Abstract
Back-surface field (BSF) design issues for n +p GaAs solar cells are addressed. An expression for the effective back-surface recombination velocity, S pp+ , valid for both homojunction and heterojunction BSFs, is derived. Computations for a typical n +p shallow junction cell demonstrate: (1) the importance of so-called bandgap narrowing effects in p + GaAs and (2) that pp + homojunction BSFs cannot attain the low (less than about 10 3 cm s −1) effective surface recombination velocities required for very high efficiency. AlGaAs/GaAs heterojunction BSFs are shown to be capable of nearly zero effective surface recombination velocity. Comparison of n +p shallow junction cells with homojunction BSFs to similar cells with heterojunction BSFs reveals an advantage of approximately 20 mV in open-circuit voltage for the heterojunction BSF. Theoretical estimates are in approximate agreement with the increase of about 30 mV that has been observed experimentally, and suggest that heterojunction BSFs may be required in order to reach the efficiency limits of n +p GaAs cells.
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