U.S. patent application number 10/041433 was filed with the patent office on 2002-05-30 for high efficiency solar photovoltaic cells produced with inexpensive materials by processes suitable for large volume production.
Invention is credited to Mowles, Thomas.
Application Number | 20020062858 10/041433 |
Document ID | / |
Family ID | 25486906 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020062858 |
Kind Code |
A1 |
Mowles, Thomas |
May 30, 2002 |
High efficiency solar photovoltaic cells produced with inexpensive
materials by processes suitable for large volume production
Abstract
A solar energy device comprising: a substrate; a photovoltaic
layer on said substrate; a back conductor in contact with said
substrate; a grid conductor in contact with said substrate; said
photovoltaic layer being of a material selected from the class
consisting of: monoclinic zinc diphosphide (also referred to as
beta zinc diphosphide and indicated by .beta.-ZnP.sub.2); copper
diphosphide (CuP.sub.2); magnesium tetraphosphide (MgP.sub.4);
.gamma.-iron tetraphosphide (.gamma.-FeP4) and mixed crystals
formed from these four materials.
Inventors: |
Mowles, Thomas; (Palo Alto,
CA) |
Correspondence
Address: |
DAVID J. GOREN
Fish & Richardson P.C.
Suite 100
2200 Sand Hill Road
Menlo Park
CA
94025
US
|
Family ID: |
25486906 |
Appl. No.: |
10/041433 |
Filed: |
October 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10041433 |
Oct 29, 2001 |
|
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07947863 |
Sep 21, 1992 |
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Current U.S.
Class: |
136/252 ;
136/265; 257/E31.026; 257/E31.126; 438/93 |
Current CPC
Class: |
H01L 31/02168 20130101;
H01L 31/022425 20130101; H01L 31/02167 20130101; Y02E 10/50
20130101; H01L 31/032 20130101; H01L 31/022466 20130101 |
Class at
Publication: |
136/252 ;
136/265; 438/93 |
International
Class: |
H01L 031/00 |
Claims
1. A photovoltaic cell comprising: (a) a thin film photovoltaic
generating layer, having a pn junction within which photogenerated
free carriers are generated, by incident light; (b) a back
conductor in electrical contact with said photovoltaic layer; and
(c) a front transparent conductor in electrical contact with said
photovoltaic layer; wherein said thin film photovoltaic generating
layer is selected from the class consisting of: .beta.-zinc
diphosphide; copper diphosphide; magnesium tetraphosphide; y-iron
tetraphosphide; and, a mixed crystal of composition
Zn.sub.pMg.sub.qFe.sub.r--Cu.sub.sP.sub.2t, where t.gtoreq.p+q+r+s,
whereby said photovoltaic cell exhibits a greatly improved average
cost per Watt-Hour over a useful life of this photovoltaic
cell.
2. A photovoltaic cell as in claim 1 wherein said thin film
photovoltaic generating layer has a thickness on the order of 1-3
microns, whereby this layer absorbs a high fraction of incident
sunlight and yet has surfaces that are sufficiently separated to
avoid significant dopant diffusion along crystallite boundaries
extending between these surfaces.
3. A photovoltaic cell as in claim 1 wherein said pn junction is a
homojunction buried sufficiently below a front surface of said thin
film photovoltaic generating layer that degradation of cell
photovoltaic conversion efficiency by outdiffusion from this layer
or by indiffusion of ambient chemicals is substantially
eliminated.
4. A photovoltaic cell as in claim 1 wherein said pn junction is a
heterojunction.
5. A photovoltaic cell as in claim 4 wherein said photovoltaic
layer is comprised of a layer of .beta.-ZnP.sub.2 and a layer of
ZnAs.sub.2 and wherein said pn junction is formed at an interface
between the ZnP.sub.2 and ZnAs.sub.2 layers.
6. A photovoltaic cell as in claim 1 further comprising a
passivation layer between an exterior surface of said cell and a
front surface of said thin film photovoltaic generating layer,
where incident light passes to said photovoltaic generating
layer.
7. A photovoltaic cell as in claim 6 wherein said passivation layer
is selected from the class consisting of BP, B.sub.6P, AlPO.sub.4,
BPO.sub.4 and ZnPO.sub.X.
8. A photovoltaic cell as in claim 7 wherein said passivation layer
is BP, whereby said passivation layer is particularly hard and
inert and can be grown from vapor in either p-doped or n-doped
compositions.
9. A photovoltaic cell as in claim 8 wherein said thin film
photovoltaic layer is .beta.-ZnP.sub.2, whereby said BP passivation
layer is physically and chemically compatible with said
.beta.-ZnP.sub.2photovolta- ic layer.
10. A photovoltaic cell as in claim 9 wherein said .beta.-ZnP.sub.2
photovoltaic layer and said BP passivation layer are in direct
contact thereby forming said pn junction at a heterojunction
between these two layers.
11. A photovoltaic cell as in claim 10 wherein said
.beta.-ZnP.sub.2, photovoltaic layer is doped n-type with boron
dopant and said BP is doped p-type with Zn dopant, thereby
utilizing the fact that boron is an n-dopant in .beta.-ZnP.sub.2,
and that Zn is a p-dopant in BP.
12. A photovoltaic cell as in claim 1 further comprising on a front
surface of said .beta.-ZnP.sub.2 layer, a layer of
degenerately-doped zinc phosphate (ZnPO.sub.X), whereby this zinc
phosphate layer functions as a passivation layer that prevents
transport of atmospheric oxygen and moisture to said diode
junction, that prevents the transport of the initial decomposition
products from said .beta.-ZnP.sub.2, layer and that functions as a
transparent conductor.
13. A photovoltaic cell as in claim 1 wherein said thin film
photovoltaic generating layer is .beta.-ZnP.sub.2.
14. A photovoltaic cell as in claim 13 wherein said
.beta.-ZnP.sub.2 layer is doped with Cu, which exhibits a low
mobility in the .beta.-ZnP.sub.2 layer.
15. A photovoltaic cell as in claim 13 wherein said front conductor
is degenerately n-doped boron phosphide (BP), which thereby
functions as a transparent conductor as well as a passivation
layer.
16. A photovoltaic cell as in claim 13 wherein said front conductor
is of a material selected from the class consisting of tin oxide
and indium tin oxide.
17. A photovoltaic cell as in claim 13 wherein said front conductor
is zinc oxide.
18. A photovoltaic cell as in claim 13 wherein said front conductor
is of a material selected from the class consisting of doped
aluminum phosphate, zinc phosphate, boron phosphide and antimony
tin oxide.
19. A photovoltaic cell as in claim 13 wherein said front conductor
is zinc metaphosphate and which functions as a transparent
conductor, whereby a very stable interface is formed between said
.beta.-zinc diphosphide layer and said zinc metaphosphate layer
because the ratio of zinc to phosphorus is the same in both
layers.
20. A photovoltaic cell as in claim 13 wherein said front conductor
is polycrystalline zinc orthophosphate, which is advantageous in
functioning as a transparent conductor, in being stable and in
being easy to grow on ZnP.sub.2.
21. A photovoltaic cell as in claim 1 further comprising an
antireflection layer on said front conductor through which incident
light is to pass to said photovoltaic layer.
22. A photovoltaic cell as in claim 21 wherein said antireflection
layer is B.sub.6P, whereby this antireflection layer is
particularly hard and inert and exhibits a low absorption for
ultraviolet light.
23. A photovoltaic cell as in claim 1 wherein said front conductor
is augmented by a grid conductor.
24. A photovoltaic cell as in claim 1 wherein said back conductor
is made of a material selected from the class consisting of copper,
aluminum, molybdenum and any alloy containing predominantly one or
more of these three materials, whereby this layer forms a low
resistance contact to the photovoltaic layer.
25. A photovoltaic cell as in claim 1 wherein said back conductor
is copper.
26. A photovoltaic cell as in claim 1 wherein said back conductor
is a copper alloy selected from the class consisting of: brass,
phosphocopper alloy, cupronickel and bronze.
27. A photovoltaic cell as in claim 1 wherein said back conductor
is thick enough that it also functions as a substrate, thereby
avoiding the need to form a separate substrate layer.
28. A photovoltaic cell as in claim 27 wherein said substrate is a
foil selected from the class consisting of: copper, aluminum,
molybdenum and any alloy containing predominantly one or more of
these three materials.
29. A photovoltaic cell as in claim 27 wherein said back substrate
has a thickness in the range 40-100 microns.
30. A photovoltaic cell as in claim 1, further comprising between
said photovoltaic layer and said back conductor an interlayer
selected from the class consisting of molybdenum, copper, silver,
lithium, gold, platinum, palladium, titanium, vanadium, chromium,
manganese, tungsten, tantalum, iron, copper and nickel
31. A photovoltaic cell as in claim 1 in which a p-type
conductivity of said photovoltaic layer has been produced by
forming this layer with a phosphorus-rich stoichiometry.
32. A photovoltaic cell as in claim 1 in which an n-type
conductivity of said photovoltaic layer has been produced by
forming this layer with a metal-rich stoichiometry to produce an
n-type layer.
33. A photovoltaic cell as in claim 1 wherein said thin film
photovoltaic generating layer is CuP.sub.2, which is a material
that uniquely has both direct and indirect bandgap states with high
solar response.
34. A photovoltaic cell as in claim 1 wherein said thin film
photovoltaic layer is MgP.sub.4.
35. A photovoltaic cell as in claim 1 wherein said thin film
photovoltaic layer is .gamma.-FeP.sub.4.
36. A photovoltaic cell as in claim 35 further comprising on a
photovoltaic layer of said .gamma.-FeP.sub.4 layer and a layer of
.alpha.-FeP.sub.4, thereby forming a heterojunction pn junction at
an interface between these two layers.
37. A photovoltaic cell as in claim 1 wherein said thin film
photovoltaic layer is a mixed crystal of composition
Zn.sub.pMg.sub.qFe.sub.pCu.sub.xP- .sub.2t.
38. A photovoltaic cell as in claim 1 wherein said thin film
photovoltaic layer was deposited by a chemical vapor deposition
(CVD) process, whereby a high quality thin film is produced.
39. A photovoltaic cell as in claim 38 wherein said CVD process is
an organometallic CVD process, whereby a high quality thin film
photovoltaic layer is produced.
40. A photovoltaic cell as in claim 39 wherein said photovoltaic
layer was deposited in a plasma-enhanced environment, thereby
enabling deposition at reduced temperatures, making this process
suitable for deposition onto high temperature stable plastic
substrates.
41. A photovoltaic cell as in claim 38 wherein said CVD process is
a microwave plasma-enhanced, CVD process, at a total chamber
pressure of 1-100 torr, whereby it can be operated at temperatures
that are low enough to enable a high temperature, stable to be used
as a substrate.
42. A photovoltaic cell as in claim 38 wherein said CVD process is
operated at a near-atmospheric pressure, thereby enabling this
process to form said photovoltaic layer on a substrate that is fed
through an open-ended CVD chamber, whereby a high volume
fabrication process can be implemented.
43. A photovoltaic cell as in claim 38 wherein process parameters
of said CVD process are selected to lie in a single phase process
region, whereby a high quality photovoltaic layer can be
formed.
44. A photovoltaic cell as in claim 38 wherein said CVD process
uses white phosphorus as a phosphorus source, whereby this low-cost
form of phosphorus produces very high quality photovoltaic
films.
45. A photovoltaic cell as in claim 1, further comprising a back
surface field formed near an interface between said back conductor
and said photovoltaic layer.
46. A photovoltaic cell comprising: (a) a film photovoltaic
generating layer of .gamma.FeP.sub.4, having a pn junction within
which photogenerated free carriers are generated by incident light;
(b) a back conductor in electrical contact with said photovoltaic
layer; and (c) a transparent front conductor in electrical contact
with said photovoltaic layer.
47. A photovoltaic cell comprising: (a) a film photovoltaic
generating layer of MgP.sub.4, having a pn junction within which
photogenerated free carriers are generated by incident light; (b) a
back conductor in electrical contact with said photovoltaic layer;
and (c) a transparent front conductor in electrical contact with
said photovoltaic layer.
48. A method of depositing, onto a substrate, a photovoltaic layer
of a material selected from the set consisting of .beta.-ZnP.sub.2,
CuP.sub.2, MgP.sub.4, .gamma.-FeP.sub.4 and mixed crystals formed
from these four materials, said method comprising the steps of: (a)
supplying, to a chemical vapor deposition (CVD) chamber, a source
of metal selected from a set consisting of Zn, Cu, Mg, Fe and a
material containing at least two of these four metals; (b)
supplying a source of phosphorus to said CVD chamber; and (c)
controlling the temperature of the substrate and the partial
pressure of gases within this reactor to lie within a phase region
which deposits a layer that includes at least one material selected
from a set consisting of .beta.-ZnP.sub.2, CuP.sub.2, MgP.sub.4,
.gamma.-FeP.sub.4 and mixed crystals formed from at least two of
these four metals and a number of phosphorus atoms that is at least
twice the total number of metal atoms.
49. A method of providing phosphorus to a chemical vapor deposition
(CVD) chamber, comprising the steps of: (a) vaporizing white
phosphorus to form gaseous phosphorus; and (b) supplying this
gaseous phosphorus to a location within said CVD chamber at which
chemical vapor deposition is to be implemented, whereby this
gaseous phosphorus is inexpensive and pure.
50. A method as in claim 49 wherein step (a) comprises: supplying
white phosphorus to a bubbler, within which this white phosphorus
is vaporized; and flowing a carrier gas through said bubbler to
carry said vaporized white phosphorus to a reaction site within
said CVD chamber.
51. A method as in claim 49 wherein step (a) comprises: feeding a
block of material containing solid white phosphorus into a hot zone
within said CVD chamber; and heating a surface of said block of
material to a temperature at which it vaporizes.
52. A method as in claim 49 wherein step (a) comprises: feeding a
block of material containing solid white phosphorus into a hot zone
remote from said CVD chamber; heating a surface of said block of
material to a temperature at which it vaporizes; and flowing a
carrier gas through said remote chamber to carry said vaporized
white phosphorus to a reaction site within said CVD chamber.
53. A method as in claim 51 wherein said block of white phosphorus
contains at least one other reactant in a concentration selected to
produce a preselected chemical composition within this CVD
chamber.
54. A method as in claim 49, further comprising the steps of:
heating said substrate to at least 300.degree. C. to decompose
gaseous phosphorus, making it available for reaction.
55. A method as in claim 49 further comprising the step of: heating
any nonsubstrate surfaces, within said CVD chamber, with which said
vaporized white phosphorus can come into contact to a temperature
sufficient to prevent deposition of said white phosphorus on such
surfaces.
56. A method as in claim 55 wherein said temperature of the
nonsubstrate surfaces is less than 200.degree. C., whereby
conventional O-rings can be used in said CVD chamber.
57. A method as in claim 49 wherein a total pressure within said
chamber is near atmospheric pressure and wherein this chamber has
an open-ended configuration, said method further comprising:
feeding an elongated substrate through this chamber to deposit a
coat onto said substrate.
58. A method as in claim 49 wherein the total pressure of gases
within said chamber during growth of said photovoltaic layer is
less than 100 Torr, thereby improving uniformity of a resulting
thin film.
59. A method as in claim 58, wherein a plasma is produced within
said reactor to enhance quality of deposited films and reduce
substrate temperature.
60. A method as in claim 59, wherein a microwave source provides
energy that excites said plasma, thereby enabling production of a
plasma at a higher pressure than is obtainable by an RF source.
61. A method as in claim 49 further comprising a step of supplying
an organometallic material containing a metal selected from the set
consisting of: Zn, Cu, Mg, Fe and a mixture containing at least two
of these four metals.
62. A method of manufacturing a photovoltaic cell comprising the
steps of: (a) supplying a source of phosphorus to a chemical vapor
deposition (CVD) chamber; (b) supplying a source of a metal
selected from the set consisting of: Zn, Cu, Mg, Fe, and mixtures
of these materials; and (c) controlling temperature of the
substrate and partial pressures of the gases within said CVD
chamber to produce on a conductive substrate a photovoltaic thin
film selected from the set consisting of: .beta.-ZnP.sub.2,
CuP.sub.2, MgP.sub.4, .gamma.-FeP.sub.4 and a mixed crystal
containing at least two of the metals selected from the set
consisting of Mg, Fe, Cu, and Zn, wherein said conductive substrate
can be a single layer of conductive material or a layer of
conductive material formed on a support layer.
63. A method as in claim 62, wherein a temperature and partial
pressure of the reactants are selected to lie within a double-phase
process region, wherein a large excess of phosphorus is used to
produce single phase growth.
64. A method as in claim 63 wherein a temperature and partial
pressures of reactants are selected to lie within a single phase
process region.
