A Strategy To Increase the Efficiency of the Dye-Sensitized TiO2 Solar

Nov 2, 2005 - The intensity of the incident light was calibrated using a KG5-filtered, monocrystalline Si reference solar cell, PVM 37 (ISO tracking n...
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J. Phys. Chem. B 2005, 109, 22513-22522

22513

A Strategy To Increase the Efficiency of the Dye-Sensitized TiO2 Solar Cells Operated by Photoexcitation of Dye-to-TiO2 Charge-Transfer Bands Eunju Lee Tae, Seung Hwan Lee, Jae Kwan Lee, Su San Yoo, Eun Ju Kang, and Kyung Byung Yoon* Center for Microcrystal Assembly, Department of Chemistry, and Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Korea ReceiVed: July 8, 2005; In Final Form: September 21, 2005

Dye-sensitized nanoporous TiO2 solar cells (DSSCs) can be classified into two types, namely, Type-I and Type-II. Type-I DSSCs are the DSSCs in which electrons are injected from the adsorbed dyes by photoexcitation of the dyes followed by electron injection from the excited dyes to TiO2 (pathway A). Type-II DSSCs are the DSSCs in which electrons are injected not only by pathway A but also by direct one-step electron injection from the dyes to TiO2 by photoexcitation of the dye-to-TiO2 charge-transfer (DTCT) bands (pathway B). The DSSCs employing catechol (Cat) or its derivatives as the sensitizers have been the typical examples of TypeII DSSCs. However, their solar energy-to-electricity conversion efficiencies (η) have never exceeded 0.7%, and the external quantum efficiencies (EQE) at the absorption maximums of the DTCT bands have never exceeded 10%. We found that the attachment of electron-donating compounds such as (pyridin-4-yl)vinyl and (quinolin-4-yl)vinyl, respectively, to Cat (designated as Cat-v-P and Cat-v-Q, respectively) leads to 2and 2.7-fold increases, respectively, in η, driven by large increases in short circuit current (Jsc). The EQE increased from 8.5 to 30% at 400 nm upon changing from Cat to Cat-v-P, at which only the DTCT band absorbs. In the case of the Cat-v-Q-sensitized DSSC, even the η obtained by exciting only the DTCT band was higher than 1%. Interestingly, the illumination of only the DTCT band resulted in the increase of fill factor from 62.6% to 72.3%. This paper provides for the first time an insight into the strategy to increase the η values of Type-II DSSCs.

Introduction Dye-sensitized TiO2 solar cells (DSSCs) have been developed as commercially compelling alternatives to the more expensive, conventional p-n junction semiconductor solar cells.1-12 A great number of dyes have been tested as sensitizers during the past 15 years in pursuit of high efficiency (η). Accordingly, the η has reached ∼11% in the laboratory scale.1 The DSSCs can be classified into two types, namely, Type I and Type II, depending on the electron-injection pathway from the dye to the conduction band (CB) of TiO2. The first pathway (pathway A) is photoexcitation of the local band of the adsorbed dye followed by electron injection from the excited dye to the CB of TiO2 as illustrated in Scheme 1A. This pathway can also be called the “two-step” electron injection pathway. The dyes that bind to the surface of TiO2 using carboxylic acid groups have been known to inject electrons to TiO2 according to pathway A. These dyes are classified as Type-I dyes, and a variety of Ru(II) complexes,1-5 coumarin derivatives,6 metal-porphyrin complexes,7-9 and others10-12 having carboxylic acid units, belong to this type. The corresponding DSSCs sensitized by Type-I dyes are classified as Type-I DSSCs. Another pathway (pathway B) is direct, “one-step” electron injection from the ground state of the dye to the CB of TiO2 by photoinduced charge-transfer (CT) excitation of the dye-to-TiO2 CT (DTCT) bands as illustrated in Scheme 1B.13,14 The dyes having enediol units have been known to bind to the surface of * To whom correspondence should be addressed. E-mail: yoonkb@ sogang.ac.kr.

SCHEME 1: Two Different Types of Electron Injection Pathways from an Adsorbed Dye to TiO2 in DSSCs

TiO2 through chelation of surface Ti(IV) ions with the enediol groups, generally giving rise to very strong DTCT bands.15-26 Notably, catechol (Cat)13,15,16 and its derivatives such as dopamine (Dop),16a,19 fluorone,20 numerous natural pigments such as bromopyrogallol red (Bpg)21 (see Chart 1), and anthocyanins having catechol moieties21-26 give strong DTCT bands in the visible region upon binding to TiO2. The related compounds such as salicylic acid,16a,16b ascorbic acid,17 and dihydroxycyclobutenedione16a also give strong DTCT bands in the visible region upon binding to TiO2. Various transition-metal cyanides have also been known to form visible CT complexes with surface Ti(IV) ions through one of the nitrile groups27-31 and hence give rise to strong DTCT bands. In close relation to this, we recently revealed that polycyclic aromatic compounds also form visible CT complexes with TiO2.32 Several reports have demonstrated that the photoexcitation of DTCT bands indeed gives rise to very fast (75%) of charge recombination occur within a few picoseconds in pathway B.15a,18,21b,22,31b In this respect, the development of the methods for increasing the true efficiency operated by pathway B is not only a big challenge for its own sake but also of great interest from the academic and practical points of view. Stemming from our experience gained in the area of achieving long-lived charge separation between the donors and acceptors encapsulated within zeolite cages and placed at the zeolitesolution interfaces,36 we have recently been interested in developing high efficiency Type-II DSSCs. By use of Cat and their novel derivatives as model Type-II dyes, we now report that the Type-II DSSC sensitized by (quinolin-4-yl)vinylattached Cat (designated as Cat-v-Q, see Chart 1) gives rise to a 3-fold increase in EQE and a more than 2-fold increase in overall η with respect to those of Cat-sensitized DSSC, despite the fact that most of the photovoltaic currents were produced by pathway B. Thus, the overall η obtained by Cat-v-Q was 1.3%, breaking for the first time the 0.7% barrier. Furthermore, after coadsorption of deoxycholic acid, the η value increased to 1.6%, demonstrating for the first time the potential of TypeII DSSCs to be developed into commercially viable DSSCs. This paper also compares the photovoltaic characteristics of the related (pyridin-4-yl)vinyl-attached catechol (designated as Catv-P) and the conventional Type-II DSSCs such as Cat, Dop, and Bpg, respectively (Chart 1). Experimental Section Materials. 3,4-Dihydroxybenzaldehyde (97%, Aldrich), 4-pyridinecarboxaldehyde (TCI), 4-quinolinecarboxaldehyde (97%, Aldrich), tetrabromomethane (Aldrich, 99%), chloromethyl methyl ether (MOMCl, tech. Aldrich), N,N-diisopropylethylamine (99.5%, Aldrich), triphenylphosphine (PPh3, 99%, Aldrich), triethyl phosphite (98%, Aldrich), potassium tert-butoxide (1.0 M in tetrahydrofuran, Aldrich), titanium(IV) isopropoxide (Ti(OiPr)4, 98+%, Acros), titanium tetrachloride (TiCl4, 99.9%, Aldrich), 2-propanol (Junsei), acetic acid (glacial, 99.99+%, Aldrich), hydroxy propyl cellulose (HPC, Mw ) 80 000, Aldrich), 4-tert-butylpyridine (TBP, 99%, Aldrich), Triton X-100 (Aldrich), acetylacetone (Aldrich, 99%), and Mucasol (Merz) were purchased and used as received. Cat (99%, Aldrich), Dop (98%, Aldrich), Bpg (TCI), and deoxycholic acid (DCA, 99%, Aldrich) were purchased and used as received. 3-Fluorocatechol (99%, Aldrich), 4-chlorocatechol (98%, TCI), 4-nitrocatechol (97%, Aldrich), 4-methylcatechol (95%, Aldrich), 3-methoxycatechol (99%, Aldrich), and 3,5-di(tert-butyl)catechol (99%, Aldrich) were purchased and used as received. 1-Hexyl-3-methylimmidazolium iodide (HMI+I-) was prepared by reacting 3-methylimmidazole with 1-hexyl iodide. CH2Cl2 (Junsei) and acetonitrile (Junsei) were distilled over CaH2, and valeronitrile (Aldrich) was used as received. Tetrahydrofuran (THF, Junsei) was distilled from Na and benzophenone. Column chromatography was performed using a Merck silica gel 60. F-doped tin-oxide glass (FTO glass, 8 Ω cm-1, thickness of FTO layer ) 100 µm, thickness of glass support ) 3 mm) was the product of Libby Owens Ford. Thermoplastic film (Surlyn) with the thickness of 100 µm was the product of DuPont. Preparation of Cat-v-P and Cat-v-Q. The two compounds were prepared according to Scheme 2. Preparation of 3,4-Dimethoxymethyloxybenzaldehyde (1). 3,4-Dihydroxybenzaldehyde (1.38 g, 10 mmol) was dissolved

