Effect of zinc impurity on silicon solar-cell efficiency

Abstract

Zinc is a major residue impurity in the preparation of solar-grade silicon material by the zinc vapor reduction of silicon tetrachloride. This paper projects that in order to get a 17-percent AM1 cell efficiency for the Block IV module of the Low-Cost Solar Array Project,1the concentration of the zinc recombination centers in the base region of silicon solar cells must be less than 4 × 1011Zn/cm3in the p-base n+/p/p+ cell and 7 × 1011Zn/cm3in the n-Base p+/n/n+ cell for a base dopant impurity concentration of 5 × 1014atoms/cm3. If the base dopant impurity concentration is increased by a factor of 10 to 5 × 1015atoms/cm3, then the maximum allowable zinc concentration is increased by a factor of about two for a 17-percent AM1 efficiency. The thermal equilibrium electron and hole recombination and generation rates at the double-acceptor zinc centers are obtained from previous high-field measurements as well as new measurements at zero field described in this paper. These rates are used in the exact dc-circuit model to compute the projections.