A novel distyryl-substituted boradiazaindacene (BODIPY) dye displays interesting properties as a sensitizer in DSSC systems, opening the way to further exploration of structure-efficiency correlation within this class of dyes

AKKAYA, Engin Umut

Dede, Yavuz

ERTEN ELA, ŞULE

Icli, Burcak

Icli, Siddik

Yılmaz, Mustafa Tuğrul

Organic Letters

English

OpenMETU (Middle East Technical University)

A Panchromatic Boradiazaindacene(BODIPY) Sensitizer for Dye-SensitizedSolar CellsSule Erten-Ela,† M. Deniz Yilmaz,‡ Burcak Icli,‡ Yavuz Dede,‡ Siddik Icli,† andEngin U. Akkaya*,§Solar Energy Institute, Ege UniVersity, Izmir, Turkey TR-35100, Department ofChemistry, Middle East Technical UniVersity, Ankara, Turkey TR-06531, andDepartment of Chemistry and UNAM-Institute of Materials Science andNanotechnology, Bilkent UniVersity, Ankara, Turkey TR-06800eua@fen.bilkent.edu.trReceived May 21, 2008ABSTRACTA novel distyryl-substituted boradiazaindacene (BODIPY) dye displays interesting properties as a sensitizer in DSSC systems, opening theway to further exploration of structure-efficiency correlation within this class of dyes.Among environment friendly and renewable sources ofenergy, solar cells have always been high on the list of likelycandidates. Unfortunately, large-scale commercialization ishampered by the production costs. Dye-sensitized solar cells1(DSSCs) appear to be highly promising alternatives to moreexpensive solar cell technologies. Considering the currentmaximal level of overall conversion efficiency (η) undersimulated sunlight for DSSCs (11%),2 improvements inefficiency and durability would certainly facilitate widespreadutilization of this technology.It is clear that there are a number of factors determiningthe efficiency of solar cells, but the structural and physicalproperties of the sensitizer are clearly important ones. As aresult, many laboratories around the world have active† Ege University.‡ Middle East Technical University.§ Bilkent University.(1) (a) Oregan, B.; Gratzel, M. Nature 1991, 353, 737–740. (b) Hagfeldt,A.; Gratzel, M Acc. Chem. Res. 2000, 33, 269–277.(2) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.;Viscardi, G.; Liska, P.; Ito, S.; Bessho, T.; Gratzel, M. J. Am. Chem. Soc.2005, 127, 16835–16847.ORGANICLETTERS2008Vol. 10, No. 153299-330210.1021/ol8010612 CCC: $40.75  2008 American Chemical SocietyPublished on Web 06/28/2008research programs in optimizing the dye component of theDSSCs. Ruthenium dyes, while holding the record forconversion efficiencies, have relatively low extinction coef-ficients, and they are also considered to be expensive andhard to purify. Motivated by the possibility of finding areplacement for metal-complex dyes, a number of chro-mophores, including coumarins,3 indolines,4 oligoenes,5merocyanines,6 hemicyanines,7 oligothiophenes,8 function-alized thiophenes,9 squaraines,10 benzothiadiazoles,11 peryl-enetetracarboxylic acid derivatives,12 diphenylaminosty-renes,13 and phthalocyanines,14 have been studied, withvarying degrees of success. For the most efficient of thesesensitizers, the overall conversion efficiency is limited to4-7.5% (Figure 2).Boradiazaindacenes (the parent dye, 1), commonly knownas BODIPY dyes, have been recognized15 as useful fluo-rescent labels for biomolecules for some time. Within thepast decade, there is much renewed interest16 in these dyes,due to the development of new avenues for derivatizationand novel applications in a highly diverse field, includingchemosensors,17 logic gates,18 light harvesters,19 energytransfer casettes,20,21 and photodynamic therapy.22 It isknown that photostability of the BODIPY dyes is signifi-cantly better23 than many sensitizers proposed. One simplederivative of BODIPY was in fact studied24 as a sensitizerwithin the DSSC context, and charge separation was clearlyobserved; however, very low overall conversion efficiencieswere obtained.We now propose that, when judiciously designed, BO-DIPY dyes have certain unique features that could make themhighly advantageous compared to most other organic dyes,and thus, they are highly promising in this regard. (i)BODIPYdyeshavehighextinctioncoefficients(70 000-80 000M-1 cm-1) and can easily be modified with any desiredfunctionalities. (ii) Absorption peak can be moved to longerwavelengths through simple modifications, keeping strongabsorption cross sections. (iii) BODIPY dyes have inherentasymmetry in charge redistribution when they undergoS0fS1 transition upon excitation, increasing the chargedensity on the meso-carbon (C-8), while decreasing it in mostother positions in