrrole, TFA, room temperature; (iv) trimethylsilylpropynal, BF3 2Et2O, DDQ, CH2Cl2, room temperature; (v) Zn(OAc)2, CH2Cl2,
3
room temperature; (vi) TBAF, THF, room temperature; (vii) n-butyl lithium, trihexylsilylchloride, THF, -78 °C (viii) 4-iodobenzoic acid, CuI, PPh3,
Pd2(dba)3, toluene/TEA (5:1), room temperature; (ix) 1, 4-iodobenzoic acid, CuI, PPh3, Pd2(dba)3, toluene/TEA (5:1), room temperature.
[5-Ethynyl-15-(trihexylsilanyl)ethynyl-10,20-bis[2,6-di(dode-
cyloxy)phenyl]porphinato]zinc (8). A solution of 7 (250 mg, 0.20
mmol) in THF (12 mL) was allowed to cool to -78 °C and n-BuLi
(1.6 M in hexane, 125 μL, 0.2 mmol) was added slowly. After the
addition was completed, the reaction mixture was allowed to
warm to room temperature. After the reaction mixture was again
cooled to -78 °C, trihexylsilyl chloride (88 μL, 0.24 mmol) was
added dropwise. After being stirred for 15 min at -78 °C, the
reaction mixture was then allowed to stir for 6 h at room
temperature. The volatiles were removed under reduced pressure
and the residue was extracted with CH2Cl2. The extract was
washed with water, dried over anhydrous MgSO4 and evaporated
to dryness in vacuo to afford deep green oily product which
was purified by silica-gel column chromatography (hexane/THF
(9:1 v/v)) to afford pure [5-ethynyl-15 (trihexylsilanyl) ethynyl-
10,20-bis[2,6-di(dodecyloxy)phenyl]porphinato]zinc, 8, (143 mg,
46% yield). 1H NMR (CDCl3): δ 9.47 (d, J = 5.0 Hz, 2H), 9.45(d,
J = 5.0 Hz, 2H), 8.69 (d, J = 10.0 Hz, 2H), 8.68 (d, J = 10.0 Hz,
2H), 7.53 (t, J = 10.0 Hz, 2H), 6.84 (d, J = 10.0 Hz 4H), 3.91(s,
1H), 3.67 (m, 8H) 1.58-1.64 (m, 6H), 1.35-1.41 (m, 6H),
1.20-1.28 (m, 12H), 0.62-1.10 (br, 75H), 0.47 (m, 8H),
0.35-0.41 (m, 8H), 0.28-0.34 (m, 8H) 0.21-0.27 (m, 8H). 13C
NMR (CDCl3): δ 159.95, 151.90, 150.76, 150.72 132.00, 131.77,
131.00, 130.66, 129.81, 120.93, 115.00, 109.22, 105.21, 100.46,
98.65, 98.29, 86.67, 82.55, 68.68, 64.52, 33.47, 31.90, 31.73, 29.49,
29.36, 29.31, 29.23, 29.05, 28.72, 28.62, 25.24, 24.41, 23.08, 22.77,
22.71, 14.25, 14.16, 13.92. MS (MALDI-TOF): m/z 1592 (Mþ)
2. A solution of 8 (102 mg, 0.064 mmol), 4-Iodobenzoic acid
(17 mg, 0.070 mmol), CuI (6 mg, 0.032 mmol), and PPh3 (16 mg,
0.064 mmol) in 10 mL of TEA/toluene (1:5 v/v) was degassed with
a N2 stream for 10 min. Pd2(dba)3 (14 mg, 0.016 mmol) was added
all at once and then stirred for 12 h. Solvent was evaporated in
vacuo and then dark residue was purified by silica-gel column
chromatography silica-gel column chromatography (dichlorome-
thane/methanol (95:5 v/v)) to afford pure 2, (66 mg, 60% yield).