65. A method as in claim 64 wherein, after growth of a layer by
process conditions within a single phase process region, said layer
is maintained in a phosphorus gas environment at a pressure
sufficient to prevent decomposition of said layer that was grown in
said single phase process region, until this layer is cooled
substantially to room temperature.
66. A method as in claim 62 wherein the ratio of phosphorus to
metal in the gaseous species within this chamber is at a minimum
value, that will permit growth, at which substantially all of the
phosphorus gas is utilized in forming a deposited layer.
67. A method as in claim 62 wherein a total pressure within said
CVD chamber is substantially atmospheric pressure.
68. A method as in claim 67, further comprising a step (d) of
supplying an inert diluent to process gases to produce a
substantially atmospheric total pressure within said reactor.
69. A method as in claim 67 wherein an elongated substrate that has
a length, along a direction of feed, that is much longer than a
width of this substrate, enters an input end of the chamber, has
layers formed thereon and exits through an output end of the
reactor.
70. A method as in claim 67 wherein said substrate has a width on
the order of or larger than a meter, whereby high volume
fabrication is possible for providing the large volume of
photovoltaic cells required for providing electrical power.
71. A method as in claim 62 wherein said substrate is heated to an
above ambient temperature selected to improve deposition
quality.
72. A method as in claim 62 wherein the total pressure of gases
within said chamber during growth of said photovoltaic layer is
less than 100 Torr, thereby improving uniformity of a resulting
thin film.
73. A method as in claim 72 wherein a plasma is produced within
said reactor to enhance quality of deposited films and reduce
substrate temperature.
74. A method as in claim 73, wherein a microwave source provides
energy that excites said plasma, thereby enabling production of a
plasma at a higher pressure than is obtainable by an RF source.
75. A method as in claim 73 wherein said substrate has a
temperature on the order of or less than 375.degree. C., whereby
this method is compatible with high temperature, stable plastic
substrates.
76. A method as in claim 73, wherein said plasma dissociates
hydrogen within said reactor, whereby this hydrogen will react with
carbon within said reactor to produce gaseous products that are
exhausted from this reactor, thereby prevent incorporation of
carbon into a film being deposited within this reactor, whereby
FeP.sub.4 films can be produced without harmful incorporation of
unwanted stable iron carbide.
77. A method as in claim 73, wherein halides are added to said
plasma to catalyze nucleation and growth of a film.
78. A method as in claim 73, wherein noble gases are added to said
plasma to activate growth of a film by transferring energy from
said plasma to said reactants used to grow a film.
79. A method as in claim 73, wherein organic materials containing
methyl, ethyl and phenyl groups are added to said plasma to reduce
a rate of growth and/or to remove or etch these films after
growth.
80. A method as in claim 62 wherein said source of metal is an
organometallic compound.
81. A method as in claim 62 wherein said source of phosphorus is
phosphine, t-butyl phosphine, bisphosphinoethane, trimethyl
phosphine and triethyl phosphine.
82. A method as in claim 62 wherein phosphorus is supplied to said
reactor as white phosphorus, said method further comprising the
step of heating said phosphorus to a temperature selected to
achieve a desired phosphorus partial pressure.
83. A method as in claim 82 wherein said white phosphorus is heated
in a bubbler and a carrier gas carries vaporized phosphorus to a
substrate on which deposition is to take place.
84. A method as in claim 82, wherein solid white phosphorus is used
as said phosphorus source by introduction directly into said
chamber or by introduction indirectly by generation at a remote
site followed by transport to said substrate.
85. A method as in claim 82 further comprising the step of heating
any nonsubstrate surface of said CVD chamber that is exposed to
said vaporized phosphorus to prevent deposition and subsequent
flaking of phosphorus.
86. A method as in claim 85 wherein a temperature of a reactor wall
is less than 200.degree. C., whereby conventional O-rings can be
used to sear said reactor.
87. A method as in claim 62 wherein doping elements are introduced
into said vapor above said substrate during growth as a hydride or
organometallic compound of said doping elements.
88. A method as in claim 87, wherein said doping element is copper,
producing a p-type conductivity.
89. A method as in claim 87, wherein said doping element is
lithium, producing p-type conductivity.
90. A method as in claim 87, wherein said doping element is boron,
producing n-type conductivity.
91. A method as in claim 87, wherein said doping element is
selected from a set consisting of aluminum, gallium, indium and
tin, which, when substituted for a metal atom yields n-type
conductivity.
92. A method as in claim 87, wherein said doping element is
selected from a set consisting of sulfur, selenium and tellurium,
which, when substituted for a phosphorus, yields n-type
conductivity.
93. A method as in claim 61 wherein a film of material selected
from the set consisting of .beta.-ZnP.sub.2, CuP.sub.2, MgP.sub.4,
.gamma.-FeP.sub.4 and a mixed crystal, containing at least two of
the metals selected from the set consisting of Mg, Fe, Cu, and Zn,
is deposited on said substrate.
94. A method as in claim 62 wherein said photovoltaic thin film is
.beta.-ZnP.sub.2.
95. A method as in claim 94 further comprising the step of: doping
said .beta.-ZnP.sub.2 layer with a dopant selected from the set
consisting of copper and lithium, producing p-type carriers.
96. A method as in claim 95 wherein said dopant is copper, which is
a preferred dopant because of its low mobility in
.beta.-ZnP.sub.2.
97. A method as in claim 94, comprising a step of forming 'a
ZnPO.sub.x passivation layer between a front surface of said
photovoltaic cell and a front surface of said thin film
photovoltaic layer.
98. A method as in claim 97 wherein said ZnPO.sub.X passivation
layer is strongly n-doped with a dopant selected from the set
consisting of Al, Ga, In and Sn.
99. A method as in claim 97 wherein said passivation layer is
sufficiently strongly doped that it functions as a transparent
conductor.
100. A method of depositing, onto a substrate, layer of ZnPO.sub.X,
said method comprising the steps of: (a) supplying, to a chemical
vapor deposition (CVD) chamber, a source Zn; (b) supplying a source
of phosphorus to said CVD chamber; (c) supplying a source of
oxygen; and (c) controlling the temperature of the substrate and
the partial pressure of gases within this reactor to deposit a
layer of material having the composition xZnO:
yP.sub.2O.sub.5,:aSiO.sub.2:bM.sub.2O.sub.3:cSnO.sub.2, where x and
y are integers and x/y is approximately 1, 2 or 3 and a, b and c
are small (or zero) and where M is a Group III element.
101. A method as in claim 100, wherein said source of zinc is
selected from the set consisting of diethylzinc and dimethylzinc
and wherein said source of phosphorus is selected from the set
consisting of: liquid P.sub.4 and P.sub.2O.sub.3.
102. A method as in claim 100, wherein said sources of zinc and
phosphorus are both solid sources, wherein said source of zinc is
selected from the set consisting of: diphenylzinc,
bis(cyclopentadienyl)zinc, bis(methylcyclopentadienyl)zinc
bis-(pentamethylcyclopentadienyl)zinc; and wherein said source of
phosphorus is selected from the set consisting of: solid white
phosphorus, solid P.sub.2O.sub.3 and organophosphates.
103. A method as in claim 100, wherein said film is produced by a
two-stage process in which a film is deposited without being fully
oxidized, followed by an oxygen treatment process.
104. A method as in claim 103, wherein said oxygen treatment,
involves plasma-enhanced oxygen species.
105. A method as in claim 94 further comprising the step of:
forming on said ZnP.sub.2 layer a zinc oxide conductive layer,
whereby this conductive oxide layer is fabricated from inexpensive,
nonpolluting materials that are readily available and are
compatible with fabrication of said ZnP.sub.2 layer.
106. A method as in claim 94 further comprising the step of:
forming on said ZnP.sub.2 layer a boron phosphide layer, which
provides an especially hard protective layer from low cost
components.
107. A method as in claim 94 wherein said ZnP.sub.2 layer is grown
at a substrate temperature in the range from 500-600.degree. C., in
a gas mixture having a phosphorus-to-zinc partial pressure ratio in
the range from 10-20 and having a sum of the zinc and phosphorus
partial pressures in the range from 1-100 Torr.
108. A method as in claim 107, wherein said gas mixture includes an
inert gas component that raises the total pressure within the
reactor, during growth of said ZnP.sub.2 layer, to substantially
atmospheric pressure.
109. A method as in claim 107 wherein said source of Zn is selected
from the set consisting of diethylzinc and dimethylzinc.
110. A method, as in claim 94, wherein said Zn and phosphorus
reactants are supplied as a rod containing white phosphorus and at
least one of the following zinc sources: diphenylzinc,
bis(cyclopentadienyl)zinc, bis(methylcyclopentadienyl)zinc and
bis(pentamethylcyclopentadienyl)zinc.
111. A method, as in claim 48, wherein said photovoltaic thin film
is CuP.sub.2.
112. A method as in claim 111 wherein said CuP.sub.2 photovoltaic
layer is formed in a plasma-enhanced environment at a substrate
temperature in the range from 400-600.degree. C., at a
phosphorus-to-copper ratio of 10-20, in a total pressure of 1-10
Torr.
113. A method as in claim 112 wherein said metal source is selected
from the set consisting of: copper phenyl acetylide, copper
hexafluoracetylacetonate, copper trifluoracetylacetonate and
cyclopentyldienylcopper triethylphosphine.
114. A method, as in claim 48, wherein said photovoltaic thin film
is MgP.sub.4.
115. A method, as in claim 114, wherein said MgP.sub.4 photovoltaic
layer is formed at a substrate temperature in the range from
400-550.degree. C., at a phosphorus-to-magnesium ratio of 15-25, in
a total pressure of 1-50 Torr.
116. A method, as in claim 115, wherein said metal source is
selected from the set consisting of: bis(cyclopentadienyl)
magnesium, bis(pentamethylcyclopentadienyl) magnesium and
bis(methylcyclopentadienyl- ) magnesium.
117. A method, as in claim 116, wherein said chamber wall is heated
to about 150.degree. C.
118. A method, as in claim 115, wherein said MgP.sub.4 layer is
formed in a plasma-enhanced environment.
119. A method, as in claim 48, wherein said photovoltaic thin film
is .gamma.-FeP.sub.4.
120. A method, as in claim 119, wherein said .gamma.-FeP.sub.4
photovoltaic layer is formed at a substrate temperature in the
range from 400-550.degree. C., at a phosphorus-to-iron ratio of
15-25 and at a total pressure of 1-50 Torr.
121. A method, as in claim 120, wherein said metal source is
selected from the set consisting of: bis(cyclopentadienyl) iron,
pentacarbonyl iron and bis(diphenylphosphino) ferrocene.
122. A method, as in claim 121, wherein said chamber wall is heated
to about 150.degree. C.
123. A method as in claim 119 wherein layer growth is nucleated
with small amounts of nucleants selected from the set consisting of
Mg and Cu.
124. A method, as in claim 48, wherein said photovoltaic thin film
is a mixed crystal having a composition of
Zn.sub.pMg.sub.qFe.sub.rCu.sub.sP.s- ub.2t, where t.gtoreq.p+q+r+s,
whereby said photovoltaic cell exhibits a greatly improved average
cost per Watt-Hour over a useful life of this photovoltaic
cell.
125. A method as in claim 124, wherein said mixed crystal
photovoltaic layer is formed in a plasma-enhanced environment at a
substrate temperature in the range from 400-550.degree. C., at a
phosphorus-to-metal ratio of 15-25, in a total pressure of 1-50
Torr.
126. A method, as in claim 125, wherein said chamber wall is heated
to about 150.degree. C.
127. A method as in claim 62, wherein said substrate is plastic,
whereby a lightweight photovoltaic device is produced.
128. A method as in claim 62 wherein said substrate is an alloy of:
iron, copper, molybdenum, aluminum or a combination of these
elements.
129. A method for forming a diode junction, wherein a transparent
conducting layer that is heavily doped with n-type dopant is
deposited upon a p-type photovoltaic layer without forming a buried
diode junction; and subjecting this structure to subsequent heat
treatment causes the dopant to outdiffuse from the transparent
conductor into a photovoltaic layer, creating a shallow diode
junction.
130. A method for forming a back surface field, wherein a p-type
photovoltaic layer is deposited upon a back conducting layer that
is heavily doped with a p-type dopant without forming a back
surface field; and subjecting this structure to subsequent heat
treatment which causes the dopant to outdiffuse from the back
conducting layer into a photovoltaic layer, creating a shallow back
surface field.
131. A method for forming a diode junction, wherein a transparent
conducting layer that is heavily doped with n-type dopant is
deposited upon a p-type photovoltaic layer without forming a buried
diode junction; wherein a p-type photovoltaic layer is deposited
upon a back conducting layer that is heavily doped with a p-type
dopant without forming a back surface field; and subjecting this
structure to subsequent heat treatment which causes the dopant to
outdiffuse from the back conducting layer into a photovoltaic
layer, creating a shallow back surface field and a diode junction,
concurrently.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of and claims priority to
U.S. application Ser. No. 07/947,863, filed on Sep. 21, 1992, the
entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates in general to photocells and the
processes of their manufacture and relates more particularly to a
class of materials, photocell structures compatible with these
materials and associated manufacturing processes that produce high
efficiency, inexpensive photocells.
CONVENTION REGARDING REFERENCE NUMBERS
[0003] In the figures, each element indicated by a reference
numeral will be indicated by the same reference numeral in every
figure in which that element appears. The first two digits of any 4
digit reference numerals and the first digit of any two or three
digit reference numerals indicates the first figure in which its
associated element is presented.
BACKGROUND OF THE INVENTION
[0004] The oil embargo of the late 1970's sensitized the world to
the problem of limited petrochemicals in the world and the
concentration of such chemicals in several regions around the world
that are unstable economically and politically. This produced a
step increase in the interest level for using renewable energy
sources such as solar power, wind power and tidal power. The recent
war between the United States and Iraq has reconfirmed the need for
a stable energy source that is not affected by political events
around the world. In addition, the desire for clean air is so
acute, that interest in nuclear power may be revived despite the
well-known radiation dangers and lack of nuclear waste treatment
methods. Unfortunately, the progress in developing alternate energy
sources has been disappointing and has shown that the development
of such technologies is very difficult.
[0005] Although there was initially a high level of hope that high
efficiency, photovoltaic cells could be manufactured to produce
directly from incident solar energy the large amounts of
electricity utilized throughout the world, the photovoltaic cells
produced up to now have been commercially viable only for special
niche markets, such as: solar powered calculators for which
consumers are willing to pay the additional cost to avoid the
problems of battery replacement; solar powered telephones for use
in areas that are remote from electrical power lines; and buildings
located in regions of the country that are sunny and sufficiently
remote from commercial power lines that solar power is a cost
effective alternative. If solar energy is to provide a significant
fraction of this country's or the world's power needs, the average
cost per Watt for solar photovoltaic cells over the life of such
cells must be reduced to a level that is competitive with the
average cost per Watt of power from existing electrical utilities
over the same period.
[0006] At the present time, the average cost per Watt of
photovoltaic power, over the life of the photovoltaic cells, is
more than five times the typical cost per Watt of electricity
produced by present day electric power plants. It is therefore
necessary to greatly reduce the cost of photovoltaic cells in order
to reduce both the purchase price of a photocell array and the
average cost of electricity produced by such cells over the useful
life of that array. For solar electric energy to be practical for
use by electrical power utilities or to be a practical alternative
for use by electrical power consumers, a photocell design must be
provided that: has a low material cost; has a highly efficient
structure; and can be manufactured in large volumes by low-cost
manufacturing processes. The design of this photocell requires an
interactive analysis of materials, cell structure and fabrication
processes. To produce low cost, efficient cells in the volumes
needed to supply a significant fraction of the world's power needs,
the manufacturing processes must provide high deposition rates and
high layer uniformity over a large area photocell.
[0007] A first significant factor in the manufacturing cost of a
solar photovoltaic cell is the cost of the materials needed to
manufacture this cell. The cost of the material in the
photosensitive, current-generating layer of the photovoltaic cell
can be a significant fraction of the cost of manufacturing such
photovoltaic cell. To efficiently convert incident radiation, this
layer must convert most of the incident solar energy into
electrical power. If the absorbance value of the photosensitive,
current-generating material is small, then its thickness must be
correspondingly large to absorb and convert most of the incident
solar energy. Because many photosensitive, current-generating
materials are relatively expensive, a significantly increased
thickness of this layer can significantly increase the total cost
of a photovoltaic cell utilizing that material.
