Effect of Different Dye Baths and Dye-Structures on ... - ACS Publications

J. Phys. Chem. C 2008, 112, 11023–11033

11023

Effect of Different Dye Baths and Dye-Structures on the Performance of Dye-Sensitized Solar Cells Based on Triphenylamine Dyes Haining Tian,† Xichuan Yang,*,† Ruikui Chen,† Rong Zhang,† Anders Hagfeldt,*,‡ and Licheng Sun*,†,§ State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular DeVices, Dalian UniVersity of Technology (DUT), 158 Zhongshan Road, 116012 Dalian, China, and School of Chemical Science and Engineering, Center of Molecular DeVices, Physical Chemistry, Organic Chemistry Royal Institute of Technology (KTH), Teknikringen 30, 10044 Stockholm, Sweden ReceiVed: February 1, 2008; ReVised Manuscript ReceiVed: April 16, 2008

A series of triphenylamine dyes were designed and synthesized as photosensitizers for the application of organic dye-sensitized solar cells (DSSCs). Different substituted phenylene units, 2,2′;5′,2′′-terthiophene (TT) and dithieno[3,2-b;2′,3′-d]thiophene (DTT) serve as the π-spacers, and the electron acceptors employ the cyanoacrylic acid or rhodanine-3-acetic acid units. Detailed investigation on the relationship between the dye structure, and photophysical, photoelectrochemical properties and performance of DSSCs is described here. By substituting the phenylene group with electron-withdrawing units as π-spacers or replacing the cyanoacrylic acid with rhodanine-3-acetic acid units as electron acceptors, the bathochromic shift of absorption spectra are achieved. The significant differences in the redox potential of these dyes are also influenced by small structure changes. Furthermore, the different dye baths for semiconductor sensitization have a crucial effect on the performance of the DSSCs due to the different absorbed amount, absorption spectra and binding modes of anchored dyes on TiO2 surface in various solvents. On the basis of optimized dye bath and molecular structure, TPC1 shows a prominent solar-to-electricity conversion efficiency (η), 5.33% (JSC ) 9.7 mA · cm-2, VOC ) 760 mV, ff ) 0.72), under simulated AM 1.5G irradiation (100 mW · cm-2). Density functional theory has employed to study the electron distribution and the intramolecular charge transfer (HOMOfLUMO) of the dyes. From the calculation results of the selected dyes, we can also find the cyanoacrylic acid unit is better than the rhodanine-3-acetic acid unit as electron acceptor. Also, the electron-withdrawing groups on phenylene units as π-spacers show the negative effect on the performance of the organic DSSCs. 1. Introduction Dye-sensitized solar cells (DSSCs) as the most promising alternatives for the photovoltaic conversion of solar energy have attracted much interested attention since Gra¨tzel and co-workers reported that in 1991.1 The photosensitizers play a crucial role for the DSSCs getting higher solar-to-electricity conversion efficiency (η) and have been actively studied by researchers all over the world. The classic ruthenium complexes, such as cisdi(thiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II), coded as N3 or N719 depending on whether it contains four or two protons,2,3 have shown the higher short-circuit current due to broad UV-vis absorption spectra and the enough long excited-state lifetime, resulted in obtaining the respectable η value (more than 10%) and higher stability.4 In recent years, the interests in organic dyes as the substitutes of noble metal complexes are increasing due to many advantages, such as diversity of molecule structures, high molar extinction coefficient, simple synthesis as well as low cost and environmental issues. Many efficient metal-free dyes for DSSCs, such as coumarin-,5 merocyanine-,6 indoline-,7 polyene-,8 hemicyanine-,9 triphenylamine-,10 and fluorene-,11 based-organic dyes * To whom correspondence should be addressed. (X.Y.) Fax: +86-41183702185. Tel: +86-411-88993886. E-mail: [email protected]. (A.H.) Fax: +46 8 7908207. Tel: +46 8 7908177. E-mail: [email protected]. (L.S.) Fax: +46 8 791 2333. Tel.: +46 8 790 8127. E-mail: [email protected]. † Dalian University of Technology (DUT). ‡ Physical Chemistry, Royal Institute of Technology (KTH). § Organic Chemistry, Royal Institute of Technology (KTH).

have been developed and showed good performance of DSSCs. Phenothiazine12 and tetrahydroquinoline13 dyes as two kinds of efficient metal-free dyes for DSSCs were reported by our groups recently. The higher η value of organic dye-sensitized solar cell based on indoline dye, 9%, has been achieved in full sunlight by Ito et al.14 Furthermore, the solid-state DSSCs based on organic dyes show higher performances than Ruthenium complexes.15 This result suggested that the commercial application of the organic dyes in DSSCs is promising. To further design and develop the more efficient metal-free dyes for DSSCs, the predominant light-harvesting abilities in visible and near-IR light of dyes are important to get a larger photocurrent response. Also, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecualr orbital (LUMO) potentials must be match with the iodine/iodide redox potential and the conduction band edge level of the TiO2 semiconductor respectively.16,17 The donor-(π-spacer)-acceptor (D-π-A) system is the basic structure for designing the metalfree dyes due to the effective photoinduced intramolecular charge transfer properties. Herein, we will report a series of metal-free dyes with triphenylamine unit as electron donor. The dyes consist of the phenylene with different substitutes as π-spacers connected to electron donor by CdC bands, and cyanoacrylic acid or rhodanine-3-acetic acid as electron acceptor, codes as TPC-dyes or TPR-dyes, respectively. TTC-dyes are constructed by adopting the 2,2′;5′,2′′-terthiophene (TT) and dithieno[3,2-b;2′,3′-d]thiophene (DTT) units to replace the

10.1021/jp800953s CCC: $40.75  2008 American Chemical Society Published on Web 06/25/2008

11024 J. Phys. Chem. C, Vol. 112, No. 29, 2008

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Figure 1. Molecular structures of triphenylamine dyes.

