Note: Descriptions are shown in the official language in which they were submitted.
1 ~ .
¦ ~ BACKGROUND OF THE I~ENTION
¦ 1. Field of the Invention
! The invention relates to photoelectrochemical
l ¦ devices, and more particularly to such devices including
¦ 5 one or more of the following features: stabilization
oE electrodes against dissolution; multi-chamber structures
permitting the use of non-transparent electrodes; multi-
¦ l photoelectrode structures, the various electrodes responsive
to differing portions of the spectrum; heat exchange
¦ means utili~ing the electrolyte for conversion of heat
energy developed therein; and multi-chamber structures
incorporating storage electrodes in combination with'
one or more of the previously discussed features.
,. . I
I 2. Background of the Invention
¦ 15 Great efort has been expended to provide alternatives
I ¦ to the finite sources of energy currently available.
One alternative recently contemplated is the generation
o electrical energy by conversion of solar radiation.
The scien~ific literature has provided several examples
20 ¦ of photoelectrochemical systems useful for the photo-
electrolysis of water or the photo-oxidation of some
'1 ¦ sult~ble redox 4pecies. The theory underlying these
¦ systems and phenomena is reasonably well understood and
is outlined, for example, in the following publications:
Gerischer, "Electrochemical Photo and Solar
, ¦ Cells Principles and Some Experiments, n Electroanaly~ical
' Chemistr r~, Vol~ 58,
' ~ , . I
, I .
' _ _ _ ~ 2 -
pp. 263-274 (1975); Manassen et al, "Electrochemical,
Solid State, Photochemical and Technological Aspects
of Photoelectrochemical Energy Converters," Nature, Vol.
263, pp. 97-100, tl976); Ellis et al, "Study of N-Type
Semiconducting Cadmium Chalcogenide-~ased Photoelectrochem-
I ical Cells Employing Polychalcogenide Electrolytes,~
J. American Chemical Society, Vol. 99, pp. 2839-48, tl977);
Wrighton et al, "Photo-Assisted Electrolysis of Water
i by Irradiation of a Titanium Dioxide Electrode," Proc.
Nat Acad. Sci.,_U S.A., Vol. 72, IJo. 4, pp. 1518-1522
i (1975); and Manassen et al U.S. Patent 4,064,326.
The photoelectrodes as generally described are
I semiconductors n-type semiconductors being photo-anodes
I I and p-type semiconductors being photo-cathodes. The
semiconductors may be large bandgap materials, for example,
. n-TiO2 or small bandgap materials, ~or example n-GaAs.
~owever, the application of photoelectrochemical semiconductor-
electrolyte systems to the conversion of solar radiation
to electrical energy suggests that semiconductors with
1 20 bandgaps near 1.4 eV will be the most efficient with
' respect to the amount of solar radiation that cen be
usefully absorbed and converted to electrical energy.
. This consideration is well known from the established
~' . theory o solid state photovoltaic devices. Until recently,
however, small bandgap materials could not be employed
as photo-anodes for example, since irradiation in the
presence of an electrolyte usually resulted in the pnoto-
dissolution of the semiconductors. Several examples
.
....... _._ . , . .. , .. ... ,.. .... ,. . . .. , _.. ~ ..... _ __. , . .......... _ _. .,
.: :
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¦ of redox couples are now known that will essentially
I l eliminate the photo-dissolution of small bandgap semicon-
; ¦ ductors.
., I
SUM~RY OF THE INVE~TI0~3
I ~
i 5 ¦ The present invention accordingly provides a complete
I cell, incorporating therein small bandgap materials and
¦ proper stabilization of the electrodes to prevent the
photodissolution thereof. Moreover, the present invention
provides for multi-chamber structures permitting the
! lo I use of non-transparent photoelectrodes, as well as manu-
¦ facturing processes for the production of such cells.
¦ Additionally, the present invention provides for the
¦ use of multi-photoelectrodes, with resulting response
I to separate portions of the electromagnetic spectrum.
¦ The above Eeatures are further combined in the present
invention with a structure permitting the use of the
¦ electrolyte as a workinq heat exchange fluid. Moreover,
`~ ¦ a charge starage feature is provided by the present invention.
i ¦ It is accordingly a primary objective of the present
¦ invention to provide an economical photoelectrochemical
I cell for the useful conversion of solar energy to electrical
_ energy.
~- It is another object to provide useful manufacturing
¦ techniques for the above.
An additional object is to provide photoelectrochemical
cells utilizing a plurality of photoelectrodes, each
being sensitive to a different portion oE the solar spectrum.
I . .
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..... ,.. ~. ..... ., .. .. ,... , .,.,,.. .... ,.,,_" ., . . .. .,~ ,,_. ,. ., ,___ _ I
.. . .
l~.Z~3~
A further object is to provide tecbnique~ for
¦ the improvement of the performance of specific photoelectrodes
by post-deposition treatments.
It is still a further object Oe the inventiOn
to provide an improved voltage output of a specific photo-
electrode by doping the semiconductor material used therein
with a suitable impurity.
Yet another objective of the invention is to provide
an improved electrolyte for a photoelectrochemical cell.
¦ It is an additional object to provide a photoelectro-
chemical cell wherein the electrolyte is used as a working
heat exchange fluid for the transfer oE ther~al energy.
Still another object is to provide an i~proved
photoelectrochemical cell capable of storage of electrical
charge via the use of a storage electrode in addition
to the photo-anode and cathode.
These and other objectives, features and advantaqes
of the present invention will become apparent upon reading
the specification, particularly in conjunction with the
attached figures.
