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

Bozic-Weber, B.

Constable, E. C.

Fürer, S. O.

Housecroft, C. E.

Troxler, L. J.

Zampese, J. A.

Biljana Bozic-Weber

Edwin C. Constable

Sebastian O. Fürer

Catherine E. Housecroft

Lukas J. Troxler

Jennifer A. Zampese

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Chemical Communications

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

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7222 Chem. Commun., 2013, 49, 7222--7224 This journal is c The Royal Society of Chemistry 2013Cite this: Chem. Commun.,2013,49, 7222Copper(I) dye-sensitized solar cells with [Co(bpy)3]2+/3+electrolyte†Biljana Bozic-Weber, Edwin C. Constable,* Sebastian O. Fu¨rer,Catherine E. Housecroft,* Lukas J. Troxler and Jennifer A. ZampeseThe hierarchical assembly of DSCs containing a new heterolepticcopper(I) complexwith a phosphonic acid anchoring ligand is described;it is shown that conventional I/I3 electrolytes may be replacedby [Co(bpy)3]2+/3+ with no loss in performance.Dye-sensitized solar cells (DSCs) are photovoltaic devices in which theresponse of a semiconductor is extended into the visible region by useof a coloured sensitizer.1 Attention is centred upon the replacement offirst generation sensitizers based on ruthenium complexes by thoseusing othermetals or organic dyes. Copper(I) complexes [Cu(Lanc)(L)]n+are leading contenders for use as dyes based on Earth abundantmetals.2 We are currently optimizing the performance of DSCs basedon copper(I) complexes and report in this paper on (i) the improve-ment of the spectral response of the dyes (ii) the stability of thesurface-bound dyes and (iii) the long-term stability of the DSC.We have developed procedures for the assembly of heterolepticcopper(I) dyes by ligand exchange between a surface-anchoredligand Lanc and [Cu(L)2]n+.3–7 Pellegrin, Daniel, Odobel and coworkers8have reported DSCs with an extended spectral response utilizing a2-(diarylaminophenyl)-1H-phenanthro[9,10-d]imidazole ancillaryligand.8 We now describe our studies of DSCs using relatedligands, which both give improved spectral characteristics andimprove the stability of the heteroleptic complexes (phen com-plexes are typically one logK unit more stable than bpy analogues).Optimal performance is achieved with phosphonic acid anchors,which both stabilize the surface-bound complex with respect tocarboxylate Lanc systems and lead to enhanced energy conversionefficiencies.4–7,9 The inclusion of an aryl spacer between bpy-unitand PO(OH)2-anchor is beneficial.7 Finally, the long term stabilityof the DSC (which may be limited by the formation of CuI in thepresence of I/I3 electrolyte) has been addressed by changing to a[Co(bpy)3]2+/[Co(bpy)3]3+ electrolyte.10 This latter change hasbeen coupled with modifications of the photoanode at themesoscopic level, specifically by changing the semiconductorfilm thickness11,12 and by post-treatment with H2O–TiCl4.13The ancillary ligand L facilitates light-harvesting and 2-(4-bromo-phenyl)-1H-phenanthro[9,10-d]imidazole14 provides a convenientstarting point for the design of a ligand family. The first designfeature is the introduction of 6,60-dialkyl or diaryl substituents tostabilize the copper(I) oxidation state.15 The second is the incorpora-tion of a triphenylamino-substituent to extend the p-conjugation andincrease the spectral response. The third is the introduction of along chain substituent which is known to suppress intermolecularaggregation by preventing extensive face-to-face p-interactionsbetween aromatic units.16 Self-assembly of dyes at the semiconduc-tor surface produces a hydrophobic monolayer that repels iodideions in the I/I3 electrolyte thereby reducing the rate of recombina-tion and increasing the open circuit voltage (Voc).17Scheme 1 summarizes our synthetic approach to a firstgeneration ligand starting from 2,9-dimethyl-1,10-phenanthro-line-5,6-dione.18 Treatment of the latter with 4-bromobenzalde-hyde and an excess of NH4OAc in ethanol yielded compound 1.The 1H and 13C NMR spectra are consistent with a tautomericequilibrium involving the imidazole NH and N sites.