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2009, 113, 14595–14597 Published on Web 07/27/2009
A Broadly Absorbing Perylene Dye for Solid-State Dye-Sensitized Solar Cells Ute B. Cappel,† Martin H. Karlsson,† Neil G. Pschirer,*,‡ Felix Eickemeyer,‡ Jan Scho¨neboom,‡ Peter Erk,‡ Gerrit Boschloo,*,† and Anders Hagfeldt† Department of Physical and Analytical Chemistry, Ångstro¨m Laboratory, Uppsala UniVersity, Box 259, 751 05 Uppsala, Sweden, and BASF SE, Speciality Chemicals Research, D-67056 Ludwigshafen, Germany ReceiVed: July 07, 2009
We present a new perylene sensitizer, ID176, for dye-sensitized solar cells (DSCs). The dye has the capability for very high photocurrents due to strong absorption from 400 to over 700 nm. Photocurrents of up to 9 mA cm-2 were achieved in solid-state DSCs employing the hole conductor 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD), with a conversion efficiency of 3.2%. In contrast, the sensitizer did not perform well in conjunction with liquid iodide/tri-iodide electrolytes, suggesting a difference in the injection and regeneration mechanisms in these two types of dye-sensitized solar cells. Dye-sensitized solar cells (DSCs) are the focus of much research due to their ability to convert sunlight to electricity at a low cost.1 DSCs using a liquid electrolyte with the iodide/ tri-iodide redox couple have reached power conversion efficiencies of over 11%.2 In comparison, solid-state DSCs (sDSCs) using the hole conductor 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) have reached efficiencies of 4-5% with dyes showing much higher conversion efficiencies in liquid electrolyte DSCs.3-6 Device performances of sDSCs are best when thin TiO2 films (∼2 µm) are used due to limitations in pore filling and losses during charge transport. With ruthenium dyes, this limits the light harvesting as they have relatively low extinction coefficients. In comparison, organic dyes have much higher extinction coefficients but often also narrower absorption spectra.7 Thus, to improve efficiencies, organic dyes with high extinction coefficients over a large range of the solar spectrum are needed. Here, we present a new perylene sensitizer, ID176, which absorbs strongly from 400 to above 700 nm. Intriguingly, this dye works well in solidstate DSCs but not in liquid electrolyte ones. Perylene dyes are promising candidates for DSC applications due to ease of synthesis, high stability, and broad absorption spectra.8 However, previously published perylenes with good efficiencies in DSCs9-11 were bound to TiO2 through ringopening of an anhydride group, resulting in a significant spectral blue shift of their absorption spectra upon attachment to TiO2. In this work, we have replaced the anhydride anchor with a carboxylic acid anchor attached to a perylene monoimide, resulting in ID176 (Figure 1). The HOMO orbital of ID176 is mainly located on the bis(phenylamine) donor, while the LUMO orbital is mainly located on the perylene core. Thus, light absorption in this dye leads to internal charge transfer from the donor to the extremely strong imide acceptor. Figure 2 shows the absorption spectrum of ID176 in dichloromethane (DCM) * To whom correspondence should be addressed. E-mail: Neil.Pschirer@ basf.com (N.G.P.);
[email protected] (G.B.). † Uppsala University. ‡ BASF SE, Speciality Chemicals Research.
10.1021/jp906409q CCC: $40.75
Figure 1. Chemical structure of ID176 (left) and the frontier molecular orbitals, HOMO (middle) and LUMO (right), calculated with DFT and B3LYP/def-SV(P).
Figure 2. Normalized UV-visible absorption of ID176 in DCM (solid line) and on TiO2 (dashed line) and emission in DCM (dotted line).
