U.S. patent application number 16/506794 was filed with the patent office on 2019-10-31 for formation of ohmic back contact for ag2znsn(s,se)4 photovoltaic devices.
The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Talia S. Gershon, Oki Gunawan, Richard A. Haight, Ravin Mankad.
Application Number | 20190334043 16/506794 |
Document ID | / |
Family ID | 59226877 |
Filed Date | 2019-10-31 |
United States Patent
Application |
20190334043 |
Kind Code |
A1 |
Gershon; Talia S. ; et
al. |
October 31, 2019 |
Formation of Ohmic Back Contact for Ag2ZnSn(S,Se)4 Photovoltaic
Devices
Abstract
Techniques for forming an ohmic back contact for
Ag.sub.2ZnSn(S,Se).sub.4 photovoltaic devices. In one aspect, a
method for forming a photovoltaic device includes the steps of:
depositing a refractory electrode material onto a substrate;
depositing a contact material onto the refractory electrode
material, wherein the contact material includes a transition metal
oxide; forming an absorber layer on the contact material, wherein
the absorber layer includes Ag, Zn, Sn, and at least one of S and
Se; annealing the absorber layer; forming a buffer layer on the
absorber layer; and forming a top electrode on the buffer layer.
The refractory electrode material may be Mo, W, Pt, Ti, TiN, FTO,
and combinations thereof. The transition metal oxide may be
TiO.sub.2, ZnO, SnO, ZnSnO, Ga.sub.2O.sub.3, and combinations
thereof. A photovoltaic device is also provided.
Inventors: |
Gershon; Talia S.; (White
Plains, NY) ; Gunawan; Oki; (Westwood, NJ) ;
Haight; Richard A.; (Mahopac, NY) ; Mankad;
Ravin; (Yonkers, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Family ID: |
59226877 |
Appl. No.: |
16/506794 |
Filed: |
July 9, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14984512 |
Dec 30, 2015 |
|
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16506794 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/022425 20130101;
H01L 31/072 20130101; H01L 31/0326 20130101; Y02E 10/50
20130101 |
International
Class: |
H01L 31/032 20060101
H01L031/032; H01L 31/072 20060101 H01L031/072; H01L 31/0224
20060101 H01L031/0224 |
Goverment Interests
STATEMENT OF GOVERNMENT RIGHTS
[0002] This invention was made with Government support under
Contract number DE-EE0006334 awarded by The Department of Energy.
The Government has certain rights in this invention.
Claims
1. A photovoltaic device, comprising: a substrate; a refractory
electrode material on the substrate; a contact material on the
refractory electrode material, wherein the contact material
comprises a transition metal oxide; an absorber layer on the
contact material, wherein the absorber layer comprises silver (Ag),
zinc (Zn), tin (Sn), and at least one of sulfur (S) and selenium
(Se); a buffer layer on the absorber layer; and a top electrode on
the buffer layer.
2. The photovoltaic device of claim 1, wherein the substrate
comprises a glass, a ceramic, a metal foil, or a plastic
substrate.
3. The photovoltaic device of claim 1, wherein the refractory
electrode material is selected from the group consisting of:
molybdenum (Mo), tungsten (W), platinum (Pt), titanium (Ti),
titanium nitride (TiN), fluorinated tin oxide (FTO), and
combinations thereof.
4. The photovoltaic device of claim 1, wherein the refractory
electrode material has a thickness of from about 0.5 micrometer to
about 2 micrometers, and ranges therebetween.
5. The photovoltaic device of claim 1, wherein the transition metal
oxide is selected from the group consisting of: titanium oxide
(TiO.sub.2), zinc oxide (ZnO), tin oxide (SnO), zinc tin oxide
(ZnSnO), gallium oxide (Ga.sub.2O.sub.3), and combinations
thereof.
6. The photovoltaic device of claim 1, wherein the transition metal
oxide comprises TiO.sub.2.
7. The photovoltaic device of claim 1, wherein the contact material
has a thickness of from about 5 nanometers to about 100 nanometers,
and ranges therebetween.
