U.S. patent application number 09/927406 was filed with the patent office on 2002-01-24 for fabrication process of solar cell.
Invention is credited to Nishida, Shoji.
Application Number | 20020009895 09/927406 |
Document ID | / |
Family ID | 17174930 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020009895 |
Kind Code |
A1 |
Nishida, Shoji |
January 24, 2002 |
Fabrication process of solar cell
Abstract
Metal-grade silicon is melted and solidified in a mold to form a
plate-shaped silicon layer and a crystalline silicon layer is made
thereon, thereby providing a cheap solar cell without a need for a
slicing step.
Inventors: |
Nishida, Shoji;
(Hiratsuka-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
17174930 |
Appl. No.: |
09/927406 |
Filed: |
August 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09927406 |
Aug 13, 2001 |
|
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|
08932708 |
Sep 18, 1997 |
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Current U.S.
Class: |
438/758 |
Current CPC
Class: |
Y02E 10/547 20130101;
Y02P 70/50 20151101; Y10S 438/955 20130101; H01L 31/068 20130101;
H01L 31/1804 20130101 |
Class at
Publication: |
438/758 |
International
Class: |
H01L 021/31 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 19, 1996 |
JP |
8-248217 |
Claims
What is claimed is:
1. A fabrication process of solar cell comprising a step of forming
a silicon layer on a crystalline silicon substrate by a
liquid-phase growth method using a metal solvent, wherein a total
concentration of impurities of a surface of said crystalline
silicon substrate is 10 ppm or more and wherein said metal solvent
is indium.
2. A fabrication process of solar cell according to claim 1,
wherein said liquid-phase growth method is carried out in a
hydrogen atmosphere.
3. A fabrication process of solar cell according to claim 1 or 2,
wherein said crystalline silicon substrate is comprised of
metal-grade silicon containing 10 ppm or more of impurities.
4. A fabrication process of solar cell comprising a step of melting
and solidifying particles of metal-grade silicon put in a mold to
form a plate-shaped metal-grade silicon substrate, and a step of
forming a silicon layer on a surface of said metal-grade silicon
substrate by a liquid-phase growth method using indium.
5. A fabrication process of solar cell according to claim 4,
wherein a material for said mold is selected from the group
consisting of carbon graphite, silicon carbide, and silicon
nitride.
6. A fabrication process of solar cell according to claim 4 or 5,
wherein a surface of said mold to be in contact with said
metal-grade silicon substrate is coated with a mold releasing agent
containing at least silicon nitride.
7. A fabrication process of solar cell comprising a step of melting
and solidifying particles of metal-grade silicon put in a mold to
form a plate-shaped metal-grade silicon substrate, a step of
dissolving a surface of said metal-grade silicon substrate with a
metal solvent and thereafter precipitating silicon in said metal
solvent on the surface of said metal-grade silicon substrate to
form a first silicon layer, and a step of further forming a second
silicon layer on a surface of said first silicon layer by a
liquid-phase growth method using indium.
8. A fabrication process of solar cell according to claim 7 wherein
a material for said mold is selected from the group consisting of
carbon graphite, silicon carbide, and silicon nitride.
9. A fabrication process of solar cell according to claim 7 or 8,
wherein a surface of said mold to be in contact with said
metal-grade silicon substrate is coated with a mold releasing agent
containing at least silicon nitride.
10. A fabrication process of solar cell according to claim 7,
wherein said first silicon layer is p-type silicon containing 10
ppm to 100 ppm of impurities.
11. A fabrication process of solar cell according to claim 7,
wherein said metal solvent is selected from the group consisting of
indium, gallium, and tin.
12. A fabrication process of solar cell according to claim 1,
wherein a thickness of said silicon layer is 10 .mu.m or more.
13. A fabrication process of solar cell according to claim 7,
wherein a thickness of said silicon layer is 10 .mu.m or more.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a fabrication process of
solar cell. More specifically, the present invention relates to a
process of fabricating a solar cell suitably applicable to a solar
cell comprising stacked thin films of polycrystalline silicon on a
cheap substrate.
[0003] 2. Related Background Art
[0004] Solar cells are widely studied and some are in practical
use, as driving energy sources of various devices or as power
supplies connected to the commercial power supply system.
[0005] A requirement for the solar cells in respect of cost is that
an element can be formed on a cheap substrate like metal. On the
other hand, silicon is normally used as a semiconductor for making
the solar cells. Among others, single-crystal silicon is most
excellent from the viewpoint of efficiency for converting light
energy to electromotive force, i.e., from the viewpoint of
photoelectric conversion efficiency. It is, however, said that
amorphous silicon is more advantageous from the viewpoints of
increase in area and decrease in cost. In recent years, use of
polycrystalline silicon is under study, for the purpose of
achieving low cost comparable to that of amorphous silicon and high
energy conversion efficiency comparable to that of single-crystal
silicon. In conventionally proposed methods as to such
single-crystal silicon and polycrystalline silicon, a plate-shaped
substrate was obtained by slicing a massive crystal. It was thus
not easy to decrease the thickness of the substrate to below 0.3
mm. Therefore, the substrate had the thickness more than necessary
for sufficient absorption of light quantity, and effective
utilization of material was not enough. Namely, the substrate
needed to be thinned more in order to further decrease the
cost.
[0006] Proposed as a production method of polycrystalline silicon
substrate with the aim of decreasing the cost was a method for
forming a silicon sheet by a spin method of pouring a liquid
droplet of molten Si into a mold. This method achieved the minimum
thickness of about 0.1 to 0.2 mm, but the decrease of thickness was
not sufficient as compared with the thickness of film (20 to 50
.mu.m) necessary and sufficient for absorption of light as
crystalline Si. In addition, this thinning method had the problem
that it became difficult for the silicon sheet itself to maintain
the strength as a substrate, so that another cheap substrate was
inevitably necessitated for supporting the silicon sheet.
[0007] A report was made about attempts to make the solar cell by
forming a substrate of metal-grade silicon and thereafter forming a
silicon layer having a thickness necessary and sufficient for
absorption of light thereon by the liquid-phase growth method (T.
