CA1085947A

CA1085947A - Multilayer organic photovoltaic elements

Info

Publication number: CA1085947A
Application number: CA287,802A
Authority: CA
Inventors: Ching W. Tang
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1977-08-02
Filing date: 1977-09-29
Publication date: 1980-09-16
Also published as: JPS5427787A; AU521118B2; DE2861508D1; EP0000829B1; EP0000829A1; JPS624871B2; AU3854378A

Abstract

MULTILAYER ORGANIC PHOTOVOLTAIC ELEMENTS

ABSTRACT OF THE DISCLOSURE
A novel multilayer, organic photoconductive composition and a photovoltaic element fabricated therefrom having enhanced conversion efficiencies are disclosed.
Porphyrinic compounds and organic photoconductive dyes comprise the several layers of the photoconductive composi-tion.

Description

1~859~

BACKGROUND OF THE INVENTION ~
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 photoconductive materials.
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 semi'conductors as in the P-N junction device. Electron-'hole pairs generated by the absorption of light in the semiconductor are separated due to the built-in field at the interfacej establishing an electrical potential.
Among chief materials used in the past for solar cells have been inorganic semiconductors, due to their ~airly i- -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 t 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 cel~s, organic photoconductors and semiconductors have been con-sidered, due to their inexpensive formation by solvent coating -and similar techniques. However, prior art organic materials have generally produced solar cells with conversion efficiencies , `~ - 2 - ~ ' . --... . ..... _.. .. .. . . , _ __ .. _ _._ _ _ . ...

.

:' only as high as about 0.05 percent at their highest, when exposed by 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. Patent 3,844,843. Still higher efficien-cies at least as high as 0.1 percent are desirable if the ce]ls 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 Conver-sion 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. Patents 3,009,006; 3,057,947;
3,507,706; 3,530,007, and IBM Technical Bulletin 18 (8), No.
2442 (January 1976). However, there is no disclosure in any of these publications that the resultant solar cells exhibit a conversion efficiency high enough for extensive practical 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 dis~closed, for example, in U.S. Patents 3,895,944 and 3,992,205. However, such charge-transport layers in the '944 patent have required the use of binders, as well as sensitizers, and in the l205 patent the layer containing phthalocyanine requires the use of another pigment admixed with it.
Phthalocyanine, a porphyrinic compound, has been 3 used in organic solar cells in the past, in contact with a . .

,.
.

- 108594~

,-layer of electron acceptors such as oxidized tetramethyl _-phenylenediamine, b-carotene, dibrominated _-phenylene-diamlne, ~-chloranil and the like. Examples are illustrated in U.S. Patent 3,057,947. However, such cells have extremely low conversion efficiencies, less than 10 9 percent, 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-con-structed from a phthalocyanine layer with or without an over-coat of malachite green, as reported, for example, in Topics in Current Chemistry, Schaefer et al, Volume 61, 1976, page ` 124, and U.S. Patent 3,789,216, issued January 29, 1974.
However, the conversion efficiency of such cells were very low -- less than 10 4 percent, as reported in Schaefer et al. ~ -A layer of porphyrin or porphyrin-like material has also been used in the past to improve already existing 20 solar cell semiconductors, such as selenium. Examples are disclosed in U.S. Patent 3,935,031. However, only inorganic r semiconductors which themselves are self-sufficient cell material, and expensive, have been suggested for such use , with porphyrin.
Pyrylium and th'lapyrylium dyes have been disclosed for use as sensitizers in photoconductive compositions, as noted, for example, in U.S. Patents 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.

. . ~ . , .. .. .. ~

108S94~

Other patents relating to the general background of organic solar cells include U. S. Patent 3,912, a31, issued October 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. Patents 3,591,374, issued July 6, 1971; 3,837,851, issued September 24, 1974; 3,840,368, issued October 8, 1974; 3,996,049, issued December 7, 1976; and 3,955,978, issued May 11, 1976.
Related Applications Commonly owned Canadian Application Serial No.
287,417 filed concurrently herewith 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.

