U.S. patent application number 14/218410 was filed with the patent office on 2014-07-17 for photovoltaic devices with an interfacial band-gap modifying structure and methods for forming the same.
This patent application is currently assigned to EGYPT NANOTECHNOLOGY CENTER. The applicant listed for this patent is Egypt Nanotechnology Center, International Business Machines Corporation. Invention is credited to Ahmed Abou-Kandil, Keith E. Fogel, Jeehwan Kim, Hisham S. Mohamed, Mohamed Saad, Devendra K. Sadana, Osama Tobail, George S. Tulevski.
Application Number | 20140196780 14/218410 |
Document ID | / |
Family ID | 45555187 |
Filed Date | 2014-07-17 |
United States Patent
Application |
20140196780 |
Kind Code |
A1 |
Fogel; Keith E. ; et
al. |
July 17, 2014 |
PHOTOVOLTAIC DEVICES WITH AN INTERFACIAL BAND-GAP MODIFYING
STRUCTURE AND METHODS FOR FORMING THE SAME
Abstract
A Schottky-barrier-reducing layer is provided between a p-doped
semiconductor layer and a transparent conductive material layer of
a photovoltaic device. The Schottky-barrier-reducing layer can be a
conductive material layer having a work function that is greater
than the work function of the transparent conductive material
layer. The conductive material layer can be a carbon-material layer
such as a carbon nanotube layer or a graphene layer. Alternately,
the conductive material layer can be another transparent conductive
material layer having a greater work function than the transparent
conductive material layer. The reduction of the Schottky barrier
reduces the contact resistance across the transparent material
layer and the p-doped semiconductor layer, thereby reducing the
series resistance and increasing the efficiency of the photovoltaic
device.
Inventors: |
Fogel; Keith E.; (Hopewell
Junction, NY) ; Kim; Jeehwan; (Los Angeles, CA)
; Sadana; Devendra K.; (Pleasantville, NY) ;
Tulevski; George S.; (White Plains, NY) ;
Abou-Kandil; Ahmed; (Elmsford, NY) ; Mohamed; Hisham
S.; (Clifton Park, NY) ; Saad; Mohamed; (White
Plains, NY) ; Tobail; Osama; (Elmsford, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Egypt Nanotechnology Center
International Business Machines Corporation |
Cairo-Alexandria
Armonk |
NY |
EG
US |
|
|
Assignee: |
EGYPT NANOTECHNOLOGY CENTER
Cairo-Alexandria
NY
INTERNATIONAL BUSINESS MACHINES CORPORATION
Armonk
|
Family ID: |
45555187 |
Appl. No.: |
14/218410 |
Filed: |
March 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12850272 |
Aug 4, 2010 |
|
|
|
14218410 |
|
|
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|
Current U.S.
Class: |
136/255 ; 438/72;
438/87; 438/98 |
Current CPC
Class: |
H01L 31/022466 20130101;
H01L 31/20 20130101; Y02E 10/547 20130101; H01L 31/028 20130101;
H01L 31/07 20130101; H01L 31/075 20130101; Y02E 10/548 20130101;
H01L 31/022483 20130101; H01L 31/1884 20130101; H01L 31/056
20141201; Y02E 10/52 20130101; H01L 31/02327 20130101; H01L
2031/0344 20130101 |
Class at
Publication: |
136/255 ; 438/98;
438/87; 438/72 |
International
Class: |
H01L 31/07 20060101
H01L031/07; H01L 31/075 20060101 H01L031/075; H01L 31/0232 20060101
H01L031/0232; H01L 31/18 20060101 H01L031/18 |
Claims
1. A photovoltaic device comprising a stack, from one side to
another side, of a transparent conductive material layer, a
Schottky-barrier-reducing layer contacting said transparent
conductive material layer, and a p-doped semiconductor layer,
wherein a Schottky barrier across said stack has a lower contact
resistance than a Schottky barrier across a comparative exemplary
stack that includes all layers of said stack less said
Schottky-barrier-reducing layer, wherein said
Schottky-barrier-reducing layer is an optically transparent layer
including an allotrope of carbon and is in direct contact with said
p-doped semiconductor layer and is in direct contact with said
transparent conductive material layer, wherein said
Schottky-barrier-reducing layer includes a same material as said
transparent conductive material layer and has a different doping
than said transparent conductive material layer.
2. The photovoltaic device of claim 1, wherein said transparent
conductive material layer includes an aluminum-doped zinc oxide
having an aluminum doping at a first dopant concentration, and said
Schottky-barrier-reducing layer includes an aluminum-doped zinc
oxide having an aluminum doping at a second dopant concentration,
wherein said first dopant concentration is greater than said second
dopant concentration.
3. The photovoltaic device of claim 1, wherein said transparent
conductive material layer includes a first fluorine-doped tin oxide
having a fluorine doping at a first dopant concentration, and said
Schottky-barrier-reducing layer includes a second fluorine-doped
tin oxide having a fluorine doping at a second dopant
concentration, wherein said first dopant concentration is greater
than said second dopant concentration.
4. The photovoltaic device of claim 1, wherein said
Schottky-barrier-reducing layer includes a material having a higher
resistivity than a material of said transparent conductive material
layer.
5. The photovoltaic device of claim 1, wherein said
Schottly-barrier-reducing layer has a work function that is greater
than a work function of said transparent conductive material layer
and is lesser than an absolute value of a Fermi level of said
p-doped semiconductor layer.
6. The photovoltaic device of claim 1, wherein a series resistance
of said photovoltaic device is equal to or less than 9
Ohms-cm.sup.2.
7. The photovoltaic device of claim 1, wherein said p-doped
semiconductor layer includes a hydrogenated p-doped
semiconductor-containing material.
8. The photovoltaic device of claim 1, further comprising: an
intrinsic semiconductor layer contacting said p-doped semiconductor
layer; and an n-doped semiconductor layer contacting said intrinsic
semiconductor layer.
9. The photovoltaic device of claim 8, wherein said intrinsic
semiconductor layer includes a hydrogenated amorphous intrinsic
semiconductor material.
10. The photovoltaic device of claim 8, wherein said n-doped
semiconductor layer includes hydrogenated n-doped amorphous
semiconductor material.
11. The photovoltaic device of claim 8, further comprising at least
one back reflector layer located on said n-doped semiconductor
layer.
12. A method of forming a photovoltaic device comprising: forming a
transparent conductive material layer on a substrate; forming a
Schottky-barrier-reducing layer on said transparent conductive
material layer, wherein said Schottky-barrier-reducing layer
includes a same material as said transparent conductive material
layer and has a different doping than said transparent conductive
material layer; and forming a p-doped semiconductor layer on said
Schottky-barrier-reducing layer, wherein a Schottky barrier across
a stack of said transparent conductive material layer, said
Schottky-barrier-reducing layer, and said p-doped semiconductor
layer has less contact resistance than a Schottky barrier across
another stack that includes all layers of said stack less said
Schottky-barrier-reducing layer.
