CA1067610A - Low cost substrates for polycrystalline solar cells - Google Patents
Low cost substrates for polycrystalline solar cellsInfo
- Publication number
- CA1067610A CA1067610A CA322,326A CA322326A CA1067610A CA 1067610 A CA1067610 A CA 1067610A CA 322326 A CA322326 A CA 322326A CA 1067610 A CA1067610 A CA 1067610A
- Authority
- CA
- Canada
- Prior art keywords
- silicon
- layer
- polycrystalline silicon
- solar cells
- deposited
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/546—Polycrystalline silicon PV cells
Abstract
ABSTRACT OF THE DISCLOSURE
Low-cost polycrystalline silicon solar cells supported on substrates are prepared by depositing successive layers of polycrystalline silicon containing appropriate dopants over supporting substrates of a member selected from the group consisting of metallurgical-grade polycrystalline silicon, graphite and steel coated with a diffusion barrier of silica, borosilicate, phosphosilicate, or mixtures thereof such that p-n junction devices are formed which effectively convert solar energy to electrical energy. To improve the conversion efficiency of the polycrystalline silicon solar cells, the crystallite size in the silicon is substantially increased by melting and solidifying a base layer of polycrystalline silicon before depositing the layers which form the p-n junction.
Low-cost polycrystalline silicon solar cells supported on substrates are prepared by depositing successive layers of polycrystalline silicon containing appropriate dopants over supporting substrates of a member selected from the group consisting of metallurgical-grade polycrystalline silicon, graphite and steel coated with a diffusion barrier of silica, borosilicate, phosphosilicate, or mixtures thereof such that p-n junction devices are formed which effectively convert solar energy to electrical energy. To improve the conversion efficiency of the polycrystalline silicon solar cells, the crystallite size in the silicon is substantially increased by melting and solidifying a base layer of polycrystalline silicon before depositing the layers which form the p-n junction.
Description
1alti7610 The present invention relates to a method of producing p-n junction semi-conductor solar cells. More particularly, the invention relates to a method of producing solar cells with the principal objective of sharply reducing production costs by depositing poly-crystalline silicon on a relatively cheap substrate such as metallurgical-grade silicon, graphite or steel.
The problem of uncovering new, abundant, cheap and non-polluting sources of energy is a problem of vital national importance. Of all energy sources, solar energy is one of the more attractive sources because of its abundant supply and because it is completely non-polluting.
An indication of the abundance of solar energy is evident by the fact that the solar power on the surface of the earth is approximately one hundred thousand times greater than the current power consumption from all energy sources.
Presently, solar energy is utilized by converting ` solar energy to thermal energy and by converting solar energy to electricity which is known as the photovoltaic system.
Both methods of utilizing solar energy are expected to aid in meeting the ever-increasing demand for clean solar energy. Currently, the silicon solar cell is the most well-;, known device in the photovoltaic system. Further, technology has advanced to the point where silicon solar cell panels which are capable of producing several kilowatts of power have been ~ed reliably in all types of space craft for manv years.
. ' ~
10~7610 Currently, silicon solar cells are manufactured by preparing polycrystalline silicon by reducing trichloro-silane with hydrogen, growing single crystals of silicon of controlled purity from the polycrystalline material, preparing silicon wafers by cutting the single crystal ingot to a thickness of at least 0.25 mm followed by polishing and etching, diffusing a dopant into the silicon wafers to form a shallow p-n junction, applying ohmic contacts to the rear surface and grid contact to the diffused surface, applying antireflecting and protective coatings to the diffused surface and finally mounting the cell into position.
This rather intricate procedure results in the current high costs of manufacturing silicon solar cells. Although the costs of production for single crystalline solar cells has recently been reduced from about $100/peak watt to about $20/peak watt, further reduction in coct of about one order of magnitude is necessary if widespread utility of solar cells is to be realized in large-scale terrestrial applicationsO
One prior art process of manufacturing semi-conductor solar cells as shown by Tarneja, et al, U.S. Patent 3,460,240 issuedAugust, 1969involves epitaxially depositingsili-con ona quaxtzsubstrate toform an N-type layer over which is epitaxially deposited two P-type silicon layers. However, this process has the disadvantage that the overall process requires the rather detailed and expensive sequence of ' iO~7~1V
steps necessary to deposit epitaxial silicon so that no significant decrease in cost of manufacture is observed.
The Jones reference, U.S. Patent 3,078,328,issued February,1963shows a methodof manufacturingsolar cellsin which a layerof siliconis grownonto a graphitesurface froma silicon melt and doped to form an N~type layer. In this growth step, silicon and carbon at the interface of the silicon and graphite layers mix to form an intermediata layer of silicon carbide. The device is completed by formation of a top p-type layer of silicon by diffusion. The reference again is disadvantaged by the complicated fabrication procedure. Thus, the cost of manufacture is unattractive . ~., from a commercial viewpoint.
Small-area polycrystalline silicon solar cells have also been fabricated by the deposition of silicon from a vapor state reactant. A polycrystalline silicon layer of ;~
a thickness of 25 - 50 ~m was deposited on silicon substrates at 900C by the reduction of trichlorosilane with hydrogen.
In this method, silicon substrates were used for convenience ,:
~:~ in order to eliminate the cracking of silicon which has been deposited on other substrates. By this procedure, 1 cm solar cells were fabricated by the successive diffusion of gallium and phosphorous to form a p-n junction about 2.5 ,~m below the surface of the device. The device had a maximum " ,, :
~ open-circuit voltage of about 0.3 V, and the greatest :. ,, ~ ~ efficiency was about O.9o/O.
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1()~i7610 A need continues to exist, therefore, for a method by which silicon semiconductor solar cells can be easily and c~eaply produced.
Accordingly, one object of the present invention is to provide a method of manufacturing polycrystalline silicon semiconductor solar cells readily and cheaply.
Another object of the invention is to provide a method of bonding polycrystalline sili-con to common substrates of substantially different chemical composition so as to form silicon semi-, ~
~ conductor solar cells.
. .:
;~ Briefly, these objects and other objects ~ of the invention, as hereinafter will become more .
readily apparent, may be obtained by a low-cost polycrystalline silicon solar cell, which compris-.
es: a steel base; at least one overlying layer of polycrystalline silicon of p-type conductivity and at least one overlying layer of polycrystalline silicon of n-type conductivity so as to define a , .............................. .
; p-n junction; and a diffusion barrier of borosili-? cate interposed between the steel base and the first polycrystalline layer.
;
Preferably, said p-n junction is formed R by first and second overlying polycrystalline sili-.
, , ~ 30 ' ,. _ S r~
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iO~;7610 con layers both of p-type conductivity and a third overlying layer of polycrystalline sili-cone of n-type conductivity.
It is also preferred that the first overlying layer is lQ - 40~m thick and has a resistivity of 0.002 - 0.003 ohm-cm, the second overlying layer is 8 - lO~m thick and has a resistivity of 0.2 - 2 ohm-cm and the third over-lylng layer is 0.2 - 0.4~ m thick and has a re-sistivity of 0.001 - 0.002 ohm-cm.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered : i ~ in connection with the accompanying drawings, ;~ wherein:-Figure 1 shows the resistivity pro-file of the p-region of polycrystalline silicon :. ,.:.
20 solar cell device supported on a graphite .i. :.::
substrate;
Figure 2 shows the current-voltage . . .
~` characteristics of a mesa diode formed from a poly-crystalline silicon, p-n junction device supported on a graphite substrate;
Figures 3A to 3F show one embodiment of ~` polycrystalline silicon, p-n junction devices support-~ ed on a graphite substrate;
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llD67610 Figure 4 is a graph of the current-voltage characteristics of one of the devices of Figure 3;
Figure 5 is the resistivity profile of the p-region of a polycrystalline silicon junction device supported on a borosilicate coated steel substrate;
- Figures 6A to 6F show one embodiment of the configuration of polycrystalline silicon, p-n .~ 10 junction solar cell devices supported on a sub- strate of borosilicate coated steel;
Figure 7 is a schematic diagram of an apparatus for the zone-melting of silicon on gra-phite substrates;
; Figure 8 is a schematic diagram of an apparatus for the unidirectional solidification of :
silicon on graphite substrates;
: Figure 9 is a graph of the dark current-: voltage characteristics of a n -silicon/p-silicon/p -~: 20 silicon (unidiretionally recrystallized)/graphite ~ solar cell;
: Figure 10 is a graph of the current-~: voltage characteristics of a n -silicon/p-silicon/
.~ p -silicon (unidirectionally recrystallized)/
: graphite solar cell under illumination with a quartz-halogen lamp equivalent to AMO
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conditions;
Figure 11 is a graph of the current-voltage characteristics of a n -silicon/p-silicon/p+-silicon (unidirectionally recrystallized)/graphite solar cell under illumination with a quartz-halogen lamp equivalent ~- to AMI conditions; and Figure 12 is a graph showing the relation between short-circuit current density and open-circuit .:
; voltage of a solar cell on metallurgical silicon/graphite ~ 10 measured under different illumination levels.
