Effect of Cooling Rate during Thermal Processes on the Electrical Properties of Cast Multi-Crystalline Silicon

Panbing Zhou,Lang Zhou,Xiuqin Wei,Naigen Zhou,Shilong Liu

doi:10.1007/s12633-021-01499-1

Abstract

Photoluminescence (PL) imaging techniques and the minority carrier lifetime test system were employed to investigate the variation of the interstitial iron (Fei) concentration, the recombination activity of structural defects and the minority carrier lifetime of cast multicrystalline silicon (mc-Si) in response to the cooling rate after heating. The results showed that when the mc-Si wafers are heated to high-temperature (1000 °C) and then cooled to ambient temperature with different cooling rate, the Fei concentration, the number of recombination active dislocations and grain boundaries increased as the cooling rate rises while the minority carrier lifetime decreased. If cast mc-Si is heated followed by faster cooling at 30 °C/s, the Fei concentration increase by 223% and the electrical activity of grain boundaries, dislocations and intragrain increase significantly, that is to say, the whole wafer is heavily contaminated with metal impurities, and present extremely low minority carrier lifetime.

Highlights

The cast multicrystalline silicon was once considered to be one of main materials for fabrication of solar cells due to its low production cost
The results showed that when the multicrystalline silicon (mc-Si) wafers are heated to hightemperature (1000 °C) and cooled to ambient temperature with different cooling rate, the Fei concentration, the number of recombination active dislocations and grain boundaries increased as the cooling rate rises while the minority carrier lifetime decreased
We studied the recombination activity of structural defects, variation of the Fei concentration and the minority carrier lifetime of cast mc-Si in response to the cooling rate after heating

Summary

Introduction

The cast multicrystalline silicon (mc-Si) was once considered to be one of main materials for fabrication of solar cells due to its low production cost. Our previous studies indicates that the minority carrier lifetime decreases with increase of cooling rate, which is referred to as thermally-induced degradation (TID) of minority carrier lifetime [8, 9]. It means that the photoelectric transformation efficiency of solar cell will be reduced potentially during the thermal processes of silicon wafer-based solar cells fabrication, such as phosphorus diffusing, annealing and electrode sintering processes. It is generally considered that the effects of the TID of minority carrier lifetime is due to the generation of smaller and more dispersed recombination centers [5,6,7], substitutional boron and other Fe-related defects [10, 11] during high temperature heating and fast cooling processes. What is the effect of these defects on the TID of minority carrier lifetime? Is there any interaction between defects and metal impurities during the cooling of the thermal process, and will this interaction affect the electrical properties of the cast mc-Si? These issues are not clear and have not been reported

Methods

Results

Conclusion