Note: Descriptions are shown in the official language in which they were submitted.
9 0 5 5
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This invention relates to a photovoltaic cell and in
particular to such a cell wherein the active ingredient of the
device is a "molecular p-n junction".
BAC~GROUND OF T~E INVENTIO~
Conventional p-n Junctions
In conventional silicon photovoltaic cells there is
a junction interfaced by "p"-type and "n"--type silicon each
respectively generated by diffusion. In one instance an
electron poor substance (for example boron) and in the other
instance an electron rich substance (for example phosphorus)
are used as impurities and are diffused into opposite sides of
a single wafer of crystaline silicon or preferably into a
single crystal of silicon. A "molecular p-n junction" is thus
created. The function of the p-n junction in such a
photovoltaic cell is to separate electrons and holes that are
respectively created by the absorption of light near the
junction.
There are two problems which arise with such p-n
junction silicon photovoltaic cells. Firstly, the junction
cannot be made very sharp and it usually extends in thickness
to over one hundred nanometers. The second difficulty is to
assure that the absorption of light will occur only at or near
the junction. With materials that are presently used this
precision is very difficult to achieve.
Photosynthesis
In nature, the primary step of photosynthesis is a
resultant of a reaction which can be contemplated to be
similiar to that of a solar cell. There is a charge
separation of an electron from a hole and this particular
separation takes place under the influence of light, solar
energy, within the highly organized reaction-centre protein
molecule. The potent molecule chlorophyll, which is
the primary absorber of sunlight in photosynthesis in nature,
is latched, or embedded within the complex of the
reaction-centre protein as an ingredient molecular structure
of living plantsO
Thus, within what is commonly known as the thylakoid
nO~5
membrane of a plant leaf, light is absorbed by a chlorophyll
entity (chl) and this emits or drives an electron across from
the chlorphyll entity of the protein molecule to an electron
acceptor, A twhich can be plastoquinone), on the other side of
the same protein molecule. Thus for this protein molecule, on
one side is an electron donor, D, and on the other side, an
electron acceptor, A.
The Invention
We have discovered that one may synthesize an
organic molecule so that it exhibits (an electron donor) a
donor portion ,D, and an (electron acceptor) an acceptor
portion, A, and when the molecule is subjected to light, the
molecule is polarized such that the donor portion becomes
excited; or, perhaps alternatively the acceptor portion
becomes excited depending upon the specific molecular
structure selected. In any event an electron is freed from
the donor molecule and thus the molecule may be appropriately
used as a free electron source.
It is thus a feature of the invention to improve the
efficiency of the p n junction by creating an analog thereof;
namely, a biological or "molecular p-n junction". In this
aspect, a light sensitive compound comprises, an electron
donor molecule, D, and an electron acceptor molecule, A,
spaced apart and inter-connected by an organic linkage of a
predetermined distance, d.
In a preferred embodiment this "molecular p-n
junction" is formed by an organic compound which has a
molecule susceptible of electron donation and another molecule
susceptible of electron acceptance, each of these two
molecules within the same compound or molecule. This molecule
~` consists of three molecular parts each of which can exist as
separate molecules. Hereinafter we will refer to these
molecular parts as the donor molecule, the acceptor molecule
and the linking chain even though they are parts of one
composite molecule. Thus the donor and acceptor molecules are
appropriately spaced apart and inter-connected by a linking
chain so that when the compound is irradiated with light, the
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electron donating or donor molecule, D, donates its electron
to the electron accepting molecule or acceptor molecule, A,
within the composite molecule.
In a preferred embodiment, these molecules are a
porphyrin (P) and a quinone (Q) inter-linked by a molecular
chain of an appropriate intra-molecular distance, d, and
formed into a single compound. This compound, if subjected to
the influence of light, is polarized to free an electron when
in the photochemically excited state. In this compound, the
porphyrin (P) and quinone (Q) are appropriately spaced so that
when the light does not irradiate the compound, the electron
does not necessarily "jump back" from the acceptor to the
donor.
An acceptable molecule having a donor and an
acceptor entity or region therein with an intramolecular
linkage is the compound of the following structure;
tu
a),10 C c~
~3 .