π-spacers of TPC-dyes. The detailed structures of the dyes are shown in Figure 1. 2. Experimental Section 2.1. Analytical Measurements. 1H NMR spectra were measured with Varian INOVA 400 MHz (U.S.A.) with the chemical shifts against tetramethylsilane. Mass spectrometry (MS) data were obtained with GCT CA156 (U.K.), HP1100 LC/MSD (U.S.A.), and LC/Q-TOF MS (U.K.). Absorption and emission spectra solution were recorded in a quartz cell with 1 cm path length on HP8453 (U.S.A.) and PTI700 (U.S.A.), respectively. Electrochemical redox potentials were obtained by cyclic voltammetry using a three-electrode cell and an electrochemistry workstation (BAS100B, U.S.A.). The working electrode was a glass carbon electrode; the auxiliary electrode was a Pt wire, and Ag/Ag+ was used as reference electrode. Tetrabutylammonium hexaflourophosphate (TBAPF6) 0.1 M was used as the supporting electrolyte in CH2Cl2. Ferrocene was added to each sample solution at the end of the experiments, and the ferrocenium/ferrocene (Fc/Fc+) redox couple was used as an internal potential reference. The potentials versus normal hydrogen electrode (NHE) were calibrated by addition of 630 mV to the potentials versus Fc/Fc+.10c The Fourier transform IR (FTIR) spectra were measured using a NEXUS FTIR spectrometer. 2.2. Fabrication of the Nanocrystalline TiO2 Solar Cells. Two kinds of titania particles (P25, Degussa, Germany and 300 nm TiO2, commercial product, China) were dispersed in terpineol, respectively, following the literature procedure.18 A layer of P25 paste (∼2 µm) was coated on the F-doped tin oxide conducting glass (TEC15, 15Ω/square, Pilkington, U.S.A.) by

screen printing and then dried for 5 min at 125 °C. This procedure was repeated six times (12 µm) and coated by a layer of 300 nm titania paste (4 µm) as scatting layer. The doublelayer electrode (area: 6 × 6 mm) was sintered at 500 °C for 30 min in air. The sintered film was further treated with 40 mM TiCl4 aqueous solution at 70 °C for 30 min, washed with ethanol and water, then annealed at 500 °C for 30 min. After the film was cooled to 40 °C, it was immersed into a 2 × 10-4 M dye solution in CH2Cl2 and maintained under dark for 12 h. The electrode was then rinsed with CH2Cl2 and dried. The hermetically sealed cells were fabricated by assembling the dye-loaded film as the working electrode and Pt-coated conducting glass as the counter electrode separated with a hot-melt Surlyn1702 film (25 µm, Dupont). The electrolyte consisting of 0.6 M 1,2dimethyl-3-propylimidazolium iodide (DMPII), 0.025 M LiI, 0.04 M I2, 0.05 M guanidium thiocyanate (GuSCN), and 0.28 M 4-tertbutylpyridine (TBP) in dry acetonitrile was introduced into the cell via vacuum backfilling by the hole in the back of the counter electrode. Finally, the hole was also sealed using Surlyn1702 film. 2.3. Fabrication the Samples for Dye Adsorbed Amount, FTIR, and Absorption Spectra on TiO2 Measurement. The preparation method of TiO2 films was same as the procedure above, the 12 µm thickness (area: 6 × 6 mm) TiO2 films without the scatter layer were sensitized for 12 h in a dye bath and were employed for the measurement of dye adsorbed amount and FTIR on TiO2 surface. The 2 µm thickness TiO2 films were sensitized for 1 h in a dye bath and were adopted for absorption spectra measurement of the dyes on TiO2 surface. 2.4. Photocurrent-Voltage Measurements. The irradiation source for the photocurrent density-voltage (J-V) measurement

Solar Cells Based on Triphenylamine Dyes

J. Phys. Chem. C, Vol. 112, No. 29, 2008 11025

SCHEME 1: Synthesis Routes of TPC1∼4, TPR1∼4 and TTC1∼2a

a

(a) (i) DMF, K2CO3, 18-crown-6 ether, rt, 2 h; (ii) THF, I2, reflux, 6 h. (b) Cynaoacetic acid, piperidine, CH3CN, reflux, 2 h. (c) Rhodanine3-acetic acid, AcONH4, AcOH, reflux, 3 h.

