6528
J. Phys. Chem. C 2007, 111, 6528-6532
Dye-Sensitized Solar Cells Made by Using a Polysilsesquioxane Polymeric Ionic Fluid as Redox Electrolyte Elias Stathatos,† Vasko Jovanovski,‡ Boris Orel,‡ Ivan Jerman,‡ and Panagiotis Lianos*,§ Technological Institute of Patras, Electrical Engineering Department, 26334 Patras, Greece, National Institute of Chemistry, HajdrihoVa 19, 1000 Ljubljana, SloVenia, and UniVersity of Patras, Engineering Science Department, 26500 Patras, Greece ReceiVed: December 21, 2006; In Final Form: February 27, 2007
Dye-sensitized solar cells have been made by using electrolytes based on a polysilsesquioxane ionic fluid. This is a nanocomposite organic-inorganic material synthesized by the sol-gel route using 1-methyl-3-[3(trimethoxy-λ4-silyl)propyl]-1H-imidazolium iodide as precursor. This compound is an imidazolium iodide derivatized with covalent attachment of a trimethoxysilane group. Condensation was obtained by formic acid solvolysis at 120 °C in the absence of water. The obtained materials have been characterized by IR, 29Si NMR and conductivity measurements. Cells have been constructed by using several variants of the above electrolyte and have been characterized by current-voltage measurements.
1. Introduction In view of the application of Dye-sensitized Solar Cells (DSC) as low-cost alternative for the conversion of solar energy, concern has been expressed as related with the use of volatile solvents to make electrolyte and with the resulting cost of sealing. Two main approaches have been adopted to solve this problem. A first approach is based on the formation of bulk heterojunctions between titania nanoparticles and p-type organic or inorganic semiconductor nanoparticles. The latter are either hole transporting conjugated polymers or organic monomers1,2 as well as inorganic salts, such as CuI and others.3-5 These are the so-called solid-state DSCs. The efficiency of such cells is generally low. One main reason for this low performance is the lack of sufficient electric conduct between titania nanoparticles making the working electrode and the p-type nanocrystallites. Especially in the case of CuI, the tendency of the crystallites of this material to grow results in decreasing the extent of heterojunction, even in cases where crystal growth inhibitors have been used.3,4 A second approach is to add gelifiers to the electrolyte thus increasing viscosity and decreasing leaking. Such gels make the so-called Quasi-solid DSCs. Quasi-solid cells have lower efficiency than liquid cells, mainly due to lower ionic conductivity, but efficiency values are still kept at satisfactory levels. For this reason they are studied with great interest. Gel electrolytes are roughly distinguished into three categories: (1) One way to make a gel electrolyte is to add organic or inorganic (or both) thickeners. Such materials may be long-chain polymers like poly (ethylene oxide) or inorganic nanoparticles like titania or silica;6-10 (2) A second way is to introduce a polymerizable precursor into the electrolyte solution and polymerize the mixture in situ.11-13 (3) Finally, a third route, which also concerns the present work, is to produce a gel incorporating the I-/I3- redox couple through the sol-gel process by using a * To whom correspondence should be addressed. E-mail: lianos@ upatras.gr. † Technological Institute of Patras. ‡ National Institute of Chemistry. § University of Patras.