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J. Phys. Chem. B, Vol. 109, No. 47, 2005 22515

SCHEME 2: Procedure for the Synthesis of Cat-v-P and Cat-v-Q

in CH2Cl2 (10 mL). N,N-Diisopropylethylamine (2.58 g, 20 mmol) was added into the above solution at 0 °C, and MOMCl (1.93 g, 24 mmol) was then slowly added into the solution keeping the temperature at 0 °C. After stirring for 30 min at 0 °C, the reaction mixture was diluted with CH2Cl2 and washed with water and brine. The organic layer was dried and concentrated in vacuo. Column chromatography over silica gel (1:4 ethyl acetate-hexane) gave solid 3,4-dimethoxymethyloxybenzaldehyde (2.17 g, 96%): 1H NMR (500 MHz, CDCl3, ppm) 9.87 (s, 1H), 7.69 (d, J ) 1.5 Hz, 1H), 7.52 (d, J ) 8 Hz, 1H), 7.29 (d, J ) 8.5 Hz, 1H), 5.35 (s, 2H), 5.29 (s, 2H), 3.54 (s, 3H), 3.53 (s, 3H); 13C NMR (500 MHz, CDCl3, ppm) 190.9, 152.7, 147.5, 131.1, 126.4, 115.9, 115.4, 95.5, 55.5, 56.4, 56.4. High-resolution mass spectrometry (HRMS) m/z for (C11H14O4 + H): calcd., 227.0919; obsd., 227.0925. Preparation of 3,4-Dimethoxymethyloxybenzyl Alcohol (2). 1 (1.13 g, 5 mmol) was dissolved in methanol (10 mL). NaBH4 (229 mg, 6 mmol) was slowly added into the above solution at 0 °C. After stirring for 3 h at 0 °C, the solvent was evaporated. After dissolving the reaction mixture in CH2Cl2 (5 mL), the solution was washed with saturated NaHCO3 solution and subsequently with brine. The organic layer was collected, dried, and concentrated in vacuo. Column chromatography over silica gel (ethyl acetate:hexane ) 1:1) gave 2 (1.12 g, 98%) as a liquid: 1H NMR (500 MHz, CDCl3, ppm) 7.14 (d, J ) 2 Hz, 1H), 7.10 (d, J ) 8 Hz, 1H), 6.92 (dd, J ) 5 Hz, 1H), 5.21 (s, 2H), 5.19 (s, 2H), 4.55 (s, 2H), 3.49 (s, 3H), 3.48 (s, 3H); 13C NMR (500 MHz, CDCl3, ppm) 147.3, 146.6, 135.7, 121.2, 116.9, 115.7, 95.5, 64.8, 56.2, 56.1. HRMS m/z for C11H16O5: calcd., 228.0998; obsd., 227.0996. Preparation of 3,4-Dimethoxymethyloxybenzylbromide (3). 2 (1.14 g, 5 mmol) was dissolved in CH2Cl2 (5 mL). PPh3 (1.31 mg, 5 mmol) and CBr4 (1.69 mg, 5 mmol) were sequentially added into the solution at 0 °C. After stirring at 0 °C for 24 h, the solvent was evaporated, and the reaction mixture was dissolved in CH2Cl2. The solution was washed with water and subsequently with brine. The organic layer was dried and concentrated in vacuo. Silica gel chromatography (ethyl acetate: hexane ) 1:2) gave 3 as a liquid (1.38 g, 95%): 1H NMR (500 MHz, CDCl3, ppm) 7.20 (d, J ) 2 Hz, 1H), 7.11 (d, J ) 8.5 Hz, 1H), 7.00 (dd, J ) 5 Hz, 1H), 5.25 (s, 2H), 5.23 (s, 2H), 4.47 (s, 2H), 3.54 (s, 3H), 3.50 (s, 3H); 13C NMR (500 MHz, CDCl3, ppm) 147.5, 147.4, 132.1, 123.5, 117.5, 116.7, 95.6, 56.5, 56.4, 33.9. HRMS m/z for C11H15BrO4: calcd., 290.0154; obsd., 290.0157.