the boradiazaindacene system (SupportingInformation). This inherent directionality of charge redistri-bution pinpoints C-8 as the optimal position of chargeinjection. (iv) The directionality observed in excitation canbe further enhanced with strategically placed electron-withdrawing and electron-donating groups, and on the basisof earlier work,25 cyanoacrylic acid and 4-N,N′-diphenyl-aminophenyl groups are to be of tremendous utility in thisregard.With these considerations, we designed compound 2(Figure 1). The synthesis is convenient; the BODIPY corewas synthesized using monoprotected terephthaldehyde understandard conditions. The green-emitting dye was then(3) Hara, K.; Sayama, K.; Ogha, Y.; Shinpo, A.; Suga, S.; Arakawa, H.Chem. Commun. 2001, 569–570.(4) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc.2004, 126, 12218–12219.(5) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.;Ai, X.; Lian, T.; Yanagida, S. Chem. Mater. 2004, 16, 1806–1812.(6) Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.; Tsukagoshi,S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 1173–1174.(7) Wang, Z.-S.; Li, F.-Y.; Huang, C.-H. Chem. Commun. 2000, 2063–2064.(8) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara,K. J. Am. Chem. Soc. 2006, 128, 14256–14257.(9) Hagberg, D. P.; Yum, J.-H.; Lee, H.; De Angelis, F.; Marinado, T.;Karlsson, K. M.; Humphry-Baker, R.; Sun, L.; Hagfeldt, A.; Gratzel, M.;Nazeeruddin, M. K. J. Am. Chem. Soc. 2008, 130, 6259–6266.(10) (a) Li, C.; Wang, W.; Wang, X.; Zhang, B.; Cao, Y. Chem. Lett.2005, 35, 554–555. (b) Burke, A.; Schmidt-Mende, L.; Ito, S.; Gratzel, M.Chem. Commun. 2007, 234–236.(11) Velusamy, M; Thomas, K. R. J.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C.Org. Lett. 2005, 7, 1899–1902.(12) Shibano, Y.; Umeyama, T.; Matano, Y.; Imahori, Y. Org. Lett. 2007,9, 1971–1974.(13) Xu, W.; Peng, B.; Chen, J.; Liang, M.; Cai, F. J. Phys. Chem. C2008, 112, 874–880.(14) He, J.; Benko, G.; Korodi, F.; Polivka, T.; Lomoth, R.; Akermark,B.; Sun, L.; Hagfeldt, A.; Sundstrom, V. J. Am. Chem. Soc. 2002, 124,4922–4932.(15) (a) Treibs, A.; Kreuzer, F. H. Liebigs Ann. Chem. 1968, 718, 208–223. (b) Haugland, R. P. The HandbooksA Guide to Fluorescent Probesand Labeling Technologies, 10th ed.; Invitrogen Corp., 2005.(16) (a) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed.2008, 47, 1184–1201. (b) Ziessel, R. Compt. Rend. Chim. 2007, 10, 622–629. (c) Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891–4932.(17) (a) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2005, 127, 10464–10465. (b) Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.; Daub, J.J. Am. Chem. Soc. 2000, 122, 968–969. (c) Coskun, A.; Akkaya, E. U.J. Am. Chem. Soc. 2006, 128, 14474–14475. (d) Zeng, L.; Miller, E. W.;Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2006, 128, 10–11.(18) Coskun, A.; Deniz, E.; Akkaya, E. U. Org. Lett. 2005, 7, 5187–5189.(19) (a) Li, F.; Yang, S. I.; Ciringh, Y. Z.; Seth, J.; Martin, C. H.; Singh,D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. L.J. Am. Chem. Soc. 1998, 120, 10001–10017. (b) Yilmaz, M. D.; Bozdemir,O. A.; Akkaya, E. U. Org. Lett. 2006, 8, 2871–2873.(20) Ulrich, G.; Goze, C.; Guardigli, M.; Roda, A.; Ziessel, R. Angew.Chem., Int. Ed. 2005, 44, 3694–3698.(21) Loudet, A.; Bandichhor, R.; Wu, L.; Burgess, K. Tetrahedron 2008,64, 3642–3654.(22) Atilgan, S.; Ekmekci, Z.; Dogan, A. L.; Guc, D.; Akkaya, E. U.Chem. Commun. 2006, 4398–4400.(23) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T. J. Am.Chem. Soc. 2005, 127, 12162–12163.(24) Hattori, S.; Ohkubo, K.; Urano, Y.; Sunahara, H.; Nagano, T.;Wada, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Fukuzumi, S. J. Phys.Chem. B 2005, 109, 15368–15375.(25) Hagberg, D. P.; Edvinson, T.; Marinado, T.; Boschloo, G.; Hagfeldt,Sun, L. Chem. Commun. 2006, 2245–2247.Figure 1. Structure and numbering of the parent boradiazaindacene1 and the structure of the target sensitizer.3300 Org. Lett., Vol. 10, No. 15, 2008condensed with N,N′-diphenylaminobenzaldehyde (preparedfrom triphenylamine via Vilsmeier formylation). Followingthe deprotection of the aldehyde group, it was condensedwith cyanoacetic acid to place electron-withdrawing cyanoand the anchoring carboxylic acid groups in the desiredpositions. An important part of the design in this dye is toplace the anchor and cyano group at the C-8 position, wherethe charge moves on excitation; a perfect placement forefficient charge injection.DFT calculations for the dye 2, at the B3PW91/6-31+G(d,p)//B3LYP/6-31G(d) level of the theory, supportdirectional movement of charge