~085947 SUMMARY OF THE INVENTION
The invention concerns solar cells having incor-porated therein'an organic, multilayer photoconductive composition, such solar cells exhi~iting enhanced,electrical response to incident light. ~, , More specifically, there is provided a photovoltaic ~, element comprising a layer of a porphyrinic compound; in contact with said layer, a layer comprising a photoconduc-tive organic dye capable of absorbing light at wavelengths between about 350 and about lO00 nm and capable of forming a rectifying ~unction with said porphyrinic compound; and an , electrode operatively connected to each of said layers, said electrode comprising a material which creates an ohmic contact '' with said layers; said element having a conversion efficiency when exposed to sunl'ight of at least about 0.02 percent.
From another aspect, the invention concerns also a multilayer photoconductive composition comprising a layer of porphyrinic comp,ound, and in contact with said layer, a layer of a photoconductive organic dye having the structùre: ~

R\ /R 3 . ".
~ Q~ (CR =CR ) m J =

~ wherein: R10 J is C or N;
Q and X are the same or different and are each O, S, or Se;
.' ' ;'~

!

lV8594`7 R8, R9 and R10 are the same or different and are each H, alkyl from 1 to about 6 carbon atoms, aryl, cyano or nitro;
- Rl, 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 Rl, 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, 10 of a cell constructed in accordance with the invention.
t DESCRIPTION OF THE PREFERRED EMBODIMENTS
. .
Although the invention is hereinafter described in - terms of its-preferred embodiment, photovoltaic elements or solar cells, it is not limited thereto. Rather, the p`hoto-conductive composition of the invention which makes such solar cells possible can also be used in any photoconductive mode or environment, for example, a photodiode, or as a ~i photoconductive element in an electrophotographic imaging process. In such other environments, the thicknesses of the 20 photoconductive layers can vary, depending on the particular r use.
As used herein, "photovoltaic element" or "cell"
- means a solid state device which converts radiation absorbed r by the element, directly to electric power. A photovoltaic element~of this invention is suitable as a terrestrial roof-top 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. .

,, '~

.. . . ...
_ . . .. . ._ ... _ _ _. ~ ................ . . _ . ~_ ~08S947 Alternatively, the cell can also be used in the current mode. The current generated in a diffuse room-light condition is about 40~A/cm2, a large enough current to be measured accurately. 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, the photoconductive composition comprises a layer of metal-free or metal-containing porphyrinic compounds, and a contact-ing layer of a photoconductive organic dye capable of forminga rectifying junction with the porphyrinic compound.
Considering first the porphyrinic compound, any such compound is operative, with or without metal, and any metal will work, such as cobalt, magnesium, zinc, palladium, nickel, copper, lead, and platinum. However, some metal phthalocyanines are preferred because of the greater conversion efficiencies which they exhibit. Examples of preferred metal phthalocyanines include copper, lead, and platinum phthalocyanine. For example, lead phthalocyanine has -produced a cell with a spectral response extending to almost 1000 nm. Further, it is preferred that the porphyrinic layer be non-crystalline, as the presence of crystals tends to provide a shorting path which can render the element inefficient.
As used herein, porphyrinic compounds is intended to mean any compound, material or synthetic which includes or derives from the basic porphyrin structure. Examples of such are disclosed in the aforesaid U.S. Patent 3,935,031.
A currently preferred class of such porphyrinic compounds is the class having the structure:

(I) . \/-=-T~

X~l _z1 lXZ
.__ .. __ ... ._ , ,, _ , wherein:
M is a metal;
Tl and T2 are both S or both C, or one of Tl and T2 is N and the other C;
xl and x2 are-the sàme or different, and are each hydrogen or . halogen, such as chlorine, fluorine, bromine and the like; and .
zl is the nuclear carbon atoms necessary to form a six-membered unsaturated ring.
A further option is to provide the porphyrinic com- ~.
pounds with the structure of (I), but in a nonmetallic complex, whereby two of the four nitrogens becomes hydrog`enated.
. It has been discovered further that the porphyrinic layer can be divided into two contiguous layers of different phthalocyanines only one of which contacts the dye layer, the other of which, in the case of a solar cell, is in ohmic contact with the electrode for the phthalocyanine. In such a case, the total thickness of the two layers of phthalocyanine .

considered together should equal the thickness that would be ` used for a single phthalocyanine layer. .
The photoconductive organic dye of the photoconduc-tive composition pref~erably ~ capable of absorbing light ~ . . . .
. :
.

