Reduction of trapping and recombination in upgraded metallurgical grade silicon: Impact of phosphorous diffusion gettering

Upgraded metallurgical grade (UMG) silicon (Si) has raised interest as an alternative material for 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 monocrystalline silicon (mono-Si) UMG and commercial high-performance multicrystalline silicon (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. 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 μs in mc-Si samples with neither slow traps nor interstitial oxygen FTIR-peaks.