Doping engineering as a method to increase the performance of purified MG Silicon during ingot crystallisation

This paper presents an overview of significant crystallisation results obtained with purified metallurgical grade silicon in the framework of the French Photosil project. Especially we show that in case of a high boron concentration in the feedstock (>2.10¹⁷ cm^-3), the higher the compensation level is, the higher the solar cells efficiency will be. Several ingots were crystallised with different concentrations of boron and phosphorus and the best solar cell efficiency (15.2%) was obtained with the highest compensated ingot. Moreover we show that this performance improvement is due to an increase of carrier lifetime which largely counterbalances the decrease of carrier mobilities, likely caused by scattering effect of ionized dopants. However, due to the different segregation coefficients of the major dopant atoms, boron and phosphorus, compensated multi-c Silicon ingots often show n-type regions, decreasing the overall material yield. Based on these findings, we suggest a novel concept of doping engineering, allowing a control of the compensation level through the entire ingot height, by introducing a well defined mix of dopant atoms (B, P and Ga) to the silicon before crystallisation. This can lead at the same time to a higher electrical performance and a higher material yield of the crystallised Silicon. As a further perspective the use of lower grade and less expensive silicon with a high electrical performance and material yield can be expected.