F. Ciszek, T. H. Wang, X. Wu, R. W. Burrows, J. Alleman, C. R.
Schwerdtfeger and T. Bekkedahl, "Si thin layer growth from metal
solution on single-crystal and cast metallurgical-grade
multicrystalline Si substrates," 23rd IEEE Photovoltaic specialists
Conference, (1993) p. 65).
[0008] In the above-stated method wherein the silicon layer was
made on the low-purity silicon substrate of metal-grade silicon,
using copper, aluminum, or tin as a metal solvent, however, the
metal as a solvent was left mainly in grain boundaries in either
case, because etchback for removing a native oxide film was carried
out in the initial stage of growth. Therefore, characteristics of
the solar cell were not sufficient. For the purpose of solving this
problem, another method was reported which used a copper and
aluminum alloy as the solvent without carrying out the etchback (T.
H. Wang, T. F. Ciszek, C. R. Schwerdtfeger, H. Moutinho, R.
Matson., "Growth of silicon thin layers on cast MG-Si from metal
solutions for solar cells," Solar Energy Materials and Solar Cells
41/42 (1996), p. 19), but this method has problems including
complex composition control of the alloy in regard to mass
production.
[0009] A method for making the substrate of metal-grade silicon is
also the same as the conventional polycrystalline process for
forming an ingot by the cast method and slicing it to obtain the
plate-shaped substrate. Such a method fails to make use of the
merit of metal-grade silicon as a cheap material.
SUMMARY OF THE INVENTION
[0010] The present invention has been accomplished in view of the
problems described above and an object of the present invention is
to provide a fabrication process of a cheap crystalline solar cell
with good characteristics.
[0011] Another object of the present invention is to provide a
fabrication process of a thin-film crystalline silicon solar cell
with good characteristics.
[0012] Another object of the present invention is to provide a
cheap solar cell necessitating no slicing step, by melting and
solidifying metal-grade silicon in a mold to form a plate-shaped
metal-grade silicon and forming a crystal silicon layer
thereon.
[0013] Still another object of the present invention is to provide
a fabrication process of solar cell comprising a step of forming a
silicon layer on a crystalline silicon substrate by a liquid-phase
growth method with a metal solvent, wherein a total concentration
of impurities of a surface of the crystalline silicon substrate is
10 ppm or more and wherein the metal solvent is indium.
[0014] A further object of the present invention is to provide a
fabrication process of solar cell comprising a step of melting and
solidifying particles of metal-grade silicon put in a mold to form
a plate-shaped metal-grade silicon substrate, and a step of forming
a silicon layer on a surface of the metal-grade silicon substrate
by a liquid-phase growth method using indium.
[0015] A still further object of the present invention is to
provide a fabrication process of solar cell comprising a step of
melting and solidifying particles of metal-grade silicon put in a
mold to form a plate-shaped metal-grade silicon substrate, a step
of dissolving a surface of the metal-grade silicon substrate in a
metal solvent and thereafter precipitating silicon in the metal
solvent on the surface of the metal-grade silicon substrate to form
a first silicon layer, and a step of forming a second silicon layer
on a surface of the first silicon layer by a liquid-phase growth
method using indium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIGS. 1A, 1B, 1C, 1D, 1E and 1F are schematic,
cross-sectional views for explaining an example of steps of a
fabrication process of solar cell according to the present
invention; and
[0017] FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G and 2H are schematic,
cross-sectional views for explaining an example of steps of another
fabrication process of solar cell according to the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] The operation of the present invention will be described
referring to FIGS. 1A to 1F and FIGS. 2A to 2H of schematic,
cross-sectional views each to show the main steps in a fabrication
process of the present invention.
[0019] In the figures, each of 101, 103, 201, and 203 designates a
mold; each of 102 and 202 metal-grade silicon; each of 104 and 204
a silicon plate (hereinafter, also referred to as "silicon sheet");
each of 105, 205, and 210 a carbon boat; each of 106 and 211
indium; each of 106 and 206 a metal solvent; 207 reprecipitated
silicon; each of 107 and 208 a silicon (active) layer; each of 108
and 209 an n.sup.+ layer.
[0020] (a-1) First, particulate metal-grade silicon are charged
into a mold having a laterally plate-shaped groove (FIG. 1A). The
silicon may be particles granules, powder or the like.
[0021] (a-2) This mold is put in an electric furnace and is kept at
a temperature higher than the melting point of silicon (approx.
1415.degree. C.) for a certain period of time, thereby melting the
metal-grade silicon. Then the temperature is lowered to solidify
the silicon to make a plate-shaped substrate (FIGS. 1B and 1C).
[0022] (a-3) The metal-grade silicon substrate thus obtained is
placed in a boat made of carbon graphite or the like, and a silicon
layer is deposited on this substrate by the liquid-phase growth
method using indium in a hydrogen atmosphere (FIGS. 1D and 1E).
[0023] (a-4) A junction is formed on the surface of this silicon
layer, thus fabricating a solar cell (FIG. 1F).
[0024] Another fabrication process of the present invention is
carried out as follows.
[0025] (b-1) Particulate metal-grade silicon are charged into the
mold having the laterally plate-shaped groove (FIG. 2A).
[0026] (b-2) This mold is put in the electric furnace and is kept
at a temperature higher than the melting point of silicon (approx.
1415.degree. C.) for a certain period of time, thereby melting the
metal-grade silicon. Then the temperature is lowered to solidify
the silicon to make a plate-shaped substrate (FIGS. 2B and 2C).
[0027] (b-3) The metal-grade silicon substrate thus obtained is
placed in the boat made of carbon graphite or the like, and the
boat is put in the electric furnace with a metal solvent, for
example indium, being on and in contact with the metal-grade
silicon substrate, thereby dissolving a surface layer of the
metal-grade silicon substrate into the solvent. Then the
temperature is lowered to make silicon saturated or supersaturated
in the solvent and silicon is again precipitated on the surface of
the metal-grade silicon substrate. The silicon precipitated at this
time is of the p-type (p.sup.+) (FIGS. 2D and 2E).
[0028] (b-4) The metal-grade silicon substrate thus obtained is
placed in the boat made of carbon graphite or the like, and a
silicon layer is further deposited on this substrate by the
liquid-phase growth method using indium in a hydrogen atmosphere
(FIGS. 2F and 2G).