[0008] Even when such low photosensitivity material is not
expensive, it can still significantly increase the cost of the
photovoltaic cell. The increased thickness of the photosensitive,
current-generating layer increases the average pathlength that the
photovoltaically generated charged species must travel to
corresponding electrodes. This produces a concomitant increase in
the electrical resistance of such layer, thereby decreasing power
conversion efficiency. In order to avoid unduly degrading the
amount of electrical power produced for a given flux of incident
solar energy, the photosensitive, current-generating layer must
have a high level of purity in order to have a high enough
conductivity that resistive losses do not significantly degrade
performance. Such increased material purity requirements can
greatly increase the cost of such solar energy cells.
[0009] Much of the research and development of solar cells has been
directed toward single crystal silicon photocells, because the
tremendous amount of knowledge about solid-state circuits
manufactured in a silicon substrate can then be applied to this
problem. Silicon also has the advantage of being a non-toxic,
readily available resource. However, crystalline silicon is a
relatively poor solar absorber, because it is an indirect bandgap
material. This means that a relatively thick crystalline layer must
be utilized to absorb a significant fraction of the incident solar
energy. Unfortunately, this increased thickness will degrade
efficiency, because of the concomitant increase in the resistance
across which the photogenerated charge carriers need to travel.
This increase in resistance because of this increased thickness
must be offset by a reduction in resistivity by the use of high
purity, high perfection silicon layers. Such layers are very costly
and therefore significantly increase the cost of single crystal
silicon photocells.
[0010] These thick layers of silicon must be made by the expensive
process of solidification from the melt in a single crystal boule
that is then sliced to form the crystalline wafer. Approximately
half of this crystal is lost during this slicing process, further
increasing the cost. Even though the silicon photocells are durable
and efficient, their cost is still prohibitively high for utility
power. Although the conventional single crystal layer growth
process can be modified to produce lower cost, polycrystalline
photocells, this change in the material structure also reduces the
efficiency of the resulting photocell, such that the resulting cost
of electric power is still too high to compete with existing
electrical utilities.
[0011] Amorphous silicon is attractive for use as the
photosensitive, current-generating layer in photocells, because its
high absorptivity for solar energy enables the photosensitive,
current-generating layer to be extremely thin, thereby reducing the
material cost of that layer arid reducing the resistive losses of
that layer. This amorphous silicon layer is also very insensitive
to impurities. This results in a very inexpensive layer that,
unfortunately, due to the nature of electricity transport in
amorphous materials, has a very low efficiency.
[0012] Although the efficiency can be increased by producing
several amorphous layers in a stacked arrangement, this also
increases the cost enough that the resulting device is not
commercially competitive. Amorphous silicon, which is actually an
alloy of hydrogen and silicon, also has a more serious weakness
that, when exposed to sunlight, hydrogen is gradually liberated,
thereby severely degrading the efficiency of the device. The
lifetime of such photocells is too short to collect enough
electricity to pay for their cost. In addition, the production of
this material is difficult, because, at the low substrate
temperature required for the growth of this amorphous phase, the
growth rate is low and the source chemicals are not fully
dissociated. This significantly increases the cost of this
material.
[0013] For the above reasons, it was important to search for
alternative materials for use in solar photovoltaic cells. Gallium
arsenide (GaAs) and aluminum gallium arsenide
(Al.sub.xGa.sub.1-xAs) have been investigated, developed and
utilized for use as solar cells. These materials have been used to
make the most efficient solar cells yet made. Unfortunately, the
cost of these devices is more than ten times the cost of silicon
devices, so that these devices, so that these devices are utilized
only when the cost of such devices is not a significant factor.
Although these devices are used for space power and high
performance solar electric race cars, they are unsuitable for
electric utility power. In addition, these materials contain
gallium, which has limited availability and contain arsenic which
is both a poison and a carcinogen. The use of this material on a
scale suitable for producing a significant fraction of the
electrical power needs of the U.S. or the world would create
tremendous environmental problems. Indeed, the tremendous volume of
photovoltaic cells that must be manufactured to provide the ability
to generate a significant fraction of our energy needs, means that
every choice of material in such photovoltaic cells must be
evaluated as to the resulting impact on the cost of materials
needed to manufacture such devices and the resulting impact on the
environment of manufacturing and/or disposing of such a tremendous
volume of these photovoltaic cells.
[0014] Cadmium telluride (CdTe) has been actively developed for
solar electric power for many years. This material has achieved
high efficiency in small area devices and research continues toward
obtaining high efficiency over large areas. However, even if the
junction efficiency and humidity degradation problems were solved,
it would still be inadvisable to use this material for terrestrial
solar electric power, because cadmium and tellurium are both
dangerous environmental poisons. In addition, tellurium is a rare
and expensive material. Such rare and expensive materials should
only be considered for use as dopants, because any other use would
make the resulting device commercially impractical and would
rapidly deplete the available quantities of such materials.
[0015] Another material currently being developed for solar
electric applications is copper indium diselenide (CuInSe.sub.2,
called CIS). Small cells of high efficiency have been made but the
process used for their growth is complex, costly, nonuniform for
large areas, and requires large amounts of the extremely toxic gas
hydrogen selenide. Indium is an expensive and very rare chemical
element, whose cost and availability have not been a problem to
date, because it has been used as a dopant. This means that only a
minute amount of this material is needed in any given device, so
that the total demand has not yet significantly depleted the amount
of this material that is available. However, if solar electric
cells using indium as a primary component were used to produce a
significant fraction of the world's power needs, the cost would
rise rapidly as the supply of this rare element became depleted.
Another problem with these solar photovoltaic cells is that
selenium is both relatively rare and toxic and its widespread use
would be inadvisable.
[0016] It is therefore necessary to locate materials, for use in
the manufacture of solar photovoltaic cells, that are abundantly
available so that the cost will be low and the available amount of
such material will not be significantly depleted, even at the
tremendous volume of solar photovoltaic cells needed to provide a
significant fraction of our power needs. These materials should
also be nontoxic, so that these volumes will not pollute the
environment. It is also important that these materials be readily
available from many sources so that there is no possibility of a
cartel controlling such resources and producing problems similar to
the oil embargo of the 1970's. These materials must be capable of
low cost deposit ion on large area substrates with high
uniformity.
SUMMARY OF THE INVENTION
[0017] In accordance with the illustrated preferred embodiment, new
materials are identified as being appropriate for use in the
manufacture of solar photovoltaic cells that are sufficiently
efficient, inexpensive and durable that they can competitively
supply a significant fraction of the world's electric power needs.
These materials were identified by considering the following
factors. The chemical elements from which the material is formed
must all be inexpensive and abundantly available, so that the huge
volumes required will not deplete the resources of such materials
or increase the cost of such materials to levels that would
prohibit widespread use of these devices. These elements should be
available throughout the world, so that a cartel cannot interfere
with reasonable, inexpensive access to these materials.
[0018] These elements must form a semiconductor material, so that a
photovoltaic diode device structure can be formed to convert
sunlight to electricity. This semiconducting material must absorb
sunlight efficiently in a very thin layer, so that only a small
amount of photovoltaic material is required for a large area device
and so that the photogenerated carriers are required to travel only
a short distance before being collected by the diode junction. This
latter benefit enables the use of lower cost, lower purity
materials than would be necessary in a thick photosensitive,
current-generating layer to keep recombination losses low, thereby
reducing material costs of the photocell. The material should also
be able to be deposited in a thin film form using processes similar
to integrated circuit techniques now in use, so that the expertise
in these fields and the manufacturing chemicals, equipment and
designs can be applied to the manufacture of these solar cells.
[0019] Semiconductors have a threshold energy for the absorption of
incident photons, known as the "bandgap", which is characteristic
of that semiconducting material. A photon that has an energy higher
than the bandgap will be strongly absorbed, whereas a photon that
has an energy lower than the bandgap will not be strongly absorbed.
Therefore, the photovoltaic semiconductor must have a bandgap that
is matched to the incident solar spectrum. If the bandgap is too
high, then fewer of the available photons will be absorbed, which
reduces the device efficiency. if the bandgap is too low, then the
voltage of the device (which is proportional to the bandgap) will
be low, which reduces the device efficiency.
[0020] The spectral distribution of the solar energy incident on
the earth outside of the earth's atmosphere differs somewhat from
the spectral distribution of the solar energy at the earth's
surface. This difference arises because of the absorbance and
reflectance of the earth's atmosphere. Therefore, the optimum
bandgap of the photovoltaic material differs according to whether
the solar cells are to be utilized above the earth's atmosphere or
at the earth's surface. As a practical matter, these two
distributions are sufficiently similar that there is negligible
impact on the choices of materials to be utilized for the
photovoltaic layer. Several independent studies have shown that
efficient solar cells can be made from a semiconductor that has a
bandgap between 1 and 2 electron volts, with the optimum being
approximately 1.5 electron volts.
[0021] Semiconductors which have a sharp transition in absorption
at the bandgap are known as "direct" bandgap materials, and those
which have more of a slope at the transitions are known as
"indirect" bandgap materials. The ideal photovoltaic materials have
a "direct" bandgap, because such materials absorb incident light in
a very thin layer (less than one micrometer thick), whereas an
"indirect" bandgap material requires a thick layer (over one
hundred microns thick) to absorb the same fraction of incident
light. Devices based upon a thin layer of direct bandgap material
are less expensive, because less material is needed and the
photovoltaic layer can be vapor deposited as a thin film.
[0022] It is difficult to deposit low cost, high quality, thick
layers from a vapor, so "indirect" bandgap materials, such as
silicon, are manufactured by solidification from a liquid melt and
then sawed into wafers for use as devices. This melt growth process
is very expensive and wasteful due to loss of material during
cutting. In addition, the resulting discrete devices (typically
less than 150 millimeters across) require additional costly
assembly into larger modules for final installation. The use of
thin films also reduces the distance that the generated carriers
must traverse before being collected. This reduces the mobility and
lifetime requirements for the current carriers in this layer, which
enables high efficiency devices to be formed from a relatively
lower quality layer. This further reduces the cost of high
efficiency devices. However, indirect materials have the advantage
of longer lifetimes of generated carriers that can produce highly
efficient devices if the purity and perfection of the thick film
can be obtained. Therefore, while indirect bandgap materials are
less likely candidates, they should also be considered if a low
cost of the thick layer is possible.
[0023] To be commercially viable for generating a significant
fraction of the electrical power needs of the United States at this
time, the resulting photovoltaic cell should maintain an efficiency
of at least 15% for a period of greater than 30 years and must be
made of materials selected such that the installation of more than
20 billion square meters of cells in less than 20 years will not
significantly deplete the supply or inflate the cost of these
materials. The total cost of these cells should be on the order of,
or less than, $50/m.sup.2, so that the average cost of electricity
from these solar photovoltaic cells over the lifetime of these
cells is comparable to the projected cost of generating that
electricity from conventional sources.
[0024] The following five materials satisfy all of the these
requirements and therefore will produce photovoltaic cells having
high efficiency at greatly reduced cost: monoclinic zinc
diphosphide (also referred to as beta zinc diphosphide and
indicated by .beta.-ZNP.sub.2,); copper diphosphide (CuP.sub.2,);
magnesium tetraphosphide (MgP.sub.4,); .gamma.-iron tetraphosphide
(.gamma.-FeP,) and mixed crystals formed from these four
materials.
[0025] Large-scale solar photovoltaic cells will typically be
manufactured as arrays of smaller cells. In order to produce a
significant fraction of our national electric power needs (on the
order of 3,000 billion kWh/year), the total area of these cells
must be on the order of 15,000 square kilometers. In order to
manufacture such a tremendous area of photovoltaic cells, it is
advantageous to utilize a reactor design that can manufacture
sheets of photovoltaic material having a width on the order of a
meter or more. Processing sheets of this size requires processes
that produce highly uniform thin films over this entire width. And
this process must operate near, or below, atmospheric pressure. If
the pressure were significantly above one atmosphere, the walls of
the equipment used for the process would have to be very thick and
heavy, the inherent difficulty and danger in such a process would
increase the effective cost of the cells produced.
[0026] All of the proposed materials are compounds that contain
more atoms of phosphorus than atoms of the metal species. They are
all known to decompose to compounds having less phosphorus content,
unless they are maintained in an atmosphere of phosphorus gas
whenever they are exposed during heating. These materials must be
deposited on a heated substrate during deposition of a thin film in
order to form a layer having the properties that approach those of
single crystals, as required for high efficiency solar devices. It
is commonly believed that crystals of these materials must be grown
at pressures from 3-10 atmospheres. The existing synthesis
literature reports very high pressure crystal growth under
phosphorus gas overpressures. These pressures are prohibitive for
the growth of large area thin films using conventional atmospheric
pressure, or vacuum, equipment.
[0027] The existing literature on the growth of .beta.-ZnP.sub.2,
crystals shows growth occurring at very high pressures (on the
order of several atmospheres) and a large excess of phosphorus to
produce zinc diphosphide instead of sesquizine phosphide (Zn.sub.3,
P.sub.2,) and therefore indicates that large area substrates cannot
be coated with ZnP.sub.2. However, a detailed analysis of the
thermodynamics of the growth of ZnP.sub.2, and the conditions of
its decomposition to Zn.sub.3, P.sub.2, has shown that in the
temperature range required by the proposed manufacturing process
for ZnP.sub.2 thin films there exists a region of obtainable
partial pressures that enables ZnP.sub.2, with nearly single
crystal properties, to be manufacture at, or below, atmospheric
pressure. This enables the production of potentially highly
efficient solar cells based upon ZnP.sub.2 using conventional
process equipment technology.
[0028] Analysis of the available literature on the other materials
(CuP.sub.2, MgP.sub.4, and FeP.sub.4) has shown (where the data
exists) that they too can be grown with near single crystal
properties at conditions obtainable with modification to the
conventional process equipment technology.
[0029] Thin films of these materials must be produced by some
means. Solidification of the liquid phase is not possible due to
the high pressures to prevent decomposition of the melt. Vacuum
evaporation techniques, while possible, are not useful because of
the nonuniformity of the deposit if done over large areas or for
long times. Chemical vapor deposition (CVD) is the preferred method
of depositing thin films of any of these materials. CVD has the
advantages of producing high purity, highly uniform thin films over
very large areas with the ability to conform to surface
irregularities, allows very high deposition rates and efficient
abrupt junctions to be formed. CVD equipment is comparatively
inexpensive and can be easily scaled to use very large substrates,
especially if an atmospheric pressure process is used. CVD is
particularly suited for adaptation to the continuous growth
processes preferred for the high throughput required to
mass-produce the large amounts of cells needed for solar electric
utility power. The cost of layers produced by CVD can be made
sufficiently low by the use of low cost source species, a process
for growth near equilibrium conditions where a near stoichiometric
gas composition can be used (reducing waste of source chemicals)
and by proper design of the gas flow system (allowing efficient
utilization of the source chemicals).
[0030] The CVD method requires a vapor transport species that has a
sufficiently high vapor pressure to transport the elements to the
substrate without condensing on the walls of the growth apparatus.
The metallic elements have transport species (in particular,
organometallic molecules) that are useful for this process and are
commercially available in high purity. The cost of these species,
if manufactured in the large volumes expected to be needed, would
be low enough to meet the cost criteria on the resulting solar
device.
[0031] A new phosphorus source has been identified that
significantly reduces the cost of manufacture and yields the high
purity layers required by all five above-identified photovoltaic
materials. This new phosphorus vapor transport species is liquid
white phosphorus and is used in a reactor having walls that are
heated to prevent condensation without decomposition. White
phosphorus is the form of phosphorus that results from smelting
phosphate ores, that is purified to make high purity red
phosphorus, and that is used to synthesize organophosphorus
compounds. It is the cheapest and highest purity form of phosphorus
and can be used directly for CVD in an appropriately designed
reactor.
[0032] Selection of the phosphorus transport species is a problem
for conventional CVD processes. Then most commonly utilized gaseous
phosphorus source for use in CVD is phosphine (PH.sub.3).
Unfortunately, it is extremely toxic (one breath at 50 ppm is fatal
half of the time) so the entire system must be absolutely
leak-tight, all process areas must be fail-safe ventilated and
monitored with phosphine detectors, thereby increasing the
downtime, facility and maintenance costs. Since it is so toxic, few
manufacturers will supply it and the cost of phosphine is very
high. In addition, phosphine does not dissociate completely at
substrate temperatures as high as 650.degree. C. and much of it
goes through the reactor without reaction. This increases the
effective cost of the process and produces a difficult exhaust
treatment and environmental safety problem. This also precludes the
use of phosphine in low substrate temperature processes. Although
trimethylphosphine and triethylphosphine can be utilized as the
phosphorus source, these two species are also quite toxic and they
allow incorporation of carbon into the thin films.
Bisphosphinoethane and tertiary butyl phosphine have recently been
introduced as commercially available phosphorus sources, but these
are about fifteen times as expensive as phosphine and therefore
contribute significantly to the manufacturing cost of the
photovoltaic cells. Research systems designed to produce phosphine
in-situ by reacting plasma produced atomic hydrogen with heated red
phosphorus have not proved useful in a production environment due
to difficulty in controlling the source reaction for uniform
repeatable processes.
