The most promising way to reduce the cost of silicon in solar cells while still maintaining performance is to utilize thin films (10 to 20 microns thick) of crystalline silicon. The method of solution growth is being employed to grow thin polycrystalline films of silicon on dissimilar substrates. The initial results indicate that, using tin as the solvent, this growth process only requires operating temperatures in the range of 800 C to 1000 C. Growth rates in the range of 0.4 to 2.0 microns per minute and grain sizes in the range of 20 to 100 microns were achieved on both quartz and coated steel substrates. Typically, an aspect ratio of two to three between the width and the Si grain thickness is seen. Uniform coverage of Si growth on quartz over a 2.5 x 2.5 cm area was observed

Barnett, A. M.

Hall, R. B.

Mcneely, J. B.

English

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Report Title
Report Type
Authors
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19711
-	 Silicon Film Solar Cell Process
-	 Quarterly
Robert B. Hall
g ames B. McNeely
Allen M. Barnett
Publication Date -
JPL Contract No. -
May 15, 1984
956769 - Report #1
This work was performer: for the Jet Propulsion Laboratory,
California Institute of Technology, and was sponsored by the
U.S. Department of Energy through an agreement with the National
Aeronautics and Space Administration.
]his report was prepared as an account of work sponsored by
an agency of the Uni'Led States Government. 	 Neither the United
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ment or any agency thereof.
(NISA -CR - 174446)
	 SILICCN E11r SCLAB CPI.L
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ABSTRACT
The most promising way to reduce the cost of silicon in
solar cells while still m3intairing performance is to utilize
thin films (10-20 microns thick) of crystalline silicon.
	 The
method of solution growth is being employed to grow thin poly-
crystalline films of silicon on dissimilar substrates.
	
