Low-Temperature Sintering of TiO 2 Colloids: Application to Flexible Dye-Sensitized Solar Cells

Transcription

1 5626 Langmuir 2000, 16, Low-Temperature Sintering of TiO 2 Colloids: Application to Flexible Dye-Sensitized Solar Cells François Pichot, J. Roland Pitts, and Brian A. Gregg* National Renewable Energy Laboratory, Golden, Colorado Received January 25, In Final Form: April 11, 2000 Colloidal TiO 2 films have been prepared in a manner suitable for use with flexible substrates. Sol-gel particles were sintered under various conditions of temperature and initial presence/absence of organic surfactants. The physicochemical properties of the resulting films are reported. Independent of the sintering temperature, films that did not initially contain organic surfactants adhered more strongly to the underlying F:SnO 2 substrate. The amount of sensitizing dyes adsorbed by a film is also sensitive to the initial presence or absence of organic surfactants and to the sintering temperature. Used as a dye-sensitized anode in photoelectrochemical cells, an open-circuit photovoltage of 647 mv and a short-circuit photocurrent density of 2 ma/cm 2 were obtained for 1 µm thick TiO 2 films sintered at 100 C; normalized to the same thickness, similar results were obtained with films initially containing surfactant and sintered at 450 C. Introduction Solar cells based on large band gap semiconductors sensitized to visible light with dyes have recently emerged as promising inexpensive alternatives to conventional photovoltaic solar cells. 1-4 Upon absorption of light, the dye adsorbed on the surface of the semiconductor injects an electron into the conduction band. The oxidized dye molecule is then regenerated by accepting an electron from a reducing agent present in solution. The cell open-circuit photovoltage is limited to the difference between the quasi- Fermi level for electrons in the semiconductor and the redox potential in the electrolyte solution. 5 Most of the work in this field has been focused on maximizing the cell efficiency. A cell with a 10% conversion efficiency of solar light to electrical power was recently reported by Graetzel et al. 2 To achieve such a high efficiency, visible light absorption by the dye must be as large as possible. Because the dye is present as a monolayer, the internal surface area of the semiconductor, hence the film thickness, must be substantial. The most efficient films, composed of ca. 15 nm TiO 2 colloids, have typically been around 8-12 µm thick. 2 The colloids composing the film must be electrically connected by sintering them together so that electrons injected anywhere in the film can be collected at the underlying conductive substrate. The method of choice for reaching these goals has been the following: After synthesis of the TiO 2 colloids via the sol-gel process, a surfactant (typically poly(ethylene glycol)-based) is added, resulting in a viscous paste that can be deposited on a substrate by blade-coating or screen printing. The exact role of the added surfactant is not clear at present. Because it renders the colloidal solution viscous, it helps in preparing crack-free thick films (10 µm) in one deposition. 6 Furthermore, the resulting sintered (1) O Regan, B.; Grätzel, M. Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Müller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. J. Am. Chem. Soc. 1993, 115, (3) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, (4) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Perera, V. P. S. Chem. Commun. 1999, 15. (5) Pichot, F.; Gregg, B. A. J. Phys. Chem. B 2000, 103, 6. (6) Barbé, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzman, F.; Shklover, V.; Grätzel, M. J. Am. Ceram. Soc. 1997, 80, films display an increased porosity, which improves ionic conductivity via better mass transport of the iodide/ triiodide redox couple. 6 Temperatures around 450 C are used to eliminate the organic compounds initially present and sinter the colloids together to obtain an electrically connected network. Dye-sensitized solar cells potentially could be used also in low power applications such as pocket calculators (5 µw) and watches (2 µw), 7 as well as photoelectrochromic windows. 