GaAs and Al/sub 0.2/Ga/sub 0.8/As solar cells with an indirect-bandgap Al/sub 0.8/Ga/sub 0.2/As emitter-heterojunction cells

Abstract

We first present a high-performance p/sup +/ Al/sub 0.8/Ga/sub 0.2/As-n GaAs heterojunction (HJ) cell, where the conventional p/sup +/GaAs emitter has been replaced by a p/sup +/ Al/sub 0.8/Ga/sub 0.2/As hetero-emitter. The total thickness of the p/sup +/ Al/sub 0.8/Ga/sub 0.2/As hetero-emitter is about 0.1 /spl mu/m, about twice that of window-layer thickness in conventional GaAs homojunction cells. The NREL-measured AM1.5G data on such a GaAs HJ cell include a V/sub OC/ of 1.029 Volts, a J/sub SC/ of 25.4 mA/cm/sup 2/, a fill-factor of 0.8279, and a total-area /spl eta/ of 21.6%. The relatively high V/sub OC/ of the cell indicates no deleterious effect in placing the Al/sub 0.8/Ga/sub 0.2/As-GaAs hetero-interface in the depletion layer. For a given set of n-GaAs base and back-surface field layer, this HJ cell has exhibited superior blue performance compared to a conventional GaAs homojunction cell (p/sup +/GaAs emitter of 0.2-/spl mu/m-thickness and doped to about 2/spl times/10/sup 18/ cm/sup -3/). Second, we describe a high-performance n/sup +/ Al/sub 0.8/Ga/sub 0.2/As-Al/sub 0.8/Ga/sub 0.2/As HJ cell, where the conventional n/sup +/ Al/sub 0.8/Ga/sub 0.2/As emitter has been replaced by a n/sup +/ Al/sub 0.8/Ga/sub 0.2/As hetero-emitter. This HJ cell has helped us to obtain a significant and reproducible improvement in the cell performance (AM1.5, /spl eta/=18.1%) over conventional Al/sub 0.8/Ga/sub 0.2/As homojunction cells (AM1.5, /spl eta/=11.1%). This Al/sub 0.8/Ga/sub 0.2/As HJ cell indicates a 42% improvement in J/sub SC/a V/sub OC/ increase of almost 110 mV, and an improved fill-factor, compared to homojunction cells. The improved J/sub SC/ is a result of higher blue-response and an enhanced red-response.