[0033] All of the proposed photovoltaic materials have high solar
absorption and low hulk density and, thus, are suitable for the
manufacture of efficient lightweight solar cells for use on solar
powered airplanes, lighter-than-air craft, satellites and other
applications where it is important to have lightweight solar power.
Because the weight and cost of the substrate of a solar cell is the
dominant fraction of the total weight and total material cost, it
is important to use lightweight, low cost substrate materials.
Plastic substrate have the advantage of being both light weight and
low cost. In order to utilize this class of substrates, the process
for depositing the solar cell materials should be at a low enough
temperature to be compatible with this choice of substrate. This
can be achieved by use of plasma enhancement to the proposed
chemical vapor deposition process which would be set to operate at
a temperature at, or below, 375 degrees Centigrade. The use of a
plasma to excite the reactant species and carrier gas enables the
production of high quality films at a lower temperature because
part of the energy required for the deposition process is supplied
by these excited species as they arrive at the substrate. This low
temperature capability enables this class of substrate materials
(plastics) to be utilized for the fabrication of solar photovoltaic
cells.
[0034] In order to produce photovoltaic cells of the highest
possible collection efficiency, the purity of the photovoltaic
layer should be as high as possible. The use of a plasma to
dissociate hydrogen gas into hydrogen atoms can be utilized to
reduce the incorporation of carbon (from the organometallic species
used to vapor transport the metal species) into the photovoltaic
layer by reacting with, and removing, any carbon that becomes
exposed during the growth process. This is particularly critical
for devices based upon iron phosphide, or mixed crystals containing
iron because the tendency for iron to form a very stable carbide.
The plasma can be used to enhance the purity of the deposited thin
film and thus the efficiency of the photovoltaic device fabricated
from the film.
[0035] It has been observed that the surface of .beta.-ZnP.sub.2,
crystal oxidize slowly in air at room temperature over a long time
and this decomposition is accelerated by temperature and in moist
environments. This process would occur at a significant rate in a
solar cell of standard design (exposed or protected by a thin
antireflection coating) under conditions of its normal operation.
In particular structures must be used to protect the critical diode
junction against such decomposition. This protection is achieved by
using a homojunction device where the junction is slightly below
the surface of the layer and by the use of a passivating layer that
prevents the transport of atmospheric oxygen and moisture to the
surface or the transport of the initial decomposition products from
the surface, thereby stopping the reaction before the junction
region is damaged. By this means, a practical cell lifetime of over
30 years can be achieved. In order for a solar cell having this
passivation layer to also have high efficiency it is necessary that
the passivating layer also has high electrical conductivity and
high transparency lo the solar spectrum. There are several
materials able to function in this multiple role. A unique and
particularly attractive one for this purpose is zinc phosphate
which has the required properties, is known to prevent
decomposition of ZnP.sub.2, and can be produced in the same reactor
utilized to produce the ZnP.sub.2 layer (possibly using the same
source species).
DESCRIPTION OF THE FIGURES
[0036] FIG. 1 illustrates the maximum efficiency of a photocell as
a function of the bandgap of its photovoltaic material.
[0037] FIG. 2A illustrates a structure of a photovoltaic cell 20
utilizing .beta.-ZnP.sub.2 as the photovoltaic layer.
[0038] FIGS. 2B-2J illustrate alternate structural embodiments of
the photovoltaic cell of FIG. 2A.
[0039] FIG. 3 is a phase diagram for ZnP.sub.2, and ZnP.sub.2, for
a P.sub.4, pressure of 5 atmosphere, a zinc partial pressure from
10 atmospheres to less than 0.001 atmosphere and a temperature
range from about 725.degree. C. to 1250.degree. C.
[0040] FIG. 4 is an alternate representation of the phase diagram
of FIG. 3 in which the data is displayed for variable zinc and
phosphorus partial pressures at several fixed temperatures.
[0041] FIG. 5 plots the ZnP.sub.2, decomposition pressure as a
function of temperature.
[0042] FIG. 6 illustrates the ideal electronic structure of the
preferred embodiment of the proposed photovoltaic device wherein
the photovoltaic layer is processed in these four regimes.
[0043] FIG. 7 plots the vapor pressure of liquid white
phosphorus.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0044] FIG. 1 illustrates the maximum efficiency of a photocell as
a function of the bandgap of its photovoltaic material. This curve
is based upon both the current obtained from the absorbed portion
of the incident solar spectrum and the voltage developed by the
cell and is peaked at about 1.45 electron Volts (eV). As seen in
the figure, .beta.-ZnP.sub.2, and CuP.sub.2, are each so close to
this peak that the efficiency of each of these two materials is
substantially optimal. CuP.sub.2, has a unique advantage by having
both an indirect bandgap of 1.4 eV which offers long lifetimes of
the generated carriers and a direct bandgap value (which is always
higher) at only 1.5 eV which would give the steep absorption edge
necessary for absorption in thin layer (characteristic of direct
absorption). In general, neglecting the details of the transport
properties, any material having a bandgap in the range from 1 to 2
eV is a strong candidate for use in photovoltaic cells to be used
for solar electric power generation. The bandgap value of 1 eV for
gamma FeP.sub.4, would indicate its usefulness in a solar
photovoltaic. Although the bandgap of MgP.sub.4, has not been
measured, its bandgap can be approximately inferred from the facts
that (1) it is black and thus its bandgap could not be greater than
2 eV (which would appear "red") or below 1 eV (which would appear
"gray" or "silvery black"); and (2) it is isostructural with
cadmium tetraphosphide (CdP.sub.4,) which is known to have a direct
bandgap near 1 eV. Because MgP, can be expected to be more ionic
than CdP.sub.4, its bandgap can be expected to be both direct and
to have a higher value closer to the optimal bandgap.
[0045] .beta.-zinc Diphosphide Embodiments
[0046] FIG. 2A illustrates a structure of a photovoltaic cell 20
utilizing .beta.-ZnP.sub.2, as the photovoltaic layer. This cell
includes a substrate 21, an insulating layer 22, a back conductor
23, a photovoltaic layer 24, a transparent conductor 25, a grid
conductor 26 and an antireflection coating layer 27. This
photovoltaic cell exhibits an improved (lower) cost and
cost-per-watt compared to existing photovoltaic cells. Although it
is designed primarily for conversion of a solar energy flux into
electrical power, it could also be utilized in other applications
that convert light of an energy absorbed by the .beta.ZnP.sub.2,
into an electrical output. Incident light 28 is transmitted through
the antireflection coating 27 and the transparent conductor 25 to
the photovoltaic layer 24, where the light is strongly absorbed by
conversion into electron-hole pairs near the front surface of this
layer and near a pn junction 29 within the diffusion length of
these carriers. This junction is formed between a highly doped
n-type (n+) region 210 and a lightly doped, or undoped (intrinsic),
p-type photovoltaic layer 24. This junction collects the electrons
generated in the p-region and the holes generated in the n+ region
to provide a current between the grid conductor 26 and the back
conductor 23. There is a highly doped p-type (p+) region 211 near
the back conductor 23 that forms an abrupt step in the p-type
concentration relative to the p-type concentration in the
photovoltaic layer 24. This concentration step intercepts any
photogenerated electrons traveling toward the back conductor 23 and
reflects them back toward the collecting junction. This structure,
known as a "back surface field", greatly increases the efficiency
of a thin film device by preventing the loss of photogenerated
electrons by recombination at the back conductor 23.
[0047] Following is a description of the function of these layers
and the factors involved in selecting appropriate materials and the
thickness of each of these layers. The n+ region 210 creates the
collection junction 29 required for the photovoltaic effect in the
photovoltaic layer 24. The transparent conductor 25 must form a low
resistance ohmic contact to the n+ region 210. The back conductor
23 must form a low resistance ohmic contact to the p+ region
211.
[0048] Substrate 21 provides the mechanical support structure
required during fabrication and use of this photovoltaic device.
This material must be strong enough to provide this necessary
support and should be sufficiently stable model the subsequent
process conditions that is it mechanical properties are not lost
during such process steps. Because of the need for low cost
photovoltaic cells, this material should be as inexpensive and as
thin as possible while providing these properties. For most
applications it is desirable that this layer be flexible. Because
the anticipated need for solar electricity is incredibly large, the
high production throughput requires that the substrate should be
able to be manufactured in the form of wide, thin sheets (on the
order of a meter wide and perhaps as thin as 40-100 microns) that
can be stored as a continuous roll from which this material can he
translated through the processing equipment for the fabrication of
the photovoltaic cells. It is advantageous that this material be
malleable, so that such rolls can be inexpensively manufactured.
The preferred choice of this material is an iron alloy such as
stainless steel because of the low cost, high current production
volumes and the commercial availability of such rolls. Other
materials would include copper alloys (brasses and bronzes),
aluminum alloys and molybdenum alloys. For applications where
extreme flexibility or light weight are required, the preferred
choice of the substrate would be films of high temperature stable
plastics such as polyimides. For applications requiring a
transparent substrates various glasses could be used though they
would be used as sheets rather than as a single roll.
[0049] The insulating layer 22 is used to electrically isolate the
device from the substrate 21 if the substrate is electrically
conducting (such as a metal) and if it is desired to make a series
contact between cells manufactured on the same substrate. It is not
needed if a plastic or ceramic substrate is used, or if a single
photovoltaic cell will cover the entire metallic substrate area and
if it is desired to o contact this single cell from the back side.
When this layer is used, it must have negligible electrical
conductivity. It must be compatible with the substrate material
such that either it can be formed by deposition on the substrate,
or by conversion of the surface of the substrate by some process,
such that it provides an intimate bond to the substrate without any
cracks or pin holes in the insulating layer. It must be thermally
stable during subsequent process steps and be chemically stable
during deposition and possible etching of the back conductor 23.
The preferred choice of this layer is silicon dioxide due to its
low cost, availability of source chemicals and advance technology
of its deposition. There are many other alternate materials
including oxides and phosphates of the potential substrate metals.
The layer need only be sufficiently thick enough to be electrically
insulating and pin-hole free. This is typically 500-5000
angstroms.
[0050] The back conductor 23 provides an electrically conductive
path parallel to the layers and out to the edge of the cell where
it can be electrically contacted by wires, or other means, to
provide power external to the cell. Therefore, it must have
negligible electrical resistance, be compatible with the insulating
layer material (i.e., low diffusivity into, and low reactivity
with, the insulator), be thermally stable during subsequent
processing, be chemically stable with respect to the material of
photovoltaic layer 24 and the conditions of deposition of that
layer, and it must form a low resistance ohmic contact to the
photovoltaic layer 24. It should also be etchable to allow
isolation of one cell from the another if more than one cell is to
be manufactured on the same substrate and the unetched back
conductor must be stable during the etching process. It must also
form a low resistance ohmic contact with an external conductor with
which it will be brought into intimate contact. Since this layer is
due of the thickest active layers in the cell (excluding the
substrate 21), it cost should be as low as possible while providing
the required electrical conductivity. The preferred choice is pure
copper but alternatives would be copper alloys (particularly the
function of the back conductor 23 was integrated with the function
of the substrate 21), aluminum (particularly for lightweight
applications) or aluminum alloys (particularly when integrated with
the substrate function for lightweight applications) or even silver
(for application where the electrical conduction is more important
than cost). It may be necessary to cover the back conductor with a
thin layer of another material to provide the required ohmic
contact lo the photovoltaic layer. Transition metals that form
phosphides with metallic conductivity would be useful for this
purpose. These would include titanium, chromium, molybdenum, iron,
copper and nickel. The thickness of this layer depends on the size
of the cell from which the current in transported with a typical
thickness being 1-10 microns.
[0051] The use of copper for the back conductor has special
advantages unique to this material. It has very high electrical
conductivity, is readily available in high purity at a reasonable
price, and is soft and malleable when highly pure (oxygen free). It
forms a low resistance ohmic contact to ZnP.sub.2 by virtue of its
ability to dope the ZnP.sub.2 p-type degenerating ZnP.sub.2 to
metallic conductivity at the interface between the two materials.
It can also form a lower phosphide Cu.sub.3P which is metallic so
that the formation of an intermetallic compound during the growth
of the ZnP.sub.2 layer will help to form the ohmic contact. Since
copper is a p-type dopant in ZnP.sub.2, it can be used to form the
p+ layer 211 by controlled out-diffusion from the substrate during
growth of the ZnP.sub.2 layer integrating the process of back
surface field formation with photovoltaic layer growth, simplifying
the process and reducing cost.
[0052] As indicated above, the photovoltaic layer 24 is beta zinc
diphosphide because this low cost material efficiently absorbs the
solar spectrum, efficiently transports the photogenerated carriers
and can be fabricated with the abrupt p/n and p/p+ junctions needed
for an efficient photovoltaic device. The critical p/n diode
junction 29 can be formed as a homojunction by a doping transition
within the layer during the layer growth or by diffusion from the
surface after the layer growth is completed. Conventional dopants
such as aluminum, gallium, indium, iron, tin, germanium, sulfur,
selenium or tellurium can be used for the n-type dopant with
gallium being preferred due to its commercial availability and lack
of tendency to form either oxides or carbides. Unconventional
dopants such as copper and lithium can be used for the p-type
dopant with copper being preferred for its lack of mobility within
the layer. The carrier concentration of undoped (intrinsic)
.beta.-ZnP.sub.2 can be adjusted from its normal slight p-type
conduction to strongly p-type by shifting the stoichiometry of the
layer to the P-rich condition by the use of phosphorus enriching,
or vacuum (zinc depleting), process or by adjusting the gas phase
composition toward a large excess of phosphorus during growth of
the layer. Use of zinc treatment after growth or growth from a gas
phase composition that is relatively rich in zinc will shift the
layer stoichiometry toward the Zn-rich condition reducing the
p-type conductivity and even converting the layer to slightly
n-type. The use of extrinsic dopants is preferred for better
control of carrier concentration. The high purity source chemicals
to produce this layer are available at a relatively low cost in
large quantities from several independent suppliers. The subsequent
processing conditions and the materials selected for adjacent
layers must be compatible with this layer. This layer must be thick
enough to absorb the incident sunlight and to fabricate separated
junctions without the dopant diffusion along the crystallite
boundaries extending between them. This thickness will be typically
1-3 microns.
[0053] The transparent conducting layer 25 must transmit sunlight
without significant absorption and must have the highest possible
electrical conductivity so that the electrical current generated in
the photovoltaic layer 24 has a low resistance path to the grid
conductor 26. It must bond intimately to the photovoltaic layer,
must form a low resistance contact to the n+ region 210, must form
a low resistance ohmic contact to the grid conductor, and must be
deposited by a method that is compatible photovoltaic layer which
does not effect the properties of the diode junction 29 in any way.
It must be thermally and chemically stable during the processes
that fabricate the grid conductor. In order to produce a photocell
with an extended lifetime, the transparent conductor should form a
continuous layer (no holes or cracks), be atmospherically stable
chemically and photochemically and be resistant to mechanical
damage. It must not be permeable to any atmospheric species that
are harmful to the photovoltaic layer. The thickness of this layer
is typically 1-3 microns.
[0054] The preferred material for the transparent conducting layer
25 is zinc phosphate which is easily formed on the zinc diphosphide
photovoltaic layer using similar equipment and, possibly, identical
chemical sources. It is chemically similar to, compatible with, and
effective for passivation of ZnP.sub.2. The zinc phosphate must be
doped strongly n-type (n+) using conventional dopants such as
aluminum, gallium and indium. Since these dopants are compatible
with the n+ layer 210 in ZnP.sub.2, this layer can be used as used
in a integrated process to form the diode junction 29 in the
photovoltaic layer 24 by controlled diffusion of the dopant from
the transparent conducting layer during or following its
deposition. Alternate choices of material for this layer will be
zinc oxide (which is compatible, well known, and inexpensive), tin
oxide (compatible, well known, though slightly more expensive),
antimony tin oxide, doped aluminum phosphate, and indium tin oxide
(where performance is more important than either cost or
availability. Boron phosphide is another possible alternative
because of compatibility with ZnP.sub.2, and its growth process,
high conductivity when nonstoichiometric (P-rich), and it extreme
hardness and chemical inertness.
[0055] The grid conductor 26 conducts current from the photovoltaic
cell to a location that can be contacted to an external load or
connected to another photovoltaic cell of similar structure. This
material must be highly electrically conductive. This material must
form a low resistance ohmic contact to the transparent conducting
layer 25. This layer is patterned on the surface of the
photovoltaic cell to provide an array of openings through which
incident light 28 can reach the photovoltaic layer 24 and to leave
a grid of conductive material that can transport current from the
edges of the regions of the photovoltaic layer that are exposed to
light, to a common current output. The exposed regions are referred
to as "active regions", because current is generated only in such
regions. The active regions are typically at least 90% of the
surface area of the photo-voltaic cell. The pattern, width,
thickness and spacing of the grid lines are a function of the
conductivity of the material in these grid lines and the
conductivity of the transparent conducting layer 25. The thickness
of the grid lines is 5-10 microns using a highly conductive
material but could be 100 times this if conductive solders are
used.
