Photocurrent Enhancement of Hemicyanine Dyes Containing RSO

Overall yields were hence improved remarkably as a result of photocurrent enhancement: BTS increased by. 66% from 3.1% to 5.1% and IDS increased by ...
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9210

J. Phys. Chem. B 2001, 105, 9210-9217

Photocurrent Enhancement of Hemicyanine Dyes Containing RSO3- Group through Treating TiO2 Films with Hydrochloric Acid Zhong-Sheng Wang,† Fu-You Li, and Chun-Hui Huang* State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking UniVersity-The UniVersity of Hong Kong Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: February 21, 2001; In Final Form: May 13, 2001

Two kinds of hemicyanine dyes, 2-[4-(dimethylamino)styryl]benzothiazolium propylsulfonate (BTS) and 2-[4(dimethylamino)styryl]-3,3-dimethylindolium propylsulfonate (IDS), were synthesized and characterized with respect to their UV-vis, fluorescence, and redox properties. Both dyes show outstanding properties for charge transfer on HCl-treated nanocrystalline TiO2 films with near 100% incident monochromatic photon-to-electron conversion efficiency (IPCE) at the maximum absorption wavelength. This treatment was found to improve short-circuit photocurrent significantly, with increases by 99% and 329% for BTS and IDS, respectively. Overall yields were hence improved remarkably as a result of photocurrent enhancement: BTS increased by 66% from 3.1% to 5.1% and IDS increased by 260% from 1.3% to 4.8%. This phenomenon can be explained by the positive shift of flat band potential for TiO2 and the increased amount of adsorbed dye molecules upon HCl treatment. The values for IPCE, short-circuit photocurrent, and overall solar energy to electricity conversion efficiency are all among the highest values respectively in the pure organic sensitizers reported so far.

Introduction With the advent of the highly efficient Gra¨tzel cell,1 there has been increasing interest in studying Ru(II)-polypyridine dyes.2-10 However, the high price of RuCl3 and the difficulties of synthesizing Ru(II)-polypyridine complexes limit the practical application of the Gra¨tzel cell at the present time. Compared with Ru(II) complexes, pure organic dyes not only are easier to prepare but also cost less. Therefore, some pure organic dyes with rich photophysical properties are expected to be promising sensitizers for nanocrystalline solar cells in view of their character of less cost and easy tailoring. Recently, Arakawa and co-workers reported an efficient photoelectric sensitizer, a kind of merocyanine dye, with 4.2% overall energy conversion yield on nanostructured TiO2 film.11 This finding indicates that pure organic dyes are promising sensitizers for nanocrystalline semiconductor electrodes. Since hemicyanine dyes with donor (D)-π conjugation bridge-acceptor (A) (D-π-A) structure exhibit multifunctions, such as nonlinear optical,12-15 fluorescence,16,17 magnetic,18 conductive,19 and photoelectric activity,20 we have systematically investigated the structure-function effects of these dyes by the Langmuir-Blodgett (LB) technique through varying donors and acceptors.20-25 In addition, through linking attaching groups such as -RSO3- to hemicyanine dyes, they can adsorb onto the surface of TiO2 successfully and show good charge-transfer properties.26,27 However, it is far from practical application due to the low monochromatic incident photon-to-electron conversion efficiency (IPCE) of less than 40%. Looking back on the developing history of the Gra¨tzel cell based on Ru(II)polypyridine complexes, one can find that surface treatment is * Correspondong author. Telephone: +86-(10)62757156. Fax: +86(10)62751708. E-mail: [email protected]. † Present address: College of Science, Shandong Agricultural University, Tai’an 271018, Shandong, PRC.

a key step for optimizing their photoelectrochemical properties. For example, Ru(II)-polypyridine complex loaded TiO2 film generates higher IPCE after TiCl4 treatment.2a Therefore, surface treatment of nanostructured TiO2 films is one of the most important factors for cell performance. Generally, IPCE for a given system can be improved through the following ways: (a) increasing adsorbed amount of dyes; (b) increasing the ratio of rate constant for electron injection to that for back charge transfer through adjusting the energy level of conduction band (CB); (c) prohibiting charge recombination between I3- ions and injected electrons. Since the title dyes adsorb onto the TiO2 surface through the electrostatic interaction between the RSO3- group and the surface Ti4+ ions, the surface without exposed Ti4+ ions may not be beneficial to adsorption, which is one of the key factors limiting the adsorbed amount. If one can increase the number of active sites on the TiO2 surface for adsorption, the adsorbed amount and hence IPCE should be improved. On the other hand, specific adsorption of cations such as H+ and Li+ on nanocrystalline TiO2 particles is known to be effective for controlling the energy level of the CB for TiO2 and hence the ability for molecular sensitizers to inject electrons into the semiconductor upon irradiation.28 In photoelectrochemical energy conversion devices employing dyesensitized titanium dioxide mesoporous electrodes, back electron transfer is generally intercepted by the use of the iodide/triiodide couple as a charge mediator. The rate for the oxidation of Iby the oxidized dye was found to depend on the nature and concentration of added cations, and efficient oxidation of I- by the oxidized dye can minimize the possibility of back electron transfer.28 On the basis of the above analysis, we approched improving IPCE for hemicyanine dyes with an RSO3- group by increasing the adsorbed amount of the dye and adjusting the flat band potential of TiO2 through treating TiO2 films with hydrochloric acid. On one hand, it may make the TiO2 surface positively charged, which increases the amount of active sites