[0029] (b-5) A junction is formed on the surface of this silicon
layer, thus fabricating a solar cell (FIG. 2H).
[0030] According to these methods, the silicon layer with less
mixture of impurities, suitable for solar cell, can be formed by
carrying out the liquid-phase growth with indium in contact with
the surface of metal-grade silicon substrate. Since the
plate-shaped metal-grade silicon substrate is directly formed by
using the mold, the time-consuming, cumbersome step of slicing the
substrate in the conventional cast method is excluded. Further,
before the liquid-phase growth with indium, the surface layer of
substrate is dissolved in the metal solvent and thereafter silicon
is reprecipitated, whereby most impurities can be removed from the
surface layer by the segregation effect. Owing to this, the silicon
layer can be formed with high quality by the liquid-phase growth on
the precipitated layer. At this time, boron B among the impurities
contained in the substrate tends to remain in the reprecipitated
layer. Thus, the reprecipitated layer becomes of the p-type
(p.sup.+). This p-type (p.sup.+) layer can be utilized as a BSF
(Back Surface Field) layer upon fabrication of solar cell.
[0031] Repetitively conducting many experiments, the inventor found
out that thin-film crystalline solar cells with good
characteristics were made by depositing a silicon layer on the
surface of a low-purity silicon substrate like metal-grade silicon
by the liquid-phase growth method using indium. The inventor also
found out that thin-film crystalline solar cells with good
characteristics were formed by depositing a silicon layer on the
surface of a plate-shaped metal-grade silicon which was formed by
melting and then solidifying in the mold, by the liquid-phase
growth method using indium. In addition, the inventor further found
that better solar cells were formed by depositing a silicon layer
by the liquid-phase growth method using indium on a layer obtained
after the surface layer of a plate-shaped metal-grade silicon
melted and then solidified in the mold was dissolved and
reprecipitated in the metal solvent. The fabrication processes of
solar cell according to the present invention will be described in
detail.
[0032] The liquid-phase growth method used in the present invention
is carried out under a hydrogen atmosphere for the purpose of
removing a native oxide film present in the surface of silicon
substrate. The temperature of growth is selected preferably in the
range of 500 to 1100.degree. C. and more preferably in the range of
700 to 1050.degree. C.
[0033] Indium used in the present invention is one having a high
purity in the range of 99.9% to 99.9999%.
[0034] The liquid-phase growth method applied in the present
invention is normally the annealing method or the temperature
difference method, but the isothermal method by the inventor
(Japanese Patent Application Laid-Open No. 6-191987) can also be
employed.
[0035] The metal-grade silicon used for the substrate for solar
cell of the present invention is one of low purity, specifically,
one containing 0.1% to 2% of impurity elements, which is cheap and
easy to use. It is also possible to reduce an amount of impurities
by forming the metal-grade silicon into particles, granules or
powder and preliminarily carrying out treatment with an acid such
as hydrochloric acid with necessity before melting it.
[0036] The mold used in the present invention is a one having a
lateral or longitudinal plate-shaped groove or a plurality of such
grooves may be provided in one mold. The material for the mold is
carbon graphite in regard to easiness of processing and price, but
the material may also be selected from any materials to which a
material for releasing silicon melted and then solidified can be
applied and which have melting points higher than that of silicon.
Silicon carbide, silicon nitride, or boron nitride can also be
used. When flow of heat is controlled upon solidification by
asymmetrically shaping the mold in the vertical direction for the
lateral groove or in the horizontal direction for the longitudinal
groove, or by providing the mold with heat-radiating plates, it is
also possible to segregate the impurities in the metal-grade
silicon on one surface side of sheet of the metal-grade silicon and
to increase the crystal grain size.
[0037] The mold releasing agent applied to the inside of the mold,
used in the present invention, is selected from those having large
contact angles and not reacting with molten silicon. A specific
example is a one containing Si.sub.3N.sub.4 as a main ingredient,
and if necessary, SiO.sub.2 or the like is added. The way to form
the coating of the mold releasing agent within the mold is a method
for spraying an organic solution or a silanol solution containing
powder of Si.sub.3N.sub.4 dispersed therein into the mold and
thermally treating it at the temperature of 400.degree. C. or more,
thereby forming the coating.
[0038] The metal solvent for dissolving and reprecipitating the
surface of metal-grade silicon substrate, employed in the present
invention, is selected from those having relatively low melting
points and sufficiently dissolving the surface layer of silicon
substrate. As examples of such metal solvents, indium, gallium, tin
and the like are preferred.
[0039] Carbon graphite is mainly used for the boat upon the
liquid-phase growth using indium and for the boat upon dissolution
and reprecipitation of the surface of metal-grade silicon sheet by
the metal solvent, used in the present invention. In addition,
silicon carbide, silicon nitride, or the like can also be applied.
The method for bringing the metal solvent into contact with the
surface of silicon sheet is mainly the slide method or the dipping
method.
[0040] The furnace used in the present invention is preferably an
electric furnace in respect of controllability. The furnace should
be a one capable of stably keeping the temperature up to above the
melting point of silicon, and is preferably a one capable of
decreasing the temperature at the rate of approximately -30.degree.
C./min or less in respect of maintaining crystallinity of the sheet
formed by solidification. The furnace used in
dissolution/reprecipitation of the surface of sheet substrate by
the metal solvent is also pursuant to the electric furnace stated
above.
[0041] The experiments conducted by the inventor for achieving the
fabrication processes of solar cell stated above will be described
in detail.
[0042] Experiment 1
[0043] This experiment was conducted to investigate the relation
between impurity concentrations of the surface of a single-crystal
substrate and characteristics of a solar cell made using a silicon
layer grown in liquid phase on the single-crystal substrate, as an
active layer (power generating layer).
[0044] P.sup.+ (100) single-crystal silicon substrates having
specific resistance of 0.01 .OMEGA..cndot.cm were intentionally
polluted with impurities of Cu, Fe, Ti, and so on, so as to prepare
substrates having four levels of impurity concentration. On each
substrate, a silicon layer was grown in liquid phase with a solvent
of indium or tin in a hydrogen atmosphere. After the liquid-phase
growth, the solar cells were fabricated and characteristics thereof
were measured. At the same time, the characteristics of the thus
fabricated solar sells were compared with standard characteristics
of a solar cell fabricated after the silicon layer was grown on a
substrate which was not polluted with impurity, in the same manner
using the solvent of indium or tin.