, An important consideration in the manufacture of silicon semiconductor solar cells is the type of support . ., - structure employed. The ideal support should be one which has the characteristics of being low in cost, light-weight, ., chemically inert to silicon at high temperatures, having ; a high electrical conductivity and a coefficient of -: .
s expansion similar to that of silicon. Furthermore, the substrate should be such that silicon strongly adheres to - the surface of the substrate, and the interface between the ~-~ 20 substrate and the silicon layer should be of low electrical resistance. In the past, refractory metals such as tantalum : .
have been used as substrates for the deposition of poly-~` crystalline silicon of high purity. Such substrates, .::
,:
however, have not been satisfactory because, from an economical point in view, they are not competitive. The `, choice of an appropriate substrate having the above-mentioned . :.
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iO67610 advantages would be a significant factor in lowering the costs of silicon solar cells, thus rendering their use more attractive.
The high cost of manufacturing silicon solar cells from semiconductor grade silicon has already been discussed.
High costs are involved by the series of process steps involved and the high cost of some of the manufacturing steps in particular. In view of these problems, the use of , , , ~ a thin layer of polycrystalline silicon containing a p-n ~, ~
junction deposited on a suitable substrate should substantially reduce material and process costs. Although the efficiency of polycrystalline silicon solar cells cannot compete with the efficiency of single crystalline cells, the . .
....
unit power costs would be many times less than that of :`
present single cyrstal cells.
It has now been discovered that polycrystalline , silicon can be satisfactorily deposited on a steel, graphite `~ or metallurgical polycrystalline silicon substrate to form a suitable silicon solar cell. The p-n junction must be ~,. .
very shallow; the n-layer (or the upper layer) is 0.2 - 0,4~m in thickness, and the p-layer (or the inner layer) is 10 - lOO~m in thickness. The order of preference of substrates is: metallurgical-grade silicon, graphite, and steel~ Although most steels will serve the purpose, metallurgical grade silicon and graphite are preferred.
x In the preparation of solar cells using either a graphite :.'' g _ ' :', : :;
~ ~ .
.. . .
~C~t;7610 or polycrystalline silicon substrate, polycrystalline silicon is deposited on an appropriate substrate by any acceptable procedure normally used for the deposition of polycrystalline silicon, such as by the thermal decomposition of silane, by the reduction of trichloro-silane or silicon tetrachloride with hydrogen at temperatures ranging from 900 C - 1200C or by the thermal decomposition of dichlorosilane. Several such layers are deposited and appropriately doped so as to form a p-n junction device by any conventional procedure known to one skilled in the art. In one embodiment, the first deposited silicon layer is doped with a p+- type dopant to achieve a p+-type layer 10 - 30~m thick and having a resistance of 0.007-0.003 ohm-cm. Doping occurs ; simultaneously with the deposition of silicon by introducing the dopant gas, e.g., diborane for p-type conductive layers and phosphine for n-type conductive layers, in proper amount into the reactant mixture. Thereafter, two successive silicon lay~rs are deposited which are p-type and then ;
n-type successively. The p-type layer is 8 - 10,4m thick :
~,~ and has a resistance of 0.2 - 2 ohm-cm, while the n-type ~i layer is 0.2 - 0.4~m thick and has a resistance of 0.001 -. ~:
~ 0.002 ohm-cm. FIGURE 1 shows the resistivity profile of i~ the structured device, except for the n-layer, which is ` obtained by the spreading resistance technique. The plot :
shows a high resistivity area in the silicon adjacent the ,.
- lQ _ ,~: ..
: ........ ' ' . :
. . . . .
10~;7fà~(1 graphite-silicon interface and i~ apparently caused by the formation of silicon carbide.
In one embodiment of the invention, the silicon devices above can be converted into mesa diodes by .
depositing titanium-silver or gold dots on the silicon ~:.
surface followed by masking and etching of the surface.
~ .
- Titanium-silver is the preferred contact material.
FIGURE 2 shows the current-voltage characteristics of one of these diodes which is exemplary of the values obtained for these diodes, wherein the "n" value calculated from the forward characteri9tics is 1.9, which is very similar to that for single crystal silicon p-n junctions.
This value indicates that the grain boundaries in polycrystalline silicon deposited on graphite supports are not significant in current conduction.
.:, By the procedure shown above, so7ar cells can be fabricated wherein the thickness of the polycrystalline silicon layer of the cell ranges from 0.001 to 0O005 cm which is at least 100 times less than the amount of silicon ~.:
employed in single crystal solar cells. The most significant advantagesare that the energy-, labor-, and material-.:. ~ .
~ consuming steps of the process of producing single crystal . .~
cells, i.e., the growth of single crystals,the preparation of the silicon wafers and the diffusion process, : ~ .
are eliminated. Although the efficiency of the poly-crystalline solar cells is less than that of single crystal iOti7610 cells, the disadvantage i9 more than countered by the many times lower unit power costs of the polycrystalline cells.
A similar procedure to that shown above can be , used to prepare polycrystalline silicon solar cells having a support structure of metallurgical polycrystalline silicon by any method known to those skilled in the art of establishing p-n junctions in deposited polycrystalline silicon. In a representative procedure, a p-type poly-crystalline layer is deposited on a wafer of silicon, which has been pretreated with hydrogen or with hydrogen . . ~ , chloride at an elevated temperature ranging from 1000C
to 1100C by the decomposition of silane with diborane as ~: ........................................................................ .
~ the dopant source. Thereafter, an upper n-type silicon layer r '. :~ ~
~ is deposited using phosphine or arsine as a dopant to yield , a structure having a shallow p-n junction. The device is completed by attaching leads to the n- and p-regions.
In another aspect of the invention, the fabrication of the cheap silicon solar cells by the decomposition of polycrystalline silicon on a steel substrate was attempted.
The problem of using steel as a substrate for polycrystalline silicon cells is complicated by several problems. The first , , ; is that a large and therefore significant difference exists . . .:;
in the thermal expansion coefficients of the materials.
; Silicon has an expansion coefficient of 4 x 10 C-l, while ::
most steels have coefficients of 14 - 16 x 10-6OC
1 , .:.
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Another factor is that high rates of diffusion of silicon and iron atoms occur into each other between adjacent layers of steel and polycrystalline silicon at the temperatures employed in the deposition process. It is known from the phase diagrams of mixtures of iron and silicon that the following iron silicides form: Fe3Si, Fe5Si3, FeSi and FeSi2. Still another factor is that at temperatures above 800C, a solid solution of iron silicides forms when silicon is deposited on steel substrates by any of the conventional silicon deposition procedures. This latter fact is substantiated by the . .
high electrical conductivity of the region.
In view of the above problems with steel as a substrate for silicon solar cells, it is apparent that another material must be applied between the deposited silicon layer and the steel substrate. Any such material (hereinafter referred to as the "diffusion barrier"~ must ., have the ability to minimize the difference in thermal expansion characteristics of the two layers and should be ~;20 chemically inert to steel and silicon at high temperatures.
' Further, the intervening layer should strongly adhere to ,;, .
both the steel and polycrystalline layers and be compatible with the manufacturing technology of the polycrystalline silicon. Thus, for example, the diffusion barrier may be `~ deposited by in-situ chemical vapor deposition prior to . .~
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~0~;7~i10 application of the silicon layer.
It has now been found that layers of films of silica, borosilicate, phosphosilicate, aluminosilicate and combinations thereof form suitable diffusion barriers between steel and silicon. If the diffusion of boron is objectionable from a borosilicate layer to the silicon layer, a borosilicate-silica double layer is necessary between the steel and si1icon layers.
Silica, borosilicate and phosphosilicate are .. . .
all inert toward silicon and steel at high temperatures, and have the added advantage of being relatively soft.
~ For instance, silica and silicon have hardness values on -~ the Moh's scale of 5 and 7, respectively. ~he thermal expansion coefficient of borosilicate can be changed as a function of its composition which makes its use :
particularly attractive. For example, a borosilicate , .
- composition containing 18 mole % boron oxide has a thermal expansion coefficient similar to that of silicon.
In polycrystalline silicon devices which have a borosilicate/steel base, borosilicate is amorphous and is not expected to significantly influence the structure , j , ,~. of the silicon deposit. Silicon which is deposited over .
borosilicate has properties similar to silicon which is deposited over silicon nitride or silicon dioxide.
Various factors such as substrate temperature, deposition :: .
~ rate and the amount of boron incorporated substantially .:
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10f~76~V
influence the micro-structure of the overlying silicon layer. Generally, silicon which is deposited at low temperatures and high rates without intentional doping consists of small crystallites, the size of which increase with increasing deposition temperature, decreasing decomposition rate and the incorporation of high concentration of boron, for example, a concentration of about 10 boron atoms/cm3. Undoped silicon film deposited at 900C at a rate of 2~m/min reveals a fiber-like structure. However, as the deposition rate is decreased to 0.2~m/min, small crystallites of silicon less than l,~m in size become dominant. Further, a notable increase in the size of the crystallites occurs by further increasing the deposition temperature to 1000 C.