10.1021/jp010667n CCC: $20.00 © 2001 American Chemical Society Published on Web 08/28/2001

Photocurrent Enhancement of BTS and IDS SCHEME 1: Structures for BTS and IDS

for dye adsorption and consequently the IPCE. Specific adsorption of H+ ions on the TiO2 surface will lead to a positive shift of flat band potential for TiO2; this change will be favorable to electron injection and reduce back electron transfer.28 In a previous communication we have reported that the maximum IPCE of 2-[4-(dimethylamino)styryl]benzothiazolium propylsulfonate (BTS) sensitized TiO2 electrode can be increased to near unity through treating TiO2 films with hydrochloric acid.29 Such a high value warrants further studies on hemicyanine dyes containing R-SO3- groups as sensitizers for solar cells. Herein, we report the systematic study of the UV-vis absorption, fluorescence emission, electrochemical, and photoelectrochemical properties of BTS and 2-[4-(dimethylamino)styryl]-3,3-dimethylindolium propylsulfonate (IDS). Photocurrent enhancement was observed upon treating nanocrystalline TiO2 electrodes with hydrochloric acid. Except at the lower open-circuit photovoltage, both dyes can rival the best sensitizer, cis-Ru(NCS)2(2,2′-bipyridine-4,4′-dicarboxylic acid)2,2a with respect to both short-circuit photocurrent and fill factor. Experimental Section Materials. Titanium tetraisopropoxide, 1,3-propane sultone, propyl carbonate (PC), 2-methylbenzothiazole, and 2-methyl3,3-dimethylindole were purchased from Acros. All other solvents and chemicals used in this work were reagent grade (Beijing Chemical Factory, China) and used without further purification. Optically transparent conducting glass (CTO glass, fluorine-doped SnO2 overlayer, transmission > 70% in the visible, sheet resistance 20 Ω/square) was obtained from the Institute of Nonferrous Metals of China. N-methyl-4-methylpyridinium iodide was available from a previous study.20 2-[4-(Dimethylamino)styryl]benzothiazolium propylsulfonate (BTS) and 2-[4-(dimethylamino)styryl]-3,3-dimethylindolium propylsulfonate (IDS) were synthesized by the aldol condensation described in the literature.30 Structures for both dyes are illustrated in Scheme 1. BTS. Elem. Anal. for C20H22N2S2O3‚H2O (calculated values in parentheses): C, 56.73 (57.12); H, 5.82 (5.75); N, 6.54 (6.66)%. IDS. Elem. Anal. for C23H28N2SO3‚H2O (calculated values in parentheses): C, 63.87 (64.16); H, 6.73 (7.02); N, 6.32 (6.51)%. Preparation of Nanocrystalline TiO2 Electrodes. The 5-µmthick nanocrystalline TiO2 films were prepared according to the procedure described in the literature.26 TiO2 films used in this work were treated as follows: TiO2 films were immersed in 0.2 mol dm-3 TiCl4 (aqueous) for 24 h followed by annealing at 450 °C for 30 min. These TiO2 films are denoted as electrode A. After electrode A was impregnated with hydrochloric acid (aqueous) for 2 h, it was withdrawn from the solution and dried under a hot air flow. This process gives electrode B.

