Method of manufacturing CIGS photovoltaic devices

Details Method of manufacturing CIGS photovoltaic devices Method of manufacturing CIGS photovoltaic devices

1998-10-15

2001-07-10

Le, Vu A. (Department: 2824)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S085000, C427S076000

Reexamination Certificate

active

06258620

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to photovoltaic cells and, more specifically, to a method of manufacturing copper-indium-gallium-diselenide (CuIn
x
Ga
1-x
Se
2
or just CIGS) photovoltaic devices using elemental selenium and without requiring complex codeposition or requiring the use of toxic H
2
Se gas.
BACKGROUND OF THE INVENTION
There is a growing consensus that the collection of solar energy and its conversion to electrical energy by means of photovoltaic devices should be included in the energy mix of the near future. The commercialization of photovoltaic devices depends on technological advances that lead to higher efficiencies, lower cost, and stability of such devices. The cost of electricity can be significantly reduced by using solar modules constructed from inexpensive thin film polycrystalline semiconductors such as copper indium diselenide (CuInSe
2
or CIS) or cadmium telluride (CdTe). Both materials have shown great promise, but certain difficulties have to be overcome before their commercialization.
Thin-film photovoltaics have two important advantages that offer the hope of achieving truly low-cost electricity production in the near future. The first key to using thin-film photovoltaics is that the material costs remain a small part of the total cell cost and the thin-film coating on large substrates can be obtained in sufficiently large volumes and at sufficiently low costs. The second key to using thin-film photovoltaics is that they hold the promise of being mass-produced in automated processing lines. Two of the leading candidates among thin film solar cells are cadmium telluride and copper indium diselenide. These thin film solar cells are developed with very simple and inexpensive processing techniques such as closed space sublimation (CSS), evaporation, and sputtering, on inexpensive window glass which also acts as a structural support for the final modules. Besides the reduced material cost, simpler and less expensive processing and simpler handling allow for a significant reduction in cost over crystalline silicon cells. However, problems also exist, slowing the thin film solar cell technology in its development and commercial use. A generic problem that is associated with all thin film solar cells is their low conversion efficiencies. The thin-film semiconductor layers are polycrystalline in nature. The inherent grain boundaries introduce regions of increased disorder and segregated impurities in large densities, resulting in a loss of photogenerated carriers due to increased recombination rates. Nevertheless, there has been continuous progress in achieving higher thin-film solar cell efficiencies during recent years despite the poor photovoltaic properties of compound materials developed by low-cost processing methods.
Gallium has been used beneficially in CIS devices, resulting in a copper-indium-gallium-diselenide (CuIn
x
Ga
1-x
Se
2
or just CIGS) structure. Gallium helps to improve the adhesion properties of the CIS films to the substrate. Also, by engineering the band gap of the CIS through Ga incorporation in the space charge region, device efficiencies have been improved further. CIGS laboratory solar cells with efficiencies in the 15-17% range have been reported by several organizations.
However, the deposition techniques used to achieve these levels of performance are either complex codeposition or a two-step process of deposition of a precursor followed by selenization with H
2
Se gas. Each of these methods has shortcomings relative to manufacturing scale-up. The codeposition technique requires tight control of the elemental sources and is quite complicated. The two-step process relaxes this issue but adds the difficulty of dealing with H
2
Se gas, a class A toxic gas, the use of which significantly adds to the complexity of using this method. Attempts at simplifing CIGS manufacturing techniques, for example, by depositing a precursor of one or more of copper, gallium, selenium, and indium and then “selenizing” that precursor by heating it in the presence of a selenium flux, have not resulted in conversion efficiencies as high as ultimately expected from such techniques.
Also, ideally, an absorber layer of a photovoltaic device must have a good bulk layer (e.g., values of space charge width of about 0.5 &mgr;m and a minority carrier diffusion length of 1-2 &mgr;m) and must have a good absorber surface (e.g., low defect density resulting in a recombination lifetimes preferably on the order of 5×10
−9
seconds or greater). Many prior art techniques focus on a manufacturing process that is an apparent compromise between achieving a good bulk layer and achieving a good absorber surface. The result is that neither the surface properties nor the bulk properties are optimized; the process is too complex to simultaneously optimize both properties at the current state of technology.
SUMMARY OF THE INVENTION
According to the present invention, CIGS devices having good conversion efficiencies (about 13%) are manufactured without using H
2
Se by first preparing a precursor and then performing a selenization step in which the precursor is heated in the presence of a selenium flux using elemental selenium as a selenium source. Different embodiments for both the precursor formation step and the selenization step are presented herein. In particular embodiments of practicing the method of the present invention, preferably a precursor containing copper, gallium, indium, and selenium is deposited onto a back contact. The copper is preferably deposited separately from the gallium, which is preferably deposited separately from the indium, which is preferably deposited in the presence of a selenium flux. This precursor is then converted into a p-type absorber layer during the selenization step in which the precursor is preferably heated in the presence of a selenium flux. The selenization step more preferably includes a two-step heating process in which the precursor is heated at a first temperature for a period of time and then heated a second temperature that is higher than the first temperature for a period of time.
An embodiment of the method of manufacturing a photovoltaic device according to the present invention includes the following steps:
(a) providing a substrate carrying a back electrode;
(b) forming a precursor on the back electrode by depositing at least one Class IB element, at least one Class IIIA element, and at least one Class VIA element on the back contact, with an atomic ratio of deposited Class IB elements to deposited Class IIIA elements in the precursor of not greater than 1.0;
(c) heating the precursor in the presence of a flux of at least one Class VIA element thereby creating a partially completed absorber layer;
(d) depositing an additional amount of at least one Class IB element onto the partially completed absorber layer, thereby changing the atomic ratio of deposited Class IB elements to deposited Class IIIA elements; and
(e) heating the partially completed absorber layer with its additional amount of Class IB element in the presence of a flux of at least one Class VIA element.
According to particular embodiments of the present invention, the precursor is formed by depositing the copper, gallium, and indium, the latter of which is preferably deposited in the presence of a selenium flux, in various orders. In a first embodiment (PI) of the precursor formation step above (step (b)), copper is deposited onto the back contact, gallium is deposited onto the copper, and then indium is deposited onto the gallium in the presence of a selenium flux.
In a second embodiment (PII) of the precursor formation step above (step (b)), indium is deposited onto the back contact in the presence of a selenium flux forming a layer of indium selenide, gallium is deposited onto the layer of indium selenide, and then copper is deposited onto the gallium. Other precursors can be used, for example, several PI sequences or several PII sequences can be deposited onto the same substrate. The PI precursor has provided

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