Patent Application
20140199805
1. Field of the Invention
The invention relates generally to photovoltaic electrodes.
2. Description of the Prior Art
Grätzel and coworkers introduced the porous, nanocrystalline dye-sensitized photovoltaic electrode (dye-sensitized solar cell, DSSC) in 1991. (Regan et al., “A Low-Cost, High Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Films,” Nature, 353, 737-740. All referenced publications and patents are incorporated herein by reference.) Derived from surfactant-templated colloid chemistry, the nanocrystalline interface improved the performance of dye-sensitized semiconductor photoelectrodes by amplifying available surface area to which sensitizing dyes can adsorb, yielding effective surface areas about 500-fold higher than the geometric areas of the film. The high effective concentration of dyes within the film, along with the further development of very efficient, broad-spectrum sensitizing dyes, results in efficient absorption of photons through much of the visible spectrum. Fast electron injection and thermalization kinetics result in efficient injection of dye electrons into the conduction band of the semiconductor film and little competition from direct recombination with the oxidized dye. Charge-transfer mediators easily permeate mesoporous nanocrystalline semiconductor films (typically anatase TiO2), recharging adsorbed oxidized dyes. The best performance to date with Grätzel cells has yielded global efficiencies of over 10% at 1 sun intensity at AM 1.5 conditions.
One of the remarkable aspects of the Grätzel cell is that the incident photon-to-current conversion efficiency (IPCE) spectrum is much broader than the solution spectra of the dyes. In particular, absorbance in the red portion of the spectrum is higher than would be inferred from solution-phase extinction coefficients of the dyes. The enhanced efficiency in the red is due to the amplified surface area of the nanocrystalline film. Sufficient absorbers are immobilized to give incident photons multiple occasions to be absorbed by TiO2-bound dye molecules, either by simple element (absorber) redundancy, or by scatter of photons within the film.
Analysis of best performance of the ruthenium-polypyridyl-based dyes, N3 and “the black dye” suggests that global efficiencies could be improved over the current benchmark of 10.4% (which has been unchanged for about 10 years) if IPCE could be increased to near unity between 700 and 900 nm.
Further increasing the specific surface area has been precluded by the current art. For one, the film architecture is fixed and presumed to be optimized. The colloid chemistry, surfactant type, and fractions of solid-to-surfactants have been rigorously explored. Increasing roughness is not an option unless a different film architecture is introduced.
Increasing film thickness has also been presumably eliminated, as most reports describe films no thicker than 12 μm being consistently achievable by the current art. This limit is likely due to two reasons. The more practical reason is that the colloidal pastes do not yield high-quality films at a thickness much greater than 10 μm, because thicker films tend to crack. The second reason is that random-walk statistics of percolative diffusion models for photoelectrons in nanocrystalline semiconductor films predict loss of electron collection efficiency in the presence of excess diffusion space. An outer boundary excessively distal from the current collector may diminish efficiency due to an increased probability of interfacial recombination events as the electron wanders through the semiconductor. The utility of thicker films will depend critically on controlling the surface character of the nanocrystalline film so as to maximize diffusion lengths of electrons within the films and increase the probability of electrons reaching the current-collecting back contact.
The invention comprises a photoelectrode comprising a conductive lead and a titania aerogel in electrical contact with the lead. The aerogel is coated with a photosensitive dye.
The invention further comprises a process of making a photoelectrode comprising the steps of: providing a conductive substrate, providing a titania aerogel paste, forming a film of the paste on the substrate, and coating the film with a dye.
A more complete appreciation of the invention will be readily obtained by reference to the following Description of the Example Embodiments and the accompanying drawings.
In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the description of the present invention with unnecessary detail.
A strategy is disclosed to fabricate high surface area, ultraporous, nanocrystalline semiconductor films for use in solar cells, with the goal of significantly bettering the performance of the state-of-the-art photoelectrochemical cells using dye-sensitized nanocrystalline semiconductor electrodes. Specifically, TiO2 aerogels are used as the photoanode material in a dye-sensitized photovoltaic electrode. The semiconductor is expressed as an aerogel due to the very high specific, active surface area and the bicontinuous pore-solid network that the aerogel architecture offers. The high surface area of aerogels allows immobilization of large amounts of sensitizing dyes within the porous volume, thus enabling superior light utilization in dye-sensitized photovoltaics, and offers a large reactive surface area for use in the absence of dyes. The continuous mesoporous network permits high diffusion rates of liquid-phase reactants to the photoelectrode surface, in both sensitized and unsensitized photoelectrodes.
