U.S. patent application number 15/777213 was filed with the patent office on 2018-11-15 for solar cell and method for preparing same.
The applicant listed for this patent is Young-kwon JUN. Invention is credited to Young-kwon JUN.
Application Number | 20180331238 15/777213 |
Document ID | / |
Family ID | 59626026 |
Filed Date | 2018-11-15 |
United States Patent
Application |
20180331238 |
Kind Code |
A1 |
JUN; Young-kwon |
November 15, 2018 |
SOLAR CELL AND METHOD FOR PREPARING SAME
Abstract
A solar cell includes a light-absorbing layer, comprising a Cu
compound or Cd compound, between two electrodes facing each other,
has an impurity material layer, comprising an impurity element to
be provided to the Cu compound or Cd compound, formed on any one
side or both sides between the two electrodes and the light
absorbing layer, and has a doping layer formed on one part of the
light absorbing layer by means of the impurity element being
diffused on the light absorbing layer.
Inventors: |
JUN; Young-kwon; (Seoul,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JUN; Young-kwon |
Seoul |
|
KR |
|
|
Family ID: |
59626026 |
Appl. No.: |
15/777213 |
Filed: |
February 20, 2017 |
PCT Filed: |
February 20, 2017 |
PCT NO: |
PCT/KR2017/001840 |
371 Date: |
May 18, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/0321 20130101;
H01L 31/0336 20130101; Y02E 10/547 20130101; H01L 31/073 20130101;
H01L 31/02963 20130101; H01L 31/0288 20130101; H01L 31/0323
20130101; H01L 31/0216 20130101; H01L 31/0327 20130101; H01L
31/0392 20130101; H01L 31/03365 20130101; Y02E 10/541 20130101;
H01L 31/1828 20130101; Y02P 70/50 20151101; Y02E 10/50 20130101;
Y02E 10/543 20130101; H01L 31/18 20130101 |
International
Class: |
H01L 31/0296 20060101
H01L031/0296; H01L 31/0216 20060101 H01L031/0216; H01L 31/073
20060101 H01L031/073; H01L 31/0224 20060101 H01L031/0224; H01L
31/0392 20060101 H01L031/0392; H01L 31/18 20060101 H01L031/18 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 18, 2016 |
KR |
10-2016-0019191 |
Claims
1. A solar cell comprising: a light absorbing layer composed of a
Cu compound or Cd compound between two electrodes facing each
other; an impurity material layer formed on any one side or both
sides between the two electrodes and the light absorbing layer and
including an impurity elements to be provided to the Cu compound or
Cd compound; and a doping layer formed on a portion of the light
absorbing layer by means of the impurity elements being diffused
into the light absorbing layer.
2. The solar cell of claim 1, wherein a p-n junction or an internal
electric field layer is formed in the Cu compound or Cd compound by
the doping layer.
3. The solar cell of claim 1, wherein the Cu compound or Cd
compound has a binary composition.
4. The solar cell of claim 3, wherein the Cu compound is
Cu.sub.xO.sub.y (wherein, x and y are any positive numbers) or
Cu.sub.xS.sub.y (wherein, x and y are any positive numbers), and
the Cd compound is Cd.sub.xTe.sub.y (wherein, x and y are any
positive numbers).
5. The solar cell of claim 1, wherein the impurity material layer
is composed of a metal oxide containing any one or more of Ti and
Si.
6. The solar cell of claim 1, exhibiting a fluctuation in current
as voltage is applied in a light irradiation state.
7. The solar cell of claim 6, wherein the fluctuation in current is
a current variation of 20% or more with respect to a voltage
variation of within 5%.
8. The solar cell of claim 6, wherein the fluctuation in current
can be reduced to as a current variation to be within 10% with
respect to a voltage variation of within 10% through polling which
intensifies an internal electric field.
9. The solar cell of claim 6, wherein the fluctuation in current
can be reduced to as the number of times a fluctuation appears
decrease through polling which intensifies the internal electric
field.
10. A method for preparing a solar cell, comprising: forming a
first electrode on a substrate; forming a light absorbing layer on
the first electrode; and forming a second electrode on the light
absorbing layer, wherein the method further comprises forming an
impurity material layer including an impurity element on the light
absorbing layer adjacent to the first electrode or the second
electrode in any one side or both sides thereof, and forming a
doping layer by diffusing the impurity element into a portion of
the light absorbing layer.