J. Phys. Chem. B, Vol. 105, No. 38, 2001 9211 Electrode C was obtained by heating electrode B at 450 °C for 30 min. The above-mentioned TiO2 films were sensitized by immersion in a 3 × 10-4 mol dm-3 solution of the dye in chloroform for 2 h. After the dye adsorption was saturated, the electrode was washed with chloroform and dried under a hot air flow. To determine the adsorbed amount of the dye molecules on the TiO2 surface, the dye was desorbed by immersing the dye-loaded TiO2 films in methanol overnight. The absorbance of desorbed solution of the dye was used to calculate the amount of the dye molecules adsorbed on the TiO2 films. Method. Film thickness was determined with a DEKTAK 3 profilometer. Elemental analysis was performed on a Carlo Erba 1106 elemental analyzer. UV-vis spectra were recorded on a Shimadzu Model 3100 UV-vis-NIR spectrophotometer. The fluorescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer. X-ray diffractometry was performed on a powder diffractometer (Rigaku Dmax-2000) using Cu KR radiation at 40 kV and 100 mA with a graphite monochromator. The film microstructure was studied using a high-resolution scanning electron microscopy (AMRAY, 1910 FE field emission microscope). Cyclic voltammetry (CV) was carried out in electrochemical cells on a Model 600 voltammetric analyzer (CH Instruments, Cordova, TN). A three-electrode cell was composed of a glassy carbon as working electrode, a platinum wire as counter electrode, and a Ag/AgCl as reference electrode. The supporting electrolyte was 0.1 mol dm-3 LiClO4 in acetonitrile, which was degassed with ultrapure nitrogen before scan. The scan rate was 0.1 V s-1. All potentials reported refer to an Ag/AgCl electrode. Flat band potentials were measured with the optical electrochemical method developed by Fitzmaurice and Gra¨tzel.31,32 A three-electrode cell was composed of the TiO2 film (A or B with 3.0 cm2 geometric surface area) working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. The above cell, which contains 0.1 mol dm-3 LiClO4 in acetonitrile as supporting electrolyte, was incorporated into the sample compartment of a UV-vis spectrophotometer. The potential scan was controlled with a Model 600 voltammetric analyzer (CH Instruments, Cordova, TN), while a Shimadzu UV-3100 spectrophotometer was used to record the absorbance changes at 780 nm. The potential for absorbance onset is regarded as the flat band potential. The photoelectrochemical experiments were carried out in a standard two-electrode system described in the literature.2,33 One drop of redox electrolyte solution was put onto the dye-loaded TiO2 film (effective area is 0.212 cm2) and penetrated inside the TiO2 film via capillary action. The redox electrolyte solution was composed of 0.1 mol dm-3 LiI, 0.6 mol dm-3 N-methyl4-methylpyridinium iodide, and 0.05 mol dm-3 I2 in PC. A platinized ITO glass as counter electrode was then clipped onto the top of dye-sensitized TiO2 working electrode to form the test cell. A 500 W xenon lamp (Ushio Electric, Tokyo, Japan) served as a white light source in conjunction with a KG4 filter (Schott, USA) and a GG420 cutoff filter (Toshiba, Japan). Monochromatic light in the range of 400-800 nm was produced by setting a KG4 filter and a suitable band-pass filter (Schott, USA) in the path of the light beam. Here, a KG4 filter was used to cut off infrared light to protect electrodes from heating, and a GG420 cutoff filter was used to filter off the light with wavelength less than 420 nm, which prevents the excitation of TiO2 films. Light intensities were measured with a Light Gauge radiometer/photometer (Coherent, Auburn, CA). Since a CTO

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TABLE 1: Absorption, Fluorescence Emission, and Redox Properties BTS IDS

λmaxa

λmaxb

λmaxc

λmaxd

λEXe

λEMe

Eoxf/V

Eredf/V

E1/2/V

557 565

529 551

461 540

473 550

560 570

590 592

0.871 0.912

0.804 0.841

0.838 0.877

a Absorption in chloroform. b Absorption in methanol. c Absorption on electrode A. d Absorption on electrode B. e Fluorescence in chloroform solution. f The supporting electrolyte was 0.1 mol dm-3 LiClO4, and potentials refer to Ag/AgCl electrode as reference. Scan rate was 0.1 V s-1.

Figure 1. UV-vis spectra for BTS-loaded TiO2 film: (a) HCl treated and (b) untreated. Absorption of bare TiO2 film together with CTO glass was deducted from the total absorption. Maximum absorbance was normalized to the same value for the purpose of comparison.

glass used in the photoelectrochemical experiments was placed in the light beam for light intensity measurement, the light adsorbed and reflected by conducting glass was corrected. Results and Discussion Photophysical Properties. Table 1 gives the data for photophysical properties of both dyes in solutions and on TiO2 films. Both dyes exhibit one π-π* transition band in the visible region due to the similar D-π-A structure. Both dyes have the same donor and different acceptor, and the maximum absorption wavelength of IDS is red shifted with reference to that of BTS in the same medium. This result indicates that IDS has a stronger acceptor than BTS. The absorption band in the visible of IDS is more intense than that of BTS in the same solvent, the former having an extinction coefficient of 7.79 × 104 dm3 mol-1 cm-1 and the latter 4.50 × 104 dm3 mol-1 cm-1 in CH3OH. The π-π* transition band depends on the solvent used and is delocalized over the entire trans chromophore. One can find from Table 1 that absorption maximum is blue shifted with increasing solvent polarity and is further blue shifted upon adsorption on nanocrystalline TiO2 films. Generally, a blue shift of the absorption band can be caused by solvatochromism or H-aggregation of dye molecules. The former is usually called “negative solvatochromism”:34 the ground-state molecule is better stabilized by solvation than the molecule in the excited state with increasing solvent polarity.34 The latter can also result in a large blue shift for absorption,35 but it is normally disadvantageous to electron injection,36 which is in contrast to the very high efficiency observed in photoelectrochemical experiments. Therefore, one can conclude that the blue shifts of absorption are attributed to solvatochromism. Interestingly, one can find from Table 1 that the visible bands in absorption spectra for BTS- and IDS-loaded TiO2 films are both red shifted to some extent upon treating TiO2 films with hydrochloric acid. As an example, the absorption spectra for BTS on TiO2 films with and without HCl treatment are shown in Figure 1. Besides the red shift of visible band, its half-width is increased

