Note: Descriptions are shown in the official language in which they were submitted.
~;24S330
--1--
This invention relates to improved back re-
flector systems and photovoltaic devices utilizing
the same. The present invention has particular
applicability to photovoltaic devices formed from
layers of amorphous semiconductor alloys. The
back reflector systems of the present invention
provide increased reflection of unabsorbed light
back into the devices in which they are employed.
One advantage of this approach is that increased
photon absorption and charge carrier generation in
the active layers is possible to provide increased
short circuit currents. Another advantage is that
the improved photoresponsive characteristics of
fluorinated amorphous silicon alloys can be more
fully realized in photovoltaic devices by prac-
ticing the present 1nvention. The invention has
its most important application in making- improved
amorphous silicon alloy photovoltaic devices of
~ ` the p-i-n configuration, either as singIe cells or
;~ 20 multiple cells comprising a p1ura11ty of single
cell units. Preferably, the doped layers of the
p-i-n cells have low absorption coefficients in
:
the wavelength regions of interest to best utilize
the back reflector of the present invention.
:
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i2i~53i30
Silicon is the basis of the huge crystalline
semiconductor inclustry and is the material which
has produced expensive high efficiency ~18 per-
cent) crystalline solar cells for space applica-
tions. For terrestrial applications, the crystal-
line solar cells typically have much lower effi-
ciencies on the order of 12 percent or less. When
crystalline semiconductor technology reached a
commercial state, it became the foundation of the
present huge semiconductor device manufacturing
industry. This was due to the ability of the
scientist to grow substantially defect-free ger-
manium and particularly silicon crystals, and then
turn them into extrinsic materials with p-type and
n-type conductivity regions therein. This was ac-
complished by diffusing into such crystalline
material parts per million of donor (n) or accep-
tor ~p) dopant materials introduced as substitu-
tional impurities into the substantially pure
2n crystalline materials, to increase their electri-
cal conductivity and to control their being either
of a p or n conduction type. ~he fabrication pro-
cesses for ~aking p-n junction crystals involve
extremely complex, time consuming, and expensive
procedures. Thus, these crystalline materials
' I ' `; ! j ~
~533~
--3--
useful in solar cells and current control devices
are produced under very carefully controlled con-
ditions by growing individual single silicon or
germanium crystals, and when p-n junctions are
required, by doping such single crystals with
extremely small and critical amounts of dopants.
These crystal growing processes produce such
relatively small crystals that solar cells require
the assembly of many single crystals to encompass
the desired area of only a single solar cell
panel. The amount of energy necessary to make a
solar cell in this process, the limitat1on caused
by the slze limitations of the silicon crystal,
and the necessity to cut up and assemble such a
crystalline material have all resulted in an im-
possible economic barrier to the large scale use
of crystalline semiconductor solar cells for
energy conversion. Further, crystalline silicon
has an indirect optical edge which results in poor
light absorption in the material. Because of the
poor light absorption, crystalline solar cells
have to be at least 50 microns thick to absorb the
incident sunlight. Even if the single crystal
material is replaced bv polyerystalline silieon
with eheaper production processes, the indireet
.,
12~5330
optical edge is still maintained; hence the mate-
rial thicl~ness is not reduced. The polycrystal-
line material also involves the addition of grain
boundaries and other defect problems, ~hich de-
fects are ordinarily deleterious.
In su~ary, crystal silicon devices have
fixed parameters which are not variable as de-
sired, require large amounts of material, are only
produceable in relatively small areas and are ex-
pensive and time consuming to produce. Devicesbased upon amorphous silicon alloys can eliminate
these crystal silicon disadvantages. An amorphous
si1icon alloy has an optical absorption edge
having properties similar to a direct gap semicon-
ductor and only a material thickness of one micron
~; or less is necessary to absorb the same amount of
sunlight as the 50 micron thick crystalline sili-
con. Further, amorphous silicon alloys can be
~ made faster, easier and in larger areas than can
crystalline silicon.
Accordingly, a considerable effort has been
made to develop processes for readily depositing
amorphous semiconductor alloys or films, each of
~; which can encompass relatively large areas, if
desired, limited only by the size ofthe deposition
:
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~Z~533~
equipment, and which could be readily doped to
form p-type and n-type materials where p-n junc-
tion devices are to be made therefrom equivalent
to those produced by their crystalline counter-
parts. For many years such work was substantiallyunproductive. Amorphous silicon or germanium
(Group IV) films are normally four-fold coordi-
nated and were found to have microvoids and dan-
gling bonds and other defects which produce a high
density of localized states in the energy gap
thereof. The presence of a high density of local-
ized states in the energy gap of amorphous silicon
semiconductor films results in a low degree of
photoconductivity and short carrier lifetime,
making such films unsuitahle for photoresponsive
applications. Additionally, such films cannot be
successfully doped or otherwise modified to shift
the Fermi level close to the conduction or valence
~: :
bands, making them unsuitable for making p-n junc-
tions for solar cell and current control~device
applications.