SCHEME 2: Synthesis Route of [4-(N,N-Diphenylamino)benzyl] Triphenylphosphonium Bromide (TPA-PPh3.Br)a

a

(a) DMF, POCl3, 45∼50 °C, 2 h. (b) NaBH4, CH2Cl2, EtOH, rt, 2 h. (c) CHCl3, PPh3.HBr, reflux, 2 h.

SCHEME 3: Synthesis Routes of the Dialdehyde Intermediatesa

a (a) AcOH, Ac2O, CrO3, H2SO4, 5∼15 °C. (b) EtOH, H2O, H2SO4, reflux 3 h. (c) DME, H2O, NaOH, Pd(PPh3)4, 100 °C, 24 h. (d) (i) n-BuLi, THF, 0 °C to rt for 1 h; (ii) DMF, -78 °C, 1 h; (iii) -78 °C to rt for 2 h; and (iv) ice water.

is an AM 1.5G solar simulator (16S-002, SolarLight Co. Ltd., U.S.A.). The incident light intensity was 100 mW · cm-2 calibrated with a standard Si solar cell. The tested solar cells were masked to a working area of 0.159 cm2. The J-V curves were obtained by linear sweep voltammetry (LSV) method using an electrochemical workstation (LK9805, Lanlike Co. Ltd.,

China). The measurement of the incident photon-to-current conversion efficiency (IPCE) was performed by a Hypermonolight (SM-25, Jasco Co. Ltd., Japan). Every dye was fabricated DSSCs for three samples, and there is about 2% error in η value of every sample. The η value and IPCE spectrum of the dyebased DSSCs were obtained based on the best cell. 3. Results and Discussion 3.1. Synthesis. The structures and synthesis routes of triphenylamine derivatives dyes are shown in Figure 1 and Scheme 1, respectively. All of the intermediates and dyes have been prepared according to several classical reactions and detailed synthetic procedures are described in Supporting Information. The electron donor unit triphenylamine (TPA) and intermediate terephthalaldehyde (M1) are commercial available andusedasreceived.[4-(N,N-diphenylamino)benzyl]triphenylphosphonium bromide (TPA-PPh3.Br) as the important intermediate was synthesized by several usual reactions and the synthesis route is shown in Scheme 2. The intermediates with dialdehyde groups were synthesized as the π-spacers and the synthesis routes are shown in Scheme 3. M2 and M3 were obtained by oxidation the corresponding dimethyl compounds. M4 was obtained from M3 under the classic Suzuki coupling condition. The synthesis of M5 and M6 was reported on our earlier paper.13a The dialdehyde intermediates were reacted with the TPA-PPh3.Br under Wittig-Honor reaction to give the dye precursors, which were further reacted with the cynaoacetic acid in the presence of piperidine or rhodanine-3-acetic acid in presence of piperidine and


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SCHEME 4: Synthesis Route of TPC5a

a (a) Glycol, p-TsOH, toluene, reflux, overnight. (b) CuCN, DMF, reflux, 2 h. (c) 2 N HCl, THF, reflux, 4 h. (d) cynaoacetic acid, TEA, toluene, reflux, 1 h.

Figure 2. Absorption spectra of selected dyes in CH2Cl2 solutions (a) and on TiO2 films (b).

TABLE 1: Absorption, Emission, and Electrochemical Properties of the Dyes absorptiona

emission

oxidation potential

dyes

λmax [nm]

at λmax [M-1 cm-1]

λmax on TiO2 [nm]

λmaxa [nm]

Eox [V] (versus NHE)b

E0-0 [V] (Abs/Em)c

Eox – E0-0 [V] (versus NHE)

TPC-1 TPC-2 TPC-3 TPC-4 TPC-5 TPR-1 TPR-2 TPR-3 TPR-4 TTC-1 TTC-2

438 441 444 425 500 468 478 480 464 424 472

37600 15200 21400 20000 12000 32100 16000 19600 29800 15500 19800

423 425 428 412 480 458 469 468 455 422 430

625 640 644 626 682 691 682 706 693 560 640

1.07 1.14 1.16 1.13 1.17 1.07 1.12 1.13 1.12 0.86 0.95

2.33 2.29 2.28 2.34 2.11 2.17 2.23 2.18 2.28 2.42 2.21

-1.26 -1.15 -1.12 -1.21 -0.94 -1.10 -1.11 -1.05 -1.16 -1.56 -1.26

a Absorption and emission spectra were measured in CH2Cl2 solution at 25 °C. b Cyclic voltammogram of the oxidation behavior of the dyes were measured in dry CH2Cl2 containing 0.1 M tetrabutylammonium hexaflourophosphate (TBAPF6) as supporting electrolyte (working electrode, glassy carbon; reference electrode, Ag/Ag+ calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference; counter electrode, Pt). c The zeroth-zeroth transition E0-0 value was estimated from the intersection of the absorption and emission spectra.

ammonium acetate (Knoevenagel condensation reaction) and converted to TPC1∼4, TPR1∼4, and TTC1∼2 (see Scheme 1). However, it is difficult to yield TPC5 by cyanogenation of TPC2 due to the decarboxyl phenomenon. For the synthesis of TPC5 (Scheme 4), the aldehyde group of TP2 was protected by the glycol to afford TP2a first. Subsequent cyanogenation of TP2a by copper cyanide yielded intermediate TP5a, which were deprotected to give TP5. However, it

is noticeable that TP5 cannot be directly obtained by TP2 reaction with copper cyanide due to the instability of aldehyde unit. Then TP5 was reacted with excess cynaoacetic acid in the presence of triethylamine (TEA) under refluxing condition in toluene to give TPC5 with a little yield. If the piperidine or ammonium acetate was employed as base catalyst in the reaction, TPC5 also cannot be obtained due to the decarboxyl phenomenon.