8. The photovoltaic device of claim 1, wherein the buffer layer
comprises a buffer material selected from the group consisting of:
copper(I) oxide (Cu.sub.2O), nickel(II) oxide (NiO), zinc telluride
(ZnTe), aluminum phosphide (AlP), molybdenum trioxide (MoO.sub.3),
cadmium telluride (CdTe), copper(I) iodide (CuI, molybdenum(IV)
oxide (MoO.sub.2), molybdenum disulfide (MoS.sub.2), molybdenum
diselenide (MoSe.sub.2), and combinations thereof.
9. The photovoltaic device of claim 1, wherein the buffer layer
comprises MoO.sub.3.
10. The photovoltaic device of claim 1, wherein the top electrode
comprises a transparent conductive oxide.
11. The photovoltaic device of claim 10, wherein the transparent
conductive oxide is selected from the group consisting of:
indium-tin-oxide, aluminum-doped zinc oxide, and combinations
thereof.
12. The photovoltaic device of claim 1, further comprising: metal
contacts on the top electrode.
13. The photovoltaic device of claim 12, wherein the metal contacts
comprise a metal selected from the group consisting of: gold (Au),
silver (Ag), aluminum (Al), nickel (Ni), and combinations
thereof.
14. A photovoltaic device, comprising: a substrate; a refractory
electrode material on the substrate, wherein the refractory
electrode material is selected from the group consisting of: Mo, W,
Pt, Ti, TiN, FTO, and combinations thereof; a contact material on
the refractory electrode material, wherein the contact material
comprises a transition metal oxide selected from the group
consisting of: TiO.sub.2, ZnO, SnO, ZnSnO, Ga.sub.2O.sub.3, and
combinations thereof; an absorber layer on the contact material,
wherein the absorber layer comprises silver (Ag), zinc (Zn), tin
(Sn), and at least one of sulfur (S) and selenium (Se); a buffer
layer on the absorber layer; and a top electrode on the buffer
layer.
15. The photovoltaic device of claim 14, wherein the refractory
electrode material has a thickness of from about 0.5 micrometer to
about 2 micrometers, and ranges therebetween.
16. The photovoltaic device of claim 14, wherein the contact
material has a thickness of from about 5 nanometers to about 100
nanometers, and ranges therebetween.
17. The photovoltaic device of claim 14, wherein the buffer layer
comprises a buffer material selected from the group consisting of:
copper(I) oxide (Cu.sub.2O), nickel(II) oxide (NiO), zinc telluride
(ZnTe), aluminum phosphide (AlP), molybdenum trioxide (MoO.sub.3),
cadmium telluride (CdTe), copper(I) iodide (CuI, molybdenum(IV)
oxide (MoO.sub.2), molybdenum disulfide (MoS.sub.2), molybdenum
diselenide (MoSe.sub.2), and combinations thereof.
18. The photovoltaic device of claim 14, wherein the top electrode
comprises a transparent conductive oxide.
19. The photovoltaic device of claim 18, wherein the transparent
conductive oxide is selected from the group consisting of:
indium-tin-oxide, aluminum-doped zinc oxide, and combinations
thereof.
20. The photovoltaic device of claim 1, further comprising: metal
contacts on the top electrode, wherein the metal contacts comprise
a metal selected from the group consisting of: Au, Ag, Al, Ni, and
combinations thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. Application Ser.
No. 14/984,512 filed on Dec. 30, 2015, the contents of which are
incorporated by reference herein.
FIELD OF THE INVENTION
[0003] The present invention relates to Ag.sub.2ZnSn(S,Se).sub.4
photovoltaic devices, and more particularly, to techniques for
forming an Ohmic back contact for Ag.sub.2ZnSn(S,Se).sub.4
photovoltaic devices.
