U.S. patent application number 10/555792 was filed with the patent office on 2006-08-17 for thin-film solar cell.
Invention is credited to John Kessler, Charlotte Platzer Bjorkman, lars Stoit.
Application Number | 20060180200 10/555792 |
Document ID | / |
Family ID | 20291248 |
Filed Date | 2006-08-17 |
United States Patent
Application |
20060180200 |
Kind Code |
A1 |
Platzer Bjorkman; Charlotte ;
et al. |
August 17, 2006 |
Thin-film solar cell
Abstract
The present invention relates to thin-film solar cells of the
CIGS-type. A characteristic feature of the invention is the use of
two integrally formed buffer layers, a first ALD Zn(O,S) buffer
layer (7) on top of the CIGS-layer (3) and a second ALD ZnO-buffer
layer (8) on top of the first (7) buffer layer. Both buffer layers
are deposited in the same process step using ALD (atom layer
deposition). The invention also relates to a method of producing
the cell and a process line for manufacturing of the cell
structure.
Inventors: |
Platzer Bjorkman; Charlotte;
(Uppsala, SE) ; Kessler; John; (Nantes, FR)
; Stoit; lars; (Uppsala, SE) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET
2ND FLOOR
ARLINGTON
VA
22202
US
|
Family ID: |
20291248 |
Appl. No.: |
10/555792 |
Filed: |
May 5, 2004 |
PCT Filed: |
May 5, 2004 |
PCT NO: |
PCT/SE04/00689 |
371 Date: |
November 7, 2005 |
Current U.S.
Class: |
136/265 ;
136/262; 257/E31.007; 257/E31.027; 438/93; 438/95 |
Current CPC
Class: |
H01L 31/0322 20130101;
Y02E 10/541 20130101; H01L 31/0749 20130101; Y02P 70/50 20151101;
Y02P 70/521 20151101 |
Class at
Publication: |
136/265 ;
136/262; 438/093; 438/095 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 8, 2003 |
SE |
0301350-5 |
Claims
1-15. (canceled)
16. A thin-film solar cell comprising a thin film of p-type
semiconductor Cu(In,Ga)(Se,S).sub.2 light absorbing layer
(CIGS-layer) (3) formed on a back electrode layer (2), a thin film
of transparent conductive metal oxide formed over the light
absorbing layer, having n-type conductivity and serving as a window
layer (5) and electrode, and an interfacial layer (6) between the
window layer and the CIGS-layer characterized by said interfacial
layer comprising a first sulphur containing buffer layer (7) ALD
grown on the CIGS-layer, and a second buffer layer (8) integrally
formed with the first buffer layer by ALD deposition and comprising
ZnO.
17. A thin-film solar cell in accordance with claim 16
characterized in that the first buffer layer (7) is ALD deposited
Zn(O.sub.x,S.sub.1-x), where x varies between 0 and 0,9, preferably
between 0,1 and 0,7
18. A thin-film solar cell in accordance with claim 16
characterized in that the first buffer layer (7) is ALD deposited
In.sub.2S.sub.3.
19. A thin-film solar cell in accordance with claim 17
characterized in that the thickness of the first buffer layer (7)
is larger than about 1 nm and less than about 30 nm.
20. A thin-film solar cell in accordance with claim 16,
characterized in that there is a graded transition of the first
buffer layer (7) into the second buffer layer (8), the sulphur
content of the first buffer layer gradually decreasing in a
direction over its thickness.