. '
~; B~IEF DESCRIPTION OF T~IE Dn~s~INGs
l PIG. la illustrates the basic structure of a photo-
., eiectrochemical cell.
FIG. lb illustrates a specific detail of the cell
of FIG. la.
FIGS. 2a and 2b provide current voltage characteristics
for a cell.
I
~ ' - 5 _
.
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~ 37~
FIG. 3 shows a plural chamber embodiment of the
! present invention.
1 ¦ FIGS. 4a and 4b illustrate a multi-photoelectrode
l ¦ embodiment.
I 5 ¦ FIG. 5 provides operating data for the cell of
FIG. 4a.
¦ PIG. 6 illustrates the utilization of the present
cell in a heat exchange environment.
! I FIGS. 7a, 7b and 7c illustrate the combination
of several embodiments incorporating therein a storage
electrode.
' .
D~T~IL~D DESCRIPTIOl~ OF THE P~EFEP~ED E~lBODI-~ENTS
- ¦ The basic embodiment is shown in FIG. la and includes
a composite electrode 8. A pair of electrodes 11 and
15 j 15, at least one of which is transparent to light, is
connected to wires 16 and 17. Wires 16 and 17 are con-
nected to the electrodes for conducting the photoelectro-
chemically generated current Erom the cell. An electrolyte
14 is surrounded by walls formed by an annular spacer
20 ¦ 13 and an epoxy structure 20. Elements 10, 11 and 12
comprise a composite electrode 8 receiving incident
radiation thereon. Transparent substrate 10 and transparent
.. , conductor 11 are commercially available as NES~ glass. Layer
' 12 comprises a scmiconductor photoeléctrode.
Element 11 is a transparent conductor to which
¦ is connected wire 16. Leads 16 and 17 are connected
I to an electrical load 19 by way of switch 18.
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74
The cell structure further includes fill-holes 22
shown as penetrating electrode 15 but clearly placeable in other
components of the present structure. The fill-holes may be placed
in the spacer section 13 or electrode 8, for example. Electro-
lyte material is injected through the fill-holes to the volume
provided therefor, and the holes may then be sealed. The seals
may bepermanently or replaceably installed in holes 22.
Lead 16 may be bonded using conductive epoxy adhesive,
to the composite electrode 8 by providing a tab 24 therein as
shown in FIG. 1 b.
As disclosed in U.S. Patents Serial Nos. 4,085,257
and 4,118,567, issued April 18 and October 3, 1978, respectively,
assigned to the assignee hereof, photosensitive suspensions may
be provided in the electrolyte or may be provided as a layer upon
the electrode 11, thereby providing a semiconductor photoelectrode
for the device. Unlike the above-mentioned applications, however,
rather than using titaniumdioxide the present invention contem-
plates the use of a narrower bandgap semi-conductor elements.
Wrighton et al.,supra, has disclosed that a single cry-
stal semiconductor electrode may be stabilized by the use ofproper redox couples within the electrolyte. To avoid the dis-
solution and decomposition of the electrode, certain chemical
species must be added to the electrolyte.
One feature of the presently preferred embodiment is the
use of polycrystalline electrode in contact with an electrolyte,
advantageously providing economic savings, as
'' .
'7'~
¦ ~ell as the desired tructural properties, permitting ~he
¦ electrodes to be transparent, or to be formed as thin films
¦ where needed.
Unlike prior art devices, such as disclosed and claimed
S ¦ in the aforementioned co-pending applications, the present
device utilizes polysulfide ions as the redox couple
in a closed loop regenerative system. The system permits I .
1. I the ions to be oxidized at the front electrode 8 responsive
¦ to photoactivity, the electrons given up thereby traveling
10 l along lead 16, switch 18, load 19 and lead 17 to the
¦ counterelectrode lS where the oxidized ions are then
¦ ¦ reduced after having migrated to the electrode by means
! of ordinary diffusion. While polysulfide ions are disclosed
. . I in the previous discussion, the general class of poly-
l chalcogenide electrolytes may be used, and polysulfide
. ions are merely used as an illustrative example.
¦ .With respect to the fabrication of the cell shown
I $n FIG. 1, the gene.ral procedure comprises the steps
! l of depositing a semiconductor thin film on a transparent
;~ 20 l conductive substrate, performing a postdeposition treatment
¦ of the coated substrate, such as heating and~or etching
; for example, attaching a lead to the transparent conducting
_ portion of the electrode, placing an inert gaskct material
. ¦ around the perimeter of tle electrode, placing a stable
1 I counterelectrode on top of the ~asket, scaling the cell
. ¦ with a non-conductive sealant to prevcnt leakage, filling
~ the cell with an electrolyte and sealing the fill-hoIes.
I ., l The lead wire 17 is attached to the counterelectrode 15
. i ¦ after placement of the counterclectrode on the gasket 13.
1~ I
. - 8 - 1.
.
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~ 2 ~37~
¦ Further considering the structure shown in FIG. la,
I semiconductor thin film layer 12 comprises a polycrystalline
¦ thin film which is at least partially transparent, for
. ¦ examplc, having a thickness preferably no greater than
; 5 ¦ 25 micrometers ~m). Semiconductor materials such as
I ¦ CdS, CdSe, CdTe or GaAs or others known to those skilled
. in the art may be used to abricate layer 12. The materials; , ¦ may be deposited by sputtering, chemical vapor deposition
! ¦ ~CVD) or vacuum evaporation of the compound and~or the
¦ constituent elements on a transparent conducting substrate
1 ~commercially available as NESA glass) comprising doped
SnO2 for example. However, the conducting substrate 10, 11
¦ may similarly comprise SnO2+In203 alloyj Cd25nO4, etc.