11. The method of claim 10, wherein the impurity material layer is
formed through a thin film process, or formed by a method of
attaching a film containing an impurity material.
12. The method of claim 11, wherein the method of attaching a film
comprises: preparing a solution by dispersing particles of the
impurity material in an organic solvent; applying the solution on
the light absorbing layer; and forming a particle layer of the
impurity material by evaporating the solvent.
13. The method of claim 11, wherein the method of attaching a film
comprises: forming a film by impregnating the particles of the
impurity material into a solvent of a thermoplastic resin and then
curing the impregnated particles; and adhering the film on the
light absorbing layer.
14. The method of claim 12, wherein the size of the particles of
the impurity material is 10 to 100 nm.
15. The method of claim 10, wherein the impurity material layer is
formed by reactive ion sputtering, and when the impurity material
layer is formed, a negative voltage is applied in a range of 0 V to
-5 V to accelerate the doping of the impurity element contained in
impurities into the light absorbing layer.
16. The method of claim 15, wherein the reactive ion sputtering
comprises: providing a target having a component of the impurity
material and injecting an inert gas and a reactive gas in a vacuum
state; and forming an oxide by generating plasma to cause the
impurity material emitted by means of an Ar ion colliding with the
target to react with oxygen plasma.
17. The method of claim 10, wherein the impurity material layer is
composed of a metal oxide, and formed by physical vapor deposition
(PVD), chemical vapor deposition (CVD), or atomic layer deposition
(ALD).
18. The method of claim 17, wherein a metal oxide layer is formed
by atomic layer deposition using a precursor containing Ti.
19. The method of claim 17, wherein the metal oxide layer comprises
any one of a Ti oxide, a composite oxide of Cu and Ti, and a
composite oxide of Cu and Si.
20. The method of claim 13, wherein the size of the particles of
the impurity material is 10 to 100 nm.
Description
TECHNICAL FIELD
[0001] The present invention relates to a solar cell and a method
for preparing the same, and more particularly, to a solar cell
structure in which an internal electric field such as a p-n
junction is formed by forming an impurity doping layer containing
Ti or Si impurities in a Cu compound or Cd compound solar cell
which includes an amorphous, polycrystalline or single crystal
solar cell to improve the photoelectric conversion efficiency of
the solar cell, and to a preparation method thereof.
BACKGROUND ART
[0002] A silicon solar cell is a crystalline solar cell including a
single crystal solar cell and a polycrystalline solar cell and has
the largest market share at present. Technologies for preparing
silicon solar cells with high efficiency at low costs are being
developed.
[0003] For the past 20 years, the most efficient silicon solar cell
in the world has been a cell with 25% efficiency using the PERL
(Passive Emitter Rear Locally Diffused) structure developed by
University of New South Wales, Australia. However, at the IEEE
Photovoltaic Specialists Conference in April 2014, Panasonic
Corporation announced that they achieved a solar cell efficiency of
25.6% by adopting a new structure. In this solar cell, a front
contact for blocking some of the sunlight entering the solar cell
is changed such that both positive and negative contacts are
located on a rear surface of the solar cell. In addition, a high
quality amorphous silicon film is formed on a crystalline silicon
wafer so as to prevent damage to a surface of the wafer, thereby
minimizing the occurrence of recombination of carriers on front and
rear surfaces, achieving an efficiency of 25.6% exceeding an
efficiency wall of 25%.
[0004] However, all the designs related to this new efficiency
record have the disadvantage of using a high quality silicon
crystal, which makes it difficult to obtain economical
efficiency.
[0005] On the other hand, a thin film solar cell technology is a
next generation solar cell technology as compared with that of a
crystalline Si solar cell. A thin film solar cell is a solar cell
which has higher efficiency than a crystalline Si solar cell and
which can be prepared at lower costs.
[0006] Many different types of thin film solar cells are being
developed, and the representative example thereof is a
CIGS(Cu(In,Ga) Se.sub.2) solar cell.