i lV8594q having wavelengths between about 350 nm and about 1000 nm.
As used herein, organic dyes are meant to include organo-metallic dyes as well. The above-referenced light absorbing capability does not mean that AmaX, the maximum absorption peak, must fall within the noted range, but rather that any of the larger absorption peaks characteristic of that particular dye should do so.
In addltion to absorption of light, the dye prefer-~ ably is one whlch forms a rectifying junction with the 10 porphyrinic compound. It has been found that certain dyes 1 -which in fact provide such a rectifying junction also have oxidation potentials which are higher than that of the porphyrinic compound used in the ad~acent layer. Such a determination of relative oxidation potential levels has been done conveniently by photoemission studies conducted as follows: The composition to be studied is coated as a film and bombarded with high energy photons from a vacuum ultraviolet light source emitting at about 1600 A.
The electrons kicked out of the composition, and their kinetic energy, are measured by an ammeter, and the kinetic energy of the electrons is subtracted to derive the minimum, or threshold, photoemission energy required by the incident beam to e~ect the electrons at zero velocity. Such studies have demonstrated a photoetnission threshold for porphyrinic compounds which is lower than the threshold for the dyes dlsclosed hereinafter, corresponding to higher oxidation potentials for the dyes than for the porphyrinic compound.
However, although the higher oxidation potential or photo-emission threshold may serve as a crude test to preliminarily 3Q identify suitable dye material, it is not known whether the level of the oxidation potential or threshold is a sufficient .

` -` 108S94~
test which by itself is determinative of the rectifying character of the dye in con~unction with the porphyrinic compound.
A further characteristic of dyes useful in this invention is that when a cell is manufactured in accordance with the invention utilizing a layer of these dyes, the cell has a conversion efficiency of at least about 0.02 percent.
As noted, any dye material having the above-noted properties can be used. Useful examples when incorporated into a solar cell are pyrylium-type dye salts which include pyrylium, thiapyrylium and selenapyrylium dye salts, and also salts of the aforementioned pyrylium-type dye salts containing condensed ring systems such as salts of benzopyrylium and naphthopyrylium dyes. Highly preferred examples have the structure-R~ ~R3 (II) Q~ CR8=CRa~ J=-~ \X Z
- .

v wherein: R10 J is C or N; t Q and X are the same or different and are each 0, S or Se;
R8, R9 and Rl0 are the same or different and are each hydrogen; alkyl from 1 to about 3 carbon atoms such as methyl, ethyl, iso-propyl and the like; aryl such as phenyl and naphthyl and including substituted aryl; cyano or nitro;
Rl, R2, R3 and R are the same or different and are each phenyl, including substituted phenyl, or alkyl or alkoxy containing from 1 to about 5 carbon atoms, such as methyl, ethyl, iso-propyl, methoxy, propoxy and the like, at least two of Rl, R2, R3 and R4 being phenyl;
m is 1 or 0 and is 0 if J is N;

. . . :
' , ,: , .

`, 1085947 .
and Z is an anionic moiety, such as perchlorate, fluoro-borate, and the like.
If Rl, R2, R3 or R are substituted phenyl, it is preferred that the substituents be located in the para posi-tion and be selected from among those which shift the blue absorptlon peak of the dye to a longer wavelength. Useful examples of such substltuents include alkyl from 1 to 3 carbon atoms and halogens such as chlorine, fluorine and the Iike.
Dyes of structure (II) above can be manufactured 10 by any convenlent method. For example, the process disclosed L
in Helvetica Chemica Octa, Volume 49, Fasciculus 7, 1966, No. 244, pages 2046 through 2049 can be used. - ¦~;
It is contemplated that another class of useful dyes is 2,4,6-trisubstituted pyrylium and thiapyrylium dye salts of the general~structure:

( III'? 5~7 n~
Rs ~x~ \R6 , 1:' ze .

. ' . ~ ' ' in which R5 and R6 are the same or different and are each alkyl from 1 to about 6 carbon atoms, such as methyl, ethyl, iso-propyl and the like; phenyl, including substituted phenyl; or a 20. 5 or 6 membered heterocyclic ring, such as thienyl, furyl, pyridyl, pyrimidinyl, thiadiazolyl or thiazolyl or pyrrolyl;
R represents an alkylamino-substituted phenyl or an alkylamino-substituted 5 or 6 membered heterocyclic ring having ~rom 1 to about 6 carbon atoms in the alkyl moiety including dialkylamino-. ~substituted and halogenated alkylamino-substituted phenyl ';

