Recent developments in multijunction solar cell research

Abstract

Power conversion efficiencies as high as 16.5% under 1 Sun, air mass 1.5 (AM 1.5) illumination have been obtained for three-terminal, two-junction 1.72 eV AlGaAs/1.15 eV GaInAs monolithic cascade solar cells. The structures were grown by metal-organic chemical vapor deposition. The increased efficiencies arise primarily from improved surface morphology in this lattice-mismatched materials system. For the individual subcells in a cascade configuration, 1 Sun, Am 1.5 efficiencies as high as 10.4% and 15.3% are obtained for 1.15 eV GaInAs and 1.72 eV AlGaAs, projecting an eventual efficiency of 25.7% for the corresponding two-junction stack. Under 400 Sun concentration, these values are projected to increase to 12%, 19% and 31% respectively. In the lattice-matched 1.72 eV AlGaAs/1.42 eV GaAs materials system, the surface morphology has proved easier to optimize. Two-junction, two-terminal cascades in which the subcells are electrically connected by high-conductance junctions have been fabricated. Open-circuit voltages V oc as high as 2.01 V and fill factors as high as 0.86 have been obtained. These fill factors, comparable with those observed for the best single-junction cells under 1 Sun, AM 1.5 conditions, demonstrate the low residual resistance of the high-conductance junction.