BACKGROUND OF THE INVENTION
[0004] Ag.sub.2ZnSn(S,Se).sub.4 (AZTSSe) is an attractive
photovoltaic absorber material. It is based on
Cu.sub.2ZnSn(S,Se).sub.4 (CZTSSe), but Ag is substituted for Cu to
suppress bulk defects. See, for example, U.S. patent application
Ser. No. 14/936,131 by Gershon et al., entitled "Photovoltaic
Device Based on Ag.sub.2ZnSn(S,Se).sub.4 Absorber," (hereinafter
"U.S. patent application Ser. No. 14/936,131").
[0005] AZTSSe is intrinsically n-type, therefore other aspects of
the device must be re-optimized. For instance, molybdenum (Mo) is
typically used as the back contact material for CZTSSe photovoltaic
devices. However, Mo is non-Ohmic to AZTSSe. Therefore, an Ohmic
back contact is needed for the formation of AZTSSe thin film
photovoltaic devices.
[0006] Further, a low-work function material is needed for making
Ohmic contact to AZTSSe. This is due to well-understood physical
principles involving Fermi level equilibration between the metallic
contact and a semiconductor; to first order, low-work function
materials tend to make Ohmic contact to n-type semiconductors
whereas high work function materials tend to make Ohmic contact to
p-type semiconductors. However, most low-work function metals are
highly reactive with sulfur and selenium. For example, an aluminum
back contact cannot be used because the annealing step may result
in the complete consumption of the aluminum metal or else the
formation of an unwanted aluminum selenide interlayer. Therefore, a
stable low-work function contact material for AZTSSe thin film
photovoltaic devices is needed.
SUMMARY OF THE INVENTION
[0007] The present invention provides techniques for forming an
Ohmic back contact for Ag.sub.2ZnSn(S,Se).sub.4 photovoltaic
devices. In one aspect of the invention, a method for forming a
photovoltaic device is provided. The method includes the steps of:
depositing a refractory electrode material onto a substrate;
depositing a contact material onto the refractory electrode
material, wherein the contact material includes a transition metal
oxide; forming an absorber layer on the contact material, wherein
the absorber layer includes silver (Ag), zinc (Zn), tin (Sn), and
at least one of sulfur (S) and selenium (Se); annealing the
absorber layer; forming a buffer layer on the absorber layer, and
forming a top electrode on the buffer layer. The refractory
electrode material may be molybdenum (Mo), tungsten (W), platinum
(Pt), titanium (Ti), titanium nitride (TiN), fluorinated tin oxide
(FTO), and combinations thereof. The transition metal oxide may be
titanium oxide (TiO.sub.2), zinc oxide (ZnO), tin oxide (SnO), zinc
tin oxide (ZnSnO), gallium oxide (Ga.sub.2O.sub.3), and
combinations thereof.
[0008] In another aspect of the invention, a photovoltaic device is
provided. The photovoltaic device includes: a substrate; a
refractory electrode material on the substrate; a contact material
on the refractory electrode material, wherein the contact material
includes a transition metal oxide; an absorber layer on the contact
material, wherein the absorber layer includes Ag, Zn, Sn, and at
least one of S and Se; a buffer layer on the absorber layer; and a
top electrode on the buffer layer. The photovoltaic device may also
include: metal contacts on the top electrode.
