An element of surprise - Efficient copper-functionalized dye-sensitized solar cells

Article abstract of DOI:10.1039/b808491b

Dye-sensitized solar cells with carboxylate-derivatized {Cu ^IL₂} complexes are surprisingly efficient and offer a long-term alternative approach to ruthenium-functionalized systems. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808491b

Chemical Communications
www.rsc.org/chemcomm
Number 32 | 28 August 2008 | Pages 3693–3808
ISSN 1359-7345
COMMUNICATION
Edwin C. Constable et al.
An element of surprise—efficient copper-functionalized dye-sensitized
solar cells

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
An element of surprise—efficient copper-functionalized dye-sensitized
solar cellsw
Takeru Bessho,a Edwin C. Constable,*b Michael Graetzel,a Ana Hernandez
Redondo,b Catherine E. Housecroft,b William Kylberg,b Md. K. Nazeeruddin,a
Markus Neuburgerb and Silvia Schaffnerb
Received (in Cambridge, UK) 20th May 2008, Accepted 24th June 2008
First published as an Advance Article on the web 8th July 2008
DOI: 10.1039/b808491b
Dye-sensitized solar cells with carboxylate-derivatized {CuIL2}
complexes are surprisingly efficient and offer a long-term alter-
native approach to ruthenium-functionalized systems.
(ii) substituents at the 6- and 60-positions to stabilize the
copper(I) state. Accordingly, we considered the two bpy
ligands 1 and 2 which differ in the extent of conjugation,
which was expected to have an influence on the absorption
spectrum. Ligand 1 was obtained by minor variations of the
literature procedure10 whereas 4 was prepared in 74% yield by
the Wittig reaction of 6,60-dimethyl-2,20-bipyridine-4,40-dicar-
baldehyde11 with Ph3PQCHCO2Me analogous to the known
preparation of the tert-butyl ester of 1.11 Hydrolysis of 4 with
LiOH in 10 : 1 thf–H2O followed by acidification with 2 M
HCl gave 2 as a white solidz whilst 1 was converted quantita-
tively to 3 upon boiling with MeOH and H2SO4. The copper(I)
complexes [Cu(L)2][PF6] (L = 3 or 4y) were prepared as red
solids by the reaction of the appropriate ligand with
[Cu(CH3CN)4][PF6] in CHCl3–MeCN. The complexes of the
free acids were prepared by reaction of the sodium salts with
copper(II) sulfate followed by reduction with ascorbic acid.9
The influence of the extended conjugation is best seen in
comparing the complexes [Cu(3)2][PF6] and [Cu(4)2][PF6]. In
MeCN solutions of the former, the MLCT band is observed
with lmax 495 nm and e 450 Mꢀ1 cmꢀ1 whereas in the more
conjugated complex [Cu(4)2][PF6] the absorption is red-shifted
to lmax 506 nm and e increases dramatically to 3650 Mꢀ1
cmꢀ1. Both of these observations indicate that the extended
conjugation in 4 has the expected effects when the complex is
compared to that with 3. This is confirmed in the complex
[Cu(2-H)2] which exhibits an MLCT absorption at 515 nm
Dye-sensitized solar cells (DSSCs) are currently under active
investigation as alternatives to silicon-based photovoltaic
devices for solar energy utilization.1 Typical DSSCs contain
a nanoparticulate semiconductor substrate functionalized with
a dye-stuff possessing a narrower band gap than that of the
semiconductor.2 In the commonest type of DSSC, the so-
called Graetzel cell, the semiconductor is anatase-like TiO2
and the dye-stuffs are ruthenium-oligopyridine complexes.3,4
It was early recognized that copper(I) complexes with 2,9-
disubstituted 1,10-phenanthroline ligands possessed similar
photophysical properties to archetypal [Ru(bpy)3]2+ salts
(bpy = 2,20-bipyridine).5–7 A significant difference between
d10 copper(I) complexes and low spin d6 ruthenium(II) com-
plexes lies in the anticipated greater lability of the former.
Nevertheless, it is surprising that, to the best of our knowl-
edge, only two reports of DSSCs with copper(I) dyes have been
reported.8,9 In this communication we show that copper(I)
complexes of 6,60-disubstituted 2,20-bipyridines are generally
effective sensitizers for TiO2 and demonstrate the tuning of the
absorption maximum and report surprisingly high incident-
photon to current efficiencies (IPCE) for DSSCs assembled
using these dyes.
with e 6740 Mꢀ1 cmꢀ1
.
Complexes of the type [CuL2]+, where L is a diimine ligand,
show a wide variety of structural variations,12 and to confirm the
geometry at the metal centre, the solid state structure of the
complex [Cu(3)2][PF6] has been determined (Fig. 1).z The Cu–N
distances are in the typical range 2.003(3)–2.039(3) A and the
bite-angles of the bpy-domains are also characteristic
(+N1–Cu1–N2, 80.93(10)1, +N3–Cu1–N4, 81.21(11)1). The
cation is distorted significantly from the idealized D2d symmetry
and the analysis introduced by White et al. for 1,10-phenanthro-
line complexes provides a convenient method of quantifying this
(provided that the two rings of each bpy ligand are approxi-
mately coplanar as in the case of [Cu(3)2][PF6] where the dihedral
angles are in the range 2–41).13 For [Cu(3)2][PF6], values of yx, yy
and yz of 97.27, 95.56 and 92.831 (with expectation values of
yx = yy = yz = 901 for D2d symmetry); the significant deviation
of yx and yy from 901 indicate rocking and wagging displace-
ments to produce a distorted trigonal pyramidal geometry.
In order to form stable copper(I) complexes which can be
bound to TiO2, the ligands must (i) possess carboxylic or
phosphonic acid substituents to link to the surface and
a Laboratory of Photonics and Interfaces (LPI), Institute of Chemical
Science and Engineering, Faculty of Basic Science, Ecole
Polytechnique Federale de Lausanne, CH-1015 Lausanne,
Switzerland
b Department of Chemistry, University of Basel, Spitalstrasse 51, CH
4056 Basel, Switzerland. E-mail: edwin.constable@unibas.ch; Fax:
+41 61 267 1005; Tel: +41 61 267 1001
w CCDC 688596. For crystallographic data in CIF format see DOI:
10.1039/b808491b
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3717–3719 | 3717