13. The method of claim 12, wherein said transparent conductive
material layer includes an aluminum-doped zinc oxide having an
aluminum doping at a first dopant concentration, and said
Schottky-barrier-reducing layer includes an aluminum-doped zinc
oxide having an aluminum doping at a second dopant concentration,
wherein said first dopant concentration is greater than said second
dopant concentration.
14. The method of claim 12, wherein said transparent conductive
material layer includes a first fluorine-doped tin oxide having a
fluorine doping at a first dopant concentration, and said
Schottky-barrier-reducing layer includes a second fluorine-doped
tin oxide having a fluorine doping at a second dopant
concentration, wherein said first dopant concentration is greater
than said second dopant concentration.
15. The method of claim 12, wherein said Schottky-barrier-reducing
layer includes a material having a higher resistivity than a
material of said transparent conductive material layer.
16. The method of claim 12, wherein said Schottky-barrier-reducing
layer has a work function that is greater than a work function of
said transparent conductive material layer and is lesser than an
absolute value of a Fermi level energy of said p-doped
semiconductor layer.
17. The method of claim 12, further comprising: forming an
intrinsic semiconductor layer on said p-doped semiconductor layer;
and forming an n-doped semiconductor layer on said intrinsic
semiconductor layer.
18. The method of claim 17, wherein said intrinsic semiconductor
layer includes a hydrogenated amorphous intrinsic semiconductor
material.
19. The method of claim 17, wherein said n-doped semiconductor
layer includes hydrogenated n-doped amorphous semiconductor
material.
20. The method of claim 17, further comprising forming at least one
back reflector layer on said n-doped semiconductor layer.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/850,272, filed Aug. 4, 2010 the entire content and
disclosure of which is incorporated herein by reference.
BACKGROUND
[0002] The present disclosure relates to photovoltaic devices, and
more particularly to photovoltaic devices including an interfacial
band-gap modifying structure and methods of forming the same.
[0003] A photovoltaic device is a device that converts the energy
of incident photons to electromotive force (e.m.f.). Typical
photovoltaic devices include solar cells, which are configured to
convert the energy in the electromagnetic radiation from the Sun to
electric energy. Each photon has an energy given by the formula
E=h.nu., in which the energy E is equal to the product of the Plank
constant h and the frequency .nu. of the electromagnetic radiation
associated with the photon.
[0004] A photon having energy greater than the electron binding
energy of a matter can interact with the matter and free an
electron from the matter. While the probability of interaction of
each photon with each atom is probabilistic, a structure can be
built with a sufficient thickness to cause interaction of photons
with the structure with high probability. When an electron is
knocked off an atom by a photon, the energy of the photon is
converted to electrostatic energy and kinetic energy of the
electron, the atom, and/or the crystal lattice including the atom.
The electron does not need to have sufficient energy to escape the
ionized atom. In the case of a material having a band structure,
the electron can merely make a transition to a different band in
order to absorb the energy from the photon.
[0005] The positive charge of the ionized atom can remain localized
on the ionized atom, or can be shared in the lattice including the
atom. When the positive charge is shared by the entire lattice,
thereby becoming a non-localized charge, this charge is described
as a hole in a valence band of the lattice including the atom
Likewise, the electron can be non-localized and shared by all atoms
in the lattice. This situation occurs in a semiconductor material,
and is referred to as photogeneration of an electron-hole pair. The
formation of electron-hole pairs and the efficiency of
photogeneration depend on the band structure of the irradiated
material and the energy of the photon. In case the irradiated
material is a semiconductor material, photogeneration occurs when
the energy of a photon exceeds the band gap energy, i.e., the
energy difference of a band gap of the irradiated material.
[0006] The direction of travel of charged particles, i.e., the
electrons and holes, in an irradiated material is sufficiently
random. Thus, in the absence of any electrical bias,
photogeneration of electron-hole pairs merely results in heating of
the irradiated material. However, an external field can break the
spatial direction of the travel of the charged particles to harness
the electrons and holes formed by photogeneration.
[0007] One exemplary method of providing an electric field is to
form a p-i-n junction around the irradiated material. As negative
charges accumulate in the p-doped region and positive charges
accumulate in the n-doped region, an electric field is generated
from the direction of the n-doped region toward the p-doped region.
Electrons generated in the intrinsic region drift towards the
n-doped region due to the electric field, and holes generated in
the intrinsic region drift towards the p-doped region. Thus, the
electron-hole pairs are collected systematically to provide
positive charges at the p-doped region and negative charges at the
n-doped region. The p-i-n junction forms the core of this type of
photovoltaic device, which provides electromotive force that can
power any device connected to the positive node at the p-doped
region and the negative node at the n-doped region.
SUMMARY
[0008] A Schottky-barrier-reducing layer is provided between a
p-doped semiconductor layer and a transparent conductive material
layer of a photovoltaic device. The Schottky-barrier-reducing layer
can be a conductive material layer having a work function that is
greater than the work function of the transparent conductive
material layer. The conductive material layer can be a
carbon-material layer such as a carbon nanotube layer or a graphene
layer. Alternately, the conductive material layer can be another
transparent conductive material layer having a greater work
function than the transparent conductive material layer. The
reduction of the Schottky barrier reduces the contact resistance
across the transparent material layer and the p-doped semiconductor
layer, thereby reducing the series resistance and increasing the
efficiency of the photovoltaic device.
[0009] According to an aspect of the present disclosure, a
photovoltaic device is provided, which includes a stack of a
transparent conductive material layer, a Schottky-barrier-reducing
layer contacting the transparent conductive material layer, and a
p-doped semiconductor layer contacting the p-doped semiconductor
layer. The Schottky barrier across the stack has a lower contact
resistance than a Schottky barrier across another stack that
includes all layers of the stack less the Schottky-barrier-reducing
layer.
[0010] According to another aspect of the present disclosure, a
method of forming a photovoltaic device is provided. The method
includes: forming a transparent conductive material layer on a
substrate; forming a Schottky-barrier-reducing layer on the
transparent conductive material layer; and forming a p-doped
semiconductor layer on the Schottky-barrier-reducing layer. The
Schottky barrier across a stack of the transparent conductive
material layer, the Schottky-barrier-reducing layer, and the
p-doped semiconductor layer has less contact resistance than a
Schottky barrier across another stack that includes all layers of
the stack less the Schottky-barrier-reducing layer.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0011] FIG. 1 is a vertical cross-sectional view of a prior art
photovoltaic device structure.
[0012] FIG. 2 is an equivalent circuit for the prior art
photovoltaic device structure of FIG. 1.
[0013] FIG. 3 is a schematic graph of an I-V curve of the prior art
photovoltaic device structure of FIG. 1.
[0014] FIG. 4 is a band diagram of a transparent conductive
material layer and a p-doped semiconductor layer in the prior art
photovoltaic device structure of FIG. 1.
[0015] FIG. 5 is a graph of an I-V curve for an exemplary prior art
photovoltaic device structure.
[0016] FIG. 6 is a vertical cross-sectional view of an exemplary
photovoltaic device structure according to various embodiments of
the present disclosure.