sol-gel precursor, like a titanium or silicium alkoxide.14-17 This precursor may be a functionalized derivative of one of the components of the electrolyte, as in the present work. This last method has been very successful since the sol-gel process leads to the formation of nanocomposite organic-inorganic materials.14 Such materials are composed of an inorganic subphase, which binds and holds the two electrodes together and seals cell and an organic subphase, which assures dispersion of ionic species and supports ionic conductivity. The whole composition is compatible with titania nanocrystalline electrode and provides good electric contact and satisfactory ionic conductivity. Such cells are easy to make. After dye-adsorption on titania electrode, it suffices to place a small drop of the sol on the surface of the electrode and then press the counter electrode on the top by hand under ambient conditions. The two electrodes bind together while the fluid sol enters into titania nanoporous structure and achieves extensive electric contact. We have successfully used various sol-gel precursors and various gel compositions: for example, tetramethylorthosilicate (TMOS) mixed with surfactant,17 Ureasils14-16 and functionalized ionic liquids.18 The present work describes the performance of a cell based on a sol-gel electrolyte made of a polysilsesquioxane ionic liquid.18 This electrolyte, which has been presented in previous publications,18,19 is shown in Figure 1A. The precursor used for the sol-gel synthesis of this material was 1-methyl-3-[3-(trimethoxy-λ4-silyl)propyl]-1H-imidazolium iodide (MTMSPI+I-) and its structure and synthesis scheme are shown in Figure 1B. When the alkoxysilane end groups are solvolyzed in the presence of formic acid they lead by polycondensation at 120 °C to the ladder-type polymeric structure of Figure 1A. This novel material, which is essentially a polymeric ionic liquid with iodine counterions has been mixed with iodine and was used to fabricate DSCs. The present work describes structural characteristics of the sol-gel electrolyte and performance of DSCs made by employing the above material. 2. Experimental Section All reagents were from Aldrich except cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II), abbreviated
10.1021/jp068812q CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007
Dye-Sensitized Solar Cells
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6529
Figure 1. Structure of Polysilsesquioxane electrolyte (A) and reaction scheme for the synthesis of the MTMSPI+I- (B).
TABLE 1: Relative Molar Content of the Components in the Electrolytes Used in the Present Work relative molar content + -
electrolyte
MTMSPI I
HCOOH
BASIC BASIC + TMOS BASIC + TMOS + IL1 BASIC + TMOS + IL2 BASIC + TMOS + NMBI + IL2
3 3 3 3 3
10 30 30 30 30
RuL2(NCS)2, which was provided by Solaronix SA (Switzerland) and were used as received. Synthesis of 1-methyl-3-[3-(trimethoxy-λ4-silyl)propyl]-1Himidazolium iodide (MTMSPI+I-) sol-gel precursor was made according to previous publications18,19 (cf. Figure 1B). Gel electrolytes were synthesized according to the following protocols: (1) 1 g of MTMSPI+I- was mixed with 0.56 g of formic acid (FA) and then 0.07 g of I2 was added under stirring. Stirring continued for about 24h under ambient conditions. Finally, it was heated for 4 h at 120 °C. This electrolyte is named BASIC; (2) 1 g of MTMSPI+I- was mixed with 0.41 g of tetramethoxysilane (TMOS) and after 5 min we added 1.29 g of formic acid. After 5 additional min, 0.07 g of I2 was added under stirring. As before, stirring continued for about 24 h under ambient conditions. Finally, it was heated for 4 h at 120 °C. TMOS was added to increase gelation and to make a more robust material. This second electrolyte is named BASIC + TMOS; (3) 1 g of MTMSPI+I- was mixed with 0.41 g of tetramethoxysilane (TMOS) and after 5 min we added 1.29 g of formic acid. After 5 additional minutes 0.07 g of I2 were added under stirring. Finally to this mixture we added 0.169 g of the ionic liquid 1-methyl-3-propylimidazolium iodide (MPI+I-). As before, stirring continued for about 24 h under ambient conditions followed by heating for 4 h at 120 °C. This electrolyte is named BASIC + TMOS + IL1; (4) 1 g of MTMSPI+I- was
TMOS 3 3 3 3
MPI+I-
0.7 1.4 1.4
NMBI
I2
0.5
0.30 0.30 0.37 0.44 0.44
mixed with 0.41 g of tetramethoxysilane (TMOS) and after 5 min we added 1.29 g of formic acid. After 5 additional minutes 0.07 g of I2 were added under stirring. Finally to this mixture we added 0.33 g of MPI+I-, i.e. twice as much as in the previous electrolyte. As before, stirring continued for about 24h under ambient conditions followed by heating for 4 h at 120 °C. This electrolyte is named BASIC + TMOS + IL2; (5) 1 g of MTMSPI+I- was mixed with 0.8 g of N-methylbenzylimidazolium (NMBI) and 0.41 g TMOS. After 5 min we added 1.29 g formic acid and after 5 additional minutes 0.07 g of I2 were added under stirring. Finally, to this mixture we added 0.33 g of MPI+I-. As before, stirring continued for about 24h under ambient conditions followed by heating for 4 h at 120 °C. NMBI was added because it is known to improve the open circuit voltage of the cell.17 This electrolyte is named BASIC + TMOS + NMBI + IL2. All above five electrolytes are fluid when are taken out from the oven. If they are left at room temperature they become solid gels. Table 1 shows the relative molar content of the components making the above five electrolytes. Our choice was to always have a ratio of 1:10 for I2:I-. Titania mesoporous films were deposited on FTO (fluorinedoped tin oxide) transparent conductive electrodes as in previous publications.14-17,20 These films are transparent, they have a thickness of about 2 µm, and they are made of anatase nanocrystals of about 15 nm diameter and about 110 m2/g active
6530 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Stathatos et al.