The
initial results indicate that, using tin as the solvent, this
growth process only requires operating temperatures in the
range of 800"C to 1000°C.	 Growth rates in the range of 0.4 to
2.0 microns per i,.inute and grain sizes in the range of 20 to
100 microns have been achieved on both quartz and coated steel
substrates.	 Typically, an aspect ratio 	 of two to three between
the width and the Si grain thickness is seen. 	 Uniform coverage
of Si growth on quartz over a 2.5 x 2.5 cm area has been observed
1
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TABLE OF CONTENTS
Page
i
11
Abstract
Table of Contents
List of Figures
I. Introduction
II. Technical Discussion
III. Conclusions
IV. Work Planned
V. New Technology
VI. References
Appendix - Time Schedule
1
3
6
8
8
9
ii
krf
LIST OF FIGURES
Figure	 Page	 Description
1	 4	 (a) Silicon crystals on quartz
( 1. 3 mm x 1. 0 min)
4	 (b) Silicon crystals on steel
( 1 .3 nom x 1 . 0 mm )
2	 5	 (a) Silicon-Film on quartz
(0.71 mm x 0.54 min)
6	 (b) Silicon-Film on steel
(0.71 mm x 0.55)
3	 7	 EDAX of Silicon-Film on steel
i i i
I.	 Introduction
The Silicon-Film process is a method for fabricating solar
cells on low-cost substrates.
	 The method is directed to the
growth of films of solar cell quality semiconductors on either
metallurgical grade (low-cost) semiconductor substrates or on
metal substrates.
	 This method is being employed to grow solar
cell quality Silicon-Films on metallurgical grade silicon sub-
strates.	 Solar cell quality layers have been demonstrated on
semiconductor grade silicon substrates using a variation of the
Silicon-Film process.
	 Epitaxial layers of similar quality using
a different growth process have been demonstrated on metallurgi-
cal grade silicon by RCA.	 These RCA-grown 1:yers have resulted
in solar cell efficiencies greater than 13%.
Thin film polycrystalline silicon has great potentia l for
achieving low cost.	 To date, polycrystalline silicon has not
been demonstrated with high efficiency in a low cost, thin film
configuration.	 The higher efficiencies for polycrystalline sili-
con are based on devices made on considerably higher cost substrates.
Thin polycrystalline silicon cells can achieve energy conver-
sion efficiencies close to those demonstrated by single crystal
material with the application of advanced optical designs and
passivation of the surfaces (including grain boundaries).
	 The
maximum performance for any solar cell design can be determined
using fundamental material and device parameters.
	 The generalized
photovoltaic solar cell is described as having five layers:
	 the
opaq!:e electrical contact, the photon-absorber minority-carrier
generator, the minority-carrier collector majority-carrier conver-
ter, the transparent electrical contact and an anti-reflection
coating.	 An optimized solar cell includes a back suface field,
BSF, and a light trapping structure (1).
The calculation of the value for the performance requires
the thickness of each layer, the carrier concentration, the
minority carrier diffusion length (which varies with the doping
level), the absorption coefficient and the surface recombination.
fhe effects of an anti-reflection coating grid reflection, series
resistance losses and other losses (which includes contact resist-
ance, shunt effects, etc.) are included (2). 	 Additionally, a
light enhancement term (due to light trapping in the silicon)
which increases short circuit current has been calculated (3).
It has been indicated that this term increases the effective
optical thickness of the material up to a factor of 50 for sili-
con (4).	 From the analysis, it can be shown that for a polycrystal-
line Si film thickness of 20 microns, solar cell efficiencies as
high as 19.4% can be achieved (5).
-1-
The Silicon-Film process is designed to significantly
reduce the silicon cost while retaining the physical character-
istics of single crystal silicon.
	 The most promising approach
for significant cost reduction is the reduction of silicon
thickness. Additional cost reductions occur due to the use of
a lower cost substrate and the utilization of continuous manu-
facturing technology.
The principal techniques for growing semiconductors onto
foreign substrates are melt growth, vapor phase growth, solid
state growth (which is often recrystallization of vapor growth)
and solution growth.
	 Several investigators have successfully
grown films of silicon from the melt on graphite or graphite-
coated ceramics (6,7).These processes offer the advantage of
relatively high growth rates.
	 The disadvantages of growing
silicon films from the melt are primarily based on the high
temperature, 1415°C, which is required to melt silicon.
	 This
temperature effectively limits the substrate selection to
graphite and ceramics. The potential problems include contami-
nation of the silicon from the substrate and the absolute temp-
erature control required for good crystal growth.
Several techniques for growing from the vapor onto foreign
substrates have also been reported (8,9,10). 	 These processes
offer the advantage of lower temperatures and acceptable growth
rates of several microns per minute.
	 The disadvantages include
small grains and contamination of the grains and grain boundaries
from the growth medium. 	 The small grain problem has been solved
by recrystallization and the contamination problem resolved by
growing a new active semiconductor layer from the vapor onto the
recrystallized substrates. (9,10).	 The recrystallization approaches
result in relatively thick and highly doped silicon initial layer
qrowth with short diffusion lengths which limit the potential for
optical enhancement.
Two hybrid solution growth approaches have been reported.
The first deposited silicon from the vapor (in vacuum) on heated
aluminum substrates (11).	 Large grains of silicon were grown
from the silicon-aluminum solution which formed.	 The silicon was
saturated with aluminum 4hich led to minority carrier diffusion
lengths that were too short for satisfactory solar cell operation.
Another approach, chemical vapor deposition of silicon onto heated
tin-coated graphite substrates, led to large grain growth from tin
solution (12).	 Apparently, diffusion of impurities from the graph-
ite into the silicon led to unacceptable low minority carrier
diffusion lengths.
This work is based on growth from saturated metal solutions.
The growth process when performed on similar or lattice matched
substrates is called liquid phase epitaxial growth.	 Liquid phase
epitaxy will, in general, produce material that forms devices
that are superior in performance to those grown by other methods
(13).	 These superior performance devices include light emitting
diodes, semicondcutor lasers, magnetic garnet bubble memories and
-2-
GaAs solar cells.
	 The improved performance of liquid phase
epitaxial devices, when compared to vapor phase or ditfused
devices, can be attributed to the exact stoichiometry control,
longer diffusion lengths, fewer deep levels and the tend ncy of
	 p
impurities to segregate to the liquid rather than the solid.
Liquid phase epitaxy also has the tendency to anneal out dislo-
cations in the substrate.
	 z
A general procedure for the anal^;is of crystal growth
from solution has been described (14).
	 For a good quality
crystalline overlayer on a substrate, the growth process can
be separated into the following four steps:
	 1) wetting, 2)
nucleation, 3) non-impinging crystal growth and 4) film crystal
growth.	 The division of this generalized procedure into its
four component parts leads to a more comprehensive analysis of
each of the required steps.	 These four steps will be discussed
and illustrated in the Technical Discussion section which follows.
Growth from saturated solutions for the growth of solar cell
quallLy silicon on metal and glass substrates offers the advantages
of low growth temperature, demonstrated large grain growth, long
minority carrier diffusion lengths and the potential for annealing
out some of the strains and dislocations caused by the dissimilar
substrates.
	 Some disadvantages are that this process has not
been previously demonstrated on dissimilar substrates and that
high through-put manufacturing technology has not been demonstrated.
II.	 Technical Discussion
Tin has been employed as the solvent for silicon since long
minority carrier lifetimes can be maintained in silicon saturated
with tin.	 Solar cells grown from saturated tin solutions onto
silicon substrates have been reported (15).
	 The semiconductor-film
process has been used to grow silicon onto 0.010 inch thick steel
and 0.040 inch thick quartz substrates.	 The steel has been coated
with silicon carbide to provide a metallurgical barrier.
	 The
quartz was also covered with silicon carbide. 	 Growth temperatures
in the range of 800°C to 1000°C were used for silicon growth.
Both ramp cooling at 0.1° to 3°C per minute and steady state growth
techniques with a temperature gradient (16) between 10 0 and 100° C
per centimeter were used.	 Growth rates for silicon ranged from
0.5 to 4 microns per minute.
Wetting, which relates to the bonding between the nutrient
medium and the substrate, plus between the crystalline semicon-
ductor and the substrate, has been accomplished on both steel and
quartz substrates.
Nucleation, which relates to the formation step of semicon-
ductor crystallites on the substrate, has also been achieved on
both the steel and quartz substrates.	 Nucleation needs to be well
controlled so that the crystal initiation and the crystal growth
steps may be separated.
- 3-
a) Silicon Crystals
on Quartz
1.3 mm x 1.0 mm
V
Non-impinging crystal growth relates to the initial stages
of growth of the crystals produced by the nucleation step.
	 The
growth rate of the crystals is determined by the flux of semi-
conductor from the nutrient medium across the convection free
boundary layer at the crystal/medium interface.
	 The growth of
non-impinging silicon grains on quartz and steel are shown in
Figure 1.
	 The silicon grains on quartz arP 49 microns tall
and 104 microns wide on 120 micron spacing. 	 The silicon grains
on steel are 48 microns wide and 33 microns thick with 140 micron
spacing.
Figure 1
b) Silicon Crystals
on Steel
1.3 mm x 1.0 mm
ORIGINAL PAGE
MACK AND WHITE PHOTOGRAPH
-4-
Film crystal growth relate ,  to the final stage of crystallite
growth after the peripheries of adjace-it crystallites have begun
to closely approach and touch each other. 	 At this stage, the
film is comprised of a set of scallop-shaped crystallites on the
substrate.
	 The semiconductor flux to these impinging crystallites
must be reduced so as to allow nutrient and impurities trapped in
the grooves to diffuse back to the bulk medium. 	 Control of the
nutrient and the interface can lead to a relatively planar growth
surface with benign boundaries.
	 Silicon - films on quartz and steel
are shown in Fis ,ire 2.
	 Continuous silicon-films on quartz for
areas greater than one square centimeter 	 have been demonstrated.
Film thickness of twenty microns plus or minus 10% have been achieved.
Silicon-films on steel have demonstrated dendritic growth with incom-
plete fill-in between the dendrites.
	