Both silica and borosilicate can be readily .
deposited on steel substrates by a variety of known chemical deposition techniques of which the following are exemplary: Silica can be deposited by (1) the oxidation of silane, or (2) by the pyrolysis of a tetraalkoxysilane, . " ~
;- 20 such as tetraethoxysilane. Borosilicate can be deposited (1) by the oxidation of a mixture of silane and diborane, or (2) by the pyrolysis of a tetraalkoxysilane-trialkoxy-;
borane mixture such as tetraethoxysilane-trimethoxyborane.
Naturally, the amounts of boron and silicon in the .. .
borosilicate are determined by the mole ratios of the silicon reactant and the boron reactant. Other suitable : .~
.~ . I
. .
10~7~i10 alkoxyboranes include triethoxyborane and tripropoxyborane, and other suitable tetraalkoxysilanes include tetramethoxy-silane and tetrapropoxysilane. The methods by which the silica and borosilicate diffusion barriers are deposited on the steel substrates are not critical, and any deposition method well known to those skilled in the art is applicable.
Phosphosilicate layers can be deposited by any suitable technique such as by the oxidation of silane and phosphine mixtures or by the hydrolysis of a silicon tetrachloride-phosphorous trichloride mixture. All of the above methods ~, . .
for depositing the above diffusion layers are compatible with current methods of depositing polycrystalline layers.
Normally, the thickness of the deposited diffusion barrier is 1 - 5,um. Normally, the borosilicate diffusion layers contain from 10 - 18% boron while phosphosilicate layers contain from 10 - 2~/o phosphorous.
By using the above diffusion barrier materials it is possible to prepare integral and tightly formed structures such as silicon/silica/steel, silicon/borosilicate/
: .;
steel and silicon/silica-borosilicate/steel which completely avoid contamination by highly conductive iron silicide.
. ~, .
Suitable steels used for the substrates of the present solar cells include Armco silicon steel and USS Vitrenamel 1, which has a carbon content less than 0.008%. Prior to . ,.:
deposition of the first polycrystalline silicon layer to eventually form the necessary p-n junction, the surface of the steel can be scavenged of oxygen by treatment with a . .~
~ . ~ . ., - :
10~7ti10 reducing agent such as hydrogen at a temperature from 600 to 1000C, preferably 900 to 1000C. Thereafter, the appropriate silica, borosilicate or phosphosilicate layer can be deposited on the steel surface.
Having generally described one aspect of the invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.
- 10 EX~MPLE 1 ' Figure 3A shows a graphite base and Figure 3B
shows the structural configuration of a solar cell having ;~ a graphite base 1, over which was deposited a p+- silicon layer 2 by the thermal decomposition of silane and diborane.
Thereafter, p-silicon layer 3 and n+-silicon layer 4 were deposited by the thermal decomposition of silane using diborane and silane as dopants, respectively. Grid contacts of Ti-Ag-Al 5 were evaporated onto the n+-siiicon surface as shown in Figures 3C and 3D. Figures 3E and 3F show another view of the device in which excess silicon deposits : ,:
were removed from the periphery of the device. The current-; voltage characteristics of the device were measured with an Aerospace Control Corp. Model 302+AMO solar simulator at room temperature, and the efficiencies achieved for a series of cells were in the range of 1.0 - 1.6%.
' : ' -', . .
10~7~10 Figure 4 is a plot of current density versus voltage for one of the above polycrystalline silicon on graphite solar cells. The open-circuit voltage, short-circuit current density, and fill factor obtained are 0.3~ V, 13 m~/Cm , and 0.44, respectively. A value of 0.85 ohms was obtained for its series resistance from the current-voltage relations under several different levels of illumination.
EX~MPLE 2 A 6 cm2 disc of p-type polycrystalline silicon having a resistivity of 0.001 ohm-cm was obtained from metallurgical grade silicon. The disc was heated under a hydrogen atmosphere at 1000C. A p-type, 10,~ m thick :;
layer of polycrystalline silicon was deposited by thermally decomposing silane with diborane as the dopant to produce a resistivity of 1 ohm-cm. Thereafter, an n-type layer :;
.....
0.2 - 0.4,~m thick and having a resistivity of 0.002 ohm-cm was deposited by thermally decomposing silane and phosphine to yield a shallow p-n junction device. Aluminum contacts .:~
were attached to the p- and n-regions. The device was subjected to light from an AM0 solar simulator and an open :. .~,, `~ circuit voltage and short-circuit current density of 0.52 V
and 15 mA/cm2, respectively, were obtained, corresponding to a conversion efficiency of 3%.
; Deposition of Borosilicate of Steel Substrates:
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.
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10~i7~;10 0~6 - 1.5 mm thick low carbon U.S. Steel Vitrenamel steel plates having a linear thermal expansion coefficient of 1.4 x 10 5OC in the temperature range of 25 - 700C were used as su~strates (Note that silicon has an expansion coefficient of 3.6 x 10-6C 1,), The steel substrates were placed in and supported on a silicon car~ide coated graphite susceptor and the susceptor was heated exothermally by an rf generator. Before silicon was deposited on the wafers, the steel substrates were heated under a - 10 hydrogen atmosphere at 1000C to remove oxygen from the , steel surface. A film of borosilicate of a thickness ranging from 3 - 5~4m was then deposited at 900C from a reactant mixture of 20 l/min hydrogen, 25 ml/min silane, 6 ml/min diborane and 250 ml/min oxygen.
Several polycrystalline solar cells were then prepared by depositing silicon on steel substrates coated with 3 - 5,~m of borosilicate. The first silicon layer was ;,' ~
~ p-type of 10 - 40~m thickness and of a resistivity of ; 0.002 - 0.003 ohm-cm, while the second layer was of p-type --, 20 silicon of 8 - 10,4m thickness having a resistivity of :
0.2 - 2 ohm-cm. The final layer was a layer of n-type silicon of 0.2 - 0.4,~m thickness and 0.001 - 0.002 ohm-cm resistivity. The resistivity profile of a device having a first p-silicon layer of 35f~m thickness of 0.002 ohm-cm : :.
;` resistivity, a second p-silicon layer of lO,~m thickness of 0.2 ohm/cm resistivity and a final n-silicon layer of ', .
.
.~ - - 19 -;......... . .. .
, ' ' ~ ' '' - ' , ~0~;7~1V
0~2f~m thickness of 0.001 ohm/cm resistivity on a borosilicate/Vitrenamel I ~ub~trate i9 shown in Figure 5.
EX~MPLE _3 Several solar cells were fabricated from the steel/borosilicate structures de~cribed above a~ shown in Figure 6A. Because borosilicate was used as the diffusion barrier, it was necessary to place the ohmic . .
contacts in the n- and p-regions of the device on the front surface of the silicon. A 2 mm wide strip 15 of the upper L0 n - and p-silicon layers was removed from the periphery of the device as shown to expose the p+- layer 12, which is .
above the borosilicate layer 11 and steel substrate 10, but below p-type layer 13 and n-type layer 14 (Figure 6B). A
:. ~.. . . .
~ thick aluminum film 16, 1 mm wide was then deposited onto .
~ the exposed p+-layer, Figures 6C and 6D. The device was :-~
:~ then annealed at 500C for 20 min. to form a low-resistance ohmic contact. Thereafter, an aluminum grid contact 17 was ... . .
~ evaporated through a metal mask onto the n+-layer, Figures :
6E and 6F.
O The electrical properties of the cells are :
exemplified by a particular cell having the following . .
configuration: 0.2~hm thick - 0.001 ohm-cm n-silicon/5~m thick - 3 ohm-cm p-silicon/15,4m thick - O.003 ohm-cm p-silicon on borosilicate/Vitrenamel I. The p~-layer was ::. i,' ~' deposited at 1000C, and the other layers were deposited at 900C. The average grain size was 2,5~m. The p-n+junction ,:
.,-' ~
..
iOtj7610 was 4.4 cm in area The current-voltage characteristics were determined under illumination by a tungsten lamp.
The power density of the surface of the device was estimated to be 80 mV/cm2. The open-circuit voltage was about 0.1 V, and the conversion efficiency was about 0.05%.
EX~MPLE 4 A USS Vitrenamel I steel plate 1 mm thick was supported on a silicon carbide coated graphite susceptor in a 55 mm ID fused silicon reaction tube. An rf generator was used to heat the susceptor externally. A 3 ~m thick ; borosilicate layer was deposited on the substrate by - decomposing a mixture of hydrogen, silane, diborane and oxygen at flow rates of 20 l/min, 25 ml/min, 6 ml/min and 250 ml/min, respectively, at 900C. Thereafter, a layer of silicon 55~m thick was deposited over the borosilicate by decomposing a mixture of hydrogen and silane at flow rates of 20 ml/min and 50 ml/min, respectively, at 900C. Both the silicon and borosilicate layers tightly adhered to the underlying substrates. Further, metallurgical ~20 examination of the cross-section of the deposited layers :~:
~; showed that no diffusion of iron occured into the silicon, ~; and similarly, no diffusion of silicon occurred into the ~ ':
steel substrate. The silicon layer overlying the boro-silicate layer was of the n-type having an electrical ?
resistivity of about 450 ohm-cm. The resistivity data . .
indicated that borosilicate effectively functions as a , .
'' _ 21 -. ' ,'`.'~
,: .