. I Layer 10, may, however, comprise plastic or a~y other
; 1S I transparent material as alternatives to glass.
The semiconductor thin film 12 may be rendered
¦ n-type or p-type by proper choice of suitable deposition
conditions, or by subsequent heat treatment or doping
j ¦ as is well known in the art. The presently preferred
embodiment contemplates the use of an n-type material.
¦ Deposition conditions or subsequent heat treatment render-
ing the material n-type may not be necessary where the
_ material ~nitially emerges as n-type, however,.
It i9 to be emphasized that the post-deposition
j 25 treatmcnt is not used expressly ~or the purpose of changing
. the majority carrier typc. That may be accomplished by
other procedures, as is known to those skilled in the art.
The present treatment has bcen found to be beneficial,
. though thc exact effects are not fully understood. It is
. .
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known that such treatment may crystallize an a~orphous film,
may increase or decrease conductivity, and may change carrier
type.
Electrolyte 14 comprises an aqueous tor other
polar solvent such as methyl or ethyl alcohol, fo~ example)
solution, which may be acidic or basic in nature. The ,
preferred eMbodiment utilizes an electrolyte containing
NaOII or ~OH, or example, to provide a basic solution
having pH greater than 7. Typically, the electrolyte
layer may have a thickness in the range of 1-10 mm and
contacts as much of each electrode as is practical.
Other thicknesses are, of course, also possibLe. ~he
electrolyte further contains ions from group VI ~poLy- ¦
chalcogenide ions~ to stabilize the photoelectrodes against
photodissoLution. SulEide (S=) or selenium ~Se ) ions
are presently used at a minimum concentration o} 0.2 ~1.
Counterelectrode 15 is made of carbon impregnated
with a metal selected from the group co~prising Pt, Co,
Ni, ~e, Pb, or Cu, or a mixture thereof. The electrode
is ordinarily fabricated by soaking in a solution of
the desired metal salt and by a subsequent heat treatment,
as is known to those skilled in the art. Alternatively,
the metal itself or a physical mixture of the metal powder,
a filler tsuch as C powder), a binder tsuch as a Teflon
susp~nsion) presscd into a metal screen ~a technique
well known in fuel cell and battery technology), or a
thin film of the desired metal is deposited on a suitable
substrate.
'.' .'
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~ 2'~?'7`~ ~
Gasket 13, preferably an inert non-conducting
¦ material, is used to insulate elcctrodes 11 and 15 _om
¦ each other. The gasket further provides the structure
or containing a liquid electrolyte. Materials such
as silicone rubber, TeElon, Glass, etc., are suitable
I for use as a gasket. As previously mentioned, the assembly
¦ is subsequently sealed. In the preferred method of
, ¦ fabrication, the assembly is edge sealed in such a manner
i that electrolyte leakage is prevented. Sealing may utilize
¦ a common, non-conducting epoxy adhesive 20, or instead
¦ use a silicone resin, or the li~e. Subsequent to the
¦ sealing, fill-holes 22 are utilized to fill the cell
with electrolyte.
¦ Of course, the amount of electrolyte used to fill
1 15 1 the cell is chosen such that the volume expansion due
i I to heating from incident insulation during operation
¦ does not lead to excessive internal pressure, i.e., the
! ¦ cell is made to be leak tight.
i Leads 16 and 17 are attached to electrode layers
2~ 11 and lS utilizing a conducting metal-filled epoxy,
¦ solder, or the like. Inasmuch as contact is advantageously
l made to conductor 11, the characteristic or ohmic contacts
_ ¦ which would have to be considered if semiconductor 12
¦ were contacted, are of no concern.
; 25 ¦ To providc ease Oe fabrication of thc contact
between lead 16 and electrode 8, a tab is provided in
I the electrodc structure as shown in FIG. lb. Specifically,
i I semiconductor photoclcctrodc layer 12 may be deposited
I
I . I
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~ 1~ 24~74
on a section o~ N~SA substrate 10 and 11, thereby leaving
an exposed portion thereof, shown as tab 24 in the figure.
Alternatively, the entire substrate may receive thin
film se~iconductor 12 thereover, and a portion of layer
12 subsequently may be etched a~ay to expose conductor
ll, thereby again forming pad 24.
In operation, the structure hereinabove described
; provides a negligible dark voltage diference between
¦ electrodes 11 and lS, less than 50 mV. Connection of
, 10 1 leads 16 and 17 to a load reduces the voltage to effectively
¦ l zero. Thus, the structure is not in itself a battery.
! No work is done thereby without irradiation. Upon irradiation, j
hDwever~ a photovoltage is generated ~in the range of -
0.3 to 0.8 volts) depending upon the radiation intensity
t 15 and spectral distribution. The voltage is generated
responsive to the interaction of the incident radiation
with the semiconductor-electrolyte junction present
at the interface of the semiconductor photoelectrode
12 and electrolyte 14. The Gerischer and Manassen et
l 20 al references, supra, outline the origin of the photovoltage
I ~ as is well known to those of ordinary skill in the art.
The generated photovoltage provides terminal ll with
a negative polarity and 15 with a positive polarity,
and provides for a flow of electrical current through
a load connccted therebetwcen. Specifically, electrons
I I are driven in the-external circuit from electrode 11
to electrodc 15, and power is thereby delivcred to the
,,, .
. j.
. - 12 -
,, _,,, _ . _ , ... . _ .. _ . . .... , .. . .. . .. _ .. .. .. . .