Preparation of Diethyl 3,4-Dimethoxymethyloxybenzylphosphonate (4). 3 (1.46 g, 5 mmol) was dissolved in P(OEt)3 (997 mg, 6 mmol), and the solution was refluxed for 24 h at 150 °C. The reaction mixture was diluted by adding CH2Cl2 (5 mL), and the solution was washed with water and subsequently with brine. The organic layer was dried and concentrated in vacuo. Column chromatography over silica gel (ethyl acetate) gave 4 (1.57 g, 90%) as a liquid: 1H NMR (500 MHz, CDCl3, ppm) 6.94 (s, 1H), 6.91 (d, J ) 8.5 Hz, 1H), 6.72 (d, J ) 8 Hz, 1H), 5.06 (s, 2H), 5.02 (s, 2H), 3.86 (m, 4H), 3.32 (s, 3H), 3.31 (s, 3H), 2.95(s, 1H), 2.87 (s, 1H), 1.09 (t, J ) 7 Hz, 6H); 13C NMR (500 MHz, CDCl3, ppm) 146.9, 146.0, 125.5, 123.5, 118.1, 116.7, 95.2, 61.7, 55.8, 33.3, 32.2, 16.2, 16.1, 16.0, 15.9. HRMS m/z for (C15H25O7P + H): calcd., 349.1416; obsd., 349.1424. Synthesis of MOM-Protected Cat-v-P and Cat-v-Q [(MOM)2-Cat-v-P and (MOM)2-Cat-v-Q]. 4 (360 mg, 1 mmol) was dissolved in THF (2 mL). Potassium tert-butoxide (KOtBu, 1.2 mmol) was added to the above solution at -78 °C. After stirring at -78 °C for 30 min, a THF solution (5 mL) of 4-pyridinecarboxaldehyde (123 mg, 1 mmol) or 4-quinolinecarboxaldehyde (157 mg, 1 mmol) was slowly added into the solution. After stirring at -78 °C for 24 h, the reaction was quenched by adding saturated NH4Cl solution (2 mL). The reaction mixture was then diluted with CH2Cl2 (5 mL), washed with water, and subsequently with brine. The organic layer was dried and concentrated in vacuo. Column chromatography over silica gel (ethyl acetate:hexane ) 1:4) gave the products as a liquid: (MOM)2-Cat-v-P (yield ) 58%) and (MOM)2-Catv-Q (yield ) 35%). 1H NMR revealed that the products are (cis and trans) geometric isomers with cis:trans isomeric ratios of 0.29:1 [(MOM)2-Cat-v-P] and 0.8:1 [(MOM)2-Cat-v-Q], respectively. trans-(MOM)2-Cat-v-P: 1H NMR (300 MHz, CDCl3, ppm) δ 8.56 (d, J ) 5 Hz, 2H), 7.38 (s, 1H), 7.34 (d, J ) 5 Hz, 2H), 7.23 (d, J ) 17 Hz, 1H), 7.17 (br s, 2H), 6.89 (d, J ) 17 Hz, 1H), 5.30 (s, 2H), 5.27 (s, 2H), 3.56 (s, 3H), 3.53 (s, 3H). cis-(MOM)2-Cat-v-P: 1H NMR (300 MHz, CDCl3, ppm) δ 8.48 (d, J ) 5 Hz, 2H), 7.19 (d, J ) 5 Hz, 2H), 7.04 (d, J ) 8 Hz, 1H), 7.01 (s, 1H), 6.85 (d, J ) 8 Hz, 1H), 6.69 (d, J ) 12 Hz, 1H), 6.43 (d, J ) 12 Hz, 1H), 5.22 (s, 2H), 5.05 (s, 2H), 3.51 (s, 3H), 3.40 (s, 3H). trans-(MOM)2-Catv-Q: 1H NMR (300 MHz, CDCl3, ppm) δ 8.58 (d, J ) 5 Hz, 1H), 8.21 (d, J ) 8 Hz, 1H), 8.13 (d, J ) 8 Hz, 1H), 7.437.75 (m, 5H), 7.20-7.30 (m, 2H), 6.86 (s, 1H), 5.32 (s, 2H), 5.29 (s, 2H), 3.58 (s, 3H), 3.55 (s, 3H). cis-(MOM)2-Cat-v-

22516 J. Phys. Chem. B, Vol. 109, No. 47, 2005 Q: 1H NMR (300 MHz, CDCl3, ppm) δ 8.80 (d, J ) 5 Hz, 1H), 8.03 (d, J ) 8 Hz, 1H), 7.43-7.75 (m, 4H), 7.20-7.30 (m, 2H), 6.95 (d, J ) 8 Hz, 1H), 6.81 (d, J ) 2 Hz, 1H), 6.72 (dd, J ) 2, 8 Hz, 1H), 5.14 (s, 2H), 4.80 (s, 2H), 3.45 (s, 3H), 3.20 (s, 3H). Preparation of trans-Cat-v-P and trans-Cat-v-Q. The cis and trans isomeric mixture of (MOM)2-Cat-v-P was dissolved in THF (5 mL). HCl (1 N, 2 mL) was added into the above solution, and the mixture was refluxed for 3 h at 70 °C. The reaction mixture was diluted with saturated solution of NaHCO3 (5 mL), and the product (mixture of cis- and trans-Cat-v-P) was extracted with CH2Cl2. The organic layer was dried over MgSO4 and concentrated in vacuo. The cis:trans ratio of the obtained Cat-v-P mixture was 0.25:1. Repeated recrystallization in hexane-CH2Cl2-MeOH gave pure trans isomer (trans-Catv-P, yield ) 16.2%, for simplicity, Cat-v-P will represent the trans isomer hereafter). Interestingly, in the case of Cat-v-Q, the cis isomer disappeared during the deprotection process and only trans isomer remained (yield ) 22.9%, for simplicity, Catv-Q will represent the trans isomer hereafter). Cat-v-P: 1H NMR (300 MHz, CD3OD, ppm) δ 8.43 (d, J ) 6 Hz, 2H), 7.52 (d, J ) 6 Hz, 2H), 7.36 (d, J ) 16 Hz, 1H), 7.10 (d, J ) 2 Hz, 1H), 6.97 (dd, J ) 2, 8 Hz, 1H), 6.92 (d, J ) 16 Hz, 1H), 6.79 (d, J ) 8 Hz, 1H); 13C NMR (500 MHz, CD3OD, ppm) 150.0, 147.0, 146.8, 145.6, 134.6, 128.6, 122.1, 121.0, 120.2, 115.3, 113.3. HRMS m/z for (C13H11O2N + H); calcd., 214.0868; obsd., 214.0868. cis-Cat-v-P: 1H NMR (300 MHz, CD3OD, ppm) δ 8.37 (d, J ) 6 Hz, 2H), 7.29 (d, J ) 6 Hz, 2H), 6.74 (d, J ) 12 Hz, 1H), 6.68 (d, J ) 8 Hz, 1H), 6.68 (d, J ) 2 Hz, 1H), 6.59 (dd, J ) 2, 8 Hz, 1H), 6.41 (d, J ) 12 Hz, 1H). Cat-v-Q: 1H NMR (300 MHz, CD3OD, ppm) 8.77 (d, J ) 5 Hz, 1H), 8.38 (d, J ) 8 Hz, 1H), 8.03 (d, J ) 8 Hz, 1H), 7.64-7.81 (m, 4H), 7.41 (d, J ) 16 Hz, 1H), 7.21 (d, J ) 2 Hz, 1H), 7.08 (dd, J ) 2, 8 Hz, 1H), 6.84 (d, J ) 8 Hz, 1H); 13C NMR (500 MHz, CD OD, ppm) 149.6, 148.0, 147.0, 145.6, 3 144.9, 136.4, 129.7, 129.0, 126.7, 126.6, 123.8, 120.4, 118.5, 116.3, 115.4, 113.5. HRMS m/z for (C17H14O2N + H); calcd., 264.1025; obsd., 264.1031. Preparation of TiO2 Gel and 1:1 TiO2-HPC Paste. TiO2 nanoparticles were prepared according to the following procedure. Distilled deionized water (160 mL) was introduced into a 500-mL round-bottom flask, and then glacial acetic acid (51 mL) was added to the water. The flask was placed in an ice bath and stirred for a few minutes to allow the solution to cool. 2-Propanol (6 mL) was added to a dropping funnel followed by Ti(OiPr)4 (24 mL). The Ti(OiPr)4 solution was slowly dripped into the cooled acetic acid solution at a rate of approximately one or two drops per second over the course of 30 min, while the solution was vigorously stirred. After the dripping was over, white precipitate was formed within the round-bottomed flask. The heterogeneous solution was heated to 80 °C using a water bath with vigorous stirring. The white precipitate disappeared upon heating the solution. The bluish, gel-like solution was further heated at the temperature for 3-4 h. The slightly milky and bluish viscose colloidal solution was transferred into a cylindrical Teflon container, which tightly fits within an autoclave. The autoclave was placed in an oven, and the temperature of the oven was increased to 230 °C. After 13 h, the temperature of the oven was allowed to cool to room temperature during the course of 24 h. The solution was sonicated for 30 min and then concentrated by evaporating water using a rotary evaporator until the TiO2 concentration became ∼12 wt %. The average particle size of the nanocrystalline TiO2, which was determined using the Scherrer equation, was 19.6 nm. To make TiO2-HPC paste, 10 g of 12 wt % aqueous TiO2