[0017] FIG. 7 is a band diagram of a junction of a single wall
carbon nanotube and a p-doped silicon material according to the
first embodiment of the present disclosure.
[0018] FIG. 8 is a graph illustrating the resistivity of
aluminum-doped zinc oxide as a function of weight percentage of
aluminum according to a second embodiment of the present
disclosure.
[0019] FIG. 9 is a band diagram for first and second exemplary
photovoltaic device structures according to the first and second
embodiments of the present disclosure.
[0020] FIG. 10A is a vertical cross-sectional view of an exemplary
photovoltaic device structure after formation of a transparent
conductive material layer according to embodiments of the present
invention.
[0021] FIG. 10B is a vertical cross-sectional view of an exemplary
photovoltaic device structure after formation of a p-doped
semiconductor layer according to embodiments of the present
invention.
[0022] FIG. 10C is a vertical cross-sectional view of an exemplary
photovoltaic device structure after formation of back reflector
layers according to embodiments of the present invention.
DETAILED DESCRIPTION
[0023] As stated above, the present disclosure relates to
photovoltaic devices including an interfacial band-gap modifying
structure and methods of forming the same, which are now described
in detail with accompanying figures. Throughout the drawings, the
same reference numerals or letters are used to designate like or
equivalent elements. The drawings are not necessarily drawn to
scale.
[0024] As used herein, a crystal structure is "microcrystalline" if
the average grain size of the material is from 1 nm to 10
microns.
[0025] As used herein, a "hydrogenated" semiconductor material is a
semiconductor material including incorporated hydrogen therein,
which neutralizes dangling bonds in the semiconductor material and
allows charge carriers to flow more freely.
[0026] As used herein, a "semiconductor-material-containing
reactant gas" refers to a gas including at least one atom of Si,
Ge, or components of a compound semiconductor material.
[0027] As used herein, an element is "optically transparent" if the
element is transparent in the visible electromagnetic spectral
range having a wavelength from 400 nm to 800 nm.
[0028] As used herein, a "Schottky-barrier-reducing" element is an
element that is located between two other elements that form a
Schottky barrier, in which the presence of the
Schottky-barrier-reducing element reduces a contact resistance of a
structure including the two other elements and the
Schottky-barrier-reducing element relative a structure including
only the two other elements.
[0029] Referring to FIG. 1, a prior art photovoltaic device
structure includes a material stack, from top to bottom, of a
substrate 110, a transparent conductive material layer 120, a
p-doped semiconductor layer 130, an intrinsic semiconductor layer
140, an n-doped semiconductor layer 150, a first back reflector
layer 160, and a second back reflector layer 170. The substrate 110
typically includes an optically transparent material. The
transparent conductive material layer 120 functions as a positive
node of the prior art photovoltaic device, and the combination of
the second back reflector layer 170 functions as a negative node of
the prior art photovoltaic device. The first back reflector layer
160 can be optically transparent, and the combination of the first
and second back reflector layers (160, 170) reflect any photons
that pass through the stack of the p-doped semiconductor layer 130,
the intrinsic semiconductor layer 140, and the n-doped
semiconductor layer 150 to enhance the efficiency of the prior art
photovoltaic device.
[0030] The p-doped semiconductor layer 130 can include an amorphous
p-doped hydrogenated silicon-containing material or
microcrystalline p-doped hydrogenated silicon-containing material.
The amorphous p-doped hydrogenated silicon-containing material or
the microcrystalline p-doped hydrogenated silicon-containing
material can be deposited by flowing a
semiconductor-material-containing reactant in hydrogen carrier gas.
In this case, hydrogen atoms are incorporated in the deposited
material of the p-doped semiconductor layer 130. The p-doped
semiconductor layer 130 can include an amorphous p-doped
hydrogenated silicon-carbon alloy or a microcrystalline p-doped
hydrogenated silicon-carbon alloy.
[0031] Referring to FIG. 2, the functionality of the prior art
photovoltaic device of FIG. 1 can be approximated by an equivalent
circuit that includes a current source, a diode, and two resistors.
The equivalent circuit of FIG. 2 approximates a unit area of the
prior art photovoltaic device of FIG. 1, which provides electrical
current that is proportional to the total irradiated area of the
prior art photovoltaic device. The photovoltaic current per unit
area generated by the prior art photovoltaic device is referred to
as a short-circuit current density J.sub.sc, i.e., the current
density generated by the prior art photovoltaic device if the
positive node and the negative node of the prior art photovoltaic
device are electrically shorted. Thus, the current source in FIG. 2
generates an electrical current with a current density of the
short-circuit current density J.sub.sc.
[0032] Power dissipation through internal leakage current is
approximated by a shunt resistance R.sub.sh. A finite value for the
shunt resistance R.sub.sh triggers an internal leakage current
through the prior art photovoltaic device of FIG. 1, and degrades
the performance of the prior art photovoltaic device. The lesser
the shunt resistance R.sub.sh, the greater is the internal power
loss due to the internal leakage current.
[0033] Power dissipation through internal resistance of the prior
art photovoltaic device of FIG. 1 is approximated by a series
resistance R.sub.s. A non-zero value for the series resistance
R.sub.s triggers Joule loss within the prior art photovoltaic
device. The greater the series resistance R.sub.s, the greater is
the internal power loss due to the internal resistance of the prior
art photovoltaic device.
[0034] Referring back to FIG. 1, a predominant portion of the
series resistance R.sub.s is the resistance of a Schottky barrier
at the interface between the transparent conductive material layer
120 and the p-doped semiconductor layer 130. The Schottky barrier
dominates the total value of the series resistance R.sub.s unless
significant defects in conductive components, e.g., the transparent
conductive material layer 120 or the first and second back
reflector layers (160, 170), causes the series resistance R.sub.s
to increase abnormally. Thus, in well-functioning prior art
photovoltaic devices of FIG. 1, the series resistance R.sub.s is
limited by the resistance introduced by the Schottky barrier at the
interface between the transparent conductive material layer 120 and
the p-doped semiconductor layer 130. In case amorphous hydrogenated
carbon-containing silicon alloy is employed for the p-doped
semiconductor layer 130, the series resistance R.sub.s of the prior
art photovoltaic device of FIG. 1 is from 20 Ohms-cm.sup.2 to 30
Ohms-cm.sup.2. In case microcrystalline hydrogenated
carbon-containing silicon alloy is employed for the p-doped
semiconductor layer 130, the series resistance R.sub.s of the prior
art photovoltaic device of FIG. 1 is from 10 Ohms-cm.sup.2 to 15
Ohms-cm.sup.2.
[0035] The potential difference between the positive node, i.e.,
the p-doped semiconductor layer 130, and the negative node, i.e.,
the n-doped semiconductor layer 150, generates an internal current
that flow in the opposite direction to the photocurrent, i.e., the
current represented by the current source having the short-circuit
current density J.sub.sc. The dark current has the same functional
dependence on the voltage across the current source as a diode
current. Thus, the dark current is approximated by a diode that
allows a reverse-direction current. The density of the dark
current, i.e., the dark current per unit area of the prior art
photovoltaic device, is referred to as the dark current density
J.sub.dark. An external load can be attached to an outer node of
the series resistor and one of the nodes of the current source. In
FIG. 2, the value the impedance of the load is the value of the
actual impedance of a physical load is divided by the area of the
prior art photovoltaic cell because the equivalent circuit of FIG.