In an attempt to minimize the aforementioned
problems involved with amorphous silicon (origi-
nally tho~ght to be elemental), W.E. Spear and
P.G. Le Comber of Carnegie Laboratory of Physics,
:
~2~S330
University of Dundee, in Dundee, Scotland, did
some work on "Substitutional ~oping of Amorphous
Silicon'l, as reported in a paper published in
Solid State Communications, Vol. 17, pp. 1193-
1196, 1975, toward the end of reducing the local-
ized states in the energy gap in amorphous silicon
to make the same approximate more closely intrin-
sic crystalline silicon and of substitutionally
doping the amorphous materials with suitable clas-
sic dopants, as in doping crystalline materials,
to make them extrinsic and of p or n conduction
types.
The reduction of the localized states was
accomplished by glow discharge deposition of amor-
phous silicon films wherein a gas of silane (SiH4)was passed through a reaction tube where the gas
was decomposed by an r.f. glow discharge and de-
posited on a substrate at a substrate temperature
of about 500-600K (227-327C). The material so
deposited on the substrate was an intrinsic amor-
phous material consisting of silicon and hydro-
gen. To produce a doped amorphous material a gas
of ~hosphine (PH3) for n-type conduction or a gas
of diborane (B2H6) for p-type conduction were pre-
mixed with the silane gas and passed through the
~ ~,
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:IL2~5330
glow discharge reaction tube under the same oper-
ating conditions. The gaseous concentration of
the dopants used was between about 5 x 10-6 and
10-2 parts per volume. The material so deposited
was shown to be extrinsic and of n or p conduction
type.
~ Ihile it was not known by these researchers,
it is now known by the work of others that the
hydrogen in the silane combines at an optimum tem-
perature with many of the dangling bonds of thesilicon during the glow discharge deposition, to
substantially reduce the density of the localized
states in the energy gap toward the end of~making
the electronic properties of the amorphous mate-
rial approximate more nearly those o the corres-
ponding crystalline material.
The incorporation of hydrogen in the above
method however has limitations based upon the
fixed ratio of hydrogen to silicon in silane, and
various Si:H bonding configurations which intro-
duce new antibonding states. Therefore, there are
basic limitations in reducing the density of
localized states in these materials.
Greatly improved amorphous silicon alloys
2~ having significantly reduced concentFations of
~ J
~Z~L5330
localized statea in the energy gaps thereof and high quality
electronic propertiea have been prepared by glow di~charge as
fully described in U.S. Patent No. 4,226,898, Amorphous
Semiconductors Equivalent to Crystalline Semiconductor~,
Stanford R. Ovshin~ky and Arun Madan which i~sued October 7,
1980, and by vapor depo~ition as fully de~cribed in U.S. Patent
No. 4,217,374, Stanford R. Ovshin~ky and Ma~atsugu Izu, which
i~sued on August 12, 1980, under the same title. A~ di~clo~ed
in the~e patents, fluorine is introduced into the amorphous
silicon ~emiconductor alloy to sub~tantially reduce the den~ity
of localized atate~ therein. Activated fluorine e~pecially
readily bonds to silicon in the amorphous body to aubstantially
decrease the denaity of localized defect state~, becau~e the
~mall size high reactivity of ~pecification of chemical bonding
of the fluorine atoms enables them to achieve a more defect-free
amorphou~ ailicon alloy. The fluorine bonda to the dangling
bond~ of the silicon and form~ what ia believed to be a
predominantly ionic ~table bond with flexible bonding angle~,
which results in a more stable and more efficient compen~ation
kh/rn
.~ .