[0056] The material for this layer must be formable into a
continuous conductive pattern, should make an intimate bond with,
must form a low resistance ohmic contact to, and be chemically
compatible with the transparent conductor. This material must also
be stable in the environment. The preferred material is aluminum.
Other choices include copper, copper alloys, and solder alloys of
zinc, lead and tin. Additional layer or layers can be included
between the grid conductor and the transparent conductor for
improved chemical compatibility, mechanical adhesion and/or
electrical contact. One or more layers may be required on top of
this layers to protect it from mechanical damage and atmospheric
chemical attack.
[0057] The antireflection coating layer 27 reduces the fraction of
incident light 28 that is reflected by the surface of the
photovoltaic cell and provided additional protection of the other
layers from mechanical damage and chemical attack by the
environment. Its optical properties and thickness are therefore
selected to prevent significant reflection from the photovoltaic
cell, its mechanical properties are selected to optimize its
protective functions and lifetime, and its chemical properties are
selected to ensure chemical compatibility with the transparent
conductor and the conductive grid. The preferred material is
magnesium fluoride. Other choices include silicon monoxide, silicon
nitride, silicon dioxide, titanium dioxide, aluminum oxide,
phosphates of aluminum and boron, and boron phosphide. This coating
can be a single layer but will often be an optical stack to
optimize its antireflection property. This coating can also have a
variable index of refraction with thickness. The thickness of a
single layer quarter-wavelength antireflection coating will be 0.2
microns.
[0058] FIGS. 2B-2J illustrate alternate structural embodiments of
the photovoltaic cell of FIG. 2A.
[0059] In FIG. 2B, the substrate 21, which must be electrically
conductive, is used for the back conductor 23 by eliminating the
insulating layer 22 and the separate back conductor 23 of FIG. 2A.
Electrical contact to the device can be made from the back side.
This embodiment is useful for small area cells as may be produced
by a pilot line.
[0060] In FIG. 2C, because the substrate 21 is electrically
non-conductive, insulating layer 22 of FIG. 2A has been eliminated.
This embodiment would be useful for cells deposited on substrates
made of ceramic, glass or plastic. This would be the preferred form
for lightweight cells deposited on plastic which are illuminated
from the top side. In the lightweight embodiment the materials of
the layers would be selected for lightness of weight rather low
cost.
[0061] In FIG. 2D, the grid conductor 26 has been deposited on a
transparent and electrically non-conductive substrate 21 prior to
fabrication of the active photovoltaic device structure. The same
active structure as in FIG. 2A is fabricated in the reverse order
upon this structure. In this embodiment the incident light 38
enters the photocell through the substrate and is most useful when
deposited upon a glass substrate. It is also useful for light
weight cells deposited on plastic where the surface exposed to the
light must be flat and smooth as would be the case for solar cells
installed on the top surface of an aircraft's wing.
[0062] In FIG. 2E, the embodiment of FIG. 2A is modified to bring
the grid conductor 26 into direct contact with the photovoltaic
layer 24 and is protected from the environment by both transparent
conductor 25 and antireflection coating 27.
[0063] In FIG. F, the embodiment of FIG. 2B is modified to bring
the grid conductor 26 into direct contact with photovoltaic layer
24.
[0064] In FIG. G, the order of fabrication the transparent
conducting layer structure of FIG. 2C is modified to bring the grid
conductor 26 into direct contact with the photovoltaic layer
24.
[0065] In FIG. 2H, the embodiment of FIG. 2D is modified to deposit
the transparent conducting layer 25 directly on the transparent
substrate 21 and serves to protect the substrate from the process
conditions used to form the subsequent grid conductor layer 26. The
incident light 28 enters the cell through the transparent
substrate. The rest of the cell layers are deposited in the reverse
order to the embodiment of FIG. 2A.
[0066] In FIG. 21, the embodiment of FIG. 2C is modified by
inverting diode junction 29 types from n+-on-p to p+-on-n and the
use of a p +-type transparent conducting layer 25 and a n+-type
back surface field region 211. This embodiment enables the use of a
lightweight back conductor 23 and an extremely durable material,
boron phosphide, for transparent conducting layer 25 and produces a
lightweight cell that is extremely durable.
[0067] In FIG. 2J, the buried p-n homojunction of FIG. 2A is
replaced by a heterojunction formed between the photovoltaic layer
24 and the transparent conductor 25. While this junction formation
method would work equally well using p-on-n or n-on-p junctions
types, this figure illustrates the embodiment that uses a p-type
photovoltaic layer 24 and an n+ type transparent conducting layer
25. This embodiment could use hard, chemically resistant boron
phosphide for the transparent conducting layer 25 to give an
extremely durable photocell. Slight adjustment of the process would
form the embodiment of FIG. 2A with an n+-type layer 210 by boron
doping.
[0068] These various embodiments illustrate that: (a) the grid
conductor 26 can make direct contact with the photovoltaic layer 24
or can make indirect contact through a transparent conductor 25;
(b) the photovoltaic layer 24 can have an inverted structure in
which the n-type layer is on the bottom and the p-type layer is on
the top; (c) the diode junction in the photovoltaic layer 24 can be
formed on the front surface at the heterojunction between the
photovoltaic layer 24 and the transparent conductor 25; (d) the
diode junction in the generating layer can be formed by replacing
part of the zinc diphosphide photovoltaic layer with a zinc
diarsenide photovoltaic layer, thereby producing a dual wavelength
absorber; (e) the photovoltaic cell can be produced in an inverted
order on a transparent substrate; (f) the photovoltaic cell can be
formed with electrical contact to the back of the cell using a
conducting substrates 21; (g) structures can be used to protect the
grid conductor 26 from environmental conditions; (h) structures are
available for use of very lightweight flexible materials for
potential space application; (i) structures are available that
enable the use of highly durable and chemically resistive materials
for applications in corrosive environments; and (j) structures are
available for use in solar powered aircraft.
[0069] .beta.-zinc Diphosphide Passivation
[0070] Tests of .beta.-zinc diphosphide have shown that it
decomposes slowly when exposed to atmospheric moisture and heat.
There is negligible decomposition under ordinary conditions, but
when a solar photovoltaic cell is exposed to heat, light and
moisture, its surface will corrode. If the photovoltaic junction is
shallow, it will be strongly affected by such surface corrosion.
Following are two different solutions to this problem.
[0071] Buried Junction Embodiment: One solution to this problem is
to form the diode junction a small distance within the photovoltaic
layer so that corrosion of the layer surface does not reach the
photosensitive diode junction. The photosensitive diode junction
must be a pn junction because any decomposition of the layer would
start at the interface between zinc diphosphide and the adjacent
surface layer and if this interface were also the junction then the
slightest decomposition there would severely degrade the junction
quality. Also, the p/n junction device will have a higher
efficiency since there are no recombination centers at the
interface as would be typical for heterojunction between two layers
of different materials. In the special case of epitaxial growth,
where the two materials have the same crystal structure and almost
identical lattice parameter (such as ZnP.sub.2 and ZnAs.sub.2) an
efficient heterojunction can be formed. However, diffused junctions
have the advantage of being less expensive to fabricate. For an
unpassivated photovoltaic cell having a buried junction, slight
surface decomposition will only slightly degrade cell efficiency
due to a small loss of optical clarity at the surface.
[0072] Passivated Junction Embodiments: The most efficient cell
would have the shallowest junction because the carriers to be
collected are strongly absorbed near the surface of a direct gap
material. The use of a shallow junction requires a minimum depth of
any surface corrosion so the use of a passivating layer between the
surface and the environment would enable the use of a shallower
junction and increase the efficiency of the cell. This passivation
layer must either prevent oxygen and moisture from reaching the
surface of the photovoltaic layer or must prevent phosphorous
oxides (that is result from by such corrosion) from escaping
through the surface of the photovoltaic layer. Almost any
continuous, pinhole-free layer of sufficient thickness having a low
moisture and oxygen, or phosphorus oxide, diffusivity will achieve
this result.
[0073] The efficiency of a shallow junction device will be low if
it is not in contact with a layer of material having a high
electrical conductivity material since ohmic losses will degrade
the efficiency. Thus, a transparent conductor 25 should be included
that also functions as a passivation layer. One particular class of
materials that are suitable for this dual function are conductive
oxides, such as zinc oxide (ZnO), tin oxide (SnO.sub.2), tin-doped
antimony oxide (Sb.sub.2, O.sub.3), or tin-doped indium oxide
(In.sub.2O.sub.3).
[0074] In order for this passivation layer to also function
efficiently as the transparent conductor, it must be relatively
thick (on the order of several microns). In general, this requires
that this layer be thicker than the photovoltaic layer. This means
that the material cost of this layer can be a significant part of
the material cost of the photovoltaic cell. This, in turn means
that the chemical components must likewise be subject to the same
limitations as the chemical components of the photovoltaic
material--namely, they should be inexpensive, abundant,
nonpolluting and not subject to control by a cartel. Zinc oxide is
the preferred choice due to these reasons.
[0075] In addition to these well known conductive oxides, zinc
phosphate is a good choice because: all of its components are
abundantly available; zinc and phosphorous sources are already
required to manufacture the .beta.-ZnP.sub.2 photovoltaic layer,
thus reducing the complexity of the fabrication apparatus and
providing the economy of commonality; its ability to be doped to
high conductivity using convent ional dopants; and its high optical
transparency. Three different zinc phosphate stoichiometries are
possible (i e., xZnO:xP.sub.2O.sub.5 where x=1, 2, or 3), thereby
increasing the ability to optimize performance. All three of these
stoichiometries have the requisite optical and physical properties
and can be doped n-type by substitution on the metal lattice, as is
typical in high efficiency conductive oxide structures. The
substitution can be selected from Group III elements (i.e., B, Al,
In, or Ga), Sn or Sb. These all are sufficiently available for use
as low concentration dopants, even at the levels required for
conductive oxides, but B, Al, Sn and Sb are the most economically
viable choices. The preferred choice is aluminum because of it low
cost and high availability.
[0076] Solid Source OMCVD of Metal Phosphides:
[0077] There is another method of sourcing for OMCVD. This uses a
solid rod that is a pressed powder or solidified melt. The
composition of the rod is the same as the vapor composition. The
rod is fed in to a region of a high temperature gradient
proportional to the desired growth rate at the substrate. The gases
from the vaporizing rod are carried to the substrate by an inert or
reactive carrier gas. The growth chamber may be at atmospheric or
reduced pressure. The walls are heated to prevent condensation of
tile reactive species. A bypass valve is installed to allow the
growth transient associated with a new rod to bypass the growth
site before growth begins.
[0078] The advantages of this system are the simplicity of the
sourcing method, its suitability a process utilizing to heated
walls and its ability to use organometallic species with a high
melting point.
[0079] This growth method is especially useful for the growth of a
continuous film such as that envisioned for solar cell
manufacturing production line. It has the further advantage that
rods can be mass produced to high tolerances at a central location
to produce identical growth results in production lines located at
widely distributed locations.
[0080] This method is particularly suited to the growth of the
proposed solar cell materials (ZnP.sub.2, CuP.sub.2, MgP.sub.4, and
FeP.sub.4). All of these have useful organometallic species that
are solid at room temperature.
[0081] In all these cases, the carrier gas will be hydrogen which
may be diluted in an inert gas.
[0082] The preferred phosphorus species for the growth of all
phosphides is white phosphorus which melts at 44.degree. C. and
boils at 280.degree. C. These are near ideal properties for this
sourcing method.
[0083] The preferred zinc species for the growth of ZnP.sub.2 is
diphenylzinc which melts at 107.degree. C. and boils at
280-285.degree. C. This is near ideal for use with white
phosphorus. Also, bis(cyclopentadienyl)zinc,
bis(methylcyclopentadienyl) zinc, bis(pentamethylcyclopentadienyl)
magnesium are all useful and available. None of these species
contains any oxygen atoms.
[0084] The preferred magnesium species for the growth of MgP.sub.4
is bis(cyclopentdienyl) magnesium which melts at 176.degree. C. and
boils at 290.degree. C. is near ideal.
Bis(pentamethylcyclopentadienyl) magnesium is also useful and
available. None of these species contains any oxygen atoms.
[0085] The preferred copper species for the growth of CuP.sub.2 is
copper(I) phenylacetylide which contains no oxygen atoms. This
compound is useful and available. There are other organometallic
copper species containing oxygen atoms where research growth of
other compounds has shown that the oxygen is not incorporated into
the films. The preferred species of this type that are useful and
available in high purity are copper(II) hexafluoroacetylacetonate
and copper (II) trifluoroacetylacetonate.
[0086] The preferred iron species for growth of FeP.sub.4 is
bis(cyclopentdienyl) iron (ferrocene) which melts at 172.degree. C.
and bis(diphenylphosphino) ferrocene which melts at 180.degree. C.
There are many other useful iron compounds but these have the
advantage in containing no oxygen atoms.
[0087] Dopants can be introduced into the phosphides in two
preferred ways: either the dopant is introduced as a solid into the
rod in a very dilute composition; or the dopant is introduced into
the carrier gas stream from a hot source bubbler through a heat
tube. The second method is preferred because it provides superior
control of the dopant concentration and also allows precise control
of the growth of abrupt and step junctions during layer growth.
[0088] Methods of OHMC Contact to Back Conductor:
[0089] A key aspect of the preferred embodiment is forming an ohmic
contact to the bus conductor when the p-type ZnP.sub.2 layer is
grown upon it. There are several methods to do this:
[0090] I. High purity oxygen free copper is the preferred material
for the back conductor. Copper can be used for the back conductor
layer or for the top layer in a back conductor stack. This is a
p-dopant in ZnP.sub.2, the interlayer (Cu.sub.3P) is metallic, and
thus an ohmic contact is formed to it. This is especially
attractive because copper is an excellent conductor and foils of it
can be obtained easily. This is an elegantly simple and effective
process. This is the preferred method to make this contact and is
useful whether of not the primary material for the back is copper.
This is useful for forming an ohmic contact to an aluminum back
conductor.
[0091] Alternate materials of this type include ordinary purity
copper for the back conductor. Copper based alloys, especially
brasses (Cu/Zn), phosphocopper alloys (Cu/P), cupronickel (Cu/Ni),
bronzes (Cu/Sn), etc., are especially useful for substrates
integrated with the back conductor (as embodied in FIG. 21) because
they have superior mechanical properties, good electrical
conductivity, and are readily available in foil form.
[0092] There is no mention in the literature of copper being used
lo make ohmic contact ZnP.sub.2 or as a substrate for growth of
ZnP.sub.2 that was later shown to give a good electrical contact.
There is no mention either of copper being the substrate for any
CVD growth of phosphides which may be explained but the perception
that phosphide of the layer would prevent ohmic contact. No
transition metal (except Au) has ever been used to contact
ZnP.sub.2 and no metal (except Al and Au) has ever been used as a
substrate for ZnP.sub.2 deposition and shown to form an electrical
contact. And these depositions where by evaporation not CVD.
[0093] 2. Molybdenum could also be used in an integrated process to
advantage because it makes an excellent substrate foil, foils are
available, it has good thermal expansion matching to ZnP.sub.2 and
can form an ohmic contact by the formation of a metallic interlayer
(MoP.sub.2).
[0094] 3. Thin interlayers of p-doping species (Cu, Ag, Li) can be
used to form an ohmic contact. These diffuse into the growing layer
forming a p+ region that I forms the contact.
[0095] 4. Thin interlayers of noble metals (Au, Pt, Pd) can be used
to form an ohmic contact. It is believed that these do react to
form metallic interlayer compounds that form ohmic contact.
[0096] 5. Interlayers of several transition metals (Ti, V, Cr, Mn,
M O, W, Ta, Fe, Co, Ni, Cu) that are known to form either low
bandgap p-type semiconducting compounds or metallic conducting
compounds can also be used to form an ohmic contact.
[0097] Transparent Conducting Layers of Zinc Phosphate:
[0098] Zinc phosphate is the preferred material system for the
conducting oxide layer. The advantages of this system are that:
[0099] 1. All of the elements are plentifully available.
[0100] 2. All of the elements can be deposited from the vapor for
an efficient and useful OMCVD process.
[0101] 3. It is possible to n-dope these phases by use of group III
elements and tin.
[0102] 4. All phases have acceptable physical and chemical
properties for use as a passivating and protecting layer.