† In situdeprotonation of 1 and reaction with n-octyl bromide yielded 2which was fully characterized.† The highest mass peak in theelectrospray mass spectrum (m/z 515.2) corresponded to the [M +H]+ ion. In contrast to 1, the NMR spectra are desymmetrized andtwo signals are observed (d 2.98 and 2.96 ppm in CDCl3) for theHMe-phen protons. Single crystals were grown from a toluene–ethylacetate solution, and Fig. 1 shows the structure of 2.† The octylchain is ordered and lies over the bromophenyl unit, the arenering of which is twisted 58.61 out of the plane of the imidazolering. The conformation of the octyl chain allows close CH  p andCH  Br contacts; the distance C29H29A  Br1 of 3.12 Å is close tothe sum of the van der Waals radii, using a value of 1.10 Å19 forH rather than the Bondi value of 1.20 Å.20 The closest CH  pcontacts are 3.30 and 3.66 Å. Buchwald–Hartwig amination of 2with diphenylamine resulted in the formation of 3; the highestmass peak in the ESI MS (m/z 604.3) arises from [M + H]+, andthe 1H and 13C NMR spectra are consistent with the structureshown in Scheme 1.Department of Chemistry, University of Basel, Spitalstrasse 51, CH4056 Basel,Switzerland. E-mail: edwin.constable@unibas.ch, catherine.housecroft@unibas.ch;Tel: +41 61 267 1008† Electronic supplementary information (ESI) available: Experimental data.Fig. S1: EQE spectra. CCDC 945455. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/c3cc44595jReceived 19th June 2013,Accepted 4th July 2013DOI: 10.1039/c3cc44595jwww.rsc.org/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 11 July 2013. Downloaded on 28/11/2016 15:13:52. View Article OnlineView Journal  | View IssueThis journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7222--7224 7223The homoleptic complex [Cu(3)2][PF6] was prepared in quanti-tative yield by reaction of [Cu(NCMe)4][PF6] with two equivalents of3. The MALDI mass spectrum showed peaks at m/z 1271.6 and666.9 corresponding to [M  PF6]+ and [M  3–PF6]+ with isotopepatterns matching those calculated. The 1H and 13C NMR spectrawere consistent with a single ligand environment, the presence ofthe octyl chain desymmetrizing the phenanthro[9,10-d]imidazoleunit.† 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) inthe UV region is consistent with the presence of two ligands in thecomplex. The spectral response of the complex extends to 600 nm,with lmax for theMLCT band at 476 nm. Reversible oxidation of theCu(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 mostprobably associated with oxidation of the Ph2N unit. No reductionprocesses are observed within the solvent accessible window.Fully masked DSCs22 were fabricated by applying a scatteringTiO2 layer to screen-printedmesoporous TiO2 (3 or 4 layers) followedby post-treatment with H2O–TiCl4,13 and then soaking the cells in asolution of anchoring ligand 4 (Scheme 2). This was followed by aperiod 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 assessedthe DSC characteristics of cells containing I/I3 electrolyte and[Cu(3)(4)]+ sensitizer supported on 3 layers of TiO2 post-treated withdifferent concentrations of H2O–TiCl4. Table 1 summarizes mea-surementsmade on the day of sealing the cells. I–V characteristics ofthe same cells were recorded one, six and eight days after sealing,and only small changes in values of JSC, VOC and Z were observed, incontrast to the ripening effects noted with other copper-basedDSCs.4,6,7 Post-treatment with 5 mmol dm3 H2O–TiCl4 results inimproved JSC and VOC values with concomitant increase