Alternatives to boron-doped Czochralski for silicon solar cell processing
A correlation of the metastable defect causing light-induced degradation in boron-doped Czochralski silicon with the concentration of oxygen and boron is firmly established. Results from a European project (TWINGO) are reported that aims at manufacturing low-cost monocrystalline base materials that do not suffer from light-induced degradation and applying laboratory concepts to industrial production with high-efficiency cells. The selected materials were float zone silicon (PV-FZTM) grown by Topsil with a refined procedure offering potential for a reduction of costs and Cz silicon grown by Pillar doped with gallium which is intended for industrial mass production. The material analysis performed at Fraunhofer ISE will be reported in this paper revealing high lifetime values for the starting materials. LFC cells were processed in a wide range of resistivities that showed no marked light-induced degradation. Efficiencies up to 20.6 % for PV-FZTM -silicon with a resistivity of 0.44 ohm cm and 19.7 % for Ga-Cz with a resistivity of 1.91 ohm cm at the tail end of the ingot have been achieved. A detailed investigation of the dependence of the cell performance on the Cz-ingot position and oxygen concentration is presented.