  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 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. 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