: 10~7~0 diffusion barrier against the diffusion of iron from the steel substrate into silicon.
EX~MPLE 5 Following the procedure of Example 4, a boro-silicate layer of 3 - 5~4m thickness was deposited over a , Vitrenamel substrate. A 30~m layer of silicon was subsequently deposited over the borosilicate layer by decomposing silane in hydrogen at 1150C. Subsequent .~ metallurgical examinations showed that the silicon/boro-silicate/steel structures effectively prevented the diffusion . :
' of iron into the silicon layer at 1150C.
.
i EX~MPLE 6 ; ,, A 7,um thick film of silica was deposited on a steel substrate at 900 C by decomposing a mixture of "~ hydrogen, silane and oxygen at flow rates of 20 l/min, 25 ml/min and 250 ml/min, respectively. ~hereafter, the temperature of the substrate was increased to 1150C, and ::
about 30~m of silicon was deposited. In Examples 5 and 6, i metallurgical examinations showed that no iron diffused from the substrate into the silicon layer.
According to another aspect of the invention, the conversion efficiency of polycrystalline silicon solar.
cells on graphite substrates can be substantially increased.
Extensive experimentation has been carried out to produce ~, such cells generally having the configuration 0.2 - O.4~4m ` O.001 - O.002 ohm-cm n-silicon/10-15,4m 0.5-2 ohm-cm ~.
., 10676~0 p-silicon/10-30~m 0.002-0.005 ohm-cm p-silicon/graphite, i.e., a low resistivity p-type silicon layer was first deposited on graphite followed by the depositions of a medium resistivity p-type silicon layer and a low resistivity n-type silicon layer. Solar cells with open-circuit voltages of up to 0.35 V and short-circuit current densities of up ~ 13 mA/cm2 were obtained under illumination with an air mass zero solar simulator, corresponding to a conversion efficiency of about 1.5%. This relatively low - 10 efficiency is associated with the relatively small size of crystallites, less than 30 micrometers on the average, in a polycrystalline silicon layer deposited by conventional chemical vapor deposition techniques. To improve the con-version efficiency of polycrystalline silicon solar cells, the crystallite size in silicon must be substantially increased, and this invention provides a simple technique to improve the conversion efficiency o polycrystalline ;,.-., :
' silicon solar cells on graphite substrates.
.~11 ` Since the solidification of a silicon melt usually yields relatively large crystallites, the recrystallization of the low resistivity p-type layer prior to the deposition of the medium resistivity p-type and low-resistivity n-type silicon layers can improve considerably the structural properties of silicon solar cells. ~owever, the recrystalli-I
~ zation of a large area silicon layer of a few micrometers - thickness on graphite substrates is extremely difficult - because of the large surface tension of molten silicon ''', , ,.
_ 23 -,,`' '.
10f~76~0 will cause the thin molten silicon layer to breakdown into discontinuous filaments. To ~tabilize the molten silicon layer, the silicon melt-substrate interfacial energy must be increased to overcome the ambient-silicon melt interfacial :, energy associated with the large surface tension of molten silicon. The silicon-substrate interfacial energy can be increased by increasing the roughness of the substrate surface, for example, by sandblasting. Thus, the recrystalli-zation of the silicon layer may be carried out by first depositing a low resistivity p-type polycrystalline silicon layer on a roughened graphite surface, followed by melting and solidification. Two approaches have been used for the recrystallization of silicon on graphite: (1) zone-melting, and (2) melting of the entire area followed by unidirectional solidification. The zone-melting process was carried out by using the apparatus shown schematically in Fig. 7.
A p+-silicon/graphite specimen of 15 cm. length and 4 cm.
width was placed in a fused silica tube in a hydrogen flow and was heated at 1200 - 1300 C by an rf generator. The .~ .
20 spacings between the turns of the rf coil were adjusted so that a 2.5 cm wide region of the specimen was above the melting point of silicon. The specimen was then pulled to allow the molten zone to traverse through the length of the specimen. Although zone-melting has produced polycrystalline silicon layers with relatively large .
~ - 24 -10fà7610 crystallites, this process involves the movement of the specimen and may not be economical for the fabrication of low cost silicon solar cells. The stationary recrystalli-zation process is therefore more desirable. This method is complicated by the fact that the density of the liquid silicon is about l~/o higher than the density of the solid.
The regions which solidify last will have a protruded surface due to the expansion of silicon on solidification.
It is therefore necessary that the solidification of molten silicon be controlled to take place from one end of the specimen to the other. This was achieved by adjusting the spacings of the rf coil so that there is a unidirectional temperature gradient along the length of the specimen, as shown in Fig. 8. The unidirectional recrystallization of . ., :
silicon on graphite is simple in operation and has produced silicon sheets with crystallites as large as two centimeters .: :,1 in length and several milimeters in width. Most crystallites are of a [110] orientation as shown by x-ray diffraction.
; An alternate and more economical approach for the :.:
preparation of silicon sheets on graphite substrates is to use metallurgical-grade silicon, at a cost of about $1/kg, : .:
as the starting material. Metallurgical-grade silicon is about 98% purity, and a major portion of the impurities in metallurgical silicon can be removed readily by the chemical treatment of the melt with chlorine or other reagents. Instead of depositing a low resistivity p-type . .
. .,:
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~0~7610 polycrystalline silicon layer on a graphite surface as discussed a~ove, the purlfied metallurgical silicon is melted on a graphite plate and unidirectionally soli-dified as shown in Fig. 8. This approach is the most inexpensive one at present for providing silicon sheet with large crystallites suitable for solar cell purposes.
Solar cells can be prepared by depositing a ; silicon film containing a p-n junction on a p+-silicon~recrystallized)/graphite or a chemically treated metallurgical silicon (recrystallized)/graphite substrate.
Two examples illustrating the solar cell fabrication techniques discussed are given below.
~- A p-type silicon layer of 0.002-0.003 ohm-cm resistivity was deposited on a roughened graphite substrate at 1150 C by the thermal reduction of trichlorosilane with hydrogen. This layer was then recrystallized by uni-directional solidification, and the recrystallized layer contained elongated crystallites of up to 2 cm in length and several millimeters in width. Subsequently, a 10-30 ~m of 0.2-1 ohm-cm p-type silicon layer and a 0.2-0.4,~m of 0.002-0.004 ohm-cm n-type layer were deposited to complete ~ the solar cell structure. The grid contact consisting ;~ of about 500 R titanium and 5,~m silver was then evaporated onto the as-deposited surface in the usual manner. The solar cell produced by this technique showed relatively '.'~
:
, 10~7610 good dark current-voltage charac~eristics as shown in Fig. 9, where the area of the cell was about 20 cm2.
These characteristics are considerably better than those of solar cells described in the previous disclosure.
Figure 10 shows the current-voltage characteristics of this cell under illumination with a quartz halogen lamp calibrated to AM0 conditions. The open-circuit voltage, ; short-circuit current density, and fill factor were found to be 0.52 V, 17 mA/cm2, and 0.55, respectively, ~10 corresponding to a conversion efficiency of about 3.7%.
~` EX~MPLE 8 Metallurgical-grade silicon was purified by :,, treating the melt with chlorine to volatilize about 5% of silicon in the form of silicon tetrachloride During this treatment, many impurity elements, such as boron and aluminum, ::; .
react preferentially with chlorine to form volatile chlorides.
,~ .
;~ Chemically treated metallurgical silicon was melted on a ; graphite plate and unidirectionally recrystallized. The resulting metallurgical silicon/graphite substrates were then used for the deposition of silicon solar cell structures by ; ~ the thermal reduction of trichlorosilane, as discussed in . ....................................................................... . .
Example 7. The characteristics of a solar cell of about 32 cm area with the configuration 1000 A SiO2/0.3-0.4 ~m 0.003 ohm-cm n-silicon/30,hm 1 ohm-cm p-silicon/metallurgical : .:
silicon (unidirectionally recrystallized)/graphite are shown ` ~' - ::
' ~
: .
. ~ ... . .. ...
in Figs. 11 and 12. Yigure 11 shows the characteristics of the cell under illumination equivalent to AMI conditions.
The open-circuit voltage, short circuit current density, and fill factor were found to be 0.53 V~ 17.5 mA/cm2, and 0.56, respectively, corresponding to an AMI efficiency of 5%. The relation between the short-circuit current density and the open circuit voltage measured under various intensities of illumination is shown in Fig. 12.
The "n" value in the diode equation and the saturation current density calculated from this plot are approximately 1.76 and 1.2 x 10-7 A/cm2, respectively. In some smaller - area solar cells (10 cm2 for example), AMI efficiencies higher than 6% have been observed.
.. ~.~ . .
Having now fully described the invention, it will be apparent to one of ordinary skill in the art that - many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.
.
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The problem of uncovering new, abundant, cheap and non-polluting sources of energy is a problem of vital national importance. Of all energy sources, solar energy is one of the more attractive sources because of its abundant supply and because it is completely non-polluting.
An indication of the abundance of solar energy is evident by the fact that the solar power on the surface of the earth is approximately one hundred thousand times greater than the current power consumption from all energy sources.