surface area. Despite the fact that the film is relatively thin, its performance20 is compatible with that of thicker depositions reported in literature. Adsorption of the dye sensitizer was made by immersing titania film in an 1 mM ethanolic solution of the dye for 12 h. In this work we used RuL2(NCS)2 as sensitizer. Fabrication of DSCs was made similarly to previous publications14-17,20 by pressing one drop of the electrolyte between the titania-dye electrode and a slightly platinized FTO counter electrode. The presence of TMOS helped to steadily hold the two electrodes together. In the case of the BASIC electrolyte alone, i.e., in the absence of TMOS (see above), binding was less robust. Before electrolyte deposition, the titania/dye electrode was heated at 100-120 °C, while the electrolyte was applied still hot and fluid. IR transmission spectra measurements were made with a Perkin-Elmer System 2000. Samples were applied on Si wafers and measured in absorbance scale. 29Si NMR spectra were recorded on a Varian Unity Plus 300 MHz spectrometer using a Doty CPMAS probe head. The Larmor frequency of the silicon nuclei was 60.190 MHz. Samples were spun about the magic angle with a frequency of 2 kHz. 29Si chemical shifts were determined using DSS (sodium 3-(trimethylsilyl)propane-1sulfonate) as an external standard and then expressed relative to TMS (tetramethylsilane) (d ) 0 ppm). Specific conductivity (σ) measurements were carried out in an electrochemical cell using platinum electrodes (the cell constant was determined with 0.1M KCl) by AC impedance measurements (SOLARTRON 1250 Frequency Response Analyzer and a SOLARTRON 1286Electrochemical Interface) in the frequency range 0.1-65000 Hz. IV curves have been recorded by connecting the cell to an external variable resistor and by measuring the current flowing through the resistor and the corresponding voltage across the resistor. Cell dimension for these measurements was 1 cm.2 Oriel 450 W Xenon lamp was used for the illumination of the samples. Illumination intensity was controlled by multiple wire grids and it was 100mW/cm2. 3. Results and Discussion The main step forward in the process of preparation of electrolytes based on a Quasi solid-state ionic liquid was the employment of formic acid (FA) catalyst.21 FA, due to its low pK value (pK ≈ 2.0) provides aprotic condensation (via solvolysis) of the MTMSPI+ I- precursor and leads to the formation of polyhedral oligomeric silsesquioxanes and, after heating, to polysilsesquioxanes. Solvolysis of various alkoxysilanes has been extensively studied in the past by Vioux and Leclerq21 and used by Sharp22 for the preparation of various water free organicinorganic hybrid gels with various water-soluble organic polymers. The mechanism of aprotic condensation under solvolysis conditions is relatively well understood and has been studied also by us in details for single methyltriethoxysilane23 and bipodal Ureasil precursor.24 The latter has been used as electrolyte in DSC cells.14 It has been demonstrated that solvolysis leads to the formation of siloxane bonds via the formation of the silyl ester intermediates instead of the silanol groups:25
Si-OR + RCOOH f Si-OOCR + ROH
(1)
ROH + RCOOH f ROOCR + H2O
(2)
Si -OOCR + ROH f ROOCR + Si -OH
(3)
Si -OR + Si -OOCR f ROOCR + Si -O- Si (4) (R)H, in the present case) In the above reactions only one of several alkoxy groups has been taken into account for simplicity. Small amount of silanol
Figure 2. Infrared spectra of the MTMSPI+ I-/HCl gel heat-treated at 150 °C for 8 h (curve A) and of the MTMSPI+ I-/FA gel prepared under solvolysis conditions (curve B). Difference spectra (curve C) correspond to the subtraction of MTMSPI+ I-/HCl from MTMSPI+ I-/FA.