. . . C~
,
where n = 1 - -> 10
preferably 2, 3, or 4
In the molecule of the aforesaid compound, one of
the electron acceptors, A, has been identified as a quinone,
. The quinone is an analog to the natural electron acceptor,
t~;
1 1B9055
plastoquinone, in photosynthesis. The quinone has the
following structure; 0
~3
u
The donor of such compound is a porphyrin, an analog
of chlororphyll and it has the following structure;
~`1~
C~ O
,
CN3
The quinone and the porphyrin are inter-linked by a
molecular chain (intramolecular linkage) according to the
following formula (or of a similiar chain containing "amide"
groups in place of the ester groups);
O
: ,~ ,.
-C- O- ( CH2 ) n~~C~CH2~
Where n=l --> 10;
;: Where n is an integer valued 1 through 10 and preferably
valued at 2, 3 or 4.
The invention ';hus contemplates an organic molecule
as an active ingredient of a photovotalic cell, the cell
comprising;
(a) a first and a second conductive material;
(b) a film of mono-molecular thickness, the fllm
possessing molecules with porphyrin and quinone
regions and hence donor and acceptor regions,
the molecules oriented so that the donor regions
are aligned and in a relative juxtaposition,
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with donor regions against one of the conductive
materialsl and the acceptor regions aligned and
adhered to the second conductive material;
(c) one of the said conductive materials being
transparent to light;
(d) electrodes attached to the conductive materials;
and;
(e) means for connecting a load across the
electrodes whereby, when the light penetrates
into the molecules of the film, electrical
energy flows through the load.
The invention also contemplates a new use, as a
photoresistor, whose resistance varies in the presence of
light, of the compound consisting;
h3
~ 1l N 1~
~G~ CU~ C~
~ o
where n = 2, 3, or 4
The invention will now be described by way of
example and with reference to the accompanying drawings in
which;
Figure 1 is a diagramatic representation of a
reaction-centre protein in which is embeded a chlorophyll
entity.
Figure 2 is a schematic of a donor D9 and acceptor A
molecular regions of a molecule separated by a molecular
linkage of distance d.
Figure 3 is a diagramatic view, for explanatory
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purposes, of the thylakoid membrane in a living plant leaf.
Figure 4 is an equation showing the polarization of
the donor and the acceptor regions under the influence of
light.
Figure 5 shows two flow paths involving the
excitation of either the donor, D, or of the acceptor, A,
regions in response to the influence of light to create a
polarized donor-acceptor molecule.
Figure 6 is an embodiment of photovoltaic cell
wherein the mono-molecular layer includes acceptor active
molecules.
Figure 7 is a photovoltaic device wherein the
mono-molecular layer includes donor active molecules.
Refering to figure 3, a cross section of a thylakoid
membrane 10 is shown. The thickness of membrane is
approximately 60 A. There is a reaction-centre protein 12
containing a chlorophyll entity chl which when excited by a
ray of light 25 excites the chlorphyll entity chl by releasing
an electron, e~ as indicated, which migrates over to the
electron acceptor region A. This entity 12 in which this
reaction takes place is known as a reaction-centre protein.
Such thylakoid membranes exist within the leaves of living
plants. The protein reaction centres are organized in an
oriented manner within the thylakoid familiar to skilled
persons in the art. The electron acceptor A has been
identified in the published literature, some of which is
` identified in Schedule "A" to this application, as a
plastoquinone or modified quinone molecule. In general~ the
natural process of photosynthesis can be represented by that
depicted in figure 2 where D is an electron donor region of a
molecule, which is excited by light, and A is an electron
acceptor region. The "solar cell" within photosynthesis
produces a voltage of about 1 volt (1 V) and has a solar power
conversion and efficiency approximating 18%, which is better
than most commercial silicon solar cells presently available.
The plant then uses this electronic power to drive the
biochemical reactions of photosynthesis and ultimately to
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store a por-tion the electrical energy as stored chemical enery
in the form of carbohydrates.
This phenomenon i9 known as the in _vo
photochemical solar energy conversion and storage reactions of
photosynthesis.