Presently, solar energy is utilized by converting ` solar energy to thermal energy and by converting solar energy to electricity which is known as the photovoltaic system.
Both methods of utilizing solar energy are expected to aid in meeting the ever-increasing demand for clean solar energy. Currently, the silicon solar cell is the most well-;, known device in the photovoltaic system. Further, technology has advanced to the point where silicon solar cell panels which are capable of producing several kilowatts of power have been ~ed reliably in all types of space craft for manv years.
. ' ~
10~7610 Currently, silicon solar cells are manufactured by preparing polycrystalline silicon by reducing trichloro-silane with hydrogen, growing single crystals of silicon of controlled purity from the polycrystalline material, preparing silicon wafers by cutting the single crystal ingot to a thickness of at least 0.25 mm followed by polishing and etching, diffusing a dopant into the silicon wafers to form a shallow p-n junction, applying ohmic contacts to the rear surface and grid contact to the diffused surface, applying antireflecting and protective coatings to the diffused surface and finally mounting the cell into position.
This rather intricate procedure results in the current high costs of manufacturing silicon solar cells. Although the costs of production for single crystalline solar cells has recently been reduced from about $100/peak watt to about $20/peak watt, further reduction in coct of about one order of magnitude is necessary if widespread utility of solar cells is to be realized in large-scale terrestrial applicationsO
One prior art process of manufacturing semi-conductor solar cells as shown by Tarneja, et al, U.S. Patent 3,460,240 issuedAugust, 1969involves epitaxially depositingsili-con ona quaxtzsubstrate toform an N-type layer over which is epitaxially deposited two P-type silicon layers. However, this process has the disadvantage that the overall process requires the rather detailed and expensive sequence of ' iO~7~1V
steps necessary to deposit epitaxial silicon so that no significant decrease in cost of manufacture is observed.
The Jones reference, U.S. Patent 3,078,328,issued February,1963shows a methodof manufacturingsolar cellsin which a layerof siliconis grownonto a graphitesurface froma silicon melt and doped to form an N~type layer. In this growth step, silicon and carbon at the interface of the silicon and graphite layers mix to form an intermediata layer of silicon carbide. The device is completed by formation of a top p-type layer of silicon by diffusion. The reference again is disadvantaged by the complicated fabrication procedure. Thus, the cost of manufacture is unattractive . ~., from a commercial viewpoint.
Small-area polycrystalline silicon solar cells have also been fabricated by the deposition of silicon from a vapor state reactant. A polycrystalline silicon layer of ;~
a thickness of 25 - 50 ~m was deposited on silicon substrates at 900C by the reduction of trichlorosilane with hydrogen.
In this method, silicon substrates were used for convenience ,:
~:~ in order to eliminate the cracking of silicon which has been deposited on other substrates. By this procedure, 1 cm solar cells were fabricated by the successive diffusion of gallium and phosphorous to form a p-n junction about 2.5 ,~m below the surface of the device. The device had a maximum " ,, :
~ open-circuit voltage of about 0.3 V, and the greatest :. ,, ~ ~ efficiency was about O.9o/O.
.
; - 4 -, . .
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1()~i7610 A need continues to exist, therefore, for a method by which silicon semiconductor solar cells can be easily and c~eaply produced.
Accordingly, one object of the present invention is to provide a method of manufacturing polycrystalline silicon semiconductor solar cells readily and cheaply.
Another object of the invention is to provide a method of bonding polycrystalline sili-con to common substrates of substantially different chemical composition so as to form silicon semi-, ~
~ conductor solar cells.
. .:
;~ Briefly, these objects and other objects ~ of the invention, as hereinafter will become more .
readily apparent, may be obtained by a low-cost polycrystalline silicon solar cell, which compris-.
es: a steel base; at least one overlying layer of polycrystalline silicon of p-type conductivity and at least one overlying layer of polycrystalline silicon of n-type conductivity so as to define a , .............................. .
; p-n junction; and a diffusion barrier of borosili-? cate interposed between the steel base and the first polycrystalline layer.
;
Preferably, said p-n junction is formed R by first and second overlying polycrystalline sili-.
, , ~ 30 ' ,. _ S r~
.:
iO~;7610 con layers both of p-type conductivity and a third overlying layer of polycrystalline sili-cone of n-type conductivity.
It is also preferred that the first overlying layer is lQ - 40~m thick and has a resistivity of 0.002 - 0.003 ohm-cm, the second overlying layer is 8 - lO~m thick and has a resistivity of 0.2 - 2 ohm-cm and the third over-lylng layer is 0.2 - 0.4~ m thick and has a re-sistivity of 0.001 - 0.002 ohm-cm.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered : i ~ in connection with the accompanying drawings, ;~ wherein:-Figure 1 shows the resistivity pro-file of the p-region of polycrystalline silicon :. ,.:.
20 solar cell device supported on a graphite .i. :.::
substrate;
Figure 2 shows the current-voltage . . .
~` characteristics of a mesa diode formed from a poly-crystalline silicon, p-n junction device supported on a graphite substrate;
Figures 3A to 3F show one embodiment of ~` polycrystalline silicon, p-n junction devices support-~ ed on a graphite substrate;
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llD67610 Figure 4 is a graph of the current-voltage characteristics of one of the devices of Figure 3;
Figure 5 is the resistivity profile of the p-region of a polycrystalline silicon junction device supported on a borosilicate coated steel substrate;
- Figures 6A to 6F show one embodiment of the configuration of polycrystalline silicon, p-n .~ 10 junction solar cell devices supported on a sub- strate of borosilicate coated steel;
Figure 7 is a schematic diagram of an apparatus for the zone-melting of silicon on gra-phite substrates;
; Figure 8 is a schematic diagram of an apparatus for the unidirectional solidification of :
silicon on graphite substrates;
: Figure 9 is a graph of the dark current-: voltage characteristics of a n -silicon/p-silicon/p -~: 20 silicon (unidiretionally recrystallized)/graphite ~ solar cell;
: Figure 10 is a graph of the current-~: voltage characteristics of a n -silicon/p-silicon/
.~ p -silicon (unidirectionally recrystallized)/
: graphite solar cell under illumination with a quartz-halogen lamp equivalent to AMO
.. i ,: ` , .,: .
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10~761V
.
conditions;
Figure 11 is a graph of the current-voltage characteristics of a n -silicon/p-silicon/p+-silicon (unidirectionally recrystallized)/graphite solar cell under illumination with a quartz-halogen lamp equivalent ~- to AMI conditions; and Figure 12 is a graph showing the relation between short-circuit current density and open-circuit .:
; voltage of a solar cell on metallurgical silicon/graphite ~ 10 measured under different illumination levels.
, An important consideration in the manufacture of silicon semiconductor solar cells is the type of support . ., - structure employed. The ideal support should be one which has the characteristics of being low in cost, light-weight, ., chemically inert to silicon at high temperatures, having ; a high electrical conductivity and a coefficient of -: .
s expansion similar to that of silicon. Furthermore, the substrate should be such that silicon strongly adheres to - the surface of the substrate, and the interface between the ~-~ 20 substrate and the silicon layer should be of low electrical resistance. In the past, refractory metals such as tantalum : .
have been used as substrates for the deposition of poly-~` crystalline silicon of high purity. Such substrates, .::
,:
however, have not been satisfactory because, from an economical point in view, they are not competitive. The `, choice of an appropriate substrate having the above-mentioned . :.
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iO67610 advantages would be a significant factor in lowering the costs of silicon solar cells, thus rendering their use more attractive.
The high cost of manufacturing silicon solar cells from semiconductor grade silicon has already been discussed.
High costs are involved by the series of process steps involved and the high cost of some of the manufacturing steps in particular. In view of these problems, the use of , , , ~ a thin layer of polycrystalline silicon containing a p-n ~, ~
junction deposited on a suitable substrate should substantially reduce material and process costs. Although the efficiency of polycrystalline silicon solar cells cannot compete with the efficiency of single crystalline cells, the . .
....
unit power costs would be many times less than that of :`
present single cyrstal cells.
It has now been discovered that polycrystalline , silicon can be satisfactorily deposited on a steel, graphite `~ or metallurgical polycrystalline silicon substrate to form a suitable silicon solar cell. The p-n junction must be ~,. .
very shallow; the n-layer (or the upper layer) is 0.2 - 0,4~m in thickness, and the p-layer (or the inner layer) is 10 - lOO~m in thickness. The order of preference of substrates is: metallurgical-grade silicon, graphite, and steel~ Although most steels will serve the purpose, metallurgical grade silicon and graphite are preferred.
x In the preparation of solar cells using either a graphite :.'' g _ ' :', : :;
~ ~ .
.. . .