[0007] A CIGS solar cell is a cell composed of a general glass
substrate-a rear electrode-a light absorbing layer-a buffer layer-a
transparent front electrode, and the like. Among the components,
the light absorbing layer which absorbs sunlight is composed of
CIGS or CIS(CuIn(S, Se).sub.2). CIGS may be used by replacing Cu,
In, and Ga which are cations, and Se which is an anion with
different metal ions or anions, respectively, each of which may be
called as a CIGS-based compound semiconductor. The representative
example thereof is Cu(In,Ga)Se.sub.2 and such a CIGS-based compound
semiconductor is a material of which the energy band gap as well as
the crystal lattice constant may be controlled by changing the type
and the composition of cations (for example: Cu, Ag, In, Ga, Al,
Zn, Ge, Sn, and the like) and anions (for example: Se and S), both
constituting the CIGS-based compound semiconductor. For example,
recently, a material such as Cu.sub.2ZnSnS.sub.4(CZTS) or
Cu.sub.2Sn.sub.xGe.sub.yS.sub.3(CTGS) (wherein, x and y are any
positive numbers) is used as a low-cost compound semiconductor
material.
[0008] However, such a composite compound semiconductor containing
Cu has a multi-component structure, and therefore, there is a
disadvantage in that it is difficult to have uniformity and
reproducibility since it is difficult to optimize the composition
by controlling each component material. In addition, a typical
structure has a limitation in improving efficiency through the
reduction in recombination of carriers, and the like.
DISCLOSURE OF THE INVENTION
Technical Invention
[0009] An object of the present invention is to provide a solar
cell structure capable of improving photoelectric conversion
efficiency by forming an internal electric field layer such as a
p-n junction in a Cu compound or Cd compound semiconductor by
doping Ti or Si impurities as a donor so as to reduce the
recombination of electrons and holes both generated in the
semiconductor by means of light absorption while improving
collection efficiency to an electrode, and a preparation method
thereof.
[0010] In addition, another object of the present invention is to
provide a Cu compound or Cd compound semiconductor solar cell
applying an internal electric field formed by the impurity doping
as a means for preventing recombination, and a preparation method
thereof.
[0011] In particular, another object of the present invention is to
provide a solar cell which enables the improvement of uniformity
and reproducibility by applying a Cu compound or Cd compound
semiconductor having a binary composition as a light absorbing
layer.
Technical Solution
[0012] The first aspect of the present invention to solve the above
mentioned task provides a solar cell including a light absorbing
layer composed of a Cu compound or Cd compound and formed between
two electrodes facing each other, an impurity material layer formed
on any one side or both sides between the two electrodes and the
light absorbing layer and including an impurity element to be
provided to the Cu compound or Cd compound, and a doping layer
formed on a portion of the light absorbing layer by means of the
impurity element being diffused into the light absorbing layer.
[0013] The second aspect of the present invention to solve the
above mentioned task provides a method for preparing a solar cell,
including forming a first electrode on a substrate, forming a light
absorbing layer on the first electrode, and forming a second
electrode on the light absorbing layer, wherein the method further
includes forming an impurity material layer including an impurity
element on the light absorbing layer adjacent to any one side or
both sides of the first electrode or the second electrode, and
forming a doping layer by diffusing the impurity element into a
portion of the light absorbing layer.
Advantageous Effects
[0014] A solar cell according to the present invention is capable
of improving efficiency by reducing the recombination of electrons
and holes generated in a semiconductor light absorbing layer and at
the same time improving the collection efficiency to an electrode
by arranging a material layer capable of impurity doping so as to
be adjacent to a light absorbing layer and forming an internal
electric field such as a p-n junction through the impurity
doping.
[0015] In addition, a solar cell according to the present invention
is capable of simplifying a process by replacing a typical
recombination preventing layer by disposing an impurity material
layer so as to be adjacent to a conductive material such as an
electrode of a cell.
[0016] In addition, according to a method for preparing a solar
cell according to the present invention, an impurity material layer
may be formed by applying a vacuum deposition method such as
reactive ion sputtering or electron beam evaporation, or by a
non-vacuum method such as electroplating, ink printing, and spray
pyrolysis.
