Effects of high temperature annealing of aluminum at the back of n(+)-p-p(+) silicon solar cells upon their spectral and electrical characteristics

Aluminum is usually deposited and alloyed at the back of n(+)-p-p(+) silico n solar cells for making a good ohmic contact and establishing a back elect ric field which avoids carrier recombination at the back surface. Typically , the aluminum thermal treatment is made at temperatures around 600 degrees C for short periods (10-30 min). However, recently it has been suggested t hat the alloyed region could act as a metallic impurity 'getterer', but sti ll little work has been done on this phenomenon. Therefore, we have made ex periments in order to observe the effect that different annealing treatment s 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 800 degrees C for 40 min, when compared to the typical aluminum thermal annealing (600 degrees C for 10 m in). From spectral response measurements. we determined that there was impr ovement of the base minority carrier effective diffusion 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 th e annealing is made at 800 degrees C for 40 min. In other words? there appe ars to be real metallic 'gettering' by the high temperature treatments, sin ce the improvement seems to be due to the reduction of the recombination ce nters in the base when iron is extracted from it. These results are encoura ging and further experiments will be made in order to improve even more the conversion efficiency of silicon solar cells, (C) 1999 Elsevier Science Lt d. All rights reserved.