Aerogels are high-surface area, highly porous (˜80-99% porosity) nanostructured materials derived from sol-gel synthesis and supercritical fluid processing methods. (Hüsing et al., “Aerogels—Airy Materials: Chemistry, Structure and Properties,” Angew. Chem. Int. Ed., 37, 23-45 (1998).) Aerogels can be made from any material that can be processed as a gel. Outstanding properties include superior surface areas (100-1000 m2/g and more specifically 150-200 m2/g for calcined TiO2) and a bicontinuous pore-solid network. Primary particles are sized between 10 to 20 nm. The pore network is primarily mesoporous, having a majority pore distribution between 5 to 50 nm. Aerogels are distinguished from the more commonly known xerogels by their relatively greater porosity, but more importantly by the continuity of the pore network throughout the solid, which facilitates diffusive mass transport at near open-medium diffusion rates. Aerogel porosity results from replacement of the pore-filling fluid with liquid carbon dioxide, for example, and subsequent supercritical extraction of the carbon dioxide. These are zero surface tension processes. Supercritical fluid extraction, or supercritical drying, of the wet gels prevents collapse of the pore structure of the wet gel that occurs when drying sol-gel-derived materials by direct evaporation of solvent (which yields xerogels).
The high surface area and fast diffusive mass-transport rates have spurred investigations into the application of aerogel materials as catalysts, battery materials, sensor materials, and supports for fuel-cell catalysts. Expression of functional materials as aerogels has yielded improvement in performance over analogous materials made by other means, and in some cases has revealed new mechanistic components in complex interfacial processes. One-, two-, or more-layer titania aerogel-based photoelectrodes may be fabricated. The high surface area and outstanding diffusional mass-transport characteristics and the approximately fixed bicontinuous, nanoscopic network of titania aerogels can be exploited to achieve IPCE values at 700 nm equivalent to or better than state-of-the-art nanocrystalline electrodes.
In one step of the process, a conductive substrate is provided. The substrate can be any substrate known in the art of photoelectrodes and equivalents thereof, including but not limited to, glass having a fluorine- or indium-doped tin oxide coating. The substrate may be transparent to facilitate the transmission of light though the substrate to the photoactive part of the photoelectrode.
In another step of the process, a titania aerogel paste is provided. The paste may comprise a titania aerogel powder, a surfactant, and a solvent. The paste may be prepared in any manner for combining the ingredients into a paste form. Methods of making titania aerogel into a paste as also known in the art, including, but not limited to, grinding the ingredients together. The pores of the aerogel may have an average size in the range of, but not limited to, about 5 nm to about 50 nm. The aerogel powder may have an average particle size in the range of, but not limited to, about 5 nm to about 20 nm. The powder may be calcined at, for example, about 400° C. or about 425° C.
Suitable surfactants include, but are not limited to, octyl phenol ethoxylate. Suitable solvents include, but are not limited to, a mixture of water and acetylacetonate.
In another step of the process, a film of the paste is formed on the substrate. The film may be formed by any process known in the art for forming a film from a paste, and include, but are not limited to, forming a layer of the paste, drying the layer, and calcining the dried layer. Calcining may be done, for example at about 475° C. or about 500° C.
In another step of the process, the film is coated with a dye. The coating may be done by any method known in the art for coating an aerogel film with a dye. One method is to apply an ethanolic solution of the dye to the film. This step may be done while the substrate is at an elevated temperature such as, but not limited to, about 70° C. to about 100° C. Suitable dyes include, but are not limited to, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (N719) and [bis(2,2′-bipyridine)][(4,4′-(n-diethyl ester-2,2′-bipyridine)]ruthenium(PF6)2 (Ru(deeb)(bpy)22+).
