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Coupling of a Copper Dye with a Copper Electrolyte: a Fascinating Springboard for Sustainable Dye-Sensitized Solar Cells Claudia Dragonetti, Mirko Magni, Alessia Colombo, Fabio Melchiorre, Paolo Biagini, and Dominique Marie Roberto ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00196 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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ACS Applied Energy Materials
Coupling of a Copper Dye with a Copper Electrolyte: a Fascinating Springboard for Sustainable Dye-Sensitized Solar Cells. Claudia Dragonetti,†, ‡ Mirko Magni,†, ‡ Alessia Colombo,†, ‡, * Fabio Melchiorre,§ Paolo Biagini,§ Dominique Roberto†, ‡ †
Dip. di Chimica dell’Università degli Studi di Milano and UdR INSTM di Milano, via Golgi 19, 20133 Milano, Italy Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), SmartMatLab Centre, Via Golgi 19, 20133 Milano, Italy § Research Center for Renewable Energy & Enviromental Istituto Donegani, ENI S.p.A., via Fauser 4, I-28100, Novara, Italy ‡
ABSTRACT: A simple new heteroleptic copper dye, bearing one 2,9-dimesityl-1,10-phenanthroline and a 6,6’-dimethyl-2,2’bipyridine-4,4’-dibenzoic acid anchoring ligand, was used as photosensitizer in dye sensitized solar cells (DSSCs). High power conversion efficiencies are reached not only with conventional I–/I3– electrolyte (40% relative conversion efficiency respect to N719) but, remarkably, also with a well-designed copper-based electrolyte. These proof-of-principle "full-copper" DSSCs open a fascinating route for environmentally friendly DSSCs. KEYWORDS: copper, redox mediators, photosensitizer, DSSCs, energy. Since the discovery of Grätzel-type DSSCs as a solution for harnessing the energy of the Sun and converting it into electricity,1 there has been an exponential growth of work to improve the photo-conversion efficiency (PCE), trying to optimize the sensitizer,2-8 and, in the last few years, the redox mediators.8-13 Except for porphyrin-sensitized14,15 and collaborative organicsensitized16 solar cells filled with Co2+/3+ based electrolytes that reach 13-14% efficiencies, the best PCEs (ca 12 %) have so far been achieved with bipyridyl complexes of ruthenium, such as benchmark cis-di(thiocyanato)bis(2,2’-bipyridine-4,4’dicarboxylate)Ru(II) (N3) and its doubly deprotonated analogue (N719),17 and related Ru(II) complexes,18-20 with the iodide/triiodide couple (I–/I3–) as electrolyte. The role of the carboxylate groups is to anchor the dye to the titania surface. The photoexcitation of the dye involves the transfer of an electron from the Ru center to the π* orbital of the carboxylated bipyridine, allowing electron injection into the TiO2 conduction band. A negative aspect of these Ru(II) complexes is the presence of the thiocyanate ligand which can be easily replaced, giving less efficient species.21 For this reason, a lot of efforts have been devoted to the preparation of thiocyanate-free ruthenium sensitizers.22-28 But, ruthenium is one of the rarest and most expensive metals and this is a drawback in the design of low-cost DSSCs. Photophysical, economic and environmental considerations make copper(I) coordination compounds interesting alternatives to ruthenium complexes.5,7,29-33 As a matter of fact, in the last few years, a phenomenal amount of work has been dedicated to their use as sensitizers in DSSCs5-8,34-36 with the conventional I–/I3– redox couple as electrolyte.
