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Notes Application of Metalloporphyrins in Nanocrystalline Dye-Sensitized Solar Cells for Conversion of Sunlight into Electricity Md. K. Nazeeruddin,*,† R. Humphry-Baker,† David L. Officer,‡ Wayne M. Campbell,‡ Anthony K. Burrell,‡ and M. Gra¨tzel*,† Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland, and Nanomaterials Research Centre and the MacDiarmid Institute for Advanced Materials and Nanotechnology, Massey University, Private Bag 11222, Palmerston North 5301, New Zealand Received February 13, 2004. In Final Form: May 4, 2004
Introduction Nanocrystalline dye-sensitized solar cells have attracted significant attention as environmentally benign and low cost alternatives to conventional solid state photovoltaic devices.1-8 The most successful charge transfer sensitizer employed so far in such cells, cis-dithiocyanatobis(2,2′bipyridyl-4,4′-dicarboxylate)ruthenium(II) (together with its various protonated forms), yields conversion efficiencies of 9-10% under air mass (AM) 1.5 solar conditions.9 The use of porphyrins as light harvesters on semiconductors is particularly attractive, given their primary role in photosynthesis and the relative ease with which a variety of covalent or noncovalent porphyrin arrays can be constructed.10 Some metalloporphyrins have been tested in the past on TiO2 semiconductors, and reasonable efficiencies have been measured.11-16 In this note, we * Authors to whom correspondence should be addressed. Email:
[email protected] (Md.K.N.). † Swiss Federal Institute of Technology. ‡ Massey University. (1) Gra¨tzel, M. Prog. Photovoltaics 2000, 8, 171. (2) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 8607-8611. (3) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Ceram. Soc. 2003, 125, 475-482. (4) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597-606. (5) Benkstein, K. D.; Kopidakis, N.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2003, 107, 7759-7767. (6) Qiu, F. L.; Fisher, A. C.; Walker, A. B.; Petecr, L. M. Electrochem. Commun. 2003, 5, 711-716. (7) He, J.; Benko, G.; Korodi, F.; Polivka, T.; Lomoth, R.; A° kermark, B.; Sun, L.; Hagfeldt, A.; Sundstrom, V. J. Am. Chem. Soc. 2002, 124, 4922. (8) Heimer, T. A.; Heilweil, E. J.; Bignozzi, C. A.; Meyer, G. J. J. Phys. Chem. A 2000, 104, 4256-4262. (9) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (10) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. Rev., accepted for publication, 2004. (11) Dabestani, R.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. B 1988, 92, 1872-1878. (12) Boschloo, G. K.; Goossens, A. J. Phys. Chem. 1996, 100, 1948919494. (13) Wamser, C. C.; Kim, H.-S.; Lee, J.-K. Opt. Mater. 2003, 21, 221224.
Figure 1. (a) Absorption spectrum and (b) emission spectrum of porphyrin 1 measured in THF solution (8 × 10-6 M).
report on the TiO2 adsorption and the spectroscopic and photovoltaic properties of five metalloporphyrins: 4-(trans2′-(2′′-(5′′,10′′,15′′,20′′-tetraphenylporphyrinato zinc(II)yl)ethen-1′-yl)-1-benzoic acid (1), 4-(trans-2′-(2′′-(5′′,10′′, 15′′,20′′-tetraxylylporphyrinato zinc(II)yl)ethen-1′-yl)-1benzoic acid (2), 4-(trans-2′-(2′′-(5′′,10′′,15′′,20′′-tetraphenylporphyrinato copper(II)yl)ethen-1′-yl)-1-benzoic acid (3), 4-(trans-2′-(2′′-(5′′,10′′,15′′,20′′-tetraphenylporphyrinato zinc(II)yl)ethen-1′-yl)-1-phenylphosphonic acid (4), and 4-(trans-2′-(2′′-(5′′,10′′,15′′,20′′-tetraphenylporphyrinato copper(II)yl)ethen-1′-yl)-1-phenylphosphonic acid (5). Results and Discussion The electronic spectra of metalloporphyrins 1-5 were recorded in tetrahydrofuran (THF) solution and compared to those of the porphyrins adsorbed onto 7 µm thick nanocrystalline TiO2 films. The absorption spectra of metalloporphyrins 1-5 in THF solution show a series of visible bands between 400 and 650 nm due to π-π* absorptions of the conjugated macrocycle. Porphyrin 1 (Figure 1) exhibits an intense Soret absorption band at 436 nm and low intensity Q-bands at 528, 565, and 602 nm.17 The intensity of the Soret band at 436 nm is almost 10 times ( ) 2.25 × 105 M-1 cm-1) higher than that of the Q-band at 565 nm ( ) 2.04 × 104 M-1 cm-1). The Soret band for porphyrin 2 is slightly red shifted (439 nm) compared to that for porphyrin 1 due to the presence of electron releasing methyl substituents. The Soret band for the copper porphyrins (3 and 5) is blue shifted by 20 nm compared to that for the zinc porphyrins (1, 2, and 4), and the corresponding low energy Q-bands are red shifted by 5 nm. The emission spectra of THF solutions of metalloporphyrins 1-5 (see Figure 1b for the emission spectrum of porphyrin 1) were obtained at room temper(14) Cherian, S.; Wamser, C. C. J. Phys. Chem. B 2000, 104, 36243629. (15) Kay, A.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. 1994, 98, 952-959. (16) Ma, T.; Inoue, K.; Yao, K.; Noma, H.; Shuji, T.; Abe, E.; Yu, J.; Wang, J.; Zhang, B. J. Electroanal. Chem. 2002, 537, 31-38. (17) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992.
