Improved High‐Purity Arc‐Furnace Silicon for Solar Cells

Abstract

Single‐crystal silicon solar cells having efficiencies within 1% of those prepared in semiconductor‐grade silicon have been fabricated in twice‐recrystallized, high purity metallurgical silicon. Produced in a 100 kW experimental arc furnace by carbothermic reduction of pure silica with pelletized carbon black, the best “solar‐grade” silicon contained 50–100 ppmw Al, 50–100 ppmw Fe, 10 ppmw Ti, 1.8 ppma B, and 2.1 ppma P. Following two Czochralski (Cz) recrystallizations, the metal impurity levels in the final ingot were indistinguishable from those in once Cz‐recrystallized semiconductor‐grade silicon, while the boron and phosphorus contents were 1.5 ppma and 0.6 ppma, respectively. The average efficiency of 4 cm2 test solar cells prepared in twice‐recrystallized “solar‐grade” materials was 13.7%, compared to 14.4% for semiconductor‐grade controls. The average efficiency of unencapsulated, production‐type 100 mm diam solar cells made in this material was 11.8%. Compared with earlier experiments, this represents a nearly 2% improvement in efficiency, which is attributed largely to the factor of two lower boron content in the starting silicon.