Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2001-05-10
2003-05-06
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S256000, C136S252000, C257S040000, C257S431000, C429S111000, C429S213000, C540S145000, C540S460000, C514S410000
Reexamination Certificate
active
06559374
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns solar cells, particularly regenerative solar cells, and light harvesting arrays useful in such solar cells.
BACKGROUND OF THE INVENTION
Molecular approaches for converting sunlight to electrical energy have a rich history with measurable “photoeffects” reported as early as 1887 in Vienna (Moser, J.
Montash. Chem.
1887, 8, 373.). The most promising designs were explored in considerable detail in the 1970's (Gerischer, H.
Photochem. Photobiol.
1972, 16, 243; Gerischer,
H. Pure Appl. Chem.
1980, 52, 2649; Gerischer, H.; Willig, F.
Top. Curr. Chem.
1976, 61, 31). Two common approaches are shown in
FIG. 1
, both of which incorporate molecules that selectively absorb sunlight, termed photosensitizers or simply sensitizers (S), covalently bound to conductive electrodes. Light absorption by the sensitizer creates an excited state, S*, that injects an electron into the electrode and then oxidizes a species in solution. The right hand side depicts a simplified
photoelectrosynthetic cell
. This cell produces both electrical power and chemical products. Many of the molecular approaches over the past few decades were designed to operate in the manner shown with the goal of splitting water into hydrogen and oxygen. Shown on the left hand side is a regenerative cell that converts light into electricity with no net chemistry. In the regenerative solar cell shown, the oxidation reactions that take place at the photoanode are reversed at the dark cathode.
The principal difficulty with these solar cell designs is that a monolayer of a molecular sensitizer on a flat surface does not absorb a significant fraction of incident visible light. As a consequence, even if the quantum yields of electron transfer are high on an absorbed photon basis, the solar conversion efficiency will be impractically low because so little light is absorbed. Early researchers recognized this problem and tried to circumvent it by utilizing thick films of sensitizers. This strategy of employing thick absorbing layers was unsuccessful as intermolecular excited-state quenching in the thick sensitizer film decreased the yield of electron injection into the electrode.
One class of thick film sensitizers is provided by the so-called organic solar cells (Tang, C. W. and Albrecht, A. C.
J. Chem. Phys.
1975, 63, 953-961). Here a 0.01 to 5 &mgr;m thick film, typically comprised of phthalocyanines, perylenes, chlorophylls, porphyrins, or mixtures thereof, is deposited onto an electrode surface and is employed in wet solar cells like those shown, or as solid-state devices where a second metal is deposited on top of the organic film. The organic layer is considered to be a small bandgap semiconductor with either n-or p-type photoconductivity and the proposed light-to-electrical energy conversion mechanisms incorporate excitonic energy transfer among the pigments in the film toward the electrode surface where interfacial electron transfer takes place. However, the importance of these proposed mechanistic steps is not clear. Increased efficiencies that result from vectorial energy transfer among the pigments have not been convincingly demonstrated. Furthermore, the reported excitonic diffusion lengths are short relative to the penetration depth of the light. Accordingly, most of the light is absorbed in a region where the energy cannot be translated to the semiconductor surface. The excitons are also readily quenched by impurities or incorporated solvent, leading to significant challenges in reproducibility and fabrication. The state-of-the-art organic solar cells are multilayer organic “heterojunction” films or doped organic layers that yield 2% efficiencies under low irradiance, but the efficiency drops markedly as the irradiance approaches that of one sun (Forrest, S. R. et al.,
J. Appl. Phys.
1989, 183, 307; Schon, J. H. et al.,
Nature
2000, 403, 408).
Another class of molecular-based solar cells are the so-called photogalvanic cells that were the hallmark molecular level solar energy conversion devices of the 1940's -1950's (Albery, W. J.
Acc. Chem. Res.
1982, 15, 142). These cells are distinguished from those discussed above in that the excited sensitizer does not undergo interfacial electron transfer. The cells often contain sensitizers embedded in a membrane that allows ion transfer and charge transfer; the membrane physically separates two dark metal electrodes and photogenerated redox equivalents. The geometric arrangement precludes direct excited-state electron transfer from a chromophore to or from the electrodes. Rather, intermolecular charge separation occurs and the reducing and oxidizing equivalents diffuse to electrodes where thermal interfacial electron transfer takes place. A transmembrane Nernst potential can be generated by photodriven electron transfer occurring in the membrane. In photoelectrosynthetic galvanic cells, chemical fuels may be formed as well. This general strategy for dye sensitization of electrodes has been employed in many guises over the years, but the absolute efficiencies remain very low. Albery concluded that an efficiency of ~13% theoretically could be achieved in an aqueous regenerative photogalvanic cell. However, efficiencies realized to date are typically less than 2%.
In 1991, a breakthrough was reported by Gratzel and O'Regan (O'Regan, B. et al.,
J. Phys. Chem.
1990, 94, 8720; O'Regan, B. and Gratzel, M.
Nature
1991, 353, 737). By replacing the planar electrodes with a thick porous colloidal semiconductor film, the surface area for sensitizer binding increased by over 1000-fold. Gratzel and O'Regan demonstrated that a monolayer of sensitizer coating the semiconductor particles resulted in absorption of essentially all of the incident light, and incident photon-to-electron energy conversion efficiencies were unity at individual wavelengths of light in regenerative solar cells. Furthermore, a global efficiency of ~5% was realized under air-mass 1.5 illumination conditions; this efficiency has risen to a confirmed 10.69% today (Gratzel, M. in “Future Generation Photovoltaic Technologies” McConnell, R. D.; AIP Conference Proceedings 404, 1997, page 119). These “Gratzel” solar cells have already found niche markets and are commercially available in Europe.
These high surface area colloidal semiconductor films (Gratzel cells) achieve a high level of absorption but also have the following significant drawbacks. (1) A liquid junction is required for high efficiency (because the highly irregular surface structure makes deposition of a solid-state conductive layer essentially impossible). (2) The colloidal semiconductor films require high temperature annealing steps to reduce internal resistances. Such high temperatures impose severe limitations on the types of conductive substrates that can be used. For example, polymeric substrates that melt below the required annealing temperatures cannot be used. (3) Significant losses are associated with transporting charge through the thick semiconductor films. These losses do not appreciably decrease the photocurrent, but have a large effect on the voltage output and thus the power is decreased significantly (Hagfeldt, A.; Gratzel, M.
Chem. Rev.
1995, 95, 49). Accordingly, there remains a need for new molecular approaches to the construction of solar cells.
SUMMARY OF THE INVENTION
The present invention provides, among other things, trans-substituted chlorins and methods of making such trans substituted chlorins. The trans-substituted chlorins may be used, among other things, as building blocks in polymers that may be incorporated into light harvesting arrays and solar cells described herein.
A light harvesting array of the present invention is useful, among other things, for the manufacture of solar cells. The light harvesting array comprises:
(a) a first substrate comprising a first electrode; and
(b) a layer of light harvesting rods electrically coupled to the first electrode, each of the light harvesting rods comprising a polymer of Formula I
Balasubramanian Thiagarajan
Lindsey Jonathan S.
Diamond Alan
Myers Bigel & Sibley & Sajovec
North Carolina State University
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