One square millimeter films
50 microns thick have been grown.
Figure 2
a) Silicon-Film on Quartz
0.11 in 	 x 0.54 in
CRIGINAL PAGE
gU1CK AND WHITE PHOTOGRAPH
-5-
F igure _2
4
, Rr" A
h) Silicon -Filar on Steel
0.71 mm x 0.55 mill
the composition of a grown layer of silicon steel was
determined by EDAX to be silicon with a trace of tin; the
data are shown in Figure 3.
III.	 Conclusions
The initial results have derionstrated all phases of the Si
growth process on steel and quartz using the solution growth
technique.	 the growth achieved to date are typically 20-70
microns high with aspect ratios (width to height) of two to
three for individual grains.
ORIGINAL PAGEBLACK AND
 WHITE PHOTOGRAPH
-6-
Ik
Figure 3
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ORIGINAL PA4;  1§
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-7-
IV. Work Planned
During the next quarter, work will continue on quantifying
the growth steps.
	 Growth runs directed toward the achievement
of uniform and complete Si-film growth over two square centimeters
will be conducted.	 Experiments will be performed to examine the
minority carrifr diffusion length of these films.
V. New Technology
A.	 Growth of polycrystalline Si on quartz and
coated steel using solution growth.
A. M. Barnett, R. B. Hall and J. B. McNeely
are the innovators of this technology which
is being initially disclosed on the date of
this re p ort, pages I through 8 .
-8-
VI.	 REFERENCES
1. David Redfield, "Multiple-Pass Thin Film Silicon Solar Cell",
Appl. Physics Letters, Vol 25, No. 11, 647, (1974).
2. M. G. Mauk, and A. M. Barnett - to be published.
3. M. G. Mauk, "Silicon Solar Cell by Solution Growth", MSEE
Thesis, Universii:y of Delaware, (1984).
4. E. Yablonovitch, "Intensity Enhancement in Textured Optical
Sheets for Solar Cells", Proc. 4 th F.uurop ean Commu _niit	 Photo-
voltaic Sola r Energy Conf . , Stresa,Fl. ReNel , A
	 ( 1982).
5. A. M. Barnett, M. G. Mauk, J. C. Zolper, I. W. Hall, W. A.
Tiller, R. B. Hall and J. B. McNeely, "ihin-Film Silicon end
GaAs Solar Cells", 11th IEEE Photovoltaic Sp ec_. Corf.,
Orlando, (1984).
6. C. Belouet, C. Texier-Hervo, M. Mautref, C. Belin, J. Paulin
and J. Schneider, "Growth of Poly-Silicon Sheets on a Carbon
Shaper by the RAD Process", J. Crysta l Gr o wth, 61, 615, (1983).
7. S. B. Schuldt, J. D. Neaps, F. M. Schmit, J. D. Zook, and B.L.
Grunq, "Large-Area Silicon-on-Ceramic Substrates for Low-Cost
Solar Cells", Conf. Re c. 15 t h IEEE Photovoltaic Spec. Conf.,
Orlando, p. 934 19^1^.	 -	 ----_._----
8. C. Feldman, N. A. Blum, F. G. Satkiewicz, "Vacuum Deposited
Po l ycrystallir.e Silicon Solar Cell for Terrestrial Use", Conf.
Rec. 14th IEEE Photovoltaic SPec. Conf., San Diego, 391,
	 1980).
9. T. L. Chu, Shirle, S. Chu,
	 C. L. Lin and R. Abderrassoul,
"High-Efficiency lhii-Film Polycrystalline-Silicon Solar Cells",
J. Appl. Pt s., CO, 919, (1979).
10. I. A.Lesk, A. Baghdadle, R. W. Gurtler, R. J. Ellis, J. A. Wise
and M. G. Coleman, "Ribbon-to-Ribbon Crysta
	