8 With low power requirements, light absorption by the dye does not need to be maximized; hence the TiO 2 film can be relatively thin. While this loss in absorption results in a proportional loss in photocurrent density, the cell photovoltage is only modestly affected. For practical reasons such as flexibility, weight, and overall device thickness, it would be advantageous to deposit these films on flexible organic substrates. For instance, we recently demonstrated that photoelectrochromic devices based on a dye-sensitized photoanode could be made on flexible substrates, 9 which may facilitate their development for retrofitting existing office windows. The introduction of organic substrates limits the available temperature range for processing the TiO 2 colloidal film: Plastics usually cannot withstand 450 C treatments without decomposing. Furthermore, the more heat resistant a plastic is, the more expensive and colored it is, as a result of the introduction of aromatic rings in its structure. We report here a comparison of various properties of TiO 2 films prepared via the traditional method and prepared without any organic surfactant and sintered at 100 C. Experimental Section Materials and Measurements. All chemical reagents, LiI (99%, Aldrich), Carbowax (Aldrich), methoxypropionitrile (Aldrich), and acetonitrile (spectrograde, Burdick & Jackson), were commercial grade. Conductive glass substrates (Libbey Owens Ford, 8 W/0, SnO 2) were washed with Liqui-Nox detergent, (7) Sommeling, P. M.; Späth, M.; van Roosmalen, J. A. M.; Meyer, T. B.; Meyer, A. F. Proceedings of the 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, (8) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383, 608. (9) Pichot, F.; Ferrere, S.; Pitts, R. J.; Gregg, B. A. J. Electrochem. Soc. 1999, 146, /la000095i CCC: $ American Chemical Society Published on Web 05/26/2000

2 Sintering of TiO 2 Colloids Langmuir, Vol. 16, No. 13, abundantly rinsed with deionized water, rinsed with ethanol, and dried in a nitrogen stream. Absorbance spectra were measured on a Hewlett-Packard 8453 diode array spectrophotometer. Current-voltage measurements were made in a two-electrode sandwich configuration using a platinum counterelectrode and were monitored with a Keithley 236 source measure unit. White light approximating AM 1.5 solar emission for the wavelength region between 400 and 800 nm was emitted from a 75 W xenon arc lamp (PTI, model A 1010). The light intensity was attenuated as necessary with neutral density filters. Current-voltage curves were generated using a program written in LabVIEW. The thickness of the TiO 2 films was measured with a Veeco profilometer model Dektak3. Scanning electron micrographs were taken with a JEOL JSM 6320F field emission scanning electron microscope. To test the adhesion of the various TiO 2 films to the underlying SnO 2:F substrate we used ultrasound disruption in the following manner: The electrodes were completely submerged in deionized water in a beaker and submitted to ultrasonic vibrations (L&R Ultrasonicator bath model T14B, 95 W) for various amounts of time. The electrodes were subsequently dried in a nitrogen stream and the presence/absence of TiO 2 film was verified by observation in a microscope with epi-illumination and visual observation of optical interference fringes. The percentage of the film still covered with the TiO 2 film was estimated by placing the film over graph paper and counting squares. TiO 2 Colloids. The TiO 2 colloidal particles were synthesized according to a literature preparation, 10 where nitric acid was substituted for acetic acid. The autoclaving step at 230 C results in the formation of nanocrystalline anatase particles. Thus, all the films reported here consist of TiO 2 in the anatase phase, even those sintered at only 100 C. Films Prepared without Surfactant. Prior to deposition, a small aliquot of the TiO 2 colloidal solution was filtered through a 450 nm filter (Acrodisc LC13, Gelman) in order to remove aggregated colloids. The substrates were then completely covered with the colloidal solution and immediately spun at speeds ranging from 800 to 3000 rpm. A narrow stripe of the TiO 2 film at the edge of the substrate was removed with a water-soaked cotton swab in order to facilitate subsequent profilometry measurements and for making electrical contacts to the electrodes. The films were finally sintered in air either at 100 C for 24 h or at 450 C for 30 min. These films are respectively referred to as T100 and T450. Films Prepared with Surfactant. These films were made using the standard procedure for dye-sensitized solar cells. A surfactant, Carbowax, was dissolved (40 wt % of dry TiO 2 content) in the TiO 2 colloidal solution. This solution was blade-coated or spin-coated on the conducting glass substrates, and the films were then sintered at 450 C for 30 min. These films are referred as CarboT450. All films (T100, T450, and CarboT450) were deposited on rigid glass/f:sno 2 substrates in order to facilitate comparisons. The low-temperature treatment is also applicable to flexible polyester/ito substrates as we demonstrated previously. 