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Sensitization of Nanostructured TiO2 by Electrostatic Coupling of Ionic Dyes to Ionic Absorbates P. K. D. D. P. Pitigala, M. K. I. Seneviratne, V. P. S. Perera, and K. Tennakone* Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka Received January 22, 2004. In Final Form: April 5, 2004 It is shown that an ionic dye Y can be electrostatically bonded to an ionic molecule X of opposite charge anchored to a TiO2 surface via suitable ligands. Dye-sensitized solid-state photovoltaic cells of the configuration n-TiO2/X-Y/p-CuSCN were constructed with X ) trihydroxybenzoic acid or mercurochrome and Y ) methyl violet. Cells of this configuration were found to be more efficient and delivered higher short-circuit photocurrents and open-circuit photovoltages compared to the cells based only on methyl violet or mercurochrome. It is suggested that this technique would be a means of extending the spectral response of dye-sensitized photovoltaic devices. The formation of a wider barrier by coupling of anionic and cationic species also improves the cell performance by suppression of recombination.
Introduction Dye sensitization of semiconductor surfaces continues to be a rich field of study from the viewpoints of fundamentals as well as applications.1-4 Different types of dye-sensitized solar cells5-10 have been demonstrated, and their efficiencies depend on fast injection of the carriers to the bands of the semiconductor and the slow back reaction.11-13 Strong electronic coupling of the dye molecule to the semiconductor surface causes fast injection.12,14 Surface chelation of the dye molecules to the semiconductor affords a way of achieving good electronic coupling compared to a van der Waals contact, and ligands that readily anchor to TiO2 and other oxide surfaces have been identified.14 We have found that ionic dyes not readily adsorbing onto the TiO2 surface can be electrostatically bonded to ionic molecules of the opposite charge that are surface chelated to TiO2 via suitable ligands and dye anchorage by this method results in efficient sensitization. The effect is demonstrated by constructing dye-sensitized solid-state solar cells using p-CuSCN as the hole collector. * To whom correspondence should be addressed. E-mail: tenna@ ifs.ac.lk. (1) McEvoy, A. J.; Gratzel, M. Dye-Sensitized Regenerative Solar Cells. In Semiconductor Electrodes and Photoelectrochemistry; Licht, S., Ed.; Encyclopedia of Electrochemistry, Vol. 6; Wiley-VCH: Weinheim, 2001; pp 397-406. (2) Hinsch, A.; Kroon, J. M.; Kern, R.; Uhlendorf, I.; Holzbock, J.; Meyer, A.; Ferber, J. Prog. Photovoltaics: Res. Appl. 2001, 9, 425. (3) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (4) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 260. (5) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (6) Tennakone, K.; Kumara, G. R. R. A.; Kumarasinghe, A. R.; Wijayantha, K. G. U.; Perera, V. P. S. Semicond. Sci. Technol. 1995, 10, 1689. (7) O’Regan, B.; Schwartz, D. T. Chem. Mater. 1995, 7, 1349. (8) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Perera, V. P. S. Chem. Commun. 1999, 9, 15. (9) Keis, K.; Magnusson, E.; Lindstrom, H.; Lindquist, S. E.; Hagfelt, A. Sol. Energy Mater. Sol. Cells 2002, 73, 51. (10) Meng, Q. B.; Takahashi, K.; Zhang, X. T.; Sutanto, I.; Rao, T. N.; Sato, O.; Fujshima, A.; Watanabi, H.; Nakamori, T.; Uragami, M. Langmuir 2003, 19, 3572. (11) Miller, R. J. D.; McLendon, G. L.; Nozik, A. J.; Schmickler, W.; Willig, F. Surface Electron Transfer Processes; VCH: New York, 1995. (12) Asbury, J. B.; Hao, E.; Wang, Y.; Gosh, H. N.; Lian, T. J. Phys. Chem. B 2001, 105, 4545. (13) Abury, J. H.; Ellingson, R. J.; Gosh, H. N.; Ferrere, S.; Nozik, A. J.; Lian, T. J. Phys. Chem. B 1999, 103, 3110. (14) Nazeeruddin, K.; Gratzel, M. Dyes for Semiconductor Sensitization. In Encyclopedia of Electrochemistry, Vol. 6; Licht, S., Ed.; WileyVCH: Weinheim, 2001; pp 407-431.