Tae et al. solution and 0.58 g of HPC were introduced into a small vial and the mixture was magnetically stirred for 2 days prior to use. The weight ratio of TiO2 and HPC in the above paste was 1:1. Preparation of Dye-Coated TiO2 Film. A large FTO glass plate was cut into smaller pieces with the size of 30 × 60 mm2. One horizontal and three evenly spaced vertical line scratches were made on the glass side of each small FTO glass plate using a glass knife to be able to divide the FTO glass plate into eight smaller pieces with the dimension of 15 × 15 mm2 after fixation of TiO2 film on the FTO side. The line-scratched FTO glass plates were washed by sonication in 3% mucasol solution, and rinsed sequentially with distilled deionized water and ethanol. In a plastic container, 2 M TiCl4 (2 mL) and distilled deionized water (78 mL) were mixed. The washed line-scratched FTO glass plates were placed within the TiCl4 solution, and the container was placed in an oven whose temperature was set at 60 °C for 30 min. The line-scratched FTO glass plates coated with TiO2 buffer layer [(TiO2)b/FTO/G-8] were washed with distilled deionized water and ethanol. The washed (TiO2)b/FTO/ G-8 plates were dried under a stream of N2. TiO2 films were deposited on (TiO2)b/FTO/G-8 plates by the doctor-blade method using Scotch tape as the spacer. The thickness of the TiO2 film was controlled by the number of Scotch tape layers. The thickness of the TiO2 layer used in this report was ∼13 µm. The air-dried TiO2 film-coated (TiO2)b/FTO/G-8 [TiO2/(TiO2)b/ FTO/G-8] plates were sintered at 450 °C for 1 h. The sintered TiO2/(TiO2)b/FTO/G-8 plate was carefully broken into 8 smaller pieces with the dimensions of 15 × 15 mm2. The areas of TiO2 layers on the sintered, small TiO2/ (TiO2)b/FTO/G [TiO2/(TiO2)b/FTO/G-1] plates were reduced to ∼8.0 × 3.0 mm2 by removing the extra TiO2 layers using a sharp-edged knife. The position of the remaining area of TiO2 on the plate was 3.0 mm away from the top edge, 9.0 mm away from the bottom edge, and 3.5 mm away from the left and right edges, respectively. Independently, each dye was dissolved in ethanol. The concentrations of dyes ranged from 0.25 to 1 mM. The TiO2/(TiO2)b/FTO/G-1 with small TiO2 area [(TiO2)sa/ (TiO2)b/FTO/G-1] plates were immersed into each dye solution for varying periods of time (15 min to 15 h) at room temperature. The dye-coated (TiO2)sa/(TiO2)b/FTO/G-1 [Dye-(TiO2)sa/(TiO2)b/ FTO/G-1] plates were washed with copious amounts of ethanol. Measurements of DTCT Bands. For the measurements of the DTCT bands, each Dye-(TiO2)sa/(TiO2)b/FTO/G-1 was cut into 9 × 15 mm2, while retaining the dye-coated small-area TiO2 layer within the center of the above plate. The smaller Dye-(TiO2)sa/(TiO2)b/FTO/G-1 plate was placed vertically on top of the bottom of a 10 × 10 × 40 mm3 rectangular cuvette. After filling the cuvette with CH3CN, the spectrum of the smaller Dye-(TiO2)sa/(TiO2)b/FTO/G-1 plate was measured with respect to CH3CN contained within the reference cuvette. The genuine DTCT band was obtained by subtracting the above spectrum with the one obtained from the colorless (TiO2)sa/ (TiO2)b/FTO/G-1 plate which was obtained by removing the dye off the TiO2 by burning the Dye-(TiO2)sa/(TiO2)b/FTO/ G-1 on a stream of hot wind (500 °C) for 15 min. Since FTO has strong absorption at λ < 380 nm, the spectrum of DyeTiO2/FTO/G was shown at the spectral region of 380 < λ e 800 nm. Measurements of Adsorbed Amounts of Dyes on TiO2. A Dye-(TiO2)sa/(TiO2)b/FTO/G-1 was immersed in 3 mL of NaOH solution (2 M) for 15 h. This caused desorption of the dye from TiO2 into the solution while keeping the TiO2 layer intact. The supernatant solution was introduced into a cuvette,

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and the spectrum of each dye in the basic solution was measured. The quantification of each dye was made based on the λmax and the molar extinction coefficient of each dye in the basic solution as follows: Cat (λmax ) 315 nm and  ) 10 000 M-1 cm-1), Dop (λmax ) 280 nm and  ) 9 900 M-1 cm-1), Bpg (λmax ) 265 nm and  ) 14 900 M-1 cm-1), Cat-v-P (λmax ) 290 nm and  ) 20 300 M-1 cm-1), and Cat-v-Q (λmax ) 304 nm and  ) 12 400 M-1 cm-1). Fabrication of Sandwich-Type Cells. Small FTO glass plates with the dimension of 15 × 15 mm2 [FTO/G-1] were prepared. Onto each small FTO glass plate two holes with the diameter of 0.75 mm were made from the FTO side to the glass side using a small diamond-coated drill tip with the diameter of 0.75 mm at the positions of (-4.5, 0 mm) and (4.5, 4.5 mm) from the center of the plate, where the horizontal and vertical edges are defined as the x and y axes, respectively. Independently, a 2-propanol solution of H2PtCl6 was prepared by dissolving H2PtCl6 (20.5 mg, 0.05 mmol) into 2-propanol (10 mL). Two drops of the 5 mM H2PtCl6 solution was dropped onto each FTO/G-1 plate having two holes [FTO/G-1H2]. After evenly spreading the H2PtCl6 solution, 2-propanol was allowed to evaporate in the atmosphere. The H2PtCl6-coated FTO/G1H2 plates were heated under a hot stream of air (380 °C) for 20 min. The Pt-coated FTO/G-1H2 (Pt/FTO/G-1H2) plates were used as counter electrodes. Dye-(TiO2)sa/(TiO2)b/FTO/G-1 plates were placed with TiO2 side facing up. Onto each Dye(TiO2)sa/(TiO2)b/FTO/G-1 plate, four narrow Surlyn tapes (width ) 1.0 mm) were placed along the top, left, and right edges of each plate and 5 mm away from the bottom edge of each plate so that the area surrounded by Surlyn tapes can also enclose the two holes of a Pt/FTO/G-1H2 plate when they are attached together. Subsequently, a Pt/FTO/G-1H2 plate was gently placed with the FTO side face down onto the Surlyn-placed Dye(TiO2)sa/(TiO2)b/FTO/G-1 plate in such a way that the two holes of the Pt/FTO/G-1H2 plate fit within the area surrounded by Surlyn tapes on the bottom plate. The sandwiched assembly was placed on a hot plate whose temperature was maintained at 130 °C and pressed for 7 min under the pressure of 1.2 N m. After being cooled to room temperature, an electrolyte solution was introduced into the void space created between the two electrodes through one of the holes. The composition and concentration of electrolyte solution were 0.6 M HMI+I-, 0.05 M I2, 0.1 M LiI, and 0.5 M TBP in the 1:1 mixed solution of acetonitrile and valeronitrile. After filling up the void space with the electrolyte solution, the holes were blocked with small glass plates using Surlyn as the adhesive. The nonoverlapped areas of Dye-sa-TiO2/Tb/FTO/G-1 and Pt/FTO/G-1H2 plates were coated with indium metal. Coadsorption of DCA onto Dye-Coated TiO2 Film. One or two Dye-(TiO2)sa/(TiO2)b/FTO/G-1 plates were dipped into an ethanol solution of DCA (50 mM, 3 mL) for 15 h. The plates were then washed with ethanol. Photovoltaic Measurements of DSSCs. The η values for the DSSCs were measured under a standard condition of one sun (100 mW cm-2, AM 1.5 filter). The IPCE spectrum or the EQE at an excitation wavelength was determined from eq 1