9 Dye Adsorption. The dye, cis-di(thiocyanato)bis(2,2 -bipyridyl-4,4 -dicarboxylate)ruthenium(ii) (Solaronix), commonly termed N3, was used as the sensitizer. TiO 2 electrodes were dipped in a 0.5 mm solution of N3 in ethanol for 12 h and subsequently rinsed with ethanol and then dipped in ethanol for 12 h to desorb any excess dye. Before being used in cells, these films were dried in a nitrogen stream. Results and Discussion Film Preparation. Depending on both the initial viscosity of the TiO 2 colloidal solution and the spinning rate, films of different thickness were obtained. If the solution was too viscous (more than approximately 20% w/w TiO 2 ), nonuniform films were obtained: The films were thicker in the middle than on the edges. The same thickness profile would result when the spinning rate was set below approximately 700 rpm. Macroscopic cracks (10) Zaban, A.; Ferrere, S.; Sprague, J.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 55. Figure 1. Metal to ligand charge transfer (MLCT) absorbance of N3 adsorbed on TiO 2 films prepared by different procedures, as a function of film thickness: b, T100 films; O, T450 films; 1, CarboT450 films. would also form below 700 rpm, when the evaporation rate could compete with centrifugal mass transfer. The maximum thickness of uniform crack-free films obtained via spin coating was 1 µm. To obtain thicker films, we employed successive spin-coating depositions, drying the films for 15 min at 100 C between applications. We do not envision any limitation in film thickness using this deposition technique. Film Morphology. Films prepared from the same initial TiO 2 colloids but deposited and thermally treated under different conditions adsorb different amounts of dye (Figure 1). The films prepared via the traditional method, i.e., initially containing surfactant and sintered at 450 C for 30 min, absorb the smallest amount of N3 dye. Others have shown that increasing the amount of surfactant in the deposition paste increases the final porosity of the sintered film. 6 In other words, the surfactant dilutes the colloids to some extent, and during the sintering process fewer colloids actually bind to each other. As a result, more void volume, and therefore less surface area per given thickness, is present in the sintered film. This explains the ratio of dye loading between T450 and CarboT450 (R T450/CarboT ). The difference between the two films without surfactant but sintered at different temperatures, T450 and T100, is also substantial. The ratio, R T100/T , is in agreement with a linear extrapolation of published data concerning TiO 2 surface area as a function of sintering temperature. 6 The loss of surface area in T450 films probably reflects an increased bridging between adjacent TiO 2 nanoparticles as compared to T100 films sintered at a lower temperature. It is worth noting that the method used here, dye adsorption, is not a direct method for surface area determination. It is possible that, in addition to the change of surface area as a function of sintering temperature, surface chemistry also plays a role in the adsorption behavior of N3 on T100 and T450. At present, it has not been determined if a difference in TiO 2 surface composition (charge, hydroxyl group concentration) between the T100 and T450 samples exist. Cross-sectional scanning electron microscopy (SEM) pictures of typical CarboT450 and T100 samples are displayed in Figure 2. The particles in the T100 film appear to be quite densely and uniformly packed. Thus the T100 film density seems approximately uniform without noticeable voids either in the bulk film or at the SnO 2 /TiO 2 interface. In contrast, the CarboT450 film shows large