21. A method of forming an interfacial layer between a thin film of
p-type semiconductor Cu(In,Ga)(Se,S).sub.2 light absorbing layer
(CIGS-layer) (3) and a thin film of n-type conductive window layer
(5) on a substrate of--a thin film solar cell structure, the method
comprising the steps of vacuum deposition of the CIGS layer in a
CIGS process chamber vacuum deposition of the CIGS layer in a CIGS
process chamber (11), characterized by transporting the substrates
with deposited CIGS layer to an ALD process chamber without
exposing the substrates to the atmosphere, and transporting the
substrates with deposited CIGS layer to an ALD process chamber
without exposing the substrates to the atmosphere, and providing
said interfacial layer in the ALD process chamber in the ALD
process chamber initially by depositing a first sulphur containing
buffer layer (6) on the surface of the CIGS-layer using atomic
layer deposition (ALD) and finally depositing a second ZnO layer
(8) on top of the first buffer layer (7) using atomic layer
deposition (ALD).
22. A method in accordance with claim 21 characterized by forming
the first and second buffer layers (7, 8) in one and the same ALD
process chamber (13).
23. A method in accordance with claim 21 characterized by forming
the first and second buffer layers in separate, but linked ALD
process chambers (13, 15).
24. A method in accordance with claim 21 characterized by providing
the first sulphur containing buffer layer (7) by exposing the
absorbing layer (3) in the ALD reaction chamber (13) alternatively
to pulses of a organo metallic zinc compound such as for example
diethyl zinc or dimethyl zinc to form a zinc containing monolayer
or to pulses of water and H.sub.2S so as to grow oxygen and sulphur
on the zinc atoms in order to form a first monolayer which is
adsorbed on top of the light absorbing layer (3), repeating said
steps to grow additional sulphur containing monolayers layers on
top of each other, and continuing this process until the first
sulphur containing buffer layer (7) of a first predefined thickness
is obtained.
25. A method in accordance with claim 24 characterized by providing
pulses of H.sub.2S in a proportion of 10-100%, preferably in a
proportion of 15-25%, and most preferred in a proportion of 10% to
the total number water- and H.sub.2S pulses.
26. A method in accordance with claim 24 characterized by providing
the second ZnO buffer layer (8) in the same manner as the first
buffer (7) layer leaving out the pulses of H.sub.2S, by continuing
to expose the first buffer layer in the ALD reaction chamber
alternatively to gas pulses of an organo metallic zinc compound,
such as for example diethyl zinc or dimethyl zinc, and pulses of
water until the second ZnO buffer layer of a second predefined
thickness is obtained.
27. A method in accordance with claim 26 characterized by
successively decreasing said proportion of sulphur as said
additional monolayers are grown so as to obtain a gradual
transition of the first buffer layer (7) into the second buffer
layer (8).
28. A process line for manufacturing a solar cell structure
comprising a thin film of p-type semiconductor
Cu(In,Ga)(Se,S).sub.2 light absorbing layer (CIGS-layer) (3) formed
on a back electrode layer (2), a thin film (5) of transparent
conductive metal oxide formed over the light absorbing layer,
having n-type conductivity and serving as a window layer and
electrode, and an interfacial layer (6) between the window layer
and the CIGS-layer, said process line comprising a vacuum
deposition chamber (11) in which the CIGS layer is vacuum deposited
on a substrate characterized by an ALD process chamber (13) for ALD
deposition of said interfacial layer by first growing a first
sulphur containing buffer layer (7) on the surface of the
CIGS-layer using ALD deposition, and finally growing a second
buffer layer (8) containing ZnO on top of the first buffer layer
using ALD deposition, and a transport chamber (12) arranged between
the vacuum deposition chamber and the ALD process chamber for
transportation of solar film blanks from the vacuum deposition
chamber to the ALD deposition chamber without exposing the blanks
to the atmosphere and for cooling the solar film blanks from the
CIGS deposition temperature to the ALD deposition temperature.
29. A process line in accordance with claim 28, characterized by an
additional ALD process chamber (15) connected to said one ALD
process chamber (13), said first mentioned ALD process chamber (13)
being used for the ALD deposition of the first buffer layer (7) and
said additional ALD process chamber (15) being used for the ALD
deposition of the second buffer layer (8).
Description
TECHNICAL AREA
[0001] The present invention relates to a thin-film solar cell
without cadmium, to a method and production line for manufacturing
such cells.
