SOLAR CELL

Solar Cell

Solar Cell

Referring to FIG. 1, the structure of an exemplar for a solar cell is comprised of a single-crystal silicon substrate 10 with thickness of approximately 50 to 250 micrometers and an interlayer 12 of germanium, or graded silicon-germanium alloy with a low germanium content at the interface between the substrate and interlayer, and a high germanium content at the upper surface. A thin layer 14 (about 4 micrometers) of GaAs is chemical vapor deposited (CVD) on the interlayer with suitable dopants for growth of single-crystal GaAs with a shallow n + /p homojunction. A back contact 16 and an obverse grid contact 18 is electroplated to complete the solar cell. An antireflection coating 20, such as silicon nitride, is deposited on the obverse side.

In both exemplars of the invention, GaAs is selected as the material for the solar cell because of the higher conversion efficiency possible (18 to 22%, compared to about 12 to 17% for Si solar cells). This would result in a smaller overall area of the solar array for the same output power. To reduce the mass and cost, the GaAs is selected to be an epitaxial thin film that is vapor deposited because films can be grown much thinner (about 4 micrometers) by vapor deposition than can be cut from a GaAs crystal grown in any other manner. The substrate for the CVD film is selected to be low-cost silicon because it has about half the density of GaAs, so that the mass of the solar cell is significantly reduced, which is important for space applications. The problem is to achieve single crystallinity in the GaAs growth by vapor deposition on single-crystal Si substrate. The solution, in accordance with this invention, is to use a Ge or graded Si/Ge interlayer fabricated, for example, by chemical vapor deposition (CVD), using the appropriate gases for providing the Ge, or graded Si/Ge, growth, namely germane or germane (GeH 4 ) and silane (SiH 4 ) in helium (He) or hydrogen (H 2 ). Although the usual carrier gas used in CVD systems is hydrogen, the carrier gas is preferably helium in order to allow epitaxial growth of Ge and Si at lower temperatures than possible with H 2 . This may be significant because of the differences of about 70% in the Ge and Si thermal expansion coefficients.

The GaAs growth is best accomplished by pyrolytic decomposition of trimethyl gallium (TMG) in the presence of arsine (AsH 3 ) with suitable gaseous sources for dopant incorporation. This method, generally known as organometallic chemical vapor deposition (OM-CVD), offers the best potential and advantages for large-scale production, lowest possible growth temperature, and lack of halide etching during growth. This is particularly so for operating in the low-pressure regime, and perhaps with glow-discharge enhancement. Subsequent processing of the GaAs layers into solar cells can follow using standard methods for metallization and antireflection coatings.

The first step in modifying the Si substrate is to grow the ...