One could also adopt a surface-chelating ionic dye as an absorbate on TiO2 so that sensitization occurs via both of the dyes. This paper also describes construction of a model dye-sensitized solid-state solar cell with two dyes where the energy conversion efficiency of the double-dye system is higher than that of the cells based on individual dyes. A main obstacle for improvement of the efficiency of dye-sensitized solar cells happens to be the narrow spectral response, a limitation of the absorption spectrum of the dye. In conventional semiconductor photovoltaics, the idea of multiple band gaps has been successfully utilized to extend the spectral response.15 One would think that use of dye mixtures would resolve the same problem in dyesensitized solar cells. Unfortunately, straightforward application of dye mixtures invariably leads to concentration quenching or insulation. Quenching and insulation can be avoided if the dyes are lightly deposited to avoid the mutual interaction of the dye molecules. Again, to achieve good light absorption cross section for each dye, the thickness of the TiO2 film needs to be increased. However, there exists an upper limit for the film thickness, that is, the electron diffusion length (Dτ)1/2 (D ) diffusion coefficient, τ ) recombination time). The strategy adopted in our model system is an attempt to circumvent the above problems. Experimental Section Nanocrystalline TiO2 films were deposited on conducting tin oxide (CTO) glass plates (0.5 × 1.5 cm2; active area, 0.25 cm2) as described previously.16 Briefly, the procedure involves spreading of a colloidal solution of titanium dioxide (prepared by hydrolysis of titanium isopropoxide in the presence of acetic acid) on CTO glass plates heated to 150 °C and sintering at 450 °C for 30 min. After cooling, the loose crust on the surface is wiped off with cotton wool and the process is repeated until a film thickness of ∼10 µm is achieved. Films of the above thickness prepared by this method have roughness factors of the order of 200-300.17 The film was washed with propan-2-ol, dried, and exposed to UV light for 20 min to burn out any organic matter contamination on the film surface. Trihydroxybenzoic acid (TA) was used as an anion-producing compound that strongly anchors to the TiO2 (15) Green, M. Mater. Sci. Eng. B 2000, 74, 118. (16) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Wijayantha, K. G. U.; Perera, V. P. S. J. Phys. D: Appl. Phys. 1998, 31, 1492. (17) Kumara, G. R. R. A.; Kaneko, S.; Okuya, M.; Tennakone, K. Key Eng. Mater.: J. Cer. Soc. Jpn. 2002, 119, 228.
10.1021/la0497999 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/06/2004
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Figure 2. I-V characteristics of (a) TiO2/MV/CuSCN and (b) TiO2/TA-MV/CuSCN.
Figure 1. Schematic diagram showing construction of the cell TiO2/MC-MV/CuSCN. Scheme 1. Anchoring of the Sodium Salt of Trihydroxybenzoic Acid to TiO2 and Attachment of the Methyl Violet Cation by Replacement of Na+ Figure 3. Photocurrent action spectra of (a) TiO2/MV/CuSCN and (b) TiO2/TA-MV/CuSCN. Table 1. Short-Circuit Photocurrent (Isc), Open-Circuit Voltage (Voc), Efficiency (η), Fill-Factor (FF), and Peak (620 nm) Incident Photon to Photocurrent Conversion Efficiency (IPCE) of (a) TiO2/MV/CuSCN and (b) TiO2/TA-MV/CuSCN cell
Isc (mA cm-2)
Voc (mV)
FF (%)
η (%)
IPCE (%)
a b
1.3 2.2
407 507
38.5 42.9
0.21 0.47
13.1 17.8
presented as Figure 1. I-V characteristics of the cells were recorded with a Keithley 2420 Source Meter and a xenon lamp at an intensity of 1000 W m-2 as the light source. An Eko Pyranometer measured the light intensities. Photocurrent action spectra were recorded using a Nikon (G 250) monochromator. surface. Methyl violet (MV) was chosen as a cationic dye, which absorbs poorly on TiO2, and mercurochrome (MC) as an anionic dye, which anchors readily to TiO2. TA was adsorbed into TiO2 films by immersing the plates in ∼10-2 M aqueous solutions containing NaOH (pH ∼ 12). The films were rinsed with water to remove unchelated TA, and an outer monolayer of MV was adsorbed by immersing them in a ∼10-3 M solution of this dye for 15 min. The amount of MV adsorbed on a TA-treated TiO2 film was estimated by noticing the depletion of the dye in the coating solution. MC-coated TiO2 films were prepared by keeping the plates immersed in an alcoholic solution of MC (∼5 × 10-4 M) for 30 min. To deposit an outer layer of MV, a MC-coated film was rinsed with ethanol and immersed in a solution of MV (∼10-3 M in ethanol) for 30 min. The amounts of adsorbed MV and MC were estimated by extraction of these dyes into alkaline solution and spectrophotometric estimation (sample plates of an identical batch were used). To form the heterojunctions n-TiO2/X-Y/pCuSCN (X ) TA or MC, Y ) MV), CuSCN (a p-type semiconductor of band gap 3.6 eV) was deposited above Y from a solution in propyl sulfide by the procedure reported earlier.18 Graphite was painted on the outer surface of CuSCN, and a gold-plated CTO glass plate served as the back contact of the photovoltaic cell. A schematic diagram showing the construction of the cell is (18) Kumara, G. R. R. A.; Konno, A.; Senadeera, G. K. R.; Jayaweera, P. V. V.; De Silva, D. B. R. A.; Tennakone, K. Sol. Energy. Mater. Sol. Cells 2001, 69, 195.