BACKGROUND OF THE INVENTION
[0002] Solar cells provide a means to produce electric power with
minimal environmental impact because it is a renewable technology.
In order to become a commercial success the solar cells need to be
efficient, to have low cost, to be durable, and not add other
environmental problems.
[0003] Today's dominant solar cell technology is based on
crystalline silicon. It fulfils many of the requirements mentioned
above but can not be produced at such low cost that electricity
generation in large scale is cost effective. It also requires
relatively large amount of energy in the production, which is an
environmental disadvantage.
[0004] Solar cells based on thin film technologies have been
developed. They offer a potential of substantial cost reductions
but have, in general, lower conversion efficiencies and less good
durability. A very promising thin film solar cell technology is
based on the semiconductor Cu(In,Ga)Se.sub.2, abbreviated CIGS,
which has demonstrated high efficiencies (16,6% in small prototype
modules [1]) and durability in operation. It remains to demonstrate
low cost in real production.
[0005] CIGS-solar cells are thin-film solar-cells with a CIGS-layer
serving as absorber of sunlight. Electron-hole-pairs are generated
therein.
[0006] A typical CIGS-solar cell is shown in FIG. 1 and comprises a
glass substrate 1 with a thickness of 2-3 mm, an Mo-back contact 2
with a thickness of 0,5-1 .mu.m, a CIGS-layer 3 of 1,5-2 .mu.m, a
CdS buffer layer 4 with a thickness of 50 nm and a ZnO window layer
5 of 0,5-1 .mu.m. An optional second buffer layer 6 may be present
between the CdS buffer layer and the window layer and has a
thickness of 50 nm.
[0007] The CIGS-layer is a Conductive Cu(In,Ga)(Se,S).sub.2
compound. The CdS buffer layer serves as protection of the
CIGS-layer. The window layer is a n-type conductive doped zinc
oxide layer. With the CIGS-layer it forms a pn-junction and serves
as a transparent front contact. The optional second buffer layer
comprises non-doped ZnO. Presently its role is not fully
understood. Statistically seen, solar cells with this second buffer
layer exhibit better properties compared to cells with a single
ZnO-layer.
[0008] The commonly used way of fabricating CIGS solar cell modules
include formation of a pn-junction and front contact according to
the following: (1) a buffer layer (typically 50 nm of CdS) is
deposited by chemical bath deposition (CBD), (2) a high resistivity
thin layer of ZnO is deposited on top of the CdS layer by
sputtering, (3) the layered structure is patterned by mechanical
scribing to open contacts for the serial interconnects, (4) a front
contact of a transparent conductive oxide (TCO) is deposited, and
(5) an additional patterning step of mechanical patterning as part
of the interconnect structure.
[0009] EP-A2-0 838 863 discloses a thin-film solar cell fabricated
on a glass substrate. The solar cell comprises a metallic back
electrode, a light-absorbing layer having on its surface a
Cu(InGa)(Se).sub.2 layer (CIGS layer), an interfacial layer
referred to as a buffer layer, a window layer and an upper
electrode. The back electrode is a p-type semiconductor and the
window layer is an n-type semiconductor. Between the back and upper
electrodes an open circuit voltage in the order of 0,2-0,8 V is
obtained when the solar cell is hit by light. Electrical current
will be generated in the p-n junction between the p- and
n-layers.
[0010] The buffer layer comprises a group II-VI compound
semiconductor containing hydroxyl groups. An exemplary compound is
Zn(O,S,OH).sub.2.
[0011] DE 44 40 878 C2 discloses a thin-film solar cell comprising
a glass substrate, a back electrode, a light absorbing layer, a
front buffer layer and a window layer. The front buffer layer
comprises a mixture of In(OH).sub.3 and In.sub.2S.sub.3 and is
applied in a wet process or a chemical vapour deposition process
(CVD) with organo metallic compounds or with atomic layer epitaxy
deposition process (ALE). The novel concept here is that the buffer
layer does not contain cadmium.