1 TABLE 1 Total concentration of impurities 1 ppm 10 ppm 100 ppm
1000 ppm Characteristics Indium AA AA AA A of solar cell solvent
Tin A B C C solvent (AA: equivalent to standard, A: slightly
inferior, B: inferior, C: considerably inferior)
[0045] The results, as shown in Table 1, showed that with use of
the solvent of tin the characteristics (conversion efficiency) were
extremely degraded when the total concentration of impurities was
10 ppm or more, whereas with use of the solvent of indium the
characteristics exhibited no great change even when the substrate
was polluted in about 1000 ppm.
[0046] This clarified that when a silicon layer was grown on a
single-crystal silicon substrate by the liquid-phase growth method
using the solvent of indium, the silicon layer exhibiting good
characteristics of solar cell was formed even when the impurity
concentration of substrate was high.
[0047] Experiment 2
[0048] This experiment was conducted to investigate the relation
between impurity concentrations of the surface of a polycrystalline
substrate and characteristics of a solar cell made using a silicon
layer grown in liquid phase on the polycrystalline substrate, as an
active layer (power generating layer).
[0049] P.sup.+ (100) polycrystalline silicon substrates having
specific resistance of 0.01 .OMEGA..cndot.cm, obtained by slicing a
cast ingot, were intentionally polluted with impurities of Cu, Fe,
Ti, and so on, so as to prepare substrates having four levels of
impurity concentration. On each substrate, the silicon layer was
grown in liquid phase with the solvent of indium or tin in the
hydrogen atmosphere. After the liquid-phase growth, the solar cells
were fabricated and characteristics thereof were measured. At the
same time, the characteristics of the thus fabricated solar cells
were compared with standard characteristics of a solar cell
fabricated after the silicon layer was grown on a substrate which
was not polluted with impurity, in the same manner with the solvent
of indium or tin.
2 TABLE 2 Total concentration of impurities 1 ppm 10 ppm 100 ppm
1000 ppm Characteristics Indium AA AA AA A of solar cell solvent
Tin A C C C solvent (AA: equivalent to standard, A: slightly
inferior, B: inferior, C: considerably inferior)
[0050] The results, as shown in Table 2, showed that with use of
the solvent of tin the characteristics (conversion efficiency) were
extremely degraded when the total concentration of impurities was
10 ppm or more, whereas with use of the solvent of indium the
characteristics exhibited no great change even when the substrate
was polluted in about 1000 ppm.
[0051] This clarified that when a silicon layer was grown on a
polycrystalline silicon substrate by the liquid-phase growth method
using the solvent of indium, the silicon layer exhibiting good
characteristics of solar cell was formed even when the impurity
concentration of substrate was high.
[0052] From the results of Experiments 1 and 2, it was found that a
silicon layer with good quality was obtained even when the silicon
layer was formed on a crystalline silicon substrate having the
impurity concentration of 10 ppm or more by the liquid-phase growth
method using the solvent of indium.
[0053] Reasons of this are possibly (1) that since indium is likely
to form an alloy with metal elements, impurities soaking out of the
substrate thus remain in the indium solvent and (2) that indium is
hard to go into the silicon layer during deposition (for example,
the indium concentration is 5.times.10.sup.14/cm.sup.3 or less in
the silicon layer grown in liquid phase at 950.degree. C.).
[0054] Experiment 3
[0055] The present experiment was conducted to investigate the
method for forming the sheet-shaped (herein after, also referred to
as "plate-shaped") metal-grade silicon by melting and solidifying
particulate metal-grade silicon.
[0056] As shown in FIG. 1A, the mold 101 was made of carbon with a
laterally plate-shaped groove and a coating of Si.sub.3N.sub.4 film
was formed on the surface of the groove for the purpose of readily
taking solidified silicon out. Particulate metal-grade silicon 102
were charged into the groove in the mold and the mold capped with
lid (mold) 103 was placed in the electric furnace. Then the mold
was kept at a fixed temperature higher than the melting point of
silicon for a certain period of time to melt the particulate
metal-grade silicon. Then the temperature of the electric furnace
was decreased gradually to solidify the metal-grade silicon.
Plate-shaped sheet 104 solidified was taken out of the mold and
elemental analysis was conducted of a region near the surface of
the sheet. Table 3 shows results of impurity analysis of the
metal-grade silicon of the raw material (particulate metal-grade
silicon) and the sheet obtained.
3 TABLE 3 Particulate Sheet-shaped Impurities metal-grade Si
metal-grade Si B 50 ppm 60 ppm Al 4500 ppm 350 ppm Ni 510 ppm 20
ppm Fe 8200 ppm 4 ppm Cr 370 ppm 2 ppm Mn 130 ppm 1 ppm Ti 250 ppm
4 ppm
[0057] The impurities decreased greatly on the whole and the total
concentration was reduced to about {fraction (1/30)}.
[0058] Grain boundaries were visualized by Secco etching, which
indicated that the crystal grain size of the obtained sheet was
increased to several mm to several cm and was equivalent to that in
the case of the silicon ingot obtained by the conventional casting
method.
[0059] Experiment 4
[0060] The present experiment was conducted to investigate the
method for forming the p.sup.+-silicon layer with less impurities
on the sheet-shaped metal-grade silicon substrate, i.e., silicon
sheet. The sheet obtained in Experiment 3 was placed in the carbon
boat 205 as shown in FIG. 2D, the metal solvent 206 of indium was
brought into contact with the sheet thereon, and the boat was
placed in the electric furnace to be kept at 1000.degree. C.,
thereby dissolving the surface layer of metal-grade silicon sheet
into the indium solvent. This state was kept for a while and after
sufficient saturation the temperature was decreased with
controlling the electric furnace, thereby again precipitating
silicon in the solvent onto the surface of the silicon sheet. After
precipitation for a fixed period of time, the indium solvent in
contact with the sheet was removed to obtain the desired silicon
precipitate layer 207.
[0061] Table 4 shows results of analysis of elements contained in
the surface of the obtained sheet, i.e., in the silicon layer
precipitated on the surface of silicon sheet.
