Theoretical analysis of solar cells based on graded band-gap structures

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Abstract
The performance of p–n graded energy gapsolar cells has been calculated for Ga1−f Al f As structures present in either the front region, the base, or both. The graded gap structure in the base is assumed to exist with a high energy gap gradient next to the junction or with a low one along a large portion of the base itself. All the possible configurations in which the builtin fields aid the minority carrier collection are analyzed with a practical theoretical model. The results indicate that the best cell has a high energy gap gradient in the front region and a low energy gap gradient in a large region of the base. This is due to an inverse window effect occurring in the base if the energy gap variation in it is too high. The optimum junction depths for the various configurations are determined as well, and it is shown how the value of these depths is not a critical parameter, as it can vary widely with little influence on the conversion efficiency of the devices. Finally, the results are analyzed versus the surface recombination velocity S and it appears that these graded energy gap structures are suitable only for S≫104 cm/sec.