~: ~ z~
The following examples of cell structures illustrate
¦ sevcral featurcs of the prescnt invcntion in conEormity
with the cell shown in FIG. la.
. ''. . .
I EXA~IPLE 1
A suitably cleaned piece such as In2O3 - SnO2
coated glass approximately 5 cm x 5 cm x 0.2 cm thick
¦ having a surface resistivity of approximately 20 ohms
per square (obtaincd co~mercially) is placed above a
CdSe ~ 1~ ZnSe (by weight) pressed powder target in a
cor.~ercial sputtering unit. The CdSe-ZnSe mixture is
sputtered at 400 watts forward po~er or 1 to 2 minutes
usinq argon gas at 10 microns pressure. The substrate
is masked so that a tab oE uncoated In2O3-SnO2 is available
Sor contacting with a silver epoxied lead. The sputtered
film and substrate are treated, post deposition, by
i heating at 375C for 15 minutes in air. A copper wire
. is then epoxied to the heat treated electrode, contacting
¦ only the In2O3-SnO2 film. A silicone rubber gasket approxi-
mately 2 mm thick is placed around the edge of the electrode.
A carbon piece approximately 5 cm x 5 cm x 0.2 cm, impregnated
l with a metal salt solution of CoC12 and thiour~a and
_, then fired at 300C in air for 10 minutes, is placed
~ over the gasket. It is believed that the deco.~position
; product is a sulfide of cobalt which acts as an electro- ¦
' 25 catalyst. The carbon electrode is provided with two
small holes drilled therethrough to allow filling of
! the cell with electro1yte. A coppcr wire is then epoxied
,,,, I . '
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to the carbon electrode and the cell is sealed on its
sides and back with a silicone adhesive (with the exception
¦ of the fill-holes). After the resin has cured, the cell
¦ is filled with a hypodermic needle with an aqueous solution
which is 1 M in NaOH, 3 M in ~Ja25 and 3 ~t in S. The
fill-holes are plugged and the cell is ready Eor use.
The I-V characteristics of a typical cell prepared in
; this manner are shown in FIG. 2.
, FIG. 2a shows the current voltage curve of a completed
¦ 10 cell as described above undcr a light intensity of 20 mw/cm2
i from a Xe arc.
FIG. 2b shows a potentiostatic current-voltage
curve of a a CdSe photoelectrode prepared as above measured
against a standard saturated calomel reference electrode
~SCE). The current density is plotted against the potential
¦ of the electrode relative to SCE. The rest potential
~Vr) is at -0.72 v vs. SCE. The current density at this
' potential is 4.5 mAfcm2 which is the shorting current.
The potential at zero current is -1.16 v vs SCE. The
open circuit voltage ~VOC) is therefore 0.44 v. The
light intensity was 20 mw/cm2. The corresponding curve
in the dark is also shown. I
, EXA~!PLE ?
l The structure is like that of Example 1, but utilizing
; pure CdSe for the semiconductor rather than CdSe ~ 13 ZnSe,
. ; as discloscd in Example 1. The observations were that
thc pure CdSe provides an increascd dark current, and
.' . . ' I .
- 14 - I
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I 1Z~13~ ~
that the open circuit voltage V decreased from 0.4
to 0.3 volt (see the potentiostatic curve, FIG. 2b).
EXA~IPLE 3
The structure essentially of Example 1, but without
, 5 1 the post deposition treatment of the doped semiconductor
film of CdSe ~ 1~ ZnSe. Without firing the ilm in air
after the sputtering deposition, the potentiostatic ISc
(shorting current~ decreased from 4.5 mA/cm2 to approxi-
: mately 0.4 mA~cm2 under similar conditions.
:, ~ . I
! lo l EXAMPLE 4
The structure essentially as disclosed for Example 1,
but in which the CdSe + 1~ ZnSe was sputtered at a 300 watt
power level. The IsC decreased from 4.5 mA/cm2 to approxi-
mately 2 mA/cm~ under similar conditions.
' I .
15 1 EXA~PLE S
. Essentially~the same structure as in Example 1,
but in which the electrolyte used was 1 M in NaOEI, 1 M
in Na2S and 1 M in S. ~he I-V characteristics under
$11umination are similar to that Oe Example 1 at low
light lntensities ~lower than 20 mW~cm2), but performance
I ~ drops off at higher light intensities.
. ReCerring =ow to FIG. 3, an alternate embodiment
of the invention is shown, wher~in the structure diEers
but oporation is essentially the same. Specifically,
a plain transparent window 30 is provided, supported
;. , .
-15-
_, .
: .
by inert gasket material 31a which also functions to
contain electrolyte 32 between window 3~ and semiconductor
photoelectrode 33. In view of the electrolyte receiving
the incident radiation before the semiconductor photoelectrode,
it follows that photoelectrode 33 need not be transparent
in this embodiment, as was required in the embodiment
of FIG. 1. ~hat is, the photoactive site in the present
embodiment is at the upper junction of photoelectrode
33 and electrolyte 32.
¦ Semiconductor material 33 further need not necessarily
be a thin film, inasmuch as the requirement for trans-
parency has been removed. Semiconductor 33 is deposited
on a conducting substrate 34 (such as Ti, Pt, Au, C,
etc.) by sputtering, CVD, vacuum evaporation of the
compound and~or the constituent elements, or electro-
deposition for example. The latter technique of electro-
deposition is not available for the first embodiment inasmuch
as destruction of the transparent conductor would result. It
is possible, however, that a transparent conductor may
ultimately be discovered capable o~ withstanding the
cathodic bias involved in the operation so that this
technigue may be used for the embodiment o~ FIG. 1 as
well. The substrate may, but need not, be a metal.