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~Z~5330
or alteration than is formed by hydrogen and other
compensating or altering agents. Fluorine also
comhines in a preferable manner with silicon and
hydrogen, utilizing the hydrogen in a more desir-
able manner, since hydrogen has several bondingoptions. Without fluorine, hydrogen may not bond
in a desirable manner in the material, causing
extra defect status in the band gap as well as in
the material itself. Therefore, fluorine is con-
sidered to be a more efficient compensating or -
altering element than hydrogen when employed alone
or with hydrogen because of its high reactivity,
specificity in chemical honding, and high electro-
negativity,
As an example, compensation may be achieved
with fluorine alone or in combination with hydro-
gen with the adclition of these element(s) in very
small quantities (e.g., fractions of one atomic
percent). ~owever, the amounts of fluorine and
hydrogen most desirably used are much greater than
such small percentages so as to form a silicon-
hydrogen-fluorine alloy. Such alloying amounts of
fluorine and hydrogen may, for example, be in the
-` ~2~5330
1 0 -
range of 1 to 5 percent or greater. It is be-
lieved that the alloy so formed has a lower den-
sity of defect states in the energy gap than that
achieved by the mere neutralization of dangling
bonds and similar defect states. Such larger
amount of fluorine, in particular, is believed to
participate substantially in a new structural con-
figuration of an amorphous silicon-containing
material and facilitates the addition of other
alloying materials, such as germanium. Fluorine,
in addition to its other characteristics mentioned
herein, is believed to be an organizer of local
structure in the silicon-containing alloy through
inductive and ionic effects. It is believed that
fluorine also influences the bonding of hydroyen
by acting in a beneficial way to decrease the den-
sity of defect states which hydrogen contributes
while acting as a density of states reducing ele-
ment. The ionic role that fluorine plays in such
an alloy is believed to be an important factor in
terms of the nearest neighbor relationships.
Amorphous silicon alloys containing fluorine
have thus demonstrated greatly improved character-
istics for photovoltaic applications as compared
: ~ :
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~Z~5330
to amorphous silicon alloys containing just hydro-
gen alone as a density of states reducing ele-
ment. However, in order to realize the full
advantage of these amorphous silicon alloys con-
taining fluorine when used to form the activeregions of photovoltaic devices, it is necessary
to assure that the greatest possible portion of
the available photons are absorbed therein for
efficiently generating electron-hole pairs.
The foregoing is important in, for example,
photovoltaic devices of the p-i-n configuration.
Devices of this type have p and n-type doped
layers on opposite sides of an active intrinsic
layer, wherein the electron-hole pairs are gener-
a~ed. They establish a potential gradient across
the device to facilitate the separation of the
electrons and holes and also form contact layers
to facilitate the collectlon of the electrons and
holes as electrical current.
Not all of the available photons are absorbed
by the active regions. While almost all of the
shorter wavelength photons are absorbed, a large
portion of the longer wavelength photons with
energies near the absorption edge of the intrinsic
semiconductor material, are not absorbed. The
: :
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:~245330
-12-
loss of these unabsorbed photons reduces the cur-
rents which can be produced. To preclude the loss
of these longer wavelength photons, back reflec-
tors, formed from conductive metals have been
employed to reflect the unused or unabsorbed light
back into the active regions of the devices.
The p and n-type layers are conductive and
preferably have a low absorption coefficient for
wavelengths near the band edge, to decrease photon
absorption in those layers. A back reflector is
therefor extremely advantageous ~7hen used in con-
junction with a p-type layer having for example a
; wide band gap forming one of the doped layers of
such a device. Back reflecting layers therefore
serve to reflect unused light back into the in-
::
trinsic region of the device to permit further
; utilization of the sun energy for generating addi-
tional electron-hole pairs. A back reflecting
layer permits a greater portion of the available
photons to pass into the active intrinsic layer
and to be absorbed therein.
Unfortunately, the best back reflectors of
the prior art have~been capable of reflecting only~
about 80 percent of the unused light of the wave-
lengths of 1nterest back into the devices in which~
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12~533C~
they are employed. Noble metals such as copperand silver, and ~etals such as aluminum, because
they are highly conductive, have been suggested as
possible back reflector materials. However, these
metals can diffuse into the semiconductor of the
devices in which they are employed and, in doing
so, adversely effect the photoresponsive charac-
teristics of the devices. As a result, thin
layers of other less conductive and less reflec-
tive metals have been employed as diffusion bar-
riers for such back reflectors. Such less conduc-
tive and reflective metals include molybdenum and
chromium. Although these metals prevent diffusion
into the semiconductor of the devices, they reduce
the reflectance of the more highly conductive
metals. Hence, there is a need for better back
reflecting systems which not only provide greater
reflection of the unused light, but also preclude
diffusion of the back reflector material into the
devices.
Applicants herein have discovered new and
improved back reflector systems which provide both
increased reflection of unused light over prior
back reflectors and protection from the back re-
~5 flector materials dlffusing into the semiconductor
3~
-14-
of the devices. The back reflectors of the pres-
ent invention can be utilized in both single cell
photovoltaic devices of the p-i-n configuration,
and multiple cell structures having a plurality of
single cell units.
~ e have found that the ahove disadvantages
may be overcome by utilizing the new and improved
back reflector systems of the present invention
which provide both increased reflection of un-
absorbed light back into the active regions of thedevices in which they are employed and protection
from diffusion of the back reflector materials
into the devices.