10.1021/la0496082 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/24/2004
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Langmuir, Vol. 20, No. 15, 2004 6515 Scheme 1
Figure 2. Absorption spectra of porphyrin 2 in (a) THF solution and (b) adsorbed onto a nanocrystalline 7 µm thick TiO2 film. The dashed line (c) is the photocurrent action spectrum obtained from a sandwich-type cell using electrolyte 1376.
Figure 3. ATR-FTIR spectra of porphyrin 2 obtained from (a) a solid sample and (b) adsorbed onto a 7 µm thick TiO2 film.
ature by exciting the Q-bands at 550 nm. The spectra show characteristic vibronic bands at 621 and 674 nm, the emission intensity of the porphyrins containing copper metal being 2 orders of magnitude lower than that of the corresponding zinc porphyrins. The visible absorption spectra of anchored porphyrin on TiO2 films (Figure 2b) show features similar to those seen in the corresponding solution spectra (Figure 2a). On the other hand, the emission spectrum of the adsorbed film is quenched as a result of electron injection from the excited singlet state of the porphyrin into the conduction band of the TiO2. The attentuated total reflection-Fourier transform infrared (ATR-FTIR) spectra of porphyrin 2, both as a solid and adsorbed onto TiO2, shown in Figure 3, are representative of porphyrins 1-3. Figure 3a shows a spectrum with a strong adsorption at 1679 cm-1 attributable to the ν(CdO) stretching mode of the carboxylic acid group. The peak at 1283 cm-1 for porphyrin 2 is due to the ν(CsO) stretching mode, which occurs at 1260 and 1269 cm-1 for porphyrins 1 and 3, respectively. The other prominent bands between 1600 and 1330 cm-1 are due to the many ν(CdC) stretching modes. Upon deprotonation of the carboxylic acid group, using tetrabutylammonium hydroxide solution, the ν(CdO) stretching mode peak disappears and two strong bands at 1616 and 1372 cm-1 due to, respectively, the asymmetric ν(sCOO-as) and symmetric ν(sCOO-s) modes appear. ATR-FTIR spectroscopy has been shown to be a powerful tool for extracting structural information of the molecules adsorbed onto a TiO2 surface.18 The ATR-FTIR spectra of the adsorbed porphyrins 1-3 on TiO2 film do not exhibit typical carboxylic acid vibrational modes, but
they all show the presence of carboxylate asymmetric ν(-COO-as) and symmetric ν(-COO-s) bands; for porphyrin 2, these occur at 1598 and 1391 cm-1, respectively (see Figure 3b). This supports binding of the porphyrins to the TiO2 rather than aggregation of the porphyrins on the surface. The possible binding modes for porphyrins containing carboxylic acid groups on a TiO2 surface are shown in Scheme 1. The unidentate coordination of the carboxylate group removes the equivalence of the two oxygen atoms, resulting in an ester type of bond formation between the carboxylic acid group and the TiO2 surface. On the basis of the ATR-FTIR measurements, this type of coordination can be ruled out, leaving only two possibilities, bridging bidentate or chelation. The difference between the asymmetric and symmetric bands in the ionic and adsorbed states can also be used as a criterion to differentiate between the various possibilities.19-21 If the difference between the carboxylate group asymmetric and symmetric stretching mode bands in the adsorbed state is less than that in the free solid state, then, the anchoring mode is either chelation or bridging bidentate, and if the opposite applies, then, the anchoring mode is unidentate. The difference between these bands in the deprotonated porphyrins (=244 cm-1) and the adsorbed porphyrins (=207 cm-1), together with other evidence that suggests that the chelation mode is unstable,22 suggests, therefore, that the carboxylate groups are bound to the TiO2 surface via a bridging bidentate mode. The performance of porphyrins 1-5 as sensitizers on nanocrystalline TiO2 electrodes (6-7 