3 5628 Langmuir, Vol. 16, No. 13, 2000 Pichot et al. Figure 2. Cross sectional SEM pictures of TiO 2 samples on SnO 2/glass substrates: A, T100 film; B, CarboT450 film. Figure 3. Results of the adhesion test performed by sonication of the films in water: b, T100 films; O, T450 films; 1, CarboT450 films. voids between clusters of densely packed particles. Therefore, there is more void volume in these films (and correspondingly less dye is adsorbed per film thickness). Also, fewer TiO 2 colloids are in contact with the SnO 2 :F substrate in the case of CarboT450 compared to T100 films. As we will discuss in the following sections, the morphology of the SnO 2 /TiO 2 interface may play an important role in mechanical as well as electrical properties of the photoelectrochemical devices. Adhesion Test. Evaluating the mechanical properties of porous thin films is a difficult task. The test we devised is rudimentary, but nonetheless instructive and surprisingly sensitive for the present comparative study. We believe that this test is probing the SnO 2 :F/TiO 2 interface based on the following observations. When films were prepared for TEM analysis (scraping with a razor blade from the SnO 2 substrate followed by sonication) only large area platelets of TiO 2 films were observed. Second, during the sonication test, the solutions did not become opalescent, as they did when nonsintered films were sonicated. Finally, when the loss of TiO 2 is observed during the sonication test, only bare SnO 2 :F and intact SnO 2 :F/ TiO 2 are apparent when observed either in the microscope or by optical interference fringes. Therefore, we infer that it is the SnO 2 :F/TiO 2 interface that constitutes the weakest point of the structure and that is disrupted first by the ultrasound treatment. As can be seen from Figure 3, the films that initially contain the organic surfactant are detached much faster upon sonication than those that do not contain surfactant. This result is consistent with the reduced SnO 2 /TiO 2 contact area of CarboT450 films observed by SEM (Figure 2) as a result of the more porous morphology of these films. We also note that T450 films do not appear to adhere more strongly than T100 films. Correlation between these observations and the long-term stability of photoelectrochemical cells based on these films remains to be done. Photoelectrochemistry. The I-V behavior of a typical cell composed of a 1 µm thick film sintered at 100 C is shown in Figure 4. Under 1 sun illumination, an opencircuit voltage of 647 mv, a short-circuit current density of 2.05 ma/cm 2, and a fill factor of 69% were obtained, yielding an overall 1.22% light-to-electricity efficiency. Similar to films sintered at 450 C, the current density of the 100 C films roughly scales linearly with light intensity (data not shown). The incident photon to current efficiencies (IPCE) of thin films prepared under different conditions are modest (Figure 5): 0.50 for CarboT450, 0.32 for T450, and 0.23 for T100. These values are low because, as discussed previously, the film thickness obtainable for the spin-coated samples is limited by the deposition technique, and the blade-coated sample was prepared as thin as possible for appropriate comparison. Better insight into the cell functioning is gained by considering the absorbed photon to current efficiencies (APCE ) IPCE/[1 - transmittance]) plotted in Figure 6. Because the optical densities of the films studied were much less than 1, the reflectivity of the back contact platinum electrode has to be taken into consideration. In good agreement with values reported in the literature, 2 the APCE for films sintered at 450 C is larger than 0.8, meaning that 8 out of 10 photons absorbed by the dye are converted and collected as electrons in the external circuit. There is little difference in APCE between the T450 and CarboT450 films. T100 films, on the other hand, display an APCE of approximately One possible explanation for the lower APCE of T100 films is a somewhat inefficient charge transport process of electrons through the TiO 2 film and/or of holes (I 3- ) through the pores of the film. The recombination of holes and electrons before they are collected at their respective substrate electrodes would yield a lower APCE. Supporting this explanation is the fact that the slope of the opencircuit voltage versus incident light intensity is 190 mv