The back reflector systems include a layer of
a highly reflective material and a layer of a
transparent material ~hich serves as both a con-
ductor and a barrier layer to prevent diffusion of
the reflective materials into the semiconductor
regions of the devices.
` 20 The highly reflective material can be a
highly reflective metallic material such as gOldr
silver, copper or aluminum, or alloys or compounds
thereof. The transparent conductor can be~a
transparent conductive oxide or a transparent con
ductive chalcogenide.
~2~S33~
The present invention provides new and im-
proved back reflector systems for use in photo-
voltaic devices. The back reflector systems of
the present invention provide increased reflection
of unabsorbed light back into the active regions
of the devices in which they are employed while
preventing diffusion of the back reflector mate-
rials into the devices.
The back reflector systems include a layer of
a highly reflective material and a layer of a
transparent conductor. The transparent conductor
layer is disposed between the device and the layer
of highly reflective material.
The highly conductive material can be a high-
ly reflective metallic material such as a highly
; reflective metal of gold, silver, copper or alumi-
:
num, or alloys thereof. The hi~hly reflective
metallic material can also be metallic compounds
such as ~Nxr TiNX, ZrNx~ Hf~xl~ or MoNx-
~ The transparent~conductor can be a transpar-
ent conductive oxide such as indium tin oxide,
::: : :
cadmlum stannate, dop~ed tln oxide, vanadium oxide,
germanium tin oxide, ferric oxide, zinc oxide, and
cuprous oxide. The transparent conductor can also
be a transparent conductive chalcogenide such as
::
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5330
-16-
zinc selenide or cadmium sulfide. It can also be
silicon carbide.
The transparent conductor serves to enhance
reflection of the unabsorbed light back into the
devices and also serves as a transparent barrier
layer to prevent diffusion of the highly reflec-
tive materials into the semiconductor regions of
the devices. The back reflector systems of the
present invention therefore provide increased bac~
reflection of unabsorbed light without de~rading
the photoresponsive characteristics of the semi-
conductor materials of the devices.
The back reflectors of the present invention
are particularly applicable in photovoltaic de-
vices of p-i-n configuration. Such devices in-
clude an intrinsic active semiconductor region
wherein photogenerated electron-hole pairs are
created and~doped regions of opposite conductivity
disposed on opposite respective sides of the in
; 20 trinsic re~ion. The active intrinsic region is
preferably an amorphous silicon alIoy body or
layer containing fluorine as a densîty of states
reducing element. The doped regions also pref-
erably include an a~orphous silicon wide band gap
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~Z~533(~
-17-
p-type alloy layer forming either the top or bot-
tom semiconductor layer of the device. In either
case, the amorphous semiconductor regions are
preferably deposited on a substrate with the layer
of highly conductive metal adjacent the substrate
and the transparent conductive oxide disposed
between the layer of highly reflective material
and the bottom doped layer.
Substantially all of the shorter wavelength
photons are absorbed in the active intrinsic
regions while only a portion of the photons having
longer wavelengths and energies near the absorp-
tion edge of the intrinsic material are absorbed.
Therefore, the thickness of the transparent con~
ductor is adjusted to optimize the reflection of
the longer wavelength photons. To that end, the
thickness of the transparent conductor is prefera-
bly determined by the relationship:
d= ~k/4
n
Where: d is the layer thickness;
is the minimum photon wavelength
to be reflected;
n is the index of refraction of the
transparent conductor; anA
k ls an odd integral multiplier.
.
i~2~5~3C~
The back reflector systems of the present
invention can also be utilized in multiple cell
devices, such as tandem cells.
Fig. 1 is a diagrammatic representation of a
glow discharge deposition systern which may be
utilized in practicing the method of the present
invention for making the photovoltaic devices of
the invention;
Fig. 2 is a sectional view of a portion of
the system of Fig. 1 taken along the lines of 2-2
therein;
Fig. 3 is a sectional view of a p-l-n photo-
voltaic device embodying the present invention;
and
Fig. 4 is a sectional view of a multiple cell
incorporating a plurality of p-i-n photovoltaie
cell units arranged in tandem configuratlon em-
bodying the present invention.
Referring now more partieularly to Fig. 1,
; 20 there is shown a glow discharge depositlon system
10 including a housing 12. The houslng 12 en-
closes a vaeuum ehamber 14 and ineludes an inlet
ehamber 16 and an outlet chamber 18. A eathode
baeking member 20 is mounted in the vaeuum chsmber
11 through an insulator 22.
:
45330
The backing member 20 includes an insulating
sleeve 24 circumferentially enclosing the backing
member 20. A dark space shield 26 is spaced from
and circumferentially surrounds the sleeve 24. A
substrate 28 is secured to an inner end 30 of the
backing member 20 by a holder 32. The holder 32
can be screwed or otherwise conventionally secured
to the backing member 20 in electrical contact
there~ith.