in efficiency.Treatment of 3 layers of TiO2 post-treated with higher concentrationshad little or no effect on the overall energy conversion efficiencies ofthe DSCs. The effect of replacing I/I3 electrolyte by [Co(bpy)3]2+/[Co(bpy)3]3+ is seen by comparing the data in Tables 1 and 2. For allthe cells in Table 1 and the top half of Table 2, the photoanodes wereleft to stand in solutions of [Cu(3)2][PF6] for 24 hours. The resultssuggest that energy conversion efficiency is increased by usinghigher concentrations of TiCl4 during post-treatment. Since dyeScheme 1 Synthesis of ligand 3. Conditions: (i) 4-bromobenzaldehyde, excessNH4OAc, 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 andH atoms omitted for clarity).Fig. 2 Absorption spectra of 3 (red) and [Cu(3)2][PF6] (blue) in CH2Cl2.Scheme 2 Structure of anchoring ligand 4.Table 1 DSC performances using [Cu(3)2][PF6], anchor 4, I/I3 electrolyte, and3 layers of TiO2; dipping time = 24 h. Measurements made on the day of sealing[TiCl4]/mmol dm3 JSC/mA cm2 VOC/mV ﬀ Z/%0 1.99 588 0.69 0.805 2.73 612 0.70 1.1815 2.00 608 0.67 0.8130 2.25 599 0.65 0.8840 2.11 591 0.64 0.8060 2.34 592 0.64 0.89Communication ChemCommOpen Access Article. Published on 11 July 2013. Downloaded on 28/11/2016 15:13:52. View Article Online7224 Chem. Commun., 2013, 49, 7222--7224 This journal is c The Royal Society of Chemistry 2013uptake by the TiO2 is affected by the time over which the photo-anodes are left to soak in solutions of the dye, we prepared a set ofDSCs with a dipping time of 64 hours. In all other respects, the DSCfabrication was the same as that of cells in which the dippingtime was 24 hours. Cell characteristics (Table 2) show that alonger soaking time leads to superior performance, and that theoptimum concentrations of H2O–TiCl4 during post-treatment are>15 mmol dm3. The external quantum efficiency (EQE) spectra forfour of the DSCs in the top part of Table 2 are shown in Fig. S1(ESI†), and confirm that the highest concentration of TiCl4 leadsto the highest EQE (E20%) and improved photon-to-current con-version efficiencies, especially at longer wavelengths.Comparison of the data in Tables 2 and 3 reveals a significantimprovement of cell performance upon increasing the thicknessof screen-printed TiO2 from 3 to 4 layers. A longer dipping time(110 hours) was used, and a maximum eﬃciency (Z) of 2.02% wasachievedwith a post-treatment of 60mmol dm3 H2O–TiCl4. Trends indata are generally maintained for measurements made with duplicatecells and give maximum values of Z with 40 or 60 mmol dm3H2O–TiCl4, but absolute values of Z show variation. Fig. 3 showscorresponding EQE spectra; a maximum photon-to-current conversionefficiency of 32% is observed. For comparison, 4-layer DSCs weremadeusing I/I3 electrolyte retaining [Cu(3)(4)]+ as dye and a dipping timeof 110 h. With post-treatment of 40 mmol dm3 H2O–TiCl4, the cellcharacteristics were JSC = 5.11 mA cm2, VOC = 574 mV, ff = 0.71 andZ = 2.08% (compared to Z = 6.90% for an analogous DSC using N719).Preliminary light-stability tests in which post-treated 4-layer DSCscontaining anchored-[Cu(3)(4)]+ dye and [Co(bpy)3]2+/3+ or I/I3 electro-lyte were continuously illuminated (100 mW cm2) indicate that allcells show similar stabilities over a 60 hour period.After optimization, the eﬃciencies of DSCs containing [Cu(3)(4)]+dye anchored onH2O–TiCl4 post-treated 4 layer TiO2 are comparableusing either [Co(bpy)3]2+/3+ or I/I3 electrolyte. This is the first reportof DSCs which combine copper(I)-based dyes and [Co(bpy)3]2+/3+electrolyte, and is a critical step towards the development of stableiodide-free copper(I) solar cells.The European Research Council (Advanced Grant 267816 LiLo),Swiss National Science Foundation and University of Basel areacknowledged for financial support. Nik Hostettler and Dr ColinMartin