Gruwtn", Proc. 12th
IEEE P hotovoltaic Sp ec. C onf., Baton Rouge, 173, (1976).
1=	 P. H. Fang and L. Ephrath, "Polycrystalline Silicon Films on
Aluminum Sheets for Solar Cell App l ications".
	
die d Physics
Letters, Vol. 25, Nu. 10, 583, (1974).
12. J. Bloem, L. J. Giling, M. W. M. Graef and H. H. C. DeMoor,
"The Growth of Polycrystalline Silicon Layers on Top of a
Tin-Coated Substrate:	 In-Situ Observations", Proc. 2nd
European Community_ Photovoltaic Solar Enerq ,_Conf., Berlin,
D. ReideI, 1S477197§ J .
13. R. L. Moon, "Liquid Phase Epitaxy", Crysta_1 Growth, 2nd ed.,
B. R. Pamplin, ed., Pergamon Press, p. 421 1
 (1980-).
-9-
VI.	 References (Cont'd)
14. A. M. Barnett, M. G. Mauk, J. C. Zolper and W. A. Tiller,
"Thin Film Depcsition for High-Performance, Low-Cost Photo-
voltaic Solar Cells", Proc. 5th European Communit
	
Photovol-
taic Solar Energy Conf., lthens, D. Reidel ,(1984 .
15. G. E. Possin, "Solution Growth of Silicon from Liquid Tin
for Application to Photovoltaic Solar Cells", Solid State
Electronics, V. 27, (2), pp. 167-176, (1984).
16. L. R. Dawson, "Liquid Phase Epitaxy", Progress in Solid State
Chemistry, Pergamon Press, Vol. 7, p. 117, (1972).
-10-
Qu--


Silicon film solar cell process

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