groups (if any) and the consequent absence of water in condensed gels beneficially affects the gel properties, enhancing, for example, the longevity and efficiency of the DSC cells.14 One of the advantages of using aprotic condensation processed at 120 °C, as in this study, lies in the fact that due to the relatively high-temperature processing of the precursor mixtures, there were no ester reaction products left in the resulting electrolytes. In addition, since the electrolyte was applied on TiO2/dye electrode when still hot and fluid, good wetting of the dye and the titania nanostructure was achieved. After cooling the electrolyte at ambient conditions, it was transformed into a solid material, which permanently sealed the electrodes. 3.1. Structural Studies of the Electrolyte. 3.1.1. IR and 29Si NMR Spectra of MTMSPI+ I-/FA Gels (BASIC). In our previous studies of MTMSPI+ I-, it was shown that MTMSPI+ Ihydrolyzed with HCl and aged for 2 weeks at ambient conditions exhibits 2% of T1, 33.5% of T2, and 63.3% of T3 species in 29Si NMR spectra. Tn signals are produced by silicon atoms bonded to n oxygen atoms. This result indicates the presence of dimmers, cyclic tetramers T4(OH)2 or open-cube like species T7(OH)318 and cube-like (T8 or T12) or ladder-like species. The sol-gel network was not completed yet and the silanol groups were still present, as inferred from the infrared spectra. Heat treatment at 80 °C of fresh MTMSPI+ I-/HCl sols for a few hours enhanced condensation and the amount of the silanol groups consequently decreased. However, heat treatment at 200 °C was required to provide fully condense electrolytes giving a unique T3 signal in 29Si NMR spectra. Absence of silanol bands and presence of bands at 1135 cm-1 and 1045 cm-1, typical for the ladder and cube- like species, supported the formation of the structure shown in Figure 1A. As mentioned above, the use of solvolysis conditions combined with heat treatment increased the extent of condensation by preferential formation of siloxane bonding instead of formation of silanol groups. Silanol groups after condensation would lead to the formation of water. For the MTMSPI+ Igels this was confirmed by comparison of the infrared spectra of MTMSPI+ I-/FA (BASIC electrolyte), heat-treated at 120 °C, with MTMSPI+ I-/HCl, heat-treated at 150 °C for 8h, both depicted in Figure 2. The second material was obtained under usual hydrolysis conditions in the presence of HCl. The difference in the two spectra showed no bands in the region where the silanol groups absorb (∼900 cm-1) but the negative absorption observed in the 3000-3500 cm-1 region provided clear evidence that the hydrolyzed samples contained more water
Dye-Sensitized Solar Cells
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6531 TABLE 2: Relative Integrated Intensity of the T and Q Signals Observed in 29Si NMR Spectra of MTMSPI+ I-/HCl (Hydrolysis) and MTMSPI+ I-/FA (Solvolysis) Electrolytes hydrolysis
solvolysis
type of 29 Si peak
δ (ppm)
integral (%)
integral (%)
T2 T3 Q2 Q3 Q4
-58.4 -67.4 -91.8 -101.6 -111.0
11.7 50.7 0.5 14.3 22.8
11.7 (22.4)a 40.6 (77.6)a 1.6 22.5 23.6
a The numbers in parentheses indicate the % amount of only T species present in electrolyte.
Figure 3. 29Si NMR spectra of MTMSPI+ I-/FA (BASIC electrolyte). The inset table presents the relative integrated intensities of the T signals.