[0012] To fabricate the solar cell with chemical wet process steps
mixed with sputtering makes the solar cell less attractive for
large scale production. To fabricate the solar cell wet processes
are mixed with the co-vapour deposition process which renders the
structure less attractive for large scale production.
[0013] An example of thin-film solar cells using a first CdZnS
buffer layer between a CIGS-layer and a window layer is disclosed
in U.S. Pat. No. 5,078,804. A second ZnO buffer layer on top of the
first buffer layer and in contact with the window layer is also
provided. The first CdZnS buffer layer comprises two layers, a high
Zn content CdZnS layer on top of a low Zn content CdZnS layer, both
of which are applied using an aqueous solution method. The second
ZnO buffer layer also comprises two layers, a low resistivity ZnO
layer on top of a high conductivity ZnO layer both of which are
applied using sputtering in an argon or oxygen/argon
atmosphere.
[0014] To manufacture the solar cell with chemical wet process
steps mixed with sputtering makes the solar cell less attractive
for large scale production. The use of toxic cadmium makes the cell
less attractive for environmental reasons. Also handling and
disposal of the hazardous wastes are costly.
[0015] U.S. Pat. No. 5,948,176 discloses a solar cell structure
comprising a first active CIGS layer on top of a metal back
contact. On top of the CIGS layer a buffer layer of n-type
conductive doped ZnO layer is deposited by a wet process using a
zinc chloride solution as doping source. A second active layer
comprising ZnO is deposited on the buffer layer. The second active
layer comprises a first high resistive ZnO layer and a second low
resistive ZnO layer on top of which a grid of front face electrodes
is sputtered.
[0016] Another example of a cadmium containing solar cell is shown
in U.S. Pat. No. 4,611,091.
SUMMARY OF THE INVENTION
[0017] One object of the invention is to reduce the number of
process steps for the manufacture of the solar cell, thereby
reducing the cost involved in their manufacture.
[0018] Another object of the invention is to replace the toxic
CdS-layer with more environmental-friendly compound.
[0019] Still another object of the invention is to replace the
CdS-layer with a material that absorbs less of the incident light
than does a CdS-layer, thereby increasing the amount of light
impinging the under-laying CIGS-layer and accordingly increasing
the photo current generated by the cell.
[0020] A further object of the invention is to replace the wet
process for manufacture of the CdS-buffer layer with a novel
process that allows for integration with the preceding (dry) vacuum
deposition of the CIGS-layer and/or integration with the following
(dry) sputter deposition of the high resistivity ZnO window
layer.
[0021] These objects are achieved with a thin-film solar cell
indicated in the preamble of claim 1, a method of producing an
interfacial layer in accordance the preamble of claim 7 and a
process line for manufacturing a solar cell structure in accordance
with the preamble of claim 14. The characteristic features of the
invention are indicated in the characterizing clause of claims 1, 7
and 14 respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a simplified sectional view of a known, typical
CIGS solar cell structure;
[0023] FIG. 2 is sectional view of a CIGS solar cell structure with
two ALD buffer layers in accordance with the invention;
[0024] FIG. 3 is a sectional view of a solar cell structure in
accordance with the invention wherein the lower of the two ALD
buffer layers has a sulphur gradient;
[0025] FIG. 4 is a schematic side view of a production line for the
manufacture of a solar cell in accordance with the invention.
