Low‐Temperature Saw Damage Gettering to Improve Minority Carrier Lifetime in Multicrystalline Silicon

M Al‐Amin,N E Grant,J D Murphy

doi:10.1002/pssr.201700268

M Al‐Amin, N E Grant + Show 1 more

Open Access

https://doi.org/10.1002/pssr.201700268

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Abstract

The minority carrier lifetime in multicrystalline silicon − a material used in the majority of today's manufactured solar cells − is limited by defects within the material, including metallic impurities which are relatively mobile at low temperatures (≤700 °C). Addition of an optimised thermal process which can facilitate impurity diffusion to the saw damage at the wafer surfaces can result in permanent removal of the impurities when the saw damage is etched away. We demonstrate that this saw damage gettering is effective at 500 to 700 °C and, when combined with subsequent low‐temperature processing, lifetimes are improved by a factor of more than four relative to the as‐grown state. The simple method has the potential to be a low thermal budget process for the improvement of low‐lifetime “red zone” wafers.

Highlights

Experimental results for saw damage gettering (SDG) are compared to sister samples with near-identical microstructures which have undergone an equivalent control anneal (CA)
In principle, to introduce an additional thermal process to use the saw damage to getter impurities prior to its removal, permanently removing impurities from the wafer. This may become a relatively low-cost way of improving poor multicrystalline silicon (mc-Si) wafers from the extrema of the ingot whose minority carrier lifetime is too short for viable cells. This approach, called saw damage gettering (SDG), has been recently studied at high temperatures (!800 C) by a group at the University of Oxford, who have demonstrated an improvement in lifetime,[3,4,5] with the best results achieved at 850 C followed by a relatively slow cool.[5]
The starting values cannot be measured in the exact same samples because of the saw damage, the value in all cases increases compared to the mean measured in the CA sample set prior to the annealing process

Summary

Introduction

Experimental results for SDG are compared to sister samples with near-identical microstructures which have undergone an equivalent control anneal (CA). [2] for a review) It is possible, in principle, to introduce an additional thermal process to use the saw damage to getter impurities prior to its removal, permanently removing impurities from the wafer. In principle, to introduce an additional thermal process to use the saw damage to getter impurities prior to its removal, permanently removing impurities from the wafer This may become a relatively low-cost way of improving poor mc-Si wafers from the extrema of the ingot whose minority carrier lifetime ( just “lifetime”) is too short for viable cells.

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Results

Conclusion