solution and on TiO2. In solution, two absorption bands can be seen, one centered at 590 nm and a sideband at 480 nm. According to time-dependent DFT calculations, these bands correspond to a transition from the HOMO to the LUMO and to a transition from the HOMO-1 to the LUMO, respectively. Upon adsorption to TiO2, only a slight blue shift in the absorption maximum, from 590 to 550 nm, is observed, while the absorption onset remains unchanged. Thus, the change of anchor was successful for obtaining a red absorbing dye when attached to TiO2. Addition of Li+ ions shifts the absorption 2009 American Chemical Society
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J. Phys. Chem. C, Vol. 113, No. 33, 2009
spectrum of the dye on TiO2 to longer wavelengths, leading to even more light harvesting in solar cells (data in S4, Supporting Information). The maximum extinction coefficient in DCM was determined to be 25000 M-1 cm-1 at 590 nm. Cyclic voltammetry of ID176 on TiO2 in a supporting electrolyte of 0.1 M tetrabutylammonium perchlorate showed a reversible oxidation at Eox ) 0.58 V versus ferrocene (1.21 V vs NHE). This is positive of both the redox potential of iodide/tri-iodide (-0.32 V vs ferrocene in methoxypropionitrile (MPN)12) and the valence band edge of spiro-MeOTAD (0.15 V vs ferrocene13). The zero-zero transition energy (E0-0) was determined from the crossing point of the normalized absorption and emission spectra in DCM (Figure 2) to be 1.85 eV. Adding Eox and E0-0 places the LUMO at -0.64 V versus NHE. This is more negative than the conduction band edge of TiO2 (approximately -0.5 V vs NHE) but does not provide a large driving force for electron injection. Dye-sensitized solar cells were fabricated on fluorine-doped tin oxide substrates. For liquid electrolyte DSCs, 7 µm thick TiO2 films (active area: 0.25 cm2) were applied by doctor blading of a colloidal TiO2 paste (Dyesol, DSL 18NR-T). For sDSCs, TiO2 blocking layers were prepared on the same substrates by spray pyrolysis.14 Following this, the TiO2 paste, diluted with terpineol, was applied by screen printing, resulting in a film thickness of 2 µm. All films were then sintered for 1 h at 450 °C, followed by a 30 min treatment in a 40 mM aqueous solution of TiCl4 at 65 °C, followed by another sintering step. The electrodes were then dyed in 0.5 mM ID176 solution in DCM. Electrolyte DSCs were assembled with a platinized counter electrode and filled with an electrolyte containing 0.1 M iodine and 0.5 M lithium iodide in MPN (cell: “liquid 1”) or an electrolyte containing 0.1 M iodine, 0.1 M lithium iodide, 0.6 M tetrabutylammonium iodide (TBAI), and 0.5 M tert-butyl pyridine (tBP) in MPN (cell: “liquid 2”). For sDSCs, spiroMeOTAD was applied by spin-coating from a solution in chlorobenzene also containing 15 mM Li(CF3SO2)2N (cell: “solid 1”) or 15 mM Li(CF3SO2)2N and 60 mM tBP (“solid 2”). Device fabrication was completed by evaporation of 200 nm of silver as the counter electrode. The active area of the solid-state DSCs was defined by the size of these contacts (4 mm × 5 mm), and the cells were masked with an aperture of 5 mm × 5.5 mm for measurements. Current-voltage characteristics at 1000 W/m2 under AM 1.5G conditions and the incident photon to current conversion efficiency (IPCE) spectra were measured for all cells. IPCE and current-voltage results are shown in Figure 3 and Table 1. The highest IPCE is obtained for the solid-state DSC when only Li(CF3SO2)2N is used as an additive to spiroMeOTAD, with a maximum of over 50% at 560 nm. Remarkably, IPCE values at 700 nm are over 30%, which is much higher than those for the best dyes in sDSCs, including ruthenium dyes at this wavelength.4-6 Upon addition of tBP, the IPCE values drop approximately 50% in intensity. Surprisingly, despite the thicker films in the electrolyte cells, the IPCEs are considerably poorer than those of the sDSCs, even in the case where no tBP is present in the electrolyte (“liquid 1”). Cell “liquid 2”, containing tBP, shows almost no photocurrent. In the electrolyte DSCs, the IPCE values correspond to the product of the injection efficiencies and the regeneration efficiencies as the absorbance of the 7 µm films is much higher than 1 for most regions of the spectrum and charge collection efficiencies are close to unity (determined by measurements of the electron diffusion length; S5, Supporting Information). Thus, the addition of tBP causes a 10-fold decrease in the injection efficiencies
Letters
Figure 3. (a) IPCE spectra and (b) current-voltage characteristics of ID176-sensitized solar cells with spiro-MeOTAD (solid) and liquid electrolyte (liquid) without tBP (1) and with tBP (2).
TABLE 1: Solar Cell Performances of sDSCs and Electrolyte DSCs with ID176 cell
Jsc calc/mA/cm2
Jsc/ mA/cm2
Voc/mV
FF
η/%
solid 1 solid 2 liquid 1 liquid 2
9.6 4.7 4.9 0.5
8.7 5.4 4.2 0.4
640 780 440 460
0.57 0.53 0.66 0.74
3.2 2.3 1.2 0.14
a Jsc calculated from the IPCE spectrum and AM1.5 G photon flux.