[0103] There are several stoichiometries and morphologies that can
be employed for conductive oxide layer:
[0104] 1. Zinc metaphosphate (ZnO:P.sub.2O.sub.5).
[0105] a. The Zn:P ratio is the same as ZnP.sub.2, which means that
the interface is more stable towards interdiffusion which yields to
desired abrupt interface.
[0106] b. The electronic configuration of ZnP.sub.2O.sub.6 is
analogous to 3SiO.sub.2. Thus, introduction of SiO.sub.2, into the
layer is possible to affect the crystal structure without effecting
the electronic structure.
[0107] c. This stoichiometry has been shown to exist in both
amorphous and crystalline forms.
[0108] 2. Polycrystalline zinc metaphosphate (2ZnO:
P.sub.2O.sub.5). The Zn:P ratio is the same as ZnP.sub.2 which
means that the interface is more stable towards interdiffusion
which produces a desired abrupt interface.
[0109] 3. Polycrystalline zinc pyrophosphate (2ZnO:
P.sub.2O.sub.5).
[0110] 4. Polycrystalline zinc orthophosphate (3ZnO:
P.sub.2O.sub.5). This is the most thermodynamically stable zinc
phosphate and is the ultimate decomposition product of ZnP.sub.2
thermal oxidation. This material is therefore the easiest phase to
grow. This phase has been shown to form a passivating layer on
ZnP.sub.2 exposed to atmospheric decomposition at a thickness of
approximately 1 micron.
[0111] The phase that will be used for the device will be
determined based upon the optimum combination of maximum optical
transparency and maximum electrical conductivity. This will be a
function of dopant introduction, process history, and silicon
introduction.
[0112] The actual conductive oxide phase will have the general
formula:
[0113] aZnO: P.sub.2O.sub.5:bSiO.sub.2,:cM.sub.2O.sub.3:dSnO.sub.2,
where M is a group III element and b, c and d are integers. This
phase is referred to generically as "zinc phosphate" (ZnPO.sub.x)
and X -1, 2, or 3.
[0114] Growth of Zinc Phosphate by OMCVD:
[0115] Liquid source OMCVD is the preferred way to grow films of
zinc phosphate for the transparent conducting layer. There are two
chemistries that can be used for the growth of these materials that
are cheap enough to be useful for low-cost solar cells:
[0116] 1. Zinc phosphate can be formed by the reaction of
organometallic zinc (DEZ or DMZ) with phosphorus trioxide
(P.sub.2O.sub.3) in the presence of oxygen (O.sub.2) in the gas
phase. This is the preferred chemistry for the deposition of
ZnPO.sub.x. The advantages of this chemistry are:
[0117] 1. The lower melting point of P.sub.2O.sub.3 (23.8.degree.
C.) allows reduced source temperature. This reduces the
specifications and cost of source equipment.
[0118] 2. The Lower boiling point of P.sub.2O.sub.3 (175.4.degree.
C.) allow lower wall temperatures. This greatly reduces the
temperature requirements of the equipment and reduces cost.
[0119] 3. The oxidation reaction of P.sub.2O.sub.3 to
P.sub.2O.sub.5 is slower and thus potentially more controllable
than the reaction of P.sub.4 to P.sub.2.sub.5
[0120] 4. The species contains most of the oxygen required by the
process thus reducing the oxygen concentration required in the gas
stream thereby further increasing the controllability of the
reaction. The advantage of the lower oxygen concentration is the
controllability of the oxidation of the zinc and doping
species.
[0121] 5. As a result, the growth process will be the most robust
and the simplest to develop.
[0122] II. An alternate reaction can be used where zinc phosphate
is formed by the reaction of organometallic zinc (diethylzinc {DEZ}
or dimethylzinc {DMZ}) with white phosphorus (P.sub.4) in the
presence of oxygen (O.sub.2) in the carrier gas phase. The
advantages of this chemistry are the commonality with the ZnP.sub.2
growth chemistry which allow integration of the two layers in the
same reactor with the least possible components. This process has
advantages for development of an integrated research reactor.
[0123] In both chemistries, dopants will be standard organometallic
compounds of Group III elements (Al, In and Ga), or boron hydride
(diborane), or tin hydride (stannane). These will be introduced
along with the zinc species.
[0124] For growth of the ZnO:P.sub.2O.sub.5 stoichiometry phase,
especially using the amorphous morphology, silicon hydride (silane)
will be used.
[0125] A two-stage process can be employed to reduce oxygen
concentration during the primary growth phase. The film will be
deposited without being fully oxidized using a low oxygen
concentration followed by an oxygen treatment process after the
organometallic are out of the gas stream.
[0126] Growth of Zinc Phosphate by Solid Source OMCVD:
[0127] Solid source OMCVD is useful for the growth of zinc
phosphate. Several methods can be employed to grow theses
phases:
[0128] I. The mixture of the zinc species and solid white
phosphorus are adjusted to give the desired stoichiometry. In this
case, the carrier gas will be oxygen typically diluted in an inert
gas. Dilute hydrogen may also be used to help dissociate the
organometallic without complete decomposition.
[0129] Since the rod for the growth of ZnP.sub.2, and the growth of
ZnP.sub.2O.sub.6 are identical with the distinction between the
phase being due to the different composition of the carrier gas,
this reduces the cost and simplifies the growth a device formed
these to materials.
[0130] The preferred zinc species for the growth of zinc phosphate
is diphenylzinc which melts at 107.degree. C. and boils at
280-285.degree. C. This is near ideal for use with white
phosphorus. Also, bis(cyclopentadienyl)zinc,
bis(methylcyclopentadienyl) zinc, bis(pentamethylcyclopentadienyl)
zinc are all useful and available.
[0131] II. An alternate method is to form the rods with a mixture
the above preferred zinc species and solid phosphorus trioxide.
Growth takes place in an oxidizing atmosphere. The advantages of
phosphorus trioxide are:
[0132] 1. Its melting which is just above room temperature
(23.8.degree. C.) simplifying rod manufacture from the smelt.
[0133] 2. It boiling point is 175.4.degree. C. which offers low
wall temperatures.
[0134] 3. It carries most of it own oxygen, reducing the required
oxygen concentration in the carrier gas.
[0135] 4. It reacts readily yet slowly with oxygen to produce the
completely oxidized form of the phosphorus pentoxide stoichiometry
(P.sub.2O.sub.5) with is desired at the substrate.
[0136] III. An alternate method is to form the rods with a mixture
of the above preferred zinc species and phosphorus pentoxide. This
method requires the use of the hexagonal modification to have a
sufficient vapor pressure for growth of the phase. This material
has low commercial availability. This is not the preferred method
for the growth of ZnPO.sub.x phases.
[0137] IV. An alternate method is to replace the preferred zinc
species with ones that contain oxygen. These species have the
general formula (RO).sub.2Zn. The advantages of these species are
that they are simpler (and thus less costly), they have a lower
formula weight (which gives a high vapor pressure), and the oxygen
content of the carrier gas is further reduced. Examples of these
species are methoxy- and ethoxy-zinc.
[0138] In all methods, dopants can be introduced into the
phosphates in two ways: either the dopant can be introduced as a
solid into the rod in a very dilute composition; or the dopant can
be introduced into the carrier gas stream from a hot source bubbler
through a heat tube. The second method is preferred because in give
superior control of the dopant concentration and also allows
precise control of the growth of abrupt and step junction during
layer growth.
[0139] Protection of the Device Using a Boron Phosphide Layer
[0140] The proposed ZnP.sub.2 solar cell will have a "durable"
option. In this form, the cell will be coated with a layer that
will mechanically and chemically protect the device from
environmental hazards.
[0141] Diamond or "diamond-like" carbon layers are frequently
proposed for device protection functions. While this may by useful
in some embodiments, it is not preferred because of the high
substrate temperature required and the slow growth rate and the
requirement for plasma activation.
[0142] Nearly the same protection can be obtained by the use of
boron phosphide (BP). This material is one of the hardest and most
inert materials known. BP can be grown from the vapor in both n-
and p-type as required by adjusting the gas phase composition.
[0143] BP is compatible with ZnP.sub.2 chemically and physically
and can be used for both protection, passivation and heterojunction
formation as required. Any BP layer will serve to protect and
passivate the ZnP.sub.2. Layers of the same type will be used for
passivation (no interfacial junction is formed). It can be used to
form a heterojunction with either n- or p-type ZnP.sub.2 by using
layers of opposite type. The preferred form of this heterojunction
is to use p-type ZnP.sub.2 and n-BP because boron is an n-dopant in
ZnP.sub.2 and Zn is a p-dopant in BP. This can be exploited to form
a homojunction device by using an integrated process where a
dopant-rich initial growth is followed by a standard layer growth.
The preferred structure of this type is to create a shallow
homojunction of n-or p-ZnP.sub.2 with a protecting layer of n-type
BP. The BP layer can be made thick and with degenerated
conductivity for use as a transparent conductor or can be made of
the proper thickness for antireflection coating (ARC) and still
serve to add conductivity.
[0144] Growth of BP is compatible with the ZnP.sub.2 growth
processes. Gaseous boron hydride(diborane), or boron chloride
(BCl.sub.3), or mixed hydride/chloride, or organoboride can be used
for the vapor boron species. In standard processes, phosphorus
vapor species comes from phosphorus hydride (phosphine) or
phosphorus chloride or organophosphide. These are compatible with
the ZnP.sub.2 surface. However, boron hydrides are preferred for
cost and process reasons and organophosphide are preferred for
safety reasons. In addition, BP can be grown by the proposed
innovative process using phosphorus gas as the phosphorus source.
This will reduce the cost of the BP layer and allow process and
equipment advantages due lo compatibility with the preferred
ZnP.sub.2 growth process (which uses this source).
[0145] BP is the preferred protective material over ZnP.sub.2
surfaces that require an especially hard inert protection.
[0146] Another possibility within this system is the use of
BP.sub.6 for antireflection layer. It is equivalently hard and
inert. It is an insulator and its advantage is that it is more
transparent to the solar spectrum and may be useful for maximum
efficiency devices where the slight UV absorption of BP may be a
problem. The lower cost and device structure advantages of BP make
it the preferred material in this system.
[0147] BP is compatible with zinc phosphate chemically and
physically and can be used for protection and for antireflection
coating. The zinc phosphate layer will be n-type and either n-or
p-type BP can be used to protect the device. If an n-type layer is
used it will aid in conductivity of the transparent layers. If a
p-type layer is used it will aid in the efficiency of the device by
reflecting electrons for the zinc phosphate layer away from the
surface and increasing the collection efficiency of the device. The
type used will depend on getting the lowest resistance with the
minimum loss. This will depend upon the thickness of both layers
which will depend upon optimizing the combined layers for both the
minimum reflectivity and minimum resistance. The exact structure
will be optimized by a detailed analysis of the overall device
performance.
[0148] Formation of the Back Surface Field by an Integrated
Process
[0149] If copper is used for the back conductor, or if a very thin
layer of either copper or lithium is deposited on the surface of
the another material that is used for the back conductor, an
integrated process is possible that will for the ohmic contact and
back surface field in one step. During growth, or subsequent heat
treatment, of the photovoltaic layer these materials will diffuse
from the region of the interface between the two layers into the
photovoltaic layer. This process will form a highly doped p+ layer
just adjacent to the interface which will aid in forming the needed
ohmic contact and can be diffused far enough into the photovoltaic
layer to act as a back surface field. This out diffusion from the
substrate can be controlled by the amount of the dopant deposited
on the back conductor before growth, the time and temperature of
the growth and subsequent heat treatment. A few minutes of heating
to a temperature of above 600.degree. C. would be sufficient. for
such a process. The photovoltaic layer must be protected by a
phosphorus over pressure during this operation. The advantages are
that this process is simpler than controlling the dopant
concentration variation in a multimodule production apparatus and
the cost will be reduced by eliminating the separate processes.
[0150] Formation of the Diode Junction by an Integrated Process
[0151] A transparent conducting layer that is heavily doped with an
n-type dopant compatible with the photovoltaic layer material is
deposited on the p-type photovoltaic layer without a diode
junction. This structure is subjected to a heat treatment either
during growth of the transparent conducting layer, or subsequent to
it, the dopant will out diffuse from the transparent conductor into
the photovoltaic layer and create a shallow p-n junction in the
photovoltaic layer near the interface between the layers. This
junction can be very shallow and yet separated from the interface
and be both efficient and stable versus corrosion. This out
diffusion can be controlled by the concentration of the dopant in
the transparent conductor just adjacent to the photovoltaic layer,
and the time and temperature of the heat treatment. Several minutes
at a temperature above 600.degree. C. would be sufficient to create
an effective junction. The advantage of the integrated process
would be a huge reduction in cost of the junction formation method
by eliminating the junction formation regime in the growth of the
photovoltaic layer which would eliminate an entire production
module and greatly simplify the production system.
[0152] Double Integrated Production System
[0153] Since the process conditions to form the integrated back
surface field and the integrated junction are very similar, it is
possible to select the conditions so that both operations could be
done simultaneously. This is an extremely cheap and easy method to
create the structure of the preferred embodiment. In this case,
there would be no doping variation during the growth of the
photovoltaic layer. The process would simplify to depositing a
uniform photovoltaic layer on a previously prepared substrate
followed by deposition of the transparent conductor and then
heating the stack for a short time to form both the back surface
field and the diode junction simultaneously. This would yield the
and cheapest production apparatus.
[0154] OMCVD Process for .beta.-ZnP.sub.2 Deposition
[0155] FIG. 3 is a phase diagram for ZnP.sub.2 and Zn.sub.3P.sub.2
for a P.sub.4 pressure of 5 atmosphere, a zinc partial pressure
from 10 atmospheres to less than 0.001 atmosphere and a temperature
range from about 725.degree. C. to 1250.degree. C. Within this
region, each of these two compounds can only be in either a solid
or a vapor phase. Curve 31 is the sublimation curve (at fixed
P.sub.4 pressure) separating the solid and vapor phases of Zn.sub.3
P.sub.2 and curve 32 is the sublimation curve (at fixed P.sub.4
pressure) separating the solid and vapor phases of ZnP.sub.2. Thus,
regions 3336, respectively, contain the following phases: solid
ZnP.sub.2 only; solid Zn.sub.3P.sub.2 only; phase containing both
solid S-ZnP.sub.2 and solid Zn.sub.3P.sub.2; and a gas phase
containing both zinc and phosphorus molecules. Point 37 is the
quadruple point between the four different phases illustrated in
that figure. Line 39 is the focus of the quadruple point as the
phosphorus pressure is varied. Point 310 is the position of the
quadruple point at 1 atmosphere of phosphorus pressure. Lines 31
and 32 will intersect at point 310 if the phosphorus pressure is
fixed at one atmosphere and they will be parallel to the direction
that they have on the figure at all pressures.
[0156] The key point of this figure is that there exists a
substantial region of zinc pressure and temperature containing only
the .beta.-zinc diphosphide phase. This will be called the "single
phase" region 33 as compared to the "double phase" region 35 where
both ZnP.sub.2 and Zn.sub.3P.sub.2 are thermodynamically stable. It
is preferred to grow ZnP.sub.2 in the single phase region because
any deposit obtained would be pure ZnP.sub.2 and there would be no
regions of the deposit having the Zn.sub.3P.sub.2 stoichiometry.
There should be no sites within any crystal in the film (which is
assumed to be polycrystalline) where a zinc or phosphorus atom
would be coordinate as it would be in Zn.sub.3P.sub.2. The quality
of the resulting film would be the lightest where the deposit is
microscopically ZnP.sub.2 solely.
[0157] It is possible to grow essentially pure ZnP.sub.2 within the
double phase region by growing the films with a large excess of
phosphorus (relative to the amount of zinc present) where the flux
to the surface would promote the growth of ZnP.sub.2, the
phosphorus rich phase, to the apparent exclusion of
Zn.sub.3P.sub.2. It is possible to grow high quality thin films of
ZnP.sub.2 under this growth regime by use of this phosphorous
excess. However, the cost of films grown under this excess of
phosphorus would be much higher than those grown in, or at least
near, the single phase region. Any phosphorus in the gas stream in
excess of that required for stoichiometric growth, will pass
through the reactor without deposition. This "wasted" phosphorus is
costly and contributes to the waste treatment problem of the
reactor exhaust and to the potential for environmental pollution
due to the growth process.
[0158] It is therefore critical to determine precisely of the
location of this region of phase-space determined by the space
variables. The variables of temperature, phosphorus partial
pressure and zinc partial pressure may be used. These have been
determined from the measured thermodynamic properties of ZnP.sub.2
(and also Zn.sub.3P.sub.2). The best available thermodynamic
information was analyzed for these variables. FIG. 3 is one
representation of this analysis (where phosphorus pressure was held
constant). Another representation is FIG. 4 where temperature is
held constant. This is a more useful representation because the
growth substrate is held at a fixed temperature. This figure shows
four single phase regions (40, 41, 42, and 43) bounded by the zinc
and phosphorus partial pressures at each of 4 fixed temperatures
(800.degree. C., 750.degree. C., 700.degree. C. and 600.degree. C.,
respectively).