[0009] A more complete understanding of the present invention, as
well as further features and advantages of the present invention,
will be obtained by reference to the following detailed description
and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A is a J-V curve for a photovoltaic device with a
low-work function transition metal oxide (TiO.sub.2) between the Mo
back contact and the AZTSSe absorber according to an embodiment of
the present invention;
[0011] FIG. 1B is a J-V curve for a photovoltaic device without a
low-work function transition metal oxide between the Mo back
contact and the AZTSSe absorber;
[0012] FIG. 2 is a cross-sectional diagram illustrating a
refractory electrode material (e.g., Mo) having been deposited onto
a substrate according to an embodiment of the present
invention;
[0013] FIG. 3 is a cross-sectional diagram illustrating a contact
material containing a stable, low-work function transition metal
oxide having been deposited onto the refractory electrode material
according to an embodiment of the present invention;
[0014] FIG. 4 is a cross-sectional diagram illustrating an AZTSSe
absorber having been formed on the contact material according to an
embodiment of the present invention;
[0015] FIG. 5 is a cross-sectional diagram illustrating a buffer
layer having been formed on the AZTSSe layer according to an
embodiment of the present invention;
[0016] FIG. 6 is a cross-sectional diagram illustrating a top
electrode having been formed on the buffer layer according to an
embodiment of the present invention;
[0017] FIG. 7 is a cross-sectional diagram illustrating metal
contacts having been formed on the top electrode according to an
embodiment of the present invention;
[0018] FIG. 8 is an image of a photovoltaic device sample prepared
using the present techniques according to an embodiment of the
present invention; and
[0019] FIG. 9 is an enlarged image of the junction between the
refractory electrode material (e.g., Mo) and the contact material
(e.g., TiO.sub.2) according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] As highlighted above, the bulk defects often encountered
with CZTSSe-based absorber materials can be eliminated if one swaps
out either Cu or Zn for a different 1+ or 2+ valence cation
(respectively). The term "CZTSSe," as used herein, refers to a
kesterite material containing copper (Cu), zinc (Zn), tin (Sn), and
at least one of sulfur (S) and selenium (Se). When silver (Ag) is
substituted for Cu, a Ag.sub.2ZnSn(S,Se).sub.4 material is formed
(abbreviated as AZTSSe). See U.S. patent application Ser. No.
14/936,131, the contents of which are incorporated by reference as
if fully set forth herein. Thus, the term "AZTSSe," as used herein,
refers to a kesterite material containing Ag, Zn, Sn, and at least
one of S and Se.
[0021] As also highlighted above, the implementation of AZTSSe
materials presents some notable challenges. For instance,
molybdenum (Mo), which is typically used as the back contact
material for CZTSSe photovoltaic devices, is unfortunately
non-Ohmic to AZTSSe due to the formation of an interfacial
MoSe.sub.2 layer (high-work function) which takes place during the
annealing step. Advantageously, provided herein are techniques for
forming Ohmic back contacts for AZTSSe photovoltaic devices.
[0022] As will be described in detail below, the present techniques
involve inserting a stable, low-work function transition metal
oxide contact material in between the Mo and the AZTSSe absorber.
We will refer to "low-work function" materials as those with a work
function below about 4.5 electronvolts (eV) and "high-work
function" materials as those with a work function above about 4.5
eV. By way of example only, suitable low-work function transition
metal oxide contact materials include, but are not limited to,
titanium oxide (TiO.sub.2), zinc oxide (ZnO), tin oxide (SnO), zinc
tin oxide (ZnSnO), and/or gallium oxide (Ga.sub.2O.sub.3). With
regard to stability, even if the transition metal oxide contact
material chosen (e.g., TiO.sub.2) reacts with S, Se to form
Ti(O,S(e)).sub.2, the work function of the oxyselenide remains low
and therefore the contact to AZTSe is still Ohmic. The present
photovoltaic devices will generally be formed as a stack containing
the following materials: Mo/low-work function transition metal
oxide contact material/AZTSSe absorber/buffer material/high-work
function (front) contact or p-type heterojunction partner.
[0023] It is hypothesized that Mo is non-Ohmic to AZTSSe due in
part to the formation of a high-work function Mo(S,Se).sub.2 layer
between the Mo and the AZTSSe. For instance, annealing a device
stack containing Mo/AZTSSe/MoO.sub.3/ITO forms a "double-diode"
indicative of the presence of a reverse junction (likely the
formation of MoSe.sub.2 between Mo and AZTSe. However, with the
addition of TiO.sub.2 between the AZTSSe and Mo (e.g.,
Mo/TiO.sub.2/AZTSSe/MoO.sub.3/ITO), the double diode feature is not
observed. This indicates that the TiO.sub.2 makes Ohmic contact to
AZTSSe, whereas Mo (with Mo(S,Se).sub.2) does not. Compare, for
example, J-V curves for sample with TiO.sub.2 (FIG. 1A) and without
TiO.sub.2 (FIG. 1B) between the Mo and AZTSSe absorber.