The homoleptic Cu(I)-complexes were the first copper coordination compounds to be used in DSSCs starting from less than 0.1% efficiency37 to reach a maximum of 3.0%, for a copper complex bearing two 6,6’-dimethyl-2,2’-bipyridine-4,4’dibenzoic acid ligands, corresponding to a 33% relative efficiency (ηrel) respect to a N719-sensitized control cell set at 100%.38,39 A big challenge was the use of heteroleptic complexes with two different diimine ligands, one with anchoring groups and the other functionalized with π-delocalized groups aimed to increase light harvesting.5-8 The actual 4.66% record efficiency, corresponding to a remarkable 63% ηrel, was obtained with a copper(I) complex having as anchoring ligand a 4,4’-dicarboxylic acid-6,6’dimesityl2,2’-bipyridine and, as ancillary ligand, a 4,4’-bis(N,Ndiethylaminestyryl)-6,6’dimethyl-2,2’-bipyridine working with the co-adsorbent chenodeoxycholic acid.40 Recently, the investigation of heteroleptic copper(I) complexes has been also extended to the application in iodine-free solar cells in which I–/I3– has been replaced with a Co2+/3+-based electrolyte. In fact a long term incompatibility of copper complexes with the common mediator has been demonstrated, due to the low solubility of CuI.41,42 By using [Co(bpy)3]2+/3+ (bpy = 2,2’bipyridine) as redox couple and a heteroleptic copper(I) complex, having ((6,6’-dimethyl-[2,2’-bipyridine]-4,4’-diyl)bis(4,1phenylene)bis(phosphonic acid) as anchoring ligand and a 2(diphenylaminophenyl)-1H-phenanthro[9,10-d]imidazole derivative ancillary ligand, as dye, Constable and co-workers prepared a fully masked DSSC with a 1.73% efficiency (25% ηrel) comparable to that obtained with the reference I–/I3– electrolyte.42 This combination of copper(I)-based dyes and [Co(bpy)3]2+/3+ electron shuttles was a critical step towards the development of stable iodide-free copper(I) solar cells.43 It just appeared that, in combination with both ruthenium(II) dyes and organic sensitizers, Cu+/2+ mediators outperform not only iodine-based electrolytes44-47 but also common Co-based redox couple.48,49 Fast dye regeneration and tunability of both redox potential and electron transfer rate make Cu mediators prone to solve the thermodynamic and kinetic dichotomies of the device operation.50 These observations, and the fact that well-designed Cu complexes may act both as dyes and mediators, prompted us to couple a simple heteroleptic copper(I) dye with homoleptic copper-based redox couples as electron shuttles in order to open the way toward, environmental friendly DSSCs. Thus, we prepared with the HETPHEN synthetic method51 the new copper(I) dye D (Chart 1) bearing one 2,9-dimesityl-1,10-
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ACS Applied Energy Materials phenanthroline (phe-L), where the mesityl groups provide the necessary steric hindrance to avoid the formation of homoleptic complexes and prevent geometric changes, and a 6,6’-dimethyl2,2’-bipyridine-4,4’-dibenzoic acid (bpy-L), chosen as anchoring ligand because the related homoleptic copper complex had a high efficiency as photosensitizer.39 The performance of D in DSSCs was studied in the presence of three different electrolytes, namely the common I–/I3– electrolyte and two couples of copper redox shuttles (E1/E2 and E3/E4, Chart 1). The Cu complexes with the 2,9-dimethyl-1,10-phenanthroline ligand (E1/E2) were prepared as reported45,47 whereas E3 and E4 with the 2-n-butyl-1,10phenanthroline were prepared from CuCl and CuSO4.5H2O, respectively (see SI).47,52
Table 1. Key optical absorption and cyclovoltammetric data for the investigated complexes. λmax/nm (103 ε /M–1 cm–1)
E1/2,Ox /V vs Fc+|Fca
c
HOMO /eVb
e
-5.19
D
478 (7.6)
0.39
E1
455 (8.0)d
0.30f
-5.10
f
-4.84
d
E2
741 (0.23)
0.04
E3
452 (6.2)d
0.11f
-4.91
f
-4.91
d
E4
756 (0.096)
0.11
a
