Novel duplex vapor-electrochemical method for silicon solar cells. Quarterly progress report No. 7, August 1, 1977--October 31, 1977

Abstract

Silicon obtained by the SiF/sub 4/-Na reaction was analyzed by spark source mass spectrometry (SSMS). A partial listing of the results is given below. The concentration units in parentheses are ppM wt. B(0.1), Al(0.8), Ga(0.06), P(0.2), F(0.1), Na(1.0), V(0.04), Mo(0.3), Ti(2.0), Zr(2.0), Mn(0.1), Ni(2.0), Zn(0.01), Cu(20.0), Cr(11.0), Fe(55.0). An excellent starting material for silicon solar cells is obtained on the basis of the low levels of B, Al, Ga, P and As. The source of the Fe and Cr is suspected to be the stainless steel container for liquid sodium. An independent check for the phosphorus content in Sample 2-19 was made by a wet colorimetric method and was found to be 90 parts per billion (ppB), in reasonable agreement with the SSMS result of 20 ppB. Silicon samples prepared from induction melted powder were evaluated for electrical properties using four-point probe conductivity and thermoelectric methods. The solid silicon in Sample 2-19 was found to be p-type and ranged in resistivity between 0.1 and 0.5 ohm cm. A stainless steel reactor was designed and fabricated to perform the SiF/sub 4/-Na reaction under P/sub SiF/sub 4// greater than 1 atmosphere. The amount of silicon produced was increased from 25 g per batch (in the glass reactor) to greater than 70 g per batch in the stainless steel reactor. The study of the effects of reaction variables such as P/sub SiF/sub 4// and maximum temperature attained on the particle size of silicon powder showed that the silicon particle size tends to grow larger with increasing pressure of the SiF/sub 4/ gas in the reaction system.