[0024] A detailed description of the present techniques is now
provided by way of reference to FIGS. 2-7 which provide an
exemplary methodology for forming an AZTSSe-based photovoltaic
device. As shown in FIG. 2, the process begins with a substrate
202. For photovoltaic device applications, suitable substrates
include, but are not limited to glass, ceramic, metal foil, or
plastic substrates.
[0025] A refractory electrode material 204 is next deposited onto
the substrate 202. Suitable refractory electrode materials include,
but are not limited to, Mo, tungsten (W), platinum (Pt), titanium
(Ti), titanium Nitride (TiN), and/or fluorinated tin oxide (FTO).
The refractory electrode material 204 can be deposited onto the
substrate 202 using, e.g., electron-beam (e-beam) evaporation,
thermal evaporation, sputtering, etc. According to an exemplary
embodiment, the refractory electrode material 204 is deposited on
the substrate 202 to a thickness of from about 0.5 micrometer
(.mu.m) to about 2 .mu.m, and ranges therebetween.
[0026] Next, as shown in FIG. 3, a contact material 302 containing
a stable, low-work function transition metal oxide is deposited
onto the refractory electrode material 204. As provided above,
suitable stable, low-work function transition metal oxides include,
but are not limited to, TiO.sub.2, ZnO, SnO, ZnSnO, and/or
Ga.sub.2O.sub.3.
[0027] The contact material 302 can be deposited onto the
refractory electrode material 204 using, e.g., e-beam evaporation,
atomic layer deposition, sputtering, etc. According to an exemplary
embodiment, the contact material 302 is deposited on the refractory
electrode material 204 to a thickness of from about 5 nanometers
(nm) to about 100 nm, and ranges therebetween.
[0028] An AZTSSe absorber 402 is then formed on the contact
material 302. See FIG. 4. As provided above, AZTSSe is a kesterite
material containing Ag, Zn, Sn, and at least one of S and Se.
Suitable techniques for forming an AZTSSe absorber are described in
U.S. patent application Ser. No. 14/936,131. For instance, as
provided in U.S. patent application Ser. No. 14/936,131, AZTSSe may
be formed using thermal evaporation in a vacuum chamber wherein Ag,
Zn, and Sn are co-evaporated from their respective sources.
Optionally, a thermal cracking cell(s) can be used for the S and
Se. For instance, as described in U.S. patent application Ser. No.
14/936,131, thermal cracking cells can be used to regulate the
ratio of the S to the Se in the AZTSSe material, so as to control
the band gap.
[0029] As also described in U.S. patent application Ser. No.
14/936,131, a sodium (Na)-containing layer (e.g., sodium fluoride
(NaF), or sodium sulfide (Na.sub.2S) or sodium selenide
(Na.sub.2Se)) can optionally be placed immediately before (or
after) the AZTSSe absorber. Na from the layer gets incorporated
into the absorber during annealing and can enhance its electrical
properties.
[0030] Prior to forming the AZTSSe, it is preferable to clean the
surface on which the AZTSSe is being formed to remove any potential
contaminants. Any standard cleaning process may be used, which can
vary depending on the electrode material being used. For instance,
an ammonium hydroxide (NH.sub.4OH) clean is suitable for a
metal-coated substrate, whereas a sulfuric acid mixed with an
inorganic oxidizer (such as NOCHROMIX.RTM. available from GODAX
Laboratories, Inc., Cabin John, Md.) is preferable for transparent
conducting oxides.
[0031] Once deposited on the substrate, the AZTSSe absorber 402 is
then annealed. Annealing improves the crystal grain structure as
well as the defect structure of the AZTSSe absorber 402, and in
some cases may be necessary to form a material having a kesterite
structure. According to an exemplary embodiment, the annealing is
carried out at a temperature of from about 400.degree. C. to about
550.degree. C., and ranges therebetween, for a duration of from
about 20 seconds to about 10 minutes, and ranges therebetween. If
present, Na (from the Na-containing layer) will incorporate into
the AZTSSe absorber 402 during this anneal.
