Artificial photosynthesis. 1. Photosensitization of ... - ACS Publications

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6272

J . Phys. Chem. 1993,97, 6272-6277

Artificial Photosynthesis. 1. Photosensitization of Ti02 Solar Cells with Chlorophyll Derivatives and Related Natural Porphyrins Andreas Kay and Michael Griitzel' Institut de Chimie Physique, Ecole Polytechnique Fsdirale de Lausanne, CH-1015 Luusanne, Switzerland Received: November 10, 1992; In Final Form: February 12, 1993

Colloidal Ti02 electrodes were photosensitized with derivatives of chlorophyll and related natural porphyrins resulting in light harvesting and charge separation efficiencies comparable to those in natural photosynthesis. The photocurrent action spectra of the electrodes correlate well with the absorption spectra of the dyes in solution. Incident photon to current efficiencies up to 83% are reached in the Soret peak at 400 nm with a 12-pm-thick Ti02 film sensitized by copper mesoporphyrin IX, which corresponds to nearly unity quantum efficiency of charge separation when light reflection losses are taken into account. Photocurrent/voltage curves of Ti02 solar cells sensitized with copper chlorophyllin show an energy conversion efficiency of 10% for the red peak at 630 nm. Under simulated sunlight illumination, an open circuit photovoltage of 0.52 V and a short circuit current density of 9.4 mA/cm2 are measured. The overall energy conversion efficiency of the cell is 2.6% under these conditions, in part limited by ohmic losses a t such high current densities. The comparison of different chlorophyllderivativesindicates that freecarboxyl groups are important for adsorption and sensitization on TiO2. However, conjugation of the carboxyl groups with the u electron system of the chromophore is not necessary for efficient electron transfer. Free bases, zinc, and even the nonfluorescent copper complexes of chlorophyllins and mesoporphyrin IX are efficient sensitizers for TiO2. Cholanic acids as coadsorbates were found to be unique in improving both photocurrent and voltage of copper chlorophyllin sensitized cells. This effect is discussed by comparison with other coadsorbates.

Introduction The photosensitization of wide-bandgap semiconductors such as Ti02 by adsorbed dyes has become more practical for solar cell applications recently with the development of porous films of very high surface area.1 Only the first monolayer of adsorbed dye results in efficient electron injection into the semiconductor, but the light-harvesting efficiency of a single dye monolayer is very small. In a porous film consisting of nanometer-sizedTi02 particles, the effective surface area can be enhanced lOOO-fold, thus making light absorption efficient even with only a dye monolayer on each particle. Nature, in fact, uses a similar means of absorption enhancement by stacking thechlorophyll-containing thylakoid membranes of the chloroplast to form the grana structures. For the photosensitization of Ti02 solar cells, Ru-bipyridine complexes have proven to be most efficient currently, due to their broad absorption spectra combined with favorable photoelectrochemical properties and high stability in the oxidized state.l However, other dyes, such as Zn-tetracarboxyphenylporphyrin2 or coumarin,' have been used as sensitizers as well. The photosensitization of a wide-bandgapsemiconductor (ZnO) by chlorophylls was first investigated by Tributsch and Calvina4 Although quantum efficienciesof up to 0.125 electron per absorbed photon were achieved in the presence of phenylhydrazine as sacrificial electron donor, the measured photocurrents were extremely small ( 5 X 1 V A l a n 2 under monochromatic illumination of unspecified intensity) due to the small light-harvesting efficiency of a dye monolayer on the semiconductor crystal. Chlorophyll monolayers were also deposited on SnOl optically transparent electrodes by means of the Langmuir-Blodgett technique, and the effect of different metal centers was investigated.5 Quantum efficienciesof up to 0.3 were reported. Much lower efficiencies result by simply dipping a SnOz electrode into a metallochlorophyll-containing electrolyte solution.6 The photosensitization of colloidal Ti02 by copper chlorophyllin has been analyzed using fluorescence quenching, flash photolysis, and microwave absorption by Kamat et al.7 Their results will be 0022-3654/93/2097-6272%04.00/0

compared with our experiments on fluorescence and transient absorption of Cu-chlorophyllin-sensitizedTi02 electrodesin the subsequent paper of this series.s Recently, photosystem I1 particles were immobilized on a ruthenium dye-sensitized Ti02 electrodein order to transfer electronsfrom water to the oxidized dye.9 In this paper, we report on the sensitization of Ti02 solar cells by chlorophylls and porphyrins with efficiencies exceeding 0.8 electron per incident photon and significant photocurrent/ voltage outputs.