Figure 2. Fluorescence emission spectra for IDS in chloroform (solid line) and on TiO2 film (dotted line). Excitation wavelength was 520 and 560 nm for solution and IDS-loaded TiO2 film, respectively.

remarkably from 55 to 90 nm upon HCl treatment. These changes are favorable to photoelectric conversion. As for IDSloaded TiO2 film, however, half-width was not changed upon HCl treatment. The adsorbed H+ ion makes the TiO2 surface positively charged, and the latter may lead to a decrease in the polarity of the immediate environment, which is responsible for the observed red shift in the absorption spectrum upon treating TiO2 films with hydrochloric acid. Fluorescence Emission. Both dyes display strong fluorescence emission at room temperature, which is in agreement with the observations for other hemicyanine dyes containing R-SO3groups.35 Although the excitation maxima are different for both dyes, the emission maxima are both located at about 590 nm as shown in Table 1. Interestingly, the fluorescence emission band in the visible region disappears when the dye is adsorbed onto a nanocrystalline TiO2 film. The emission spectra for IDS as an example are shown in Figure 2. This phenomenon can be explained by the efficient electron injection from the excited singlet state of the dye to the CB of TiO2, and suggests that both dyes are good sensitizers on nanostructured TiO2 films. Energy LeVels of HOMO and LUMO. To thermodynamically judge the possibility of electron transfer from the excited dye to the CB of TiO2, CV was performed to determine the redox potentials (Table 1). Redox potentials (0.838 and 0.877 V vs Ag/AgCl for BTS and IDS, respectively), calculated by averaging the related oxidation and reduction potentials, are roughly regarded as energy levels of HOMO. The absorption maximum (462 nm, 2.69 eV for BTS; 540 nm, 2.30 eV for IDS) in the visible region corresponds to the gap between the HOMO and LUMO of the dye, and then the energy levels of LUMO can be estimated to be -1.85 and -1.42 V (vs Ag/AgCl) for BTS and IDS, respectively. Obviously, the excited-state energy levels for both dyes are higher than the bottom of the CB (-1.19 V vs Ag/AgCl; see Table 2), indicating electron injection should be possible thermodynamically. In addition, both dyes show reversible features, suggesting they should be expected to be regenerative dyes in dye-sensitized solar cells.

Photocurrent Enhancement of BTS and IDS

J. Phys. Chem. B, Vol. 105, No. 38, 2001 9213 TABLE 3: Effect of HCl Treatment on the Adsorbed Amount of Dyes on the TiO2 Surface molecules adsorbed per unit area/ 1016 molecules cm-2 film BTSa IDSb

electrode A

electrode B

electrode C

1.92 0.60

3.53 2.69

2.24 0.70

a 0.2 mol dm-3 HCl treating TiO2 film for 2 h. b 0.15 mol dm-3 HCl treating TiO2 film for 2 h.

Figure 3. SEM micrographs of 5-µm-thick TiO2 film: (a) electrode A and (b) electrode B (0.15 M hydrochloric acid was used to treat the TiO2 film).

TABLE 2: Flat Band Potentials of TiO2 Film with and without HCl Treatment c(H+)/mol dm-3 Vfb/V (vs Ag/AgCl)

0 -1.19

0.01 -0.96

0.15 -0.94

1.0 -0.93

Characterization of Nanocrystalline TiO2 Films. Morphology of Particles in Nanocrystalline Films. SEM micrographs (Figure 3) show that the particles become a little smaller upon treating TiO2 film with hydrochloric acid: the average size of the particle changed from about 12 nm in electrode A to about 10 nm in electrode B. With a little smaller particle size, HCltreated TiO2 film should have a slight larger surface area than an untreated one. Since nanocrystalline TiO2 film is well-known for its acid-resisting property, it is rational that the treatment of hydrochloric acid influences the particle size to only a slight extent. In addition, half-width of XRD peaks for nanocrystalline TiO2 film become broadened a little upon HCl treatment. Since the smaller the particle size, the wider the half-width of XRD peak, the XRD result is in agreement with that of SEM micrographs. Effect of HCl Treatment on Flat Band Potential. Flat band potential is a key factor limiting the solar cell performance because the relative energy level of excited dye molecule and CB for TiO2 significantly influence electron injection and back charge-transfer.28,32 Optical electrochemistry was performed on different kinds of TiO2 films used in this work, and the results are summarized in Table 2. A more than 200 mV positive shift