[0017] In addition, according to a method for preparing a solar
cell according to the present invention, a binary compound
semiconductor containing Cu or Cd is applied as a light absorbing
layer so that the light absorbing layer may be further simplified
to facilitate the control of physical properties and be stably
maintained. As a result, the efficiency of a solar cell is expected
to be maintained for a longer time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic view showing a cross-sectional
structure of a Cu compound semiconductor solar cell including an
impurity material layer according to an embodiment of the present
invention.
[0019] FIG. 2 is a graph showing a result of measurement of a
change in photoelectric conversion current, that is, a
short-circuit current according to the application of a reverse
bias in a light irradiation state in a Cu compound solar cell
including an impurity material layer according to an embodiment of
the present invention.
[0020] FIG. 3 shows a case (A) in which a current-voltage
characteristic is measured in a light irradiation state in a Cu
compound solar cell including an impurity material layer according
to an embodiment of the present invention.
[0021] FIG. 4 shows a case (B) in which a current-voltage
characteristic is measured after poling by applying a negative
voltage (-5 V) in a Cu compound solar cell including an impurity
material layer according to an embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] Hereinafter, the configuration and the operation of
embodiments of the present invention will be described with
reference to the accompanying drawings.
[0023] In describing the present invention, a detailed description
of related known functions and configurations will be omitted when
it may unnecessarily make the gist of the present invention
obscure. Also, when a certain portion is referred to "include" a
certain element, it is understood that it may further include other
elements, not excluding the other elements, unless specifically
stated otherwise.
[0024] The present invention is characterized in providing a solar
cell including a light absorbing layer composed of a Cu compound or
Cd compound and formed between two electrodes facing each other, an
impurity material layer formed on any one side or both sides
between the two electrodes and the light absorbing layer and
including a donor element to be provided to the Cu compound or Cd
compound, and a doping layer formed on a portion of the light
absorbing layer by means of the donor element being diffused into
the light absorbing layer.
[0025] A p-n junction or an internal electric field layer may be
formed in the Cu compound or Cd compound by the doping layer.
[0026] A light absorbing layer material may include, for example, a
binary compound containing Cu and having an energy band gap of
1.0-2.1 eV, such as CuO, Cu.sub.2O, CuS, and Cu.sub.2S as a p-type
semiconductor. Cu.sub.xO.sub.y, and Cu.sub.xS.sub.y (x and y are
any positive numbers) may be preferably used. In addition,
Cd.sub.xTe.sub.y (wherein, x and y are any positive numbers) may
also be preferably used.
[0027] The impurity material layer may include a material which
belongs to a group IV or containing an element having four valence
electrons or an oxidation number of +4, and may be preferably
composed of a metal oxide containing any one or more of Ti and
Si.
[0028] In addition, the solar cell may exhibit a fluctuation in
current as voltage is applied in a light irradiation state. The
fluctuation in current may be a current variation of 20% or more
with respect to a voltage variation of within 5%. In addition, the
fluctuation in current can be reduced to as a current variation to
be within 10% with respect to a voltage variation of within 10%
through polling which intensifies an internal electric field. That
is, according to the present invention, the fluctuation in current
can be reduced to as a current variation to be within a certain
range with respect to a voltage variation through polling which
intensifies the internal electric field of the doping layer. In
addition, the fluctuation in current can be reduced to as the
number of times a fluctuation appears decrease through polling
which intensifies the internal electric field.
[0029] In addition, the present invention is characterized in
providing a method for preparing a solar cell, including forming a
first electrode on a substrate, forming a light absorbing layer on
the first electrode, and forming a second electrode on the light
absorbing layer, wherein the method further includes forming an
impurity material layer including a donor element on the light
absorbing layer adjacent to any one side or both sides of the first
electrode or the second electrode, and forming a doping layer by
diffusing the donor element into a portion of the light absorbing
layer.
[0030] The impurity material layer may be formed by a vacuum
deposition process such as physical vapor deposition (PVD),
chemical vapor deposition (CVD), and atomic layer deposition (ALD),
a non-vacuum thin film process such as plating, ink printing, and
spray pyrolysis, or by a method of attaching a film containing an
impurity material.
[0031] The method of attaching a film may include preparing a
solution by dispersing particles of the impurity material in an
organic solvent, applying the solution on the light absorbing
layer, and forming a particle layer of the impurity material by
evaporating the solvent.