Experimental Section Preparation of Ti02 Electrodes. Nanostructured Ti02 films were prepared by spreading a colloidal Ti02 dispersion on SnO2 conductiveglass,as reported earlierIb (method B of ref 1b). Films obtained in this manner of 12-pm thickness are only translucent and scatter light strongly, especially at short wavelengths. Therefore, transparent films of 7-pm thickness were prepared** for optical studies and treated with aqueous T i c 4 solution as beforelb (method B). Chlorophyll Derivatives. Chlorophyll a was obtained from Sigma. A mixture of chlorophylls a and b was extracted from spinach with methanol and partially purified by precipitation with dioxane/water."J Treatment of a diethyl ether solution of the mixture with dilute HCl gave the magnesium-free pheophytins. The hydrolysis of the phytyl ester bond with HCl gave pheophorbides a and b, which were separated by extraction with diethyl ether at their respective HCl numbers.ll Mg-chlorin e6 was prepared by saponificationof chlorophyll.12 Chlorin e6 trimethyl ester was obtained by transesterification of pheophytin a with NaOCH3 in THF.1' Copper was introduced into the ester in diethyl ether solution by addition of cupric acetate saturated acetic acid. Saponificationof the trimethyl esters with hot KOH in methanol gave free base and Cu-chlorin e6. Cu-2-aoxymesoisochlorin e4 was isolated from commercial Cu-chlorophyllin (Merck, K&K, or Sigma) by extraction of an aqueous solution with 1-butanoland column chromatography on silica gel with toluene/acetic acid (1O:l). 0 1993 American Chemical Society

Photosensitizationof Ti02 Solar Cells

Porphyrins. Mesoporphyrin IX dimethyl ester was obtained from Sigma. Copper was insertedin benzene solution fromcupric acetatesaturatedaceticacid (severaldaysat room temperature).14 Zinc was introduced in acetone solution from zinc acetate saturated methanol (2 min under reflux). The esters were hydrolyzed in acetone with KOH/methanol under reflux. The purity of all dyes was checked by thin-layer chromatography on silica gel with water/l-butanol/acetic acid (5050:1, upper phase) as eluent. Dye Adsorption and Experimental Setup. The Ti02 electrodes (1-cm2 active surface area) were fired 30 min at 550 OC in air and dipped still hot (=80 "C) in 1 mL of an ethanolic solution of the dye (-5 X le5M) and any coadsorbate. After at least 5 h of soaking, the electrode was dried in a stream of dry air, wetted immediatelywith electrolyte (0.5 M KI, 40 mM I2 in 80% ethylenecarbonate,20%propylene carbonate),and pressed against a platinum foil counter electrode. Illuminationoccurred through the transparent SnO2 substrate with light from a xenon lamp (Cermax300 W) passing through a high-intensity monochromator (Kratos GM 252, spectral bandwidth AA = 10 nm). For the determination of action spectra, the short circuit current of the cell was divided with an analog divider by the signal of a thermopile, measuring the light power, as well as by the output of a potentiometer connected to the monochromator, representing the wavelength of the light. The absolute light intensity incident on the solar cell was determined with a radiometer (YSI-Kettering Model 65A). Current/voltage curves were measured by scanning the voltage of the Ti02 electrode with respect to the platinum counter electrode (Thompson precision potentiostat 25 1 in a twoelectrodeconfiguration). Thus, these curves represent the overall performance of the solar cell. For white light illumination, a xenon lamp (Osram 450 W) was used in combinationwith a heat reflection filter (Schott type 113) to simulate solar radiation.

Results and Discussion Chlorophyll. Chlorophyll a (Figure la) does not adsorb efficiently on Ti02 from solvents such as ethanol, acetone, THF, or pyridine, due to the weak interaction of its ester and keto carbonyl groups with the hydrophilic oxide surface. However, it does adsorb from less polar solvents such as diethyl ether or hexane. The photocurrent action spectrum (Figure la) shows peaks corresponding to the absorption spectrum in solution, but the photocurrent rises earlier below 500 nm. The Qyband, which corresponds to the lowest excited singlet state, is strongly broadened and red-shifted. This may be caused by interaction with the polar Ti02 surface as well as by aggregation of the chlorophyll1s due to its high concentration on the electrode. The sharp rise below 400 nm is due to bandgap excitation of the Ti02 itself, as seen by comparison with an unsensitized electrode (Figure la). The photocurrentefficiency (IPCE: incident photon to current efficiency = number of electrons per incident photonlb) is low; e.g., IPCE670 = 3.5% in spite of an optical density A670 = 0.3 for theelectrodeof Figure la. Since the transmitted light is reflected by the platinum counter electrode and traverses the electrode a second time, the effective absorbanceis A670 = 0.6, corresponding to 75% light absorption in the red peak. Without addition of pyridine to the chlorophyll a solution in ether, the IPCE is even lower, possibly due to enhanced dye aggregation, which can be reduced by an axial magnesium ligand like pyridine.16 Pheophytin, the free base of chlorophyll, shows similar low adsorption and photosensitization behavior on TiO2. Pheophorbide. Hydrolysis of the phytyl ester bond of pheophytin results in a free carboxyl group (Figure lb) and thus in a much stronger adsorption in TiO2, allowing adsorption from ethanol. The photocurrent action spectrum of pheophorbide a corresponds toits absorptionspectrumin solution, which resembles the spectrum of pheophytin a, but the peaks are red-shifted by