IPCE (%) ) (1240/λex) × (Jsc/Iinc) × 100

(1)

where Jsc is the short-circuit photocurrent (A cm-2), Iinc is the incident light intensity (W cm-2), and λex is the excitation wavelength. Instruments. 1H and 13C NMR spectra were recorded on a Varian Inova 300- or 500-MHz NMR spectrometer. The chemical shifts are reported with respect to that of tetrameth-

ylsilane in CDCl3 and CD3OD. High-resolution mass spectra were obtained from a JEOL JMS 700 installed in the Korea Institute of Basic Science, Kyungpook National University. SEM images of TiO2 films were obtained from a field emission scanning electron microscope (Hitachi S-4300) at an acceleration voltage of 10-20 kV. A platinum/palladium alloy (in the ratio of 8 to 2) was deposited with a thickness of about 15 nm on top of the samples. X-ray diffraction patterns for the characterization of TiO2 nanocrystals were obtained from a Rigaku D/MAX-1C with the monochromatic beam of Cu KR. The UVvis spectra of the samples were recorded on a Shimadzu UV3101PC. The diffuse reflectance UV-vis spectra of the solid samples were obtained using an integrating sphere. Currentvoltage (J-V) curves were obtained from a Keithley 2400 source meter under the light produced from an Oriel 191 solar simulator equipped with an AM 1.5 filter. The light source was an Oriel 1000-W Xe lamp. The applied light intensity was 100 mW cm-2. The intensity of the incident light was calibrated using a KG5-filtered, monocrystalline Si reference solar cell, PVM 37 (ISO tracking number 1006), which was calibrated at the National Renewable Energy Laboratory in Golden, Colorado, USA. Each data in the Tables represents the average value obtained from three samples. The IPCE spectra for the cells were measured on an IPCE measuring system (PV Measurements), in which monochromatic light is generated using a tungsten light source filtered by a grating monochromator and order-sorting filters. The light was modulated with a mechanical chopper (Digirad C-980). A broadband light is simultaneously applied to the sample cell to bring the cell into the end-use conditions. The current generated by the sample cell at a specific voltage (generally V ) 0) is converted to a voltage by a transimpedance amplifier and measured with a lock-in amplifier (Stanford Research Systems SR810) synchronized to the mechanical chopper. The photon flux of the monochromatic light was determined by measuring the signal produced by a calibrated photodiode and comparing the signal to the photodiode’s known spectral response information. Cyclic voltammetry measurements were performed on a Bioanalytical Systems, Inc. CV-50W voltammetric analyzer using a standard three-electrode cell with a platinium wire working electrode (electrode area, 5.0 cm2), a platinum gauze counter electrode, and an Ag/AgCl (sat. KCl) reference electrode in DMF containing a 0.1 M (Bu)4NClO4 electrolyte. The sweep rate was 0.1 V s-1. Quantum mechanical calculations of the frontier orbitals of (H2O)2(OH)2Ti-Cat, (H2O)2(OH)2Ti-Cat-v-P, and (H2O)2(OH)2Ti-Cat-v-Q were done with jaguar v 5.5 suite (Schro¨dinger, Portland, US).37 B3LYP38 flavor of density functional theory (DFT) calculations were adopted with LACVP** basis set, which uses Hay-Watt effective core potential and basis set for Ti39 and 6-31G** basis sets for other atoms (H, C, O, and N). Results and Discussion Photovoltaic Parameters of Type-II DSSCs Sensitized by Cat, Dop, and Bpg. To compare the photovoltaic parameters of Cat-v-P and Cat-v-Q with those of the typical dyes that have been employed in Type-II DSSCs under the same experimental condition, we prepared a series of Type-II DSSCs sensitized by Cat, Dop, and Bpg, respectively. The UV-vis spectrum of each dye in solution (the local band of each dye) and the diffuse-reflectance UV-vis spectrum of bare TiO2 (the local band of TiO2) are shown in each panel of Figure 1, together with the UV-vis spectrum of the corresponding Dye-TiO2/ FTO/G (the composite spectrum of the local bands of each dye

22518 J. Phys. Chem. B, Vol. 109, No. 47, 2005

Tae et al. TABLE 1: Photovoltaic Parameters of the Type-II DSSCs Sensitized by the Dyes Shown in Chart 1 dye

Voc (mV)

Jsc (mA cm-2)

FF

η (%)

Aaa

Cat Dop Bpg Cat-v-P Cat-v-Q N719

557 452 445 561 562 750

1.51 0.40 0.79 2.74 3.53 12.4

66.5 67.8 63.1 68.1 66.5 66.6

0.6 0.1 0.2 1.1 1.3 6.2

0.17 0.01 0.10 0.07 0.08 0.09

a

Figure 1. The UV-vis spectrum of a Type-II dye in acetonitrile (dotdashed line), the diffuse-reflectance UV-vis spectrum of bare TiO2 (dashed line), the UV-vis spectrum of the corresponding Dye-(TiO2)sa/ (TiO2)b/FTO/G-1 plate (solid line), and the corresponding IPCE curve of the cell (square-dashed line) with Cat (A), Dop (B), and Bpg (C), as the dye.

and TiO2 and the corresponding DTCT band). The corresponding IPCE curve was overlaid on the UV-vis spectrum of each Dye-TiO2/FTO/G. In the cases of Cat and Dop, the onsets of the local bands were 300 and 332 nm, respectively, while the onset of TiO2 appeared at ∼390 nm. Therefore, the new absorption bands that appear at 400 e λ < 700 nm represent only the DTCT bands. This means that, in the above two cases, the corresponding local bands do not overlap with the DTCT bands in the visible region. Yet, the IPCE curves very closely follow the corresponding traces of DTCT bands. This unambiguously demonstrates that, in the visible region (λ g 400 nm), the photocurrents are generated only by pathway B. Furthermore, the close matching between the IPCE curve and DTCT band indicates that the electron injection efficiency is the function of the absorption probability. The EQEs at 400 nm were 8.5 and 4.2%, respectively, which are about maximum EQEs obtainable by only pathway B in the cases of Cat- and Dop-sensitized DSSCs, respectively. In both cases, the EQE rapidly increased at ∼390 nm at which the onset of the local band of TiO2 begins, due to additional photocurrent generation by TiO2. The photovoltaic parameters of the Cat- and Dop-sensitized DSSCs are listed in Table 1. The open-circuit voltage (Voc) and the Jsc of Cat-sensitized DSSC were significantly higher than those of Dop-sensitized DSSC. The corresponding η values were 0.6 and 0.1%, respectively. Although both values are still relatively very small compared to that of N719-sensitized Type-I

Adsorbed amount in µmol cm-2.