- 12 - ' .. . , :
. ~

-.~ 108S94~

dialkylaminopyridyl, dialkylaminofuryl, dialkylaminothienyl, dialkylaminopyrimidinyl, dialkylaminothiadiazolyl or dialkyl-aminothiazolyl; X is oxygen, selenium or sulfur and z~ is an anion such as perchlorate, fluoroborate, and the like.
Examples of such compounds, particularly wherein at least one of R5, R6 and R7 is heterocyclic, are described and claimed in commonly owned Canadian Application Serial No. 282,176, filed July 6, 1977, by D.P. Specht et al, entitled "Sensitizers for Photoconductive Compositions", and in Research Disclosure, Volume 157, May 1977, Publication No. 15742, published by Industrial Opportunities, Limited, Homewell, Havant, Hampshire, PO9 lEF, United Kingdom.
Representative useful dyes having structures of the type described above include:
4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyrylium perchlorate, 4-[(2,6-dimethoxy-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyrylium perchlorate, 4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenyl-thiapyrylium perchlorate, 4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenyl-pyrylium fluoroborate, 4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylselenapyrylium perchlorate, 4-[(2,6-diphenyl-4H-selenin-4-ylidene)methyl]-2,6-diphenylselenapyrylium perchlorate, 4-[(2,6-diphenyl-4H-pyran-4-ylidene~methyl]-2,6-diphenyl-selenapyrylium perchlorate, .

-' 1085947 4-[(2,6-diethyl-4H-thiapyran-4-ylidene~methyl]-2,6- .
diphenylthiapyrylium perchlorate, 4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diethoxythiapyrylium perchlorate,

2,6-diphenyl-4-[(-2,6-dip.henyl-4H-pyranylidene)amino]
pyrylium perchlorate, 2,6-diphenyl-4-(4-dimethylaminophenyl)thiapyrylium hexafluorophosphate, ~,~-diph~nyl-4-(4-diphenylaminophenyl)thiapyrylium 10 perchlorate, 2,6-diphenyl-4-(4-dipropylaminophenyl)thiapyrylium perchlorate, 4-{[2,6-di(~-methylphenyl)-4H-thiapyran-4-ylidene]methyl}-2,6-diphenylthiapyrylium perchlorate, .
4-{[2,6-di(~-fluorophenyl)-4H-thiapyran-4-yliderl~]~ethyl}-2,6-diphenylthiapyrylium perchlorate, 4-{[2,6-di(_-fluorophenyl)-4H-thiapyran-4-ylidene]methyl}-2,6-di(_-fluorophenyl)thiapyrylium perchlorate, 4-{[2,6-di(_-methylphenyl?-4H-thiapyran-4-ylidene]methyl~-20. 2,6-di(_-methylphenyl)thiapyrylium ~erchlorate.
It has been discovered further that a layer comprising a mixture of two different dyes of structure (II), or a dye of structure (II) with a dye of structure (III) forms a useful rectifying ~unction ln con~unction with the phthalocyanine-i .
type layer~ In fact, in some instances synergism has been :
demonstrated in that the conversion efficiency of the mixture . exceeds that obtainable from usin~ either one Or the dyesabove, presumably because the dyes complement each other in .. the mixture.

, . :

- 14 ~

108594~

Thus, the dye layer needs only comprise a dye of the type described, or a mixture of such dyes. In addition, however, other materials, compounds, compositions and the like can be added, provided that they do not significantly decrease the conversion efficiency of the cell, such as by altering the oxidation level of the dye layer, preventing the layer from forming a rectifying contact with the porphyrinic r layer, or creating sinks for the light-generated carriers conducted by the dye.

The thickness of the total photoconductive composi- I
tion comprising the porphyrinic and dye layers to~gether is an lmportant aspect of the photovoltaic elements of the inven-tion, at least if maximum conversion efficiencies are desired.
It has been found that efficiencies begin to decrease drastically for thickness in excess of about 0.5 microns, apparently due to failure of the light to penetrate to the region ad~acent the rectifying ~unction or to increased series resistance.
Minimum thickness for the individual layers appears to be dictated more by coating techniques and the minimum that can be used without shorting. Useful devices of improved efficiency have been constructed with thicknesses for each of the phthalocyanine and dye layers as low as about 100 A.
Greater thicknesses can be used for other photocon-ductive usès or embodiments.
Currently preferred thicknesses for each Or the two layers, for optimum solar cell results, are about 300 to 500 A. If unequal thicknesses are to be used, it ls preferred that the thinner layer be adjacent the window electrode, described hereinafter, to permit proper illumination of the ~ rectifying junction.
.
. ~ ' ~ - 15 - L