~C~t;7610 or polycrystalline silicon substrate, polycrystalline silicon is deposited on an appropriate substrate by any acceptable procedure normally used for the deposition of polycrystalline silicon, such as by the thermal decomposition of silane, by the reduction of trichloro-silane or silicon tetrachloride with hydrogen at temperatures ranging from 900 C - 1200C or by the thermal decomposition of dichlorosilane. Several such layers are deposited and appropriately doped so as to form a p-n junction device by any conventional procedure known to one skilled in the art. In one embodiment, the first deposited silicon layer is doped with a p+- type dopant to achieve a p+-type layer 10 - 30~m thick and having a resistance of 0.007-0.003 ohm-cm. Doping occurs ; simultaneously with the deposition of silicon by introducing the dopant gas, e.g., diborane for p-type conductive layers and phosphine for n-type conductive layers, in proper amount into the reactant mixture. Thereafter, two successive silicon lay~rs are deposited which are p-type and then ;
n-type successively. The p-type layer is 8 - 10,4m thick :
~,~ and has a resistance of 0.2 - 2 ohm-cm, while the n-type ~i layer is 0.2 - 0.4~m thick and has a resistance of 0.001 -. ~:
~ 0.002 ohm-cm. FIGURE 1 shows the resistivity profile of i~ the structured device, except for the n-layer, which is ` obtained by the spreading resistance technique. The plot :
shows a high resistivity area in the silicon adjacent the ,.
- lQ _ ,~: ..
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. . . . .
10~;7fà~(1 graphite-silicon interface and i~ apparently caused by the formation of silicon carbide.
In one embodiment of the invention, the silicon devices above can be converted into mesa diodes by .
depositing titanium-silver or gold dots on the silicon ~:.
surface followed by masking and etching of the surface.
~ .
- Titanium-silver is the preferred contact material.
FIGURE 2 shows the current-voltage characteristics of one of these diodes which is exemplary of the values obtained for these diodes, wherein the "n" value calculated from the forward characteri9tics is 1.9, which is very similar to that for single crystal silicon p-n junctions.
This value indicates that the grain boundaries in polycrystalline silicon deposited on graphite supports are not significant in current conduction.
.:, By the procedure shown above, so7ar cells can be fabricated wherein the thickness of the polycrystalline silicon layer of the cell ranges from 0.001 to 0O005 cm which is at least 100 times less than the amount of silicon ~.:
employed in single crystal solar cells. The most significant advantagesare that the energy-, labor-, and material-.:. ~ .
~ consuming steps of the process of producing single crystal . .~
cells, i.e., the growth of single crystals,the preparation of the silicon wafers and the diffusion process, : ~ .
are eliminated. Although the efficiency of the poly-crystalline solar cells is less than that of single crystal iOti7610 cells, the disadvantage i9 more than countered by the many times lower unit power costs of the polycrystalline cells.
A similar procedure to that shown above can be , used to prepare polycrystalline silicon solar cells having a support structure of metallurgical polycrystalline silicon by any method known to those skilled in the art of establishing p-n junctions in deposited polycrystalline silicon. In a representative procedure, a p-type poly-crystalline layer is deposited on a wafer of silicon, which has been pretreated with hydrogen or with hydrogen . . ~ , chloride at an elevated temperature ranging from 1000C
to 1100C by the decomposition of silane with diborane as ~: ........................................................................ .
~ the dopant source. Thereafter, an upper n-type silicon layer r '. :~ ~
~ is deposited using phosphine or arsine as a dopant to yield , a structure having a shallow p-n junction. The device is completed by attaching leads to the n- and p-regions.
In another aspect of the invention, the fabrication of the cheap silicon solar cells by the decomposition of polycrystalline silicon on a steel substrate was attempted.
The problem of using steel as a substrate for polycrystalline silicon cells is complicated by several problems. The first , , ; is that a large and therefore significant difference exists . . .:;
in the thermal expansion coefficients of the materials.
; Silicon has an expansion coefficient of 4 x 10 C-l, while ::
most steels have coefficients of 14 - 16 x 10-6OC
1 , .:.
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Another factor is that high rates of diffusion of silicon and iron atoms occur into each other between adjacent layers of steel and polycrystalline silicon at the temperatures employed in the deposition process. It is known from the phase diagrams of mixtures of iron and silicon that the following iron silicides form: Fe3Si, Fe5Si3, FeSi and FeSi2. Still another factor is that at temperatures above 800C, a solid solution of iron silicides forms when silicon is deposited on steel substrates by any of the conventional silicon deposition procedures. This latter fact is substantiated by the . .
high electrical conductivity of the region.
In view of the above problems with steel as a substrate for silicon solar cells, it is apparent that another material must be applied between the deposited silicon layer and the steel substrate. Any such material (hereinafter referred to as the "diffusion barrier"~ must ., have the ability to minimize the difference in thermal expansion characteristics of the two layers and should be ~;20 chemically inert to steel and silicon at high temperatures.
' Further, the intervening layer should strongly adhere to ,;, .
both the steel and polycrystalline layers and be compatible with the manufacturing technology of the polycrystalline silicon. Thus, for example, the diffusion barrier may be `~ deposited by in-situ chemical vapor deposition prior to . .~
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~0~;7~i10 application of the silicon layer.
It has now been found that layers of films of silica, borosilicate, phosphosilicate, aluminosilicate and combinations thereof form suitable diffusion barriers between steel and silicon. If the diffusion of boron is objectionable from a borosilicate layer to the silicon layer, a borosilicate-silica double layer is necessary between the steel and si1icon layers.
Silica, borosilicate and phosphosilicate are .. . .
all inert toward silicon and steel at high temperatures, and have the added advantage of being relatively soft.
~ For instance, silica and silicon have hardness values on -~ the Moh's scale of 5 and 7, respectively. ~he thermal expansion coefficient of borosilicate can be changed as a function of its composition which makes its use :
particularly attractive. For example, a borosilicate , .
- composition containing 18 mole % boron oxide has a thermal expansion coefficient similar to that of silicon.
In polycrystalline silicon devices which have a borosilicate/steel base, borosilicate is amorphous and is not expected to significantly influence the structure , j , ,~. of the silicon deposit. Silicon which is deposited over .
borosilicate has properties similar to silicon which is deposited over silicon nitride or silicon dioxide.
Various factors such as substrate temperature, deposition :: .
~ rate and the amount of boron incorporated substantially .:
:
. ,~.:
~ - 14 -::
;
10f~76~V
influence the micro-structure of the overlying silicon layer. Generally, silicon which is deposited at low temperatures and high rates without intentional doping consists of small crystallites, the size of which increase with increasing deposition temperature, decreasing decomposition rate and the incorporation of high concentration of boron, for example, a concentration of about 10 boron atoms/cm3. Undoped silicon film deposited at 900C at a rate of 2~m/min reveals a fiber-like structure. However, as the deposition rate is decreased to 0.2~m/min, small crystallites of silicon less than l,~m in size become dominant. Further, a notable increase in the size of the crystallites occurs by further increasing the deposition temperature to 1000 C.
Both silica and borosilicate can be readily .
deposited on steel substrates by a variety of known chemical deposition techniques of which the following are exemplary: Silica can be deposited by (1) the oxidation of silane, or (2) by the pyrolysis of a tetraalkoxysilane, . " ~
;- 20 such as tetraethoxysilane. Borosilicate can be deposited (1) by the oxidation of a mixture of silane and diborane, or (2) by the pyrolysis of a tetraalkoxysilane-trialkoxy-;
borane mixture such as tetraethoxysilane-trimethoxyborane.
Naturally, the amounts of boron and silicon in the .. .
borosilicate are determined by the mole ratios of the silicon reactant and the boron reactant. Other suitable : .~
.~ . I
. .
10~7~i10 alkoxyboranes include triethoxyborane and tripropoxyborane, and other suitable tetraalkoxysilanes include tetramethoxy-silane and tetrapropoxysilane. The methods by which the silica and borosilicate diffusion barriers are deposited on the steel substrates are not critical, and any deposition method well known to those skilled in the art is applicable.
Phosphosilicate layers can be deposited by any suitable technique such as by the oxidation of silane and phosphine mixtures or by the hydrolysis of a silicon tetrachloride-phosphorous trichloride mixture. All of the above methods ~, . .
for depositing the above diffusion layers are compatible with current methods of depositing polycrystalline layers.
Normally, the thickness of the deposited diffusion barrier is 1 - 5,um. Normally, the borosilicate diffusion layers contain from 10 - 18% boron while phosphosilicate layers contain from 10 - 2~/o phosphorous.
By using the above diffusion barrier materials it is possible to prepare integral and tightly formed structures such as silicon/silica/steel, silicon/borosilicate/
: .;
steel and silicon/silica-borosilicate/steel which completely avoid contamination by highly conductive iron silicide.
. ~, .
Suitable steels used for the substrates of the present solar cells include Armco silicon steel and USS Vitrenamel 1, which has a carbon content less than 0.008%. Prior to . ,.:
deposition of the first polycrystalline silicon layer to eventually form the necessary p-n junction, the surface of the steel can be scavenged of oxygen by treatment with a . .~
~ . ~ . ., - :
10~7ti10 reducing agent such as hydrogen at a temperature from 600 to 1000C, preferably 900 to 1000C. Thereafter, the appropriate silica, borosilicate or phosphosilicate layer can be deposited on the steel surface.
Having generally described one aspect of the invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.
- 10 EX~MPLE 1 ' Figure 3A shows a graphite base and Figure 3B
shows the structural configuration of a solar cell having ;~ a graphite base 1, over which was deposited a p+- silicon layer 2 by the thermal decomposition of silane and diborane.
