Copper(i) dye-sensitized solar cells with [Co(bpy)3] 2+/3+ electrolyte

Article abstract of DOI:10.1039/c3cc44595j

The hierarchical assembly of DSCs containing a new heteroleptic copper(i) complex with a phosphonic acid anchoring ligand is described; it is shown that conventional I^-/I₃^- electrolytes may be replaced by [Co(bpy)₃]^2+/3+ with no loss in performance.

Full text of DOI:10.1039/c3cc44595j

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Copper(I) dye-sensitized solar cells with [Co(bpy)3]2+/3+
electrolyte†
Cite this: Chem. Commun., 2013,
49, 7222
Biljana Bozic-Weber, Edwin C. Constable,* Sebastian O. Furer,
¨
Catherine E. Housecroft,* Lukas J. Troxler and Jennifer A. Zampese
Received 19th June 2013,
Accepted 4th July 2013
DOI: 10.1039/c3cc44595j
www.rsc.org/chemcomm
The hierarchical assembly of DSCs containing a new heteroleptic mesoscopic level, specifically by changing the semiconductor
copper(I) complex with a phosphonic acid anchoring ligand is described; film thickness11,12 and by post-treatment with H2O–TiCl4.13
it is shown that conventional Iꢀ/I3 electrolytes may be replaced
by [Co(bpy)3]2+/3+ with no loss in performance.
The ancillary ligand L facilitates light-harvesting and 2-(4-bromo-
phenyl)-1H-phenanthro[9,10-d]imidazole14 provides a convenient
starting point for the design of a ligand family. The first design
ꢀ
Dye-sensitized solar cells (DSCs) are photovoltaic devices in which the feature is the introduction of 6,60-dialkyl or diaryl substituents to
response of a semiconductor is extended into the visible region by use stabilize the copper(I) oxidation state.15 The second is the incorpora-
of a coloured sensitizer.1 Attention is centred upon the replacement of tion of a triphenylamino-substituent to extend the p-conjugation and
first generation sensitizers based on ruthenium complexes by those increase the spectral response. The third is the introduction of a
using other metals or organic dyes. Copper(I) complexes [Cu(Lanc)(L)]n+ long chain substituent which is known to suppress intermolecular
are leading contenders for use as dyes based on Earth abundant aggregation by preventing extensive face-to-face p-interactions
metals.2 We are currently optimizing the performance of DSCs based between aromatic units.16 Self-assembly of dyes at the semiconduc-
on copper(I) complexes and report in this paper on (i) the improve- tor surface produces a hydrophobic monolayer that repels iodide
ment of the spectral response of the dyes (ii) the stability of the ions in the Iꢀ/I3ꢀ electrolyte thereby reducing the rate of recombina-
surface-bound dyes and (iii) the long-term stability of the DSC.
We have developed procedures for the assembly of heteroleptic
tion and increasing the open circuit voltage (Voc).17
Scheme 1 summarizes our synthetic approach to a first
copper(I) dyes by ligand exchange between a surface-anchored generation ligand starting from 2,9-dimethyl-1,10-phenanthro-
ligand Lanc and [Cu(L)2]n+.3–7 Pellegrin, Daniel, Odobel and coworkers8 line-5,6-dione.18 Treatment of the latter with 4-bromobenzalde-
have reported DSCs with an extended spectral response utilizing a hyde and an excess of NH4OAc in ethanol yielded compound 1.