A compound, which may be synthesized, and which is
capable of creating donor and acceptor regions has the
following structureO
~U,~
c~ ~(c~ 3~
~3 ,,
C~
where n = 2, 3, or 4
In our paper entitled "Intramolecular Photochemical
Electron Transfer in a Linked Porphyrin-Quinone Molecule as a
Model for the Primary Step of Photosynthesis", Nature-July 11,
1980, page 286, we disclosed the foresaid compound. In
figures 2 and 3 of that paper, and particularily in figure 3,
thereof evidence is given that such molecules are polarized
under the influence of light so that the left hand portion of
the molecule, which is the porphyrin, P, becomes the electron
donor, D, while the quinone, Q, the right hand side of the
molecule, becomes the electron acceptor, A. Depending upon
the length of P-Q compound, which is dependent upon the value
of n, stability or instability of the electron transfer is
achieved when the compound is under the influence of light.
In the compound there studied n=3.
; We now conceive that utilizing, for example, the
aforesaid compound within a mono-molecular film layer, and
organizing that mono-molecular layer such that the molecules
are ju~tapositioned and oriented wherein the acceptor and
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donor regions (porphyrin and quinone regions) are oriented in
relative juxaposition, such a mono-molecular film will, under
the influence of light directed normal to the film surface,
result in an electron charge separation according the equation
of figure 4.
If alternative compounds are selected having
properties consistent with the aforesaid compound, two
possible reactions will occur. Referring to figure 5, either
the reaction on the left hand side thereof or on the right
hand side thereof are possible in creating a po~arized D+-A-
compound. That is, depending on the D-A compound properties,
either the donor portion is excited by the light to the D*
state to emit an electron, or the acceptor portion of the
molecule is excited by light to A* state to extract an
electron from the donor portion.
In either flow path the common entity of a polarized
D+ - A- molecule is achieved.
Thus we perceive a new type of photovoltaic cell
utilizing as the active ingredient an organic molecule while
generating an EMF under the influence of light and comprising
this molecule into a mono-molecular oriented layer or film.
Thus, referring again to figure 5, either the D, or
A can absorb light. In the former case (which is a situation
analagous to photosynthesis), D functions as aphotochemical
reducing agent, whereas in the latter case, A functions as a
photochemical oxidizing agent.
As one of the inventors has earlier shown, J.R~
Bolton, Science Vol 202, page 735 (1978), the optimum
wavelength for conversion of sunlight t at least at the earth's
surface, to work - electricity - is approximately 840
nanometers. Thus D (or A) should absorb strongly in the
visible and near infrared region of -the spectrum if good
operating efficiency is to be achieved.
The distance, d, between D and A is determined by
; the length of the linkage and for the aforesaid compound
isdependent upon the value of n, where n is an integer. The
value of n therefore will determine the distance, d, between D
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..
. . .
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~ ~ ~9055
and A, a factor which will be important in establishing the
efficiency of the electron charge separation unde~ light.
If the distance, d, between D and A is too short,
e.g., when n=0, the electron transfer may occur in light but
there will be high probability of a back electron transfer
before the electron can be picked off from the acceptor, A. In
this case the acceptor cannot be coupled effectively to an
external electric circuit to cause electrons to flow. Thus
the efficiency in this case will be low.
On the other hand, if the distance, d, between D and
A is too great, e.g. when n has a very high value, this will
require an electron transfer over a large distance from the
donor to the acceptor and hence there will be a low
probability that such a transfer will occur in light and again
efficiency will be lost. The optimum value of the distance,
d, between the acceptor and the donor D, will be found in the
vicinity or the preferred range of 15 ~ to 25 A, as when n= 2,
3 or 4, although in certain applications n may have an
acceptable range of values of 1 through 10, whereupon
distances of 10 ~ to 40 ~ are possible. The distance, d, can
also be affected by the conformation of the molecule, which
preferably, in the mono-layer of the film, F, the D-A molecule
would be in the stretched confirmation.
Another important design criterion is the difference
in the electro-chemical potential between the D and A regions.
~nder all circumstances the reaction;
D - A = D+ - A-
must be uphill in energy (i.e~ a positive Gibbs Energy Change
or an overall negative change in the standard electro-
chemical potential ~ E). If ¦~ E¦ is too small theefficiency will be low; however, as ¦~ E¦ approaches
about (Eg~ 0.4)V (here Eg¦ is the energy of the excited
state), the thermodynamic limit is approached and efficiency
again will drop. Eg at 840 nanometers is approximately
1.5eV; hence,¦~ E¦ should be approximately 0.8 to 1.0V for
optimal efficiency.
Now referring to figures 6 and 7 and to the solar
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cells according to our invention utilizing a D-A molecule as
the active ingredient, such D-A molecule is arranged into an
oriented film generally shown as region F interposed between a
transparent conductive material 31 (31') and a solid
conductive material 30 (30').