[0032] In addition, the method of attaching a film may include
forming a film by impregnating the particles of the impurity
material into a solvent of thermoplastic resin and then curing the
impregnated particles, and attaching the film on the light
absorbing layer.
[0033] At this time, the size of the particles of the impurity
material may be 10 nm to 100 nm, and a preferable size of the
particles is about 50 nm.
[0034] The impurity material layer is formed by reactive ion
sputtering, and when the impurity material layer is formed, a
negative voltage may be applied in a range of 0 V to -5 V to
accelerate the doping of the donor element contained in impurities
into the light absorbing layer.
[0035] At this time, the reactive ion sputtering may include
providing a target having a component of the impurity material and
injecting an inert gas and a reactive gas in a vacuum state, and
forming an oxide by generating plasma to cause the impurity
material emitted by means of an Ar ion colliding with the target to
react with oxygen plasma.
[0036] The impurity material layer is composed of a metal oxide
preferably including any one of a Ti oxide (Ti.sub.xO.sub.y), a
composite oxide of Cu and Ti (Cu.sub.xTi.sub.yO.sub.z), and a
composite oxide of Cu and Si (Cu.sub.xSi.sub.yO.sub.z), and may be
formed by physical vapor deposition (PVD), chemical vapor
deposition (CVD), or atomic layer deposition (ALD).
[0037] The Ti oxide may be formed by atomic layer deposition using
a precursor containing Ti.
[0038] In addition, the doping layer may be formed through a heat
treatment simultaneously forming an impurity doping layer.
[0039] In addition, the impurity material layer may be applied to
replace a typical insulating layer of Al.sub.2O.sub.3, and the like
which is used as a recombination preventing layer in a compound
semiconductor solar cell.
EXAMPLE
[0040] FIG. 1 is a schematic view of a Cu compound solar cell
including an impurity material layer for doping an impurity
material according to an embodiment of the present invention.
[0041] As shown in FIG. 1, a Cu compound solar cell according to an
embodiment of the present invention includes a substrate, an Al
electrode formed on the substrate, a Cu compound semiconductor
layer formed on the Al electrode and serving as a light absorbing
layer, an impurity material layer formed on the Cu compound
semiconductor layer, a transparent electrode formed on the impurity
material layer, and an Al grid formed on the transparent electrode,
and a diffusion layer of a donor element such as Ti or Si which the
impurity material layer includes is formed in a predetermined
region from an interface in contact with the impurity material
layer to the inside of the Cu compound semiconductor layer.
[0042] A preparation process of a solar cell having the
above-described structure is as follows.
[0043] First, as a substrate, soda lime glass having a thickness of
3 mm was used.
[0044] Next, as a rear electrode, an Al thin film was formed to a
thickness of about 1 to 2 .mu.m by using a sputtering method. As a
rear electrode material, a conductive material such as Mo and W may
be used in addition to Al. In addition, a conductive material such
as Al may be formed by applying a low-cost non-vacuum method such
as electroplating, ink printing, and spray pyrolysis in addition to
a vacuum deposition method such as sputtering.
[0045] Thereafter, on the Al thin film, a thin film of
Cu.sub.xO.sub.y or Cu.sub.xS.sub.y (wherein, x and y are any
positive numbers), which is a p-type semiconductor, was formed to a
thickness of 1 to pm by applying a sputtering method which has a
high deposition rate to form a light absorbing layer composed of a
binary Cu compound semiconductor.
[0046] Specifically, the light absorption layer is formed by using
a material having a purity of 99.99% or more and having a compound
composition containing Cu, O, or S as a sputtering target, and by
performing a deposition step of an electric polarization layer by
reactive ion sputtering, the step of which is divided into four
sections for each time period. First, a sputtering target material
is provided using a material containing Cu or S, for example, Cu,
or CuS, and the like. Thereafter, Ar, as a carrier gas, and O.sub.2
or S.sub.2, as a reaction gas, are injected. Next, plasma is
generated to emit metal atoms from the target material using Ar
ions. Finally, a Cu compound containing Cu and O, or Cu and S is
formed by means of oxygen or sulfur ions generated from the
reaction gases reacted with the emitted metal atoms to form a Cu
compound thin film containing Cu.sub.xO.sub.y or
Cu.sub.xS.sub.y.