Fig. 1 Solid state structure of the [Cu(3)2]+ cation present in
[Cu(3)2][PF6] showing the numbering scheme adopted; hydrogen
atoms have been omitted for clarity, thermal ellipsoids depicted at
50% probability. Selected bond lengths (A) and angles (1): Cu1–N1,
2.009(2); Cu1–N2, 2.016(2); Cu1–N3, 2.039(3); Cu1–N4, 2.003(3);
N1–Cu1–N2, 80.93(10); N1–Cu1–N3, 122.14(10); N2–Cu1–N3,
119.26(10); N1–Cu1–N4, 134.76(11); N2–Cu1–N4, 123.77(11);
N3–Cu1–N4, 81.21(11).
Preliminary experiments indicated that {CuL2} complexes
with 1 and 2 bound strongly to TiO2 nanoparticles whereas
complexes with the ester 4 did not bind (Fig. 2). Surprisingly,
[Cu(3)2][PF6] also gave good dye-modified surfaces and we
attribute this to in situ hydrolysis of the ester.
Fig. 3 IPCE (top) and I–V (bottom) curves for DSSCs prepared with
copper(I) complexes of 1 (--) and 2 (—) at light intensity of 100%, 50%
and 10% sun, respectively.
Solar cells were constructed using a standard protocol opti-
mized for the production of ruthenium-sensitized dye-cells14 and
evaluated using a standard procedure.15 In Fig. 3 we present the
photovoltaic action spectra and current–voltage characteristics for
devices fabricated with complexes of 1 and 2. We do indeed
observe significantly enhanced incident-photon to collected-
electron quantum efficiency (IPCE) with complex 2 when com-
pared to the complex 1. By considering the molar extinction
coefficients of the two complexes, in the device containing complex
1 we would expect a lower IPCE, and in our measurements we do
observe this, with the IPCEs of the device containing the complex
2 being 50% compared to the complex 1, which yielded only 38%.
Fig. 3 (bottom) shows the current–voltage curves measured
under simulated air mass (AM) 1.5 solar illumination at an
intensity of 100, 50 and 10 mW cmꢀ2 and in the dark. We
observe a slightly enhanced short-circuit current density and a
reasonable decrease in the open-circuit voltage for the complex
2 compared to the complex 1, resulting in power conversion
efficiencies of 2.3 and 1.9%, respectively. We also tested the
effects of solvent on dye deposition and the addition of
chenodeoxycholic acid,4 which is known to reduce dye loading
while having a relatively small effect on the short-circuit
photocurrent and improving the photovoltage. The data ob-
tained after deposition from ethanol solution are superior to
those from acetonitrile or tert-butanol solutions. Adding
chenodeoxycholic acid did not improve the efficiency com-
pared to comparable cells without chenodeoxycholic acid.
Table 1 shows current–voltage characteristics data for
solar cells derivatized with copper(I) complexes of 1 and 2,
Fig. 2 Binding to TiO2 of 10ꢀ3 M {CuL2} complexes with (from left
to right) 1, 3, 2 and 4. The FTO (fluorine doped tin oxide) conducting
glass slides were coated with 6–7 mm TiO2 nanoparticles and then
immersed in solutions (MeOH for 1, CHCl3 for 3 and 4, and 1 : 9
DMF–CHCl3 for 2) for a period of 12 h.
which were obtained with
supported on FTO conducting glass using an electrolyte
solution containing 0.6 N-methyl-N-butylimidazolium
iodide, 0.03 M iodine, 0.1 M LiI, 0.1 M guanidinium
a nanocrystalline TiO2 film
M
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This journal is The Royal Society of Chemistry 2008
3718 | Chem. Commun., 2008, 3717–3719