The Journul of Physical Chemistry, Vol. 97, No. 23, 1993 6273 about 10 nm. The optical density of the electrode in the Qu band was A675 = 0.7 corresponding to 96% light absorption in the red peak, including the effect of the reflecting counter electrode. At A = 630nm, theelectrodehadanabsorbanceA6M=0.21,resulting in still 62% light harvesting at this wavelength. This explains the reduced peak ratio in the action spectrum as compared to that in the absorption spectrum. Without addition of deoxycholic acid to the dye solution the coloration was even stronger; however, the photocurrent efficiency was actually smaller. In spite of 96% light absorption, only 25% of the incident red photons produce an electron flowing in the external circuit. The quantum efficiency (electrons per absorbed photon) is slightly higher, since some of the incident light is already lost by reflection from the electrodesurface. It reaches the highest value reported so far for the sensitization of a semiconductor by a chlorophyll derivative,s yet it is far behind the nearly unity efficiency of primary charge separation in natural photosynthesis. It may be argued that the propionic acid side chain of pheophorbide is not a useful group for attachment on TiO2, since it presents an insulating barrier for electron injection from the excited dye molecule into the conduction band of Ti02. We therefore investigated chlorophyll derivatives which have a carboxyl group in conjugation with the 'K electron system of the tetrapyrrole macrocycle. Chlorophyllin. Alkaline hydrolysis of chlorophyll not only saponifies the phytyl ester bond but also opens the cyclopentanone ring with formation of two additional carboxyl groups (Figure IC). Chlorophyllin a, which is the Mg complex of chlorin e6, therefore adsorbs well on Ti02. The absorption spectrum in ethanol depends strongly on the pH of the solution. As prepared, the three carboxyl groups are dissociated and the absorption maxima lie at 418,522,602, and 645 nm. In contrast to that of chlorophyll, the Qr band of chlorophyllin is much weaker than the Soret band, due to opening of the cyclopentanonering. The fluorescence is strong with maxima at 654 and 705 nm. Protonation of the carboxyl groups with excess acetic acid or deoxycholic acid results in a shift of the Qr band to 657 nm, while the other bands are only weakly red shifted (422, 528, 570,610 nm). However, the spectrum changes further with time, giving rise to a 688-nm peak, a Soret band shifted to 428 nm with a shoulder at 382 nm, and new peaks at 586 and 644 nm. The fluorescence is about half as strong and shifted to 702 and 758 nm. Addition of base reverts the spectrum to that of the starting salt within a few minutes. This behavior has been ascribed to dimerization of the un-ionized chlorophyllin causing a bathochromic shift of the Q,band by exciton interactions,12.17although this explanation has been called into question by others.'* A chlorophyll dimer with a red-shifted Q,band also constitutes the special pair in the reaction center of photosyntheticbacteria and green plants. The highest photosensitization efficienies are obtained after adsorption from a solution containing 20 mM deoxycholic acid (Figure IC). The action spectrum shows contributions from all spectral forms of chlorophyllin a (the shoulder at 450 nm is due to chlorophyllin b ) . The efficiency at 688 nm is low, although this is the main solution species in the presence of acid. Without deoxycholic acid, the electrodeis much more strongly colored and the action spectrum correspondsto the deprotonated species, but the efficiency is poor (7% at 648 nm). Addition of pyridine in order to prevent aggregation of the dye has no effect. On increasing the deoxycholic acid concentration to 100 mM, the efficiency in the Q, band region drops below 10% while the other bands are barely affected (Figure IC). The depression of the red peak is also apparent in the absorption spectrum of the electrode but is completely reversible on desorption of the dye from Ti02 with 0.2 M acetic acid in ethanol. Probably, the Qu band is further red-shifted and broadened by interaction with the polar, high refractive index TiO2.