DSSC compared in the last entry of Table 1 (6.2%), the η value of Cat-sensitized DSSC was significantly higher than that of Dop-sensitized DSSC, consistent with the fact that the Voc and Jsc of Cat-sensitized DSSC were significantly higher than those of Dop-sensitized DSSC. The adsorbed amounts of Cat and Dop onto the TiO2 films under our experimental conditions were 0.17 and 0.01 µmol cm-2, respectively. Thus, the adsorbed amount of Dop was much less than that of Cat. This fact reveals that the adsorbed amounts of catechol derivatives onto TiO2 vary depending on their nature, and this is likely to be one of the reasons for the η of Dop-sensitized DSSC being much poorer than that of Cat-sensitized DSSC. Interestingly, however, their fill factors (FFs) were similar (∼67) in both cases. In the case Bpg, the absorption tail of the local band extended up to 700 nm, while the absorption maximum (λmax) in the visible region was 485 nm (Figure 1). The Bpg-adsorbed TiO2 films showed an additional band with λmax at ∼560 nm consistent with the report of Ghosh and the co-workers.21b This new absorption band was assigned as the DTCT band by Ghosh and the co-workers.21b Thus, in the case of Bpg, there is no spectral region in which only DTCT band absorbs since both local and DTCT bands strongly overlap in the visible region. In any case, the IPCE curve closely followed the trace of the spectrum of Bpg-adsorbed TiO2 film, indicating that even in this case the photocurrent is generated not only by pathway A but also by pathway B in the visible region. Therefore, the Bpgsensitized DSSC is a typical example in which both pathways operate in the whole spectral region. In this respect, Bpgsensitized DSSC is distinguished from Cat- and Dop-sensitized DSSCs. The photovoltaic parameters of the Bpg-sensitized DSSC are listed in Table 1 (entry 3). Its Voc was similar to that of Dopsensitized DSSC. However, its Jsc value was placed between those of Dop- and Cat-sensitized DSSCs. The FF was slightly smaller than those of Cat- and Dop-sensitized DSSCs. The overall η lied between those of Cat- and Dop-sensitized DSSCs. Interestingly, in the above three DSSCs, a correlation exists between the adsorbed amount and η, with both increasing in the order: Cat > Bpg > Dop. Photovoltaic Parameters of Cat-v-P-Sensitized DSSC. In the case of Cat-v-P, the λmax of the local band appeared at 343 nm with the molar extinction coefficient of 24 500 M-1 cm-1 in CH3CN as shown in Figure 2A. The onset of the spectrum was 400 nm. The Cat-v-P-adsorbed TiO2 gave an orangecolored CT band with the λmax of ∼500 nm which is ∼50-60 nm red-shifted with respect to that of Cat, indicating that, as expected, Cat-v-P is a better donor than Cat. In support of this, the oxidation potential of Cat-v-P (0.91 V vs Ag/AgCl in DMF, scan rate ) 0.1 V s-1) shifted by 150 mV to the negative with respect to that of Cat (1.06 V vs Ag/AgCl in DMF). The onset of the Cat-v-P-to-TiO2 DTCT band was 700 nm. The IPCE curve of Cat-v-P-sensitized DSSC nearly superimposed with the DTCT band in the visible region as shown in Figure 2A, indicating that the photocurrent generated in the

Efficiency of the Dye-Sensitized TiO2 Solar Cells

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22519 SCHEME 3: Proposed Mechanism Partially Responsible for the Increase in Efficiency and EQE in DSSCs Sensitized by Cat-v-P and Cat-v-Q

Figure 2. The UV-vis spectrum of a Type-II dye in (dot-dashed line), the diffuse-reflectance UV-vis spectrum of bare TiO2 (dashed line), the UV-vis spectrum of the corresponding Dye-(TiO2)sa/(TiO2)b/FTO/ G-1 plate (solid line), and the corresponding IPCE curve of the cell (square-dashed line) with Cat-v-P (A) and Cat-v-Q (B) as the dye.

Figure 3. The calculated electron densities of the HOMO and LUMO of (H2O)2(OH)2Ti-Cat (A), (H2O)2(OH)2Ti-Cat-v-P (B), and (H2O)2(OH)2Ti-Cat-v-Q (C).

visible region arises purely by pathway B and that the efficiency of electron injection by DTCT is linearly proportional to the extinction coefficient of the DTCT band. The IPCE value at 400 nm was 30%, which corresponds to a more than 3-fold increase with respect to that of Cat-sensitized DSSC. It is also important to note that this IPCE value is the highest ever observed in the Type-II DSSCs. This also demonstrates the potential of Type-II DSSCs to be developed into commercially viable DSSCs.

Although the Voc (561 mV) value was only slightly higher than that of Cat-sensitized DSSC (557 mV), the obtained Jsc (2.7 mA cm-2) was 1.8 times higher than that of Cat-sensitized DSSC (1.5 mA cm-2) (Table 1). However, the FF was nearly the same with that of Cat-sensitized DSSC. The overall η was 1.1, which was again 1.8-times higher than that of Cat-sensitized DSSC. Since Voc and FF are nearly the same, it is concluded that the increase in η is caused by the increase in Jsc. Most interestingly, the adsorbed amount of Cat-v-P in the cell was only 0.07 µmol cm-2, which is only ∼40% of that of Cat in the Cat-sensitized DSSC. On the basis of the fact that η increased with increasing the adsorbed amount of dye in the cases of Cat, Dop, and Bpg, the above result indicates that the electron injection efficiency from Cat-v-P to TiO2 is about 4.5 times higher than that of Cat to TiO2 per molecule. This means that the attachment of v-P to Cat leads to a large (4.5-fold) increase in electron injection efficiency. As a possible means to account for the above phenomenon, the highest-occupied molecular orbitals (HOMOs) and lowestunoccupied molecular orbitals (LUMOs) of Ti(IV)-coordinated Cat, Cat-v-P, and Cat-v-Q were calculated based on density functional theory, and the results are compared in Figure 3. For this calculation, the Ti(IV) ion was chelated to a catechol or a catechol moiety and coordinated to two H2O molecules and two OH- groups as in the report of Prezhdo and co-workers.40 In the case of Ti-Cat, while the electron density was delocalized almost entirely over the Cat in the HOMO, the electron density completely shifted to Ti(IV) and the surrounding H2O and OH ligands consistent with the DTCT nature of the HOMO-LUMO transition as shown in Figure 3A. This phenomenon is well explained in the report of Prezhdo and co-workers.40 In the case of Ti-Cat-v-P, however, while the HOMO is delocalized over the entire Cat-v-P, the LUMO is localized largely on Ti and to a certain extent on Cat-v moiety as well, showing that a charge shift occurs to a certain degree from the P moiety to Ti-Cat-v moiety, as shown in Figure 3B. On this basis, we propose that intramolecular consecutive charge shift from P to Ti-Cat-v moiety plays an important role in contributing to such a dramatic increase in the electron injection efficiency from Cat-v-P to TiO2, as illustrated in Scheme 3, where D1 and D2 stand for Cat-v and P, respectively. This means that eventual photoinduced electron transfer occurs from P to TiO2. The above situation can better be described as the typical acceptor (A)-primary donor (D1)-secondary donor (D2) triad system where A ) TiO2, D1 ) Cat-v moiety, and D2 ) P, in which eventual charge separation occurs between A and D2 via charge shift between D1 and D2. Efficient charge separation has also been observed between TiO2 and D2 upon DTCT excitation of TiO2-D1-D2 triad systems where D1 ) Dop and D2 ) biological systems such as a DNA double helix, biotin, or