Results and Discussion On exposure of TiO2 films to an aqueous solution of TA containing NaOH, TA chelates to TiO2 via two hydroxyl groups. The presence of NaOH prevents involvement of the carboxylate group in the chelation process, because of the affinity of this acidic group to Na+ (Scheme 1). In the absence of NaOH, TA could also attach to TiO2 via a hydroxyl and a carboxylate ligand; however, these films do not readily adsorb MV. On subsequent exposure of the TA-treated film (in an alkaline solution of TA) to a solution of the cationic dye MVCl, MV cations bond to TA anions anchored to TiO2, eliminating NaCl (Scheme 1). Figure 2 compares the I-V characteristics of the two cells n-TiO2/ TA-MV/p-CuSCN and n-TiO2/MV/p-CuSCN. It is seen that short-circuit photocurrent (Isc), open-circuit voltage (Voc), efficiency (η), and fill-factor (FF) are higher in the former cell compared to the latter where MV directly affixes to TiO2 (Table 1). The photocurrent action spectra of the two cells are shown in Figure 3, and the incident photon to photocurrent conversion efficiency (IPCE) at the absorption peak of MV (620 nm) is higher in the first cell (Table 1). The quantities of MV adsorbed on TiO2 (positively charged N atoms of organic bases attach to the
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Figure 5. I-V characteristics of (a) TiO2/MC-MV/CuSCN, (b) TiO2/MC/CuSCN, and (c) TiO2/MV/CuSCN.
Figure 4. Schematic diagram showing the relative positions of conduction bands (CB) and valence bands (VB) of TiO2 and CuSCN and the ground and excited levels of MC (S°1, S/1) and MV (S°2, S/2) and charge transfer on excitation: (a) MC and (b) MV. Scheme 2. Anchoring of Mercurochrome to TiO2 and Attachment of the Methyl Violet Cation by Replacement of Na+
acidic sites of the oxide surfaces19) and TiO2/TA were found to be 1.5 × 10-8 and 3.7 × 10-8 mol cm-2, respectively. The degree of adsorption of MV is weaker on the bare TiO2 surface than on TiO2/TA. The coverage of MV in TiO2/ TA-MV is of the order of a single monolayer (estimated “area” of the MV molecule ∼ 0.9 nm2), whereas that on bare TiO2 is ∼0.4 of a monolayer. The ratio of the Isc values of TiO2/MV and TiO2/TA-MV ()0.6) happens to be higher than the ratio of the surface concentration of MV in the two cases ()0.4). Presumably, this is an indication that the surface protection against recombination is incomplete even in the presence of TA. Electron transfers generating the photocurrent can be understood as follows: excitation of the chromophore in MV will release an electron to the conduction band of TiO2 through the TA bridge and a hole to the valence band of CuSCN, that is,
hν + TiO2/TA-MV/CuSCN f TiO2/TA-MV*/CuSCN f TiO2(e-)/TA-MV/CuSCN(h+) (1) (19) Kanan, S. M.; Lu, Z.; Cox, J. K.; Bernhardt, G.; Tripp, C. P. Langmuir 2002, 18, 1707.