1
2-(diarylaminophenyl)-1H-phenanthro[9,10-d]imidazole ancillary The H and 13C NMR spectra are consistent with a tautomeric
ligand.8 We now describe our studies of DSCs using related equilibrium involving the imidazole NH and N sites.† In situ
ligands, which both give improved spectral characteristics and deprotonation of 1 and reaction with n-octyl bromide yielded 2
improve the stability of the heteroleptic complexes (phen com- which was fully characterized.† The highest mass peak in the
plexes are typically one logK unit more stable than bpy analogues). electrospray mass spectrum (m/z 515.2) corresponded to the [M +
Optimal performance is achieved with phosphonic acid anchors, H]+ ion. In contrast to 1, the NMR spectra are desymmetrized and
which both stabilize the surface-bound complex with respect to two signals are observed (d 2.98 and 2.96 ppm in CDCl3) for the
carboxylate Lanc systems and lead to enhanced energy conversion HMe-phen protons. Single crystals were grown from a toluene–ethyl
efficiencies.4–7,9 The inclusion of an aryl spacer between bpy-unit acetate solution, and Fig. 1 shows the structure of 2.† The octyl
and PO(OH)2-anchor is beneficial.7 Finally, the long term stability chain is ordered and lies over the bromophenyl unit, the arene
of the DSC (which may be limited by the formation of CuI in the ring of which is twisted 58.61 out of the plane of the imidazole
presence of Iꢀ/I3ꢀ electrolyte) has been addressed by changing to a ring. The conformation of the octyl chain allows close CHꢁ ꢁꢁp and
[Co(bpy)3]2+/[Co(bpy)3]3+ electrolyte.10 This latter change has CHꢁꢁꢁBr contacts; the distance C29H29Aꢁ ꢁꢁBr1 of 3.12 Å is close to
been coupled with modifications of the photoanode at the the sum of the van der Waals radii, using a value of 1.10 Å19 for
H rather than the Bondi value of 1.20 Å.20 The closest CHꢁ ꢁꢁp
Department of Chemistry, University of Basel, Spitalstrasse 51, CH4056 Basel,
contacts are 3.30 and 3.66 Å. Buchwald–Hartwig amination of 2
Switzerland. E-mail: edwin.constable@unibas.ch, catherine.housecroft@unibas.ch;
with diphenylamine resulted in the formation of 3; the highest
Tel: +41 61 267 1008
mass peak in the ESI MS (m/z 604.3) arises from [M + H]+, and
† Electronic supplementary information (ESI) available: Experimental data.
1
the H and 13C NMR spectra are consistent with the structure
Fig. S1: EQE spectra. CCDC 945455. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3cc44595j
shown in Scheme 1.
c
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Fig. 2 Absorption spectra of 3 (red) and [Cu(3)2][PF6] (blue) in CH2Cl2.
Scheme 2 Structure of anchoring ligand 4.
probably associated with oxidation of the Ph2N unit. No reduction
processes are observed within the solvent accessible window.
Fully masked DSCs22 were fabricated by applying a scattering
TiO2 layer to screen-printed mesoporous TiO2 (3 or 4 layers) followed
by post-treatment with H2O–TiCl4,13 and then soaking the cells in a
solution of anchoring ligand 4 (Scheme 2). This was followed by a
period of 24, 64 or 110 hours soaking in solutions of [Cu(3)2][PF6].
During this period, the surface-anchored heteroleptic complex
[Cu(3)(4)]+ was formed (see experimental details).†ꢀWe first assessed
the DSC characteristics of cells containing Iꢀ/I3 electrolyte and
[Cu(3)(4)]+ sensitizer supported on 3 layers of TiO2 post-treated with
different concentrations of H2O–TiCl4. Table 1 summarizes mea-
surements made on the day of sealing the cells. I–V characteristics of
the same cells were recorded one, six and eight days after sealing,
and only small changes in values of JSC, VOC and Z were observed, in
contrast to the ripening effects noted with other copper-based
DSCs.4,6,7 Post-treatment with 5 mmol dmꢀ3 H2O–TiCl4 results in
improved JSC and VOC values with concomitant increase in efficiency.
Treatment of 3 layers of TiO2 post-treated with higher concentrations
had little or no effect on the overall energy conversion efficiencies of
Scheme 1 Synthesis of ligand 3. Conditions: (i) 4-bromobenzaldehyde, excess
NH4OAc, EtOH, 91%; (ii) NaH, DMF, n-octyl bromide, 75 1C, 15 h; (iii) Ph2NH,
Pd(bda)2/PtBu3, toluene, reflux overnight, 52%.†
Fig. 1 Structure of compound 2 (ellipsoids plotted at 40% probability level and
H atoms omitted for clarity).