Thereafter, p-silicon layer 3 and n+-silicon layer 4 were deposited by the thermal decomposition of silane using diborane and silane as dopants, respectively. Grid contacts of Ti-Ag-Al 5 were evaporated onto the n+-siiicon surface as shown in Figures 3C and 3D. Figures 3E and 3F show another view of the device in which excess silicon deposits : ,:
were removed from the periphery of the device. The current-; voltage characteristics of the device were measured with an Aerospace Control Corp. Model 302+AMO solar simulator at room temperature, and the efficiencies achieved for a series of cells were in the range of 1.0 - 1.6%.
' : ' -', . .
10~7~10 Figure 4 is a plot of current density versus voltage for one of the above polycrystalline silicon on graphite solar cells. The open-circuit voltage, short-circuit current density, and fill factor obtained are 0.3~ V, 13 m~/Cm , and 0.44, respectively. A value of 0.85 ohms was obtained for its series resistance from the current-voltage relations under several different levels of illumination.
EX~MPLE 2 A 6 cm2 disc of p-type polycrystalline silicon having a resistivity of 0.001 ohm-cm was obtained from metallurgical grade silicon. The disc was heated under a hydrogen atmosphere at 1000C. A p-type, 10,~ m thick :;
layer of polycrystalline silicon was deposited by thermally decomposing silane with diborane as the dopant to produce a resistivity of 1 ohm-cm. Thereafter, an n-type layer :;
.....
0.2 - 0.4,~m thick and having a resistivity of 0.002 ohm-cm was deposited by thermally decomposing silane and phosphine to yield a shallow p-n junction device. Aluminum contacts .:~
were attached to the p- and n-regions. The device was subjected to light from an AM0 solar simulator and an open :. .~,, `~ circuit voltage and short-circuit current density of 0.52 V
and 15 mA/cm2, respectively, were obtained, corresponding to a conversion efficiency of 3%.
; Deposition of Borosilicate of Steel Substrates:
., ~ - 18 -..
.
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10~i7~;10 0~6 - 1.5 mm thick low carbon U.S. Steel Vitrenamel steel plates having a linear thermal expansion coefficient of 1.4 x 10 5OC in the temperature range of 25 - 700C were used as su~strates (Note that silicon has an expansion coefficient of 3.6 x 10-6C 1,), The steel substrates were placed in and supported on a silicon car~ide coated graphite susceptor and the susceptor was heated exothermally by an rf generator. Before silicon was deposited on the wafers, the steel substrates were heated under a - 10 hydrogen atmosphere at 1000C to remove oxygen from the , steel surface. A film of borosilicate of a thickness ranging from 3 - 5~4m was then deposited at 900C from a reactant mixture of 20 l/min hydrogen, 25 ml/min silane, 6 ml/min diborane and 250 ml/min oxygen.
Several polycrystalline solar cells were then prepared by depositing silicon on steel substrates coated with 3 - 5,~m of borosilicate. The first silicon layer was ;,' ~
~ p-type of 10 - 40~m thickness and of a resistivity of ; 0.002 - 0.003 ohm-cm, while the second layer was of p-type --, 20 silicon of 8 - 10,4m thickness having a resistivity of :
0.2 - 2 ohm-cm. The final layer was a layer of n-type silicon of 0.2 - 0.4,~m thickness and 0.001 - 0.002 ohm-cm resistivity. The resistivity profile of a device having a first p-silicon layer of 35f~m thickness of 0.002 ohm-cm : :.
;` resistivity, a second p-silicon layer of lO,~m thickness of 0.2 ohm/cm resistivity and a final n-silicon layer of ', .
.
.~ - - 19 -;......... . .. .
, ' ' ~ ' '' - ' , ~0~;7~1V
0~2f~m thickness of 0.001 ohm/cm resistivity on a borosilicate/Vitrenamel I ~ub~trate i9 shown in Figure 5.
EX~MPLE _3 Several solar cells were fabricated from the steel/borosilicate structures de~cribed above a~ shown in Figure 6A. Because borosilicate was used as the diffusion barrier, it was necessary to place the ohmic . .
contacts in the n- and p-regions of the device on the front surface of the silicon. A 2 mm wide strip 15 of the upper L0 n - and p-silicon layers was removed from the periphery of the device as shown to expose the p+- layer 12, which is .
above the borosilicate layer 11 and steel substrate 10, but below p-type layer 13 and n-type layer 14 (Figure 6B). A
:. ~.. . . .
~ thick aluminum film 16, 1 mm wide was then deposited onto .
~ the exposed p+-layer, Figures 6C and 6D. The device was :-~
:~ then annealed at 500C for 20 min. to form a low-resistance ohmic contact. Thereafter, an aluminum grid contact 17 was ... . .
~ evaporated through a metal mask onto the n+-layer, Figures :
6E and 6F.
O The electrical properties of the cells are :
exemplified by a particular cell having the following . .
configuration: 0.2~hm thick - 0.001 ohm-cm n-silicon/5~m thick - 3 ohm-cm p-silicon/15,4m thick - O.003 ohm-cm p-silicon on borosilicate/Vitrenamel I. The p~-layer was ::. i,' ~' deposited at 1000C, and the other layers were deposited at 900C. The average grain size was 2,5~m. The p-n+junction ,:
.,-' ~
..
iOtj7610 was 4.4 cm in area The current-voltage characteristics were determined under illumination by a tungsten lamp.
The power density of the surface of the device was estimated to be 80 mV/cm2. The open-circuit voltage was about 0.1 V, and the conversion efficiency was about 0.05%.
EX~MPLE 4 A USS Vitrenamel I steel plate 1 mm thick was supported on a silicon carbide coated graphite susceptor in a 55 mm ID fused silicon reaction tube. An rf generator was used to heat the susceptor externally. A 3 ~m thick ; borosilicate layer was deposited on the substrate by - decomposing a mixture of hydrogen, silane, diborane and oxygen at flow rates of 20 l/min, 25 ml/min, 6 ml/min and 250 ml/min, respectively, at 900C. Thereafter, a layer of silicon 55~m thick was deposited over the borosilicate by decomposing a mixture of hydrogen and silane at flow rates of 20 ml/min and 50 ml/min, respectively, at 900C. Both the silicon and borosilicate layers tightly adhered to the underlying substrates. Further, metallurgical ~20 examination of the cross-section of the deposited layers :~:
~; showed that no diffusion of iron occured into the silicon, ~; and similarly, no diffusion of silicon occurred into the ~ ':
steel substrate. The silicon layer overlying the boro-silicate layer was of the n-type having an electrical ?
resistivity of about 450 ohm-cm. The resistivity data . .
indicated that borosilicate effectively functions as a , .
'' _ 21 -. ' ,'`.'~
,: .
: 10~7~0 diffusion barrier against the diffusion of iron from the steel substrate into silicon.
EX~MPLE 5 Following the procedure of Example 4, a boro-silicate layer of 3 - 5~4m thickness was deposited over a , Vitrenamel substrate. A 30~m layer of silicon was subsequently deposited over the borosilicate layer by decomposing silane in hydrogen at 1150C. Subsequent .~ metallurgical examinations showed that the silicon/boro-silicate/steel structures effectively prevented the diffusion . :
' of iron into the silicon layer at 1150C.
.
i EX~MPLE 6 ; ,, A 7,um thick film of silica was deposited on a steel substrate at 900 C by decomposing a mixture of "~ hydrogen, silane and oxygen at flow rates of 20 l/min, 25 ml/min and 250 ml/min, respectively. ~hereafter, the temperature of the substrate was increased to 1150C, and ::
about 30~m of silicon was deposited. In Examples 5 and 6, i metallurgical examinations showed that no iron diffused from the substrate into the silicon layer.
According to another aspect of the invention, the conversion efficiency of polycrystalline silicon solar.
cells on graphite substrates can be substantially increased.
Extensive experimentation has been carried out to produce ~, such cells generally having the configuration 0.2 - O.4~4m ` O.001 - O.002 ohm-cm n-silicon/10-15,4m 0.5-2 ohm-cm ~.
., 10676~0 p-silicon/10-30~m 0.002-0.005 ohm-cm p-silicon/graphite, i.e., a low resistivity p-type silicon layer was first deposited on graphite followed by the depositions of a medium resistivity p-type silicon layer and a low resistivity n-type silicon layer. Solar cells with open-circuit voltages of up to 0.35 V and short-circuit current densities of up ~ 13 mA/cm2 were obtained under illumination with an air mass zero solar simulator, corresponding to a conversion efficiency of about 1.5%. This relatively low - 10 efficiency is associated with the relatively small size of crystallites, less than 30 micrometers on the average, in a polycrystalline silicon layer deposited by conventional chemical vapor deposition techniques. To improve the con-version efficiency of polycrystalline silicon solar cells, the crystallite size in silicon must be substantially increased, and this invention provides a simple technique to improve the conversion efficiency o polycrystalline ;,.-., :
' silicon solar cells on graphite substrates.