of flat band potential was observed when nanocrystalline TiO2 film was treated with hydrochloric acid, which is attributed to the specific adsorption of H+ ions at the TiO2 surface.32 With increasing concentration of hydrochloric acid from 0.01 to 1 mol dm-3, the flat band potential decreases by only 0.03 V. Although the flat band potential can be adjusted easily in aqueous solution by adjusting pH, water as solvent is disadvantageous to photoelectric conversion. However, treating TiO2 film with hydrochloric acid followed by drying is no doubt a good way to decrease the flat band potential in nonaqueous aprotic solution. The positive shift of the flat band potential is expected to be favorable to electron injection and to reduce charge recombination between dye cations and electrons injected.28 Effect of HCl Treatment on Adsorbed Amount. The amount of adsorbed dye molecules is increased outstandingly upon HCl treatment of TiO2, and the results are listed in Table 3. One can see from Table 3 that the adsorbed amount of dye in electrode B is increased by 83% and 350% for BTS and IDS, respectively, compared with that in each corresponding electrode A. Interestingly, the amount of adsorbed BTS is higher than that of adsorbed IDS in each type of electrode (A, B, or C). This phenomenon can be interpreted by the fact that BTS has less steric hindrance than IDS because the latter has two more methyl groups than the former. If HCl is removed from TiO2 film by heating again at 450 °C for 30 min, the adsorbed amount of dye is decreased by a factor of 2/3 and 1/4 for BTS and IDS, respectively, compared with that in the corresponding electrode B, but still higher than that in the corresponding A. This result indicates that the total surface area of TiO2 film is increased to some extent upon HCl treatment followed by annealing at 450 °C. In fact, the smaller particle size upon HCl treatment confirmed by SEM and XRD corresponds to an increase in total surface area. After HCl treatment, besides the native active adsorbing sites (Ti4+), the newly formed surface sites (H+) are also active for dye adsorption. As a consequence, dye molecules can be adsorbed in a more compact way, thus increasing the adsorbed amount remarkably. Principally, photocurrent is increased with increasing adsorbed amount of dye; therefore, more adsorbed dye molecules are partly responsible for photocurrent enhancement. Photoelectrochemistry: Photocurrent Action Spectra. Photocurrent action spectra for the dye-loaded TiO2 films are shown in Figure 4, where the incident monochromatic photonto-electron conversion efficiency (IPCE), defined as the number of electrons generated by light in the external circuit divided by the number of incident monochromatic photons, is plotted as a function of excitation wavelength. IPCE is derived by

IPCE (%) )

1240Isc (µA cm-2) λ (nm) Pin (W m-2)

(1)

where Isc is the short-circuit photocurrent density generated under illumination of incident monochromatic light, whose

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TABLE 4: Effect of Concentration of Hydrochloric Acid on Cell Performance Based on IDSa c(H+)/M Isc/mA cm-2 Voc/mV FF η (%)

0

0.01

0.05

0.15

0.20

0.25

0.30

0.35

0.50

3.5 483 0.728 1.34

9.68 489 0.682 2.57

10.31 481 0.698 3.75

14.95 442 0.669 4.80

16.49 420 0.627 4.71

17.31 435 0.580 4.74

17.75 423 0.572 4.66

17.59 418 0.576 4.59

17.67 378 0.581 4.21

a 5-µm-thick TiO2 films were used. Thin-layer sandwich-type solar cells were illuminated under 92.2 mW cm-2 white light from a xenon lamp, and a GG420 filter together with a KG4 filter was used to cut off infrared light and light with wavelength less than 420 nm.

Figure 4. Photocurrent action spectra for (a) BTS and (b) IDS. IPCE was corrected for the absorption and reflection by CTO glass.

wavelength and power are λ and Pin. The dye-loaded TiO2 film was illuminated from the front side through the conducting glass support. IPCE values reported are corrected for the absorption and reflection of incident light by the glass. On electrode A (without HCl treated), BTS generated 61.2% of maximum IPCE, whereas IDS only yielded 26.9% partly due to the poor light harvesting resulting from the low adsorbed amount. On electrode B (HCl treated), both BTS and IDS yielded maximum IPCE near unity. The remarkable increase in IPCE from electrode A to B is attributed to both the increase in adsorbed amount and the positive shift of the flat band potential. Comparing the two factors, we believe the latter is the key point for the remarkable improvement of photocurrent generation. IPCE is the product of light-harvesting efficiency (LHE(λ)), the quantum yield of electron injection (φinj(λ)), and the efficiency of collecting the injected charge (ηc(λ)) at the back contact, and it is expressed as