Figure 3. J-V curves and IPCE spectra of DSSCs based on TPC1 sensitized in different solutions.

TABLE 2: Photovoltaic Performance of DSSCs Based on TPC1 Sensitized in Different Solutionsa b

solvent

JSC [mA · cm-2]

VOC [mV]

ff

η [%]

TPC1

CH2Cl2 MeCN EtOH THF DMF

9.7 9.4 9.1 8.2 5.6

760 720 709 663 579

0.72 0.68 0.66 0.67 0.62

5.33 4.59 4.23 3.61 2.00

dye

IPCE [%] (λmax) 87 83 84 73 53

(480 (460 (460 (440 (440

nm) nm) nm) nm) nm)

a Irradiation light AM 1.5G simulated solar light (100 mW · cm-2) at room temperature; working area, 0.159 cm2; electrolyte, 0.6 M DMPII/0.025 M LiI/0.04 M I2/0.05 M GuSCN/0.28 M TBP in dry MeCN. b The concentration of TPC1 dye is 2 × 10-4 M in different dye baths.

TABLE 3: Vas, Vs, and ∆V Data (cm-1) of All the Samples sample TPC1 sodium salt anchored TPC1 via anchored TPC1 via anchored TPC1 via anchored TPC1 via anchored TPC1 via

CH2Cl2 MeCN EtOH THF DMF

Vas(COO-)

Vs(COO-)

∆V

1618 1621 1621 1618 1618 1648

1375 1387 1389 1390 1386 1382

243 234 232 228 232 266

Figure 5. Absorption bands of the anchored TPC1 on TiO2 surface via different solutions.

Figure 6. Possible binding modes for caboxylate unit on TiO2 surface.

Figure 4. Adsorbed amount of the anchored TPC1 on TiO2 films via different solutions.

3.2. Absorption, Emission Spectra, and Electrochemical Properties of the Dyes. The absorption spectra of selected dyes in CH2Cl2 and on TiO2 films are shown in Figure 2, and others are described in the Supporting Information (Figures S1, S2). The characteristic data are collected in Table 1. All of the dyes with a strong absorption maximum in the visible region

corresponding to intramolecular charge transfer absorption are observed.20 For TPC-dyes and TPR-dyes, introducing the substitutes to π-spacers decreases the extinction coefficients (ε). However, employing the electron-withdrawing substitutes to π-spacers of TPC-dyes and TPR-dyes have shown the bathochromic shift absorption spectra and therefore improved the light-harvesting range. For example, TPC5 dye with terephthalonitrile as π-spacer gives a broader absorption band with the absorption maximum at 500 nm than other dyes. Comparing TPR-dyes with TPC-dyes, the introduction of rhodanine-3acetic acid as electron acceptor contributed to the bathochromic shift of the absorption spectra. It is because that rhodanine-3acetic acid has the stronger electron-withdrawing ability than cyanoacrylic acid unit. The π-spacers, 2,2′;5′,2′′-terthiophene (TT) and dithieno[3,2-b;2′,3′-d]thiophene (DTT) are also introduced to the dye molecules to give TTC1 and TTC2, respectively and are attempted to increase the light-harvesting range in visible region. However, TTC1 dye has shown a broad absorption band at 400∼600 nm but with a small absorption maximum at 424 nm.


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Figure 7. FTIR spectra of neat TPC1 (a), TPC1 sodium salt (b), anchored TPC1 on TiO2 via CH2Cl2 (c), and DMF (d).

Figure 8. Possible dimerized structure of neat TPC1.

TABLE 4: Photovoltaic Performance of DSSCs Based on the Triphenylamine and N719 Dyesa dyeb

JSC [mA · cm-2]

VOC [mV]

ff

η [%]

TPC1 TPC2 TPC3 TPC4 TPC5 TPR1 TPR2 TPR3 TPR4 TTC1 TTC2 N719c

9.7 9.4 8.1 8.1 1.3 7.8 3.0 3.4 3.1 6.5 7.3 13.5

760 668 704 630 480 590 525 526 524 583 570 703

0.72 0.69 0.68 0.68 0.70 0.71 0.73 0.72 0.73 0.73 0.65 0.67

5.33 4.36 3.86 3.49 0.44 3.27 1.14 1.29 1.19 2.75 2.76 6.33

a Irradiation light, AM 1.5G simulated solar light (100 mW · cm-2) at room temperature; working area, 0.159 cm2; electrolyte, 0.6 M DMPII/0.025 M LiI/0.04 M I2/0.05 M GuSCN/ 0.28 M TBP in dry MeCN. b Dye bath: CH2Cl2 solution (2 × 10-4 M). c Dye bath: MeCN/t-BuOH (50/50, v/v) solution (3 × 10-4 M).