ꢀ
the DSCs. The effect of replacing Iꢀ/I3 electrolyte by [Co(bpy)3]2+/
The homoleptic complex [Cu(3)2][PF6] was prepared in quanti-
tative yield by reaction of [Cu(NCMe)4][PF6] with two equivalents of
3. The MALDI mass spectrum showed peaks at m/z 1271.6 and
666.9 corresponding to [M ꢀ PF6]+ and [M ꢀ 3–PF6]+ with isotope
[Co(bpy)3]3+ is seen by comparing the data in Tables 1 and 2. For all
the cells in Table 1 and the top half of Table 2, the photoanodes were
left to stand in solutions of [Cu(3)2][PF6] for 24 hours. The results
suggest that energy conversion efficiency is increased by using
higher concentrations of TiCl4 during post-treatment. Since dye
1
patterns matching those calculated. The H and 13C NMR spectra
were consistent with a single ligand environment, the presence of
the octyl chain desymmetrizing the phenanthro[9,10-d]imidazole
unit.† Fig. 2 compares the solution absorption spectra of 3 and
[Cu(3)2][PF6]. The approximate doubling of the extinction coeffi-
cients of the bands (assigned to p* ’ p and p* ’ n transitions) in
the UV region is consistent with the presence of two ligands in the
complex. The spectral response of the complex extends to 600 nm,
with lmax for the MLCT band at 476 nm. Reversible oxidation of the
Cu(I) centre in [Cu(3)2][PF6] occurs at +0.42 V, compared to +0.50 V
(vs. Fc/Fc+ in CH2Cl2) for [Cu(dmp)2][PF6] (dmp = 2,9-dimethyl-1,10-
phenanthroline).21 A second oxidation process at +0.63 V is most
Table 1 DSC performances using [Cu(3)2][PF6], anchor 4, Iꢀ/I3ꢀ electrolyte, and
3 layers of TiO2; dipping time = 24 h. Measurements made on the day of sealing
[TiCl4]/mmol dmꢀ3
JSC/mA cmꢀ2
VOC/mV
ff
Z/%
0
5
15
30
40
60
1.99
2.73
2.00
2.25
2.11
2.34
588
612
608
599
591
592
0.69
0.70
0.67
0.65
0.64
0.64
0.80
1.18
0.81
0.88
0.80
0.89
c
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Table 2 DSC performances using [Cu(3)2]+, anchor 4, [Co(bpy)3]2+/3+ and 3
H2O–TiCl4, but absolute values of Z show variation. Fig. 3 shows
corresponding EQE spectra; a maximum photon-to-current conversion
efficiency of 32% is observed. For comparison, 4-layer DSCs were made
using Iꢀ/I3ꢀ electrolyte retaining [Cu(3)(4)]+ as dye and a dipping time
of 110 h. With post-treatment of 40 mmol dmꢀ3 H2O–TiCl4, the cell
layers TiO2. Measurements were made on day of sealing the cell
[TiCl4]/mmol dmꢀ3
JSC/mA cmꢀ2
VOC/mV
ff
Z/%
Cells dipped into solution of [Cu(3)2][PF6] for 24 h
0
5
15
30
40
60
1.15
1.75
1.62
1.56
2.20
3.17
380
535
564
590
542
596
0.49
0.53
0.56
0.61
0.55
0.63
0.22
0.50 characteristics were JSC = 5.11 mA cmꢀ2, VOC = 574 mV, ff = 0.71 and
0.51
0.56
0.66
Z = 2.08% (compared to Z = 6.90% for an analogous DSC using N719).
Preliminary light-stability tests in which post-treated 4-layer DSCs
1.19 containing anchored-[Cu(3)(4)]+ dye and [Co(bpy)3]2+/3+ or Iꢀ/I3ꢀ electro-
lyte were continuously illuminated (100 mW cmꢀ2) indicate that all
0.73
Cells dipped into solution of [Cu(3)2][PF6] for 64 h
0
5
15
30
40
60
2.28
1.83
3.44
3.48
2.87
2.28
586
611
628
619
621
586
0.55
0.62
0.70
0.70
0.68
0.55
cells show similar stabilities over a 60 hour period.
0.69
1.50
1.51
1.21
0.73
After optimization, the efficiencies of DSCs containing [Cu(3)(4)]+
dye anchored on H2O–TiCl4 post-treated 4 layer TiO2 are comparable
using either [Co(bpy)3]2+/3+ or Iꢀ/I3ꢀ electrolyte. This is the first report
of DSCs which combine copper(I)-based dyes and [Co(bpy)3]2+/3+
electrolyte, and is a critical step towards the development of stable
iodide-free copper(I) solar cells.
The European Research Council (Advanced Grant 267816 LiLo),
Swiss National Science Foundation and University of Basel are
acknowledged for financial support. Nik Hostettler and Dr Colin
Martin recorded 500 MHz NMR spectra, and Gino Gu¨nzburger and
Table 3 DSC performances with [Cu(3)2]+, anchor 4, [Co(bpy)3]2+/3+ and 4 layers
TiO2; dipping time 110 h. Measurements made on day of sealing
[TiCl4]/mmol dmꢀ3
JSC/mA cmꢀ2
VOC/mV
ff
Z/%
0
5
15
30
40
60
3.01
3.33
2.39
3.39
5.05
4.95
545
505
391
427
578
610
0.51
0.51
0.51
0.52
0.59
0.67
0.83
0.85
¨
0.48 Ewald Schonhofer assisted with DSC stability tests.
0.75
1.73
Notes and references
2.02
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Fig. 3 EQE spectra for five of the DSCs listed in Table 3.
7224 Chem. Commun., 2013, 49, 7222--7224
c
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