Abstract
In order to improve the conversion efficiency (up to a final target over 20% of conversion efficiency) of the photovoltaic cell with multi-crystalline silicon, it is necessary to optimize the casting conditions of silicon ingot such as purifications of mold, mold coating material, atmosphere gas and solidification rate. We have investigated the influence of solidification conditions on the microstructure by the experimental measurement of molten silicon temperature and the simulation of molten silicon flow and temperature distribution in rectangular crucible. It was detected that large supercooling in crucible bottom was effective in enlarging the crystal grain size. High quality silicon ingot was obtained by controlling the solidification rate. The diffusion length of the wafer (thickness 5mm, resistivity 0.65Ω·cm, at bare state) was attained to 350μm or longer from this silicon ingot. Consequently, the conversion efficiency of the cell reached to 18.3% (cell area 25cm2) and 20.3%(cell area 4cm2) using this silicon wafer.