With [NBu4][PF6] 0.1 M, on GC electrode at 0.2 V s–1. HOMO = –e[E1/2,Ox+4.8]. e = unitary charge; 4.8 = potential of Fc+|Fc couple versus vacuum.54 cIn CH2Cl2. dIn CH3CN. eIn CH3CN/CH2Cl2 25/1, E1/2 Fc+|Fc = 0.40 V vs SCE. fIn CH3CN, E1/2 Fc+|Fc = 0.39 V vs SCE. b
The electrochemical response of D was compared with that of the related free ligands, working in N,N-dimethylformamide, DMF (Figure 1). Both the dicarboxyl bipyridine anchoring ligand, bpy-L, and the sterically hindered phe-L showed CV characteristics in good agreement with similar compounds,27,50 with bpy-L being more easily reduced thanks to the presence of electron withdrawing COOH groups. D shows two subsequent chemically and electrochemically quasi-reversible cathodic peaks due to electron transfers centred on the ligands, probably on the electron poorer bpy-L. On the anodic side the non-canonical signal with peak potential at 0.37 V vs Fc+|Fc can be attributed to the Cu(I)/Cu(II) electron transfer, whose chemical reversibility (backward peak at about –0.26 V vs Fc+|Fc) is hampered by interactions with coordinating solvent. In fact when less donating solvents (CH3CN > CH2Cl2) were employed the same Cu-centred process progressively assumes a higher degree of chemical and electrochemical reversibility (Figure S3) and shifts at more positive potentials.50
-0.5 0.5
s )
1 unit
D
3
-1
j/(cv ) /(A cm dm mol V
The visible region absorption spectrum (SI, Figure S1) of D in CH2Cl2 shows a broad band centred at 478 nm (Table 1), due to a metal-to-ligand charge transfer transition (MLCT), along with a shoulder at higher wavelengths in agreement with the fingerprint of other copper(I) complexes with 2,9-disubstitutedphenathrolines.29,53 Excitation at 478 nm of D in air-equilibrated CH2Cl2 generates an emission band of low intensity at 559 nm (Figure S2). The two Cu(I) mediators (E1 and E3) are characterized by a MLCT around 450 nm, with molar extinction coefficients comparable to those of D, that could hamper light harvesting by D-sensitized TiO2 photoanode, negatively affecting the overall conversion efficiencies. The related oxidized forms E2 and E4 exhibit a very low broad band at 750 nm, attributable to dd transitions and negligible respect to the 100-fold more intense MLCT of D (Table 1). A cyclic voltammetry study (CV) on glassy carbon electrode (GC) was performed to clarify the electrochemical behaviour of both the D dye and the E1/E2 and E3/E4 redox couples. Key energetic levels were also determined to establish the suitability of D as sensitizer for TiO2 photoanodes and to verify the level matching between dye and electron mediators, mandatory for DSSC to work.
phe-L
-2
Chart 1. Chemical structures of the investigated dye (D) and of the two couples of copper-based redox mediators (E1/E2 and E3/E4).
bpy-L
0.5
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-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
+
E vs Fc |Fc /V
Figure 1. CV patterns of D and related free-ligands. Sample concentration 5.5·10–4 M in DMF with [NBu4][PF6] 0.1 M, GC electrode, 0.2 V s–1 scan rate potential. Estimation of the energy level of the highest occupied molecular orbital (HOMO) and of the excited state (U(D*|D+)) of the dye is important to judge its applicability in DSSCs. The HOMO of D was evaluated in the same solvent used in the photoelectrochemical devices (CH3CN, Table 1) while its excited state energy level was estimated according to Equation 155 in CH2Cl2, due to the less propensity of the solvent to quench emission signals, using E1/2,Ox = 0.47 V vs Fc+|Fc (in CH2Cl2 with 0.1 M [NBu4][PF6]) and U0–0 = 2.38 eV (the 0–0 transition energy estimated as U0–0 = hc/λcross by the wavelength at the cross point, λcross (Figure S2), between normalized absorption and emission spectrum in CH2Cl2).56
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ACS Applied Energy Materials
U U ( D * D + ) = −e − 0−0 − E1 / 2,Ox + 4.8 e
Equation 1
