Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
1999-09-28
2001-11-27
Chapman, Mark (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S264000, C136S265000, C204S192250, C204S192260
Reexamination Certificate
active
06323417
ABSTRACT:
FIELD OF THE INVENTION
The present invention involves manufacture of I-III-VI semiconductor materials for use in photovoltaic cells, the semiconductor materials manufactured by the method, and photovoltaic devices incorporating the semiconductor materials. In a significant aspect, the present invention is directed toward the manufacture of semiconductors of CIS and variations of CIS, such as CIGS and CIGSS.
BACKGROUND OF THE INVENTION
I-III-VI compound semiconductor materials have received significant attention for use in photovoltaic cells because of the desirable band gap provided by many of those materials. Several photovoltaic cell designs have been proposed using such semiconductors, with a common cell design incorporating a p-type layer of the I-III-VI semiconductor material forming a heterojunction with an n-type semiconductor material, such as cadmium sulfide or cadmium zinc sulfide.
A number of different I-III-VI semiconductor materials have been proposed for use in photovoltaic cells. Some examples include AgInS
2
, AgGaSe
2
, AgGaTe
2
, AgInSe
2
, AgInTe
2
, CuGaS
2
, CuInS
2
, CuInTe
2
, CuAlS
2
, and CuGaSe
2
. Most attention, however, has been focused on copper indium diselenide (CuInSe
2
) and variations of copper indium diselenide in which a portion of the indium is replaced with one or more of aluminum and gallium and/or a portion of the selenium is replaced with sulfur and/or tellurium. Copper indium diselenide is commonly referred to as “CIS.” Two promising variations of CIS that have been proposed include CuIn
x
Ga
1−x
Se
2
(commonly referred to as “CIGS”) and CuIn
x
Ga
1−x
Se
y
S
2−y
(commonly referred to as “CIGSS”). These and other I-III-VI semiconductors may be manufactured for use in photovoltaic cells according to the present invention.
A variety of techniques have been proposed for fabricating various I-III-VI semiconductor materials for use in photovoltaic cells. One technique is to coevaporate all of the I, III and VI components and then react the components to form the desired semiconductor material. This technique has been used to produce small-area photovoltaic cells with very high efficiencies, in excess of 15%, but the technique is complex. Simultaneous deposition from a number of sources is difficult to control, and scale-up for a commercial operation is a significant problem.
Another technique is to sequentially deposit precursors for the desired semiconductor material and then heat the deposited precursors to form the desired semiconductor material. Relative to coevaporation, sequential deposition is less complex and easier to control and is, therefore, easier to scale up for a commercial operation. A significant problem with sequential deposition techniques, however, is that resulting photovoltaic cells tend to have relatively low efficiencies.
One variation on the sequential deposition technique for making CIS involves “selenization” of predeposited copper and indium films to form the final semiconductor material. This selenization is accomplished by reacting the metal precursors with a reactive selenium-containing gas, typically hydrogen selenide. Although the selenization process has resulted in somewhat improved photovoltaic cell efficiencies, hydrogen selenide is a very hazardous gas, which makes operation of the process problematic.
Another significant problem with all of the fabrication techniques, and especially with sequential deposition techniques, is a lack of good adhesion of the I-III-VI semiconductor material to adjoining layers, and especially to a molybdenum film often used as a back contact for CIS photovoltaic devices. Without good adhesion, delaminations can occur which significantly impair the performance and reliability of the photovoltaic cells.
A need exists for new techniques to fabricate I-III-VI semiconductors for use in photovoltaic cells, and especially for fabrication techniques that are easily scaled up for use in a commercial operation, that permit the manufacture of photovoltaic cells having reasonably high efficiencies, that avoid the use of highly hazardous gases and that promote good adhesion of the semiconductor material to adjoining layers.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for making I-III-VI semiconductor materials for use in photovoltaic modules in a manner that is less complex from a control standpoint than coevaporation techniques. It is another object of the present invention to provide a method for making I-III-VI semiconductor layers that is scalable for manufacture of large-area films. It is another object of the present invention to provide a method for making I-III-VI semiconductor materials in a manner to promote good adhesion of the semiconductor layer for use in photovoltaic cells. It is a further object of the present invention to provide a method for making I-III-VI semiconductor materials which can be incorporated into photovoltaic cells to provide relatively high cell efficiencies. It is yet another object of the present invention to provide a method for making I-III-VI semiconductor materials which avoids the use of highly hazardous hydrogen selenide gas.
These and other objects are addressed by the method of the present invention through a sequential deposition process for making a layer of I-III-VI semiconductor material for use in photovoltaic cells, with the method involving sequential deposition of components, with at least one III component, typically in elemental form, being deposited prior to deposition of a I component, which is also typically in elemental form. Following the sequential deposition, the I-III-VI semiconductor material is formed, incorporating the previously deposited III and I components. In one preferred embodiment of the method, a III component is converted from elemental form to a compound form, in the form of a chalcogenide compound including the III component, prior to deposition of a I component. The conversion of the III component from elemental form to a compound form may effectively be accomplished through an initial heat treatment prior to deposition of a I component. Following deposition of the I component, the final semiconductor material may then be formed in a final heat treatment step. The method, therefore, typically includes at least two heat treatments. A VI component is often provided during the final heat treatment as a predeposited film of an elemental VI component. Also, or alternatively, a VI component, typically in elemental form, may be supplied during the final heat treatment, and/or during the initial heat treatment, as a vapor.
To convert the III components from elemental form to compound form in a chalcogenide compound, a chalcogen, or VI component, must be present during the initial heat treatment. According to the present invention, the chalcogen may be effectively supplied during the initial heat treatment in the form of one or more films of the desired elemental chalcogen that have been separately deposited, prior to the initial heat treatment, adjacent to the film of the elemental component. The present invention, therefore, typically involves the deposition of a sequence of films, typically including only elemental components, prior to the initial heat treatment. Also, or alternatively, a VI component, typically in elemental form, may be supplied during the initial heat treatment in vapor form.
In one preferred embodiment, after the initial heat treatment and prior to the final heat treatment, additional precursor films are sequentially deposited to facilitate formation of the final desired semiconductor material. These additional films typically include at least a film with a I component, typically in elemental form. These additional films may also include one or more films with a VI component, typically also in elemental form, and/or one or more films with a III component, typically also in elemental form. Intermediate processing between the initial heat treatment and the final heat treatment may include one or more additional heat treatment steps.
Pre
Gillespie Timothy J.
Lanning Bruce R.
Marshall Craig H.
Chapman Mark
Lockheed Martin Corporation
Swidler Berlin Shereff & Friedman, LLP
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