The present disclosure relates to solar cells, and in particular to dye materials used in dye-sensitized semiconductor and organic solar cells including a method for fabricating such dye materials directly from coal.
Worldwide energy demand will reach about 28 terawatts per year by 2050. In 2008, annual worldwide consumption of energy was about 14 terawatts of energy which means that worldwide energy production must be doubled to meet projected demands for energy. At present, there exist three main options for meeting the projected 14 terawatts of energy demand: fossil fuel, nuclear power, and renewable energy sources. A primary issue related to the use of fossil fuel to produce carbon neutral energy is carbon sequestration. Producing 14 terawatts of energy from fossil fuels requires securely storing approximately 25 billion metric tons of CO2/year. If nuclear power is employed to produce 14 terawatts of energy, an additional 1-gigawatt electric nuclear fission plant must be brought on-line every day for the next 50 years to meet projected demand. Recent safety issues associated with the operation of nuclear power plants are of great concern, especially in view of the problems experienced by nuclear power plants in Japan in the wake of the recent earthquake. Among the possible renewable energy sources that may be utilized at present, about 0.5 terawatts per year may be produced from worldwide hydroelectric resources, 2 terawatts per year from all tides and ocean currents, 12 terawatts per year from geothermal integrated over all the land area, and 2-4 terawatts per year from wind power. However, about 36,000 terawatts per year could be potentially obtained from solar energy. Therefore, one of the most attractive options to meet the 14 terawatts challenge is to develop technology designed to harvest solar energy.
Currently, most of the commercial solar cells are silicon solar panels. These types of commercial solar cells can be rather expensive to produce and transport and must be handled with delicate care. However, organic solar cells (OSCs) and dye-sensitized solar cells (DSCs) are an excellent option for commercial solar cells. These types of solar cells use organic materials and may be manufactured to be very thin, lightweight, and flexible. To make OSCs and DSCs economically competitive with other alternative energy sources, these solar cells must be developed to have reduced cost and enhanced efficiency. To date, researchers have focused their attention on improving the efficiency of these solar cells. Since 1997, the optimal cell efficiency has only slowly increased from about 10% to the current 12.3% for DSCs; the cell efficiency of OSCs has remained steady at about 5%.
There exist significant challenges in improving the cell efficiency of OSCs and DSCs. A need exists to develop alternative strategies for developing technology based on OSCs and DSCs at a reduced cost so that solar cell technology may become a major producer of energy to fulfill the projected additional worldwide energy demand of 14 terawatts by 2050 and beyond.
In one aspect, a dye-sensitized solar cell that includes a coal-based dye material is provided. It has been discovered unexpectedly that coal-based dye materials such as powdered coal and coal derivatives absorb light energy over a wide range of wavelengths including the visible and near-IR spectra. Further, the aromatic organic structures typically present in the coal-based dye materials impart an electron transfer capability that makes these materials suitable for use in a dye-sensitized solar cell.
In another aspect, a dye-sensitized solar cell is provided that includes a coal-based dye material, an anode sheet, a cathode sheet, and an electrolyte. The anode sheet includes a fluoride-doped tin dioxide coating attached to a first substrate and a TiO2 layer attached to the fluoride-doped tin dioxide coating opposite to the first substrate. The coal-based dye material also includes a plurality of anchor groups that are covalently bonded to the TiO2 layer opposite to the fluoride-doped tin dioxide coating of the anode sheet.
The cathode sheet includes a platinum coating attached to a second substrate. The cathode sheet is situated over the anode sheet such that the coal-based dye material is sandwiched between the TiO2 layer and the platinum coating. The electrolyte is disposed between the platinum coating and the coal-based dye material.
The dye-sensitized solar cell disclosed herein overcome many of the limitations of existing dye-sensitized solar cell designs. In particular, the coal-based dye material is relatively easy to produce and readily available at low cost, resulting in a solar cell that is economically competitive relative to other forms of alternative energy production.
In yet another aspect, a method of producing a dye-sensitized solar cell is provided that includes spin coating a thin film comprising an oxide semiconductor onto a conductive glass substrate. The method further includes forming a sensitized thin film by submerging the thin film in a suspension that includes a powdered coal comprising a plurality of anchor groups suspended in a solvent to covalently bond the anchor groups to the thin film. In addition, the method includes covering the sensitized thin film with a catalyst-coated counter electrode and situating an electrolyte between the sensitized thin film and the catalyst-coated counter electrode to form the dye-sensitized solar cell.
Additional objectives, advantages and novel features will be set forth in the description which follows or will become apparent to those skilled in the art upon examination of the drawings and detailed description which follows.
Corresponding reference characters indicate corresponding elements among the view of the drawings. The headings used in the figures should not be interpreted to limit the scope of the claims.
The organic solar cells (OSCs) and related methods of producing OSCs as provided herein involve developing economically competitive OSCs and dye-sensitized solar cells (DSCs) based on a novel technical approach and strategy distinguished from prior art strategies that researchers have used in the past to improve cell efficiency. Rather than improving cell efficiency with conventional methods, the method described herein produces dyes that are much less costly than existing dyes. Dyes, a critical component of OSCs and DSCs, are currently expensive and difficult to synthesize. Thus, it is commercially useful to develop a method that can produce dye materials cheaply and in substantial quantities for solar cells. If this can be achieved without requiring higher cell efficiency than what is currently available, solar cell technology can be economically competitive with other energy technologies.