Figure 4. 29Si NMR spectra of MTMSPI+ I-/TMOS hydrolyzed with HCl and heat treated for 2 h at 80 °C (solid line) and MTMSPI+ I-/TMOS electrolyte hydrolyzed with FA (dashed line).
than the samples prepared under the solvolysis conditions, contrary to the fact that the hydrolyzed gels were treated for longer time at higher temperature. For the assessment of the water content in electrolytes we relied on the spectra of Figure 2. Even though, there were no silanol bands in both materials, small bands attributed to water have been detected. We believe that the water has been incorporated in the gels during the manipulation of samples while performing the spectra measurements. In addition, the siloxane bands at 1143 and 1039 cm-1 demonstrated that a more condensed structure formed in the case of MTMSPI+ I-/FA. However, as seen in Figure 3, the presence of T2 signal observed in the 29Si NMR spectra suggested that fully condensed structure was never obtained and open cube-like T7(OH)3 or tetrameric T4(OH)2 species (28.4%) were still present in MTMSPI+ I-/FA gels (BASIC electrolyte). 3.1.2. IR and 29Si NMR Spectra of MTMSPI+ I-/FA/TMOS/ IL2 Gels (BASIC + TMOS + IL2). The influence of the addition of TMOS and MPI+ I- to the MTMSPI+ I- was studied with the help of 29Si NMR, shown in Figure 4 and Table 2. These spectra contain the characteristic T and Q species from which we inferred that condensation process of MTMSPI+ I- was realized in the presence of TMOS. T signals are produced by materials ensuing from alkyltrialkoxysilane species while Q
signals are similar to T signals but produced by materials ensuing from tetralkoxysilane species. As in the previous paragraph, comparison with corresponding hydrolyzed species26 has been again made in order to judge the effect of the presence of both TMOS and ionic liquid to the degree of condensation of the solvolyzed gel. Inspection of the relative intensities of the corresponding signals (Table 2) revealed that T3 was the dominant condensation species of the electrolyte BASIC + TMOS + IL2 prepared under solvolysis conditions (40.6%). In this respect, BASIC + TMOS + IL2 behaves similarly to the MTMSPI+ I-/TMOS/HCl (hydrolyzed) gels. In both gels, the amount of T3 species was almost twice as large as with the Q4 or Q3 condensation species of TMOS (Q4 ) 23.6%, Q3 ) 22.5%). The amount of T3 species in the BASIC + TMOS + IL2 was approximately 10% smaller while the inverse was detected for the Q3 species. Q2 was small for both hydrolyzed and solvolyzed gel. Although the real picture is impossible to conceive, it is very likely that the open cube-like species (T4(OH)2 and T7(OH)3) became part of the ensemble of silicon atoms, forming rings or chains all bound to at least one silicon atom of Q4 type. The corresponding extent of self-condensation versus cross-polymerization could not be established because of the absence of QT and TQ species.27,28 The structure is far from regular as indicated by the broadness of the NMR signals. Beyond doubt, the gel electrolytes could be ranked among nanocomposite organic-inorganic materials composed of both organic and inorganic clusters. Indeed, they were formed from clusters of the positively charged polysilsesquioxane embedded in TMOS matrix, while the ionic liquid serves as a percolating phase enhancing transport of ions. Our NMR results suggested that the silica network stemming from TMOS is not intertwined with the silsesquioxane network but formed its own network, relatively weakly disturbed by the presence of the charged silsesquioxanes. The proposed structure of the existing two phases is typical for nanocomposites.26 It is interesting to note that the extent of condensation of TMOS in the BASIC + TMOS + IL2 electrolyte was higher than the corresponding condensation in MTMSPI+ I-/TMOS/HCl gels, probably due to the presence of the ionic liquid MPI+I-. In conclusion, IR and 29Si NMR spectra show that FA solvolysis produces betterbehaving gel electrolytes than HCl-hydrolysis. 3.2. Conductivity and Cell Performance. The ionic conductivity of polysilsesquioxane electrolytes was studied and the results are presented in Table 3, which also summarizes all data obtained by IV characterization of the cells fabricated with each of the above five electrolytes. The BASIC electrolyte, i.e., the polysilsesquioxane ionic liquid mixed with iodine gave a relatively low cell efficiency. This is obviously due to low ionic conductivity since this electrolyte contains no solvent and conductivity is realized only by a Grotthus-type mechanism.29 Addition of TMOS caused a further decrease of short circuit
6532 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Stathatos et al.