.. __ _ . .. .. _ . . . . . . .. _, __, . ... . ~

108S94r~ :

In accordance with a ma~or aspect of the invention, the above-described photoconductive composition can be used in a solar cell or photovoltaic element. For such a device, electrodes are operatively connected, one to the porphyrinic layer and the other to the dye layer. Although the most common, and preferred, construction is one in which the electrodes are in physical contact with their respective photoconductive layers, this need not always be the case.
For example, the porphyrinic layer which contributes to the 10 formation of the rectifying ~unction can be spaced away from its electrode by a layer of a different porphyrinlc compound as descrlbed above.
The electrodes for the cell are selected to form an ohmic contact with the ad~acent layer. As used herein, "ohmic L
contact" does not mean a physical contact necessarily, but instead refers to the relative work function values of the two materials. Because the interface between the materials of the photoconductive composition provides the necessary rectifying ~unction, the electrode materials are selected to t 20 provide an ohmic contact. The electrode adjacent to the porphyrinic layer preferably has a high work function, while ¦-the one ad~acent to the dye layer preferably has a low work function. In addition, at least one of the electrodes must be a window electrode, that is, be at least semitransparent to useful light.
It has been found that a useful material for the i-electrode ad~acent to the porphyrinic layer is a glass or a transparent film such as poly(ethylene terephthalate) coated with a semitransparent layer of indium tin oxide, tin 30 oxide, or nickel This material not only has a hlgh work _ - 16 -1013S94~

function, but its semitransparency makes it highly useful as the window electrode. Examples of such materials having a glass support are Nesa~ and Nesatron~ glass manufactured by PPG Industries and having a sheet resistance of about 10 to 50 ohms/square and an optical transmittance:of about 80 . percent, for visible light.
The opposite electrode can be metal with.a low work function, such as indium, silver, tin, aluminum or the like, and can be semitransparent or opaque. Silver is preferred for minimum los.s in conversion efficiency upon aging.
As shown in the Figure, such a photovoltaic cell as described above comprises a laminar array 10 of a window electrode 12 comprising a transparent support 14 and a semitransparent layer 16 of indium tin oxide, tin oxide or nickel; a layer 18 of a porphyrinic compound, either metal-free or metallic; a layer 20 of a photoconductive dye of proper oxidation level; and an electrode 22 of a metal having a sufficient work function as to form an ohmic contact .-. 20 with dye layer 20. It will be appreciated that the dimensions of the Figure have been exaggerated for clarity. Preferred i~
thicknesses for the layers comprise, for layer 16, 0.5 microns to about 5 microns; for layer 18, 100 to 2500 A; for layer 20j 100 to 2500 A; and for.electrode 22, 100 to 2,000 A. As noted above, in preferred constructions the combined thick-nesses of layers 18 and 20 do not exceed about 0.5 micron, for maximum efficiencies.
Wires 24 represent leads contacting the electrodes to connect the cell to a load circuit, as is conventional. r ~ .

. .. . ..... . . ................. .. .. .. . i . .

- , . .
, Such photovoltaic elements constructed from the materials described above have been found to produce markedly superior conversion efficiencies, at least as much as about ~
0.02 percent when exposed to sunlight, and even as high as ~-0.5 percent.
Any suitable coating technique can be used to manufacture the photoconductive composition and/or solar - cell. For example, any coating technique can be used wherein the two layers forming the rectifying ~unction are coated from 10 two different solvents, one upon the other, the solvent of one being a poor solvent for the other. In this manner, a well-defined interface between the two layers will be maintained.
An alternative and highly preferred method is to vapor deposit the porphyrinic layer on a clean, i.e., polished, window electrode, us-ing sources of porphyrinic compounds which are reasonably free of decomposable or volatile materials, and thereafter solvent coat the dye layer as by spin coating at f between about 1,000 and about 10,000 rpm from one or more of the following-solvents: 1,2-dichloroethane, dichloromethane, 20 and mixtures of the two. For pyrylium dyes, a particularly useful solvent mixture has been, by weight percent, 49 percent 1,2-dichloroethane, 49 percent dichloromethane, and 2 percent 1,1,1,3,3,3-hexafluoroisopropyl alcohol. A currently preferred process for polishing the Nesatron~ glass, if used as the window electrode-, comprises rubbing the Nesatron~
surface with a cotton flannel wetted with a suspension of an alumlna or other abrasive, or by polishing in a spinning disc, usually for a few minutes. The polished Nesatron~ ;
glass was then sonicated in a 1:1 H20/isopropyl alcohol bath for about half an hour to remove the abrasive particles, ' . ' ' ' ~ '' .

