Graded-band-gap AlGaAs solar cells for AlGaAs/Ge cascade cells

Abstract

The development of a graded-emitter Al/sub 0.8/Ga/sub 0.92/As cell that is to be incorporated in an AlGaAs/Ge cascade cell and that has essentially equaled the performance predicted by initial modeling is described. The Al/sub x/Ga/sub (1-x)/As is graded from x=0.08 to 0.18 in a 0.25 mu m thick emitter. The best measured efficiency for the structure under AM0 conditions is 19.5% using an Al/sub 2/O/sub 3/ single-layer antireflection coating. Optimization of the growth process using minority carrier properties has produced improved AlGaAs material quality and, therefore, better cells. Preliminary radiation resistance measurements suggest that AlGaAs cells retain greater fractions of V/sub oc/ and I/sub sc/ than GaAs cells when irradiated with 1 MeV electron fluences greater than 10/sup 15//cm/sup 2/, and graded-emitter cells performed the best in the AlGaAs group, although the grading in the tested cells was not optimized.<>