Photoelectrode 33 and substrate 34 include registered
holes therein, permitting the electrolyte to communicate
betwcen the chambers above and below the photoelectrode.
As discussed supra, the fact that the electrolyte is
exposed to radiation in the upper chamber permits a departure
from the thin film, transparency requirement o~ the
.
- 16 -
.. . . .. . . . ,, . ,. ... .. _ , _.,.. .. .. .... .. ~ . .... ., . .. _. _ . ...
37~ !
photaelectrode in the first emhodiment. Similarly, substrate
34 neod not be transparent.
The cell includes a second, lower chamber, between
the photoelectrode and the substrate therefor and the
counterelectrode 36. Holes 35 permit the electrolyte
to communicate between the two chambers. Gasket 31b
provides for separation between the substrate 34 and
counterelectrode 36, as well as orming a portion of
the chamber for containin~ the electrolyte between the
substrate and the counterelectrode
Holes 35 are used in ehe present embodiment to
permit the oxidized ions to travel to the counterelectrode
for reduction. Of course, an alternative embodiment
could be provided permitting a passageway external to
the cell, rather than within the cell as shown herein.
It is essential for the purposes of the present embodiment,
however, that the electrolyte contacting the semiconductor
photoelectrode be in contact with the electrolyte contactinq
the counterelectrode.
Wires 37 and 38 are shown connected to the photoelectrode
(specifically, by way of illustration, to the metal substrate
thereof) and to the counterelectrode. Wires 37 and 1
communicate the power generated within the cell by way
o~ switch 39 to electrical load 40.
................... 'l l EXAMPLE 6
The photoelectrode is fabrlcated as follows: ¦
a piece of titanium sheet approximately 0.04 cm thick
. . I .
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~ I - 17 -
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1~ 2 ~ ` 7~ 1
and having the dimensions of 2 cm x 5 cm, with several
small holes drilled therein, is held above a CdSe pressed
powder target in a commercial sputtering unit. The CdSe
is sputtered at 400 watts forward power for five minutes.
S The CdSe/Ti electrode i5 ~hen heat treated for 15 minutes
I , I at 4250C in air. Referring to FIG. 3, a copper wire
I is attached to the Ti and the cell is assembled as follows:
; on glass cover 30 is placed silicone rubber gasket 31a.
I ¦ The CdSe/Ti electrode fabricated as above is placed on
¦ the gasket, and gas~et 31b placed over the combination.
~he carbon electrode, imDregnated with cobalt sulfide
as in Example 1, is placed thereover, and the cell sealed.
The aqueous electrolyte is the injected into the cell,
¦ and the fill-holes sealed. The following potentiostatic
performance parameters are typical of such a photoelectrode:
Voc ~open circuit voltage) 0.35
' ISc (shortlng current)~Y3 mA/cm2
- I PmaX (maximum power output) 0.3 mW/cm2 ~at
j a light intensity of 20 mW~cm2)
~ 20 ~tsolar to electrical po~er conversion efficiency)
,' , 1.5~ ' 1
F.F. ~Eill factor) 35~ ¦
l . I
j EXA~IPL~ 7
, ¦ The structure is similar to that of Example 6
- 25 1 but rather than sputtering, the CdSe was electro-deposited
¦ from an aqueous solution of 1 gm CdSO4, 0.2 gm SeO2 in
¦100 ml 1 N1125O4. Thc properly cleancd Ti is madc the
I
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,
1~.2~'7 ~ ~
¦ cathode during electrodeposition, and several microns
j of CdSe are electrodeposited at a current density of
60 mA/cm2 for lS0 seconds. ~he film is rinsed in de-
ionized water and heat treated in air at 425C for five
minutes. Under potentiostatic conditions with an electrolyte
l M in l~aOH, l ~t in Na2S, l M in S electrolyte and 20 m~Y/cm2
irradiation from a Xe arc, the open circuit voltage is
0.4 volt and the short circuit current is 3 mA/cm2,
. I
l EXA~;rLE 8
! lo l The structure is essentially similar to that of
Example 7 but with a different post deposition treatment:
the CdSe film is buried in powdered CdSe, fired in a
¦ N2 atmosphere at 700C for l0 minutes and subsequently
¦ ¦ in air at 425C for l0 minutes. ~he open circuit voltage
i lS was 0.6 volt rather than 0.4 volt due to a substantial
decrease in dark current. Of course, where the substrate
is glass the treatment at 700C cannot be done, inasmuch
i ¦ as the glass would melt.
EXAMPLE 9
¦ 20 1 The structure is essentially similar to Example 8,
! but in which the filM, after firLng, is etched in a 50g
~ ~Cl + H2O solution for five seconds. ~ film which gave
; l milliamp per syuare centimeter shorting current before
etching gave a current of 3 miIliamps per square centimeter
after otching and the ill factor improved.
~, . .
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.. . . . _ . _ . . .. . _ _ ,_ _ . . _ _ .. . .. .. .... _ , . . .. . . .
~ .Z43~7~
FIG. 4a provides yet a third embodiment of the
¦ present invention and illustrates a situation wherein
three electrodes are provided in a single cell. The
¦ cell includes a composite electrode 41, comprising a
¦ transparent substrate 41a, along with a transparent con-
¦ ductor 41b and a first semiconductor film 41c. The semi-
¦ conductor utilized in the present em~odiment is CdS,
a film passing radiation of wavelength in excess of
I 5,500 A. A chamber is formed having gasket segments
l 42a and 42b for separating counterelectrode 43 from the
first semiconductor film 41c and from a second photoelectrode
I comprising a second semiconductor film 45. The second
I ¦ semiconductor film comprises, for example, CdSe, material
responsive to radiation of wavelengths less than 7,200 A.