TABLE 3: Data Obtained by J-V Characterization of the DSCs Made by Using Each of Five Different Electrolytes electrolyte BASIC BASIC + TMOS BASIC + TMOS + IL1 BASIC + TMOS + IL2 BASIC + TMOS + NMBI + IL2
η σ Voc Jsc (mS/cm) (mV) (mA/cm2) ff (%) 0.1 0.012 0.27 0.32 0.32
500 490 500 510 660
4.8 4.1 5.7 7.4 7.5
0.69 0.69 0.70 0.70 0.70
1.7 1.4 2.0 2.6 3.1
current density Jsc and subsequently of cell efficiency. This result was expected since, in the presence of TMOS, the electrolyte was no more a fluid. The cell using BASIC + TMOS electrolyte is essentially a Solid DSC, but in a sense different from the purely solid-state DSC containing inorganic hole conductors, described in the Introduction. Addition of MPI+I-, that is of a monomeric ionic liquid, increased current and cell efficiency as a function of MPI+I- concentration. In all these cases, the open circuit voltage did not substantially change. The increase or decrease of efficiency was only the result of current variation. It is obvious that the presence of a fluid increases ionic conductivity by allowing for ion mobility.30 Finally, in the last case, a relatively large increase of Voc was observed, as expected in the presence of NMBI, and this was reflected in the large increase of cell efficiency. These solvent-free redox electrolytes make cells, which are stable for several months without apparent variations and without loss of efficiency. Stability of the electrolytes themselves was verified by IR and silicon NMR. This beneficial outcome is first of all due to the absence of liquid electrolyte. It is also due to the stability of the polysilsesquioxane ionic fluid. 4. Conclusions Quasi-solid-state DSCs have been constructed by using a new polymeric ionic fluid as electrolyte. The electrolyte was synthesized by the sol-gel route using MTMSPI+I- as precursor. MTMSPI+I- was synthesized by derivatizing methylimidazolium with triethoxysilane. Condensation of this material in the presence of formic acid and in the absence of water (organic acid solvolysis) led to Si-O-Si-O-type polymerization and formation of a polysilsesquioxane-type structure. This structure was characterized by IR and 29Si NMR spectroscopy. When this material was mixed with iodine it served as redox electrolyte for DSCs. Five different electrolytes have been made by introducing various additives. The presence of TMOS resulted in more robust materials. Addition of monomeric imidazolium iodide increased ionic conductivity, short-circuit current and overall cell efficiency. Presence of NMBI increased open-circuit voltage and cell efficiency. DSCs made by the above material are robust and easy to assemble. Their efficiency is relatively low (3.1%) but there is ground for improvement. Possible,