When the dyes are attached to TiO2 surface, the absorption spectra of these dyes are blue-shifted more or less as compared to that in solutions (Figure 2b) because of strong interactions between the dyes and the semiconductor surface. This kind of interaction also can lead to form the aggregated state of the dyes on semiconductor surface.19 In general, the large blueshift of the dye on TiO2 compared with that in solutions could have a more tendency to form aggregation state on TiO2, especially H-type aggregation. For higher efficiency of DSSCs, the monolayer dyes anchoring onto the TiO2 surface is neces-

sary. One can find all of the dyes with small blue-shift values besides TTC2 (42 nm). This result means that most of the dyes could be attached to TiO2 surface with quasi monolayer state, thus subsequent semiconductor sensitization for DSSCs test is without the addition of the chenodeoxycholic acid (CDCA) to suppress the dye aggregation in dye baths. In DSSCs, the dye is excited by the absorbing photon and then forms oxidized dye after injection of the electron into the conducting band (CB) of TiO2. Then the oxidized dye accepts electron from iodine/iodide to be regenerated. Therefore, to be effectively reduced by the electrolyte the HOMO level of the dye must be more positive than the reducing potential of iodine/ iodide, 0.4 V versus NHE. The first oxidation potential (Eox) corresponding to the HOMO level of the dye was measured in CH2Cl2 by cyclic voltammetry (CV), and the data are summarized in Table 1. The HOMO levels of these dyes are positive enough comparing with that of iodine/iodide, indicating that the oxidized dyes could be reduced effectively by electrolyte and then regenerated. From the values, we can find the small change of the π-spacers shows a great effect on the HOMO levels. The introduction of the electron-withdrawing substitutes in π-spacers made the HOMO levels of dyes more positive. For example, TPC5 showed a more positive HOMO value, 1.17 V (versus NHE), than that of TPC1, 1.07 V (versus NHE). The increase of HOMO levels of dyes could show a negative effect on DSSCs performance due to the broader gap between the HOMO level and the redox potential of iodine/iodide leading to the more waste of energy. Comparing the TPR-dyes with TPC-dyes, we can find that the presence of rhodanine-3-acetic acid unit in dyes exhibited little changes in HOMO levels of the dyes. When the TT and DTT units were employed to act as the π-spacers, the HOMO levels of TTC1 and TTC2, 0.86 and 0.95 V (versus NHE), respectively, have shown them to be



Figure 9. J-V curves and IPCE spectra of DSSCs based on selected dyes.

Figure 10. Dark current of DSSCs based on TPC1 and TPR1.

Figure 11. Reference solar irradiance spectra of AM 1.5G solar light (solid line) and the calculated spectral photocurrent density of DSSC based on TPC1 dye (dash line).

sufficiently more negative than that of TPC1, 1.07 V (versus NHE) because of π-conjugate effect of TT and DTT units. The LUMO levels of these dyes were calculated by Eox E0-0, where E0-0 is the zeroth-zeroth energy of the dyes estimated from the intersection between the absorption and emission spectra; the data were collected in Table 1. To effectively inject the electron into the CB of TiO2, the LUMO levels of the dyes must be sufficiently more negative than the conducing band energy (Ecb) of semiconductor, -0.5 V (versus NHE).21 From the LUMO values, we can find that all of the dyes can complete the process of electron injection into CB of

TiO2 to form the oxidized dyes. Noticeably, the relatively large energy gaps between the LUMO of the dye and Ecb of semiconductor allow for the treatment the 4-tert-butylpyridine (TBP) to the electrolyte, which shift the Ecb of the TiO2 negatively about 0.3 V and consequently improve the opencircuit voltage and total conversion efficiency.8 3.3. Optimization of the Solvents for Electrode Sensitization. To our knowledge, the dyes in different solvents exhibit diversified interaction between the dyes and solvents,19 which could cause changes of the physical and chemical properties between the dyes and semiconductor surface. Therefore, the suitable solvent for semiconductor sensitization is important to obtain prominent solar-to-electricity conversion efficiency (η). To optimize the dye baths of these dyes for TiO2 sensitization, TPC1 in different solutions (CH2Cl2, MeCN, EtOH, THF, and DMF) was employed to sensitize TiO2, and then the photovoltaic performance of TPC1-based DSSCs was measured. Figure 3 shows the dependence of the J-V curves and IPCE spectra on different dye baths of TPC1-based DSSCs, and corresponding photovoltaic data were collected in Table 2. From the data, one can find the big differences in the performance of TPC1-based DSSCs fabricated in various dye-bath solutions. When CH2Cl2 solution of TPC1 was introduced to sensitize the semiconductor electrode, the DSSCs obtained the best η value, 5.33%, than that in other solutions. A broad feature in the action spectrum (300–650 nm) with IPCE values more than 85% of TPC1-based DSSCs via CH2Cl2 made it possible to obtain the higher photocurrent. However, the DSSCs sensitized in DMF gave the poor performance (η ) 2.0%) and showed narrow IPCE spectrum (300–600 nm) with low IPCE value (53% at 440 nm). To further investigate why the solvents could cause such a different performance of DSSCs, the dye adsorbed amount and absorption spectra of TPC1 on TiO2 surface sensitized in different solvents were measured and are shown in Figures 4 and 5, respectively. By desorbing the dye into a basic solution, the dye absorbed amount was estimated by measuring the absorption spectrum of the resultant solution. The TiO2 surface concentrations of TPC1 sensitized in CH2Cl2, MeCN, EtOH, THF, and DMF were determined to be 4.46 × 10-7, 4.39 × 10-7, 3.36 × 10-7, 3.25 × 10-7, and 0.69 × 10-7 M · cm-2, respectively. The result shows an interesting fact that the decrease of photocurrent density of TPC1-based DSSCs via different solutions is in direct ratio with the decreasing adsorbed amount. It is possibly because the increase of dye adsorbed amount led to action spectrum broadening and IPCE value increasing.22For example, the amount of anchored dye in DMF


