Further analysis of aluminum alloying for the formation of p+ regions in silicon solar cells (original) (raw)
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Solid-State Electronics, 1999
Aluminum is usually deposited and alloyed at the back of n + -p-p + silicon solar cells for making a good ohmic contact and establishing a back electric ®eld which avoids carrier recombination at the back surface. Typically, the aluminum thermal treatment is made at temperatures around 6008C for short periods (10±30 min). However, recently it has been suggested that the alloyed region could act as a metallic impurity`getterer', but still little work has been done on this phenomenon. Therefore, we have made experiments in order to observe the eect that dierent annealing treatments of the aluminum have on the characteristics of solar cells. In this paper, we show that short-circuit current and open-circuit voltage both improve when aluminum is annealed at temperatures of 8008C for 40 min, when compared to the typical aluminum thermal annealing (6008C for 10 min). From spectral response measurements, we determined that there was improvement of the base minority carrier eective diusion length due to the high temperature aluminum annealing. Furthermore, from DLTS measurements we have determined that the iron concentration in the base is reduced when the annealing is made at 8008C for 40 min. In other words, there appears to be real metallic`gettering' by the high temperature treatments, since the improvement seems to be due to the reduction of the recombination centers in the base when iron is extracted from it. These results are encouraging and further experiments will be made in order to improve even more the conversion eciency of silicon solar cells. #
Solar Energy Materials and Solar Cells, 2000
Impurity gettering is an essential process step in silicon solar cell technology. A widely used technique to enhance silicon solar cell performance is the deposition of an aluminum layer on the back surface of the cell, followed by a thermal annealing. The aluminum thermal treatment is typically done at temperatures around 6003C 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 e!ect the above process has on dark and illuminated I}< cell characteristics is reported in this paper. We report results on silicon solar cells where annealing of aluminum was done at two di!erent temperatures (6003C and 8003C), 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 8003C causes improvement of the electrical cell characteristics. We have also made evident that for temperatures below &250 K, the predominant recombination process for our cells is trap-assisted carrier tunneling for both annealing temperatures, but it is less accentuated for cells with annealing of aluminum at 8003C. For temperatures above &250 K, the recombination proceeds through Shockley}Read}Hall trap levels, for cells annealed at both temperatures. Furthermore, it seems from DLTS measurements that there is gettering of iron impurities introduced during the fabrication processes. The transport of impurities from the bulk to the back surface (alloyed with aluminum) reduces the dark current 0927-0248/00/$ -see front matter