[0047] The light absorbing layer may be formed to a thickness of
about 1 to 5 .mu.m by the sputtering, under the conditions of a
process temperature of 200.degree. C. or less, a process pressure
of 2 mTorr, an Ar flow rate of 20 to 50 sccm, a O.sub.2 flow rate
of 10 to 30 sccm, and a direct current voltage of 500 to 800 V. The
most preferable process conditions of 300.degree. C. and 30 minutes
may be applied to form a light absorbing layer of a thickness of
about 2 .mu.m containing a Cu compound such as Cu.sub.xO.sub.y or
Cu.sub.xS.sub.y.
[0048] The light absorbing layer may also be formed by applying a
low-cost non-vacuum method such as electroplating, ink printing,
and spray pyrolysis in addition to a vacuum deposition method such
as sputtering.
[0049] Thereafter, on the light absorption layer, an impurity
material layer is formed to a thickness of about 100 nm or less by
deposition.
[0050] When the thickness of the impurity material layer is less
than 10 nm, doping amount is insufficient, and when greater than
100 nm, resistance is increased due to a residual thickness.
Therefore, it is most preferable to form the impurity material
layer at a thickness of 10 to 100 nm.
[0051] Specifically, when an oxide is applied as the impurity
material, a thin film of a Ti oxide, a Cu and Ti composite oxide,
or a Cu and Si composite oxide is formed to a thickness of about 50
nm by an RF sputtering method, which is advantageous in deposition
rate, to form an impurity material layer.
[0052] Specifically, when forming the thin film of a Cu and Ti
composite oxide, a material having a purity of 99.99% or more and
having a compound composition containing Cu, Ti, and S is used as
the sputtering target.
[0053] The deposition step of the impurity material layer by a
reactive ion RF sputtering method is divided into four sections for
each time period. First, a sputtering target material is provided
using a material containing Cu, TI, and O, for example,
CuTiO.sub.3, and the like. Thereafter, Ar as a carrier gas, and
O.sub.2 as a reaction gas, are injected. Next, plasma is generated
to emit metal atoms from the target material using Ar ions.
Finally, an oxide containing Cu and Ti is formed by means of oxygen
ions reacted with the emitted metal atoms to form a thin film
containing a Cu and TI composite compound
(Cu.sub.xTi.sub.yO.sub.z).
[0054] The impurity material layer of a thickness of about 10 nm to
100 nm may be formed by the RF-sputtering under the conditions of a
process temperature of 200.degree. C. or less, a process pressure
of 5 mTorr, an Ar flow rate of 20 to 50 sccm, a O.sub.2 flow rate
of 10 to 30 sccm, an AC frequency of 2.5 to 3 MHz, a voltage of 300
to 500 V, and time of within 10 minutes. Preferably, an impurity
material layer of 50 nm including a Cu.sub.xTi.sub.yO.sub.z
composite oxide such as CuTiO.sub.3 and the like may be formed by
applying the conditions of 200.degree. C. and 4 minutes.
[0055] In addition, when forming a thin film of a Cu and Si
composition oxide (Cu.sub.xSi.sub.yO.sub.z), an impurity material
layer may be formed in a similar manner as in the case of a
Cu.sub.xTi.sub.yO.sub.z thin film.
[0056] In addition, a p-n junction or an internal electric field
may be formed by forming a Cu.sub.xTi.sub.yO.sub.z thin film or a
Cu.sub.xSi.sub.yO.sub.z thin film on a Cu compound semiconductor,
or by a method in which Ti atoms or ions are diffused to be doped
into a portion of the light absorbing layer through a heat
treatment after the formation.
[0057] On the other hand, in the case of forming a Ti oxide
(Ti.sub.xO.sub.y) thin film, an impurity material layer may be
formed through, for example, a step of adsorbing a precursor in
which a Ti compound is mixed to the light absorbing layer by using
atomic layer deposition, and a step of forming an oxide by
oxidizing the adsorbing layer of the precursor in which a Ti
compound is mixed.