Table 1 Current–voltage characteristics data derivatized with copper(I) complexes of 1 and 2 and a comparison with the ruthenium dye N71916,a
L
VOC/mV
Jsc/mA cmꢀ2
FF (%)
Ef (%)
IPCE(max) (%)
nm
1
566
556
767
5.25
5.9
17.7
0.64
0.7
0.71
1.9
2.3
9.7
38.6
50.1
87
470
470
550
2
N719
a
7.4 + 4.4 mm double layer sensitized nanocrystalline TiO2 film on FTO conducting glass; electrolyte: 0.6 M N-methyl-N-butylimidazolium
iodide, 0.03 M I2, 0.1 M LiI, 0.1 M guanidinium thiocyanate and 0.5 M tert-butylpyridine in 15 : 85 (v/v) valeronitrile–acetonitrile. VOC = open
circuit potential, Jsc = short circuit current, FF = fill factor, Ef = power conversion efficiency, IPCE = incident photon to current efficiency.
(500 MHz, CDCl3): d/ppm 8.26 (s, 2H), 7.74 (d, 2H, J 15.8 Hz), 7.56
thiocyanate and 0.5 M tert-butylpyridine in a 15 : 85 (v/v)
(s, 2H), 6.83 (d, 2H, J 16.1 Hz), 3.87 (s, 6H), 2.26 (s, 6H). UV-Vis,
mixture of valeronitrile and acetonitrile. The data represent
l
max/nm (emax/Mꢀ1 cmꢀ1): 255 (94 200), 324 (24 200), 508 (3650).
ESMS m/z: 767.5 ([Cu(4)2]+). Found: C, 49.29; H, 4.15; N, 5.71. Calc.
for C40H40CuF6N4O8Pꢂ3H2O: C, 49.67; H, 4.79; N, 5.79%.
z C32H32CuF6N4O8P, M = 809.14, monoclinic, space group P21/c, purple
plates, Z = 4, a = 10.8110(2), b = 20.0824(4), c = 16.3507(3) A,
b = 98.681(1)1, V = 3509.3(1) A3, Dc = 1.531 Mg mꢀ3, m(Mo-Ka) =
0.755 mmꢀ1, T = 173 K, 7729 reflections collected. Refinement of 469
parameters using 4831 reflections with I 41.5s(I) converged at final R1 =
0.0505 (R1 all data = 0.0890), wR2 = 0.0564 (wR2 all data = 0.0779),
gof = 1.241.w
the optimized results for cells, measured using 7.4 + 4.4 mm
double layer sensitized TiO2 films. In order to reduce scattered
light from the edge of the glass electrodes of the dyed TiO2
layer, a light shading mask was used on the DSSCs, so that the
active area of the DSSC was fixed at 0.2 cm2.
The copper complexes {CuIL2} are surprisingly effective as
sensitizers for DSSCs. Although these initial results are not
comparable with state of the art ruthenium dyes such as N719,
they indicate that with iterative chemical optimization, sensitizers
comparable to ruthenium complexes might be prepared. How-
ever, the ‘‘Techno-Economic’’ analyses of the two sensitizers
clearly show that even though the efficiency of the copper
complex is 4 times lower than that of the ruthenium sensitizer
N719,16 the cost is an order of magnitude lower. We are currently
developing new copper-based dyes that are (i) red-shifted and (ii)
more efficient and evaluating copper-based versus organic17 or
state of the art ruthenium18 dyes for next generation devices.
In conclusion, we have shown that copper(I)-based complexes
may effectively replace ruthenium(II) complexes in DSSCs and
that the tuning methods applied to the latter are also effective in
optimizing the behaviour of copper-sensitized systems.
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Notes and references
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¨
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Chem. Commun., 2008, 3717–3719 | 3719