According to this equation, the estimated U(D*|D+) is –2.89 eV, a value that should guarantee sufficient driving force to efficiently inject electrons into the underlying TiO2 conduction band, placed at ca –3.5 eV (calculated using E1/2Fc+|Fc = 0.63 V vs NHE).57 As mentioned for D, and reported for related complexes,46,47,49,50 the chemically reversible and electrochemically quasi-reversible electron transfer for E3 and E4 can be attributed to a metal-centred process which brings to a quite limited rearrangement of the coordination sphere around the Cu atom (Figure S4). The quite perfectly superimposable CV patterns of E3 and E4 suggests a mutual interconversion of the two species. The level energy matching between D and Cu-based redox mediators is promising, being the driving force for dye regeneration 0.09 eV and 0.28 eV for E1/E2 and E3/E4, respectively (Table 1). Interestingly, the E3/E4 couple offers a well-balanced oxidation half-wave potential, E1/2,Ox, that could thermodynamically assure an efficient dye recovery without overwhelming the output photovoltage of the device. Dye-sensitized solar cells were fabricated using FTO glass coated TiO2 sensitized with D as photoanode, a platinized FTO counter electrode and an electrolyte solution containing the conventional I–/I3– redox couple, the known E1/E2 and the novel E3/E4 couple (SI). Results of the investigated thin film DSSCs are presented in Table 2 together with those obtained with the Ru(II) benchmark N719 and the previously reported related homoleptic copper(I) dye bearing two 6,6’-dimethyl-2,2’bipyridine-4,4’-dibenzoic acid ligands, Cu(bpy-L)2+.39 In addition to the absolute photoconversion efficiency (η), Table 2 reports the efficiency relative to a cell based on N719 dye and the standard I–/I3– electrolyte set at 100% (ηrel). It appeared that the novel heteroleptic copper complex D bearing one 2,9-dimesityl-1,10-phenanthroline and a 6,6’dimethyl-2,2’-bipyridine-4,4’-dibenzoic acid anchoring ligand (bpy-L), acts as a more efficient dye than the related homoleptic complex Cu(bpy-L)2+, due to a higher photocurrent density (jsc; Table 2) as expected for a better directionality for electron injection in the TiO2 conduction band.5-8 It is worth pointing out that D is much more performing that the related heteroleptic complex bearing the same 2,9-dimesityl-1,10-phenanthroline but 2,2’-biquinoline-4,4’-dicarboxylic acid as anchoring ligand (η = 0.49%; ηrel = 7.5%) and characterized by lower molar extinction coefficients,58 confirming the particular goodness of 6,6’dimethyl-2,2’-bipyridine-4,4’-dibenzoic acid to anchor the dye on the titania surface.39 With the simple heteroleptic dye D, we wanted to check the possibility to fabricate a “full-copper" DSSC as a sustainable route for environmentally friendly DSSCs. Remarkably, substitution of the common I–/I3– redox couple by the well-known E1/E2 couple leads to a fair efficiency (Table 2) that is increased by a factor of 1.5 by using the novel E3/E4 couple (η = 2.0 %; ηrel = 22%) due to a much better jSC and FF. The lower shortcircuit photocurrent observed with the E1/E2 couple can be attributed to a more competitive light harvesting with D, as indicated by a higher molar extinction coefficient of the absorption band around 450 nm (Table 1), in agreement with a lower IPCE curve at 475 nm (Figure 2). In addition to their absorption in the visible region, a limitation of these Cu complexes mediators could be their low solubility which hampers to use an electrolyte solution as concentrated as the conventional I–/I3– solution. In fact, in the case of E1/E2 and E3/E4 redox couples, we succeeded in preparing acetonitrile