Positions of the ground (S°2) and excited (S/2) states of MV and the band positions of TiO2 and CuSCN (Figure 4) permit the above charge transfers energetically. Apart from enhancement of the dye adsorption, the TA bridge seems to act as a barrier suppressing the recombination of the separated charges. We believe that the first cell performs better compared to the second, at least partly because of this effect. To test this hypothesis further, we adjusted the level of MV coverage on TiO2/TA-MV/CuSCN to obtain the same Isc value as that of TiO2/MV/CuSCN and found that the Voc of this cell attains a value (497 mV) of the same order of magnitude as that of a cell with full coverage of MV (Table 1). This observation suggests that the TA barrier effectively suppresses recombinations that lower the Voc. There are two possible types of recombinations. (1) The geminate combination of the separated electron hole pair across the dye layer. (2) Electron-hole recombination at the points where the TiO2 and CuSCN surfaces touch each other. The bridge connecting TA to MV is expected to suppress recombination of the first type. If the voids in the dye layer were covered with TA, recombinations of the second type would also be mitigated. In the second experiment, we replaced TA by the dye MC, which is a sodium salt of a xanthene anion. The dye MC surface chelates to TiO2 via hydroxyl ligands (adsorption involving the carboxylate group is ruled out as the anchorage remains stable even in an alkaline solution). On treatment of MC-adsorbed TiO2 film with a solution of MVCl, the methyl violet cation replaces Na+ (Scheme 2). Dyes deposited are found be in an equimolecular ratio as expected from the stoichiometry of Scheme 2. The cell TiO2/MC-MV/CuSCN effectively responded to light absorbed by both MC and MV, and we summarize the charge separation process as follows:
hν1 + TiO2/MC-MV/CuSCN f TiO2/MC*-MV/CuSCN f TiO2(e-)/MC-MV/CuSCN(h+) (2) hν2 + TiO2/MC-MV/CuSCN f TiO2/MC-MV*/CuSCN f TiO2(e-)/MC-MV/CuSCN(h+) (3) where ν1 and ν2 are light frequencies absorbed by MC and MV and excitation of either dye results in injection of electrons to TiO2 and holes to CuSCN. A schematic diagram (Figure 4) depicting the relative positions of the bands of TiO2 and CuSCN and the energy levels of the dyes MC and MV indicate that the above charge transfers are allowed energetically. Figure 5 shows the I-V characteristics of the cell TiO2/ MC-MV/CuSCN. It is seen that Isc, Voc, and η are
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Langmuir, Vol. 20, No. 12, 2004 5103 Table 3. Incident Photon to Photocurrent Conversion Efficiencies (IPCEs) of the Cells: (a) TiO2/MC-MV/ CuSCN, (b) TiO2/MC/CuSCN, and (c) TiO2/MV/CuSCN at the Absorption Peak Positions of MC (550 nm) and MV (620 nm)
Figure 6. Photocurrent action spectrum of (a) TiO2/MC-MV/ CuSCN, (b) TiO2/MC/CuSCN, and (c) TiO2/MV/CuSCN. Table 2. Short-Circuit Photocurrent (Isc), Open-Circuit Voltage (Voc), Efficiency (η), and Fill-Factor (FF) of (a) TiO2/MC-MV/CuSCN, (b) TiO2/MC/CuSCN, and (c) TiO2/MV/CuSCN cell
Isc (mA cm-2)
Voc (mV)
FF (%)
η (%)
a b c
4.6 2.4 1.3
629 603 407
47.3 40.9 38.5
1.37 0.6 0.21
significantly higher for the MC-MV system compared to the cells based on the individual dyes MC and MV (Table 2). The photocurrent action spectrum (Figure 6) of the MC-MV cell shows two peaks at 550 and 620 nm corresponding to the contributions from the two dyes MC and MV, respectively. Interestingly, the IPCEs at the above peak absorption positions of MC and MV are higher in the MC-MV cell compared to the cells with MC or MV only (Table 3). The wider barrier (MC-MV compared to MC or MV) seems to suppress the recombination of the separated electron-hole pair. In a heterostructure of the form n/D1D2/p, where D1 and D2 are layers of two dyes sandwiched between n- and p-type semiconductors, excitation of D1 could result in electron injection to the n-region with the formation of the cation D1+. Unless the positive charge on D1+ is quickly transferred to the p-region, D1+ and e- would recombine. The efficient transfer of the positive charge on D1+ to the p-region through D2 requires electronic coupling between
cell
IPCE (%) (λ ) 550 nm)
IPCE (%) (λ ) 620 nm)
a b c
22.2 15.2 4.6
13.8 13.1
D1 and D2. An analogous situation holds on excitation of D2. In the present system, D1 and D2 are ionically bonded. Furthermore, when two dye layers are in contact, the dissipative quenching processes,
D1* + D2 f D1 + D2
D1 + D2* f D1 + D2 D1* + D2* f D1 + D2 (4)
could take place and the rate of charge injection must be faster than quenching for charge separation to occur. Quenching can be prevented by separation of the two by an ultrathin barrier20 or by strong electronic coupling of the two dyes as in the present case. Conclusion The dyes MC and MV are intrinsically poor sensitizers, giving efficiencies of 0.60% and 0.21%, respectively, when used singly to form solid-state dye-sensitized photovoltaic cells. However, efficiency increases 1.37% in the combined system. Broadening of the spectral response will increase the Isc and therefore the efficiency. The present investigation also suggests that the dye double layer acts as a more effective barrier that suppresses recombination. We believe that combination of more efficient sensitizers in anionic and cationic forms and with appropriate absorption spectra is a promising method for broadening of the spectral response of dye-sensitized solar cells. LA0497999 (20) Perera, V. P. S.; Pitigala, P. K. D. D. P.; Jayaweera, P. V. V.; Bandaranayake, K. M. P.; Tennakone, K. J. Phys. Chem. B 2003, 107, 13758.