2 describes the functionality of a unit area of the prior art
photovoltaic cell.
[0036] Referring to FIG. 3, a schematic graph of an I-V curve of
the prior art photovoltaic device structure of FIG. 1 is shown. The
bias voltage V is the voltage across the load in the equivalent
circuit of FIG. 2. The open circuit voltage Voc corresponds to the
voltage across the load as the resistance of the load diverges to
infinity, i.e., the voltage across the current source when the load
is disconnected. The inverse of the absolute value of the slope of
the I-V curve at V=0 and J=J.sub.sc is approximately equal to the
value of the shunt resistance R.sub.sh. The inverse of the absolute
value of the slope of the I-V curve at V=V.sub.oc and J=0 is
approximately equal to the value of the series resistance R.sub.s.
The effect of the dark current is shown as an exponential decrease
in the current density J as a function of the bias voltage V around
a non-zero value of the bias voltage.
[0037] The operating range of a photovoltaic device is the portion
of the I-V curve in the first quadrant, i.e., when both the bias
voltage V and the current density J are positive. The power density
P, i.e., the density of power generated from an unit area of the
prior art photovoltaic device of FIG. 1, is proportional to the
product of the voltage V and the current density J along the I-V
curve. The power density P reaches a maximum at a maximum power
point of the I-V curve, which has the bias voltage of V.sub.m and
the current density of J.sub.m. The fill factor FF is defined by
the following formula:
F F = J m .times. V m J sc .times. V oc . ( Eq . 1 )
##EQU00001##
The fill factor FF defines the degree by which the I-V curve of
FIG. 3 approximates a rectangle. The fill factor FF is affected by
the series resistance R.sub.s and the shunt resistance R.sub.th.
The smaller the series resistance R.sub.s, the greater the fill
factor FF. The greater the shunt resistance R.sub.th, the greater
the fill factor FF. The theoretical maximum for the fill factor is
1.0.
[0038] The efficiency .eta. of a photovoltaic device is the ratio
of the power density at the maximum power point to the incident
light power density P.sub.s. In other words, the efficiency .eta.
is given by:
.eta. = J m .times. V m P s . ( Eq . 2 ) ##EQU00002##
Eq. 2 can be rewritten as:
.eta. = J sc .times. V oc .times. FF P s . ( Eq . 3 )
##EQU00003##
Thus, the efficiency h of a photovoltaic device is proportional to
the short circuit current density J.sub.sc, the open circuit
voltage V.sub.oc, and the fill factor FF.
[0039] The efficiency .eta. of a photovoltaic device depends on the
spectral composition of the incident light. For solar cells, the
efficiency is calculated under a standard radiation condition
defined as 1 sun, which employs the spectrum of the sunlight.
[0040] Referring to FIG. 4, a band diagram illustrates the band
bending in the p-doped semiconductor layer 130 in the prior art
photovoltaic device structure of FIG. 1 due to the transparent
conductive material layer 120. Materials currently available for
the transparent conductive material layer 120 are n-type materials.
A Schottky barrier exits at the interface between the transparent
conductive material layer 120 and the p-doped semiconductor layer
130. The valence band and the conduction band of the p-doped
semiconductor layer 130 bend downward at the interface between the
transparent conductive material layer 120 and the p-doped
semiconductor layer 130.
[0041] In case the transparent conductive material layer 120 is an
aluminum-doped zinc oxide, the work function of the transparent
conductive material layer 120 is about 4.5 eV. In other words, the
Fermi level E.sub.F is at 4.5 eV below the vacuum level. Other
typical materials for the transparent conductive material layer 120
also have a work function of about 4.5 eV.
[0042] In case the p-doped semiconductor layer 130 includes an
amorphous hydrogenated silicon carbon alloy, the band gap of the
p-doped semiconductor layer 130 is around 1.85 eV. The difference
between the Fermi level and the valence band of the amorphous
hydrogenated silicon carbon alloy is about 1.0 eV. This is a
significant energy barrier, and is the cause of the predominant
component of the series resistance R.sub.s from 20 Ohms-cm.sup.2 to
30 Ohms-cm.sup.2 in the prior art photovoltaic device of FIG.
1.
[0043] Referring to FIG. 5, the significant series resistance
R.sub.s in the prior art photovoltaic device of FIG. 1 can be
manifested as humps in an I-V curve in case the p-doped
semiconductor layer 130 includes an amorphous hydrogenated silicon
carbon alloy. The portion of the I-V curve in the fourth quadrant
can be obtained by applying an external voltage across the positive
and negative terminals of the prior art photovoltaic device of FIG.
1. The hump in the first quadrant can adversely affect the fill
factor FF, and consequently affect the efficiency r adversely.
[0044] FIG. 6 is a vertical cross-sectional view of an exemplary
photovoltaic device structure according to various embodiments of
the present disclosure. The photovoltaic device structure includes
a material stack, from top to bottom, of a substrate 10, a
transparent conductive material layer 20, a
Schottky-barrier-reducing layer 22, a p-doped semiconductor layer
30, an intrinsic semiconductor layer 40, an n-doped semiconductor
layer 50, a first back reflector layer 60, and a second back
reflector layer 70.
[0045] The substrate 10 is a structure that provides mechanical
support to the photovoltaic structure. The substrate 10 is
transparent in the range of electromagnetic radiation at which
photogeneration of electrons and holes occur within the
photovoltaic structure. If the prior art photovoltaic device is a
solar cell, the substrate 10 can be optically transparent. The
substrate 10 can be a glass substrate. The thickness of the
substrate 10 can be from 50 microns to 3 mm, although lesser and
greater thicknesses can also be employed.
[0046] The transparent conductive material layer 20 includes a
material that is transparent in the range of electromagnetic
radiation at which photogeneration of electrons and holes occur
within the photovoltaic device structure. If the photovoltaic
device structure is employed as a solar cell, the transparent
conductive material layer 20 can be optically transparent. For
example, the transparent conductive material layer 20 can include a
transparent conductive oxide such as a fluorine-doped tin oxide
(SnO.sub.2:F), an aluminum-doped zinc oxide (ZnO:Al), or indium tin
oxide. The thickness of the transparent conductive material layer
20 can be from 300 nm to 3 microns, although lesser and greater
thicknesses can also be employed.
[0047] The Schottky-barrier-reducing layer 22 includes a material
that reduces the Schottky barrier through the stack of the
transparent conductive material layer 20, the
Schottky-barrier-reducing layer 22, and the p-doped semiconductor
layer 30 relative a stack (not shown) consisting of a first layer
that is the same as the transparent conductive material layer 20
and a second layer that is the same as the p-doped semiconductor
layer 30. Thus, the presence of the Schottky-barrier-reducing layer
22 reduces the Schottky barrier of the stack of the transparent
conductive material layer 20, the Schottky-barrier-reducing layer
22, and the p-doped semiconductor layer 30 relative to the stack
consisting of the first layer and the second layer.