Kay and Gratzel

6274 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 pheophorbide a

500 600 wavelength [nml

80

700

Mg-chlorin e6

400

500 600 wavelength [nm]

460

500

700

700 500 600 700 400 wavelength [nm] wavelength [nm] Figure 1. Photocurrent action spectra of Ti02 (12 pm P25) electrodes sensitized with chlorophyll derivatives (solid lines) and absorption spectra of the dyes in solution (dashed lines); IPCE = incident photon to current efficiency. (a) Chlorophyll u adsorbed from diethyl ether containing 0.1 M pyridine, upper curve enlarged 4 times; curve on the left: photoresponse of the Ti02 electrode without sensitizer. (b) Pheophorbide u/ethanol + 2 mM deoxycholic acid. (c) Mg-chlorin e6; absorption spectra in ethanol: (-) deprotonated form, (- -) immediately after addition of 5 mM acetic acid, (- -) after 24 h; action spectra after adsorption from ethanol containing 20 mM (-) and 100 mM (- -) deoxycholic acid, respectively. (d, e) Ht,Cu-chlorin es/ethanol+ 20 mM deoxycholicacid. (f) Cu-2-a-oxymesoisochlorined/ethanol+ 100 mM deoxycholicacid; absorption spectra: (- -) in ethanol, (-) on a transparent Ti02 electrode (thickness 7 pm, ,4630 = 0.91), (- -) same electrode without sensitizer; (--) percentage of light absorbed by the dye on the transparent Ti02 electrode (vertical scale &100%); (- -) action spectrum of this electrode, (-) action spectrum on a light-scattering Ti02 electrode (12 pm P25).

--

-

-

--

H-rin e+ After removal of thecentral Mgz+, the solution spectrum of chlorophyllin shows only a weak red-shifted peak a t 697 nm in the presence of deoxycholic acid (Figure Id). This

-

-

peak does not appear in the photocurrent action spectrum, which otherwisecompares well with theabsorption spectrum insolution. The shoulder a t 435 nm is again due to the chlorophyllin b

Photosensitization of Ti02 Solar Cells derivative. The efficiency is quite high even in the green part of the spectrum, where the extinction coefficient of H2-chlorin e6 is small. This demonstrates the high light-harvesting efficiency on the colloidal Ti02 film. Cu-ChIorin e+ Copper chlorophyllin is produced on a large scale as a water-soluble dye for application in food coloring, cosmetics, and medicine.19 Copper is introduced to make the dye more stable against photooxidation by reducing its excited-state lifetime.8 However, commercial Cu-chlorophyllin is a mixture of several degradation products, almost all of which are lacking the carboxyl group in conjugation with the r electron system. To avoid decarboxylation reactions, all three carboxyl groups have to be protected by esterification before Cu2+ insertion and can only be saponified afterward.20b Cu-chlorin e6 prepared in this way, although not fluorescent? is an excellent photosensitizerfor Ti02 (Figure le). Up to70%oftheincidentphotonsareconverted into electrons which can be measured as current in the external circuit. Cu-2-a-Oxymeaoi~hlorin e4. The commercial Cu-chlorophyllin is made by hot saponification of raw chlorophyll in the presence of a copper salt. One of the major products is Cu-2or-oxymesoisochlorin e4, which lacks the ring carboxyl group and has a hydrated vinyl group.20 It can be separated from the other saponification products by chromatography. In spiteof its weaker adsorption on silica gel and TiO2, due to the lack of one carboxyl group, its photosensitization efficiency is as high as that of Cuchlorin e6 (Figure lf, solid curve). Thus, the carboxyl group in conjugation with the P electron system of the tetrapyrrole macrocycleis not necessary for good electron injection. The action spectrum again compares well with the solution spectrum. The shoulder at 440 nm is absent due to removal of the chlorophyllin b analog by chromatography. Figure If also shows the absorption spectrum on a transparent Ti02 electrode (see Experimental Section). It correspondsto the solution spectrum except for the residual scattering background from the Ti02 substrate. The action spectrum reflects the percentage of light absorbed by the sensitizer, as calculated from the absorption spectrum of bare and dye-coated electrode. The photocurrent efficiency is higher with the nontransparent (P25) Ti02 electrode in low-absorption regions of the dye due to its increased layer thickness (12 pm compared to 7 pm) and the additional increase in effective optical thickness due to light scattering.' Hr,Zn-,Cu-Mesoporphyrin IX. Porphyrinsare distinguished from chlorins by an additional double bond in one of the pyrrole rings, which is the reason for their red color. Protoporphyrin IX is the precursor in the biosynthesis of chlorophyll. Its Fe2+complex forms the oxygen-binding prosthetic group of hemoglobin. Its derivatives also occur in redox-active cytochromes and enzymes. Oddly enough, nature does not employ red porphyrins as photosynthetic pigments. Protoporphyrin IX has two propionic acid groups and adsorbs well on TiO2. Here, we report only the similar results obtained with its hydrogenated derivative mesoporphyrin IX, in order to avoid complications arising from the reactive vinyl groups. The free base has a moderately high photocurrent efficiency with peaks corresponding to the solution spectrum (Figure 2a). The spectral response is very broad and extends to 680nm. Insertion of Zn2+ increases the peak IPCE to nearly 80% (Figure 2b); however, the cutoff occurs already at 630nm. The Cu2+complex, if adsorbed from THF, has an even higher IPCE of 83% in the Soret peak (Figure 2c), whichcorrespondstonearlyunity quantum efficiency (electrons per absorbed photon) if reflection losses are taken into account. This confirmstheobservationthat conjugation of the attaching carboxyl groups with the P electron system of the chromophore is not necessary for efficient electron injection. Adsorption of Cu-mesoporphyrin IX from ethanol gives a maximum IPCE of only 70% but a higher open circuit photo-