[0058] More specifically, in the case of forming a Ti.sub.xO.sub.y
thin film,
tetrakis(dimethylamino)titanium(TDMAT:Ti[N(CH.sub.3).sub.2].sub.4),
tetrakis(diethylamido)
titanium(TDEAT:Ti[N(C.sub.2H.sub.5).sub.2].sub.4),
tetrakis(ethylmethylamido)titanium(TEMAT:Ti[N (C.sub.2H.sub.5)
(CH.sub.3)].sub.4), titanium
tetraisopropoxide(TTIP:Ti[OCH(CH.sub.3).sub.2].sub.4), and the like
may be used as a Ti compound precursor.
[0059] The atomic layer deposition step is performed by repeating a
process divided into four sections for each time period.
[0060] First, an adsorbing layer of a precursor material (a Ti
compound) is formed by adsorbing the precursor material using a Ti
compound precursor having Ar as a diluent gas (the first step).
Then, by-products and residual gas are removed using an Ar gas (the
second step). Next, plasma is generated while oxygen is injected to
be subjected to an oxidation reaction with the adsorbing layer (the
third step). Finally, by-products and residual gas are removed
using an Ar gas (the fourth step) to form a TiO.sub.2 thin film or
an oxide film of a Ti compound.
[0061] For example, the first step is performed for 0.3 to 5
seconds, the second step is performed for 10 to 20 seconds, the
third step is performed for 3 to 5 seconds, and the fourth step is
performed for 10 to 20 seconds. The four steps are considered to be
one cycle, and by repeating 100 to 500 cycles according to a film
formation thickness and a film formation rate (about 0.1 nm/sec)
under a reaction temperature of 100 to 300.degree. C., the impurity
material layer may be formed to a thickness of 50 nm.
[0062] Preferably, under a temperature of 200.degree. C., an
impurity material layer of about 50 nm composed of an oxidized
material of a Ti compound is formed by causing TiO.sub.2 and a Cu
compound semiconductor formed by applying 500 cycles of atomic
layer deposition which is composed of 1 second of the first step,
10 seconds of the second step, 3 seconds of the third step, and 5
seconds of the fourth step to react with each other chemically. At
this time, a gas injection rate of 50 sccm may be applied at each
step, and in the first step, hydrogen (H.sub.2) gas may be
simultaneously applied together with the Ti compound precursor.
[0063] In addition, a p-n junction or an internal electric field
may be formed by forming a TiO.sub.2 layer on the Cu compound
semiconductor, or by a method in which Ti atoms and ions of the
TiO.sub.2 layer are diffused to be doped into the light absorbing
layer through a heat treatment after the formation.
[0064] In addition, the impurity material layer may be formed by
applying a low-cost non-vacuum method such as electroplating, ink
printing, and spray pyrolysis in addition to a vacuum deposition
method such as sputtering.
[0065] Next, as an upper portion electrode of the solar cell, a
transparent electrode is formed by depositing a transparent
conductive material on the electric polarization layer. At this
time, as the transparent conductive material, a material such as
indium tin oxide (ITO), zinc oxide (ZnO), aluminum-doped zinc oxide
(Al-doped ZnO), and fluorine-doped tin oxide (F-doped SnO.sub.2)
may be formed by sputtering.
[0066] Finally, as an additional upper portion electrode, Ag is
printed by screen printing and then firing heat treated to form an
Ag grid, completing the preparation of the solar cell.
[0067] On the other hand, the impurity material layer is formed by
first forming an electrode pattern, and then before performing the
firing heat treatment, forming an impurity material on a front
surface, a rear surface, or both surfaces by a vacuum deposition
method such as sputtering, or a low cost non-vacuum method such as
electroplating, ink printing, and spray pyrolysis. Thereafter, the
firing heat treatment is performed to complete the preparation of
the solar cell.
[0068] In addition, the impurity material layer may be formed using
a method of first forming an electrode pattern, and then attaching
a particle layer or a film containing the impurity material on a
front surface, a rear surface, or both surfaces so as to be in
contact with the electrode before or after the firing heat
treatment.
[0069] In this case, a method of dispersing impurity particles
containing a complex oxide such as CuTiO.sub.3 or CuSiO.sub.3 in an
organic solvent such as acetone or toluene, coating the solution by
a spray method, and evaporating the solvent to form a particle
layer of impurities, and a method of impregnating strong impurity
particles with a solvent of thermoplastic resin having a softening
point of 120.degree. C. or higher, such as polyethylene,
polystyrene, and polyphenylene ether, followed by hardening to form
a polymer film, and then laminating the film on a surface of the
solar cell may be applied. At this time, the size of the impurity
particles may be 10 nm to 100 nm, preferably about 50 nm.