4 TABLE 4 Impurities Precipitate Si layer B 10 ppm Al 3 ppm Ni
<5 ppm Fe 2.5 ppm Cr 0.6 ppm Mn <0.2 ppm Ti <1 ppm
[0062] From Table 4, it was confirmed that the total concentration
of impurities in the precipitated silicon layer was further
decreased to {fraction (1/20)} or less of that of the sheet-shaped
metal-grade silicon.
[0063] Determination of pn was made by the thermoelectromotive
force method, which showed that the precipitated silicon layer 207
was of the p-type (p.sup.+).
[0064] Experiment 5
[0065] In the present experiment, a silicon layer was formed on the
metal-grade silicon sheets obtained in Experiments 3 and 4 by the
liquid-phase growth method using the indium solvent, and surface
morphology thereof was investigated.
[0066] The sheet substrate 104 made in Experiment 3 was placed in
the boat of carbon graphite as shown in FIG. 1D and the silicon
layer 107 with a thickness of 30 .mu.m was formed at the
temperature-decreasing rate of -1.degree. C./min with the growth
start temperature of 950.degree. C. and the supercooling
temperature of 4.degree. C. in the hydrogen atmosphere. The silicon
sheet having the formed silicon layer 107 will be referred to as
sheet 1. In the same manner, the sheet substrate having the
precipitated silicon layer (p.sup.+) 207 made in Experiment 4 was
placed in the boat of carbon graphite as shown in FIG. 2F, and the
silicon layer 208 with a thickness of 30 .mu.m was formed on the
p.sup.+ layer 207 of the sheet substrate at the
temperature-decreasing rate of -1.degree. C./min with the growth
start temperature of 950.degree. C. and the supercooling
temperature of 4.degree. C. in the hydrogen atmosphere. The silicon
sheet having the formed silicon layer 208 will be referred to as
sheet 2.
[0067] After formation of the silicon layers 107 and 208, the
surfaces of silicon layers 107 and 208 were observed with an
optical microscope and a scanning electron microscope. Results of
the observation showed that in either case the relatively flat
silicon layer 107 and 208 were obtained and each of them was
equivalent to the surface of the silicon sheet 104 or the p.sup.+
layer 207. In addition, crystal grain sizes of the silicon layer
107 and 208 were also close to those of the silicon sheets 104 and
204 as being a ground layer. Further, etch pit densities in the
surfaces of the silicon layers 107 and 208 thus grown were
approximately 1.times.10.sup.5 pits/cm.sup.2 and approximately
2.times.10.sup.4 pits/cm.sup.2, respectively.
[0068] Experiment 6
[0069] In the present experiment, thin-film solar cells were made
using as an active layer the silicon layer 107 on the sheet 1 and
the silicon layer on the sheet 2 formed in Experiment 5.
[0070] P was implanted into the surfaces of silicon layers 107 and
208 under the conditions of 80 keV and 1.times.10.sup.15/cm.sup.2
by the ion implantation method and the silicon layers were annealed
at 950.degree. C. for 30 minutes, thereby forming the n.sup.+
layers 108 and 209, respectively.
[0071] After that, transparent electrodes (ITO (0.085 .mu.m)) were
formed on the n.sup.+ layers 108 and 209, respectively, and
collector electrodes (Cr (0.02 .mu.m)/Ag (1 .mu.m)/Cr (0.004
.mu.m)) were formed on each transparent electrode, successively by
vacuum vapor deposition.
[0072] Al was evaporated on the back surface of sheet 1 and sheet 2
as being a substrate, thereby forming the back electrode.
[0073] For the thin-film crystalline solar cells thus fabricated,
I-V characteristics were measured under AM 1.5 (100 mW/cm.sup.2)
illumination. As the result of measurement, the solar cell using
the sheet 1 and having the cell area of 2 cm.sup.2 demonstrated the
open-circuit voltage of 0.56 V, the short-circuit photocurrent of
27 mA/cm.sup.2, the curve factor of 0.75, and the conversion
efficiency of 11.3%. The solar cell using the sheet 2 and having
the cell area of 2 cm.sup.2 demonstrated the open-circuit voltage
of 0.57 V, the short-circuit photocurrent of 28 mA/cm.sup.2, the
curve factor of 0.78, and the conversion efficiency of 12.4%.
[0074] From the results of the experiments as discussed above, it
is clear that a silicon layer can be obtained with good quality
when the silicon layer is made on the crystalline silicon substrate
having the impurity concentration of 10 ppm or more by the
liquid-phase growth method using the solvent of indium, and that
the thin-film crystalline solar cells can be made with good
characteristics by melting and solidifying the particulate
metal-grade silicon to form the silicon sheet, dissolving and
reprecipitating the surface thereof with the metal solvent to form
the p-type (p.sup.+) silicon layer 207, and thereafter forming a
silicon layer thereon by the liquid-phase growth method using the
solvent of indium.
[0075] The thickness of the silicon layer made in the present
invention is preferably 10 .mu.m or more, more preferably from 10
.mu.m to 100 .mu.m, and further more preferably from 10 .mu.m to 50
.mu.m, from the viewpoint of efficient absorption of light.
EXAMPLES
[0076] The fabrication processes of solar cell according to the
present invention will be described in more detail with examples,
but it is noted that the present invention is intended to be
limited to these examples.
Example 1
[0077] In the present example, a silicon layer was deposited on a
low-purity silicon wafer obtained by slicing an ingot using the
liquid-phase growth method using indium, and a thin-film solar cell
was made using it as an active layer.
[0078] An ingot was pulled up from the raw material of metal-grade
silicon of purity 98% by the CZ (Czochralski) method and it was
sliced in a wafer of 0.5 mm thick to obtain a metal-grade silicon
substrate. The elemental analysis was conducted for the region near
the surface of the metal-grade silicon substrate thus made, thereby
obtaining the results of Table 5.
5 TABLE 5 Impurities Metal-grade Si substrate B 8 ppm Al 2 ppm Ni
<5 ppm Fe 1 ppm Cr 0.6 ppm Mn <0.2 ppm Ti <1 ppm
[0079] The crystal grain size of the metal-grade silicon substrate
was several mm to several cm and the specific resistance thereof
was 0.05 .OMEGA..cndot.cm (p-type). The metal-grade silicon
substrate thus made was placed in the carbon boat, and a silicon
layer 50 .mu.m thick was formed at the temperature-decreasing rate
of -2.degree. C./min with the growth start temperature 950.degree.