22520 J. Phys. Chem. B, Vol. 109, No. 47, 2005

Figure 4. Linear relationship between the oxidation potentials of Cat, Cat-v-P, and Cat-v-Q and the absorption maximums of the corresponding DTCT bands.

avidin.41 In close relation to the above, the CT excitation of the quantum efficiency of photoinduced electron transfer from D1 and A placed within zeolites or across zeolite-solution interfaces significantly increases upon attaching D2 next to D1 due to consecutive intramolecular charge shift from D1 to A followed by electron transfer from D2 to D1+.42 In any case, the calculated HOMO and LUMO of Ti-Cat-v-P confirm the DTCT nature of the ∼500-nm band. In addition to the above explanation, we also propose that the red-shift of the DTCT band caused by the increase in the donor strength of the dye by attaching a more electron rich substituent (v-P) to Cat leading to a substantial increase in the absorption of visible light contributes to the increase in the electron injection efficiency. Thus, we conclude that both the consecutive charge shift leading to the retardation of back electron-transfer rate discussed above and the red-shift of the DTCT band as a result of increasing the donor strength of the dye leading to the increase in the absorption of visible light are responsible for the increase in η. Photovoltaic Parameters of Cat-v-Q-Sensitized DSSC. In the case of Cat-v-Q, the λmax of the local band appeared at 383 nm with the molar extinction coefficient of 21 600 M-1 cm-1 in CH3CN as shown in Figure 2B. The onset of the spectrum was 435 nm. The Cat-v-Q-adsorbed TiO2 was purple, and the λmax of the DTCT band was ∼580 nm, which is about ∼80 nm red-shifted with respect to that of Cat-v-P. The onset of the Cat-v-Q-to-TiO2 DTCT band was 774 nm, which is also red-shifted with respect to that of Cat-v-P by 74 nm. The above red shifts indicate that, as expected, Cat-v-Q is a better donor than Cat-v-P. In support of this, the oxidation potential of Catv-Q (0.83 V vs Ag/AgCl in DMF, scan rate ) 0.1 V s-1) shifted by 80 mV to the negative with respect to that of Cat-v-P (0.91 V vs Ag/AgCl in DMF). Interestingly, the plot of the λmax of the DTCT band with respect to the oxidation potential of the dye for Cat, Cat-v-P, and Cat-v-Q gave a linear relationship as shown in Figure 4. This result is rather unexpected considering the fact that the Mulliken’s CT theory holds for a weak donor-acceptor interaction between a donor and an acceptor33 but not for the case of Cat-binding TiO2 since there exists a strong interaction (binding) between the dye and TiO2. Indeed, when we plotted the λmax of the DTCT band with respect to the oxidation potential of the dye for Cat, 3-fluorocatechol, 4-chlorocatechol, 4-nitrocatechol, 4-methylcatechol, 3-methoxycatechol, and 3,5-di(tertbutyl)catechol, such a linear relationship was not observed. Although a systematic study is needed to elucidate the intriguing phenomenon of the linear relationship, it indicates that Cat, Catv-P, and Cat-v-Q belong to a homologous series of dyes that bind to TiO2 in a same manner. The IPCE curve of Cat-v-Q-sensitized DSSC also nearly superimposed with the corresponding trace of DTCT band in

Tae et al. the region (435 e λ < 800 nm) in which only the DTCT band absorbs, indicating that the photocurrent generated in the visible region arises by pathway B and that the efficiency of electron injection by DTCT simply follows the extinction coefficient of DTCT band. The IPCE value at 440 nm was 25%, which is ∼3 times higher than that of Cat-sensitized DSSC. The measured Voc (562 mV) of Cat-v-Q-sensitized DSSC was again slightly higher than that of Cat-sensitized DSSC (557 mV) but was essentially the same with that of Cat-v-Psensitized DSSC (561 mV) (Table 1). This indicates that the attachment of (pyridin -4-yl)vinyl or (quinolin-4-yl)vinyl does not affect Voc. However, the obtained Jsc (3.53 mA cm-2) was more than 2.3 times higher than that of Cat-sensitized DSSC (1.51 mA cm-2). The FF was still the same with that of Catsensitized DSSC. The overall η was 1.3, which was ∼2.2 times higher than that of Cat-sensitized DSSC. The fact that Voc and FF remain nearly the same also suggests that the increase in η is driven by the increase in Jsc. In the case of Cat-v-Q-sensitized DSSC, the adsorbed amount of Cat-v-Q in the cell was 0.08 µmol cm-2, which is only 47% of that of Cat in the Catsensitized DSSC. The above result therefore indicates that the electron injection efficiency from Cat-v-Q to TiO2 is about 4.7 times higher than that of Cat to TiO2 per molecule. The calculated HOMO and LUMO of Ti(IV)-coordinated Cat-v-Q also showed that a charge shift occurs from the Q moiety to Ti-Cat-v moiety, as shown in Figure 3C. Therefore, as in the case of Cat-v-P-sensitized DSSC, we propose that the intramolecular consecutive charge shift illustrated in Scheme 3 giving rise to eventual charge separation between TiO2- and Q+ arising from A-D1-D2 triad system, partially contributes to the dramatic increase in the electron injection efficiency from Cat-v-Q to TiO2, since by this way the lifetime of chargeseparated state is elongated, giving rise to a higher chance to the electrolyte (I-) to reduce the oxidized dye. In addition, we also propose that the large red-shift of the DTCT band caused by the increase in the donor strength of the dye contributes to the increase in the electron injection efficiency. The above two sets of results (Cat-v-P and Cat-v-Q) clearly demonstrate that the attachment of a second donor group (P or Q) to Cat leads to a dramatic increase in electron injection efficiency. Interestingly, the conjugating bridge, the vinyl group, also behaves similarly as Cat. Effect of Adsorbed Amounts of Cat, Cat-v-P, and Catv-Q on η. Interestingly, the effect of adsorbed amount of dye on the photovoltaic parameters was dramatically different depending on the type of dye. Thus, while the overall η increased with the adsorbed amount and saturated at a certain amount in the case of Cat-sensitized DSSC, the reverse behavior was observed in the cases of DSSCs sensitized by Cat-v-P and Cat-v-Q, respectively, as shown in Table 2. Although the reason the above two sets of dyes show different behaviors is not clear at this stage we propose that Cat-v-P and Cat-v-Q have higher propensities to aggregate between the neighboring dyes upon increasing the concentration of the dyes on the surface and the aggregation is responsible for the decrease of the ηs. This report thus demonstrates for the first time that the adsorbed amount of Type-II dye sensitively affects the photovoltaic parameters of the DSSCs and the relationship between the adsorbed amount and the η is sensitively governed by the nature of the dye. Effect of Coadsorption of DCA on Photovoltaic Parameters of Type-II DSSCs. It has been well established that the coadsorption of DCA leads to an increase in η to a certain extent.6b,7,9 We also coadsorbed DCA onto Type-II-dye adsorbed

Efficiency of the Dye-Sensitized TiO2 Solar Cells

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22521

TABLE 2: Effect of Adsorbed Amount of Dye on the Photovoltaic Parameters of DSSCs Sensitized by Cat, Cat-v-P, and Cat-v-Q dye Cat

Cat-v-P Cat-v-Q

a

Aaa

Voc (mV)

Jsc (mA cm-2)

FF

η (%)

0.05 0.10 0.18 0.25 0.31 0.05 0.09 0.17 0.09 0.15 0.21

552 573 552 568 549 579 538 520 557 526 507

0.59 1.36 1.69 1.55 1.69 2.59 2.05 1.55 3.19 2.46 1.71

65.8 64.3 65.4 63.2 61.2 70.2 66.0 61.5 70.1 65.9 61.6

0.2 0.5 0.6 0.6 0.6 1.0 0.7 0.5 1.3 0.9 0.5

Adsorbed amount in µmol cm-2.