for the electrolyte DSCs. This can be explained by tBP shifting the conduction band of TiO2 to more negative potentials, hindering injection.12 Comparing the sDSC and the electrolyte DSC without tBP, the injection efficiency is at least a factor of 2 lower in the electrolyte cell. In the electrolyte DSC, the IPCE maximum corresponds to the HOMO-1 to LUMO transition at 470 nm, where a side shoulder appears in the IPCE for the sDSCs. This suggests that for the electrolyte DSC, dye regeneration is more efficient to the HOMO-1 than that to the HOMO. These observations suggest a difference in the injection and regeneration mechanisms in the solid-state DSC when compared to the electrolyte DSC, resulting in higher short-circuit currents (Jsc) in the sDSCs for this particular dye. One difference between spiro-MeOTAD and iodide/tri-iodide as redox mediators is that the regeneration of oxidized dye molecules by spiroMeOTAD is about an order of magnitude faster than the regeneration by iodide.15 Therefore, spiro-MeOTAD might be able to more effectively intercept the recombination of oxidized dye with electrons in TiO2 than iodide.16 An alternative explanation is that reduction of excited dye molecules by spiroMeOTAD may precede electron injection into TiO2. Such a mechanism has been recently suggested for the indolene dye, D149.17 For our system, such a mechanism is feasible as the energy of the LUMO of ID176 is very close to the conduction band edge of TiO2 and injection might be limited or slow. Studies of the system by ultrafast spectroscopy to investigate which one of these mechanisms applies are ongoing. The higher photocurrents in the sDSCs combined with an increase in open-circuit voltage (Voc) lead to a 2- to 3-fold higher
Letters power conversion efficiency (η) of cells “solid 2” and “solid 1” compared to that of cell “liquid 1”. A 140 mV increase in Voc is observed between cells “solid 1” and “solid 2”, confirming the negative shift of the conduction band by tBP. As adding tBP decreases the quantum efficiency of the dye, the overall best efficiency was achieved in a sDSC without tBP with a value of 3.2%. To our knowledge, this is the best efficiency reported for solid-state DSCs, which do not contain this volatile additive, and demonstrates that tBP is not necessary for good solar cell performances. In summary, we have presented a perylene dye absorbing as broad as ruthenium dyes in DSCs with a much higher extinction coefficient in the red part of the solar spectrum. The dye performs much better in solid-state DSCs than in those using iodide/tri-iodide liquid electrolytes. This result demonstrates that dyes, which have not been successfully applied in electrolyte DSCs, can still be candidates for solid-state DSCs, as different injection and regeneration mechanisms might apply. Acknowledgment. We acknowledge the financial support of the BMBF (Bundesministerium fu¨r Bildung und Forschung) and thank Anke Handreck and Alfred Kuhn (BASF SE) for sample preparation. Supporting Information Available: Detailed results and technical data on synthetic procedures, DFT calculations, characterization of ID176, and devices. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269.
J. Phys. Chem. C, Vol. 113, No. 33, 2009 14597 (2) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (3) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583. (4) Snaith, H. J.; Moule, A. J.; Klein, C.; Meerholz, K.; Friend, R. H.; Gra¨tzel, M. Nano Lett. 2007, 7, 3372. (5) Wang, M.; Xu, M.; Shi, D.; Li, R.; Gao, F.; Zhang, G.; Yi, Z.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. AdV. Mater. 2008, 20, 4460. (6) Yum, J.; Hagberg, D. P.; Moon, S.; Karlsson, K. M.; Marinado, T.; Sun, L.; Hagfeldt, A.; Nazeeruddin, M. K.; Gra¨tzel, M. Angew. Chem., Int. Ed. 2008, 47, 1. (7) Mishra, A; Fischer, M. K. R; Ba¨uerle, P. Angew. Chem., Int. Ed. 2009, 48, 2474. (8) Herrmann, A.; Mullen, K. Chem. Lett. 2006, 35, 978. (9) Ferrere, S.; Gregg, B. A. New J. Chem. 2002, 26, 1155. (10) Edvinsson, T.; Li, C.; Pschirer, N.; Scho¨neboom, J.; Eickemeyer, F.; Sens, R.; Boschloo, G.; Herrmann, A.; Mu¨llen, K.; Hagfeldt, A. J. Phys. Chem. C 2007, 111, 15137. (11) Li, C.; Yum, J.; Moon, S.; Herrmann, A.; Eickemeyer, F.; Pschirer, N. G.; Erk, P.; Scho¨neboom, J.; Mu¨llen, K.; Gra¨tzel, M.; Nazeeruddin, M. K. ChemSusChem 2008, 1, 615. (12) Boschloo, G.; Ha¨ggman, L.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13144. (13) Cappel, U. B.; Gibson, E. A.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2009, 113, 6275. (14) Peng, B.; Jungmann, G.; Ja¨ger, C.; Haarer, D.; Schmidt, H. W.; Thelakkat, M. Coord. Chem. ReV. 2004, 248, 1479. (15) Bach, U.; Tachibana, Y.; Moser, J.-E.; Haque, S. A.; Durrant, J. R.; Gra¨tzel, M.; Klug, D. R. J. Am. Chem. Soc. 1999, 121, 7445. (16) Tatay, S.; Haque, S. A.; O’Regan, B; Durrant, J. R.; Verhees, W. J. H.; Kroon, J. M.; Vidal-Ferran, A.; Gavin˜a, P.; Palomares, E. J. Mater. Chem. 2007, 17, 3037. (17) Snaith, H. J.; Petroza, A.; Ito, S.; Miura, H.; Gra¨tzel, M. AdV. Funct. Mater. 2009, 19, 1.
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