Improved semiconductors for photovoltaic solar cells. Second quarterly progress report, September 18, 1976--December 18, 1976. [CuInSe/sub 2/

Abstract

The objective of this research program is the investigation of copper indium selenide (CuInSe/sub 2/) films as a promising potential semiconductor material for low cost, mass produced thin film photovoltaic solar cells. The program emphasis during this quarter has been on the development of techniques for producing single phase CuInSe/sub 2/ films deposited by vacuum evaporation from three independent vapor sources (Cu, In, and Se). Achievement of single phase chalcopyrite CuInSe/sub 2/ is reported by utilizing post-deposition baking in an Se environment at 500/sup 0/C and in high vacuum at 350/sup 0/C to 400/sup 0/C. Electrical properties of the deposited P-type films are presented which show a strong dependence on film composition. Copper rich films are reported to be of low resistivity, low hole mobility, and high carrier concentration while with increasing In content the reverse is noted. The optimum composition is shown to produce films with average values of: resistivity - 20 to 80 ohm cm; hole mobility - 1.3 to 1.8 cm/sup 2//V-sec; and concentrations - 0.5 to 2 x 10/sup 17/ cm/sup -3/. Reproducibility in these values is shown and indicates that the deposition method is well controlled. The electrical properties of pure copper selenide films aremore » described and suggest possible application as a low cost, ohmic contact material to the P-type CuInSe/sub 2/. Initial film stability test results are discussed and imply device passivation/encapsulation against the environment, especially moisture, will be necessary. Typical optical absorption vs wavelength behavior for the CuInSe/sub 2/ thin films is presented and interpreted in terms of the absorption edge and free carrier absorption. This data is used to calculate a band-gap energy of 1.04 eV and an estimated absorption coefficient of 3 x 10/sup 4/ cm/sup -1/ at a wavelength of one micron.« less