The resulting film may have a thickness of, but not limited to, about 2 μm, about 10 μm, about 20 μm, about 30 μm, about 40 μm, and any thickness in between these values. The film may also be made by forming a plurality of layers from the paste. Such layers may have thicknesses of, but not limited to, about 0.2 μm to about 10 μm. The short wavelength advantages of more finely ground aerogel powders can be added to the long wavelength, long path length advantages of sintered films as long as electrical conductivity between aerogel pieces is good. Preliminary data show that this approach is effective at converting light to electricity through a broader range of the visible spectrum than either the fine powder-derived thin films or the coarse powder-derived thick films. Pastes derived from either coarsely or finely ground aerogels can be used for the second, third, or nth layer.
The resulting structure may be useful as a photoelectrode. The photoelectrode may be made by other processes than those described here and comprises a conductive lead and a titania aerogel coated with a photosensitive dye. The lead can be the substrate as previously described and the aerogel may be in the form of a film coated on the substrate, as previously described. The film may comprise a powder of the titania aerogel or a monolithic aerogel. The photoelectrode can also comprise an electrolyte in contact with the aerogel and a cathode in contact with the electrolyte.
Recently published work focusing on improving performance of dye-sensitized photovoltaic performance in the red portion of the spectrum has included adding a layer of 400-nm titania colloids to improve light scattering within the film (Nazeeruddin, M. K. et al. “A Swift Dye Uptake Procedure for Dye Sensitized Solar Cells,” Chem. Commun. (2003) 1456-1457) and similarly, addition of a colloidal layer that acts as a photonic bandgap material, creating a stop band and also improving light scattering within the film (Nishimura, N. et al. “Standing Wave Enhancement of Red Absorbance and Photocurrent in Dye-Sensitized Titanium Dioxide Photoelectrodes Coupled to Photonic Crystals,” J. Am. Chem. Soc. 125 (2003) 6306-6310). The processing flexibility lent by the pre-formed TiO2 aerogels may allow for accessing thicker films without losing electrical connectedness to the current-collecting FTO contact. The solid part of the nanoscopic aerogel pore-solid network is continuous and therefore electrically “self-wired.” Since as-prepared TiO2 aerogels are millimeter-sized pieces, which can be ground as coarsely or as finely as desired, films tens of micrometers thick can be readily made. Longer path lengths are more critical at longer wavelengths.
Analysis of contemporary colloidal TiO2 films show specific surface areas of about 105 to 125 m2/g are achieved in the best performing films, comparable to values of 80-100 m2/g for the best performing aerogel films produced thus far. Use of lower calcination temperatures may result in still higher specific surface areas in the aerogel films.
Titania aerogels can offer advantages in processing flexibility in that they have a pre-programmed architecture that is similar to the architecture, both in terms of surface area and percentage porosity, to the nanocrystalline films typically used in Grätzel -type DSSCs. While the microscopic density of these films may be similar to that of colloid-derived nanocrystalline films, the macroscopic density of our course films may be somewhat lower and can result in somewhat less dye immobilized in the first 2-3 μm of film near the current collector. More translucent films derived from more finely ground aerogel powders may cut down scattering somewhat, as well as facilitating more absorption of shorter wavelength light closer to the current collector, yielding better performance at higher intensities.
Titania aerogels may also serve as effective top layers on conventional nanocrystalline films. As the conventional nanocrystalline films are nearly optimized for performance at full solar intensities, an additional layer, which better harvests photons in the red portion of the spectrum, may be advantageous, and perhaps may perform better than the colloidal scattering layers now employed.
Having described the invention, the following examples are given to illustrate specific applications of the invention. These specific examples are not intended to limit the scope of the invention described in this application.