TABLE 3: Effect of Coadsorbed DCA on the Photovoltaic Parameters of DSSCs Sensitized by the Dyes Shown in Chart 1a Voc (mV) Jsc (mA‚cm-2)

dye Cat Dop Bpg Cat-v-P Cat-v-Q

554 (557) 470 (452) 473 (445) 568 (561) 579 (562)

1.28 (1.51) 0.31 (0.40) 1.10 (0.79) 3.08 (2.74) 4.29 (3.53)

FF

η (%)

Aab

69.7 (66.5) 68.2 (67.8) 68.5 (63.1) 69.5 (68.1) 62.6 (66.5)

0.5 (0.6) 0.1 (0.1) 0.4 (0.2) 1.2 (1.1) 1.6 (1.3)

0.11 (0.17) 0.01 (0.01) 0.04 (0.10) 0.05 (0.07) 0.07 (0.08)

a Numbers in the parentheses stand for the corresponding values obtained in the absence of DCA (from Table 2) for easier comparison. b Adsorbed amount in µmol‚cm-2.

TiO2 films. As summarized in Table 3, Voc and Jsc, and FF somewhat increased upon coadsorption of DCA except Cat- and Dop-sensitized DSSCs. As a result, the overall η increased by 100% (Bpg), 9% (Cat-v-P), and 23% (Cat-v-Q), respectively. Thus, in the case of Cat-v-Q-sensitized DSSC, the η increased to 1.6%, which is again the highest value ever observed from Type-II-dye-sensitized DSSCs. Considering that the η of N-719sensitized DSSC prepared under our experimental condition was only 6.2%, which is about a half of that achieved by Gra¨tzel’s group1 we believe that the η of Cat-v-Q-sensitized DSSC can be increased over 3% under an optimized condition. This result demonstrates the potential of Type-II DSSCs to be developed into commercially viable DSSCs. While the DCA-induced increases in Voc were not high, that is, only 6% (Bpg), 1% (Cat-v-P), and 3% (Cat-v-Q), respectively, the increases in Jsc were substantial, that is, 39% (Bpg), 12% (Cat-v-P), and 22% (Cat-v-Q), respectively. This shows that the DCA-induced increases in η are actually driven by the increase in Jsc, despite the fact that the adsorbed amounts of the dyes decreased by 60% (Bpg), 29% (Cat-v-P), and 13% (Cat-v-Q), respectively, upon adsorption of DCA. This suggests that the suppression of back electron transfer from the injected electrons residing on TiO2 to I3- plays an important role in giving rise to the above increase in Jsc. The adsorption of DCA did not affect much on the FF. Minimum η Arising Purely from Pathway B. To obtain the minimum η arising purely from pathway B, we placed a

400-nm (in the case of DSSC sensitized by Cat, Dop, and Catv-P) or 435-nm cutoff filter (in the case of Cat-Q) on top of each DSSC during the J-V curve measurements to excite only the corresponding DTCT bands (but not the local bands). Consistent with the decrease in the excitation wavelength range, the overall η decreased substantially, by up to 40%. The cell parameters under the condition of partial illumination are summarized in Table 4. Interestingly, while Voc values decreased significantly (by ∼7-18%) upon sensitization of only the DTCT bands in the cases of Cat- and Dop-sensitized DSSCs, only slight reductions in Voc (by ∼2%) were observed from the DSSCs sensitized by Cat-v-P and Cat-v-Q upon excitation of only the corresponding DTCT band. In other words, when Cat-v-P and Cat-v-Q were sensitizers, Voc remained nearly intact as opposed to the cases where Cat and Dop were sensitizers. In contrast to Voc, Jsc values decreased rather monotonically by large amounts (30-41%) regardless of the type of dye upon illuminating only the DTCT bands, consistent with the decrease in the number of electrons injected from the dyes to TiO2 caused by the reduction in the band of excitation wavelength. We propose that the above sensitizer-dependent different behavior of Voc and Jsc originates from the difference in the back electron-transfer rate. Thus, in the cases of Cat and Dop, most of the electrons injected from the dyes to TiO2 by pathway B are transferred back to the dyes due to the faster back electron transfer rates, and only small portions of electrons that were trapped within deep trap sites, which are lower in energy state, survive the back electron-transfer process, leading to the arrival to the FTO glass. In contrast, as a result of slower back electrontransfer rates, even the electrons injected to the sites of TiO2 with higher energy states can manage to arrive in FTO glass. Interestingly, while the FFs of Cat- and Dop-sensitized DSSCs decreased by 6 and 15%, respectively, those of DSSCs sensitized by Cat-v-P and Cat-v-Q increased by 3 and 15%, respectively, upon excitation of only the DTCT bands. This phenomenon was highly reproducible. In fact, FF is a function of Voc and is supposed to increase with increasing Voc.43 Therefore, the above phenomenon is quite unusual from the facts that the Voc values remained nearly intact or even slightly decreased, and that Jsc decreased. The reasons giving rise to the above unprecedented phenomenon are not clear at this stage and the elucidation of such reasons is the subject of future studies. Overall, this report demonstrates that the attachment of a second donor such as P or Q to Cat via a vinyl bridge leads to a dramatic increase in η (by 2- and 2.7-fold, respectively), revealing for the first time the potential of Type-II DSSCs to be developed into commercially viable DSSCs. The EQEs at the maximum λmax values of the DTCT bands were 30% (Catv-P) and 25% (Cat-v-Q), respectively, which correspond to 3.5- and 2.9-fold increases with respect to that of Cat-sensitized DSSC. The above increases in η are in fact driven by the increases in Jsc. We propose that both the consecutive charge shift from the secondary donor to a primary donor leading to the retardation of back electron-transfer rate and the red shift of the DTCT band caused by the increase in the donor strength

TABLE 4: Photovoltaic Parameters of the DSSCs Sensitized by Cat, Dop, Cat-v-P, and Cat-v-Q under the Condition of Only DTCT Excitationa dye

λexb

Voc (mV)

∆c

Jsc (mA‚cm-2)

∆c

FF

∆c

η (%)

Cat Dop Cat-v-P Cat-v-Q

g400 g400 g400 g435

517 (554) 385 (470) 555 (568) 568 (579)

-7 -18 -2 -2

0.80 (1.28) 0.21 (0.31) 2.17 (3.08) 2.52 (4.29)

-38 -32 -30 -41

65.2 (69.7) 57.5 (67.8) 71.3 (69.5) 72.3 (62.6)

-6 -15 3 15

0.3 (0.5) 0.1 (0.1) 0.9 (1.2) 1.0 (1.6)

a Numbers in the parentheses stand for the corresponding values obtained under the condition of full-range illumination (from Table 3) for comparison. b Excitation wavelength in nm.. c Difference in %.

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