[0026] FIG. 5 is an alternative embodiment of the production line
shown in FIG. 4,
[0027] FIG. 6 is a diagram showing the efficiency of a CIGS solar
cell with two ALD buffer layers in accordance with the
invention
[0028] FIG. 7 is a diagram showing the quantum efficiency of the
CIGS solar cell with two ALD buffer layers in accordance with the
invention compared to a CIGS solar cell having just one CdS buffer
layer;
[0029] FIG. 8 is a diagram showing the efficiency of CIGS solar
cell in accordance with the invention compared to a CIGS solar cell
having just one ALD Zn(O,S) buffer layer, and
[0030] FIG. 9 is a diagram showing the efficiency of CIGS solar
cell in accordance with the invention compared to a CIGS solar cell
having just one ALD ZnO layer, and to a CIGS solar cell having just
one CdS buffer layer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] A CIGS-cell in accordance with the present invention is
shown in FIG. 2. It comprises the usual glass substrate 1, the back
contact layer 2 of molybdenum, the CIGS-layer 3 and the window
layer 5. The usual CdS-buffer layer is replaced with two buffer
layers, a first buffer layer containing Zn(O,S) deposited on the
CIGS-layer and a second buffer layer 8 deposited on the first one
and containing ZnO.
[0032] This is done in one process step in accordance with the
invention. At first the Zn(O,S)-layer is deposited by atomic layer
deposition (ALD) and immediately following this the ZnO-layer is
deposited by A/D in the same process chamber. In effect, it can be
considered that the two layers are replaced with one single layer
of Zn(O,S), where no sulphur is added during the latter part of the
deposition.
[0033] It is the inventive ALD deposition in the same chamber that
makes possible the combination of a Zn(O,S)-layer and a
ZnO-layer.
[0034] The function of the Zn(O,S)-layer is to make the surface of
the under-laying active CIGS-layer electronically passive. The
CIGS-layer surface contains defects that are active unless the
Zn(O,S)-layer is not present. They also have a negative influence
on the properties of the under-laying CIGS-layer. The Zn(O,S)-layer
will make these defects electronically passive and will to a great
extent, if not completely, reduce their influence on the
CIGS-layer. At present it is not fully understood what other
properties and functions the Zn(O,S)-layer has.
[0035] The function of the ZnO-layer is to physically protect the
under-laying very thin Zn(O,S) layer. The thickness of the
Zn(O,S)-layer is less than about 30 nm (=30.times.10.sup.-9 m or
300 .ANG.). Experiments have shown that the thickness of the
ZnO-layer does not appear to be critical; its thickness is
generally about 50-100 nm.
[0036] The ZnO- and Zn(O,S)-layers are in this embodiment of the
invention two separate, distinct layers. The ZnO-layer is
integrally formed with the sulphur containing Zn(O,S)-layer and
vice versa. Together the two layers appear as a single unit.
[0037] In a second embodiment shown in FIG. 3 the sulphur content
of the upper part of Zn(O,S)-layer gradually decreases and finally
vanishes, indicating the first monolayer of a ZnO-layer of the
desired thickness. In other words there is a sulphur gradient over
the thickness of the combined ZnO- and Zn(O,S)-layers. In this
embodiment there are no distinct layers but the Zn(O,S)-layer
transforms into the ZnO-layer, and vice versa Nevertheless, the
Zn(O,S)-layer is integral with ZnO-layer.
[0038] The ALD process for deposition of the Zn(O,S)- and
ZnO-layers is in accordance with another aspect of the present
invention integrated with a preceding process for deposition of the
CIGS-layer. In FIG. 4 a production line in accordance with the
invention for manufacturing a solar cell structure of the invention
is shown to comprise a conventional CIGS-production line comprising
inlet chamber 9, a transport and heating chamber 10, a CIGS-process
chamber 11, a transport and cooling chamber 12, and an outlet lock
chamber 13. In accordance with the invention the lock chamber is
used as ALD process chamber for deposition of the ZnO- and
Zn(O,S)-layers. Following the ALD process chamber there is an
optional exit lock 14. The exit lock may also be a part of the ALD
chamber.
[0039] Substrates provided with the sputtered back contact Mo-layer
are loaded one at a time into the inlet chamber. In the transport
chamber they are accelerated and heated to about 500.degree. C.
They will thereby arrive in a serial stream, close together, to the
CIGS-process chamber where they are deposited in sequence in line.