µm) was determined from measurements on photovoltaic cells using an electrolyte with a composition of 0.6 M M-methyl-N-butyl imidazolium iodide, 0.05 M iodine, 0.05 M LiI, and 0.5 M tert-butylpyridine in a 50:50 (v/v) mixture of valeronitirile and acetonitrile (1376). The photovoltaic data obtained in two different solvents (THF and ethanol) are given in Table 1. The data show that the nonprotic solvent THF is slightly better than the protic solvent ethanol. Also, it is clear from these data that the Zn derivatives exhibit superior efficiencies to the corresponding Cu derivatives and that the porphyrins with carboxylate binding groups outperform those with more strongly bound phosphonate binding groups. Overall, porphyrin 2 is clearly the most efficient. The photocurrent action spectrum obtained from a TiO2 film coated with a monolayer of porphyrin 2 used in a sandwich cell under illumination by simulated AM 1.5 solar light is shown in Figure 2c. As expected, the shape of the action spectrum clearly follows the shape of the (18) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451-455. (19) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1998, 14, 2744. (20) Deacon, G. B.; Phillips, R. Coord. Chem. Rev. 1989, 33, 227250. (21) Mehrotra, R. C.; Bohra, R. Metal Carboxylates; Academic Press: New York, 1983. (22) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gra¨tzel, M. J. Phys. Chem. B 2000, 104, 1300.
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Table 1. Photovoltaic Performance of Nanocrystalline TiO2 Films Sensitized by Porphyrin Dyes Using Electrolyte 1376a
a The composition of electrolyte 1376 is 0.6 M butylmethylimidazolium iodide (BMII), 0.05 M I , 0.1 M LiI, and 0.5 M tert-butylpyridine 2 in 1:1 acetonitrile and valeronitrile.
absorption spectrum in the solution and that of the porphyrin adsorbed on TiO2. The incident monochromatic photon-to-current conversion efficiency (IPCE) is 75% for the Soret band and 50-60% for the Q-band peaks. From the overlap integral of this curve with the standard global AM 1.5 solar emission spectrum, a short circuit photocurrent density of 10 mA/cm2 is obtained, a value that is comparable to the measured value 9.7 mA/cm2. The open circuit voltage and fill factor, respectively, 660 ( 50 mV and 0.75 ( 0.05, correspond to an overall conversion efficiency of 4.8%, making it the most efficient metalloporphyrin sensitizer reported to date. Figure 4 shows the light intensity dependence of the photocurrent density for porphyrin 2. The excellent linearity exhibited for light intensities between 0.1 and 1.0 sun confirms the absence of porphyrin aggregates on the surface, a result that is in agreement with the ATR-FTIR data. Despite the fact that the anchoring group does not participate in the π-π* excitation that is responsible for the visible bands, electronic coupling of the excited state to the Ti (3d) conduction band through the conjugated carboxylic acid group is strong enough that charge injection is very efficient. The IPCE value for porphyrin 2 of 75% shows that the excited electron transfer process is comparable in efficiency to that of ruthenium polypyridyl sensitizers. This observation is supported by recent work by Durrant et al., which showed that the dyes cisdithiocyanatobis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) (N3) and Zn-TCPP adsorbed on TiO2 have
Figure 4. Photocurrent-voltage characteristics of a nanocrystalline photoelectrochemical cell sensitized with porphyrin 2 under various incident radiation conditions. The green curve shows the dark current.
almost indistinguishable electron injection and recombination kinetics.23 Conclusions ATR-FTIR measurements show that carboxylate groups are absorbed onto the TiO2 surface by bidentate chelation. (23) Tachibana, T.; Haque, S. A.; Mercer, I. P.; Durrant, J. R.; Klug, D. R. J. Phys. Chem. B 2000, 104, 1198-1205.