Correspondingly, the contact resistance through the stack of the
transparent conductive material layer 20, the
Schottky-barrier-reducing layer 22, and the p-doped semiconductor
layer 30 is less than the contact resistance of the stack
consisting of the first layer and the second layer. In other words,
the Schottky barrier across the stack of the transparent conductive
material layer 20, the Schottky-barrier-reducing layer 22, and the
p-doped semiconductor layer 30 has lower contact resistance than a
Schottky barrier across another stack that includes all layers of
the stack less the Schottky-barrier-reducing layer 22. The various
embodiments of the present disclosure differ by the composition of
the material in the Schottky-barrier-reducing layer 22.
[0048] According to a first embodiment, the Schottky-barrier
inducing layer 22 is an optically transparent layer including an
allotrope of carbon. In this embodiment, the exemplary photovoltaic
device structure is referred to as a first exemplary photovoltaic
device structure. In one case, the Schottky-barrier-reducing layer
22 can be a single wall carbon nanotube layer. A carbon nanotube is
an allotrope of carbon with a cylindrical nanostructure. A single
wall carbon nanotube is a carbon nanotube that does not contain any
other carbon nanotube therein, and is not contained in another
carbon nanotube. Thus, a single wall carbon nanotube is a single
strand of carbon nanotube that stands alone by itself without
including, or being included in, another carbon nanotube. The
cylindrical arrangement of carbon atoms in a single wall carbon
nanotube provides novel properties that make the carbon nanotube
potentially useful in many applications. The diameter of a single
wall carbon nanotube is on the order of a few nanometers, while the
length of the single wall carbon nanotube can be from tens of
nanometers to tens of centimeters. The chemical bonding of a single
wall carbon nanotube is composed entirely of sp2 bonds, similar to
the bonding of graphite. This bonding structure is stronger than
the sp3 bonds found in diamonds, providing the single wall carbon
nanotube with their unique strength. The single wall carbon
nanotube has a work function on the order of 5 eV, which is greater
than the work function of most transparent conductive material
employed for photovoltaic devices.
[0049] In another case, the Schottky-barrier-reducing layer 22 can
be a graphene layer. Graphene is a structure consisting of carbon
as a two-dimensional sheet. A graphene monolayer has a thickness of
about 0.34 nm, i.e., which is approximately the atomic diameter of
a single carbon atom. A graphene layer can exist as a monolayer of
a two-dimensional sheet. Alternately, a graphene layer can exist as
a stack of a plurality of two-dimensional monolayers of carbon,
which do not exceed more than 10 monolayers and is typically
limited to less than 5 monolayers. Graphene provides excellent
in-plane conductivity. Semiconductor devices employing graphene
have been suggested in the art to provide high-density and
high-switching-speed semiconductor circuits. Carbon atoms are
arranged in a two-dimensional honeycomb crystal lattice in which
each carbon-carbon bond has a length of about 0.142 nm.
[0050] According to a second embodiment, the
Schottky-barrier-reducing layer 22 includes a same material as the
transparent conductive material layer 20. However, the
Schottky-barrier-reducing layer 22 has a different doping than the
transparent conductive material layer. The difference in the doping
between the transparent conductive material layer 20 and the
Schottky-barrier-reducing layer 22 is set such that the presence of
the Schottky-barrier-reducing layer 22 reduces the Schottky barrier
between the transparent conductive material layer 20 and the
p-doped semiconductor layer 30. In this embodiment, the exemplary
photovoltaic device structure is referred to as a second exemplary
photovoltaic device structure.
[0051] In one case, the transparent conductive material layer 20
includes an aluminum-doped zinc oxide having an aluminum doping at
a first dopant concentration, and the Schottky-barrier-reducing
layer 22 includes an aluminum-doped zinc oxide having an aluminum
doping at a second dopant concentration. In this case, the first
dopant concentration is greater than the second dopant
concentration.
[0052] In another case, the transparent conductive material layer
20 includes a first fluorine-doped tin oxide having a fluorine
doping at a first dopant concentration, and the
Schottky-barrier-reducing layer 22 includes a second fluorine-doped
tin oxide having a fluorine doping at a second dopant
concentration. In this case, the first dopant concentration is
greater than the second dopant concentration.
[0053] The p-doped semiconductor layer 30 includes an amorphous or
microcrystalline p-doped semiconductor-containing material. In some
cases, the p-doped semiconductor layer 30 can include a
hydrogenated amorphous or microcrystalline p-doped
semiconductor-containing material. The presence of hydrogen in the
p-doped semiconductor layer 30 can increase the concentration of
free charge carriers, i.e., holes, by delocalizing the electrical
charges that are pinned to defect sites.
[0054] A hydrogenated p-doped semiconductor-containing material can
be deposited in a process chamber containing a
semiconductor-material-containing reactant gas a carrier gas. To
facilitate incorporation of hydrogen in the hydrogenated p-doped
semiconductor-containing material, a carrier gas including hydrogen
can be employed. Hydrogen atoms in the hydrogen gas within the
carrier gas are incorporated into the deposited material to form an
amorphous or microcrystalline hydrogenated p-doped
semiconductor-containing material of the p-doped semiconductor
layer 30. The thickness of the p-doped semiconductor layer 30 can
be from 3 nm to 30 nm, although lesser and greater thicknesses can
also be employed.
[0055] The p-doped semiconductor layer 30 can include a
silicon-containing material, a germanium-containing material, or a
compound semiconductor material. In one embodiment, the p-doped
semiconductor layer 30 includes a silicon-containing material. The
microcrystalline p-doped hydrogenated semiconductor-containing
material can be a microcrystalline p-doped hydrogenated
silicon-carbon alloy. In this case, a carbon-containing gas can be
flown into the processing chamber during deposition of the
microcrystalline p-doped hydrogenated silicon-carbon alloy. The
atomic concentration of carbon in the microcrystalline p-doped
hydrogenated silicon-carbon alloy of the p-doped semiconductor
layer can be from 1% to 90%, and preferably from 10% to 70%. In
this case, the band gap of the p-doped semiconductor layer 30 can
be from 1.7 eV to 2.1 eV.
[0056] The intrinsic semiconductor layer 40 includes an intrinsic
hydrogenated semiconductor-containing material. The intrinsic
hydrogenated semiconductor-containing material is deposited in a
process chamber containing a semiconductor-material-containing
reactant gas a carrier gas including hydrogen. Hydrogen atoms in
the hydrogen gas within the carrier gas are incorporated into the
deposited material to form the intrinsic hydrogenated
semiconductor-containing material of the intrinsic semiconductor
layer 40. The intrinsic hydrogenated semiconductor-containing
material can be amorphous or microcrystalline. Typically, the
intrinsic hydrogenated semiconductor-containing material is
amorphous. The thickness of the intrinsic semiconductor layer 40
depends on the diffusion length of electrons and holes in the
intrinsic hydrogenated semiconductor-containing material.