The cathode backing member 20 includes a well
34 into which is inserted an electrical heater 36
for heating the backing member 20 and hence the
substrate 28. The cathode backing member 20 also
includes a temperature responsive probe 38 for
measuring the temperature of the backing member
20. The temperature probe 38 is utilized to con-
trol the energization of the heater 36 to maintain
the backin~ member 20 and the substrate 28 at any
desired temperature.
The system 10 also includes an electrode 40
which extends from the housing 12 into the vacuum
chamber 14 spaced from the cathode backing member
20. The electrode 40 includes a shield 42 sur-
rounding the electrode 40 and which in turn car-
ries a substrate 44 mounted thereon. The elec-
trode 40 includes a well 46 into which is inserted
~ILZ4533~)
-20-
an electrode heater 48. The electrode 40 also
includes a temperature responsive probe 50 for
measuring the temperature of the electrode 40 and
hence the substrate 44. The probe 50 is utilized
to control the energization of the heater 48 to
maintain the electrode 40 and the substrate 44 at
any desired temperature, independently of the mem-
ber 20.
A glow discharge plasma is developed in a
space 52 between the substrates 28 and 44 by the
power generated from a regulated ~.F., ~.C. or
D.C. power source coupled to the cathode backing
member 20 across the space 52 to the electrode 40
which is coupled to ground. The vacuum chamber 14
is evacuated to the desired pressure by a vacuum
pump 54 coupled to the chamber 14 through a parti-
cle trap 56. A pressure gauge 58 is coupled to
the vacuum system and is util~zed to control the
pump 54 to maintain the system 10 at the desired
pressure.
The inlet chamber 16 of the housing 12 pre-
ferably is provided with a plurality of conduits
60 for introducing materials into the system 10 to
be mixed therein and to be deposited in ~he cham-
ber 14 in the glow:discharge plasma space 52 upon
, .
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~Z~533(~
-21-
the substrates 28 and 44. If desired, the inlet
chamber 16 can be located at a remote location and
~he gases can be premixed prior to being fed into
the chamber 14. The gaseous materials are fed
into the conduits 60 through a filter or other
purifying device 62 at a rate controlled by a
valve 64.
When a material initially is not in a gaseous
form, but instead is in a liquid or solid form, it
can be placed into a sealed container 66 as indi-
cated at 68. The material 68 then is heated by a
heater 70 to increase the vapor pressure thereof
in the container 66. A suitable gas, such as
argon, is fed through a dip tube 72 into the mate-
rial 68 so as to entrap the vapors of the material68 and convey the vapors through a filter 62' and
a valve 64' into the conduits 60 and hence into
the system 10.
The inlet chamber 16 and the outlet chamber
18 preferably are provided with screen means 74 to
confine the: plasma in the chamber 14 and princi-
pally between the substrates 28 and 44.
The materials fed through the conduits 60 are
mi~ed in the inlet chamber 16 and then fed into
the glow discharge space 52 to maintain the plasma
. .
~Z4533~
-22-
and deposit the alloy on the substrates with the
incorporation of silicon, fluorine, oxygen and the
other desired alterant elements, such as hydrogen,
and/or dopants or other desired materials.
In operation, and for depositing layers of
intrinsic amorphous silicon alloys, the system 10
is first pumped down to a desired deposition pres-
sure, such as less than 20 mtorr prior to deposi-
tion. Starting materials or reaction gases such
as silicon tetrafluoride (SiF4) and molecular
hydrogen (H2) and/or silane are fed into the inlet
chamber 16 through separate conduits 60 and are
then mixed in the inlet chamber. The gas mixture
is fed into the vacuum chamber to maintain a par-
tial pressure therein of about .6 torr. A plasmais generated in the space 52 between the sub-
strates 28 and 44 using either a DC voltage of
greater than 1000 volts or by radio frequency
power of about 50 watts operating at a frequency
of 13.56 ME~z or other desired frequency.
In addition to the intrinsic amorphous sili-
con alloys deposited in the manner as described
above, the devices of the present invention as
illustrated in the various embodiments to be des-
cribed herelnafter also utilize doped amorpùous
.., ,, . ~
~Z~53313
-23-
silicon alloys including wide band gap p amorphous
silicon alloys~ These doped alloy layers can be
p, p+, n, or n+ type in conductivity and can be
formed by introducing an appropriate dopant into
the vacuum chamber along with the intrinsic start-
ing material such as silane (SiH4) or the silicon
tetrafluoride (SiF4) starting material and/or
hydrogen and/or silane.
