« PreviousContinue »
 Appl. No.: 907,361
 Filed: May 18,1978
Related U.S. Application Data
 Continuation-in-part of Ser. No. 821,115, Aug. 2, 1977, abandoned.
 Int. CI.* H01L 31/06
 U.S. CI 136/89 NB; 357/8;
 Field of Search 136/89 NB; 357/8, 30;
250/211 J, 212
 References Cited
U.S. PATENT DOCUMENTS
3.009.006 11/1961 Kostelec 136/89
3,009,981 11/1961 Wildi et al 136/89
3,057,947 10/1962 Calvin et al 136/89
3,507,706 4/1970 Webb 136/89
3.530.007 9/1970 Golubovic 136/89
3,591,374 7/1971 Seiis 96/1.6
3,789,216 1/1974 Komp 250/211 R
3,837,851 9/1974 Shattuck et al 96/1.5
3,840,368 10/1974 Ikeda et al 96/1.5
3,844,843 10/1974 Kay et al 136/206
3,895,944 7/1975 Wiedemann et al 96/1.5
3,912,931 10/1975 Gravisse et al 250/458
3,935,031 1/1976 Adler 136/206
3,938,994 2/1976 Reynolds et al 96/1.6
3,955,978 5/1976 Rochlitz et al 96/1.5
3,992,205 11/1976 Wiedemann 96/1.6
3,996,049 12/1976 Rochlitz 96/1.5
3,997,342 12/1976 Bailey 96/1.6
4,106,951 8/1978 Masi 136/89 NB
R. L. Gamblin et al., "Solar Cell Utilizing Organic Photoconductor", IBM Tech. Disc. Bull, vol. 18, p. 2442, (1976).
L. W. Davies, "Prospects for the Direct Conversion of Solar Energy to Electricity", AWA Tech. Rev., vol. 15, pp. 139-142, (1974).
"Topics In Current Chemistry", vol. 61, Springer-Verlag (1976), p. 124.
F. B. Kaufman "Efficient Two-Layered Organic Photovoltaic Device", IBM Tech. Disc. Bull., vol. 19, pp. 367-368, (1976).
N. F. Yannoni, "Organic Materials Research", pp. 175-184 of Solar Cells, Gordon & Breach Science Publishers, London (1971).
T. K. Mukherjce, "Photocurrents & Photopotentials In Organic Solids", Cont. Record, IEEE Photospecialists Conf. (1967), pp. 7-23.
A. K. Ghosh et al., "Photovoltaic & Rectification Prop-
erties of Al/Mg Phthalocyanine Ag Schottley-Barrier
Cells", /. Appl. Phys., vol. 45, pp. 230-236, (1974).
R. C. Nelson, "Sensitization of Photoconductivity In
Cadmium Sulfide", /. Opt. ScL Am., vol. 46, pp. 13-16,
Primary Examiner—Aaron Weisstuch
Attorney, Agent, or Firm—Dana M. Schmidt
A novel multilayer, organic composition, a photovoltaic element fabricated therefrom having enhanced conversion efficiencies, and their use to generate power, are disclosed. Compounds with generally planar polycyclic nuclei such as organic photoconductive dyes comprise the several layers of the composition.
17 Claims, 1 Drawing Figure
MULTILAYER ORGANIC PHOTOVOLTAIC
RELATED APPLICATIONS 5
This application is a continuation-in-part application of U.S. Ser. No. 821,115, filed on Aug. 2, 1977, now abandoned.
BACKGROUND OF THE INVENTION io
1. Field of the Invention
This invention relates to photovoltaic elements useful for converting light and particularly for converting solar energy into electrical energy. The invention features the use of organic compounds.
2. State of the Prior Art
So-called Schottky barrier or P-N junction photocells rely upon the fact that a built-in potential exists at the metal/semiconductor interface as in the Schottky device or at the junction between the P-type and N-type semiconductors as in the P-N junction device. Electronhole pairs generated by the absorption of light in the semiconductor are separated due to the built-in field at the interface, establishing an electrical potential.
Among chief materials used in the past for solar cells 25 have been inorganic semiconductors, due to their fairly high conversion efficiencies which have been as high as 12 to 15 percent, for example, for silicon. However, such devices have proven to be very expensive to construct, due to the melt and other processing techniques necessary to fabricate the semiconductor layer. As a result, such devices have had extensive practical utility only in the field of space exploration, and not in terrestrial applications.