There are at least about 92 existing metal-free dyes used in the production of existing solar cells. Representative chemical structures of some of these existing dyes are illustrated in Table 1 below. As shown in Table 1, a common feature of these dyes is the conjugation of aromatic rings. A suitable dye for OSCs may be selected on the basis of one or more factors including, but not limited to, the ability to absorb light in the red and near infrared region, ease of synthesis, and good photo and redox stabilities during extended use.
The inventors have discovered unexpectedly that chemically unmodified coal functions as an excellent dye in DSCs. As described herein, solar cells fabricated using coal and/or its direct derivatives are referred to as coal solar cells. Coal is a mixture of compounds, and its structure and composition typically varies by location and type. However, it is well-known that coal typically includes a large percentage of conjugated aromatic rings. The light absorption properties of known representative coal sub-structures are evaluated below.
For use in coal solar cells, one may fabricate coal-based dyes through physical processes such as grinding or chemical processes such as acid washing to make functionalized coal derivatives. As coal is a far cheaper material than any of the existing metal-free dyes typically used in DSCs, the processes to fabricate coal derivatives for solar cells may render coal-based dyes much less expensive than other compounds. For example, coal containing high organic sulfur content, such as Illinois coal, has been found to be an excellent candidate for solar cells, as demonstrated below.
As a fossil fuel, coal is consumed predominantly as an energy source. The inventors have discovered unexpectedly that coal may be used to capture solar energy and convert the captured solar energy to electricity when included in the manufacture of a DSC. In this manner, coal may be used as an energy carrier rather than an energy source. In addition, when the coal solar cells reach the end of their usefulness or efficiency, the coal that was used in solar cells may be collected and either recycled or burned for energy.
In an aspect, the method of manufacturing coal solar cells relies on the solar energy absorption capability of coal and its derivatives as solar cell materials. A particularly critical feature of these coal-based dyes includes the electron transfer capability of the excited electrons upon absorption of the solar energy.
In general, the coal-based dye for use in a DSC may be selected in order to have a LUMO energy level that is above Fermi energy level of the TiO2 substrate. In an aspect, the LUMO energy of the coal-based dye may have a LUMO energy that falls above about −3 eV to about −4.2 eV, depending on the specific TiO2 substrate used in the DSC. Further, the coal-based dye may be selected in order to have a HOMO energy level that is below the HOMO energy level of the I−/I3− electrolyte. In an aspect, the HOMO energy of the coal-based dye may be above about −5 eV, and may vary if a different electrolyte is used.
The treatment of the coal is a critical aspect that may affect the performance of coal dyes in coal solar cells. DSCs fabricated using coal dyes produced using different treatments may be used to test the effect of coal treatment on cell performance. Coal has been shown to contain many functional groups as well as many aromatic local structures which may be responsible for its light absorbing properties. Non-limiting examples of these functional groups include hydroxyls, ethers, aryl ethers, thiols, and thioethers. The hydroxyls and thiols may be utilized as anchoring groups to attach the coal molecular structure to the TiO2 during the fabrication of a DSC. The ethers may be cleaved with a hard acid including but not limited to HCl or H2SO4, and the aryl ethers may be cleaved with AlCl3 and the resulting hydroxyls may be used as anchoring groups. In addition, thioethers may be cleaved using lithium and phenothaline.
The DSCs may be constructed using coal derivatives in a variety of forms. Non-limiting forms of coal derivatives suitable for use in the construction of DSCs include aryl ether cleaved, thioether cleaved, and any combination thereof. In an aspect, the anchoring groups used to bond the coal-based dye to the metal oxide layer in the DSC may be selected to enhance electron transport between the dye and the metal oxide in order to enhance the overall efficiency of the DSC.
Fabricating a DSC using coal may be accomplished by spin coating a thin film of an oxide semiconductor such as TiO2 onto a conductive glass such as F:SnO conductive glass. The TiO2 may then be annealed and submerged in a suspension of powdered coal suspended in a solvent such as a 50:50 vol % acetonitrile:butanol solution. The sensitized TiO2 may then be covered with the catalyst-coated counter electrode and an I−/I3− liquid electrolyte may be injected between the counter electrode and TiO2 layers. A non-limiting example of an electrolyte suitable for use in the DSC is a solution of 0.5M potassium iodide mixed with 0.05M iodine in water-free ethylene glycol. Other non-limiting examples of suitable DCS electrolytes include a CoIII/CoII solution or a solid electrolyte.
The following examples illustrate various aspects of the invention described herein.
To characterize the light absorption properties of coal, UV-visible light absorption spectra of a series of aromatic compounds representative of chemical structures occurring in coal were computed. Although the overall molecular structure of coal is large and relatively complex, isolated coal substructures were examined to assess their light harvesting potential.
Because coal is a typically a mixture of at least several aromatic compounds, we calculated the UV-visible light absorbance spectrum for a 1:1 mixture of benzene (compound 9) and naphthalene (compound 10), summarized in
Additional light adsorption calculations were performed to assess the effect of sulfur on the absorption of sunlight.
Feasibility studies of coal as a dye material for DSCs were made using representative aromatic organic molecules shown in
Two prototype solar cells were assembled to demonstrate the feasibility of producing DSCs using coal as a dye material. Photographs of the two prototype solar cells are shown in
It should be understood from the foregoing that, while particular embodiments have been illustrated and described, various modifications can be made thereto without departing from the spirit and scope of the invention as will be apparent to those skilled in the art. Such changes and modifications are within the scope and teachings of this invention as defined in the claims appended hereto.
This is a non-provisional patent application that claims priority to U.S. Provisional Patent Application Ser. No. 61/466,749, filed on Mar. 23, 2011, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | |
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61466749 | Mar 2011 | US |