C. and the supercooling temperature 7.degree. C. in the hydrogen
atmosphere by the liquid-phase growth method using the solvent of
indium.
[0080] Then thermal diffusion of P was effected at the temperature
of 900.degree. C. from a diffusion source of POCl.sub.3 into the
surface of the silicon layer to form an n.sup.+ layer, thereby
obtaining a junction depth of about 0.5 .mu.m. A dead layer in the
surface of the n.sup.+ layer thus formed was removed by etching,
thereby obtaining the junction depth of about 0.2 .mu.m having a
moderate surface concentration. Further, the transparent conductive
film, ITO about 0.1 .mu.m thick was formed on the n.sup.+ layer by
electron beam vapor deposition, and the collector electrode (Cr
(0.02 .mu.m)/Ag (1 .mu.m)/Cr (0.004 .mu.m)) was further formed
thereon by vacuum vapor deposition. The back electrode was made by
evaporating Al on the back surface of the sheet as being the
substrate.
[0081] For the thin-film crystalline solar cell thus fabricated,
the I-V characteristics were measured under AM 1.5 (100
mW/cm.sup.2) illumination and the solar cell with the cell area of
2 cm.sup.2 demonstrated the open-circuit voltage of 0.57 V, the
short-circuit photocurrent of 31 mA/cm.sup.2, the curve factor of
0.77, and the conversion of efficiency of 13.6%.
Example 2
[0082] In the present example, the particulate metal-grade silicon
was melted and solidified to make a sheet of metal-grade silicon, a
silicon layer was deposited thereon by the liquid-phase growth
method using indium, and the thin-film solar cell was made using
the silicon layer as an active layer. The fabrication process will
be described according to the procedures of the fabrication steps
shown in FIGS. 1A to 1F.
[0083] (1) Particulate metal-grade silicon (purity 98%) was soaked
in a mixture solution of hydrochloric acid and hydrogen peroxide
heated at 120.degree. C. to leach out impurities, followed by
washing with water and drying. Then the silicon was charged into
the groove in the mold of carbon graphite as shown in FIG. 1A. At
this time the coating for release of mold was formed on the
internal surface of the groove of mold by preliminarily applying a
silanol solution containing Si.sub.3N.sub.4 powder dispersed
therein to the inside of mold and thermally treating it at
400.degree. C.
[0084] (2) The mold was put in the electric furnace to be kept at a
constant temperature (1500.degree. C.) higher than the melting
point of silicon. After a lapse of appropriate time (30 minutes to
1 hour), it was annealed at the temperature-decreasing rate of
-10.degree. C./min to effect solidification, thereby obtaining a
sheet of metal-grade silicon (silicon sheet) 104.
[0085] Table 6 shows results of elemental analysis for the region
near the surface of the silicon sheet 104 thus made. From Table 6,
it was assured that the impurities in the silicon sheet were
decreased greatly as compared with the metal-grade silicon of raw
material. The crystal grain size of the silicon sheet was several
mm to several cm and the specific resistance thereof was 0.01
.OMEGA..cndot.cm (p-type).
6 TABLE 6 Impurities Si sheet B 12 ppm Al 30 ppm Ni <5 ppm Fe 1
ppm Cr 0.8 ppm Mn <0.2 ppm Ti <1 ppm
[0086] (3) The silicon sheet thus made was placed in the carbon
boat as shown in FIG. 1D, and a silicon layer 50 .mu.m thick was
formed at the temperature-decreasing rate of -1.5.degree. C./min
with the growth start temperature 950.degree. C. and the
supercooling temperature 5.degree. C. in the hydrogen atmosphere by
the liquid-phase growth method using the solvent of indium.
[0087] (4) Thermal diffusion of P was effected at the temperature
of 900.degree. C. from the diffusion source of POCl.sub.3 into the
surface of the silicon layer to form the n.sup.+ layer, thereby
obtaining the junction depth of about 0.5 .mu.m. Then the dead
layer in the surface of the n.sup.+ layer thus formed was removed
by etching, thereby obtaining the junction depth of about 0.2 .mu.m
having a moderate surface concentration.
[0088] (5) Further, an ITO film approximately 0.1 .mu.m thick was
made as a transparent electrode on the n.sup.+ layer by electron
beam vapor deposition.
[0089] (6) The collector electrode (Cr (0.02 .mu.m)/Ag (1 .mu.m)/Cr
(0.004 .mu.m)) was made on the transparent electrode by vacuum
vapor deposition.
[0090] (7) Al was evaporated on the back surface of the silicon
sheet as being the substrate, thus forming the back electrode.
[0091] For the thin-film crystalline solar cell thus fabricated,
the I-V characteristics were measured under AM 1.5 (100
mW/cm.sup.2) illumination. As the result of measurement, the solar
cell with the cell area of 2 cm.sup.2 demonstrated the open-circuit
voltage of 0.56 V, the short-circuit photocurrent of 30
mA/cm.sup.2, the curve factor of 0.78, and the conversion
efficiency of 13.1%.
Example 3
[0092] In the present example, a p.sup.+ silicon layer was formed
by bring the silicon sheet into contact with the metal solvent 206
of tin, a silicon layer was deposited thereon by the liquid-phase
growth method using indium, and the thin-film solar cell was made
using the silicon layer as an active layer. The fabrication process
will be described according to the procedures of the fabrication
steps shown in FIGS. 2A to 2H.
[0093] (1) Partculate metal-grade silicon was soaked in the mixture
solution of hydrochloric acid and hydrogen peroxide heated at
120.degree. C. to leach out impurities, followed by washing with
water and drying. Then the silicon was charged into the groove in
the mold of carbon graphite as shown in FIG. 2A. At this time the
coating for release of mold was formed on the internal surface of
the groove of mold by preliminarily applying the silanol solution
containing Si.sub.3N.sub.4 powder dispersed therein to the inside
of mold and thermally treating it at 400.degree. C.