Typically, the thickness of the intrinsic semiconductor layer 40 is
from 100 nm to 1 micron, although lesser and greater thicknesses
can also be employed.
[0057] The intrinsic semiconductor layer 40 can include a
silicon-containing material, a germanium-containing material, or a
compound semiconductor material. In one embodiment, the intrinsic
semiconductor layer 40 includes a silicon-containing material. The
semiconductor material of the intrinsic semiconductor layer 40 can
be amorphous intrinsic silicon.
[0058] The n-doped semiconductor layer 50 includes an n-doped
semiconductor-containing material. The n-doped semiconductor layer
50 can be a hydrogenated material, in which case an n-doped
hydrogenated semiconductor-containing material is deposited in a
process chamber containing a semiconductor-material-containing
reactant gas a carrier gas including hydrogen. The n-type dopants
in the n-doped semiconductor layer 50 can be introduced by in-situ
doping. Alternately, the n-type dopants in the n-doped
semiconductor layer 50 can be introduced by subsequent introduction
of dopants employing any method known in the art. The n-doped
semiconductor layer 50 can be amorphous or microcrystalline. The
thickness of the n-doped semiconductor layer 50 can be from 6 nm to
60 nm, although lesser and greater thicknesses can also be
employed.
[0059] The n-doped semiconductor layer 50 can include a
silicon-containing material, a germanium-containing material, or a
compound semiconductor material. In one embodiment, the n-doped
semiconductor layer 50 includes a silicon-containing material. The
semiconductor material of the n-doped semiconductor layer 50 can be
amorphous n-doped silicon.
[0060] The first back reflector layer 60 includes a transparent
conductive material that is transparent in the range of
electromagnetic radiation at which photogeneration of electrons and
holes occur within the photovoltaic device structure. If the
photovoltaic device structure is employed as a solar cell, the
first back reflector layer 60 can be optically transparent. For
example, the first back reflector layer 60 can include a
transparent conductive oxide such as a fluorine-doped tin oxide
(SnO.sub.2:F), an aluminum-doped zinc oxide (ZnO:Al), or indium tin
oxide. Since such transparent conductive oxide materials are n-type
materials, the contact between the first back reflector layer 60
and the n-doped semiconductor layer 50 is Ohmic, and as such, the
contact resistance between the first back reflector layer 60 and
the n-doped semiconductor layer 50 is negligible. The thickness of
the back reflector layer 60 can be from 25 nm to 250 nm, although
lesser and greater thicknesses can also be employed.
[0061] The second back reflector layer 70 includes a metallic
material. Preferably, the metallic material has a high reflectivity
in the range of electromagnetic radiation at which photogeneration
of electrons and holes occur within the photovoltaic device
structure. The metallic material can include silver, aluminum, or
an alloy thereof. The thickness of the second back reflector layer
70 can be from 100 nm to 1 micron, although lesser and greater
thicknesses can also be employed.
[0062] Because the resistance due to the Schottky barrier between
the transparent conductive material layer 20 and the p-doped
semiconductor layer 30 is the predominant component of a series
resistance in properly constructed (i.e., non-defective)
photovoltaic devices, the reduction of the Schottky barrier results
in a dramatic reduction in the series resistance in the exemplary
photovoltaic device structure according to the present disclosure
compared to prior art photovoltaic device structures. For example,
while the prior art photovoltaic device structure of FIG. 1 has a
series resistance from 20 Ohms-cm.sup.2 to 30 Ohms-cm.sup.2 if an
amorphous hydrogenated carbon-containing silicon alloy is employed
for a p-doped semiconductor layer 130, or has a series resistance
from 10 Ohms-cm.sup.2 to 15 Ohms-cm.sup.2 if a microcrystalline
hydrogenated carbon-containing silicon alloy is employed for the
p-doped semiconductor layer 130, photovoltaic device structures
according to the present disclosure can have a series resistance
less than 9 Ohms-cm.sup.2. Various samples of photovoltaic device
structures according to the various embodiments of the present
disclosure demonstrated a series resistance from 4 Ohms-cm.sup.2 to
9 Ohms-cm.sup.2.
[0063] Referring to FIG. 7, a graph illustrates the J-V
characteristics of a junction of single wall carbon nanotubes and a
p-doped silicon material according to the first embodiment of the
present disclosure. The J-V characteristics of the junction of the
single wall carbon nanotube and a p-doped silicon material
approximate the J-V characteristics of the junction between the
Schottky-barrier-reducing layer 22 and the p-doped semiconductor
layer 30 within the first exemplary photovoltaic device structure
for the case in which the Schottky-barrier-reducing layer 22
includes a layer of single wall carbon nanotubes and the p-doped
semiconductor layer 30 includes the p-doped silicon material. The
J-V characteristics show a diode forward voltage drop of about 0.1V
and an estimated internal resistance of about 2 Ohms-cm.sup.2. The
J-V characteristics illustrate a lower contact resistance between
the Schottky-barrier-reducing layer 22 and the p-doped
semiconductor layer 30 than the contact resistance between any
transparent conductive material layer known in the art and a
p-doped semiconductor layer known in the art.
[0064] Referring to FIG. 7, a band diagram of a junction of a
single wall carbon nanotube and a p-doped silicon layer illustrates
the mechanism for the reduction in the contact resistance between
the Schottky-barrier-reducing layer 22 and the p-doped
semiconductor layer 30 according to the first embodiment of the
present disclosure. The band bending .PHI..sub.b in the p-doped
semiconductor layer 30 is the same irrespective of any selected
energy level, i.e., the amount of band bending the same for the
vacuum level, for the valence band E.sub.v, and for the conduction
band E.sub.c. The various energy gaps between different energy
levels in the band diagram are related by the following
equations:
.eta.=E.sub.F-E.sub.V.apprxeq.0 (Eq. 1)
.PHI..sub.SWCNT+.eta.+.PHI..sub.b=.chi..sub.e+E.sub.g Si (Eq.
2)
.PHI..sub.SWCNT=4.97 eV (Eq. 3)
.chi..sub.e=4.05 eV (Eq. 4)
E.sub.g Si=1.12 eV (Eq. 5)
Solving the above equations, the value of band bending .PHI..sub.b
is 0.2 eV, which is substantially smaller than an equivalent band
bending of about 0.7 eV at an interface between a typical
transparent conductive material and a p-doped silicon material. The
smaller band bending at the interface between
Schottky-barrier-reducing layer 22 and the p-doped semiconductor
layer 30 reduces the Schottky barrier, and consequently the
accompanying contact resistance, compared with a Schottky barrier
between a transparent conductive material and a p-doped silicon
material.
[0065] Single wall carbon nanotubes are transparent. See, for
example, Hu et al., "Percolation in Transparent and Conducting
Carbon Nanotube Networks," Nanoletters, 2004, Vol. 4, No. 12, pp.