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Figure 12. Frontier molecular orbitals of the HOMO and LUMO calculated with DFT on a B3LYP/6-31+G(d) level of the selected dyes.

solution is the least and the photocurrent of DSSC fabricated in DMF is the lowest, 5.6 mA · cm-2. From the absorption spectra of TPC1 on TiO2 surface, we can find some slight changes among these sensitized TiO2 films in different dye baths. When TiO2 film was sensitized in EtOH solution of TPC1, the absorption spectrum was broader than that of the film sensitized in THF. This phenomenon could cause the narrower IPCE spectrum of TPC1-based DSSCs from THF than that of DSSCs constructed in EtOH, although the adsorbed amount of anchored-TPC1 in THF and EtOH is almost the same. The solvent effect on DSSC performance is complicated. One possible reason is the different binding modes between TiO2 surface and anchored dye from different solvents. The carboxylate functional groups of dyes play a crucial role in interfacial binding between dye and semiconductor. To achieve high quantum yields of electron injection into semiconductor, the dye should be in intimate contact with the semiconductor surface. The resonance FTIR technique provides a direct probe by allowing for the acquisition of vibrational data of dye on TiO2 surfaces. In general, a carboxylate group can coordinate to the TiO2 surface in main three ways 23,25 (Figure 6): unidentate mode, chelating mode, and bridging bidentate mode. DeaconPhilips rule24 is helpful to determine the binding modes of the dye on TiO2 surface by calculating the frequency separation, ∆V ) Vas(COO-) - Vs(COO-), between the asymmetric (Vas) and symmetric stretching (Vs) modes of the carboxylate unit. The ∆V values are in the order of unidentate > ionic form ≈ bidentate bridging > bidendate chelating. The FTIR spectra of selected samples from our dye systems were measured as shown in Figure 7, and Vas, Vs, and ∆V data of all the samples were collected in Table 2 for comparison. For TPC1 sodium salt, the asymmetric and symmetric stretching band of carboxylate group appeared at 1618 and 1375 cm-1, which contributed to a ∆V value at 243 cm-1. Notably, there was no resonant band in the region from 1700 to 1750 cm-1 in IR spectrum of neat TPC1, but a band of medium intensity at 1619 cm-1 appeared, which can be assigned to the coupled asymmetrical stretch. The

result suggested that TPC1 possible behaved with dimerized structure in solid state10b (see Figure 8). When the dye was anchored to TiO2 surface via various solvents, different ∆V values were also obtained. The anchored dye via CH2Cl2, MeCN, EtOH, and THF showed the similar ∆V values, 231, 232, 228, and 232 cm-1, respectively. On the basis of this comparison and general conclusions, the bidentate bridging or bidendate chelating structure is suggested for the majority binding modes between semiconductor surface and anchored dye from CH2Cl2, MeCN, EtOH and THF. However, these IR spectra are almost consistent with that of neat TPC1. It means that the interaction between semiconductor surface and anchored dye via these solvents are also similar with the intermolecular action of neat TPC1. So we can conclude that the binding modes between the semiconductor surface and the anchored dye via CH2Cl2, MeCN, EtOH, and THF should be the bidentate bridging structures. Distinguishingly, when DMF was introduced to act as the solvent for TiO2 sensitization, the increase of ∆V value, 266 cm-1, was obtained and this suggested that a unidentate structure probably was in the presence of TPC1-adsorbed TiO2. However, the peak of carbonyl group was not observed at 1700-1750 cm-1. The results suggested that TPC1 was attached on TiO2 films probably by the mixture structure of unidentate and bidentate coordination. The recent investigation from other research groups indicated the bidentate structure is superior to the unidentate structure in the stability of the anchored dye and interfacial quantum yields of electron injection due to the intimate contact with the semiconductor surface.25 Our results reveal that the TPC1-based DSSCs exhibited the poorer performance in DMF than in the other four tested solvents for semiconductor sensitization. For TPC1 dye, all the observation suggested that CH2Cl2 is the most suitable solvent for electrode sensitization due to large adsorbed amount and the excellent surface binding modes of anchored dye leading to the broader IPCE spectrum and higher IPCE values of DSSC, which contribute to the prominent solarto-electricity conversion efficiency.