[0094] (2) The mold was put in the electric furnace to be kept at a
constant temperature (1460.degree. C.) higher than the melting
point of silicon. After a lapse of appropriate time (30 minutes to
1 hour), it was annealed at the temperature-decreasing rate of
-6.degree. C./min to effect solidification, thereby obtaining a
sheet of metal-grade silicon (silicon sheet) 204.
[0095] (3) After the silicon sheet solidified was taken out, the
silicon sheet thus fabricated was placed in the carbon boat as
shown in FIG. 2D. Then the boat was put in the electric furnace
with the metal solvent of tin being brought into contact with the
silicon sheet thereon. The inside of the electric furnace was kept
at 1050.degree. C. to dissolve the surface layer of the silicon
sheet in the tin solvent. This state was maintained for several
hours to effect sufficient saturation, and then the temperature was
decreased at the rate of -3.degree. C./min with controlling the
electric furnace, thereby again precipitating silicon in the
solvent onto the surface of the silicon sheet. After the
precipitation of one hour, the boat was slid to take the tin
solvent away, thus obtaining a desired silicon precipitate layer
207.
[0096] Table 7 shows results of analysis of elements contained in
the surface of the obtained sheet, i.e., in the silicon layer 207
precipitated on the surface of the silicon sheet.
7 TABLE 7 Impurities Precipitate Si layer B 6 ppm Al 0.4 ppm Ni
<5 ppm Fe 1.4 ppm Cr 0.03 ppm Mn <0.2 ppm Ti <1 ppm
[0097] From Table 7, it is apparent that the amount of impurities
is decreased greatly in the silicon precipitate layer 207 as
compared with the particulate metal-grade silicon of raw material.
The thickness of the reprecipitated silicon layer obtained was
about 60 .mu.m from observation of cross section by SEM/EDX.
[0098] Determination of pn by the thermoelectromotive force method
showed that the precipitate silicon layer was of the p-type
(p.sup.+).
[0099] (4) The silicon sheet thus made was placed in the carbon
boat as shown in FIG. 2F, and a silicon layer 40 .mu.m thick was
further formed at the temperature-decreasing rate of -25.degree.
C./min with the growth start temperature 930.degree. C. and the
supercooling temperature 70.degree. C. in the hydrogen atmosphere
by the liquid-phase growth method using the solvent of indium.
[0100] (5) Thermal diffusion of P was effected at the temperature
of 900.degree. C. from the diffusion source of POCl.sub.3 into the
surface of the silicon layer to form a n.sup.+ layer, thereby
obtaining the junction depth of about 0.5 .mu.m. Then the dead
layer in the surface of the n.sup.+ layer thus formed was removed
by etching, thereby obtaining the junction depth of about 0.15
.mu.m having a moderate surface concentration.
[0101] (6) Further, an ITO film approximately 0.1 .mu.m thick was
made as a transparent electrode on the n.sup.+ layer by electron
beam vapor deposition.
[0102] (7) The collector electrode (Cr (0.02 .mu.m)/Ag (1 .mu.m)/Cr
(0.004 .mu.m)) was made on the transparent electrode by vacuum
vapor deposition.
[0103] (8) Al was evaporated on the back surface of the silicon
sheet as being the substrate, thus forming the back electrode.
[0104] For the thin-film crystalline solar cell thus fabricated,
the I-V characteristics were measured under AM 1.5 (100
mW/cm.sup.2) illumination. As the result of measurement, the solar
cell with the cell area of 2 cm.sup.2 demonstrated the open-circuit
voltage of 0.58 V, the short-circuit photocurrent of 31
mA/cm.sup.2, the curve factor of 0.76, and the conversion
efficiency of 13.7%.
Example 4
[0105] The present example is different from Example 3 in that the
solar cell was made using the metal solvent of gallium and the mold
of SiC.
[0106] The fabrication process will be described according to the
procedures of the fabrication steps of FIGS. 2A to 2H.
[0107] (1) The mold of SiC as shown in FIG. 2A was made, and
thereafter the silanol solution containing Si.sub.3N.sub.4 powder
dispersed therein was applied onto the internal surface of the
groove in the mold and was then thermally treated at 600.degree.
C., thereby forming the coating for release of mold.
[0108] (2) Particulate metal-grade silicon was soaked in the
mixture solution of hydrochloric acid and hydrogen peroxide heated
at 120.degree. C. to leach out impurities, followed by washing with
water and drying. Then the silicon was charged into the groove in
the mold.
[0109] (3) The mold was put in the electric furnace to be kept at a
constant temperature of 1480.degree. C. After a lapse of about 40
minutes, it was annealed at the temperature-decreasing rate of
-7.5.degree. C./min to effect solidification, thereby obtaining a
sheet of metal-grade silicon (silicon sheet) 204.
[0110] (4) After the silicon sheet solidified was taken out, the
silicon sheet thus fabricated was placed in the carbon boat as
shown in FIG. 2D. Then the boat was put in the electric furnace
with the metal solvent of gallium being brought into contact with
the silicon sheet thereon. The inside of the electric furnace was
kept at 650.degree. C. to dissolve the surface layer of the silicon
sheet in the gallium solvent. This state was maintained for several
hours to effect sufficient saturation, and then the temperature was
decreased at the rate of -4.degree. C./min with controlling the
electric furnace, thereby again precipitating silicon in the
solvent onto the surface of the silicon sheet. After the
precipitation of 30 minutes, the boat was slid to take the gallium
solvent away, thus obtaining a desired silicon precipitate layer
207.
[0111] Table 8 shows results of analysis of elements contained in
the surface of the obtained sheet, i.e., in the silicon layer 207
precipitated on the surface of the silicon sheet.
8 TABLE 8 Impurities Precipitate Si layer B 8 ppm Al <0.1 ppm Ni
<5 ppm Fe 0.2 ppm Cr <0.01 ppm Mn <0.2 ppm Ti <1
ppm
[0112] From Table 8, it is apparent that the amount of impurities
is decreased greatly in the silicon precipitate layer 207 as
compared with the particulate metal-grade silicon of raw material.
The thickness of the reprecipitated silicon layer obtained was
about 40 .mu.m from observation of cross section by SEM/EDX.
[0113] Determination of pn by the thermoelectromotive force method
showed that the preciptate silicon layer was of the p-type
(p.sup.+).
