Doping optimization of solar grade (SOG) silicon ingots for increasing ingot yield and cell efficiency

Abstract

In the near future, SoG will become the principal material for photovoltaic ingot production as it requires much less energy for purification compared to silicon grades using gas transformation and purification (usually Siemens process or equivalent also used for electronic-grade preparation). In this study, several kinds of silicon have been compared with different dopant contents (mainly boron and phosphorus). Ingot yield and cell efficiency have been optimized for each source of silicon at a commercial level (450kg ingots) using boron or gallium doping. Starting from the resistivity specification given by the cell process, the doping level has been adjusted in order to maximize the ingot silicon yield (weight of silicon bricks used for wafer cutting/weight of silicon ingot). After doping adjustment, ingot quality has been checked, i.e. brick resistivity and lifetime of minority carriers, and wafers have been processed to solar cells. Doping optimization has led to comparable ingot yields and cell efficiencies using SoG and silicon purified by Siemens process or equivalent. The study has been implemented at the Kazakhstan Solar Silicon Plant in Ust-Kamenogorsk using Kazakhstan SoG, SoG has been received from a European manufacturer and polycrystalline silicon has been purified using the Siemens process. Directional solidification furnaces have been manufactured by ECM Technologies, France.