solutions till to 0.017 M for the corresponding Cu2+ complex, while in the standard I–/I3– couple the oxidized form is 0.04 M. Therefore, in order to better compare the performances of Cu+/Cu2+ and I–/I3– electrolytes, we fabricated cells with a 2.4 fold more diluted I–/I3– concentration respect to the traditional one. Surprisingly a similar efficiency (η = 3.6 %; ηrel = 40%) was obtained upon dilution but it was reached with a significant decrease of the jsc which compensate for a considerable increase of the Voc (Table 2). This unexpected observation suggests that in Cu-based electrolytes an optimization of the concentration of the components should be performed to understand the entire potentiality of the related DSSCs. Further work is in progress in our laboratory on this fascinating topic. Table 2. Photoelectrochemical performance of DSSCs. Electrolyte, Dye
[oxidizer M]a
mAcm–2
VOC V
FF
η %b
ηrel %c
jSC
N719d
I–/I3– e,f [0.04]
15.4
0.80
71
8.9
100 %
Cu(bpyL)2+ d
I–/I3– e,f [0.04]
7.3
0.59
69
3.0
33 %
D
–
I /I3
– e,f
[0.04]
9.0
0.61
63
3.5
39 %
D
I–/I3–
e,g
[0.017]
8.2
0.67
65
3.6
40 %
D
h
E1/E2 [0.017]
4.7
0.75
36
1.3
15 %
D
E3/E4 h [0.017]
6.3
0.61
53
2.0
22 %
a
b
Concentration of the oxidizer in M. There is a decrease of the efficiencies in the range 25-30% by working in the presence of a 16mm2 black mask. cRelative efficiency respect to a N719sensitized control cell set at 100%. dReference 39. e0.28M tertbutylpyridine in 15/85 (v/v) mixture of valeronitrile/acetonitrile. f 0.65M N-methyl-N-butylimidazolium iodide, 0.025M LiI, 0.04M iodine; in the case of N719, 0.05M guanidinium thiocyanate is present. g0.26M N-methyl-N-butylimidazolium iodide, 0.01M LiI, 0.017M iodine. h0.17M Cu(I) : 0.017M Cu(II) + 0.1M LiTFSI in CH3CN.
Figure 2. IPCE for DSSCs sensitized with D and I–/I3– 0.04 M oxidizer (a), I–/I3– 0.017M oxidixer (b), E1/E2 (c), and E3/E4 (d) as electrolyte. Taking into account that the conventional redox couple I–/I3– has many problems, such as complicated two-electron redox chemistry and corrosiveness, and that although Co2+/3+ complexes are outer-sphere one-electron redox systems with tunable redox potentials, they bring some issues associated with their slow mass transport and large internal reorganization energy between the high-spin d7 and low-spin d6 states which costs driving force for dye regeneration, clearly well-designed Cu+/2+ complexes are promising alternative electron shuttles.48,49 Wonderfully, the present work shows for the first time that they can be combined to copper dyes to give performant solar cells. In conclusion, not only this work confirms the goodness of 6,6’-dimethyl-2,2’-bipyridine-4,4’-dibenzoic acid to anchor Cudyes on the titania surface, but, most importantly it shows that it is
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possible to fabricate environmentally friendly “full copper” DSSCs. Remembering that the efficiency of DSSCs based on Cudyes or Cu-mediators increased by a factor of ca 50 and 100 from their first use, the already good efficiencies of the newborn full copper DSSCs are expected to rapidly increase. We are currently working to improve the solar light harvesting of the Cu-dyes, with well-designed functional groups, and to increase the Voc, by looking for an optimal concentration of the Cu-based electrolytes in the presence of additives such as tert-butylpyridine, as commonly employed with conventional I-/I3- electrolyte.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthesis and characterization of dye and electrolytes; electrochemical measurements; fabrication of dye-sensitized solar cells. (PDF)
AUTHOR INFORMATION Corresponding Author
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT A. C. thanks Università degli Studi di Milano (Piano Sostegno alla Ricerca 2015–17-LINEA 2 Azione A – Giovani Ricercatori) for financial support. We thank Dr Daniele Marinotto (CNR) and Dr Stefano Chiaberge (ENI), for emission and mass spectra, and both Regione Lombardia and Fondazione Cariplo for the use of instrumentation purchased through the SmartMatLab Centre project (2014). The “Ministero degli Affari Esteri e della Cooperazione Internazionale” is also acknowledged (bilateral project Italy-India, Prot. nr. MAE0104617) for financial support.
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