[0114] (5) The silicon sheet thus made was placed in the carbon
boat as shown in FIG. 2F, and a silicon layer 40 .mu.m thick was
further formed at the temperature-decreasing rate of -1.0.degree.
C./min with the growth start temperature 940.degree. C. and the
supercooling temperature 6.degree. C. in the hydrogen atmosphere by
the liquid-phase growth method using the solvent of indium.
[0115] (6) Thermal diffusion of P was effected at the temperature
of 900.degree. C. from the diffusion source of POCl.sub.3 into the
surface of the silicon layer to form an n.sup.+ layer, thereby
obtaining the junction depth of about 0.5 .mu.m. Then the dead
layer in the surface of the n.sup.+ layer thus formed was removed
by etching, thereby obtaining the junction depth of about 0.2 .mu.m
having a moderate surface concentration.
[0116] (7) Further, an ITO film approximately 0.1 .mu.m thick was
made as a transparent electrode on the n.sup.+ layer by electron
beam vapor deposition.
[0117] (8) The collector electrode (Cr (0.02 .mu.m)/Ag (1 .mu.m)/Cr
(0.004 .mu.m)) was made on the transparent electrode by vacuum
vapor deposition.
[0118] (9) Al was evaporated on the back surface of the silicon
sheet as being the substrate, thus forming the back electrode.
[0119] For the thin-film crystalline solar cell thus fabricated,
the I-V characteristics were measured under AM 1.5 (100
mW/cm.sup.2) illumination. As the result of measurement, the solar
cell with the cell area of 2 cm.sup.2 demonstrated the open-circuit
voltage of 0.56 V, the short-circuit photocurrent of 30
mA/cm.sup.2, the curve factor of 0.76, and the conversion
efficiency of 12.8%.
Example 5
[0120] The present example is different from Example 3 in that the
solar cell was made using the metal solvent of indium and the mold
of Si.sub.3N.sub.4.
[0121] The fabrication process will be described according to the
procedures of the fabrication steps of FIGS. 2A to 2H.
[0122] (1) The mold of Si.sub.3N.sub.4 as shown in FIG. 2A was
made. Particulate metal-grade silicon was soaked in the mixture
solution of hydrochloric acid and hydrogen peroxide heated at
120.degree. C. to leach out impurities, followed by washing with
water and drying. Then the silicon was charged into the groove in
the mold. The mold was put in the electric furnace to be kept at a
constant temperature of 1460.degree. C. After a lapse of one hour,
it was annealed at the temperature-decreasing rate of -5.degree.
C./min to effect solidification, thereby obtaining a sheet of
metal-grade silicon (silicon sheet) 204.
[0123] (2) After the plate-shaped silicon sheet solidified was
taken out, the silicon sheet thus obtained was placed in the carbon
boat as shown in FIG. 2D. Then the boat was put in the electric
furnace with the metal solvent of indium being brought into contact
with the silicon sheet thereon. The inside of the electric furnace
was kept at 800.degree. C. to dissolve the surface layer of the
silicon sheet in the indium solvent. This state was maintained for
several hours to effect sufficient saturation, and then the
temperature was decreased at the rate of -3.degree. C./min with
controlling the electric furnace, thereby again precipitating
silicon in the solvent onto the surface of the silicon sheet. After
the precipitation of two hours, the boat was slid to take the
indium solvent away, thus obtaining a desired silicon precipitate
layer 207.
[0124] Table 9 shows results of analysis of elements contained in
the surface of the obtained sheet, i.e., in the silicon layer 207
precipitated on the surface of the silicon sheet.
9 TABLE 9 Impurities Precipitate Si layer B 5 ppm Al <0.1 ppm Ni
<5 ppm Fe 0.1 ppm Cr <0.01 ppm Mn <0.2 ppm Ti <1
ppm
[0125] From Table 9, it is apparent that the amount of impurities
is decreased greatly in the silicon precipitate layer 207 as
compared with the particulate metal-grade silicon of raw material.
The thickness of the reprecipitated silicon layer obtained was
about 40 .mu.m from observation of cross section by SEM/EDX.
[0126] Determination of pn by the thermoelectromotive force method
showed that the precipitate silicon layer was of the p-type
(p.sup.+).
[0127] (3) Liquid-phase growth took place on the surface of this
silicon sheet, i.e., on the silicon layer 207 precipitated, using
another indium solvent different from that used in the previous
steps of dissolution and reprecipitation. That is, a silicon layer
50 .mu.m thick was further formed at the temperature-decreasing
rate of -1.degree. C./min with the growth start temperature
950.degree. C. and the supercooling temperature 40.degree. C. in
the hydrogen atmosphere.
[0128] (4) Thermal diffusion of P was effected at the temperature
of 900.degree. C. from the diffusion source of POCl.sub.3 into the
surface of the silicon layer to form an n.sup.+ layer, thereby
obtaining the junction depth of about 0.5 .mu.m. Then the dead
layer of the surface of the n.sup.+ layer thus formed was removed
by etching, thereby obtaining the junction depth of about 0.2 .mu.m
having a moderate surface concentration.
[0129] (5) Further, an ITO film approximately 0.1 .mu.m thick was
made as a transparent electrode on the n.sup.+ layer, made in step
(4), by electron beam vapor deposition.
[0130] (6) The collector electrode (Cr (0.02 .mu.m)/Ag (1 .mu.m)/Cr
(0.004 .mu.m)) was made on the transparent electrode by vacuum
vapor deposition.
[0131] (7) Al was evaporated on the back surface of the silicon
sheet as being the substrate, thus forming the back electrode.
[0132] For the thin-film crystalline solar cell thus fabricated,
the I-V characteristics were measured under AM 1.5 (100
mW/cm.sup.2) illumination. As the result of measurement, the solar
cell with the cell area of 2 cm.sup.2 demonstrated the open-circuit
voltage of 0.58 V, the short-circuit photocurrent of 31
mA/cm.sup.2, the curve factor of 0.78 and the conversion efficiency
of 14.0%.
[0133] Effects of the Invention
[0134] As described above, the present invention permits the
thin-film crystalline silicon solar cells with good characteristics
to be fabricated by the simple steps without slicing of an ingot.
Therefore, it can make the mass-producible, cheap, good-quality,
thin-film solar cells commercially available.
* * * * *