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The photovoltaic data establish that, for porphyrins with carboxylic binding groups, the Zn containing diamagnetic metalloporphyrins have very high incident monochromatic photon-to-current conversion efficiencies compared to those observed for the Cu containing paramagnetic metalloporphyrins. In addition, porphyrins with a phosphonate anchoring group show lower efficiencies than those with a carboxylate anchoring group. Porphyrin 2 has the highest conversion efficiency so far obtained with porphyrin-type sensitizers on nanocrystalline films. These findings open up new avenues for improving further the efficiency of nanocrystalline injection solar cells for practical utility, by engineering suitable porphyrins with a small band gap and which absorb in the visible and near IR regions of the solar spectrum. Experimental Section Synthesis. The synthesis and characterization of metalloporphyrins 1-5 will be published elsewhere.24 TiO2 Electrode Preparation. TiO2 anatase nanoparticles of 16 nm were prepared by hydrolysis of titanium(IV) isopropoxide.25 Nanocrystalline TiO2 films of 7 µm thickness were deposited onto transparent conducting glass (TEC-15; the glass had been coated with a fluorine-doped stannic oxide layer; sheet resistance, 12-15 Ω/cm2) by screen printing. These films were dried at 150 °C for 20 min and then were sintered at 500 °C for 20 min. The heated electrodes were impregnated with a 0.05 M titanium tetrachloride solution in a water saturated desiccator for 30 min at 70 °C and then were washed with distilled water. The 0.05 M titanium tetrachloride solution was prepared by adding titanium tetrachloride to ice to make a 2 M solution. This solution was then cooled to -20 °C before being diluted to 0.05 M. The electrodes were heated at 520 °C for 20 min and were then allowed to cool to 50 °C before dipping them into the dye solution. Dye solutions were prepared in the concentration range (1-3) × 10-4 M in THF or ethanol, and the electrodes were immersed in the solutions for 18-22 h. tert-Butyl pyridine of 2-5 vol % was added to the dye solutions to prevent aggregation of porphyrin (24) Officer, D. L.; Campbell, W. M. Manuscript in preparation, 2004. (25) Nazeeruddin, M. K.; Pe’chy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Le, C.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613.
molecules on the TiO2 surface.26 The dye-coated electrodes were rinsed quickly with acetonitrile to remove any unadsorbed dye. Analytical Measurements. UV-vis and fluorescence spectra were recorded on a Cary 5 spectrophotometer and a Spex Fluorolog 112 spectrofluorometer, respectively, using a 1 cm path length quartz cell. The FTIR spectra for all the samples were measured using a Digilab 7000 FTIR spectrometer. The ATR data were taken with the “Golden Gate” diamond anvil ATR accessory (Graseby-Specac) using, typically, 64 scans at a resolution of 2 cm-1. The IR optical bench was flushed with dry air throughout the measurements, and the mechanical force keeping the samples in contact with the diamond window was identical for all samples. No ATR correction has been applied to the data. Some spectra showed artifacts due to attenuation of light by the diamond window in the 2000-2350 cm-1 region of the spectrum. The dye-coated TiO2 films were rinsed in THF and dried at 150 °C before measuring the spectra. The FTIR spectra of the anchored dyes were obtained by subtracting the spectrum of the blank TiO2 films from the IR spectrum of the dye-coated TiO2 films of the same thickness. Photovoltaic Measurements. Photoelectrochemical data were obtained using a 450 W xenon light source focused to give 1000 W/m2, the equivalent of one sun at AM 1.5, at the surface of the test cell. A sandwich cell was assembled by using the dyeanchored TiO2 film as the working electrode and conducting glass coated with chemically deposited platinum from 0.05 M hexachloroplatinic acid as the counter electrode. A thin layer of electrolyte 1376 was introduced into the interelectrode space, and the electrodes were tightly held in a cell holder.
Acknowledgment. We acknowledge financial support of this work by the Swiss Federal Office for Energy (OFEN), U.S. Air Force Research Office under Contract No. F61775-00-C0003, NANOMAX (Contract No. ENK6CT-2001-00575 of the fifth RTD framework program by EU and funded by OFES, Bern), and the New Zealand Foundation for Research, Science and Technology New Economy Research Fund contracts MAUX0014 and MAUX0202. We thank Dr. Marie Jirousek and P. Comte for their kind assistance in obtaining photovoltaic data. LA0496082 (26) Nazeeruddin, M. K.; Humphry-Baker, R.; Gra¨tzel, M.; Wo¨hrle, D.; Schnurpfeil, G.; Schneider, G.; Hirth, A.; Trombach, N. J. Porphyrins Phthalocyanines 1999, 3, 230.