The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6275 80 .

--

H,-mesoporphyrin IX ::

\ 80

i

A

Zn-mesoporphyrin IX

-

-~

Cu-mesoporphyrin IX

460

560

660

I60

wavelength [nm] Figure 2. Photocurrent action spectra of Ti02 electrodes sensitized with porphyrin derivatives (solid lines) and absorption spcctra of the dyes in solution (dashed lines): (a, b) Hr,Zn-mesoporphyrin IX/ethanol + 20 mM deoxycholic acid; (c) Cu-mesoporphyrin IX/THF deoxycholic acid.

+ 20

mM

voltage (490 mV) than from THF (350 mV). In both cases, deoxycholic acid was present as coadsorbate, causing a positive shift of the conduction band edge of Ti02 by protonation of the semiconductor surface. This shift seems to be more pronounced in THF, resulting in a larger driving force for electron injection from the excited dye yet a lower photovoltage. Photocurrent/Voltage Curves. While the photocurrent effi-

-

Kay and Griitzel

6276 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993

-

10

b (x10)

3a s:C‘

-6e 4

-

3 -

TABLE I: Effect of Coadsorbates on the Dye Coverage and PhotoelectrochemicalProperties of TiO~-Cu-2-a-oxy~isocesoisochlorin e4 Solar C e l rb

a(x20)

A

t: 4 -

’x-

s 2E

a 0.0 0.1

0.2 0.3 0.4 0.5 0.6 photovoltage [VI Figure 3. Photocurrent/voltage curve of a Ti02 electrode (0.5cm2active surface area) sensitized with Cu-2-a-oxymesoisochlorine4 (adsorbed fromethanolcontaining 20 mM chenodeoxycholic acid). The electrolyte was 50% ethylene carbonate, 50% acetonitrile, 0.5 M tetrapropylammoniumiodide,and 40mM iodine: (a)under monochromatic illumination (A = 630 nm, P = 0.75 mW/cm2); (b, c) under white light illumination of 10 and 100 mW/cm2, respectively. ciency gives an indication of the quantum efficiency of the charge separation step, it does not contain any information on the energy conversion efficiency. For this, the electrical power output given by the product of photocurrent and photovoltage has to be compared with the light power input. The photocurrent/voltage curve of a Cu-2-a-oxymesoisochlorine4 electrode under monochromaticillumination (A = 630nm, P = 0.75 mW/cm2) isshown in Figure 3a. At the maximum power point, the electrical output is Pa = 0.23 mA/cm2 X 0.34 V = 0.078 mW/cm2, corresponding to an energy conversion of 10.4% in the red peak. The relatively low efficiency is mainly due to the low photovoltage of 0.42 V at open circuit, compared to a photon energy of 1.97 eV at 630 nm. The discrepancy is even larger for shorter wavelength light. This is one reason for the still lower total energy conversion efficiency of 2.6% under simulated sunlight illumination (Figure 3c). In addition, the high current density of up to 9.4 mA/cm* results in ohmic losses, due to the series resistance of the solar cell. At 10%white light intensity,the energy conversion efficiency therefore improves to 3.1% (Figure 3b). Coadsorption of Cumesoporphyrin IX does not enhance the polychromatic efficiency of the electrode by improving green light absorption, which is already quite good with Cu-2-a-oxymesoisochlorine4 alone. Still, the overlap of the photocurrent action spectrum with the solar emissionspectrum is more complete with a recently described Ru complex,lb which exhibits 6&85% photocurrent efficiency over the whole visible spectrum, with a tail extending up to 800 nm. Effect of Coadsorbates. Adsorption of a limited amount of Cu-2-a-oxymesoisochlorine4 from pure ethanol (50 nmol in 1 mL) onto 1 cm2 of Ti02 results in complete uptake of the dye by the outer part of the Ti02 layer, while its inner part remains uncolored. The photocurrent efficiency is very low since the inner Ti02 layer, facing the SnO2 conducting substrate, remains insulating in the absence of electron injection from an adsorbed dye. Only after addition of another 50 nmol of dye does the electrode become homogeneously colored, yet the efficiency is still rather low (Table I; column d (Table Id)). The dye concentration on the Ti02 electrode, as calculated from the absorbance change of the dipping solution ((630 = 3.7 X 104 L mol-’ cm-l), is 96 nmol/cm2. The Ti02 powder used for electrode preparation (Degussa P 25) has a primary particle size around 25 nm and a specific surface area of 55 m2/g, giving a roughness factor of 1100 for a coating of 2 mg of Ti02/cm2. Thus, the real surface concentration of the dye is 87 pmol/cm2, corresponding to a surface area of 1.9 nm2/molecule. Since the chromophore has a diameter of roughly 1 nm, there should be enough surface area available even for a flat adsorption of the tetrapyrrole macrocycle on Ti02. Hence, the low efficiency is