4 Sintering of TiO 2 Colloids Langmuir, Vol. 16, No. 13, Figure 4. Current-voltage curve for a 1 µm thick T100 TiO 2 film sensitized with N3: continuous line, in the dark; dotted line, under 75 mw/cm 2 white light illumination. The electrolyte is composed of 0.5 M LiI, 0.05 M I 2, and 0.2 M tert-butylpyridine in acetonitrile; the counterelectrode is a platinum-coated SnO 2 electrode. The insert shows the visible absorption spectrum of the sensitized film. Figure 6. Absorbed photon to current efficiency spectra of same N3-sensitized TiO 2 films as in Figure 5: b, 1.7 µm thick CarboT450 film (A MLCT ) 0.27); O,1µm thick T450 film (A MLCT ) 0.15); 1, 0.9 µm thick T100 film (A MLCT ) 0.24). Figure 5. Photoaction spectra (IPCE) of TiO 2 films sensitized with N3: b, 1.7 µm thick CarboT450 film (A MLCT ) 0.27); O, 1 µm thick T450 film (A MLCT ) 0.15); 1, 0.9 µm thick T100 film (A MLCT ) 0.24). The electrolyte was composed of 0.5 M LiI, 0.05 MI 2, and 0.2 M tert-butylpyridine in methoxypropionitrile. per decade (see Figure 7), yielding a rectification coefficient of 3.2. This value is higher than those of dye-sensitized solar cells based on TiO 2 films sintered at 450 C, usually between 1 and While the mechanism for charge separation and transport in dye-sensitized cells is believed to be different than that for solid-state photovoltaic cells 5 (from the study of which, most theoretical models derive), the rectification coefficient still may have some relevance to charge transport. Large rectification coefficients have been correlated to the large number of surface electronic states within the porous TiO 2 film. 11 The density of such surface states, which can act as recombination sites, may decrease at the higher sintering temperature. At low temperatures, the necking between adjacent TiO 2 colloids will presumably not be as pronounced as for films sintered at higher temperatures. Such a diminished contact area (11) Liu, Y.; Hagfeldt, A.; Xiao, X.-R.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1998, 55, 267. (12) Huang, S. Y.; Schlichthörl, G.; Nozik, A. J.; Grätzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, (13) Sodergern, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S.-E. J. Phys. Chem. 1994, 98, Figure 7. Open circuit voltage as a function of incident white light intensity for N3-sensitized T100 TiO 2 films (b, 600 nm thick film; O, 900 nm thick film). Both linear regressions have a slope of 190 mv per decade. The electrolyte was composed of 0.5 M LiI, 0.05 M I 2, and 0.2 M tert-butylpyridine in methoxypropionitrile. between neighboring colloidal particles could lead to an increase in surface recombination between TiO 2 conduction band electrons and holes present in the surrounding electrolyte. The fill factor of low-temperature films ranges between 60 and 66% and is always lower than for films of the same thickness prepared under the same conditions but higher sintering temperature. This is consistent with the assumption that charge transport, whether electrons and/or holes, in the T100 films is not as efficient as for T450 and CarboT450films. Conclusion We have shown that dye-sensitized solar cells can be made suitable for flexible substrates by applying a low sintering temperature (100 C) to TiO 2 colloidal films in the absence of organic surfactant. The photoelectrochemical behavior of these films sensitized with the N3 dye is similar to those of conventional systems in which the films are sintered at 450 C. The APCE of sensitized films sintered at 100 C is lower than that of classic hightemperature films. However, as a result of the lowtemperature treatment, these films have a larger internal surface area and can therefore adsorb more dye for an equivalent film thickness. This effective increase in light absorption coefficient compensates somewhat for the efficiency loss. We also show that, regardless of the

5 5630 Langmuir, Vol. 16, No. 13, 2000 Pichot et al. sintering temperature, films prepared without organic surfactant tend to adhere better to the underlying F:SnO 2 substrate, a factor which may be important for the durability of devices made from these films. Acknowledgment. We thank Dr. Rick Matson for taking the SEM pictures. This work was supported by the US Department of Energy. We thank the following for financial support: The National Center for Photovoltaics (F.P.), NREL s FIRST program (F.P., R.P.), and the Office of Science, Division of Basic Energy Sciences, Chemical Sciences Division (B.G.). LA000095I