1H NMR (CDCl3): δ 9.68 (d, J = 4.5 Hz, 2H), 9.63(d, J = 4.5 Hz,
2H), 8.91 (d, J = 4.5 Hz, 2H), 8.86 (d, J = 4.5 Hz, 2H), 8.24 (d,
J = 8.0 Hz, 2H), 8.08 (d, J = 8.0 Hz, 2H), 7.73(t, J= 8.5 Hz, 2H),
7.03 (d, J = 8.5 Hz 4H), 3.86 (m, 8H) 1.75-1.80 (m, 6H),
1.54-1.57 (m, 6H), 1.38-1.45 (m, 12H), 0.78-1.22 (br, 75H),
0.64 (m, 8H), 0.40-0.57 (br, 24H). MS (MALDI-TOF): m/z
1713 (M þ Hþ)
3. A solution of 7 (90 mg, 0.070 mmol), 4-iodobenzoic acid
(19 mg, 0.076 mmol), 2,6-diethyl-4,4-difluoro-1,3,5,7-tetramethyl-
8-(4-iodophenyl)-4-bora-3a,4a-diaza-sinadcene (1) (38 mg, 0.076
mmol) CuI (6 mg, 0.071 mmol), and PPh3 (37 mg, 0.140 mmol) in
10 mL of TEA/toluene (1:5 v/v) was degassed with N2 stream for
10 min. Pd2(dba)3 (32 mg, 0.035 mmol) was added all at once and
then stirred for 12 h. Solvent was evaporated in vacuo and then
dark residue was purified by silica-gel column chromatography
silica-gel column chromatography (dichloromethane/methanol
(95:5 v/v)) to afford pure 3, (70 mg, 55% yield). 1H NMR
(CDCl3): δ 9.82 (d, J = 4.5 Hz, 2H), 9.76 (d, J = 4.5 Hz, 2H),
9.01 (d, J = 4.5 Hz, 2H), 9.00 (d, J = 4.5 Hz, 2H), 8.28 (d, J = 7.5
Hz, 2H), 8.25 (d, J = 8.0 Hz, 2H) 8.13 (d, J = 7.5 Hz, 2H), 7.84
(t, J = 8.5 Hz, 2H), 7.61 (d, J = 8.0 Hz, 2H), 7.15 (d, J = 8.5 Hz
4H), 4.0 (t, J = 6.0 Hz, 8H), 2.70 (s, 6H), 2.47 (q, J = 6.5 Hz 4H),
1.62 (s, 6H) 1.25-1.33 (m, 8H), 1.01-1.24 (m, 38H), 0.87-0.98
3762 DOI: 10.1021/la9031927
Langmuir 2010, 26(5), 3760–3765

Lee and Hupp
Article
(m, 20H), 0.76-0.82 (m, 8H), 0.65-0.73 (m, 16H), 0.54-0.62
(m, 8H). MS (MALDI-TOF): m/z 1808 (Mþ)
Preparation of Dye-Sensitized Solar Cells. Photoelec-
trodes were prepared on 12 Ω cm-2 FTO-coated glass
(Hartford glass). Blocking layers of TiO2 were deposited using
400 ALD cycles of titanium isopropoxide (Aldrich), TIP, and
water as precursors. TiO2 was grown at 200 °C using reactant
exposure times of 1 and 0.02 s for TIP and H2O, respectively, and
nitrogen purge times of 10 s between exposures. A transparent
TiO2 nanoparticle layer was prepared by doctor blading a paste
of TiO2 nanoparticles (DSL 18NR-T, Dyesol) on the FTO. A
scattering layer of TiO2 nanoparticles (WER4-O, Dyesol)
was subsequently deposited on top of the transparent layer. The
resulting electrodes were annealed at 500 °C in air for 30 min. The
thicknesses of the photoelectrodes were approximately 12 μm,
measured using a Tencor P10 profilometer. After annealing, the
TiO2 electrodes were immediately immersed in a 0.2 mM solution
of 2 and 3 in THF. After 2 h, they were rinsed with THF and
dried with N2. A Surlyn frame (25 μm) was sandwiched between
the open-pore side of the membrane and a platinized FTO
electrode. Light pressure was applied at 130 °C to seal the cell.