column

Id

(nmol/ IPCE& @A/ Vme cm2) (96) cm2) (mV)

solvent/coadsorbatP 7 42 100 490 a pyridine 30 51 370 380 methanol b 14 100 190 440 tetrahydrofuran C 25 96 330 430 d ethanol 49 740 460 45 e 40 mM cholic acid 41 56 5 mM deoxycholic acid 700 430 f 43 58 820 440 20 mM deoxycholic acid g 68 840 490 19 100 mM deoxycholic acid h 68 20 mM chenodeoxycholic acid 25 940 480 i 57 9 740 490 k 20 mM lithocholic acid 690 490 59 1 100 mM ursodeoxycholic acid 13 12 53 670 490 m 10 mM dehydrocholic acid 390 490 31 35 n 500 mM Triton X-100 25 45 670 400 100 mM acetic acid 0 25 300 530 1 M glycerol 33 P 42 27 320 520 9 10 mM sorbitol 45 38 410 520 r 5 mM D(+)-glucose 21 49 100 mM cyclohexane620 360 S carboxylic acid 28 51 650 380 t 100 mM l-adamantaneacetic acid 19 480 360 39 U 5 mM R(-)-cyclohexylhydroxyacetic acid 16 580 400 V 44 1 mM D(-)-quinic acid The electrodes were dipped in 1 mL of solvent (ethanol for e-v) containing 50 nmol(100 nmol for c and d) of Cu-2-a-oxymesoisochlorin e4. Dye surfaceconcentration on the electrode. Photocurrent efficiency on the Q,, band at 630 nm. e Short circuit photocurrent under white light illumination of 10 mW/cmz(approximately 10%of full sunlight). e Open circuit photovoltage under white illumination of 10 mW/cm2 (approximately 10% of full sunlight). probably not due to multilayer formation. Also, absorption and action spectra of the electrode show no indication of chlorophyllin aggregation in the pores of the electrode. This is in accordance with the observation that, contrary to the coordinatively unsaturated magnesium complexes, copper chlorophylls do not tend to coordinative aggregation.21 Adsorption from the more polar methanol results in homogeneous coloration even with only 50 nmol of dye/cm2 of TiO2, yet the efficiency is still low (Table I, column b). Less polar solvents, like acetone, propylene carbonate, dioxane, pyridine, or THF (Table Ia,c), give inhomogeneouscoloration and even lower efficiencies than ethanol. Both photocurrent and voltage can be improved appreciably by addition of cholanic acids to the ethanolic dye solution (Table Ie-m). The effect also occurs on preadsorption of cholanic acids, while postadsorption on an already colored electrode is less effective. The common feature of these bile acids is a steroid skeleton with a flexible carboxylic acid side chain and one to three hydroxyl groups on one side of the steroid backbone. This makes them chiral amphiphilic molecules with a hydrophobic and a hydrophilic side. They form chiral micelles in aqueous solution and channel-type inclusion compounds with many guest molecules in the solid state.22 Cholanic acids are expected to adsorb on the Ti02 surface with their carboxyl and hydroxyl functions. However, rather high concentrations are necessary to compete with the adsorption of Cu-2-a-oxymesoisochlorine4 (at 50 pM), which has two carboxyl groups (Table Ie-m). The decrease in surface concentration of the dye by the %pacer effect” of cholanic acids is not sufficient for enhanced photosensitization efficiency since other surfactants known to prevent chlorophyllaggregation, like Triton X-100, although decreasing the dye coverage, barely improvethe electrodeperformance (Table In). The same observation has been made with LangmuirBlodgett films of Cu-chlorophyll on SnO2 electrodes, where the photosensitization efficiency is not enhanced by two-dimensional