Solar Cells Based on Triphenylamine Dyes 3.4. Photovoltaic Performance of DSSCs Based on the Three Series of Dyes. The photovoltaic properties of the solar cells fabricated from all these organic dye-sensitized TiO2 electrodes were measured under simulated AM 1.5G irradiation (100 mW · cm-2). The open-circuit photovoltage (VOC), shortcircuit photocurrent density (JSC), fill factor (ff), and solar-toelectrical energy conversion efficiencies (η) are listed in Table 4. J-V characteristics and IPCE spectra of the devices based on the selected dyes are shown in Figure 9. The optimized sensitization conditions of these dyes are determined to be 2 × 10-4 M in CH2Cl2, and the electrolyte is 0.6 M DMPII, 0.025 M LiI, 0.04 M I2, 0.05 M GuSCN, and 0.028 TBP in dry acetonitrile. Depending on the different π-spacers and electron acceptors, these triphenylamine dyes show distinguishing and interesting results. In general, TPC-dyes show narrower IPCE spectra and higher IPCE values and energy conversion efficiencies than the corresponding TPR-dyes, which employ the rhodanine-3-acetic acid as electron acceptor. The result is in agreement with our recent investigation of the dyes with cyanoacrylic acid units giving faster and more effective electron injection from LUMO to CB of semiconductor than that with rhodanine-3-acetic acid units,12 which is further demonstrated by IPCE spectra and density functional theory (DFT) calculations. The IPCE is expressed as26

IPCE(λ) ) LHE(λ)Φinj · ηc ) LHE(λ) · (λ)ET where Φinj is the quantum yield of charge injection from the dye excited-state into TiO2, ηc is the efficiency of the collecting the injected charge at the back contact, and Φ(λ)ET is defined as an electron transfer yield, which is a product of the electron injection yield and the charge collection efficiency. This means that the TPC-dyes have much higher (Φ(λ)ET) values considering similar unity LHE value for these dyes. Because JSC can be calculated by integrating the product of the incident photon flux density and the cell’s IPCE over wavelengths used for light absorption by the dye, the lower Φ(λ)ET and corresponding lower IPCE values will be responsible for the low JSC and performance of TPR-dyes. Also, we can find that the TPR-dyes give lower voltage values than TPC-dyes. For example, the η values of DSSCs based on TPC1 and TPR1 are 5.33% (JSC ) 9.7 mA · cm-2, VOC ) 760 mV, ff ) 0.72) and 3.27% (JSC ) 7.8 mA · cm-2, VOC ) 590 mV, ff ) 0.71), respectively. Figure 10 shows the current-voltage curves under dark conditions of the DSSCs based on TPC1 and TPR-1, and the onset for TPR-1-based DSSCs was obviously more negative than that for the TPC1based DSSCs, which indicates that the back-electron-transfer process corresponding to the reaction between the conductionband electrons in the TiO2 and I- ion in the electrolyte occurs more easily in DSSCs based on TPR-dyes than in the DSSC based on TPC1 dyes.27 The argument could be supported by the poor electron injection of TPR-dyes, which has a negative effect on the competition process between the circuit current and dark current. The different spatial array between TPR-dyes and TPC-dyes on TiO2 surface could also cause different effect on suppressing dark current.22 Therefore, it seems that the rhodanine-3-acetic acid unit is not the suitable constructional moiety for the dyes getting a higher η value, although the rhodanine-3-acetic acid unit increases the light-harvesting capability. When the TT and DTT units are introduced to the TTC1 and TTC2 dyes, respectively, the DSSCs based on the two dyes give poor energy conversion efficiency, 2.75% for TTC1 and

J. Phys. Chem. C, Vol. 112, No. 29, 2008 11031 2.76% for TTC2, due to the lower photocurrent and photovoltage values. The poor light-harvesting ability of TTC1 dyes could cause the lower photocurrent value. However, for TTC2, the dye aggregation could be the main reason.13a From the CV data, we can find that the HOMO energy levels of the two dyes are relatively negative in comparison to that of the TPC-dyes. The result leads to the reduction of the energy gap between oxidation potential of the dyes and redox potential of electrolyte, which could decrease regeneration efficiency of the oxidized dye by I- and thus result in the increasing of the dark current and poor performance of the DSSCs based on the two dyes. For TPC-dyes or TPR-dyes, the introduction of different substitutes on the phenylene subunit as π-spacers also shows the huge distinguish of DSSCs performance. The decreasing molar extinction coefficients (ε) of the dyes with substitutes on the phenylene subunit could cause the dissatisfactory lightharvesting efficiency, which lead to lower photocurrent. Furthermore, the electron-withdrawing substitutes on the phenylene subunit may be the barrier of the electron injection into TiO2 CB and thus show the lower η value. For example, the η value of DSSC based on TPC5 is only 0.44% (JSC ) 1.3 mA · cm-2, VOC ) 480 mV, ff ) 0.70). This result indicates the electronwithdrawing substitutes on π-spacers have negative effect on DSSCs performance. However, for TPC4 dye, although the thiophene units are not the electron-withdrawing groups, the narrow visible spectrum response range probably is the main reason for the poor photovoltaic performance. 3.5. Photocurrent Density of DSSC Based on TPC1 Calculations. To validate the reliability of the photovoltaic data measured by our solar simulator, the calculated photocurrent density (J′SC) (JSC) of DSSC based TPC1 dye, 9.6 mA · cm-2, was obtained by integrating the curve of calculated spectral photocurrent density (Icell(λ)) versus wavelength (λ) (shown in eq 1), which is in agreement with the experimental value (JSC), 9.7 mA · cm-2. This result indicates the mismatch between the simulated light and AM 1.5G nature light is very small. Icell(λ) can expressed as the product of spectral intensity of solar light (Isun(λ)) and IPCE (λ) (see eq 2). Isun(λ) can be obtained by the spectral photon irradiance (Win (λ)) of AM 1.5G solar emission, and the formula is described in eq (3).

