Impact of iron contamination on minority carrier lifetime of multicrystalline silicon solar cells

Abstract

This thesis presents the qualitative analysis of the variation of effective lifetime of minority carrier in n/p-type Si solar cell due to different recombination mechanisms associated with it. There is a direct relation between the minority carrier lifetime and the solar cell efficiency. The higher lifetime of minority carrier results in increased conversion efficiency of silicon solar cell. As silicon wafers are endowed with different crystal defects and grain boundaries, a reduced minority carrier lifetime is observed for both bulk and surface recombination processes that occur through these defects. A detailed knowledge of the role of recombination processes on carrier lifetime will facilitate the design of high efficiency solar cell. A comparative study has been made to analyze the impact of interstitial iron in minority carrier lifetime of multicrystalline silicon (mc-Si). It is shown that iron plays a negative role and is considered very detrimental for minority carrier recombination lifetime. The analytical results of this study are aligned with the spatially resolved imaging analysis of iron rich mc-Si. A numerical study has been carried out to extract bulk recombination lifetime of minority carrier in Fe contaminated p-type compensated silicon solar cell. It has demonstrated that the compensation will lead to a substantial increase in both intrinsic and Shockley-Read-Hall (SRH) lifetime for minority carrier in p-Si. To understand the role of deliberate phosphorus doping in the minority carrier lifetime of iron contaminated boron-phosphorus-compensated p-type solar grade silicon, another numerical study has been performed. This study confirmed that compensation results a significant increase in bulk lifetime of minority carrier. The gain in carrier lifetime is predicted due to the shift in Fermi energy level, carriers screening and reduction in net equilibrium hole concentration. The bulk lifetime of minority carrier reaches its maximum for phosphorus concentration around 1015 cm-3 if the boron concentrations remain fixed at 1017 cm-3. The utmost importance of this result is the control of compensation level that will facilitate strong improvements in silicon solar cell efficiencies.