Photosensitizationof Ti02 Solar Cells dilution with lipids.” The higher photocurrent can rather be explained by a positive shift of the conduction band edge of Ti02 in the presence of acid, resulting in a larger driving force for electron injection from the excited dye.* In fact, the addition of acetic acid to the ethanolic dye solution also increases the photocurrentyet slightly decreases theopen circuit voltage (Table Io). On the other hand, addition of base or even short exposure of the electrode to ammonia vapor drastically decreases the injection efficiency. The effect is most prominent in the case of Cu-2-CY-oxymesoisochlorine4, since its excited-state oxidation potential lies just above the conduction band edge of TiOz, so small changes in the position of the latter strongly affect the injection efficiency.* In addition, the excited-state potential of chlorophyllin might be influenced by interaction of its hydrophobic tetrapyrrole macrocycle with the nonpolar backside of the adsorbed cholanic acid molecule. Tuning of the redox potential of chlorophyll is in fact very important in natural photosynthesis, where shifts of several hundreds of millivolts are effected by the surrounding protein matrix.23 The photovoltage is limited by the dark current, arising from reduction of iodine on the electrode at negative potentials, and can thereforebe enhanced by adsorption of molecules which render the approach of iodine to the Ti02 surface more difficult. The higher photovoltage in the presence of coadsorbed polyhydroxy1 compounds like glycerol, sorbitol, or glucose (Table I p r ) can be rationalized by this insulation mechanism. With cholanic acids, the photovoltage gain is less pronounced due to the counteracting conduction band shift in the presence of acid. Carboxylic acids with bulky hydrocarbon skeletons only increase the photocurrent efficiency but not the voltage (Table Is-u). This also holds for quinic acid, which competes with chlorophyllin adsorption even at low concentration (Table Iv). Further investigated coadsorbates are water; urea; EDTA; paraldehyde;and myristic, oleic, 8-hydroxystearic,pyroglutamic, camphanic, and 5,5-diethylbarbituric acids: noneof them improve both photovoltage and current as cholanic acids do.

Conclusion The photosensitization of colloidal Ti02 electrodes by chlorophyll derivatives has been shown to give high photocurrent quantum yields, approaching the unity efficiency of primary charge separation in natural photosynthesis. Also, the efficient light harvesting in plants was successfully imitated by adsorbing monolayers of the chlorophyll derivatives on nanostructured Ti02 films. Contrary to natural photosynthesis, where most chlorophyll molecules act as antennae and transfer the excitation energy to a small number of reaction centers, each molecule on the Ti02 electrode can be photoelectrochemically active. Therefore, photosensitization is possible even with the nonfluorescent Cuchlorophyllin, which cannotundergo energy transfer. Mechanistic aspects of the sensitization process shall be investigated in the subsequent paper of this seriese8 The practical usefulness of the chlorophyll solar cell is limited at present by the rather low energy conversion efficiency in sunlight. Ruthenium dyes are more promising in this regard, due to their better light-harvesting properties, especially in the green spectral region.1b They are also preferred in view of longterm stability: electron injection into the semiconductor only

The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6277 changes the oxidation state of the ruthenium center, while chlorophyll is oxidized to a cation radical? which is susceptible to side reactionsbefore being rereduced. Contrary to chlorophyll in plants, which is renewed each springtime and even undergoes continuousturnover during thevegetationperiod, a photosensitizer for solar cells has to be stable for many years. Nevertheless, a C~-2-~~-oxymesoisochlorin e4 sensitized Ti02 electrodedelivered an electrical charge of 200 C/cm2 in 18 h of illumination, corresponding to lo5 turnovers per dye molecule. The photocurrent dropped to half of the initial value during this time while the edges of the electrode turned gray, probably due to the intrusion of oxygen into the not perfectly sealed cell.