The CIGS-process chamber has sources of Cu, In, Ga and Se for
deposition of the CIGS-layer by vapour deposition. After growing
the CIGS-layer to the desired thickness the substrates enter the
transport and cooling chamber where they are cooled to the desired
ALD-process temperature of about 120.degree. C. in vacuum or in an
inert environment before they enter the ALD-process chamber. In the
ALD process chamber the substrates are processed in parallel as
indicated in FIG. 4, or one by one.
[0040] The ALD process chamber has sources of H.sub.2S, diethyl
zinc and water. Each source comprises a valve through with the
corresponding gaseous compound is injected into the process chamber
for a predetermined time. A "pulse" of the corresponding gaseous
compound will thus be given.
[0041] Before the ALD process starts, the ALD process camber is
flushed with nitrogen gas so as to purge the reaction chamber.
[0042] The ALD process starts by injecting a pulse of diethyl zinc.
On the surface of the CIGS-layer a monolayer of zinc containing
molecules is absorbed. Thermodynamically the process is so
controlled that a thin monolayer of Zn molecules will result at the
temperature existing; no further mono layers will result even if
additional diethyl zinc pulses are given.
[0043] Next a pulse of H.sub.2O or H.sub.2S is injected. Following
this another pulse of diethyl zinc is injected. Next a pulse of
H.sub.2O or H.sub.2S is given. This will result in the growth of
oxygen and sulphur on the monolayer containing zinc molecules. A
first monolayer of Zn(O,S) results.
[0044] These steps are cycled to grow additional Zn(O,S)
mono-layers on top of each other until the first Zn(O,S)-layer of
the desired thickness is obtained. The mono-layers will grow in a
very controlled manner and they grow over edges down into pits and
irregularities that may exist in the CIGS-layer.
[0045] The thickness of the Zn(O,S)-layer is controlled by
selecting the number of cycles diethyl zinc pulses alternating with
water and H.sub.2S.
[0046] Each second pulse is thus of diethyl zinc and between these
pulses of H.sub.2O and H.sub.2S are injected. Expressed in another
way one can say that pulses of diethyl zinc are alternating with
pulses of H.sub.2O and H.sub.2S. The order in which H.sub.2O and
H.sub.2S alternate need not be H.sub.2O--H.sub.2S
--H.sub.2O--H.sub.2S etc. but may vary. For example one pulse of
H.sub.2S may be followed of several pulses of H.sub.2O in a row.
Further, the scheme according to which pulses of H.sub.2O and
H.sub.2S are injected need not be regular, but may vary.
[0047] Although not described above it should be understood that
the ALD process chamber has to be cleaned before a pulse of a new
compound is injected. To this end the ALD process chamber is
flushed with nitrogen gas.
[0048] Applicant has found that the first pulses of diethyl zinc
may not adhere to the CIGS-layer. For this reason it is important
to prevent the substrates from being exposed to the atmosphere
during their transport from the CIGS process chamber to the ALD
process chamber.
[0049] It appears to exist an incubation time until the
Zn(O,S)-layer starts to grow on the CIGS-layer. It is vital that
each pulse of a compound shall cover the entire area of the
substrate and be allowed to react with the uppermost monolayer
thereon. The reaction will saturate if the pulse is sufficiently
long. Therefore the duration of each pulse must be adapted to
achieve this. Once a monolayer starts to grow it will grow to the
same thickness over the entire surface of the under-laying layer.
The reaction is thus self-regulating and the process is cycled
until the desired thickness of the Zn(O,S)-layer is achieved.
[0050] Applicant has found that a solar cell with having a
coefficient of efficiency of 16% results if the following scheme is
followed: every second pulse is always diethyl zinc; between the
zinc pulses a total of five pulses of H.sub.2O and H.sub.2S are
given, four of these being H.sub.2O and one being H.sub.2S. For
example:
Zn--H.sub.2O--Zn--H.sub.2O--Zn--H.sub.2O--Zn--H.sub.2O--Zn--H.sub.2S--Zn--
- etc.