[0159] It can be observed that at a temperature as high as
800.degree. C. there is a substantial single phase region 40 for
the growth of ZnP.sub.2 where the total pressure is less than an
atmosphere. Therefore, by appropriate use of an inert diluent, an
atmospheric pressure vapor deposition process can be implemented to
deposit a layer of .beta.-zinc diphosphide, thereby enabling the
manufacture of wide sheets (for example, a meter wide) of
photovoltaic cells, having a .beta.-zinc diphosphide photovoltaic
layer. At a temperature of 800.degree. C., the total pressure at
the quadruple point is about 0.1 atmosphere (76 torr). This
indicates that at temperatures as high as 800.degree. C., single
phase zinc diphosphide can be grown using plasma enhanced CVD using
microwave (2.45 GHz) plasmas excitation sources. These sources
operate effectively in the pressure range from 1 to 100 torr total
pressure.
[0160] At a temperature of 600.degree. C. (considering region 43),
the total pressure at the quadruple point is about 0.001 atmosphere
(0.76 torr). This indicates that at temperatures as high as
600.degree. C., zinc diphosphide can be grown using plasma enhanced
CVD using radio-frequency (13.56 MHz) plasma excitation sources.
These sources operate effectively in the pressure range of 1 to 10
torr total pressure.
[0161] There are other phase-space variables that can be used lo
define the boundary pf the one phase region for ZnP.sub.2 growth.
These are temperature, total pressure and phosphorus/zinc partial
pressure ration. It will be observed that locus in the quadruple
point at a fixed temperature is given by line 44. The ratio of
phosphorus (P.sub.4) to zinc (Zn) atoms is 6 at the quadruple point
under all conditions (i.e., along line 44). This is the minimum
ratio that will permit growth of single phase ZnP.sub.2. Thus, a
reactor that is designed to grow single phase ZnP.sub.2 will
operate near to, but to the right of, line 44. Line 45 represents a
typical growth condition were the phosphorus to zinc partial
pressure ratio is 10. A reactor operating at this ratio will grow
single phase ZnP.sub.2 provided the total pressure is appropriate
to the substrate temperature. Because growth at the exact quadruple
point condition would yield multiple phases, this excess phosphorus
is used to move the growth conditions away from the quadruple point
into the single phase region but without a substantial cost
increase due to large excesses of phosphorus. It also allows for
fluctuations in the temperature of the substrate and partial
pressures of the species while maintaining the single phase growth
condition over the range of fluctuation experienced.
[0162] The exact location in phase space of the single phase region
must ultimately be determined by detailed growth experiments. The
thermodynamic theory used to project these conditions are based
upon a precise determination of the vapor pressures and
decomposition conditions of the compounds involved (in this case,
ZnP.sub.2, and Zn.sub.3P.sub.2) at the temperatures of interest.
The data used to project these regions are the best available but
they are not accurate enough at the temperatures of use
(400-650.degree. C.) to define these regions without further
experimentation. The information presented here is the best
available and gives the process whereby the best growth condition
will be determined. The basic data for this analysis is shown in
FIG. 5 which plots the decomposition pressure of ZnP.sub.2. These
lines 51A and 51B, are two determinations of this information near
the temperature range of their measurements. The region to the left
of lines 51 is the region of zinc diphosphide stability 52 and the
region to the right is the region of Zn.sub.3P.sub.2 stability 53.
The growth of ZnP.sub.2 must occur within region 52 and the layer
after growth must be maintained in the conditions of this region at
all times during processing. Whenever the ZnP.sub.2 layer is
heated, there must be an overpressure of phosphorus gas sufficient
to maintain the stability of the surface which is given by the
bounds of region 52. Also, after the ZnP.sub.2, layer is grown, it
must be maintained within region 52 while it is cooled down lo low
temperature (roughly, below 100-200.degree. C.). In practice,
decomposition is prevented by using a large excess of phosphorus
pressure, but during growth the minimum pressure that stabilizes
the phase is used because that is the lowest cost condition. Thus,
the best growth conditions would be within the region 52 very near
the lines 51. Thus, the total pressure at the growth condition
expressed by line 45 would run nearly parallel to lines 51 and
within the region 52. This offers a method to approximate the
target growth condition for materials where the complete
thermodynamic information is not available.
[0163] The basic process to grow the photovoltaic layer (as seen by
the substrate) is (1) the substrate is introduced into the reaction
chamber; (2) the reaction chamber is purged with a clean inert gas
(typically nitrogen) to remove any traces of atmosphere entering
with the substrate; (3) the chamber is purged with high purity
hydrogen (the preferred carrier gas) to remove any residual
nitrogen; (5) the substrate is heated in hydrogen to the process
temperature; (6) the photovoltaic layer is grown using the
predetermined conditions for single-phase growth; (7) the layer is
cooled down while maintaining the phosphorus overpressure until
cooled; (8) the chamber is purged with high purity hydrogen to
remove any residual phosphorus; (9) the chamber is purged with
clean nitrogen to remove any residual hydrogen; and (10) the
completed layer is removed from the process chamber.
[0164] Fabrication of the structure of the preferred embodiment
will require four sequential growth regimes interconnected by
critical transitions during growth of the photovoltaic layer. The
first regime will establish the ohmic contact to the back conductor
23 by growing p-type material where the concentration of the
p-dopant will be sufficiently high at the initiation of growth to
degenerate the photovoltaic semiconductor and produce the ohmic
electrical contact needed to the back conductor 23. The second
regime will grow the back surface field region 211. The p-dopant
concentration is reduced somewhat so that a region of strongly
p-doped (p+) of normal (nondegenerate) semiconductor will be grown
to a thickness greater than the range of quantum mechanically
tunneling electrons in this material (greater than 100 angstroms).
The p-dopant concentration will drop abruptly to form the back
surface field at the beginning of the third regime which will
deposit most of the thickness of the photovoltaic layer 24. This
region will be weakly p-type and must be of the best quality. The
fourth regime will grow the strongly n-doped (n+) region 210 on the
photovoltaic device. This region starts with the formation of an
abrupt transition from p-type to n-type conductivity creating the
collecting junction 29. The concentration of n-dopant is increased
rapidly during the growth of this layer to provide a weak front
surface field and ends at the surface of the photovoltaic layer
with a concentration of n-dopant that may degenerate the
semiconductor at the surface. FIG. 6 illustrates the ideal
electronic structure of the preferred embodiment of the proposed
photovoltaic device wherein the photovoltaic layer is processed in
these four regimes.
[0165] In production, the "continuous" substrate will not be
exposed to the atmosphere once it is introduced into the production
apparatus and it will be cleaned by a plasma enhanced vapor process
prior to deposition of any layers. The cleaned substrate will be
translated through the apparatus and through adjacent modules with
each having a different process. The basic modules required for
fabricating the preferred embodiment are (1) back conductor
deposition; (2) p-type photovoltaic layer deposition; (3) n-type
photovoltaic layer deposition; (4) transparent conductor
deposition; (5) patterned grid conductor deposition; and (6)
antireflection coating deposition.
[0166] Zinc is easily transported to the substrate by an
organometallic zinc, such as dimethyl- and diethyl-zinc, which have
high vapor pressures, low decomposition temperatures, high
availability and relatively low cost.
[0167] The phosphorous source can be any of several
commercially-available sources, such as phosphine, t-butyl
phosphine, bisphosphinoethane, trimethyl-phosphine or
triethylphosphine. For reduced temperatures, there may be only
partial decomposition of these phosphorus sources at the wafer, so
that their relative partial pressures must be increased by an
amount that corrects for such partial decomposition to produce the
desired concentration of phosphorous at the substrate surface.
Although phosphine can be used, it is not favored because of the
added costs and risks arising due to its high toxicity.
[0168] The preferred phosphorous source is white phosphorous,
because it is very inexpensive, is readily available in high
purity, has relatively low toxicity and has a relatively low
decomposition temperature (near 300.degree. C.). It has never been
used in OMCVD before, perhaps because it requires a relatively high
temperature (44.degree. C.) to melt, requires an even higher
temperature to provide the required vapor pressure of phosphorus
for the OMCVD process, requires a "hot-wall reactor" and a
"hot-wall panel" to be discussed below.
[0169] FIG. 7 plots the vapor pressure of liquid white phosphorus.
This figure gives the vapor pressure of phosphorus in the heated
source chamber ("bubbler") as a function of the bubbler
temperature. The phosphorus vapor is carried to the reaction
chamber by an inert carrier gas which dilutes the phosphorus
concentration which is also diluted by further by mixing with other
gases. The actual partial pressure of the phosphorus chamber is
known from the bubbler temperature and the dilution factors during
transport. The temperature of any surface to which the phosphorus
is exposed must be higher than the condensation temperature for the
actual partial pressure of the phosphorus as given in the figure.
Thus, growth at a partial pressure of phosphorus of 10 torr must
have a chamber wall heated to over 130.degree. C. Growth at the
maximum useful phosphorus pressure of 300 torr would require a
maximum wall temperature of 250.degree. C. This is difficult with
current reactor technology and contradicts one of the original
advantages of OMCVD processes--namely, that it use cold-wall
reactors in which the walls are either at room temperature or are
water-cooled, such as in the vicinity of the hot zone of the
reactor. Conventional system components, such as mass flow
controllers, valves, pressure controllers, will not work at these
elevated temperatures. For example, typical O-rings cannot be used
above 200.degree. C. The cost of such a customized reactor design
increases equipment costs, but is more than made up in production
savings, especially for a product of the volume anticipated for
photocells that can compete effectively against electrical power
provided by electrical utilities. The advantages of this system
will also be applicable to other uses, so that the cost of these
reactors will drop significantly when they are needed in commercial
quantities.
[0170] The best conditions for the growth of zinc diphosphide
layers occur in the range of 500-600.degree. C., with a
phosphorus/zinc partial pressure ratio of 10-20, and with a total
pressure of reacting species (zinc and phosphorus) in the 1-100
torr range. For the atmospheric pressure process, an inert gas,
such as hydrogen, is supplied to raise the overall pressure in the
reactor to atmospheric pressure, so that meter-wide substrate
layers can be processed within a reactor, without the expense of
enclosure walls being strong enough to produce a process pressure
significantly different from atmospheric pressure. For the plasma
process where excitation by microwave excitation is preferred, the
hydrogen gas would be supplied to bring the total pressure to the
10-100 torr range.
[0171] There are two available p-type doping elements (copper and
lithium). Copper can be purchased in extreme purity as copper
hexafluoroactylacetonate and cyclopentadienyl copper trietheyl
phosphine (preferred). The vapor pressure of these species is low
and the chamber walls may have to be heated slightly to prevent
condensation when p+-layers are grown. Lithium is available
commercially as cyclopentadienyl lithium, methyl lithium
(preferred) and phenyl lithium.
[0172] There are many available n-type doping elements in the form
of vapor transportable compounds, the most useful being boron
(hydride), aluminum (organometallic), gallium (organometallic,
preferred), indium (organometallic), and tin (hydride).
[0173] Copper Diphosphide Embodiments:
[0174] With respect to the preferred embodiment and the selection
of alternative materials for layers other than the photovoltaic
layer, copper diphosphide is undistinguishable from the
.beta.-ZnP.sub.2 embodiments. This is due to the similarity of
within this class of materials in that both are p-type
intrinsically and have a similar value of the bandgap. However,
CuP.sub.2 has a unique bandgap structure that makes it particularly
attractive for solar photovoltaic devices having low cost and high
efficiency. CuP.sub.2 has an indirect bandgap of about 1.4 eV which
is sufficient for a high efficiency because of the longer free
carrier lifetimes in indirect bandgap materials but one would
project from that single fact that devices made from it would have
to be very thick (on the order of 100 microns) to be efficient and
as such would also be more expensive. However, the direct bandgap
of CuP.sub.2 is at 1.5 eV which indicates that it will also have
excellent absorption and be thinner (on the order of one micron).
Thus, CuP.sub.2 has the best of both properties, a long free
carrier lifetime and a thin cell structure, indicating a very
efficient and very low cost photovoltaic device is possible.
[0175] The photovoltaic layer 24 in this preferred embodiment is
copper diphosphide because this low cost material efficiently
absorbs the solar spectrum, efficiently transports the
photo-generated carriers and can be fabricated with the abrupt p/n
and p/p+ junctions needed for an efficient photovoltaic device. The
critical p/n diode junction 29 can be formed as a homojunction by a
doping transition within the layer during the layer growth or by
diffusion from the surface after the layer growth is completed.
[0176] Copper diphosphide has a unique crystal structure where the
copper atoms reside in pairs in an octahedral site. As such, while
the formal valence of each copper atom is +2, conventional doping
rules based upon it are less likely to apply and the most likely
substitutional dopants will be small and should be able to bind
covalently with like atoms. Thus for standard substitutional
doping, boron would be the preferred n-type dopant and lithium
would be the preferred p-type dopant. The tendency for large
dopants would be to displace the pair of atoms and conventional
substitution would not occur. Assignment of a formal valence of +1
to each copper atom would indicate magnesium and, possibly, zinc
would be an effective n-dopants. Dopant on the phosphorus atom
lattice using sulfur, selenium or tellurium can be used for the
n-type dopant by conventional substitution.
[0177] Controlling the doping and the crystal structure by
controlling the stoichiometry by phosphorus, copper and vacuum
treatment may prove useful for forming high efficiency
structures.
[0178] The subsequent processing conditions and the materials
selected for adjacent layers must be compatible with this layer.
This layer must be thick enough to absorb the incident sunlight and
to fabricate separated junctions without the dopant diffusion along
the crystallite boundaries extending between them. This thickness
will be typically 1-3 microns.
[0179] Growth of CuP.sub.2 by OMCVD
[0180] CuP.sub.2 can be grown by OMCVD with a few adjustments. The
only reported measurement of the decomposition pressure of this
material indicates the atmospheric pressure growth is likely at
temperatures as higher than 700.degree. C. and that plasma enhanced
growth using microwaves is possible as 650.degree. C. and/or up to
550.sub.c using RF excitation. From this data, the optimal growth
can be expected in the range of 400-600.degree. C. at a
phosphorus/copper ratio of 10-20 and a total pressure of 1-10
torr.
[0181] The species used to transport the copper to the growth site
are either copper hexafluoroacetylacetonate or
cyclopentadienylcopper triethylphosphine because both are readily
available in high purity commercially. The first has the advantage
of a higher vapor pressure (preferred) and the second has the
advantage of not having oxygen in the molecule. The higher vapor
pressure is important because, to achieve target pressures on the
order of 1-10, torr requires heating of the lines and chamber to
the order of 100.degree. C. For atmospheric pressure growth,
hydrogen will be used to raise the total system pressure to near
one atmosphere and the plasma enhanced growth will use a hydrogen
balance to bring the total system pressure to the 10 to 100 torr
range. Microwave excited plasmas are preferred because the reactant
partial pressures are so high that total pressure at the higher end
of the temperature requires this source. Either excitation could be
used at the lower end of the substrates temperature range.
[0182] The phosphorus species for use with ZnP.sub.2 are also
useful for CuP.sub.2 and the preferred phosphorus species is white
phosphorus. The dopant used will be that organometallic or hydride
forms of the elements discussed in the embodiment section for this
material.
[0183] Magnesium Tetraphosphide Embodiments:
[0184] With respect to the preferred embodiment and the selection
of alternative materials for layers other than the photovoltaic
layer, copper diphosphide is indistinguishable from the
.beta.-ZnP.sub.2 embodiments. The principle differences can arise
form the amount of unknown information concerning MgP4. It has not
been confirmed that MgP4 is a direct bandgap material although the
data that is available supports this conclusion. If MgP.sub.4 where
shown to be indirect, then the layer thickness projected for the
preferred embodiment must be increased to 100-250 microns. It has
not been verified that MgP.sub.4 will be p-type in its intrinsic
conductivity although the isostructural material CdP.sub.4 and most
phosphides of this type have p-type conductivity due to vacancies
on the metal lattice. If this material were shown to be n-type then
the doping requirements for growth of the layer would change but
the preferred embodiment will be as that for ZnP.sub.2. Assuming
these two unknowns are as predicted, the preferred embodiment of
this device is identical to that of ZnP.sub.2. Although the value
of the bandgap is not unknown, it is within the range that would
yield a device with good efficiency provided it is within the
projected range of 1-2 eV.