¦ Contained within the chamber formed between electrode
¦ 41c, gasket 42a, counterelectrode 43, gasket 42b and
¦ electrode 45 is an electrolyte 44. The semiconductor film
¦ 45 is deposited on a conducting substrate 46.
¦ Electrolyte 44 is chosen to pass radiation having
~ 20 l wavelengths within the range of wavelengths passed byI I the first semiconductor and those effective on the second
semiconductor. Particularly, where the first semiconductor
l ls cadmium sulfidc, having an orange appearance and passing
; ¦ radiation with wavelengths exceeding 5,500 A, an orange
¦ electrolyte, a polysulfide electrolyte, is chosen.
I Thus, the photoactive site formed at thc interface between
¦ the electrolyte and the first semiconductor elcctrode
¦ is responsive to radiation oE wavelengths shorter than
I
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Z ~ 3'~4
i 5,500 A. Any radiation oE wavelength greater than 5,500 A
.. . I is passed by the CdS and through the electrolyte to the
: . second semiconductor film. At the interface between
. ¦ the electrolyte and the second semiconductor film, where
¦ 5 the interface i5 responsive to wavelengths shorter than
¦ ~,200 A, all radiation passed by the first photoactive
I . site ls lncident on the second photoactive site.
The second semiconductor Eilm provides a seco.nd
electrical output of the cell, in addition to the first
¦ output taken from the transparent conductor as in ~IG.
1. Leads 48, 49 and 50 are connected to the transparent
: conductor, the counterelectrode 43 and the second semi-
l conductor 45. Switching means 52`and 53 are provided
¦. ¦ to enable connection of the electrical power on leads
48 and 50 to load 51 in any combination. Thus, either
one or both of switches 52 and 53 may be closed to provide
.~ ~ . power to load 51.
- . As discussed in connection with PIG. 3, second
semicondùctor film 45 need not be transparent, inasmuch
as any radiation has already passed through the electrolyte
.¦ and the photoactive junction thereof prior ta impinging
on the ilm itsel~ ¦
I . . From the prcceding discussion, it is concluded
that the semiconductor film having a smaller bandgap
li.e., responsive to radiations of longer wavelengths)
. ¦ must be in the back Oe the cell. Othert~ise, if such
~. a film were provided at the first photoactive sitet then
: I only radiation of insufEicient energy to excite the . .' ..
.
. , - 21 - ~
. , , . .. . _._ ._.... - .-,.. ._.. ,__. ~.. . . . _, ,, _,._ _,. .... .
1~ '7~ ~
¦ s~aller bandgap (of wavelengths greater than 7,200 A
¦ in this example) would be passed thereby. This low energy
¦ radiation would then impinge on the photoactive site
¦ at the semiconductor having even a larger bandgap energy,
¦ and accordingly not cause any reaction thereat.
The presently provided switching arrangement permits
the two photoelectrodes to be employed separately or
¦ to be tied together inasmuch as both are photo-anodes.
¦ An alternative example to the present structure
10 ¦ utilizes CdSe as the front electrode and CdTe, a smaller
¦ bandgap ~aterial at the back, along with a polyselenide
¦ elcctrolyte. As previously mentioned, the electrolyte
is chosen to have a similar cut-off to the first electrode.
¦ FIG. S shows the I-V curves for a cell of the type oE
15 ¦ FIG. 4a hereinabove described utilizing a sputter deposited
¦ CdS film on a conducting Indium oxide coated glass substrate
¦ as a front photo-anode~ and a sputter deposited CdSe
film on a titanium substrate as the second photo-anode.
¦ The electrolyte utilized was 1 M in NaOH, 1 M in Na2S,
1 M in S in water. An annular ring of carbon was used
as a counterelectrode, and silicone rubber was the ~as~et.
The cell was irradiated by a Xe arc at an intensity of
2a mw/cm2. Curve 5a shows the I-V curve for the CdS
photo-anode, and curve Sb shows the I-V curve for the
25 l CdSc photo-anode. Curve 5c shows the I-V curve when
the two photo-anodes are connccted in parallcl illustrating
the advantageous esult obtaincd with this e~mple.
. I
- 22 -
., .. I ..,
.. _ .
~ 2~7~1
¦ FIG. 4b shows another embodiment where the two
¦ semiconductor layers are deposited on opposing sides
i ¦ of a single substrate. The substrate is transparent
¦ and is coated on both sides with a transparent conductor.
! 5 l The counterelectrode may readily be made as large as
- ¦ the photo-anodes, ~hich is ~nown to be beneficial.
¦ Although the radiation has to traverse an electrolyte
layer beEore reachin~ the first photoelectrode ~403),
I it now has to pass only through the transparent conductors
¦ 10 l ~404 and 406) and the transparent substrate (405) before
reaching the second photo-anode (qO7).
Yet another embodiment of the present invention
. , ¦ is illustrated in FIG. 6, wherein the photoelectrode
I I chemical cell is shown as simultaneously performing the
~ 15 ¦ two functions of electrical power generation and solar
I i ¦ heat absorption. Thus, the illustrated structure comprises
¦ a composite electrode 60, having the transparent substrate,
; I conductor and semiconductor photoelectrode combination
discussed in connection with the previous embodiments.