IPCE ) LHE(λ) φinj(λ) ηc(λ)

(2)

One can see from eq 2 that increased adsorbed amount contributes to IPCE by increasing LHE(λ). However, prior to HCl treatment, the maximum absorbance of the dye on TiO2 is more than 3 and 1 for BTS and IDS, respectively, so LHE(λ) at maximum absorption can be increased by no more than 10%, which is far less than the percentage of increase in IPCE upon HCl treatment. Nevertheless, IPCE at other wavelengths with very low extinction coefficient can be increased to a large extent due to the increased adsorbed amount. One can conclude from the above analyses that the increased adsorbed amount cannot be responsible for the increased IPCE completely. It is inferred from eq 2 that the other two factors may be improved upon HCl treatment. As for TiCl4-treated TiO2 films, the increase of the necking between the particles will facilitate the percolation of the electrons from one particle to the other. Consequently, it can be assumed that electron collection efficiency in these films will be close to unity37 as the TiO2 electrodes used in this work

were all pretreated with TiCl4 solution. Apart from increasing the adsorbed amount, the adsorbed H+ ion can decrease the position of the flat band by more than 0.2 V, which is the key reason for resulting in such a remarkable increase in IPCE. Since the difference of the excited state energy level of the dye and the flat band potential of TiO2 becomes larger with the decrease of the flat band, the driving force for electron injection is enhanced and charge recombination between dye cations and electrons injected is minimized.28 IPCE near 100% at maximum absorption indicates that all three efficiencies must be near 100% at the same wavelength upon treating TiO2 films with HCl. Of course, a time-resolved laser technique would be needed to give more direct evidence to prove which process is actually improved upon HCl treatment. Effect of Concentration of Hydrochloric Acid on Cell Performance. To seek an optimal concentration of HCl to treat TiO2 film, performance parameters for IDS-based solar cells using TiO2 films untreated and treated with different concentrations of HCl were measured, and the results are listed in Table 4. Fill factor (FF) is defined as

FF ) VoptIopt/VocIsc

(3)

where Vopt and Iopt are respectively voltage and current for maximum power output, and Voc and Isc are open-circuit photovoltage and short-circuit photocurrent, respectively. The overall yield (η) is expressed by

η ) (FF Voc Isc)/Pin

(4)

where Pin is the power of incident white light. One can see from Table 4 that Isc is increased drastically in the beginning of HCl treatment, is increased gradually from 0.01 to 0.3 mol dm-3, and finally remains almost the same in the range of 0.25-0.5 mol dm-3 hydrochloric acid concentration. Voc remains almost constant from untreated to 0.05 mol dm-3 HCl treated TiO2 film, and decreases gradually with further increasing the concentration of hydrochloric acid. FF decreases gradually with increasing concentration of hydrochloric acid due to the ohm loss resulting from the increased Isc. Balancing all the three factors, overall yield increases with the increase of concentration for hydrochloric acid, attains the highest level at 0.15-0.25 mol dm-3 hydrochloric acid concentration, and then decreases gradually. The change of Isc and Voc can be explained by the positive shift of the flat band potential. With the decrease of the flat band potential, open-circuit photovoltage will be decreased because the latter relates to the difference of the flat band potential and the redox potential of I3-/I- couple in the electrolyte.1 One can see from Table 4 that no noticeable changes in open-circuit photovoltage were observed for treated TiO2 with concentrations of HCl up to 0.05 mol dm-3. The remarkably increased short-circuit photocurrent results from the increased electron injection and/or the reduced charge recombination, which certainly leads to an increase in photovoltage

Photocurrent Enhancement of BTS and IDS

J. Phys. Chem. B, Vol. 105, No. 38, 2001 9215

Figure 5. Effect of concentration of hydrochloric acid on the performance of BTS-based solar cells. The intensity of the incident white light from a xenon lamp was 92.2 mW cm-2.

Figure 6. I-V curves for (a) BTS and (b) IDS. Solar cells were illuminated under white light of 92.2 mW cm-2 from a xenon lamp. To treat TiO2 films for BTS and IDS, 0.2 and 0.15 mol dm-3 HCl, respectively, were used.

and compensates for the voltage loss due to the decreased flat band potential (see eq 5).2a

Voc )

()(

Iinj kT ln e ncbket[I3-]

)