2513-2517.
[0066] Referring to FIG. 8, a graph illustrates the resistivity of
aluminum-doped zinc oxide as a function of weight percentage of
aluminum according to a second embodiment of the present
disclosure. The resistivity of aluminum-doped zinc oxide depends on
the dopant concentration of aluminum. A high concentration of
aluminum decreases resistivity of aluminum-doped zinc oxide, and
shifts the Fermi level close to the conduction band edge.
Conversely, a low concentration of aluminum increases resistivity
of aluminum-doped zinc oxide, and shifts the Fermi level away from
the conduction band toward the mid-gap level, i.e., the energy
level midway between the valence band edge and the conduction band
edge.
[0067] In case aluminum-doped zinc oxide is employed for the
transparent conductive material layer 120 in the prior art
photovoltaic device structure of FIG. 1, the concentration of
aluminum in the transparent conductive material layer 120 is
constant. A homogeneous heavily-aluminum-doped zinc oxide layer may
be employed for the transparent conductive material layer 120 in
the prior art photovoltaic device structure of FIG. 1. On the one
hand, if a heavily-aluminum-doped zinc oxide layer contacts a
p-doped semiconductor layer 30, the presence of the Fermi level
close to the conduction band edge of the heavily-aluminum-doped
zinc oxide layer causes significant the band bending in the p-doped
semiconductor layer 30, and the Schottky barrier between the
p-doped semiconductor layer 30 and the heavily-aluminum-doped zinc
oxide layer. At the same time, the low resistivity of the
heavily-aluminum-doped zinc oxide layer reduces internal resistance
of the photovoltaic device structure that includes the
heavily-aluminum-doped zinc oxide layer. On the other hand, if a
lightly-aluminum-doped zinc oxide layer contacts a p-doped
semiconductor layer 30, the presence of the Fermi level close to
the middle of the band gap of the lightly-aluminum-doped zinc oxide
layer reduces the band bending in the p-doped semiconductor layer
30 relative to the band bending in the case of a
heavily-aluminum-doped zinc oxide layer. The Schottky barrier
between the p-doped semiconductor layer 30 and the
lightly-aluminum-doped zinc oxide layer is correspondingly
decreased. However, the high resistivity of the
lightly-aluminum-doped zinc oxide layer increases internal
resistance of the photovoltaic device structure that includes the
heavily-aluminum-doped zinc oxide layer. The thickness of the
Schottky-barrier-reducing layer 22 can be from 1 nm to 10 nm in
this case. When lightly-aluminum-doped zinc oxide is located at the
interface between p-type semiconductor and heavily-aluminum-doped
zinc oxide, the lightly-aluminum-doped zinc oxide reduces Schottky
barrier heights and heavily-aluminum-doped zinc oxide reduces sheet
resistance of entire zinc oxide stacks.
[0068] In one example of the second embodiment, the transparent
conductive material layer 20 includes an aluminum-doped zinc oxide
having an aluminum doping at a first dopant concentration, and the
Schottky-barrier-reducing layer 22 includes an aluminum-doped zinc
oxide having an aluminum doping at a second dopant concentration.
The first dopant concentration is greater than the second dopant
concentration. The high aluminum concentration of the transparent
conductive material layer 20 provides low internal resistance in
the second exemplary photovoltaic device structure. At the same
time, the low aluminum concentration of the
Schottky-barrier-reducing layer 22 provides a low Schottky barrier,
and correspondingly, a low contact resistance between the
Schottky-barrier-reducing layer 22 and the p-doped semiconductor
layer 30. In one example, the first dopant concentration is
selected to be in the range from 2.0% atomic concentration or
greater, and the second dopant concentration is selected to be in
the range between 0% and 2.0%, although different ranges may be
selected for the first and second dopant concentrations.
[0069] In case fluorine-doped tin oxide is employed for the
transparent conductive material layer 120 in the prior art
photovoltaic device structure of FIG. 1, the concentration of
fluorine in the transparent conductive material layer 120 is
constant. A homogeneous heavily-fluorine-doped tin oxide layer or a
homogeneous lightly-fluorine-doped tin oxide layer may be employed
for the transparent conductive material layer 120 in the prior art
photovoltaic device structure of FIG. 1. On the one hand, if a
heavily-fluorine-doped tin oxide layer contacts a p-doped
semiconductor layer 30, the presence of the Fermi level close to
the conduction band edge of the heavily-fluorine-doped tin oxide
layer causes significant the band bending in the p-doped
semiconductor layer 30, and the Schottky barrier between the
p-doped semiconductor layer 30 and the heavily-fluorine-doped tin
oxide layer. At the same time, the low resistivity of the
heavily-fluorine-doped tin oxide layer reduces internal resistance
of the photovoltaic device structure that includes the
heavily-fluorine-doped tin oxide layer. On the other hand, if a
lightly-fluorine-doped tin oxide layer contacts a p-doped
semiconductor layer 30, the presence of the Fermi level close to
the middle of the band gap of the lightly-fluorine-doped tin oxide
layer reduces the band bending in the p-doped semiconductor layer
30 relative to the band bending in the case of a
heavily-fluorine-doped tin oxide layer. The Schottky barrier
between the p-doped semiconductor layer 30 and the
lightly-fluorine-doped tin oxide layer is correspondingly
decreased. However, the high resistivity of the
lightly-fluorine-doped tin oxide layer increases internal
resistance of the photovoltaic device structure that includes the
heavily-fluorine-doped tin oxide layer. The thickness of the
Schottky-barrier-reducing layer 22 can be from 1 nm to 50 in this
case, although lesser and greater thicknesses can also be
employed.
[0070] In another example of the second embodiment, the transparent
conductive material layer 20 includes a fluorine-doped tin oxide
having a fluorine doping at a first dopant concentration, and the
Schottky-barrier-reducing layer 22 includes a fluorine-doped tin
oxide having a fluorine doping at a second dopant concentration.
The first dopant concentration is greater than the second dopant
concentration. The high fluorine concentration of the transparent
conductive material layer 20 provides low internal resistance in
the second exemplary photovoltaic device structure. At the same
time, the low fluorine concentration of the
Schottky-barrier-reducing layer 22 provides a low Schottky barrier,
and correspondingly, a low contact resistance between the
Schottky-barrier-reducing layer 22 and the p-doped semiconductor
layer 30. In one example, the first dopant concentration is
selected to be in the range from 2.0% atomic concentration or
greater, and the second dopant concentration is selected to be in
the range between 0% and 2.0%, although different ranges may be
selected for the first and second dopant concentrations.
[0071] In some cases, the Schottky-barrier-reducing layer 22 can
include a material having a higher resistivity than a material of
the transparent conductive material layer 20 if the conductivity of
the Schottky-barrier-reducing layer 22 increases with the dopant
concentration in the Schottky-barrier-reducing layer 22, i.e., if
the resistivity of the Schottky-barrier-reducing layer 22 decreases
with the dopant concentration in the Schottky-barrier-reducing
layer 22.