recorded 500 MHz NMR spectra, and Gino Gu¨nzburger andEwald Scho¨nhofer assisted with DSC stability tests.Notes and references1 K. Kalyanasundaram, Dye-sensitized Solar Cells, EPFL Press, Lausanne,2010.2 B. Bozic-Weber, E. C. Constable and C. E. Housecroft, Coord. Chem.Rev., 2013, DOI: 10.1016/j.ccr.2013.05.019.3 A. H. Redondo, E. C. Constable andC. E. Housecroft,Chimia, 2009, 63, 205.4 B. Bozic-Weber, V. Chaurin, E. C. Constable, C. E. Housecroft,M. Meuwly, M. Neuburger, J. A. Rudd, E. Scho¨nhofer andL. Siegfried, Dalton Trans., 2012, 41, 14157.5 B. Bozic-Weber, E. C. Constable, C. E. Housecroft, P. Kopecky,M. Neuburger and J. A. Zampese, Dalton Trans., 2011, 40, 12584.6 B. Bozic-Weber, S. Y. Brauchli, E. C. Constable, S. O. Fu¨rer, C. E.Housecroft and I. A. Wright, Phys. Chem. Chem. Phys., 2013, 15, 4500.7 B. Bozic-Weber, S. Y. Brauchli, E. C. Constable, S. O. Fu¨rer,C. E. Housecroft, F. J. Malzner, I. A. Wright and J. A. Zampese,Dalton Trans., 2013, DOI: 10.1039/c3dt51416a.8 M. Sandroni, M. Kayanuma, A. Planchat, N. Szuwarski, E. Blart,Y. Pellegrin, C. Daniel, M. Boujtita and F. Odobel, Dalton Trans.,2013, DOI: 10.1039/c3dt50852h.9 P. Pe´chy, F. P. Rotzinger,Md. K. Nazeeruddin, O. Kohle, S.M. Zakeeruddin,R. Humphry-Baker andM. Gra¨tzel, J. Chem. Soc., Chem. Commun., 1995, 65.10 T. W. Hamann, Dalton Trans., 2012, 41, 3111.11 S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boschloo andA. Hagfeldt, J. Am. Chem. Soc., 2010, 132, 16714.12 E. A. Gibson, A. L. Smeigh, L. Le Pleux, L. Hammarstro¨m, F. Odobel,G. Boschloo and A. Hagfeldt, J. Phys. Chem. C, 2011, 115, 9772.13 S.-W. Lee, K.-S. Ahn, K. Zhu, N. R. Neale and A. J. Frank, J. Phys. Chem.,2012, 116, 21285; P. M. Sommeling, B. C. O’Regan, R. R. Haswell, H. J. P.Smit, N. J. Bakker, J. J. T. Smits, J. M. Kroon and J. A. M. van Roosmalen,J. Phys. Chem. B, 2006, 110, 19191; B. C. O’Regan, J. R. Durrant,P. M. Sommeling and N. J. Bakker, J. Phys. Chem. C, 2007, 111, 14001.14 D. W. Allen, J. Hawkrigg, H. Adams, B. F. Taylor, D. E. Hibbs andM. B. Hursthouse, J. Chem. Soc., Perkin Trans. 1, 1998, 335.15 W. W. Brandt, F. P. Dwyer and E. D. Gyarfas, Chem. Rev., 1954, 545, 959.16 Y. Ooyama and Y. Harima, Eur. J. Org. Chem., 2009, 2903.17 Md. K. Nazeeruddin, E. Baranoﬀ and M. Gra¨tzel, Sol. Energy, 2011,85, 1172.18 R. H. Zheng, H. C. Guo, H. J. Jiang, K. H. Xu, B. B. Liu, W. L. Sun andZ. Q. Shen, Chin. Chem. Lett., 2010, 1270.19 R. S. Rowland and R. Taylor, J. Phys. Chem., 1996, 100, 7384.20 A. Bondi, J. Phys. Chem., 1964, 68, 441.21 M. K. Eggleston, D. R. McMillin, K. S. Koenig and A. J. Pallenberg,Inorg. Chem., 1997, 36, 172.22 H. J. Snaith, Energy Environ. Sci., 2012, 5, 6513.Table 2 DSC performances using [Cu(3)2]+, anchor 4, [Co(bpy)3]2+/3+ and 3layers TiO2. Measurements were made on day of sealing the cell[TiCl4]/mmol dm3 JSC/mA cm2 VOC/mV ﬀ Z/%Cells dipped into solution of [Cu(3)2][PF6] for 24 h0 1.15 380 0.49 0.225 1.75 535 0.53 0.5015 1.62 564 0.56 0.5130 1.56 590 0.61 0.5640 2.20 542 0.55 0.6660 3.17 596 0.63 1.19Cells dipped into solution of [Cu(3)2][PF6] for 64 h0 2.28 586 0.55 0.735 1.83 611 0.62 0.6915 3.44 628 0.70 1.5030 3.48 619 0.70 1.5140 2.87 621 0.68 1.2160 2.28 586 0.55 0.73Table 3 DSC performances with [Cu(3)2]+, anchor 4, [Co(bpy)3]2+/3+ and 4 layersTiO2; dipping time 110 h. Measurements made on day of sealing[TiCl4]/mmol dm3 JSC/mA cm2 VOC/mV ﬀ Z/%0 3.01 545 0.51 0.835 3.33 505 0.51 0.8515 2.39 391 0.51 0.4830 3.39 427 0.52 0.7540 5.05 578 0.59 1.7360 4.95 610 0.67 2.02Fig. 3 EQE spectra for five of the DSCs listed in Table 3.ChemComm CommunicationOpen Access Article. Published on 11 July 2013. Downloaded on 28/11/2016 15:13:52. View Article Online

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

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

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