(5)

where Iinj is the flux of charge resulting from dye-sensitized electron injection, ncb is the concentration of electrons at the surface of TiO2, and ket is the rate constant for triiodide reduction by injected electrons. However, when increasing the concentration of hydrochloric acid for treating TiO2 films further, the increased photocurrent is not large enough to compensate the voltage loss resulting from the further decreased flat band potential. Consequently, open-circuit photovoltage is decreased gradually with increasing concentration of HCl from 0.05 mol dm-3. Balancing all factors limiting the cell performance, 0.15 mol dm-3 HCl solution was used to treat nanocrystalline TiO2 films and optimal cell performance was obtained under this treatment. Similarly, the optimal concentration of HCl was determined to be 0.2 mol dm-3 for BTS-based solar cells, as seen in Figure 5. It was found that 0.15-0.2 mol dm-3 HCl was an optimal concentration for treating TiO2 films to obtain optimal output performance of the solar cell. Above this range, short-circuit photocurrent is still increased up to about 20 mA cm-2, but the increased short-circuit photocurrent is not able to compensate for the loss in open-circuit photovoltage and fill factor. Briefly, overall yield increased up to 0.15 (for IDS) or 0.2 mol dm-3 (for BTS) HCl treatment and then decreased with increasing concentration of HCl further. To confirm the positive effect of H+ on cell performance, dye-sensitized electrode C was also tested. It is found that a little change in cell performance was observed in electrode C compared with electrode A. This result proves that photocurrent cannot be improved significantly if HCl is removed from TiO2 films. In other words, the presence of H+ ions on the TiO2 surface is the key factor for the remarkable improvement of short-circuit photocurrent. When 0.1 mol dm-3 LiOH was selected as a base to treat nanocrystalline TiO2 films, the solar cell only generated 1.84 mA cm-2 short-circuit photocurrent and 460 mV of open-circuit photovoltage. Since Li+ is a potential-determining ion that has been proved to be beneficial to electron injection,28,32 this result indicates that the adsorbed OH- ion is unfavorable to photocurrent generation. As we know,

Figure 7. Relationship between light intensity and the short-circuit photocurrent generated by IDS-sensitized electrode B. To treat TiO2 films, 0.15 mol dm-3 HCl was used.

the presence of OH- on the TiO2 surface will lead to a negative shift of flat band potential for TiO2, and this is disadvantageous to electron injection.31,32 Therefore, the decrease in short-circuit photocurrent was observed upon treating TiO2 with LiOH. This experimental result is good counter evidence for the dramatic positive effect of H+ ion on photocurrent generation. Photocurrent-Voltage Characteristics. At optimal concentration of HCl for treating TiO2 films, HCl-treated TiO2 electrodes gave more outstanding output performance than untreated TiO2 electrodes did, as seen in Figure 6. Under illumination of white light (92.2 mW cm-2) from a Xe lamp, untreated TiO2 electrodes generated 8.3 and 3.5 mA cm-2 Isc, 3.07% and 1.34% of η for BTS and IDS, respectively, while HCl-treated TiO2 electrodes produced 16.5 and 15.0 mA cm-2 Isc, 5.09% and 4.80% of η for BTS and IDS, respectively. One can see from Figure 6 that the improvement both in Isc and η upon HCl treatment is very remarkable. Upon HCl treatment, short-circuit photocurrent is increased to a large extent as a result of increased IPCE. Besides the high values of short-circuit photocurrent, the value for fill factor near 0.7 is also impressive. Effect of light intensity on short-circuit photocurrent and open-circuit photovoltage was also investigated. Figure 7, as an example, shows the linear relationship between light intensity and short-circuit photocurrent generated by IDS-based solar cells. This linearity indicates that the photocurrent generated is

9216 J. Phys. Chem. B, Vol. 105, No. 38, 2001

Wang et al. cis-Ru(II)(NCS)2(2,2′-bipyridine-4,4′-dicarboxylic acid)2,2a shortcircuit photocurrent remained almost unchanged, whereas opencircuit photovoltage was decreased to some extent upon HCl treatment. The above results show that the position of the flat band potential for nanocrystalline TiO2 can be moved positively upon HCl treatment. In particular, HCl treatment can improve the photoelectric properties of those dyes with poor electron injection efficiency, but cannot efficiently increase the shortcircuit photocurrent for those dyes with highly efficient electron injection. Conclusions

Figure 8. Relationship between the logarithm of light intensity and open-circuit photovoltage generated by IDS-sensitized electrode B. To treat TiO2 films, 0.15 mol dm-3 HCl was used.