In an effort to reduce the cost of solar cells, organic photoconductors and semiconductors have been considered, due to their inexpensive formation by solvent coating and similar techniques. However, prior art organic materials have generally produced solar cells with conversion efficiencies only as high as about 0.05 percent at their highest, when exposed to incident sunlight at an intensity of 100 mW/cm2. An example of such a material is crystal violet, as described, for example, in U.S. Pat. No. 3,844,843. Still higher efficiencies at least as high as 0.1 percent are desirable if the cells are to have practical terrestrial use, notwithstanding their inexpensive cost of manufacture. An efficiency of 0.3 percent was reported as being achieved through the use of an undisclosed dopant, as noted in "Prospects for Direct Conversion of Solar Energy to Electricity," AWA Technical Review, Volume 15, No. 4,1974, footnote 3, but none of the materials used has been disclosed.
Solar cells utilizing other organic photoconductive materials are disclosed in U.S. Pat. Nos. 3,009,006; 3,057,947; 3,507,706; 3,530,007; and IBM Technical Disclosure Bulletin 18 (8), page 2442 (January 1976). However, there is no disclosure in any of these publications how to manufacture a solar cell which exhibits a conversion efficiency high enough for extensive practi- 60 cal terrestrial use, i.e., greater than about 0.1 percent.
Multilayer photoconductive compositions have been formulated in the past, for xerographic application, using porphyrinic compounds overlayered with a charge-transport layer, as disclosed, for example, in 65 U.S. Pat. Nos. 3,895,944 and 3,992,205. However, such charge-transport layers in U.S. Pat. No. 3,895,944 have required the use of binders, as well as sensitizers, and in
U.S. Pat. No. 3,992,205 the layer containing phthalocyanine requires the use of another pigment admixed therewith.
Phthalocyanine, a porphyrinic compound, has been used in organic solar cells in the past, in contact with a layer of electron acceptors such as oxidized tetramethyl p-phenylenediamine, /J-carotene, dibrominated pphenylenediamine, p-chloranil and the like. Examples are illustrated in U.S. Pat. No. 3,057,947. However, such cells have extremely low conversion efficiencies, less than 10~7 percent (power output, col. 3, line 69, divided by 100 milliwatt input) for several reasons. First, the acceptors are not dyes and therefore do not absorb radiation in the visible spectrum as well as dyes do. Second, the layers are formed by pressing techniques and, as such, require thicknesses which are far too large for efficient solar cells.
Multilayer photoelectric cells have been constructed from a phthalocyanine layer with or without an overcoat of malachite green, as reported, for example, in Topics in Current Chemistry, Springer-Verlag, Volume 61, 1976, page 124, and U.S. Pat. No. 3,789,216, issued Jan. 29, 1974. However, the conversion efficiency of such cells were very low—less than 10-4 percent, as reported in Springer-Verlag.
A layer of porphyrin or porphyrin-like material has also been used in the past to improve already existing solar cell semiconductors, such as selenium. Examples are disclosed in U.S. Pat. No. 3,935,031. However, only expensive inorganic semiconductors which themselves are self-sufficient cell materials have been suggested for such use with porphyrin.
Pyrylium and thiapyrylium dyes have been disclosed for use as sensitizers in photoconductive compositions, as noted, for example, in U.S. Pat. Nos. 3,938,994 and 3,997,342. No mention is made in these patents, however, as to the dye being useful with an adjacent layer of porphyrinic compound.
Other patents relating to the general background of organic solar cells include U.S. Pat. No. 3,912,931, issued Oct. 14, 1975.
Other patents relating to the general background of photoconductor compositions having a charge generating layer and a separate layer including a charge transport compound include U.S. Pat. Nos. 3,591,374, issued July 6, 1971; 3,837,851, issued Sept. 24, 1974; 3,840,368, issued Oct. 8, 1974; 3,996,049, issued Dec. 7, 1976; and 3,955,978, issued May 11, 1976.
U.S. Pat. No. 4,125,414 based on commonly owned U.S. application Ser. No. 885,926, filed on Mar. 13, 1978, a continuation-in-part application of U.S. Ser. No. 821,117, now abandoned filed on Aug. 2,1977 by C. W. Tang et al: entitled "Organic Photovoltaic Elements," discloses elements comprising an organic photoconductive layer which includes pyrylium-type dyes together with a binder and a photoconductor. A preferred method of making such a composition features the formation of a discrete discontinuous phase in a continuous phase. A very thin nucleating layer of copper phthalocyanine can also be used with this photoconductive layer, but it does not form a rectifying junction.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an organic solar cell using a multilayer organic composition, such a cell having improved conversion efficiencies. 5
It is another object of the invention to provide certain novel multilayer organic photoconductive laminates.