on excitation (SupportingInformation). The HOMO is primarily comprised of πframework of the BODIPY and the styryl groups, withsignificant contributions from π electrons of the dipheny-lamino substituent (Figure 3); whereas the LUMO is veryclearly confined to the π system of the anchor group. (SeeSupporting Information for the nearly identical orbitalpictures obtained with other methods.) This overall picture,found at all levels considered, shows a very clear electrondensity shift from the π framework of the BODIPY coreand the styryl substituents to the pendant carboxyphenylmoiety upon a vertical excitation of the π-π* type.Compound 2 has a relatively broad absorption peak ataround 700 nm in chloroform solution, consistent with itscharge transfer (ICT) characteristics (Figure 4). The molarextinction coefficient at the peak wavelength was found tobe 69 500 M-1 cm-1. Two additional transitions at shorterwavelengths result in a panchromatic performance for thisdye. Cyclic voltammetry of the compound 2 (SupportingInformation) revealed a HOMO energy level of 5.088 eVand a LUMO energy level of 3.517 eV, which is atsufficiently high energy for thermodynamically favorableinjection of electrons to nanocrystalline TiO2-CB. The dye-sensitized cell device was constructed in analogy to literature(for experimental details, see the Supporting Information).Characterization of the sandwich type cell (ESI) yieldedshort-circuit current (JSC) of 4.03 mA/cm2, open-circuitvoltage (VOC) of 562 mV, and fill factor (FF) of 73.5%,giving an η of 1.66%. IPCE data (Figure 5) reveal a nearconstant photon-to-current conversion efficiency with a peakFigure 2. Schematic drawing of a dye-sensitized solar cell: theBODIPY dye is placed under the F-doped SnO2 layer and adsorbedto nanocrystalline TiO2.Figure 3. Schematic representation of the frontier orbitals (left,HOMO; right, LUMO) of compound 2 computed at the B3PW91/6-31+G(d,p)//B3LYP/6-31G(d) level of theory. The surfaces aregenerated with an isovalue at 0.02.Figure 4. Overlayed absorption spectrum of the dye 2 in chloroform(red dashes) and the photocurrent action spectrum (black solid line)obtained under simulated sunlight (AM 1.5). Nearly constantphoton-to-current conversion level between 400 and 800 nm isimpressive but, at the same time, an indication of aggregation onthe TiO2 surface.Figure 5. Current versus voltage in the constructed cell. Voc ) 562mV, Jsc ) 4.03 mA/cm2, FF ) 0.735, η ) 1.66%. Cell’s activesurface area was 0.159 cm2, and the illumination intensity was 100mW/cm2 at 1.5 AM conditions.Org. Lett., Vol. 10, No. 15, 2008 3301value of 22% at 750 nm. This peak value, to the best of ourknowledge, corresponds to the largest reported monochro-matic incident photon-to-current conversion efficiency in thenear IR region (i.e., at 750 nm) for organic dyes, and secondto only a single tris(thiocyanato)terpyridyl-ruthenium com-plex dye.26 In addition, we would like to point out that thereported η value was obtained without any of the additivestypically used (e.g., cheno; 3a,4a-dihydroxy-5b-cholic acid)for enhancing conversion efficiencies.In conclusion, we have demonstrated that boradiaza-indacenes or BODIPYs can be transformed into satisfactorysensitizers especially in the longer wavelength region of thevisible and near-IR region of the solar spectrum. Furtherstructural optimization reducing the chances of aggregationand enhancing the directionality of charge redistribution inthe excited state is very likely to yield more efficientsensitizers. Our work to that end is in progress.Acknowledgment. The authors gratefully acknowledgevaluable advice and guidance provided by Prof. Dr. M.Gratzel and Dr. S. M. Zakeeruddin in EPFL, Switzerland,and State Planning Organization (DPT) and Turkish Acad-emy of Sciences (TUBA) for funding.Supporting Information Available: Synthesis proce-dures, computational details, molecular orbital plots, detailsof cell construction and characterization, and CV data forthe dye. This material is available free of charge via theInternet at http://pubs.acs.org.OL8010612(26) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.;Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover,V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem.Soc. 2001, 123, 1613–1624.3302 Org. Lett., Vol. 10, No. 15, 2008

A panchromatic boradiazaindacene (BODIPY) sensitizer for dye-sensitized solar cells

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A panchromatic boradiazaindacene (BODIPY) sensitizer for dye-sensitized solar cells

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