Acknowledgment. This work has been supported by a grant from the SwissFederal Office of Energy (OFEN: “Office Fedbra1 de 1’Energie”). References and Notes (1) (a) ORegan, B.; Gritzel, M. Narure (London) 1991,335,737. (b) Nazeeruddin, M. K.; Kay, A.; Rodicio, J.; Humphry-Baker, R.; Mlller, E.; Liska, P.; Vlachopoulos, N.; Gritzel, M. J. Am. Chem. Soc., in press. (2) (a) Kalyanasundaram,K.; Vlachopoulos,N.; Krishnan,V.; Monnier, A.; Gritzel, M. J. Phys. Chem. 1987, 91, 2342. (3) Enea, 0.;Moser, J.; GrHtzel, M. J. Electround. Chem. 1989, 259, 59. (4) (a) Tributsch, H.; Calvin, M. Photochem. Phorobiol. 1971, 14,95. (b) Tributsch, H. Photochem. Photobiol. 1972, 16, 261. (5) (a) Miyasaka, T.; Watanabe, T.; Fujishima, A,; Honda, K. J. Am. Chem. Soc. 1978,100,6657. (b) Miyasaka, T.; Watanahe, T.; Fujishima, A.; Honda, K. Nature 1979, 277, 638. (c) Miyasaka, T.; Watanabe, T.; Fujishima,A.; Honda, K. Photochem. Photobiol. 1980,32,217. (d) Watanabe, T.; Fujishima, A.; Honda, K. In Energy Resources through Photochemistry and Catalysis; Gritzel, M., Ed.; Academic Pres: New York, 1983. (e) Watanabe, T.; Machida, K.; Suzuki, H.; Kobayashi, M.; Honda, K. Coord.

Chem. Rev. 1985, 64,207. (6) Takehara, K.; Imai, H.; Ide, Y . Mem. Fac. Sci., Kyushu Univ. Ser. C 1987, 16, 1. (7) (a) Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J. Phys. Chem. 1986,90,1389. (b) Kamat, P. V.; Chauvet, J.-P. Radfar.Phys. Chem. 1991, 37, 705. (8) Kay, A,; Humphry-Baker, R.; Gritzel, M., to be published. (9) Rao, K. K.; Hall, D. 0.; Vlachopoulos, N.; GrHtzel, M.; Evans, M. C. W.; Seibert, M. J. Phorochem. Photobiol. B 1990, 5, 379. (10) Iriyama, K.; Ogara, N.; Takamiya, A. J. Biochemistry 1974,76,901, (1 1) Fischer, H.; Stern, A. Die Chemie des Pyrrols; Leipzig, 1940; Vol. 2, Part 2. (12) Oster, G.; Broyde, S. B.; Bellin, J. S . J . Am. Chem. Soc. 1964, 86, 1309, 1313. (13) Fuhrhop,J.-H.;Smith, K. M. In PorphyrfnsandMerulloporphyrfns; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 777. (14) Verma, A. L.; Mendelsohn, R.; Bernstein, H. J. J. Chem. Phys. 1974, 61, 383. (15) (a) Goedheer, J. C. In The Chlorophylls; Vernon, L. P., Seely, G. R., Eds.; AcademicPress: New York, 1966; p 147. (b) Katz, J. J.; Bowman, M. K.; Michalski, T. J.; Worcester, D. L. In Chlorophylls; Scheer, H., Ed.; CRC: Boca Raton, 1991; p 211. (16) Katz, J. J.; Shipman, L.; Cotton, T. M.; Janson, T. R. In The Porphyrinr; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 5, p 401. (17) Bucks, R. R.; Boxer, S . G. J . Am. Chem. Soc. 1982,104, 340. (18) Davis, R. C.; Pearlstein, R. M. Nature (London) 1979, 280, 413. (19) Kephart, J. C. Econ. Bot. 1955, 9, 3. (20) (a) Strell, M.; Kalojanoff, A.; Zuther, F. Arzneimirrel-Forsch.1955, 5,640; 1956,6,8. (b) Zuther, F. Neue Reaktionenin der Chlorophyllchemie, Dissertation, Mlnchen, 1955. (21) Boucher, L. J.; Katz, J. J. J . Am. Chem. Soc. 1967, 89, 4703. (22) Herndon, W. C. J. Chem. Educ. 1967, 44, 724. (23) Watanabe, T.; Kobayashi, M. In Chlorophylls; Scheer, H., Ed.; CRC: Boca Raton, 1991; p 287.