In figure 6 the film, F, consists of D-A molecules
in juxtaposition as an oriented mono-molecular layer film, F,
preferably in the stretched conformation The D-A molecules
are oriented so that the donor ends D, are toward the solid
conducting electrode 30. Thus the D-A molecules are oriented
so that the donor, D, ends are towards the conducting
electrode 30, while the acceptor, A, ends are toward the
transparent conducting film 31. The film 31, is layered on a
glass sheet or other transparent sheet 33. When light 25
penetrates the glass 33 and the transparent conductor film 31,
it excites the acceptors A, as in the right-hand side of
Figure 5, and this extracts an electron from the molecularly
. attached donor, D. Electrons flow into the juxtapositioned
conductor 45, and into the external circuit 50, and load RL,
to be returned to the electrode 40 and the conductor 30 to the
donor regions, D.
Referring to figure 7, a solid conducting electrode,
for instance a good conductor such as silver material 30',
forms one of the conductors or conductive materials, and a
second transparent conductor 31', for example one formed of
another good conductor but of transparent material such as
tin oxide, is layered on a transparent sheet of insulating
material such as glass 33'. Alternatively the transparent
conductor 31' could be a ~ery thin coating of metal such as
aluminium such that a high fraction of the incident light is
transmitted through this layer. The acceptors are oriented to
be juxtaposed against the solid (non-transparent) conductive
~ material 30'.
- Rays of light 25, penetrate through the glass 33'
and the transparent conductive material 31' and the donor
regions, D, of the film F, which are adjacent thereto causing
an electron to be freed from the molecule, whereupon the
,
.
' ~' ' - ' '
.
. .
0 ~ 5
electron is passed to the acceptor, A, and to the solid
conductor 30' and hence to to the external circuit 50, and the
external load, RL.
In a variation of the embodiments of figures 6 and 7
the conductive material 30t30') and 31(31') may either or both
be semi-conductors.
It should now be appreciated that a D-A molecule
therefore, and particularly the compound
CU~
C~ R~ C
C~
is an organic material possessing properties of a "molecular
p-n junction."
: 30
.~r
. .
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5 5
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SUPPLEMENTARY DISCLOSURE
As clearly indicated on page 6 of the disclosure,
molecular chain ~intermolecular linkaye) which for
supplementary disclosure purposes shall be more specifically
defined as a linking structure, may include amides in place of
the specific ester groups already disclosed.
More particularly all linking structures, whether
containing amides or ester groups should be covalent and be a
strong absorber of visible and near-infrared light as the
latter will increase the electron transfer yield.
Of the amide containing linking structures, two specific
linkiny structures that include such amides are:
O O
C--C H2--
'~--(C:H2)--1~
~ H
where n = 2, 3 or 4
and
~/o \~
N~ ~ C--C H2--
Either of these two linking structures may alternatively
be used in place and instead of the earlier disclosed ester
containing linking structures, since both of these amide
linking structure, are covalent and of acceptable length
(between 10 A and 40 A).
'i~,
~ 3 ~9055
SCHEDULE A
(Relevant Publications)
1. Kong, J.L.Y. ~ Loach, P.A. in Frontiers of Biological
Energetics - Electrons to Tissues, Vol. 1 (eds Dutton,
P.L., Leiyh, J.S. & Scarpa, A.) 73-82 (Academic, New York,
1978). Abstract No. THPMA 12 of the 7th Annual Meeting of
the American Society for Photobiology, Asilomar (1979);
J. Het Chem. (in the press).
2. Kong, J.L.Y. and Loach, P.A. in Journal of Heterocyclic
Chemistry, June 1980, 737 "Synthesis of Covalently-Linked
Porphyrin-Quinone Complexes
3. Bolton, James R., Science, 17 November, 1978, Vol. 202,
pp. 705-711 "Solid Fuels".
4. Bolton, James R., Ann Rev. Energy. 1979. 4:353-401
"Photochemical Conversion and Storage of Solar Energy".
5. Bolton, James R., Ho, Te-Fu, McIntosh Alan R., Nature,
July 17, 1980. "Intramolecular Photchemical Electron
Transfer In a Linked Porphyrin-Quinone Molecule as a Model
for the Primary Step of Photosynthesis".