J′SC )

∫300∞ Icell(λ) dλ

Icell(λ) ) Isun(λ) · IPCE(λ) Isun(λ) )

λ · Win(λ) 1240

(1) (2) (3)

Figure 11 shows Win (λ) of AM 1.5G solar light28 and Icell(λ) of DSSC based on TPC1 dye. 3.5. Molecular Orbital Calculations. To get a further insight into the big difference in performance of DSSCs based on these TPA dyes, density functional theory (DFT) calculations29 were performed at a B3LYP/6-31+G(d) level for the geometry optimization.30,31 The frontier MOs of the selected dyes reveals that HOMOfLUMO excitation moves the electron density distribution from TPA moiety to cyanoacrylic acid or rhodanine3-acetic acid moiety via different π-spacers (see Figure 12). The effective intramolecular charge transfer (ICT) process indicates that the electron injection from the excited dyes to TiO2 CB is feasible. However, the introduction of cyano-groups into phenylene as the π-spacer of the TPC5 dye, a portion of electrons is pulled to the cyano-groups at the LUMO level, which could block the electron injection, and thus yield lower photocurrent. This phenomenon is similar with the anthraquinone dyes reported before by our group.32 Furthermore, the LUMO

11032 J. Phys. Chem. C, Vol. 112, No. 29, 2008 electron density geometry distribution of TPR1 is mainly concentrated on rhodanine framework, especially on the carbonyl and thiocarbonyl, and resulting in the position of LUMO isolated from the -COOH anchoring group due to the presence of -CH2- group.12 Consequently, the TPR1 dye prevents electrons from being effectively injected into TiO2 conduction band via the carboxyl group.33 These results indicate that dyes with rhodanine-acetic acid units cannot give effective and fast electron injection from LUMO level to TiO2 CB in comparison to the similar dyes but with cyanoacrylic acid units. TTC1 dye also shows the ideal ICT process. However, the relative flexible molecular could cause the more wastage of the energy. 4. Conclusion By changing the different π-spacers and electron acceptor, three series of triphenylamine dyes were designed and synthesized as the sensitizers for DSSCs applications. The introduction of different electron-withdrawing substitutes on phenylene units as the π-spacers or rhodanine-acetic acid as electron acceptor in the molecular structures can give bathochromic shifts of absorption spectra. Also, the significant differences in the redox potential of these dyes are caused by small structure changes. Furthermore, the various dye baths from different solvents also give different performance of the DSSCs due to the dye adsorbed amount, absorption spectra and binding modes of the anchored dyes on TiO2. FTIR study showed that the TPC1 dye can be anchored on the TiO2 surface in bidentate bridging mode from CH2Cl2 dye bath and gave a prominent solar-toelectricity conversion efficiency (η), 5.33% (JSC ) 9.7 mA · cm-2, VOC ) 760 mV, ff ) 0.72), while the same dye gave only 2.00% η value if the dye bath was changed to DMF due to the presence of the unidentate binding mode and less dye adsorbed amount on TiO2. The substituted phenylene with electron-withdrawing units or rhodanine-acetic acid in dyes affords a negative effect on the DSSCs performances. DFT calculations have been performed on the dyes, which show the effective intramolecular charge transfer (HOMOfLUMO) of the excited dyes. In addition, the result also reveals that the electron-withdrawing units on π-spacers suppress the electron injection from the LUMO level to TiO2 CB, and the dyes containing cynaoacrylic acid anchor groups have much better orbital overlap with TiO2 conduction band than those dyes containing rhodanine-3-acetic acid. Acknowledgment. We thank the National Natural Science Foundation of China (Grant 20633020), the Ministry of Science and Technology (MOST), Ministry of Education (MOE), the Program for Changjiang Scholars and Innovation Research Team in university (PCSIRT), the Swedish Energy Agency, the K&A Wallenberg Foundation, and the Swedish Research Council for financial support of this work. The authors are grateful to Tannia Marinado and Daniel P. Hagberg at Royal Institute of Technology (KTH), Sweden, for helpful discussions. Supporting Information Available: The synthesis details, absorption spectra in solutions and on TiO2 films, photovoltaic properties and IPCE spectra of the triphenylamine dyes, FTIR spectra of TPC1 on TiO2 surface via different solvents, and reference spectral irradiance data of AM 1.5G sunlight. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature. 1991, 353, 737–740.

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