Gettering effects by aluminum upon the dark and illuminated I-V characteristics of N+-P-P+ silicon solar cells

Impurity gettering is an essential process step in silicon solar cell techn ology. A widely used technique to enhance silicon solar cell performance is the deposition of an aluminum layer on the back surface of the cell, follo wed by a thermal annealing. The aluminum thermal treatment is typically don e at temperatures around 600 degrees C for short times (10-30 min). Seeking a new approach of aluminum annealing at the back of silicon solar cells, a systematic study about the effect the above process has on dark and illumi nated I-V cell characteristics is reported in this paper. We report results on silicon solar cells where annealing of aluminum was done at two differe nt temperatures (600 degrees C and 800 degrees C), and compare the results for cells with and without aluminum alloying. We have shown that annealing of the aluminum in forming gas at temperatures around 800 degrees C causes improvement of the electrical cell characteristics. We have also made evide nt that for temperatures below similar to 250 K, the predominant recombinat ion process for our cells is trap-assisted carrier tunneling for both annea ling temperatures, but it is less accentuated for cells with annealing of a luminum at 800 degrees C, For temperatures above similar to 250 K, the reco mbination proceeds through Shockley-Read-Hall trap levels, for cells anneal ed at both temperatures. Furthermore, it seems from DLTS measurements that there is gettering of iron impurities introduced during the fabrication pro cesses. The transport of impurities from the bulk to the back surface (allo yed with aluminum) reduces the dark current and increases the effective dif fusion length as determined from dark I-V characteristics and from spectral response measurements, respectively. All these effects cause a global effi ciency improvement for cells where aluminum is annealed at 800 degrees C as compared to conventional cells where the annealing was made at 600 degrees C. (C) 2000 Elsevier Science B.V. All rights reserved.