A solution of 0.60 M butylmethylimidazolium iodide, 0.03 M I2,
0.10 M guanidinium thiocyanate, 0.50 M 4-tert-butylpyridine
and 0.1 M LiI in 3 mL of acetonitrile-valeronitrile (85:15)
was introduced into the TiO2 cells. Additional Surlyn and a
microscope cover slide sealed the electrolyte into the cell. Cells
were illuminated through an Oriel 81092 filter using the Xe lamp
and excitation monochromator of a Jobin Yvon fluorimeter.
Broadband excitation using the grating at zeroth order was
98 mW cm-2. Optical power was measured with an Ophir 3A-
SH radiometer. A CH Instruments 1200A potentiostat was used
to measure current-voltage curves.
Electrochemical Measurements. Cyclic voltammetry was
carried out on a CH Instruments CH900 electrochemical analyzer
using a conventional three-electrode system consisting of a plati-
num working electrode, a platinum wire electrode, and a pseudo
Ag/AgCl reference electrode. Ferrocene/ferrocenium (þ0.64 V vs
NHE) was used as an internal reference. All measurements were
performed at ambient temperature under nitrogen atomosphere
in dry deoxygenated 0.1 M CH2Cl2 solution of tetrabutylammo-
nium hexaflorophosphate.
Figure 1. (a) UV-vis absorption spectra of 1, 2, and 3 in toluene.
(b) Fluorescence excitation spectra of 1, 2, and 3 with fixed
emission at 650 nm in toluene.
compared to those of zinc tetraphenylporphyrin in toluene,20
reflecting extended π-conjugation. Similar results have been
reported in other ethyne-linked porphyrins.21 The spectrum of 3
exhibits characteristic absorption bands for zinc porphyrin (ZnP)
(Soret band: λmax= 457 nm, Q-bands: λmax= 586, 648 nm) and
bodipy (λmax= 528 nm). The Soret band and Q-bands of 3 are
red-shifted compared to those of 2, indicating the porphyrin
moiety is perturbed by the modification. Specifically, the bodipy
moiety absorbs in the blue-green, a region where the porphyrin
absorbs weakly. In terms of spectral coverage, this absorption
enhances the light-harvesting capability of compound 3. The
fluorescence excitation spectra of compounds 1, 2 and 3 were
measured in toluene by scanning the excitation from 700 to 400
nm with fixed emission at 650 nm corresponding to ZnP. The
excitation profile of 3 is well matched with its absorption
spectrum, indicating that the efficient singlet-state energy transfer
from bodipy to ZnP takes place in compound 3.22 Control
experiments with 2 reveal an excitation spectrum that is well
matched with its absorption spectrum, and (as expected) lacking a
feature between 500 and 550 nm.
Results and Discussion
Synthesis. The synthesis of 2 and 3 is depicted in Scheme 1 and
is based on previously reported porphyrin synthetic strategies.18
The important synthetic intermediate 7 was preparedin 15% yield
from the Lewis acid catalyzed condensation of meso-substituted
dipyrromethane 6 and trimethylsilylpropynal followed by Zinc
metalation and deprotection of trimethylsiliyl group. Compound
8 was prepared in 46% yield by reaction between monolithiated
porphyrin and trihexylsilyl chloride. Title compounds 2 and 3
were prepared in 60 and 55% yield, respectively by Pd-catalyzed
Sonogashira couplings. Structures of the new compounds were
characterized by various spectroscopic methods including 1H
NMR, 13C NMR and MALDI-TOF mass spectra. The bulky
alkoxyphenyl substituents were introduced at two of the four
meso-positions to avoid aggregation19 which is major factor in
decreasing the efficiency of sensitized photocurrent generation.
Absorption Spectra and Fluorescence Excitation Profiles.
Figure 1 shows absorption spectra and fluorescence excitation
profiles of various chromophores in toluene. Compound 2
exhibits typical porphyrin absorption features with a strong Soret
band at 452 nm and moderate Q bands at 582 and 639 nm.
