Multichromophoric dye-sensitized solar cells based on supramolecular zinc-porphyrin...perylene-imide dyads

Article abstract of DOI:10.1039/c2cc33120a

Multichromophoric dye-sensitized solar cells (DSCs) based on self-assembled zinc-porphyrin...peryleneimide dyads on TiO₂ films display more efficient light-to-electrical energy conversion than DSCs based on individual dyes. Higher efficiency of

Full text of DOI:10.1039/c2cc33120a

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COMMUNICATION
Multichromophoric dye-sensitized solar cells based on supramolecular
zinc-porphyrinꢀ ꢀ ꢀperylene-imide dyadsw
Dillip K. Panda,z Flynt S. Goodson,z Shuvasree Ray, Rachel Lowell and Sourav Saha*
Received 1st May 2012, Accepted 6th July 2012
DOI: 10.1039/c2cc33120a
Multichromophoric dye-sensitized solar cells (DSCs) based on
self-assembled zinc-porphyrinꢀ ꢀ ꢀperyleneimide dyads on TiO2
films display more efficient light-to-electrical energy conversion
than DSCs based on individual dyes. Higher efficiency of multi-
chromophoric dyes can be attributed to co-sensitization as well
as vectorial electron transfer that lead to better electron–hole
separation in the device.
Ease of construction, low manufacturing cost, and improved
dyes and electrolytes render dye-sensitized solar cells (DSCs)1
an attractive device for light-to-electrical energy conversion.
Most DSCs developed thus far involve a single chromophore
unit—usually a Ru(II) complex, zinc-porphyrin (ZnP), zinc-
phthalocyanine (ZnPc), or perylenediimide (PDI) derivative—
that displays intense visible absorption, albeit within narrow
wavelength regions, leaving the rest of the solar spectra,
Scheme 1 Schematic representation of chemisorption of PyPMI-2 on
TiO2-coated FTO electrode followed by self-assembly of
especially in the longer wavelength regions, unused for energy
conversion.2 To overcome this limitation and improve the
light-harvesting efficiency, co-sensitization of TiO2 particles
with multiple chromophores that can absorb wide regions of
light has been introduced.3 Covalent attachment of two or
more chromophores4 makes the synthesis of multichromophoric
dyes a challenging process. Alternatively, different dyes can be
immobilized on TiO2 surfaces via chemisorption and physisorption
processes.5 However, if the electron donor and acceptor
chromophores attached to the TiO2 surface are randomly
distributed, it hampers the possibility of vectorial electron
transfer6 from the donor-chromophore via acceptor-chromophore
to n-type TiO2 particles, a process that is crucial for optimal
charge-separation and photocurrent generation. Although
metal–ligand coordination has been employed to anchor
ZnP onto TiO2 surfaces7 as well as to assemble supramolecular
dyads and triads based on ZnPc and dipyridyl-PDI (DPPDI)
in solutions,8 this strategy has been rarely exploited to construct
multichromophoric DSCs.
a
a
ZnP-1ꢀ ꢀ ꢀPyPMI-2 dyad via metal–ligand coordination. The dyad-
functionalized TiO2–FTO surface is used as a working electrode in
the DSC.
In this communication, we demonstrate that coordination
of ZnP-1 with N-pyridyl-peryleneimide-2 (PyPMI-2) leads to
the formation of a supramolecular ZnP-1ꢀ ꢀ ꢀPyPMI-2 dyad on
the TiO2 surface (Scheme 1). DSCs composed of this dyad on a
TiO2-coated F:SnO2 (FTO) photoanode, platinised indium-
tin-oxide (Pt/ITO) cathode, and Iꢁ/I3ꢁ redox mediator generate
a much higher open-circuit voltage (VOC), short-circuit current
(JSC), and energy conversion efficiency (Z = 1.1 ꢂ 0.06%)
under 1.5 air-mass (AM) conditions than DSCs made of
individual dyes.