It is a related object of the invention to provide a solar cell which is both inexpensive to produce and sufficiently efficient as to be useful in terrestrial applica- 10 tions.
Other objects and advantages will become apparent upon reference to the following Summary and Description of the Preferred Embodiments, when read in light of the attached Drawing. 15
SUMMARY OF THE INVENTION
The invention concerns solar cells and an organic, multilayer composition thereof, such solar cells exhibiting enhanced electrical response to incident light.
More specifically, there is provided an improved photovoltaic element comprising a first layer of an organic, electron donor material; a second layer of an organic electron acceptor material in contact with said first layer; at least one of said materials being capable of 25 absorbing light at wavelengths between about 350 and about 1000 nm and both of said materials being capable of forming a rectifying junction between them; and electrodes in operative ohmic contact with at least part of said layers. The improvement in the cells is that each of the materials comprises a compound containing a generally planar polycyclic nucleus, and when laminated together, the layers produce a total combined thickness no greater than about 0.5 microns and a conversion efficiency for said element of at least about 35 0.02% when exposed to an AM2 light source.
In another aspect of the invention, each of the organic materials of such a cell comprises a compound containing a generally planar, polycyclic nucleus having a surface area of at least about 40 square angstroms 40 and a width of at least about 5 angstroms, and the combined thickness of said layers does not exceed about 0.5 microns.
The invention also concerns the use of such cells to generate electric power from incident radiation.
In another aspect, the invention concerns also a multilayer photoconductive laminate comprising a layer of an organic electron donor compound containing a generally planar polycyclic nucleus, and in contact with said layer, a layer of a photoconductive organic dye 50 having the structure:
Q and X are the same or different and are each O, S, or Se;
R8, R9 and R10 are the same or different and are each H, alkyl from 1 to about 3 carbon atoms, aryl, cyano or nitro; R1, R2, R3 and R4 are the same or different and are each phenyl, or alkyl or alkoxy containing from 1 to about 5 carbon atoms, at least two of R1, R2, R3 and R4 being phenyl; m is 1 or 0; and Z© is an anion.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is a sectional view, partly schematic, of a cell constructed in accordance with the invention.
DESCRIPTION OF THE PREFERRED
Although the invention is hereinafter described in terms of its preferred embodiment, photovoltaic elements or solar cells, it is not limited thereto. Rather, the composition of the invention which makes such solar cells possible can also be used in a photoconductive mode or environment other than solar cells, for example, in a photodiode or as a photoconductive element in an electrophotographic imaging process. In such other environments, the thicknesses of the layers can vary from that desirable for solar cells, depending on the particular use. Also, only one electrode need be disposed in ohmic contact, in most photoconductive uses.
As used herein, "photovoltaic element" or "cell" means a solid state device which converts radiation absorbed by the element, directly to electric power. A photovoltaic element of this invention is suitable as a terrestrial rooftop generator or as a light-level measuring device. As a light-level measuring device, the cell can be used both at high and low light levels. The cell exhibits a moderately high open circuit voltage of about 300-500 mV.
Alternatively, the cell can also be used in the current mode. The current generated in a diffuse room-light condition is about 40 uA/cm2, a large enough current to be measured accura'tely. The current can thus become a measure of the light intensity, and the calibrated cell can be used as an exposure meter and find application in cameras.
In accordance with one aspect of the invention, organic solar cells are provided with conversion efficiencies heretofore unattainable, that is, at least about 0.02% and as high as 1%. This is achieved by the use of a layer of an organic electron donor material in contact with a layer of an organic electron acceptor material, each of which comprises a compound with a generally planar polycyclic nucleus. Together the layers have a combined thickness that does not exceed about 0.5 microns.
The terms "electron donor" and "electron acceptor" are used to describe the respective materials' electron affinity, particularly when the materials are considered as a pair. Thus, an electron donor material, hereinafter "donor material," has a relative low electron affinity, and an electron acceptor, hereinafter "acceptor material," has a relatively high electron affinity. As such, a donor material tends to act as a p-type material whereas an acceptor material tends to act as an n-type material. The two materials when layered together form a rectifying junction between them, and at least one of them is capable of absorbing light at wavelengths between about 350 and about 1000 nm.