1. Technical Field
The present invention relates to a method of fabricating a copper indium gallium selenide (CIGS) thin film for solar cells through co-vacuum evaporation and a CIGS thin film for solar cells fabricated by the same. More particularly, the present invention relates to a CIGS thin film for solar cells fabricated using a simplified co-vacuum evaporation process without deterioration in physical properties of crystal growth of the thin film and band-gap grading by Ga distribution while reducing process costs through simplification of process steps and adjustment of evaporation elements.
2. Description of the Related Art
Recently, importance on development of next-generation clean energy sources has increased due to the depletion of fossil fuel reserves. Thereamong, a solar cell is a device that converts solar energy directly into electricity. Solar cells can serve as an energy source to solve energy problems in the future, since they do not emit pollutants and rely upon the sun.
A solar cell may be divided into a variety of kinds depending upon materials used in a light-absorption layer, and the most currently available solar cell is a silicon solar cell. However, since silicon prices have been rising due to shortage of high purity silicon, a thin film type solar cell is drawing attention. The thin film type solar cell is fabricated to a thin thickness and thus contributes to less material consumption and light weight, thereby providing a wide application range. Studies have been actively made as to amorphous silicon, CdTe, CIS or CIGS as a material for such a thin film solar cell.
CIS or CIGS is a I-III-VI compound semiconductor material and has higher conversion efficiency than any other thin-film solar cell materials experimentally prepared. Particularly, since the CIS or CIGS can be fabricated to a thin thickness of 10 microns or less and can be stably operated even after long term use, the CIS or CIGS is expected to be a low-cost, high-efficiency solar cell material as an alternative to silicon.
Particularly, a CIS thin film is a direct transition type semiconductor that can be made thinner, is suitable for optical-conversion due to a band-gap of 1.04 eV, and exhibits high optical-absorption coefficient. CIGS is an alternative to CIS in which a portion of In is substituted with Ga or Se is substituted with S in order to improve low open circuit voltage of CIS.
A CIGS thin film can be generally fabricated by vacuum deposition or non-vacuum coating. Particularly, vacuum deposition may include co-evaporation, in-line evaporation, a two-step process (precursor-reaction), and the like. Thereamong, co-evaporation has been traditionally used to fabricate a high-efficiency CIGS thin-film solar cell. However, co-evaporation has a problem of difficulty in commercialization due to a complicated process and difficulty in fabrication of a large area solar cell. To solve this problem, a two-step (deposition/selenization) process facilitating mass production has been developed.
Among current CIGS deposition processes, a high efficiency technique having an efficiency of 20% or more is a three-step co-vacuum evaporation process developed by NREL wherein, in a first step, In, Ga, and Se are deposited, in a second step, Cu and Se are deposited excessively relative to a normal amount to promote crystal growth, and in a third step, In, Ga, and Se are further supplied to a thin film to adjust a composition of the CIGS thin-film. Particularly, a Cu—Se compound formed in the second step has a low melting point of about 523° C. or less, thereby causing crystal growth of the CIGS thin-film through induction of liquid-phase sintering of the compound. Thus, it has been known that a V-type distribution of Ga in the composition of the thin-film is obtained through such a three-step process, thereby providing efficiency improvement through band-gap grading.
However, considering mass production, since regulation of flux of elements in each step is complex and thin-film deposition requires a long time, the three-step process has a drawback of high process cost. Therefore, there is a need for a method of fabricating a CIGS thin film for solar cells through a simplified process capable of realizing crystal growth and band-gap grading by Ga composition distribution, while simplifying process steps and significantly reducing a film-deposition time.