[0185] The photovoltaic layer 24 in this preferred embodiment is
magnesium tetraphosphide because it is a low cost material which is
expected to efficiently absorb the solar spectrum, efficiently
transport tile photo-generated carriers and can be fabricated with
the abrupt p/n and p/p+ junctions needed for an efficient
photovoltaic device. The critical p/n diode junction 29 can be
formed as a homojunction by a doping transition within the layer
during the layer growth or by diffusion from the surface after the
layer growth is completed.
[0186] The crystal structure is similar to that of CuP.sub.2 as far
as the phosphorus lattice is concerned but each octahedral space
contains only one atom thus normal valence rules apply with an
expected formal valence of +2 on the magnesium atom. Thus the
conventional dopant useful for ZnP.sub.2 are also useful for
MgP.sub.4. The preferred dopants in this case are lithium for
p-type and boron for n-type due to the small size of the magnesium
atom.
[0187] Controlling the crystal stoichiometry by phosphorus,
magnesium, and vacuum treatment may prove useful for forming high
efficiency structures.
[0188] The subsequent processing conditions and the materials
selected for adjacent layers must be compatible with this layer.
This layer must be thick enough to absorb the incident sunlight and
to fabricate separated junctions without the dopant diffusion along
the crystallite boundaries extending between them. This thickness
will be typically 1-3 microns, unless it is discovered to be and
indirect material as mentioned above.
[0189] Growth of MgP.sub.4 by OMCVD:
[0190] There is little information available on the properties of
MgP.sub.4 other than crystal structure determination. The
decomposition pressure and the optical and electrical property are
not accurately known. There are several reasons to project the
MgP.sub.4 can be grown by OMCVD with a few adjustments. The crystal
structure of MgP.sub.4 is isostructural with CuP.sub.2 with respect
to the phosphorus lattice structure. There are planes of continuous
nets of interlocking phosphorus rings all composed solely of
phosphorus atoms. The metal atoms lie between the planes and bond
the planes together. It is known that the decomposition pressures
of these materials are dependent upon the properties of the
phosphorus lattice structure and that primary vapor species are
phosphorus atoms removed from the phosphorus layers. Thus, the
decomposition pressure of materials having a similar phosphorus
lattice structure would be similar. This is supported by the
observation that MgP.sub.4 has been grown in an ampule near
atmospheric pressure at about 600.degree. C. and CdP.sub.4, at
about 500.degree. C. Thus, like CuP.sub.2, MgP.sub.4 should grow
under pressure and temperature conditions obtainable by OMCVD. From
this analysis the optimal growth condition for MgP.sub.4 are
projected to be in the range of 400-550.degree. C. at a
phosphorus/magnesium ratio of 15-25 and a total pressure of 1-50
torr.
[0191] The species used to transport magnesium to the growth site
are bis(cyclo-pentadienyl) magnesium and
bis(methylcyclopentadienyl) magnesium where the first is preferred
due to better known properties and longer use in as an high purity
OMCVD source. The vapor pressure of both of these species or too
low for effective use in a room temperature reactor. The optimum
pressure of 1-50 torr requires heating of the lines and chamber to
the order of 150.degree. C. For atmospheric pressure growth,
hydrogen will be used to raise the total system pressure to near
one atmosphere and the plasma enhanced growth will us a hydrogen
balance to bring the total system pressure to the 10 to 100 torr
range. Microwave excited plasmas are necessary because the reactant
partial pressures are so high that the total pressure needed would
require this plasma excitation source.
[0192] 'The phosphorus species for use with ZnP.sub.2 are also
useful for MgP.sub.4 and the preferred phosphorus species is white
phosphorus. The dopant used will be that organometallic or hydride
forms of the elements discussed in the embodiment section for this
material.
[0193] One unique feature of MgP.sub.4 to be noted is that a
catalyst is necessary to synthesize the material from the elements
in sealed ampule at pressures near one atmosphere. The
organometallic growth process is expect to provide that catalytic
effect without any additional external catalyst because the species
arrive at the growth site already bonded to a foreign atom and the
bond is switch to a different material during the layer growth. For
an additional catalytic effect the plasma enhanced growth method
will be used. Because the plasma produces great numbers of free
radicals of all species, this is an excellent catalyst. If more
catalysis is needed (and this is highly unlikely) then additional
species such as chlorine, bromine arid methane can be added to the
plasma gas to insure sufficient species capable of catalysis. These
techniques assure that a low pressure OMCVD growth method is useful
for this material. This means that the processing methods and
equipment expressed for ZnP.sub.2 would also be applicable to
MgP.sub.4.
[0194] Gammairon Tetraphosphide Embodiments:
[0195] With respect to the preferred embodiment and the selection
of alternative materials for layers other than the photovoltaic
layer gamma iron tetraphosphide is indistinguishable from the
.beta.ZnP.sub.2 embodiments. The principle differences can arise
form the amount of unknown information concerning
.gamma.-FeP.sub.4. It has not been confirmed that .gamma.-FeP.sub.4
is a direct bandgap material and the meager data initially
available indicates that it may be an indirect gap materials. If
.gamma.-FeP.sub.4 where shown to be indirect, then the layer
thickness projected for the preferred embodiment must be increased
to 100-250 microns. It has not been verified that
.gamma.-FeP.sub.4will be a p-type in it intrinsic conductivity
although it has a structure that is closely related to both
MgP.sub.4 and CuP.sub.2, and most phosphides of this type have
p-type conductivity due to vacancies on the metal lattice. If
.gamma.-FeP.sub.4 proves to be n-type then the preferred embodiment
will be identical but the doping requirements during growth of this
layer would have to be adjusted. Assuming these two unknowns are as
indicated, the preferred embodiment of this device is identical to
that of ZnP.sub.2 with the layer thickness increased as indicated.
The value of the bandgap of the gamma phase is known to be near 1
eV. Thus, the maximum efficiency is reduced and the cost (due to
the layer thickness change) is increased. Though less attractive
than other materials in this class, the low cost and wide
availability of iron promote its further development.
[0196] The photovoltaic layer 24 in this preferred embodiment is
gamma iron tetrasphophide because this low cost material which is
expected to efficiently absorb the solar spectrum, efficiently
transport the photo-generated carriers and can be fabricated with
the abrupt p/n and p/p+ junctions needed for an efficient
photovoltaic device. The critical p/n diode junction 29 can be
formed as a homojunction by a doping transition within the layer
during the layer growth or by diffusion from the surface after the
layer growth is completed.
[0197] The crystal structure is similar to that of MgP.sub.4, as
far as the phosphorus lattice is concerned, but each octahedral
space contains only one atom thus normal valence rules apply with
an expected formal valence of +2 on the iron atom. Thus the
conventional dopants useful for ZnP.sub.2 are also useful for
y-FeP.sub.4. The preferred dopants in this case are copper for
p-type and aluminum for n-type due lo the size of the iron
atom.
[0198] Controlling the crystal stoichiometry by phosphorus,
magnesium, and vacuum treatment may prove useful for forming high
efficiency structures.
[0199] The subsequent processing conditions and the materials
selected for adjacent layers must be compatible with this layer.
This layer must be thick enough to absorb the incident sunlight and
to fabricate separated junctions without the dopant diffusion along
the crystallite boundaries extending between them. This thickness
will be typically 1-3 microns, unless it is discovered to be and
indirect material as mentioned above.
[0200] Growth of .gamma.-FeP.sub.4, by OMCVD:
[0201] There is little information available on the properties of
y-FeP.sub.4, other than crystal structure determination. The
decomposition pressure and the optical and electrical property are
not accurately known. There are several reasons to project the
.gamma.-FeP.sub.4 can be grown by OMCVD with a few adjustments. The
crystal structure of .gamma.-FeP.sub.4 has a similar structure to
MgP.sub.4 and CuP.sub.2 with respect to the phosphorus lattice
structure. These are planes of continuous nets of interlocking
phosphorus rings all composed solely of phosphorus atoms. The metal
atoms lie between the planes and bond the planes together. The
difference is in the pattern by which the rings are arrayed. It is
known the decomposition pressure of these materials are dependent
upon the properties of the phosphorus lattice structure and that
primary vapor species are phosphorus atoms removed from the
phosphorus layers. Thus, the decomposition pressure of materials
having a similar phosphorus lattice structure would be similar.
This is supported by the observations that MgP.sub.4 has been grown
in a ampule near atmospheric pressure at about 600.degree. C. and
CdP.sub.4 grew at above 500.degree. C. at a similar pressure. Thus,
like MgP.sub.4, .gamma.-FeP.sub.4 should grow under pressure and
temperature conditions obtainable by OMCVD. From this analysis the
optimal growth condition for .gamma.-FeP.sub.4 are projected to be
in the range of 400-550.degree. C. at a phosphorus/magnesium ratio
of 15-25 and a total pressure of 1-50 torr.
[0202] The species used to transport iron to the growth site are
bis(cyclopentadienyl) iron and pentacarbonyl iron where the later
is preferred due it higher vapor pressure. The vapor pressure of
the carbonyl is sufficient for use in a room temperature reactor.
The optimum pressure of 1-50 torr would require heating of the
lines and chamber to the order of 150.degree. C. if the dienyl were
used. For atmospheric pressure growth, hydrogen will be used to
raise the total system pressure to near one atmosphere and the
plasma enhanced growth will us a hydrogen balance to bring the
total system pressure to the 10 to 100 torr range. Microwave
excited plasmas are required because the reactant partial pressures
are so high that the total pressure needed would require this
plasma excitation source.
[0203] The phosphorus species for use with ZnP.sub.2 are also
useful for .gamma.-FeP.sub.4 and the preferred phosphorus species
is white phosphorus. The dopant used will be that organometallic or
hydride forms of the elements discussed in the embodiment section
for this material.
[0204] One unique feature of .gamma.-FeP.sub.4 to be noted is that
the alpha and beta phase are not useful for solar energy
applications because the bandgaps are too low. Thus, the main
challenge is to control nucleation of the film to produce only the
gamma phase. The can be effected by the substrate, the growth
conditions, and by the introduction of alloying agents such as
magnesium, copper and (possibly) zinc. Since the crystal structure
of the phosphorus lattice of y-FeP.sub.4 is similar to the
MgP.sub.4 and CuP.sub.2 lattice, the addition of small amounts of
these elements would effect the morphology of the .gamma.-FeP.sub.4
layer and would influence the nucleation. This manipulation could
promote the desired structure or a influence crystal
morphology.
[0205] A catalyst would be helpful to promote growth of this higher
phosphidephase as it would for MgP.sub.4 and could also effect the
crystal morphology also. The techniques elaborated for MgP.sub.4
apply to .gamma.-FeP.sub.4 also and extend to influencing the
crystal morphology.
[0206] Mixed Crystal Embodiments:
[0207] Since the proposed photovoltaic materials all belong to a
class of materials having similar properties, the crystals formed
by alloys (continuously variable ratio) or mixed crystals (fixed
ratio) made by codeposition of two or more of the proposed
materials are useful. It is known that the crystals or alloys
formed between two crystals having similar properties have
properties intermediate to or similar to the crystals "mixed" to
form them. It is also known that the thermodynamic stability and
thus resistance to corrosion are also superior in mixed crystals
compared to the crystals used to form them. Thus mixed crystals of
the general formulae Zn.sub.X Mg.sub.YCu2.sub.Z Fe.sub.UP2.sub.W,
where x, y, z, u, w=0,1,2, . . . (small whole numbers, where at
least two of X, Y, Z and U are nonzero and where w is nonzero)
would be useful for low cost high-efficiency photovoltaic devices
of the preferred embodiment structure. Since any one of the
individual compounds can be made using very similar conditions by
OMCVD as described, the mixed crystals can also be fabricated
directly by this method by simultaneous growth.
[0208] ZnP.sub.2: MgP.sub.4 Solar Cells
[0209] A similar reason to the usefulness of CuP.sub.2:ZnP.sub.2
solar cells applies to ZnP.sub.2:MgP.sub.4 mixed crystals for solar
cells. They will have a discrete stoichiometry. They will be
similar to the pure phases and therefore useful for solar cell
application. The will have improved thermodynamic stability and
thus increase resistance to decomposition. They could provide
superior performance as thin film solar cells.
[0210] The higher thermodynamic stability of these mixed crystals
will mean that they have a lower decomposition pressure, require
less phosphorus overpressure to stabilize the phase and are thus
easier to grow by any means particularly by OMCVD.
[0211] There is no mention of any Zn/Mg/P crystal in the literature
having the "higher" stoichiometry (where there is more phosphorus
atoms than the sum of other metal atoms). In particular crystals
where the phosphorus stoichiometry is equal to or greater than 2
times the sum of the other metal atoms.
Appendix
[0212] CuP.sub.2,: ZnP.sub.2, Solar Cells
[0213] I claim that a Cu.sub.XZn.sub.1-XP.sub.2 mixed crystals will
be useful for solar cells because both end compounds are useful and
that the mixed crystals could have superior properties to either
such as resistance to decomposition, increased radiative
recombination lifetime and better optical absorption.
[0214] CuP.sub.2, and ZnP.sub.2 have very different crystal
structures and as such do not form homogeneous solid solutions with
each other (x will have a particular invariant value or values).
There will be crystals having discrete stoichiometries in the phase
space between. Normally, the properties of the mixed crystals will
be similar to the pure crystals. But the mixed crystals will have a
higher thermodynamic stability than either of the pure phases and
thus be more resistant to decomposition than either ZnP.sub.2 or
CuP.sub.2.
[0215] The higher thermodynamic stability of these mixed crystals
will mean that they have a lower decomposition pressure, require
less phosphorus overpressure to stabilize the phase and are thus
easier to grow by any means particularly by OMCVD.
[0216] Mixed crystal compositions near CuP.sub.2 (x=1) would have
different crystal structure and properties than mixed crystal near
ZnP.sub.2 (x=0) or mixed crystals near CuZnP.sub.2, (x=0.5) and the
superior composition must be determined by experiment. However, any
crystals formed within the entire range of composition would by
useful for solar cell application.
[0217] Resistance to corrosion (a potential problem with both
CuP.sub.2, and ZnP.sub.2,) is expected lo be improved by mixed
crystallization as mixed crystals generally show this behavior.
Mixed crystals near CuP.sub.2, are expected to have superior
lifetimes and slightly better optical absorption. Mixed crystals
near ZnP.sub.2 are cheaper and easier to grow. Intermediate mixed
crystals (x=0.5) may have unique properties not directly apparent
in either end compound. OMCVD growth of all these phases is
possible but very different regimes of growth are required.
[0218] CuP.sub.2,: MgP.sub.4 Solar Cells
[0219] A similar reason to the usefulness of CuP.sub.2: ZnP.sub.2,
solar cells applies to CuP.sub.2,: MgP.sub.4 mixed crystals for
solar cells. They will have a discrete stoichiometry. They will be
similar to the pure phases and therefore useful for solar cell
application. The will have improved thermodynamic stability and
thus increase resistance to decomposition. They could provide
superior performance as thin film solar cells.
[0220] The higher thermodynamic stability of these mixed crystals
will mean that they have a lower decomposition pressure, require
less phosphorus overpressure to stabilize the phase and are thus
easier to grow by any means particularly by OMCVD.
[0221] Because the crystal structure of CuP.sub.2 is closer to that
of MgP.sub.4, than ZnP.sub.2 and it is expected that these mixed
crystal will be more like the pure phase than would be expected
from "ZnCuP.sub.2".
[0222] FeP.sub.4 Heterojunction Device
[0223] Alpha FeP.sub.4 (0.32 eV) has a very similar crystal
structure to gamma FeP.sub.4 (I eV). The primary difference is in
the stacking of the layers.
[0224] It is possible to create a heterojunction solar cell in this
system by depositing the gamma phase on a substrate of the alpha
phase and doping each phase to produce an electrical junction at
their interface. The advantage of the cell is that the two
different material each absorb a different segment of the solar
spectrum offering a higher current generation efficiency than would
a single absorber. This device would more useful as a broad-band
infrared absorber than for a solar cell because the energy gaps are
not ideal for the solar spectrum.
[0225] Mg.sub.XFe.sub.1-xP.sub.4 Solar Cells
[0226] MgP.sub.4 and FeP.sub.4 have similar crystal structures. The
difference is in the stacking orders.
[0227] Mixed crystals are very likely. The properties of these will
reflect the pure crystal properties. Thus, mixed crystals near
MgP.sub.4, are likely to useful for solar applications because the
stacking sequence of MgP.sub.4, is very similar to that of gamma
(solar) FeP.sub.4. Mixed crystals near FeP.sub.4, will have
indeterminate properties because any of several FeP.sub.4, stacking
orders may be produced. Thus, these high FeP.sub.4 content crystals
would require Mg to catalyze a gamma type crystal phase to be of
interest to the solar application.
* * * * *