[0051] With the indicated figures H.sub.2S is given in a proportion
of 1 to 5 (i.e. 20%) to the total number of water and H.sub.2S
pulses. A solar cell of almost equal the same coefficient of
efficiency is obtained if the proportion of H.sub.2S pulses to the
total number of H.sub.2O and H.sub.2S pulses is 1:6 (17%).
[0052] Solar cells with an efficiency coefficient varying between
13-16% result if the H.sub.2S is given in a proportion from
10-100%, preferably in a proportion of 15-25% of the total number
of water and H.sub.2S pulses. If the ratio of H.sub.2S pulses to
H.sub.2O plus H.sub.2S pulses is 10% it is possible to obtain solar
cells with a coefficient of efficiency in the order of about 15%.
If H.sub.2S pulses only are given (corresponding to 100%) the
resulting solar cell will have a coefficient of efficiency of
13%.
[0053] Applicant has thus found that the first buffer layer should
comprise Zn(O.sub.x,S.sub.1-x), where x varies between 0 and 0,9,
preferably between 0,1 and 0,7.
[0054] Another way of expressing the manner in which the Zn(O,S)
buffer layer is grown is to say that pulses of diethyl zinc
alternate with pulses of water; some of the latter being replaced
with pulses of hydrogen sulphide. Thus, by varying the proportion
of H.sub.2S pulses the properties of the resulting solar cell is
controlled.
[0055] As indicated above the thickness of the Zn(O,S)-layer is
less than about 30 .mu.m. Applicant has found that excellent solar
cells are obtained if the thickness is so thin as just 1 nm.
[0056] When the Zn(O,S)-layer of the predefined thickness has been
grown the process is repeated, now without any pulses of H.sub.2S,
and the process continues to generate ZnO. Giving alternate pulses
of diethyl zinc and water a first mono-layers of ZnO is grown on
top of the surface of the Zn(O,S)-layer. Continuing to give
alternate pulses of diethyl zinc and water additional mono-layers
of ZnO will grow on top of each other and the process is cycled
until a ZnO-layer of the desired thickness results.
[0057] Generally speaking the layers will grow at a controlled
speed when the above indicated ALD process is followed. The
duration of the pulses will depend on the volume of the ALD process
chamber.
[0058] After completion of the ZnO-layer the substrates are
transported to the exit lock from which they are delivered to a
patterning station which is followed by a station for depositing
the front contact and to a station for the additional patterning
step as mentioned in the introductory portion of the
description.
[0059] The above described ALD method is well suited for deposition
of the combined Zn(O,S) and ZnO sulphur gradient layer shown in
FIG. 3. At the end of the process for growing the Zn(O,S)-layer the
number of H.sub.2S pulses is gradually decreased mono-layer for
mono-layer until no H.sub.2S pulses is present. Finally diethyl
zinc pulses alternating with water pulses only are injected thereby
continuing to build ZnO-monolayers on top of each other until the
desired thickness of the ZnO-layer is achieved.
[0060] The process line described above is harmonized in that the
vacuum deposition process for growing the CIGS-layer and the ALD
processes for growing the two Zn(O,S) and ZnO-layers all are dry
processes.
FURTHER EMBODIMENTS OF THE INVENTION
[0061] Instead of diethyl zinc it is possible to use dimethyl zinc
or another organo metallic zinc compound. It is also possible to
use a organo metallic indium compound.
[0062] Applicant has found that if the substrates in the ALD
chamber have a temperature of about 120.degree. C. the two buffer
layers can be grown at this temperature.
[0063] The Zn(O,S)-layer can be deposited at a temperature of
160.degree. C., and the ZnO-layer at 120.degree. C. Although a
temperature of 120.degree. C. of the substrates in the ALD process
chamber is preferred, the two buffer layers may be deposited at a
temperature as high as about 250.degree. C. and it may also be as
low as 100.degree. C. A preferred temperature range is
100-130.degree. C. for the two buffer layers 7, 8.