Because of complementary electronic and absorption
properties ZnP, PMI, and PDI derivatives were chosen as
chromophore units for multichromophoric DSCs. To prevent
self-aggregation and improve the solubility of these chromo-
phores in nonpolar solvents, 4-tert-butylphenyl groups were
introduced at all four meso-positions of ZnP and 4-tert-
butylphenoxy groups were attached to all four bay-positions
of PMI and PDI derivatives (Scheme S1, ESIw). The N-pyridyl
group of PyPMI-2 is designed to serve as an axial ligand of
ZnP-1 to form the supramolecular ZnP-1ꢀ ꢀ ꢀPyPMI-2 dyad.
The remaining anhydride unit of PyPMI-2 was exploited for
its chemisorption on the TiO2 surface.9
Department of Chemistry and Biochemistry and Integrative
NanoScience Institute, Florida State University, 95 Chieftan Way,
Tallahassee, FL 30306-4390, USA. E-mail: saha@chem.fsu.edu;
Tel: +1 850 645 8616
w Electronic supplementary information (ESI) available: Synthesis,
characterization, Ka calculations from NMR data, ESIMS, UV/Vis
spectra, DSC construction, and photocurrent measurements. See DOI:
10.1039/c2cc33120a
z These authors made major and equal contributions.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8775–8777 8775

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Table
1 EOx and ERed (mV) of ZnP-1, PyPDICy-3, and
ZnP-1ꢀ ꢀ ꢀPyPDICy-2 dyad (vs. Ag/AgCl in CH2Cl2)
Entry
E1
E2
E1
E2
Ox
Ox
Red
Red
ZnP-1
PyPDICy-3
ZnP-1ꢀ ꢀ ꢀPyPDICy-3
+830
+1340
+780
+1140
—
+1220
ꢁ1000
ꢁ780
ꢁ820
ꢁ1670
ꢁ950
ꢁ970
formation (m/z = 2044.7) from ZnP-1 (901.3) and PyPDICy3
(1142.4).
The UV/Vis spectrum of the ZnP-1ꢀ ꢀ ꢀPyPDICy-3 dyad in
CH2Cl2 displays (Fig. S2, ESIw) absorption peaks at 420 (lmax),
450, 545, and 575 nm, which is essentially a linear combination
of ZnP-1 (420 (lmax), 550, and 570 nm) and PyPDICy-3
(450, 545, and 575 (lmax) nm) spectra, indicating that such a
complex formation does not change the HOMO–LUMO gap of
individual dyes.
Cyclic voltammetry measurements (Table 1 and Fig. S3, ESIw)
demonstrate that coordination of ZnP-1 with PyPDICy-3 renders
the former a better electron donor, as E1 decreases from
Ox
+830 mV (vs. Ag/AgCl in CH2Cl2) for free ZnP-1 to +780 mV
Fig. 1 1H NMR titration spectra of PyPDICy-3 with ZnP-1
in the dyad, whereas the reduction of PyPDICy-3 in the
complex (E1
(CD2Cl2).
= ꢁ820 mV vs. Ag/AgCl in CH2Cl2) requires
Red
To determine the binding affinity (Ka) from 1H NMR
titration experiments (Fig. 1), PyPMI-2 was replaced with
PyPDICy-3 to avoid competition between the pyridine ligand
a slightly more negative potential than for the free ligand
(ꢁ780 mV). These changes in redox properties indicate that
coordination of ZnP-1 with pyridine lone-pair electrons of
PyPDICy-3 enhances the electron density of the former, while
diminishing the electron density of the latter.
