Reduction of Trapping and Recombination in Upgraded Metallurgical-Grade Solar Silicon: Impact of Phosphorous Gettering
2021
Upgraded Metallurgical-Grade (UMG) silicon (Si) has raised interest as an
alternative material for silicon solar cells due to its low cost, low
environmental impact and low CAPEX. Maximum cell efficiencies at the level of
those obtained from high purity poly-Si have been reported. However, a higher
defect density and the compensated doping character result in UMG-based cell
efficiencies varying over wider ranges in frequency distribution charts. In
this report we characterize mc-Si UMG samples with different defect densities,
comparing them with mono-Si UMG and commercial high-performance mc-Si samples,
analysing the impact of carrier trapping by means of photoconductance (PC)
decay measurements, and its evolution after applying a phosphorous diffusion
gettering (PDG) process to the wafers. When analyzing the decay time constant
of the PC measurements, slow (66.8+-14.3 ms) and fast (16.1+-3.5 ms) traps are
found in mc-Si samples, while no evidence of trapping is found in mono-UMG
samples. Slow traps are effectively removed after the PDG process, while fast
traps do remain. The influence of dislocations clusters and the possible role
of oxygen, as revealed by Fourier-transform infrared spectroscopy (FTIR) is
discussed. Finally, the improvement in minority carrier lifetime due to the PDG
treatment is reported for each sample type, reaching values up to 140 us in
mc-Si samples with no slow traps neither interstitial oxygen FTIR-peaks.
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