Absorption bands of compound 2 are red-shifted by 30-50 nm
(20) Takahashi, K.; Komura, T.; Imanaga, H. Bull. Chem. Soc. Jpn. 1989, 62,
386–391.
(21) (a) Anderson, H. L. Chem. Commun. 1999, 2323–2330. (b) Lee, C.-W.; Lu,
H.-P.; Lan, C.-M.; Huang, Y.-L.; Liang, Y.-R.; Yen, W.-N.; Liu, Y.-C.; Lin, Y.-S.; Diau,
E. E-G.; Yeh, C.-Y. Chem.;Eur. J. 2009, 15, 1403–1412.
(22) (a) Haugland, R. P.; Yguerabide, J.; Stryer, L. Proc. Natl. Acad. Sci. U.S.A.
1969, 63, 23–30. (b) Stryer, L.; Haugland, R. P. Proc. Natl. Acad. Sci. U.S.A. 1967, 58,
719–726. (c) Hsiao, J.-S.; Krueger, B. P.; Wagner, R. W.; Johnson, T. E.; Delaney, J. K.;
Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.; Donohoe, R. J. J. Am.
Chem. Soc. 1996, 118, 11181–11193.
(18) Lee, C.-H.; Lindsey, J. D. Tetrahedron 1994, 50, 11427–11440.
(19) Splan, K. E.; Hupp, J. T. Langmuir 2004, 20, 10560–10566.
Langmuir 2010, 26(5), 3760–3765
DOI: 10.1021/la9031927 3763

Article
Lee and Hupp
Figure 3. Cyclic voltammograms of 1 (a), 2 (b), and 3 (c) in
CH2Cl2 containing 0.1 M (TBA)PF6 (scan rate = 0.1 V/s).
Figure 2. (a) UV-vis absorption spectra of 2 and 3 on TiO2 and
color photos of the electrods (inset). (b) Fluorescence excitation
spectra of 2 and 3 with fixed emission at 650 nm on TiO2. The
intense peak between 630 and 670 nm is artifactual and arises from
scattering of the excitation beam.
Absorption spectra of 2 and 3 on TiO2 are similar to the
corresponding solution spectra (Figure 2a) but exhibit a small
blue shift, presumably reflecting slight aggregation.23 To prove
energy transfer from bodipy to ZnP occurs on the TiO2 surface,
fluorescence excitation spectra of 2 and 3 were measured on TiO2
in air with fixed emission at 650 nm. As shown in Figure 2b, the
excitation spectrum includes a feature (530 nm) assignable to
bodipy absorption.
Electrochemical Properties. Electrochemical properties of 1,
2 and 3 have been examined by cyclic voltammetric method in
CH2Cl2 solutions with tetrabutylammonium hexafluorophosphate
((TBA)PF6) as the supporting electrolyte. CVs of 1 and 2 show a
reversible redox wave at 1.26 and 0.92 V vs NHE, respectively,
which are obviously attributed to redox reaction at the bodipy and
zinc porphyrin centers (Figure 3, parts a and b). The CV of 3 is a
sum of those of 1 and 2 as expected with redox waves at 0.96 and
1.29 V vs NHE (Figure 3c). Excited oxidation potentials of 2 and 3
are calculated from the ground-state oxidation potentials, and
zero-zero excitation energy E0-0, according to eq 1
Figure 4. (a) Photocurrent action spectra of 2 and 3 sensitized
solar cells (b) J-V curves of 2 and 3 sensitized solar cells under AM
1.5 illumination.
þ
þ
ꢁ
EðS =S Þ ¼ EðS =SÞ -E0 -0
ð1Þ
where E(Sþ/S) is the first oxidation potential of the sensitizer
measured by cyclic voltammetry and E0-0 is the zero-zero
excitation energy obtained by the intersection of the normalized
absorption and emission spectra. The excited-state oxidation
potentials of 2 and 3 are -1.02 and -0.95 V vs NHE, respectively.