1
and the anhydride terminal. The H NMR spectrum of free
ZnP-1 (CD2Cl2, 298 K) displays a singlet at 8.98 ppm corre-
sponding to Hz protons on the pyrrole rings and two doublets
at 8.14 and 7.79 ppm that represent Hy and Hx on 4-tert-
butylphenyl rings, respectively. The 1H NMR spectrum of free
PyPDICy-3 (CD2Cl2, 298 K) displays two doublets at 8.73 and
7.25 ppm that represent Ha and Hb of the N-pyridyl ring
and two singlets at 8.17 and 8.11 ppm that correspond to Hc
and Hd on the PDI core, respectively. Protons of 4-tert-butyl-
phenoxy groups (He, Hf, Hg, and Hh) appear as doublets at the
7.25–7.29 and 6.83–6.85 ppm region. 1H NMR titration
(Fig. 1) of PyPDICy-3 with ZnP-1 (CD2Cl2, 298 K) caused
upfield shifts of the ZnP-1’s Hz signal to 8.91 ppm (Dd =
ꢁ0.07 ppm) and of the Hx signal to 8.12 ppm (Dd =
ꢁ0.02 ppm) at a 1 : 1 ZnP-1/PyPDICy-3 ratio (2 mM each),
indicating a complex formation. Dramatic upfield shifts of
N-pyridyl and PDI core protons due to the shielding effects of
perpendicular ZnP-1 ring-current confirm its axial coordina-
tion with ZnP-1. For instance, PyPDICy-3’s Hb becomes very
broad and shifts to 6.32 ppm (Dd = ꢁ0.93 ppm), Hc to
7.95 ppm (Dd = ꢁ0.22 ppm), and Hd to 8.06 ppm (Dd =
ꢁ0.06 ppm). The Ha signal disappears, which is characteristic
of axial coordination of pyridyl ligands with ZnP.8,10 Protons
on 4-tert-butylphenoxy groups of PyPDICy-3 also shift slightly
upfield. Addition of >1 equiv. of ZnP-1 to PyPDICy-3 did not
change the NMR spectrum any further, indicating a fast
equilibrium of the complexation process at room temperature.
For the construction of working electrodes, PyPMI-2 was
anchored through its anhydride end by immersing a TiO2-coated
(a 5 mm thick layer of 20 nm particles, Solaronix) FTO substrate
into a PyPMI-2 solution (0.15 mM in CH2Cl2).9 After washing
off the unbound dyes, the PyPMI-2-functionalized TiO2 surface
was immersed into ZnP-1 solution (2 mM in CH2Cl2) to form
the ZnP-1ꢀ ꢀ ꢀPyPMI dyad on the surface (Scheme 1). To make
a control device, only ZnP-1 was attached to a pyridine-4-
carboxylic acid functionalized TiO2/FTO surface via similar
metal–ligand coordination. To complete the construction of
DSCs, the Pt/ITO surface was used as a counter electrode and
ꢁ
the Iꢁ/I3 couple (1.0 M LiI + 0.06 M I2 in propylene
carbonate (PC) or MeCN) as a redox mediator (Scheme 1).
Immobilization of ZnP-1, PyPMI-2, and the ZnP-1ꢀꢀꢀPyPMI
dyad on TiO2-coated FTO surfaces was confirmed by UV/Vis
spectroscopy (Fig. 2a). The dyad-functionalized TiO2/FTO surface
displays both ZnP-1 and PyPMI-2 peaks at 420 and 550 nm
regions, respectively, whereas individual ZnP-1 and PyPMI-2
functionalized surfaces display their characteristic absorption
peaks. In the absence of PyPMI-2 or pyridine-4-carboxylic acid
on the TiO2 surface, ZnP-1 does not bind to the surface and can be
easily washed away.
Finally, light-to-electrical energy conversion by these DSCs
was demonstrated by current–voltage (I–V) measurements
(Fig. 2b). To understand the effects of multichromophoric dyes
on light-harvesting properties of DSCs, i.e., VOC, JSC, fill-factor
(FF), and Z of the ZnP-1ꢀ ꢀ ꢀPyPMI-2 dyad-based device were
compared to those of DSCs made of individual ZnP-1 and
PyPMI-2 dyes (Table 2). All three devices show similar I–V
curves in the dark and under the same visible light source
(100 mW cmꢁ2). ZnP-1ꢀ ꢀ ꢀPyPMI-2 dyad-based DSC displays
The Ka of a 1 : 1 ZnP-1ꢀ ꢀ ꢀPyPDICy-3 complex (4.5 ꢃ 105 Mꢁ1
,
CD2Cl2, 298 K) was derived from NMR chemical shifts of
the diagnostic Hb signal using the nonlinear least-squares fit method
(see ESIw).11 This Ka value is consistent with that measured for
a similar ZnPcꢀ ꢀ ꢀimidazole coordination complex.10 ESIMS
further confirms (Fig. S1, ESIw) the ZnP-1ꢀ ꢀ ꢀPyPDICy3 dyad
c
8776 Chem. Commun., 2012, 48, 8775–8777
This journal is The Royal Society of Chemistry 2012

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based DSC than the PyPMI-2 based device could be attributed
to the insulating nature of pyridine-4-carboxylic acid that binds
ZnP-1 on TiO2 in comparison with direct chemisorption of
PyPMI-2 on the electrode (via Ti–O bond formation) that
facilitates electron injection.