not limited by diffusion of the iodide or triiodide ions within the nanocrystalline network up to current densities of about 20 mA cm-2. Pore size influences photocurrent generation significantly.37 If pore size is large enough, transport kinetics will be fast enough to regenerate (i.e., reduce) the dye. Otherwise, diffusion of the electrolyte will be the limiting step for photocurrent generation; namely, short-circuit photocurrent tends to become saturated with increasing light intensity. Open-circuit photovoltage is proportional to the logarithm of light intensity, as seen in Figure 8. According to eq 5, opencircuit photovoltage is proportional to the logarithm of electrons injected. Since the short-circuit photocurrent increases linearly with increasing light intensity, electrons injected should be increased linearly with increasing light intensity. Consequently, open-circuit photovoltage relates to the logarithm of light intensity linearly. This linearity indicates that charge recombination between electrons injected and I3- ions in nanopores does not become serious with increasing light intensity. Otherwise, open-circuit photovoltage should tend to be saturated at high light intensity in the studied range. Stability. Since photostability is a vital parameter for sustained cell operation, we studied cell performance changes under illumination of white light. For continuous illumination of 10 h, short-circuit photocurrent increased a little for the first 30 min and remained almost constant in the remaining time. On the other hand, open-circuit photovoltage increased by about 50 mV for the first 30 min and then remained almost unchanged. Since the solar cell was not sealed, long-term stability was not tested. However, a thin-layer sandwich solar cell was set under daylight for more than 2 months, and photodegradation of the dyes was not observed because no noticeable change was observed in their absorption spectra. Applicability of HCl Treatment. To investigate the applicability of HCl treatment, other hemicyanine dyes containing an RSO3- group were also tested. For example, (E)-N-(3-sulfopropyl-4-[2-(4-dimethylaminophenyl)ethenyl]pyridinium or (E)N-(3-sulfopropyl-4-[2-(4-dimethylaminophenyl)ethenyl]quinolinium sensitized solar cells26 were also improved through HCl treatment; the former increased by 20% from 4.2% to 5.16% and the latter increased by 48% from 2.42% to 3.59%, with respect to overall yield. These results show that HCl treatment is favorable to improving the photoelectric properties for hemicyanine dyes with an RSO3- group, although not all dyes of this type were tested. Regarding other kind of dyes, such as

The presence of H+ ions on the TiO2 surface significantly influences photoelectrochemical properties for hemicyanine dyes containing an RSO3- group. This surface treatment of nanocrystalline TiO2 films with hydrochloric acid is proved to increase the adsorbed amount of dyes such as BTS and IDS as well as the short-circuit photocurrent of the corresponding solar cell. Since HCl treatment makes the TiO2 surface positively charged, the adsorbed amount of dyes is increased due to the increased adsorption sites, which, therefore, leads to a more compact arrangement of dyes on the TiO2 surface. The increased adsorbed amount is partly responsible for the photocurrent enhancement, especially for the wavelength region where extinction coefficient is very low. The major reason for the photocurrent enhancement is the positive shift of the flat band potential of nanostructured TiO2 film upon HCl treatment. In contrast to photocurrent enhancement, open-circuit photovoltage is decreased to some extent upon treating TiO2 with hydrochloric acid, which is reasonable in view of the decreased difference between the flat band potential of TiO2 and the redox potential of I3-/I- couple.38 The significant improvement of short-circuit photocurrent upon treating nanocrystalline TiO2 film with HCl solution indicates that surface treatment is very important in solar cell optimization. Our results confirm that hemicyanine-group dyes are powerful candidates for dye-sensitized solar cells. These findings broaden the scope of dyestuff for solar cells and aid the development of Gra¨tzel cells. Since the cost of hemicyaninegroup dye is much lower than that of ruthenium polypyridine complex, the overall cost of dye-sensitized solar cells could be further reduced. Furthermore, due to the ease of tailoring, more efficient hemicyanine dyes as solar sensitizers could be designed easily after studying the structure-efficiency relationship. Acknowledgment. The authors thank the State Key Program of Fundamental Research (G1998061308), the NNSFC (20023005 and 59872001), and the Doctoral Program Foundation of Higher Education (99000132) for financial support of this work. References and Notes (1) O’Regen, B.; Gra¨tzel, M. Nature (London) 1991, 353, 737. (2) (a) Nazeeruddin, M. K.; Kay, A.; Rodicicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (b) Nazeeruddin, M. K.; Humphry-Baker, R.; Gra¨tzel, M.; Murrer, B. A. Chem. Commun. 1998, 719. (c) Nazeeruddin, M. K.; Pe´chy, P.; Gra¨tzel, M. Chem. Commun. 1997, 1705. (3) Cahen, D.; Hodes, G.; Gra¨tzel, M.; Guillemoles, J. F.; Riess, I. J. Phys. Chem. B 2000, 104, 2053. (4) So¨dergren, S.; Hagfelt, A.; Olsson, J.; Lindquist, S.-E. J. Phys. Chem. 1994, 98, 5552. (5) (a) Wang, Z.-S.; Huang, C.-H.; Zhang, B.-W.; Hou, Y.-J.; Xie, P.H.; Qian, H.-J.; Ibrahim, K. New J. Chem. 2000, 24, 567. (b) Wang, Z.-S.; Huang, C.-H.; Hou, Y.-J.; Xie, P.-H.; Zhang, B.-W.; Cheng, H.-M. Chem. Mater. 2001, 13, 678. (6) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. J. Phys. Chem. B 1997, 101, 2591.

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