€
€
(23) (a) Akins, D. L.; Ozcuelik, S.; Zhu, H.-R.; Guo, C. J. Phys. Chem. 1996,
100, 14390–14396. (b) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys.Chem. B
1998, 102, 1528–1538. (c) Khairutdinov, R. F.; Serpone, N. J. Phys. Chem. B 1999,
103, 761–769. (d) Osuka, A.; Maruyama, K. J. Am. Chem. Soc. 1988, 110, 4454–4456.
3764 DOI: 10.1021/la9031927
Langmuir 2010, 26(5), 3760–3765

Lee and Hupp
Article
These are sufficiently negative of the conduction-band-edge energy
of TiO2 in contact with the DSSC electrolyte (ca. -0.5 V vs NHE)
that electron injection should be straightforward. In addition the
ground- state oxidation potentials are sufficiently positive that
regeneration with iodide should be both thermodynamically and
kinetically feasible.
of 627 mV, and a fill factor (FF) of 0.73, yielding an overall
conversion efficiency (η), derived from the equation η = Jsc
ꢀ
Voc ꢀ FF/input radiation power, of 1.55%. On the other hand,
the 2-sensitizedcell gives a Jsc of2.1 mA/cm-2, Voc of613 mV, and
FF of 0.66, yielding an η of 0.84%. The η value of the 3-sensitized
cell is nearly twice that of the 2-sensitized cell. The higher
efficiency of 3-sensitized cell is primarily attributable to improved
light harvesting by 3.
Photoelectrochemical Properties. Dye-sensitized solar cells
were fabricated using 2 and 3 as photosensitizers for nanocrystal-
line TiO2. A coating of 20 nm-sized TiO2 particles was first screen-
printed on a fluorine-doped SnO2 (FTO) conducting glass elec-
trode and then over coated with 400 nm-sized light scattering
particles. The TiO2 electrode was then immersed in 2 and 3 dye
solution in THF at room temperature for 2 h. Platinized FTO
conducting glass was used as counter electrode. The incident
monochromatic photon-to-current conversion efficiencies
(IPCEs) of cells based on 2 and 3 as sensitizers are shown in
Figure 4a. The IPCE spectra are similar to the corresponding
absorption spectra on TiO2 (Figure 2a) as well as in toluene
(Figure 1a). The IPCE spectrum of 3 is slightly red-shifted
compared to that for 2, consistent with the absorption spectrum
of 3. Notably, the 3-sensitized cell shows greater solar spectral
coverage compared to that of the 2-sensitized cell with absorption
between 500 and 550 nm, consistent with the notion that efficient
energy-transfer-mediated electron injection is achieved by the
bodipy moiety in the 3-sensitized cell. Figure 4b shows the
photocurrent-voltage (J-V) curves of the cells. Under AM 1.5
illumination, the 3-sensitized cell exhibits a short circuit photo-
current density (Jsc) of 3.4 mA/cm-2, an open circuit voltage (Voc)
Conclusion
A zinc porphyrin-bodipy dyad, 3, exhibiting efficient energy
transfer from the bodipy subunit to the porphyrin subunit, has
been synthesized. The photovoltaic properties of 2 and 3 were
evaluated by incorporation into dye sensitized solar cells. The zinc
porphyrin-bodipy (3) dyad based cell showed nearly twice the
efficiency of a zinc porphyrin (2) based reference cell in large part
due to the improved solar spectral coverage of 3 by bodipy.
Further improvement of the power conversion efficiency in
porphyrin-bodipy dyad based solar cells should be possible by
designing bodipy derivatives that have higher molar extinction
coefficients and more red-shifted absorption.
Acknowledgment. We thank the Korean Electric Power
Research Institute and the U.S. Department of. Energy’s Office
of Science (Grant No. DE-FG87ER13808) for support of our
work. We also gratefully acknowledge support for Chang Yeon
Lee via a Korea Research Foundation Grant funded by the
Korean Government (MOEHRD) (KRF-2007-357-C00058).
Langmuir 2010, 26(5), 3760–3765
DOI: 10.1021/la9031927 3765