[0070] In addition, during or after the formation of the impurity
material layer, poling may be applied to enhance the internal
electric field by applying reverse bias to the substrate. At this
time, a reverse bias voltage is within a range of the reverse
breakdown voltage of a Cu compound semiconductor diode, and may be
preferably a negative voltage within 0 to -5 V.
[0071] FIG. 2 is a graph showing a result of measuring a
short-circuit current while applying reverse bias in a light
irradiation state in a Cu compound solar cell including an impurity
material layer according to an embodiment of the present invention,
and showing that the short-circuit current increases as polling is
increased by the reverse bias.
[0072] As a result, the present invention exhibits an effect of
increasing photoelectric conversion current by forming an internal
electric field such as a p-n junction through a process of doping
impurities into a Cu compound semiconductor to which an impurity
material layer is adjacent so that the recombination of
photo-excited charge carriers is reduced.
[0073] On the other hand, due to the characteristics of an internal
electric field layer, when the change in current is measured while
increasing or decreasing the range of voltage including operating
voltage in a light irradiation state, the direction and the
magnitude of an electric field in the internal electric field layer
changes according to the applied voltage. Therefore, as shown in
FIG. 3, current does not increase or decrease regularly with
respect to the change in voltage, and irregular fluctuations
appear.
[0074] The magnitude of such fluctuations may vary such that a
variation of 20% to 120% (increase or decrease) in current may be
exhibited, for example, with respect to a voltage variation
(increase or decrease) of within 5%.
[0075] The irregular fluctuations are reduced if polling is
performed which intensifies the internal electric field by applying
an arbitrary reverse voltage or negative voltage to the internal
electric field layer. As shown in FIG. 4, for example, a current
variation can be reduced to as 10% or less with respect to a
voltage variation of within 10%, and the number of times that a
fluctuation, that is, an irregular increase or decrease, appears,
is reduced.
[0076] On the other hand, in a Cu compound solar cell including an
impurity material layer according to the embodiment of the present
invention, as the number of types of components of a light
absorbing layer is decreased from CIGS of four-component system,
CIS(CuInS.sub.2) of three-component system, Cu.sub.2S of binary
system, and the like, the reproducibility and the uniformity in the
physical properties of the compound may be improved when the light
absorbing layer is formed. Therefore, it is advantageous in terms
of the uniformity, reproducibility and performance optimization of
a solar cell that a binary Cu compound semiconductor of the present
invention is applied as a light absorbing layer compared with a
case in which a multi-component system is applied.
[0077] In addition, although a solar cell in which a binary
semiconductor containing Cu is applied as a light absorbing layer
is described in an embodiment of the present invention, a solar
cell including a binary semiconductor light absorbing layer
containing Cd, such as CdTe and Cd.sub.xTe.sub.y (wherein, x and y
are positive numbers) also has effects of forming an internal
electric field, and increasing an efficiency by introducing an
impurity material layer.
[0078] In addition, in the embodiment of the present invention,
since a light absorbing layer composed of a binary semiconductor
containing Cu or Cd, such as Cu.sub.xO.sub.y, Cu.sub.xS.sub.y, and
Cd.sub.xTe.sub.y (wherein, x and y are any positive numbers) is a
p-type semiconductor, a solar cell having an impurity material
layer capable of doping a donor element is described. However, when
a light absorbing layer is composed of an n-type semiconductor, as
in the case of forming a doping layer in a light absorbing layer
through an impurity material layer capable of doping an acceptor
element, an internal electric field such as a p-n junction is
formed to reduce the recombination of electrons and holes generated
in a semiconductor light absorbing layer and at the same time, the
collection efficiency to an electrode is improved, thereby
increasing the efficiency of a solar cell.
[0079] The present invention includes a case in which the method
for forming an internal electric field layer by doping an impurity
layer proposed in an embodiment is applied to a single type solar
cell composed of only a single element such as Si or Ge, in
addition to a compound solar cell.
* * * * *