Preparation of TiO2 aerogel—Titania aerogels were prepared in a manner similar to that described by Dagan et al., “TiO2 Aerogels for Photocatalytic Decontamination of Aquatic Environments,” J. Phys. Chem. 97, 12651-12655 (1993). An ethanolic solution of titanium (IV) isopropoxide was added to a stirred mixture of H2O, ethanol, and a catalytic amount of nitric acid (typically 63 mg of 70% nitric acid), yielding a firm, clear gel in minutes. The gel was subsequently aged (typically overnight), rinsed with acetone multiple times over several days, and loaded under acetone into a supercritical dryer (Fisons Bio-Rad E3100) and rinsed with liquid CO2 before taking the liquid CO2 above its critical temperature and pressure (Tc=31° C., Pc=7.4 MPa). The supercritical drier was vented to atmospheric pressure, and the carbon dioxide was released as a gas. The titania aerogels were removed from the dryer and heated in a vacuum oven to remove water at about 100° C. and residual organics at about 200° C., and then calcined in a muffle furnace at 350-425° C., to yield coarse, translucent white pieces, millimeters in size. The titania aerogels were ground to a white powder with an agate mortar and pestle and characterized for surface area and porosity using nitrogen physisorption measurements (at 77K) using a Micrometrics ASAP 2010 accelerated surface area and porosimetry system. Inspection of the nitrogen physisorption isotherm reveals a mesoporous material with pores that are open at both ends. Pore distribution for the aerogel, computed using density functional theory analysis software, is shown in
Porosity and surface area data for representative titania aerogels are summarized in Table 1. Titania aerogels calcined at 425° C. are mesoporous, nanocrystalline anatase materials that are ˜70% porous with specific surface areas of ˜140 m2/g. The nitrogen physisorption isotherm is characteristic of a mesoporous material with pores that are open at both ends. The pore-size distribution for the titania aerogel is shown in
X-ray diffraction of titania aerogels calcined at 425° C. is shown in
Preparation of aerogel film—Calcined titania aerogel films were constructed by adapting aerogels to the methods of Nazeeruddin et al., “Conversion of Light to Electricity by cis-X2Bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) Charge Transfer Sensitizers (X=Cl−, Br−, I−, CN−, and SCN−) on Nanocrystalline TiO2 Electrodes,” J. Am. Chem. Soc. 115, 6382-6390 (1993). The preparation called for (1) the grinding of 12 g of Degussa P25 with about 4 mL of water and 0.4 mL of acetylacetone (which serves to prevent re-aggregation of particles) in a mortar and pestle, followed by (2) incremental addition of 16 mL of water with continued grinding, followed by the addition of 0.2 mL Triton-X 100. The composite paste was then spread on fluorine-doped tin oxide-coated glass (FTO) and fired at 450-550° C. in air. Titania aerogel preparations were limited to about 1.5 g per batch, primarily by the volume capacity of the supercritical dryer, so the Nazeeruddin procedure was appropriately scaled. The textures of the paste were varied from very viscous to very watery. Viscous to moderately viscous pastes (Method A) resulted from using 0.6 g of TiO2 aerogel, 0.66 mL of water and about 0.22 mL of 50 mg/mL of Triton-X 100 (Aldrich) in water. Ten to 50 μL of acetylacetone (Aldrich) were added to the paste just as the grinding was begun. More water (˜0.5-1 mL) was added to thin the paste sufficiently for making films. This paste was spread with a glass pipette onto fluorine-doped tin oxide-coated glass (Pilkington Glass) masked with tape (˜60-μm thick),
Thick films derived from Method A were topologically rough and nearly opaque. A typical thick film was uneven with features comprising 40-50-μm-thick plateaus and valleys in the 10-μm range. A thinner film made by Method B was more continuous but equally rough. Qualitatively tuning the viscosity of the pastes to intermediate values generated rough but continuous films of intermediate thickness. A two-layer film derived from a thin first layer and a thicker second layer had a rough, continuous topology and a thickness of ˜30 μm.
Scanning electron microscopy, shown in
Dye coating—Films were coated with dye by soaking in mM ethanolic solutions of cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (which goes by aliases, including: RuL2(NCS)2:2 TBA, or Ruthenium 535 bis-TBA, or N719) from Solaronix (Switzerland), or mM solutions of [bis(2,2′-bipyridine)][(4,4′-(n-diethyl ester-2,2′-bipyridine)]ruthenium(PF6)2, abbreviated Ru(deeb)(bpy)22+ (a gift from Johns Hopkins University) in CH3CN, overnight. The films were typically removed from the furnace during cool-down from the calcination while still at ˜80° C. to minimize the level of adsorbed water in the film and then soaked in the sensitizer solutions while still warm.