... , ..... .. ___ . . .. , . ___ _ . .... .. .. ~ . , .. . ..... _, _ _ . . . . _ _ . . . .
: : .
'. : ~ ', ~: .

lV8594q and then rinsed thoroughly with distilled water. The polished Nesatron~ glass appears relatively clean in a strong light.
The electrode for the dye layer can be applied by conventional vapor deposition.

Examples The following examples further illustrate the nature of the invention. In each case, a modified Kodak 600 slide pro~ector, together with appropriate glass filters and a water filter, was used to provide a simulated light source of AM2 spectrum, as defined in H. J. Hovel, "Solar Cells", 1975. The light incident on the cell had an intensity of 75 mW/cm which was calibrated against a standard silicon cell having a short-circuit current output of 21.5 mA/cm2 at 75 mW/cm2, AM2. The current-voltage characteristics of each cell were traced by applying an external voltage to the cell in either polarity. The voltage across the cell and the ~ -.current through it (measured by a Keithley 602 multimeter) were simultaneously traced in an x-y recorder. ~ -.
Example 1 A cell as shown in the Figure was fabricated in the following manner:
(a) A piece of Nesatron~ glass about 1 inch by 1 inch was polished and thoroughly cleaned. r (b) A 400 A thick copper-phthalocyanine film was depos-ited on the Nesatron~ glass by vapor sublimation in a 1 x 10 5 - torr vacuum.

r ... ~ . 1~
- 19 _ .. . . .

108S94~
i .

(c) A 400 A thiok layer of a photoconductive dye, 4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenyl-thiapyrylium perchlorate, was spin-coated on top of the copper phthalocyanine layer. This dye film appears to be homogeneous and very uniform.
.
(d) The top electrode, indium, was evaporated on top of ;~
the two-layer organic component to complete the cell structure.

Under the simulated AM2 illumination described above (75 mW/cm2), the cell developed an open-circuit voltage 10 of 0.36 volt, a short-circuit current of 2 mA/cm , and a fill factor of 0.47. The power conversion efficiency was 0.45 per-cent.

-Examples 2 th~ough 7 A class of dyes having the formula: r !
0~ ,0 Q~ C=~~X ClO k 0 H 0 . ~.
i'~
wherein 0 is phenyl, and X and Q are any two of the three i elements O, S, Se, were tested as the photoconductive materlal in a solar cell prepared as described for Example 1. The Cu-phthalocyanine layer deposited by vapor sublimation was O
20 about 300 to 500 A thick, and the dye layer deposited on the Cu-phthalocyanine by spin-coating was also about 300 to 500 A. Table I lists the photovoltalc output of the cells when tested under simulated Air-Mass-2 illumination.
~ .

~ .

/, . _. . . , _ 1o8S947 TABLE I
.

Dye . 2 Conversion Example Q X Voc (volts) Isc (mA/cm ) Efficiency, %
2 0 0 0.55 0.9 0.29

3 0 S 0.45 1.5 0.43

4 S S 0.36 2.0 0.45 : 5 S Se 0.31 . 1-5 0.28 6 Se Se 0.24 2.0 0.28 7 0 Se 0.42 0.4 0.10 , . i~' - :, Examples 8 through 11 A class of dyes having the formula: .
. ' R13 0 H ,0-R1Z :
R13 0, - ~-\0 R12 CIO4 wherein R12 and R13 are para substituents selected from H, l:.
CH3, F, were tested as described in Example 1 in a photovoltaic c cell of the configuration described in Example 1. Table II
lists the output of these cells.
,:

'' ~

. ~ :

, .
- 21 - .

., ~ , . . . :
'. ' ' ' . :' ~:. ~ ' - .