For n or p doped layers, the material can be
doped with 5 to lO0 ppm of dopant materials as it
is deposited. For n+ or p~ doped layers, the
material is doped with 100 ppm to over l percent
of dopant material as it is deposited. The n
dopants can be phosphorus, arsenic, antimony, or
bismuth. Preferably, the n doped layers are
deposited by the glow discharge decomposition of
at least silicon tetrafluoride (SiF4) and phos-
phine (PH3). Hydrogen and/or silane gas (SiH4)
may also be added to this mixture.
The p dopants can be boron, aluminum, gal
lium, indium, or thallium. Preferably, the p
doped layers are deposited by the glow discharge
decomposition of at least silane and diborane
(B2H6) or silicon tetrafluoride and diborane. To
533~
-2~-
the silicon tetrafluoride and diborane, hydrogen
and/or silane can also be added.
In addition to the foregoing, and in accor-
dance with the present invention, the p-type
layers are formed from amorphous silicon alloys
containing at least one band gap increasing ele-
ment. For example, carhon and/or nitrogen can be
incorporated into the p-type alloys to increase
the band gaps thereof. A wide band gap p amor-
phous silicon alloy can be formed for example by agas mixture of silicon tetrafluoride (SiF4),
silane (SiH4), diborane (B2H6), and methane
(CH4). This results in a p-type amorphoas silicon
alloy having a wide band gap.
The doped layers of the devices are deposited
at various~temperatures depending upon the type of
material deposited and the substrate used~ For
aluminum substrates, the upper temperature should
not be above about 600C and for stainless steel
it could be above about 1000C. For the intrinsic
and doped alloys initially compensated with hydro-
gen, as for example those deposited from silane
gas starting material, the substrate temperature
should be less than about 400C and preferably
between 250C and 350C.
~:4533(~
-25-
Other materials and alloying elements may
also be added to the intrinsic and doped layers to
achieve optimized current generation. These other
materials and elements will be described herein-
after in connection with the device configurationsembodying the present invention illustrated in
Figs. 3 and 4.
~ eferring now to Fig. 3, it illustrates in
sectional view a p-i-n device embodying the pres-
ent invention. The device 110 includes a sub-
strate 112 which may be glass or a flexible web
formed from stainless steel or aluminum. The sub-
strate 112 is of a width and length as desired and
preferàbly 5 to 10 mils thick.
In accordance with the present invention, a
layer 114 of highly reflective material is depos-
ited upon the substrate 112. The layer 114 lS
deposited by vapor deposition, which is a rela-
tively ~ast deposition process. The layer 114
preferably is a highly reflective metallic mate-
rial such as silver, gold, alu~inum, or copper or
alloys thereof. The highly reflective material
can also be a highly reflective metallic compound
such as WNX, TiNX, ZrNx, HfNX, or MoNx. Deposited
over the layer 114 is a layer 115 of a transparent
:~IL2~533(3
-26-
conductor. The transparent conductor can be a
transparent conductive oxide (TC0) deposited in a
vapor deposition environment and, for example, may
be indium tin oxide (IT0), cadmium stannate
(Cd2SnO4) zinc oxide, cuprous oxide, vanadium
oxide, germanium tin oxide, ferric oxide, or tin
oxide (SnO2). The transparent conductor layer 115
can also be formed silicon carbide, or a transpar-
ent conductive chalcogenide such as cadmium sul-
fide or zinc selenide. The layer 114 of highlyreflective material and the layer 115 of transpar-
ent conductor form a back reflecting system in
accordance with the present invention.
The substrate 112 is then placed in the glow
discharge deposition environment. A first doped
wide band gap p-type amorphous silicon alloy layer
116 is deposited on the layer 115 in accordance
with the present invention. The layer 116 as
shown is p+ in conductivity. The p+ region is as
20 thin as possible on the order of 50 to 500 ang-
stroms in thickness which is sufficient for the p+
region to make good ohmic contact with the trans-
parent conductive oxide layer 115. The p+ region
also serves to establish a potential gradient
across the device to facilitate the collection of
'''' ' ` '
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~Z~5330
-27-
photo induced electron-hole pairs as electrical
current. The p~ region 116 can be deposited from
any of the gas mixtures previously referred to for
the deposition of such material in accordance with
the present invention.
A body of intrinsic amorphous silicon alloy
118 is next deposited over the wide band gap
p-type layer 116. The intrinsic bod 118 is rela-
tively thick, on the order of 4500A, and is depos-
ited from silicon tetrafluoride and hydrogenand/or silane. The intrinsic body preferably con-
tains the amorphous silicon alloy compensated with
fluorine where the majority of the electron-hole
pairs are generated. The short circuit current of
the device is enhanced by the combined effects of
the back reflector of the present invention and
the wide band gap of the p-type amorphous silicon
alloy layer 116.
