Computer modeling of two-junction, tandem solar cells is performed with an emphasis on exploring the sensitivity of cell design and performance to important, practical parameters such as subcell connectivity, incident spectrum, junction temperature, concentration ratio and top cell quantum efficiency. The accuracy of the model is verified by comparing calculated bandgap<iependent, normalized conversion efficiency temperature coefficients with those measured experimentally for state-of-the-art, singlejunction cells. Examples of the effects of operational parameter variations are presented. Tandem designs based on independently connected subcells are shown to have several advantages.
Based on the modeling work, novel, low-bandgap, InP-based devices have been developed which appear promising for bottom cell applications in two-junction tandems. In particular, epitaxially grown, high-performance p/n homojunctions in Ga0.47In0.s3As layers lattice matched to InP substrates have been fabricated. The results of performance testing the Ga0.47In0.s3As cells under mild concentration ratios suggest that a practical efficiency of at least 35% is possible for a GaAs/Ga0.47In0.s~As mechanically stacked, twojunction tandem cell which is independently connected and operated under a concentration ratio of 500 suns (ASTM E891-87 direct spectrum, 25 Β°C).