[0064] In an alternate embodiment of the process line shown in FIG.
5, there are two separate ALD process chambers 13 and 15 for the
deposition of the respective buffer layers 7 and 8. If the two
buffer layers are deposited at different temperatures each ALD
process chamber is held at the respective temperature. This will
increase the throughput since the cooling time of the substrates
will be reduced compared to the case in which the substrates are
allowed to cool in the same chamber before the ZnO-layer is
deposited.
[0065] Above it has been indicated that the first buffer layer,
i.e. the Zn(O,S)-layer, contains zincoxysulfide. During the ALD
deposition process a secondary phase may develop, for example zinc
hydroxide, and accordingly be present in the buffer layers.
[0066] According to a further embodiment of the invention the first
buffer layer may comprise ALD deposited indiumsulfide
In.sub.2S.sub.3 and the second buffer layer the usual ALD deposited
ZnO as described above. The ALD deposited In.sub.2S.sub.3 is
manufactured in the same manner as described above, replacing the
diethyl zinc pulses with a organo metallic indium compound such as
indium acetyl acetonate, In(acac).sub.3.
[0067] FIG. 6 is a current density (mA/cm.sup.2)-voltage (V) graph
of a solar cell provided with a first Zn(O,S)-buffer layer and a
second ZnO-buffer layer in accordance with the invention.
Characteristic properties of the cell is a open circuit voltage
V.sub.OC of 684 mV, a fill factor FF of 74%, a short circuit
current I.sub.SC of 32,0 mA/cm.sup.2 and an efficiency of 16, 0%.
The solar cell was provided with an anti-reflective coating. These
properties demonstrate that the solar cell of the invention has an
equal or even superior performance than a common CIGS-cell with a
CdS-buffer layer.
[0068] FIG. 7 is a diagram showing the quantum efficiency QE at
different wavelengths for a solar cell provided with a first
Zn(O,S)-buffer layer and a second ZnO-buffer layer in accordance
with the invention compared with a common CIGS-cell having a
CdS-buffer layer. None of the cells had an anti-reflective layer.
As appears the solar cell in accordance with the invention has a
higher QE in the blue region of the spectrum.
[0069] FIG. 8 is a graph similar to that of FIG. 6 wherein a solar
cell provided with a first Zn(O,S)-buffer layer and a second
ZnO-buffer layer in accordance with the invention is compared to a
solar cell having only the first Zn(O,S)-buffer layer. As appears
from Table 1 below the cell having two buffer layers in accordance
with the invention has superior properties TABLE-US-00001 TABLE 1
Voc Isc FF Efficiency (mV) (mA/cm.sup.2) (%) (%) U128d2, ALD 538
31.3 69.7 11.7 Zn(O,S)/ZnO U128d1, ALD 389 19.3 24.7 1.9 Zn(O,S)
I.sub.sc is the short circuit current and FF is the fill
factor.
[0070] FIG. 9 is a graph similar to the graph of FIG. 8 wherein a
solar cell provided with a first Zn(O,S)-buffer layer and a second
ZnO-buffer layer in accordance with the invention is compared to a
common CIGS-cell using a single CdS-buffer layer and to a CIGS-cell
with only the second ZnO-buffer layer. As appears from the
properties listed in Table 2 below the cell in accordance with the
invention has an equal performance with the CIGS-cell using a
CdS-buffer layer. TABLE-US-00002 TABLE 2 Voc Isc FF Efficiency (mV)
(mA/cm.sup.2) (%) (%) ALD ZnO 401 34.4 60.7 8.4 CdS 566 32.0 74.6
13.5 ALD 542 34.8 71.5 13.5 Zn(O,S)/ZnO
REFERENCES
[0071] [1] M Green, K Emery, D King, S Igari and W Warta, Solar
Cell Efficiency Tables (Version 21), Prog. Photovolt: Res. Appl.
2003; 11, p. 39-45
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