Deposited on the intrinsic body 118 is a fur-
ther doped layer 120 which is of opposite conduc-
tivity with respect to the first doped layer 116.
It comprises an n~ conductivity amorphous silicon
alloy and may also have a wide band gap. The ~+
layer 120 is deposited from any of the gas mix-
tures previously referred to for the deposition of
,,
12~S33~
-28-
such material. The n+ layer 120 is deposited to a
thickness between 50 and 500 angstroms and serves
as a contact layer.
A transparent conductive oxide (TCO) layer
122 is then deposlted over the n+ layer 120. The
TCO layer 122 can also be deposited in a vapor
deposition environment and, for example, may be
indium tin oxide (ITO), cadmium stannate
(Cd2SnO4), or doped tin oxide (SnO2).
On the surface of the TCO layer 122 is depos-
ited a gri~ electrode 124 made of a metal having
good electrical conductivity. The grid may com-
prise orthogonally related lines of conductive
material occupying only a minor portion of the
area of the metallic region, the rest of which is
to be exposed to solar energy. For example, the
grid 124 may occupy only about from 5 to 10~ of
the entire area of the TCO layer 1220 The grid
electrode 124 uniformly collects current from the
TCO layer 122 to assure a good lo~ series resis-
tance for the device.
To complete the device llO, an anti-reflec-
tion (AR) layer 126 is applied over the grid elec-
trode 124 and the areas of the TCO layer 122
between the grid electrode areas. The AR layer
5330
-29-
126 has a solar radiation incident surface upon
which impinges the solar radiation. For example 7
the AR layer 126 may have a thickness on the order
of magnitude of the wavelength of the maximum
energy point of the solar radiation spectrum/
divided by four times the index of refraction of
the anti-reflection layer 126. A suitable AR
layer 126 would be zirconium oxide of about 500A
in thickness with an index of refraction of 2.1.
In an alternative form, the TCO layer 12~ can also
serve as an anti-reflection layer and the anti-
reflection layer 126 may then he eliminated and a
suitahle encapsulant may be substituted in its
place.
It is not necessary that the transparent con-
ductor layer 115 and TCO layer 122 be formed from
the same material. The TCO layer 122 must be able
to transmit incident radiation of both short and
long wavelength. ~lowever, since essentially all
of the shorter wavelength radiation will be
absorbed in the intrlnsic region 118 during the
first pass therethrough, the transparent conductor
layer 115 need only be transmissive of longer
wavelength radiation, for example, light having
wavelengths of about 6000A or longer.
~;4533~
-30-
The thickness of the layer 115 of transparent
conductor, here a transparent conductive oxide,
can be adjusted to optimize the reflectance
enhancement of the layer 115. For example, the
layer 115 preferably has a thickness determined by
the relationship:
d= ~k/4/n
Where: d is the thickness of layer
115;
~ is the minimum photon wave-
length to be reflec~ed;
n is the index of refraction of
the transparent conductor;
and
k is an odd integral multi-
plier.
Nearly all of the photons having shorter
wavelengths will be absorbed by the active intrin-
sic layer 118. As a result, and as previously
explained, the major portion of the photons which
are not absorbed have longer wavelengths. These
photons may have wavelengths of about 6000A for
example and~longer. For a transparent conductive
oxide of, for example, indium tin oxide which has
an index of refraction of about 2.0 at these
longer wavelengths, and with k being preferably
equal to 1, the thickness of layer 115 should be
about 750A.
: : :
~.~4533~
Any one of the highly reflective materials
previously mentioned may be used in conjunction
with the indium tin oxide layer of 750A. However,
of the reflective materials previously mentioned,
copper is the least expensive and exhibits good
reflectivity for the longer wavelengths of 6000A
or greater. With this combination of materials
and thickness of the indium tin oxide of 750A,
there can be expected at least 97 percent reflec-
tion of all of the unused light back into thesemiconductor regions of the device 110. Addi-
tionally, because the transparent conductive oxide
also serves as a transparent barrier layer, dif-
fusion of the copper, or any of the other highly
reflective materials when employed, into the
semiconductor regions of the device 110 is pre-
vented.
As previously mentioned, the band gap of the
intrinsic layer 118 can be adjusted for a particu-
lar photoresponse characteristic with the incor-
; poration of band gap decreasing elements. As a
further alternative, the band gap of the intrinsic
body 118 can be graded so as to be gradually
increasing from the p+ layer 116 to n+ layer 120
. . ,
~Z~5330
For example, as the intrinsic layer 118 is deposited, one
or more band gap decreasing elements such as germanium, tin,
or lead can be incorporated into the alloys in gradually
decreasing concentration. Germane gas (GeH~¦ for example
can be introduced into the glow discharge deposition cham~er
from a relatively high concentration a-t first and gradually
diminished thereafter as the intrinsic layer is deposited to
a point where such introduction is terminated. The
resulting intrinsic body will thus have a band ~ap
descreasing element, such as germanium, therein in gradually
decreasing concentrations from the p-~ layer 116 to~ards t~e
n+ layer 120.