`` 108S947 , TABLE II
. .
,', ' 2 Conversion Example_ Dye Voc (volts) Isc,(mA/cm ) Eff.iciency, %
8 R12=H,R13=CH3 ' 0.33 1.75 0.31 9 R12=H R13=F 0.30 2.15 0-35 10 R12=R13=F 0.20 1.6 0.17 ., ' 11 R12=R13=CH3 - 4 1.3 0.28 Examples, 12 through 19 Metal-free phthalocyanine and a number of metal- ~
phthalocyanines were used in a photovoltaic cell of the con-figuration described in Example 1. Phthalocyanine films of thickness ranging from 300 to 500 A were deposited on.clean Nesatron~ ~lass, and a 400 to 500 A thick film of 4-[(2,6-', diphenyl-4H-thiapyran-4-yIidene)methyl]~2,6-diphenylthiapyrylium ,.' '~
perchlorate was deposited on top of each phthalocyanine film by spin-coating. Indium was used as the top electrode.
Table III lists the output of these cells comprising various '~
phthalocyanlne layers.

' ' . '' ' ' .
~, .

. .
. ~,, - ~

.
' - 22 - , ' ., . . . .. .. . ... .... . . ..... ,,,~ . ~, .
.

~(3 85947 TABLE III

~ Conversion Example Phthalocyanine Voc (volts) Isc (mA/cm') Efficiency, %

12 Metal-free 0.25 0.18 0.02 13 Co 0.20 0-35 3 14 Ni 0.25 o.80 0.09 Cu o.36 2.00 0.45 16 Zn 0.36 1.10 0.14 17 Pb 0.35 3.50 0.50 1018 Pd 0.42 0.75 0.14 19 Pt o.38 1.25 0.21 '~' .

~.
EXam~Ple ?
.. . .
A mixturé of photoconductive dyes was used in the photovoltaic cell of the configuration described in Example 1. '~-A 400 to 500 A-thick Cu-phthalocyanine film was deposited on clean Nesatron~ glass by vapor sublimation. Then a 400 to 500 A thick film, containing a 1:1 mixture of 4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenylpyrylium perchlorate and 4-~(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenyl-thiapyrylium perchlorate, was spin-coated on top of the Cu-phthalocyanine layer. Indium was the top electrode. Under '-simulated Air-Mass-2 illumination, the cell developed an open-circuit voltage of 0.43 volt, a short-circuit current of 2 mA/cm2, and a fill factor of 0.44, giving an efficiency of 0.5 percent.

.:
i~

' ~ ".
: - 23 -~ ' , .

. _ . .. . . . .. . . , . . _ .. . . . .. ..
.
- .
. .

~V8594~

Example 21 Example 20 was repeated, except that the mixture of dyes comprised a l:1 mixture of 4-~(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthlapyrylium per-chlorate and 2,6-diphenyl-4-(4-dimethylaminophenyl)thiapyrylium perchlorate. The conversion efficiency was found to be about 0.5 percent.
' Example 22 In a cell configuration as described in Example l, Nesatron~ glass/copper-phthalocyanine (400 A)/photocondùctive dye/Ag, where the photoconductive, dye was a 400 to 500 A
fllm of 4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyrylium perchlorate and where silver was used as the top electrode; the cell developed an open-circuit voltage of 0.38 volt, a short-circuit current of 1.8 mA/cm2 and a fill factor of 0.4, giving a conversion efficiency of 0.36 percent. The cell was quite stable under prolonged illumination. On sub~ecting the cell to a 90-hour exposure to simulated Air-Mass-2 illumination, the cell reached a steady state efficiency of 0.23 to 0.25 percent, with no evidence of further degradation.

.
Example 23 A cell was fabricated as described for Example l, but the dye used was the following:

~ .
. ~ . ,~.

'i .

. ` ~08s947 ,I O I

I~O,I I~O I

The cell had a Voc = 0.500 V, Isc = 0.2 mA, a.fill factor of 0.28, and an efficiency of 0.05 percent.
.'~
Exam~le 24 Example 1 was repeated, except that the dye used was the following~

- ~ ~CH ( CH
6~S~ ~--C=-~ \0 3 2 C 10 \. /- I' ,, . i~

The cell had a Voo of about 0.5 V, an Isc of about 0.24 mA, a fill factor of about 0.34, and a conversion efficiency of 10 about 0.05 percent. . I .

Examples 25 through 27 Cells were fabricated as described for Example 1, - except that dyes of the following structure were used:

0\ R /0 - :
~Q/\ ~--C=-\ - \X Cl 04 , ~
0 \0' '''.

, . ~

, . _ . _ . . .. _ ~,, _ _ _ . ,, . _, _,,,,,, ,, .. , _ . _ _ . . _ lV8594~
,, .