modification of the organic groups attached to the polysilsesquioxane backbone may be helpful in this direction. References and Notes (1) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortels, F.; Salbeck, J.; Spreitzer, H.; Graetzel, M. Nature 1998, 395, 583. (2) Kron, G.; Egerter, T.; Werner, J. H.; Rau, U. J. Phys. Chem. B 2003, 107, 3556. (3) Kumara, G. R. A.; Konno, A.; Shiratsuchi, K.; Tsukahara, J.; Tennakone, K. Chem. Mater. 2002, 14, 954. (4) Meng, Q.-B.; Takahashi, K.; Zhang, X.-T.; Sutanto, I.; Rao, T. N.; Sato, O.; Fujishima, A.; Watanabe, H.; Nakamori, T.; Uragami, M. Langmuir 2003, 19, 3572. (5) O’Hayre, R.; Nanu, M.; Schoonman, J.; Goossens, A.; Wang, Q.; Graetzel, M. AdV. Funct. Mater. 2006, 16, 1566. (6) Stergiopoulos, T.; Arabatzis, I.; Katsaros, G.; Falaras, P. Nano Lett. 2002, 2, 1259. (7) Wang, H.; Li, H.; Xue, B.; Wang, Z.; Meng, Q.; Chen, L. J. Am. Chem. Soc. 2005, 127, 6394. (8) Wang, P.; Zakeeruddin, M. S.; Comte, P.; Exnar, I.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (9) Wang, P.; Zakeeruddin, M. S.; Moser, J. E.; Nazeeruddin, K. M.; Sekiguchi, T.; Graetzel, M. Nat. Mater. 2003, 2, 402. (10) Dai, Q.; MacFarlane, D. R.; Howlett, P. C.; Forsyth, M. Angew. Chem., Int. Ed. 2005, 44, 313. (11) Suzuki, K.; Yamaguchi, M.; Hotta, S.; Tanabe, N.; Yanagida, S. J. Photochem. Photobiol. A: Chem. 2004, 164, 81. (12) Murai, S.; Mikoshiba, S.; Sumino, H.; Hayase, S. J. Photochem. Photobiol. A: Chem. 2002, 148, 33. (13) Nogueira, A. F.; De Paoli, M.-A.; Montanari, I.; Monkhouse, R.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2001, 105, 7517. (14) Stathatos, E.; Lianos, P.; Lavrencic Stangar, U.; Orel, B. AdV. Mater. 2002, 14, 354. (15) Stathatos, E.; Lianos, P.; Surca Vuk, A.; Orel, B. AdV. Funct. Mater. 2004, 14, 45. (16) Stathatos, E.; Lianos, P.; Jovanovski, V.; Orel, B. J. Photochem. Photobiol. A: Chem. 2005, 169, 57. (17) Stathatos, E.; Lianos, P.; Zakeeruddin, M. S.; Liska, P.; Graetzel, M. Chem. Mater. 2003, 15, 1825. (18) Jovanovski, V.; Orel, B.; Jese, R.; Surca Vuk, A.; Mali, G.; Hocevar, S. B.; Grdadolnik, J.; Stathatos, E.; Lianos, P. J. Phys. Chem. B 2005, 109, 14387. (19) Jovanovski, V.; Orel, B.; Jesˇe, R.; Mali, G.; Stathatos, E.; Lianos, P. Int. J. Photoenergy 2006, article ID 23703, p. 1-8 (20) Stathatos, E.; Lianos, P.; Tsakiroglou, C. Microporous Mesoporous Mater. 2004, 75, 255. (21) Vioux, A.; Leclerq, D. Heterogeneous Chem. ReViews 1996, 3, 65. (22) Sharp, K. G. J. Sol-Gel Sci. and Tech. 1994, 2, 35. (23) Orel, B.; Jesˇe, R.; Vilcnik, A.; Sˇ tangar, U.-L. J. Sol-Gel Sci. and Tech. 2005, 34, 251. (24) Orel, B.; Jesˇe, R.; Sˇ tangar, U.-L.; Grdadolnik, J.; Puchberger, M. J. Non-Cryst. Solids 2005, 351, 530. (25) Stathatos, E.; Lianos, P.; Orel, B.; Surca Vuk, A.; Jesˇe, R. Langmuir 2003, 19, 7587. (26) Orel, B.; Jese, R.; Surca Vuk, A.; Jovanovski, V.; Slemenik Persˇe, L.; Zumer, M. J. Nanosci. Nanotechnol. 2006, 6, 382. (27) Van Bommel, M. J.; Bernards, T. N. M.; Boonstra, A. H. J. NonCryst. Solids, 1991, 128, 231. (28) Prabakar, S.; Assink, R. A.; Raman, N. K.; Myers, S. A.; Brinker, C. J. J. Non-Cryst. Solids, 1996, 202, 53. (29) Kubo, W.; Murakloshi, K.; Kitamura, T.; Yoshigo, S.; Haruki, M.; Hanabusa, K.; Shirai, H.; Wada, Y.; Yanagida, S. J. Phys. Chem. B. 2001, 105, 12809. (30) Jovanovski, V.; Stathatos, E.; Orel, B.; Lianos, P. Thin Solid Films 2006, 511-512, 634.