Properties of coated films—Photoaction spectra were taken using a home-built photoelectrochemical cell 90 schematically shown in
Preliminary IPCE data using Ru(deeb)(bpy)2(PF6)2 as a sensitizer were gathered while determining effects of film processing and thickness on performance. Photoaction spectra of thick TiO2 aerogel films sensitized with Ru(deeb)(bpy)2(PF6)2 in 0.5 M LiI/0.050 I2/CH3CN are shown in
Sensitization with N719—Efficiency depends on both film thickness and excitation intensity. To make more meaningful comparisons to the current state of the art, RuL2(NCS)2:2 TBA, or N719 dye was introduced to the films.
The methods were further modified to attempt to fully exploit the advantage of being able to pre-program the aerogel architecture (particularly porosity and surface chemistry) before casting as a film. Films were cast from pastes derived from aerogels that were calcined initially at lower temperatures ˜350° C., to decrease the crystalline content of the films before subsequent calcinations. The reasoning was that titania (and oxides generally) treated at lower temperatures are richer in surface —OH groups and surface water, which leave the surface active towards further condensation. Upon subsequent calcinations, such surfaces can readily condense with those of other micrometer-sized aerogel pieces within the film. Calcination of the first cast layer was then performed at 400° C., again to keep the layer “active” towards thermally driven condensation chemistry with the second layer. After addition of the second layer, with a post-treatment with TiCl4 still remaining, it was calcined at 425 ° C. After soaking in ˜1 M aq. TiCl4, the multilayer film was calcined at 470° C. and sensitized with N719. Photoaction spectra in
Experiments at higher light fluence—Preliminary experiments performed on a solar simulator yielded photocurrents of ˜1.5 mA/cm2 at 0.5 cm2 electrodes, compared to over 20 mA/cm2 for the state-of-the-art nanocrystalline electrodes. Very good open-circuit photovoltages of 0.75 V were measured under the same conditions, which is approximately equivalent to those in the best nanocrystalline films. Integration of current-voltage curves gave global efficiency of roughly 0.2%. Given the IPCE values measured at the same electrodes, the photocurrents generated under simulated sunlight were somewhat puzzling. Intensity-dependent IPCE studies on sensitized 1- and 2-layer aerogel films were performed. Photoaction spectra at intensities of 0.5-2 mW/cm2, with ˜0.25-cm2 spot sizes and excitation linewidths of about 15 nm (estimated from the slit widths in the spectrometer) yielded the relatively high IPCE values shown in
The photoaction spectra in
The difference between efficiencies achieved at low light fluence and at higher intensities is likely wavelength-dependent as well as intensity-dependent.
Alternately, the coarse nature of the films, while probably making them more scattering in nature, may also allow penetration of light sufficiently deep into the films such that the electrons are injected into films at distances from the current-collecting FTO contact that are greater than the electron diffusion lengths. Peter et al., J. Phys. Chem. B 104, 949-958 (2000) showed through modeling and intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) that IPCE should be independent of intensity, due to the fact that lifetimes and diffusion lengths of injected electrons are intensity dependent in opposite senses, unless the distance of the electron injection from the current collector exceeds diffusion lengths of the electrons. In such cases an electron concentration profile is generated that peaks within the film, and electrons can diffuse both towards and away from the current collector. The backscattering explanation seems more likely, given that 700-nm light penetrates the film more deeply (thus resulting in possible electron injection at all distances from the current collector) than the shorter wavelengths, yet does not seem to suffer such losses in efficiency at higher intensities.
It is also possible, given the thick cell geometry, that a depletion layer is generated at the counter electrode at higher intensities. Current-time plots at higher light intensities (not shown) reveal a current that decays to a steady-state over seconds to minutes, depending on wavelength and intensity of light.
Barbé et al., J. Am. Ceram. Soc. 80, 3157-3171 (1997) report a loss of efficiency at 1 Sun intensity compared to 1/10 Sun intensity when using films with average pore sizes of 4 nm, while realizing no losses when the average pore size is closer to 20 nm. They attribute this loss to mass-transport limitations in the smaller-pore film. Here, the average pore size is ˜8 nm before treatment of the film with TiCl4, and may shrink somewhat upon treatment of the film with TiCl4. It is possible that the combination of small pore size and thick films may conspire to create a mass-transport bottleneck in the films at higher intensity.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described.
This application is a divisional application is U.S. patent application Ser. No. 11/052,887 filed on Feb. 9, 2005.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11052887 | Feb 2005 | US |
| Child | 14065452 | US |