In conclusion, we have self-assembled ZnP, PMI, and PDI
dyes via metal–ligand coordination and immobilized them on
TiO2 surfaces to construct multichromophoric DSCs. Broad
light-absorbing ZnP-1ꢀ ꢀ ꢀPyPMI-2 dyad-based DSCs display
higher VOC, JSC, and efficiency (Z = 1.1 ꢂ 0.06%) under
1.5 AM conditions than those made of individual dyes. To
improve the efficiency of DSCs, we have undertaken a supra-
molecular assembly of multichromophoric DSCs using red
and NIR absorbing dyes.
Fig. 2 (a) UV/Vis spectra of TiO2–FTO surfaces functionalized with
the ZnP-1ꢀ ꢀ ꢀPyPMI-2 dyad (green), PyPMI-2 (red), and ZnP-1 (violet).
(b) I–V characteristics of DSCs composed of the ZnP-1ꢀ ꢀ ꢀPyPMI-2
dyad (green), PyPMI-2 (red), and ZnP-1 (violet) showing photocurrent
generation (JSC and VOC) under 1.5 AM conditions (dotted lines) and
negligible dark currents (solid lines).
We acknowledge Drs Richard Schaller, Seth Darling, and
Daniel Rosenmann (Argonne National Laboratory (ANL))
for providing Pt/ITO counter electrodes. Use of the Center for
Nanoscale Materials at ANL was supported by the U. S.
Department of Energy, Office of Basic Energy Sciences, under
Contract No. DE-AC02-06CH11357.
Table 2 Performance of DSCs based on the ZnP-1ꢀ ꢀ ꢀPyPDICy-2
dyad ZnP-1, and PyPMI-2 dyes under standard illumination
(100 mW cmꢁ2
)
Dye composition
J
SC/mA cmꢁ2 VOC/mV FF (%) Z (%)
ZnP-1ꢀ ꢀ ꢀPyPMI-2 5.51
410
390
340
49
57
60
1.10 ꢂ 0.06
0.72 ꢂ 0.04
0.09 ꢂ 0.02
Notes and references
PyPMI-2
ZnP-1
3.21
0.47
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dielectric constant of PC (64) than of MeCN (37.5), which
facilitates better charge separation in the former devices.12
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1
the *ZnP-1 excited state through PyPMI-2 to TiO2 or from
ZnP-1 via the 1*PyPMI-2 excited state to TiO2 would generate
+
ZnP-1ꢄ radical cations and inject electrons to n-type semi-
conducting TiO2 via PyPMI-2ꢄꢁ radical anions. Characterization
of these photophysical processes will be conducted through
transient absorption spectroscopy measurements. Nevertheless, the
greater efficiency of the dyad-based DSC over the individual dyes
suggests that both pathways perhaps contribute to light-to-
electrical energy conversion in the dyad system, as these
chromophores absorb light in entirely different regions (Fig. 2b).
In addition, vectorial electron transfer in the ZnP-1ꢀꢀꢀPyPMI-2–
TiO2 device leads to better charge separation than one-step
PET from 1*ZnP-1 or 1*PyPMI-2 to the TiO2 surface in
monochromophoric devices. Lower VOC and JSC of the ZnP-1
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8775–8777 8777