Gallium-arsenide solar cells for use with concentrated sunlight

The performance of GaAs/Ga1-xAlxAs solar cells with graded-band-gap and window structures has been compared using standard theoretical modelling techniques. At high levels of solar concentration, superior power-conversion efficiencies are obtained with the graded-band-gap structures. The realisation of band-gap grading requires a precise control of gradation of the aluminium content during epitaxy, which is conveniently achieved using a vapour-phase technique. To grow good-quality Al-containing compounds, we have adopted the metallo-organic chemical-vapour-deposition (m.o.c.v.d.) process for the present work. Our system is designed for a rapid throughput of large-area slices, and complete multilayer structures can be prepared in under two hours. Solar-cell structures of the window type have been prepared bythe m.o.c.v.d. method, and also by liquid-phase epitaxy (l.p.e.). Test cells have been fabricated using in-house processing technology, and solar-cell performance has been assessed at concentration ratios up to 600. The performance of the l.p.e.- grown cells fits closely to that predicted, with the collection efficiency reaching 95#x0025;, Voc = 0.98 V and a fill factor of 0.8 at 1 sun.