Table IV lists the output of these cells for various substitutions at Q, X, and R.

TABLE IV

Conversion Example Q X R Voc (volts) Isc (mA/cm2) Efficiency, %

S S CH3 0.25. 0.4 0.036 26 0 0 C-N 0.4 0.4 o.o66 27 o 0 1 0.44 0.28 0.05 1:
/ \

~o`- I~o`l~ ~ ;

.

Exam~le ?8 ~ .
10A photovoltaic element was prepared and tested as described in Example 1, except that the dye layer, at a thick- j~
ness of about 400 A, was comprised of: , - eS~ 0 ~---N~ 3 C I O

~This element was found to have a Voc of about 0.52 V, an Isc of about 1 mA/cm2, and a fill factor of 0.40, producing a converslon efficiency of about 0.27 percent.
The invention has been described in detail with particular reference to preferred embodiments thereof, but F

.

~ 26 - .

- 108594~7 ~

it will be understood that variations and modifications can be effected within the spirit and scope of the inven- ~
tion. ::
,~ , , ' . ~,,~
" ' " ' ' ' ' '. ,' : , ' , ~ . ~ , ' '' ' ' ,''' ';

- ?
.' ' ' ' ' ' ~,, 1..
l~:

r --...
, ~ .

,r - 27 - , :

.. ~
., , . , :

Claims

WHAT IS CLAIMED IS:

1. A photovoltaic element comprising, a layer of a porphyrinic compound;
in contact with said layer, a layer comprising a photoconductive organic dye capable of absorbing light at wavelengths between about 350 and about 1000 nm and capable of forming a rectifying junction with said porphyrinic compound;
and an electrode operatively connected to each of said layers, said electrode comprising a material which creates an ohmic contact with said layers;
said element having a conversion efficiency when exposed to sunlight of at least about 0.02 percent.

2. An element as defined in Claim 1, wherein said dye is a pyrylium, thiapyrylium or selenapyrylium dye.

3. A photovoltaic element of enhanced conversion efficiency, comprising, in laminar array, an electrode comprising a semitransparent layer of nickel, indium tin oxide or tin oxide on a trans-parent support;
a layer of lead or copper phthalocyanine;
a layer of 4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyrylium perchlorate;
and a layer of indium.

4. A photovoltaic element comprising a layer of a compound having the structure:

wherein:
M is a metal;
T1 and T2 are both S or both C, or one of T1 and T2 is N and the other C;
X1 and X2 are the same or different, and are each halogen or hydrogen; and Z1 is the nuclear carbon atoms necessary to form a six-membered unsaturated ring.
in contact with said layer, a layer comprising a photoconductive organic dye capable of absorbing light at wavelengths between about 350 and about 1000 nm and capable of forming a rectifying function with said phthalocyanine;
and an electrode operatively connected to each of said layers, said electrode comprising a material which creates a non-rectifying junction with said layers;
said element having a conversion efficiency when exposed to sunlight of at least about 0.02 percent.

5. An element as defined in Claim 4, wherein said dye is a pyrylium, thiapyrylium or selenapyrylium dye.

6. An element as defined in Claim 4, wherein T1 and T2 are carbon and X1 and X2 are hydrogen.

7. A photoconductive composition, comprising a layer of phthalocyanine or metal phthalo-cyanine, and in contact with said layer, a layer of a photoconductive organic dye having the structure:

wherein:
J is or N;
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 6 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.THETA. is an anion.

8. A composition as defined in Claim 7, wherein said dye has a photoemission threshold which is higher than that of said phthalocyanine.

9. A composition as defined in Claim 7, and further including a layer of metal operatively connected to at least one of said layers, said metal being selected to make an ohmic contact with said one layer.

10. A composition as defined in Claim 7, wherein said dye is a thiapyrylium dye.

11. A photoconductive composition, comprising a layer of phthalocyanine or metal phthalo-cyanine, and in contact with said layer, a layer of a photoconductive organic dye having the structure:

in which R5 and R6 are the same or different and are each alkyl from 1 to about 6 carbon atoms, phenyl, or a 5 or 6-membered heterocyclic ring;
R7 represents an alkylamino-substituted phenyl or an alkylamino-substituted 5 or 6-membered heterocyclic ring having from 1 to about 6 carbon atoms in the alkyl moiety;
X is oxygen, selenium or sulfur; and Z.THETA. is an anion.