IV-B. SOLAR ENERGY COLLECTION SYSTEM 1. Solar Array IV.B.l.a SOLAR CELL TECHNOLOGY S. Gaudiano, F. L. Baiamonte Experiment Systems Division (1) INTRODUCTION One of the most fundamental and necessary elements of the SPS is the solar cell, a photovoltaic device which converts incident radiation from the sun into electrical energy. Solar arrays compromise the largest and heaviest portion cf the SPS. It is therefore necessary that there be a clear understanding of this device since improvements within it radically affect the configuration of the SPS design and its cost. This section of the report involves the technological factors associated with the cells including their operational limitations, materials, and fabrication techniques. The discussion is based on the use of silicon as the primary solar cell semiconductor material, but also includes comparisons with other candidate materials such as gallium arsenide and cadmium sulfide where applicable. (2) THEORY OF OPERATION A semiconductor material is a poor conductor of electricity which has a conductivity greater than insulators but less than metals (figure IV.B.l.a.l). Its conductivity ranges from approximately 10"6 (ohm-cm)-l to 10$ (ohm-cm)~l. The energy gap between the valence band and the conduction band is small enough (figure IV.B.l.a.2) such that an appreciable number of thermally excited electrons from the valence band can move into the conduction band. The electrons which are excited to the conduction band leave an empty state in the valence band which is referred to as a hole. Further, there may be nonsemiconductor atoms present with energy levels in the forbidden band, near the conduction band, which can donate an electron to the conduction band. The electrical characteristics of the semiconductor can be altered by doping it with impurity atoms having the desired electrical properties. If the semiconductor contains more electrons than holes, it is said to be n-type, and if there are more holes than electrons, it is said to be p-type. The electrochemical potential for charged carriers in p-type semiconductors differs from n-type semiconductors. It is this potential difference which gives rise to an electric field between n-type and p-type regions in a semiconductor material. Electrons in the p-type material are swept by the built-in field of the junction to the n-type material, and since the holes are of opposite charge, they flow to the p-type silicon. As radiant (photon) energy impinges upon the semiconductor, electron-hole pairs are generated and they migrate through the semiconductor until they are either collected or recombine with other holes
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