¦ A gasket seal 61 is shown between the electrode 60 and
counterelectrode 62. Electrolyte 63 is contained within
the chamber formed by the electrode, the counterelectrode
. and the gasket, and leads 64 and 65 provide the electrical
~l energy Erom the electrodes, via switch 66, to load 67,
~ ! 25 as previously described.
I ¦ In addition to the electrical generation of the
cell, a second function is performed by the present structure.
Specifically, electrolyte 63 experiences an increase ~n
'. '
- 23 -
~.. s~ . .. ..... . ... .
~L~ 2~7~
I temperature as a result o~ heat exchangc with the other
; ¦ structural co~poncnts of the cell and due to absorption
¦ of long wavelength infrared radiation. The electrolyte
is circulated by means of piping 68 to heat exchanger
¦ 69 by way of pump 70.
~hus, the electrolyte is effectively used as a
¦ primary heat exchange fluid circulated by the pump 70.
¦ l As a result oE the present structure, the system is capable
I of converting solar energy into useful energy capable
¦ of doing work by two mechanisms. Firstly, electrical
¦ energy is directly generated by the photoelectrochemical
¦ processes previously described. Secondly, heat energy
; is generated for hot water or space heating applications,
for example. Of course, the use of the cell for heating
~5 applications is not limited to the structure shown in
FIG. 6, but may similarly be provided for the previously
described embodiments.
Referring now to FIG. 7, three cell structurès
are disclosed which incorporate therein a storage electrode.
; 20 Prior to description of the structure utilized,
,, the following is provided by way of explanation. In
normal operation of a photoelectrochemical cell of thc
. type described, a first electrode, the counterelectrode,
- is utilized having essentially constant potential, determined
by the redox couple in the electrolyte, regardless of
;~ , the state Oe its irradiation. That is, the electrode
may be in the dark or in the light, and maintain the
~ same potential. nPotential" as utilized herein is
- - 24 -
..
. __ . . . .,,.. ,.. _ _ ... ., .. _ _ .... ..... _ _. . . ...... _ _......... ... . _ ... ... . ....
~ 2~
esstentially with respect to a reference electrode, such as a
saturated calomel electrode (SCE) for example.
A second electrode, the photoelectrode, is utilized
having a potential variation with respect to SCE responsive to
irradiation. Thus, a CdSe electrode may show a potential of -0.7
V with respect to SCE in the dark, and may have a potential of
-1.1 V with respect to SCE in the light. A carbon electrode pos-
sesses a potential of -0.7 V with respect to SCE both in dark and
light conditions. If a third electrode is provided which can
undergo a reversible redox reaction, is compatible with the elec-
trolyte and has a redox potential greater in magnitude than the
potential of the counterelectrode, but less in magnitude than the
potential of the irradiated photoelectrode, then a photoelectro-
chemical cell of the type described can be converted into a stor-
age cell.
; A structure embodying a storage electrode is disclosed
in U.S. Patent Serial No. 4,118,546 issued October 3, 1978,
"Triple Electrode Photogalvanic Cell with Energy Storage Capabili-
ty," assigned to the assignees hereof. A publication by Manassen
et al in the Journal of the Electrochemical Soclety, Vol. 124,
pp. 532-4, 1977, elaborates also on photoelectrochemical energy
conversion and storage cells. The present embodiment of the in-
stant invention incorporates a structure similar to that described
in V. S. Patent Serial No. 4,118,546, with the addition of a semi-
permeable membrane and utilizing for illustrative purposes a
photo-anode of cadmium selenide~
X
- 25 -
2 ~ ~`7~
': .' ~ .
¦ a carbon counterelectrod~ and a storage electrode comprised
¦ eithcr of Ag/~g2S or Cd/Cd(O~1)2. The Ag/Ag2S electrode
¦ has a redox potential of -0.9 volt vs. SC~ in basic solution,
¦ while the Cd/Cd~O~l)2 electrode has a redox potential
¦ of -1.0 volt vs. SCE. It is apparent that an irradiated
cadmium selenide electrode at a potential of -1.1 volts
¦ vs. SCE will be able to charge both of these electrodes,
¦ 0.2 volt is available to charge the Ag/Ag2S electrode
I and 0.1 volt is available to charge the CdJCd(OI~)2 electrode.
¦ To stabili~e the cadmium selenide electrode sulfur
must be added to the sodium sulfide electrolyte giving
I a polysulfide solution. The silver electrode, however,
¦ corrodes in polysulfide solution forming silver sulfide.
¦ This effectively discharges the storaqe electrode.
l Therefore, the Ag/-~g2S electrode must be separated from
the polysulfide electrolyte by an ion permeable membrane
which allows no polysulfide ions or sulfur to migrate
into the sulfide electrolyte used for the Ag/Ag2S electrode.
l Similarly, the CdiCd(OH)2 electrode corrodes even in
¦ ordinary sulfide electrolyte forming cadmium sulfide.
¦ This storage electrote must be used in a potassium hydroxide
. I or sodium hydroxide electrolyte separated by a permeable
selective membrane that stops sulfide diffusion. For
. l the two examples above in the charged state one obtains
l a battery with a voltage of 0.2 volt with a Ag~Ag25 carbon
co~bination and the battery with a voltage of .3 volt
with a CdJCd(OIIJ2 carbon combination.
I .
, . ~,,
I ~.2-~ ~7~L
I .
¦ Other examples of storagc electrodes with suitable
¦ redox potentials may be found in thc book by w. ~1. Latim~r,
The Oxidation States of the Elements and Their Potentials
¦ in Aq~eous Solutions, published by Prentice-Hall, Incorporated.