Referring now to Fig. 4, a multiple cell device
150 is there illustrated in sectional view which is arranged
in tandem configuration. The device 150 comprises two
single cell units 152 and 154 arranged in series relation
~s can be appreciated, plural single cell units o~ more
than two can be utilized.
cr/ `
.
.
~L24533~
The device 150 includes a substrate 156
formed fro~ a metal having good electrical conduc-
tivity such as stainless steel or aluminum, for
example. Deposited on the substrate 156 is a back
reflector system embodying the present invention
which includes a layer 157 of highly reflective
material which may be formed from the materials
and by the processes as previously described. To
complete the back reflector, a layer 159 of a
transparent conductor such as a transparent con-
ductive oxide is deposited onto the highly reflec-
tive material layer 157. The layer lS9 can be
formed from any of the transparent conductors and
deposited to an optimized thickness as previously
described.
The first cell unit 152 includes a first
doped p+ amorphous silicon alloy layer 158 depos-
ited on the transparent conductive oxide layer
159. The p+ layer is preferably a wlde band gap
p-type amorphous silicon alloy in accordance with
the present invention. It can be deposited from
.
any of the previously mentioned starting materials
for depositing such material.
Deposited on the wide band gap p+ layer 158
25 lS a first intrinsic amorphous silicon alloy body
.,
. .
~Z~533~
-3~-
160. The first intrinsic alloy body 160 is pref-
erably an amorphous silicon-fluorine alloy.
Deposited on the intrinsic layer 160 is a
further doped amorphous silicon alloy layer 162.
5 It is opposite in conductivity with respect to the
conductivity of the first doped layer 15~ and thus
is an n+ layer. It may also have a wide band gap,
The second unit cell 154 is essentially iden-
tical and includes a first doped p~ layer 164, an
intrinsic body 166 and a further doped n+ layer
168. The device 150 is completed with a TCO layer
170, a grid electrode 172, and an antireflection
layer 174.
The band gaps of the intrinsic layers are
preferably adjusted so that the band gap of layer
166 is greater than the band gap of layer 160. To
that end, the alloy forming layer 166 can include
one or more band gap increasing elements such as
nitrogen and carbon. The intrinsic alloy forming
the intrinsic layer 160 can include one or more
; band gap decreasing elements such as germanium,
tin, or lead.
It can be noted from the figure that the
intrinsic layer 160 of the cell is thicker than
the intrinsic layer 166. This allows the entire
S33~3
usuable spectrum of the solar energy to be utilized for
generating electron-hole pairs.
Although a tandem cell embodlment has been shown
and described herein, the unit cells can also be isolate~
from one another with oxide layers for example to form a
stacked multiple cell. Each cell could include a pair of
collection electrodes to facilitate the series connection
of the cells with external wiring.
As a further alternative, and as mentioned wîth
respect to the single cells previously describedt one or
more of the intrinsic bodies of the unit cells can include
~lloys having graded band gaps~ Any one or more OI the band
gap increasing or decreasing elements previously mentioned
can be incorporated into the intrinsic alloys for this
purpose.
As can be appreciated from the foregoing, the
present inven-tion provides new and improved back reflector
systems for use, for example, in photovoltaic cells. The
back reflectors not only
35 -
c.r~
12~533~
increase the amount of unused light reflected back
into the semiconductor regions of the cells, but
also serve to prevent diffusion of the back
reflector materials into the semiconductor
regions. As examples of the effectiveness of the
new and improved back reflectors of the present
invention, with a transparent conductive oxide of
indium tin oxide, reflectivities of 98.5 percent,
97 percent, and 90 percent are obtainable when
highly reflective metals of silver, copper, and
aluminum, respectively are used therewith as com-
pared to reflectivities of 80% for silver alone,
74% for copper alone, and 70~ for aluminum alone.
For each embodiment of the invention des-
cribed herein, the alloy layers other than the
intrinsic alloy layers can be other than amorphous
layers, such as polycrystalline layers. (By the
term "amorphous" is meant an alloy or material
which has long range disorder, although it may
have short or intermediate order or even contain
at times some crystalline inclusions.)
Modifications and variations of the present
invention are possible in light of the above
teachings. It is therefore, to be understood that
1~4533~
-37-
within the scope of the appended claims the inven-
tion may be practiced otherwise than as specif-
ically described.
:
: :
~' :
~'