.~11 ` Since the solidification of a silicon melt usually yields relatively large crystallites, the recrystallization of the low resistivity p-type layer prior to the deposition of the medium resistivity p-type and low-resistivity n-type silicon layers can improve considerably the structural properties of silicon solar cells. ~owever, the recrystalli-I
~ zation of a large area silicon layer of a few micrometers - thickness on graphite substrates is extremely difficult - because of the large surface tension of molten silicon ''', , ,.
_ 23 -,,`' '.
10f~76~0 will cause the thin molten silicon layer to breakdown into discontinuous filaments. To ~tabilize the molten silicon layer, the silicon melt-substrate interfacial energy must be increased to overcome the ambient-silicon melt interfacial :, energy associated with the large surface tension of molten silicon. The silicon-substrate interfacial energy can be increased by increasing the roughness of the substrate surface, for example, by sandblasting. Thus, the recrystalli-zation of the silicon layer may be carried out by first depositing a low resistivity p-type polycrystalline silicon layer on a roughened graphite surface, followed by melting and solidification. Two approaches have been used for the recrystallization of silicon on graphite: (1) zone-melting, and (2) melting of the entire area followed by unidirectional solidification. The zone-melting process was carried out by using the apparatus shown schematically in Fig. 7.
A p+-silicon/graphite specimen of 15 cm. length and 4 cm.
width was placed in a fused silica tube in a hydrogen flow and was heated at 1200 - 1300 C by an rf generator. The .~ .
20 spacings between the turns of the rf coil were adjusted so that a 2.5 cm wide region of the specimen was above the melting point of silicon. The specimen was then pulled to allow the molten zone to traverse through the length of the specimen. Although zone-melting has produced polycrystalline silicon layers with relatively large .
~ - 24 -10fà7610 crystallites, this process involves the movement of the specimen and may not be economical for the fabrication of low cost silicon solar cells. The stationary recrystalli-zation process is therefore more desirable. This method is complicated by the fact that the density of the liquid silicon is about l~/o higher than the density of the solid.
The regions which solidify last will have a protruded surface due to the expansion of silicon on solidification.
It is therefore necessary that the solidification of molten silicon be controlled to take place from one end of the specimen to the other. This was achieved by adjusting the spacings of the rf coil so that there is a unidirectional temperature gradient along the length of the specimen, as shown in Fig. 8. The unidirectional recrystallization of . ., :
silicon on graphite is simple in operation and has produced silicon sheets with crystallites as large as two centimeters .: :,1 in length and several milimeters in width. Most crystallites are of a [110] orientation as shown by x-ray diffraction.
; An alternate and more economical approach for the :.:
preparation of silicon sheets on graphite substrates is to use metallurgical-grade silicon, at a cost of about $1/kg, : .:
as the starting material. Metallurgical-grade silicon is about 98% purity, and a major portion of the impurities in metallurgical silicon can be removed readily by the chemical treatment of the melt with chlorine or other reagents. Instead of depositing a low resistivity p-type . .
. .,:
' ,;
. ~. .. . ......
~0~7610 polycrystalline silicon layer on a graphite surface as discussed a~ove, the purlfied metallurgical silicon is melted on a graphite plate and unidirectionally soli-dified as shown in Fig. 8. This approach is the most inexpensive one at present for providing silicon sheet with large crystallites suitable for solar cell purposes.
Solar cells can be prepared by depositing a ; silicon film containing a p-n junction on a p+-silicon~recrystallized)/graphite or a chemically treated metallurgical silicon (recrystallized)/graphite substrate.
Two examples illustrating the solar cell fabrication techniques discussed are given below.
~- A p-type silicon layer of 0.002-0.003 ohm-cm resistivity was deposited on a roughened graphite substrate at 1150 C by the thermal reduction of trichlorosilane with hydrogen. This layer was then recrystallized by uni-directional solidification, and the recrystallized layer contained elongated crystallites of up to 2 cm in length and several millimeters in width. Subsequently, a 10-30 ~m of 0.2-1 ohm-cm p-type silicon layer and a 0.2-0.4,~m of 0.002-0.004 ohm-cm n-type layer were deposited to complete ~ the solar cell structure. The grid contact consisting ;~ of about 500 R titanium and 5,~m silver was then evaporated onto the as-deposited surface in the usual manner. The solar cell produced by this technique showed relatively '.'~
:
, 10~7610 good dark current-voltage charac~eristics as shown in Fig. 9, where the area of the cell was about 20 cm2.
These characteristics are considerably better than those of solar cells described in the previous disclosure.
Figure 10 shows the current-voltage characteristics of this cell under illumination with a quartz halogen lamp calibrated to AM0 conditions. The open-circuit voltage, ; short-circuit current density, and fill factor were found to be 0.52 V, 17 mA/cm2, and 0.55, respectively, ~10 corresponding to a conversion efficiency of about 3.7%.
~` EX~MPLE 8 Metallurgical-grade silicon was purified by :,, treating the melt with chlorine to volatilize about 5% of silicon in the form of silicon tetrachloride During this treatment, many impurity elements, such as boron and aluminum, ::; .
react preferentially with chlorine to form volatile chlorides.
,~ .
;~ Chemically treated metallurgical silicon was melted on a ; graphite plate and unidirectionally recrystallized. The resulting metallurgical silicon/graphite substrates were then used for the deposition of silicon solar cell structures by ; ~ the thermal reduction of trichlorosilane, as discussed in . ....................................................................... . .
Example 7. The characteristics of a solar cell of about 32 cm area with the configuration 1000 A SiO2/0.3-0.4 ~m 0.003 ohm-cm n-silicon/30,hm 1 ohm-cm p-silicon/metallurgical : .:
silicon (unidirectionally recrystallized)/graphite are shown ` ~' - ::
' ~
: .
. ~ ... . .. ...
in Figs. 11 and 12. Yigure 11 shows the characteristics of the cell under illumination equivalent to AMI conditions.
The open-circuit voltage, short circuit current density, and fill factor were found to be 0.53 V~ 17.5 mA/cm2, and 0.56, respectively, corresponding to an AMI efficiency of 5%. The relation between the short-circuit current density and the open circuit voltage measured under various intensities of illumination is shown in Fig. 12.
The "n" value in the diode equation and the saturation current density calculated from this plot are approximately 1.76 and 1.2 x 10-7 A/cm2, respectively. In some smaller - area solar cells (10 cm2 for example), AMI efficiencies higher than 6% have been observed.
.. ~.~ . .
Having now fully described the invention, it will be apparent to one of ordinary skill in the art that - many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.
.
',: .
:
:
: ,.
, ~ , , . , :, ~ - 28 _ ':-~
:
; :.' ~ : .. -
Claims (3)
1. A low-cost polycrystalline silicon solar cell, which comprises:
a steel base;
at least one overlying layer of polycrystalline silicon of p-type conductivity and at least one overlying layer of polycrystalline silicon of n-type conductivity so as to define a p-n junction; and a diffusion barrier of borosilicate interposed bet-ween said steel base and said first polycrystalline layer.
a steel base;
at least one overlying layer of polycrystalline silicon of p-type conductivity and at least one overlying layer of polycrystalline silicon of n-type conductivity so as to define a p-n junction; and a diffusion barrier of borosilicate interposed bet-ween said steel base and said first polycrystalline layer.
2. The solar cell of Claim 1, wherein said p-n junction is formed by first and second overlying polycrystalline silicon layers both of p-type conductivity and a third overlying layer of polycrystalline silicon of n-type conductivity.
3. The solar cell of Claim 2, wherein said first over-lying layer is 10 - 40µm thick and has a resistivity of 0.002 -0.003 ohm-cm, said second overlying layer is 8 - 10µm thick and has a resistivity of 0.2 - 2 ohm-cm and said third overlying layer is 0.2 - 0.4µm thick and has a resistivity of 0.001 -0.002 ohm-cm.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA322,326A CA1067610A (en) | 1975-05-12 | 1979-02-26 | Low cost substrates for polycrystalline solar cells |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/576,488 US3961997A (en) | 1975-05-12 | 1975-05-12 | Fabrication of polycrystalline solar cells on low-cost substrates |
| CA251,831A CA1068805A (en) | 1975-05-12 | 1976-05-05 | Low cost substrates for polycrystalline solar cells |
| CA322,326A CA1067610A (en) | 1975-05-12 | 1979-02-26 | Low cost substrates for polycrystalline solar cells |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1067610A true CA1067610A (en) | 1979-12-04 |
Family
ID=27164451
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA322,326A Expired CA1067610A (en) | 1975-05-12 | 1979-02-26 | Low cost substrates for polycrystalline solar cells |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1067610A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2971086A1 (en) * | 2011-01-31 | 2012-08-03 | Inst Polytechnique Grenoble | STRUCTURE SUITABLE FOR FORMATION OF SOLAR CELLS |
-
1979
- 1979-02-26 CA CA322,326A patent/CA1067610A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2971086A1 (en) * | 2011-01-31 | 2012-08-03 | Inst Polytechnique Grenoble | STRUCTURE SUITABLE FOR FORMATION OF SOLAR CELLS |
| WO2012104535A3 (en) * | 2011-01-31 | 2013-03-28 | Institut Polytechnique De Grenoble | Structure for forming solar cells |
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