[0072] Referring to FIG. 9, a band diagram for first and second
exemplary photovoltaic devices structure illustrates the mechanism
for according to the first and second embodiments of the present
disclosure. The Schottly-barrier-reducing layer 22 has a work
function that is greater than a work function of the transparent
conductive material layer by a work function differential .DELTA..
The work function of the Schottly-barrier-reducing layer 22 is
lesser than an absolute value of the Fermi level E.sub.F of the
p-doped semiconductor layer 30, i.e., the energy difference between
the vacuum level and the Fermi level energy E.sub.F for the p-doped
semiconductor layer 30.
[0073] In the absence of the Schottky-barrier-reducing layer 22, a
direct contact between a transparent conductive material layer and
a p-doped semiconductor layer causes an energy barrier equivalent
to .DELTA.+.PHI..sub.b, i.e., the sum of the difference in the work
functions of the transparent conductive material layer 20 and the
Schottky-barrier-reducing layer 22 and the band bending .PHI..sub.b
at the interface between the Schottky-barrier-reducing layer 22 and
the p-doped semiconductor layer 30. Because the probability of
finding a hole within the valence band of the p-doped semiconductor
layer is determined by the Fermi-Dirac statistics, the probability
of finding a hole in a surface region of the p-doped semiconductor
layer 130 at the interface with the transparent conductive material
layer 20 decreases almost exponentially with .DELTA.+.PHI..sub.b.
Thus, the Schottky barrier and the contact resistance are
significant. In the present disclosure, the total energy barrier is
broken into two separate barriers, each of which is less
significant than a combined energy barrier. Consequently, the
Schottky barrier and the contact resistance in the combined stack
of the transparent conductive material layer 20, the
Schottky-barrier-reducing layer 22, and the p-doped semiconductor
layer 30 according to the first and second embodiments of the
present disclosure are reduced compared to prior art.
[0074] FIG. 10A-10C are sequential vertical cross-sectional views
that illustrate a manufacturing process for forming the exemplary
photovoltaic device structure of FIG. 6. Referring to FIG. 10A, the
substrate 10 includes a material that is transparent in the range
of electromagnetic radiation at which photogeneration of electrons
and holes occur within the photovoltaic structure as describe
above. The transparent conductive material layer 20 is formed on
the substrate 10, for example, by deposition.
[0075] Referring to FIG. 10B, the Schottky-barrier-reducing layer
22 is deposited for example, by chemical vapor deposition,
evaporation, or any other known methods of deposition. In case the
Schottky-barrier-reducing layer 22 includes an allotrope of carbon,
methods known in the art for forming such an allotrope of carbon
can be employed. In case the Schottky-barrier-reducing layer 22
includes a transparent conductive material having a different
dopant concentration than the transparent conductive material layer
20, methods for forming the transparent conductive material layer
20 can be modified to alter the dopant concentration in the
Schottky-barrier-reducing layer 22.
[0076] Referring to FIG. 10C, the p-doped semiconductor layer 30 is
deposited in a process chamber containing a
semiconductor-material-containing reactant gas and a carrier gas.
The p-doped semiconductor layer 30 is formed on the
Schottky-barrier-reducing layer 22 in the presence of the
semiconductor-material-containing reactant and the carrier gas in a
chemical vapor deposition. In case the carrier gas includes
hydrogen, the p-doped semiconductor layer 30 includes a
hydrogenated p-doped semiconductor material. The chemical vapor
deposition process can be plasma enhanced chemical vapor process
(PECVD) performed at a deposition temperature from 50.degree. C. to
400.degree. C., and preferably from 100.degree. C. to 350.degree.
C., and at a pressure from 0.1 Torr to 10 Torr, and preferably from
0.2 Torr to 5 Torr.
[0077] The semiconductor-material-containing reactant gas includes
at least one atom of silicon, germanium, or a component
semiconductor material of a compound semiconductor material such as
GaAs. The p-type dopants in the p-doped semiconductor-containing
material of the p-doped semiconductor layer 30 can be introduced by
in-situ doping. Alternately, the p-type dopants in the
microcrystalline p-doped hydrogenated semiconductor-containing
material can be introduced by subsequent introduction of dopants
employing any method known in the art such as plasma doping, ion
implantation, and/or outdiffusion from a disposable diffusion
source (e.g., borosilicate glass).
[0078] The material of the p-doped semiconductor layer 30 can be a
p-doped hydrogenated silicon-carbon alloy. In this case, a
carbon-containing gas can be flown into the processing chamber
during deposition of the p-doped hydrogenated silicon-carbon
alloy.
[0079] Subsequently, the intrinsic semiconductor layer 40 is
deposited on the p-doped semiconductor layer 30, for example, by
plasma-enhanced chemical vapor deposition. In case the intrinsic
semiconductor layer 40 includes an intrinsic hydrogenated
semiconductor-containing material, hydrogen gas is supplied into
the process chamber concurrently with a
semiconductor-material-containing reactant gas. The intrinsic
hydrogenated semiconductor-containing material can be amorphous or
microcrystalline.
[0080] The n-doped semiconductor layer 50 is deposited on the
intrinsic semiconductor layer 40, for example, by plasma-enhanced
chemical vapor deposition. In case the n-doped semiconductor layer
50 includes an n-doped hydrogenated semiconductor-containing
material, hydrogen gas is supplied into the process chamber
concurrently with a semiconductor-material-containing reactant gas.
The material of the n-doped semiconductor layer 50 can be amorphous
or microcrystalline.
[0081] The n-type dopants in the n-doped semiconductor layer 50 can
be introduced by in-situ doping. For example, phosphine (PH.sub.3)
gas or arsine (AsH.sub.3) gas can be flown into the processing
chamber concurrently with the semiconductor-material-containing
reactant gas if the n-doped semiconductor layer 50 includes an
n-doped silicon-containing material or an n-doped
germanium-containing material. If the n-doped semiconductor layer
50 includes an n-doped compound semiconductor material, the ratio
of the flow rate of the reactant gas for the Group II or Group III
material to the flow rate of the reactant gas for the group VI or
Group V material can be decreased to induce n-type doping.
Alternately, the n-type dopants in the n-doped semiconductor layer
50 can be introduced by subsequent introduction of dopants
employing any method known in the art.
[0082] The first back reflector layer 60 is deposited on the
n-doped semiconductor layer 50 employing methods known in the art.
The first back reflector layer 60 includes a transparent conductive
material. The second back reflector layer 70 is subsequently
deposited on the first back reflector layer 70, for example, by
electroplating, electroless plating, physical vapor deposition,
chemical vapor deposition, vacuum evaporation, or a combination
thereof. The second back reflector layer 70 can be a metallic
layer.
[0083] While the present disclosure has been particularly shown and
described with respect to preferred embodiments thereof, it will be
understood by those skilled in the art that the foregoing and other
changes in forms and details can be made without departing from the
spirit and scope of the present disclosure. It is therefore
intended that the present disclosure not be limited to the exact
forms and details described and illustrated, but fall within the
scope of the appended claims.
* * * * *