¦ An example of a membrane used successfully with
¦ the above two examples of storage electrodes is one manu-
¦ factured by the Dupont Company and ~nown as N~FION.
¦ Referring specifically to FIG. 7a, a multi-chamber
¦ structure is shown incorporating a transparent cover
10 l 100, such as glass for example, and a gasket 101a forming
i ¦ portions of a chamber for electrolyte 102. A photoelectrode ~
¦ ¦ comprising a semiconductor film 103 on a conducting substrate
! -¦ 105 forms the remainder of the chamber, with holes 104
provided through the semiconductor film and conducting
¦ substrate to provide communication of the electrolyte
I ¦ between the two chambers on either side of the electrode.
I Gasket 101b is shown forming the second chamber with
i ¦ electrode 103-105, and in conjunction with a semi-permeable
¦ l me~brane 106. The electrolyte contained in bath chambers
¦ 20 l is the same, and the previous description of opcration
of FIG. 3 is applicable hereto. Semi-permeable membrane
106 is provided to separate the electrolyte utili~ed
_ in conjunction with the photoelectrode from the electrolyte
. used in conjunction with the storage electrode. The
. membrane is selected to permit selectivc ionic passage
for the current generation os charge storage desired
oE the present embodiment.
'.'
.
~ 7~
l Counterelectrode 107 is placed above the semi-permeable
¦ me~brane. The third chamber of the device is formed
¦ by membrane 106, gasket lOlc and
¦ storage electrode 108. The chamber stores electrolyte
I 109, which usually differs from electrolyte 102. ~he
¦ electrolyte 109 is selected such that the storage electrode
¦ will not corrode, or self dischar~e. ~owever, ~nown
¦ examples of such electrolytes will not stabilize the
l photoelectrode. Those which stabilize the photoelectrode,
¦ corrode the storage electrodes. Accordingly, membrane
¦ 106 is utilized in the present embodiment to permit
both stabilization o the photoelectrode and non-cor~osive
¦ use of the storage electrode.
It is to be noted that while counterelectrode
¦ 107 is shown as being the storage chamber of the cell,
I it may similarly be
¦ on both sides of the membrane
concurrently.
¦ Leads 110, 112, and 114 are shown connected to
l the conducting substrate of the photoelectrode, the storage
I ¦ electrode and the counterelectrode, respectively, to
conduct electrical energy therefrom to load 116 by way
l of switches llB and 120. In operation, closure of switch
_ I 118 permits delivery Oe power to lead 116 under normal
l photoactivity. Similarly, closure of switch 120 permits
¦ delivery of power to the load from the storage function
of the cell, while closure of both switches simultaneously
¦ may be used to provide power from both cell functions
¦ to the load.
.' . l
.
,.
As described in the above-identified U.S. Patent Serial
No. ~,118,546, a ~umper 122 may be provided to facilitate the
storage function in the present cell.
Referring now to FIG. 7b, the embodiment shown essenti-
ally uses the structure of the embodiment of FIG. 1 in conjunction
with the storage electrode assembly described along with FIG. 7a.
Specifically, a transparent substrate 200 is used in conjunction
with a transparent conductor 201 and a semiconductor film 202.
A gasket 203a and 203b is utilized as to form the electrolyte
chamber, and counterelectrode 204 is shown as being within that
chamber. Semi-permeable membrane 205 is provided to complete
the chamber, and electrolyte 206 is stored therein.
A second chamber is formed by membrane 205. gasket 203c,
and storage electrode 207. A properly selected electrolyte 208
is contained within t~e chamber and performs the storage function
in the manner previously described. Additionally, the load and
switching arrangement shown in FIG. 7a is repeated herein. It is
to be understood, of course, that a~y switching means is contem-
plated, and the power is to be deli~ered to any load and not just
to a resistive load.
FIG. 7c provides yet a further modification of the
storage electrode st~ucture, utilizing a transparent substrate
300, a transparent conductor 301 and a photoelectrode composed of
the semiconductor film 302 as in FIG. 1.
~,
.
X - 29 -
~ . .
` 7a~
G:skct 303~ is use to ~orm ~ first chambcr containing
electrolyte 304 th~rein, and counterelectrode 305 is
shown as having holes 306 therein. A second chamber,
in communication with the first cha~ber through holes
306, is formed by counterelectrode 305, gasket 303b,
and semi-permeable membrane 307. Finally, a chamber
~s formed for the electrolyte used in conjunction with
the storage electrode, the cha~ber being formed by membrane
307, gasket 303c, and storage electrode 309 for containing
electrolyte 308. A typical switching arrangement and
load, similar to that described in conjunction with FIG.
7a is shown, with the wires 310, 312 and 314 being connected
to the photoelectrode conductor, the counterelectrode,
and the storage electrode, respectively. The storage feature
described above may be combined with other aspects o~ the
present invention, including but not limited to the structure
of Figs. 4a and 4b, for example.
Clearly, the structures disclosed hereinabove
may be cylindrical, or may have other shapes. Any materials
disclosed are for illustrative purposes only, and do
not limit the invention.
The preceding speciEication describes, by way of
illustration and not of limitation, a preferred embodiment
of the invention. Equivalent variations of the described
embodiment will occur to those skilled in the art. Such
variations, modifications, and equivalents are within the
scope of the invention as recited with greater particularity
in the following claims, when interpreted to obtain the
benefits of all equivalents to which the invention i5
fairly entitled.
I .
l l -30-
I .,
.. . . , , ~ ~ :
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.