[0032] According to an exemplary embodiment, the anneal is
performed in an environment containing excess chalcogen, e.g.,
excess S and/or Se. See, for example, U.S. Pat. No. 8,642,884
issued to Mitzi et al., entitled "Heat Treatment Process and
Photovoltaic Device Based on Said Process" (hereinafter "U.S. Pat.
No. 8,642,884"), the contents of which are incorporated by
reference as if fully set forth herein. As described in U.S. Pat.
No. 8,642,884, a heat treatment process involving sulfurization or
selenization passivates the layers/interfaces of the device and/or
suppresses phase decomposition.
[0033] A buffer layer 502 is then formed on the AZTSSe absorber
402. See FIG. 5. As provided above, AZTSSe is intrinsically n-type.
Accordingly, modifications to the device stack may be needed. For
instance, regarding the buffer layer, materials traditionally used
as a buffer such as cadmium sulfide (CdS) might not be appropriate.
Accordingly, suitable alternative buffer materials for use with
AZTSSe absorbers include, but are not limited to, copper(I) oxide
(Cu.sub.2O), nickel(II) oxide (NiO), zinc telluride (ZnTe),
aluminum phosphide (AlP), molybdenum trioxide (MoO.sub.3), cadmium
telluride (CdTe), copper(I) iodide (CuI, molybdenum(IV) oxide
(MoO.sub.2), molybdenum disulfide (MoS.sub.2), and/or molybdenum
diselenide (MoSe.sub.2). The semiconductors (e.g., Cu.sub.2O, CuI,
etc.) are "p-type heterojunction partners" while some of the
materials in the list are simply high-work function contacts (e.g.,
MoO.sub.3). According to an exemplary embodiment, the buffer layer
502 is deposited using chemical bath deposition, thermal or e-beam
evaporation, atomic layer deposition, electrodeposition or
sputtering to a thickness of from about 5 nm to about 200 nm, and
ranges therebetween.
[0034] As shown in FIG. 6, a top electrode 602 can then be formed
on the buffer layer 502. According to an exemplary embodiment, the
top electrode 602 is formed from a transparent conductive oxide
(TCO), such as indium-tin-oxide (ITO) and/or aluminum (Al)-doped
zinc oxide (ZnO) (AZO)). By way of example only, the top electrode
602 can be deposited onto the buffer layer 502 using
sputtering.
[0035] Metal contacts 702 may be formed on the top electrode 602.
See FIG. 7. According to an exemplary embodiment, the metal
contacts 702 are formed from gold (Au), silver (Ag), aluminum (Al)
and/or nickel (Ni). The metal contacts 702 may be formed on the top
electrode 602 using a process such as e-beam or thermal
evaporation.
[0036] The present techniques are further described by way of
reference to the following non-limiting examples. A photovoltaic
device was prepared according to the present techniques including a
glass substrate, and Mo as the refractory electrode material 204. A
20 nm thick layer of TiO.sub.2 was then deposited (using e-beam
evaporation) as the contact material 302 on the Mo. Following the
above process flow, an AZTSSe absorber 402 was formed on the
TiO.sub.2, followed by a MoO.sub.3 (buffer layer 502) and Au metal
contacts 702. An image 800 of the resulting device is shown in FIG.
8.
[0037] An enlarged view of the Mo/TiO.sub.2 interface is provided
in FIG. 9. As shown in FIG. 9, there might still be the formation
of some MoSe.sub.2 beneath the TiO.sub.2 layer. However, this does
not ruin the contact to the underlying metal (in this case Mo) due
to the presence of the TiO.sub.2. The MoSe.sub.2, if any, merely
adds to the series resistance.
[0038] Although illustrative embodiments of the present invention
have been described herein, it is to be understood that the
invention is not limited to those precise embodiments, and that
various other changes and modifications may be made by one skilled
in the art without departing from the scope of the invention.
* * * * *