Practical considerations in tandem cell modeling

• Published in: Solar cells 1989-10, Vol.27 (1), p.191-204
• Main Authors: Wanlass, M.W., Emery, K.A., Gessert, T.A., Horner, G.S., Osterwald, C.R., Coutts, T.J.
• Format: Article
• Language: English
• Subjects: Applied sciences; Energy; Exact sciences and technology; Natural energy; Solar energy; Solar thermal conversion
• ISSN: 0379-6787; 1878-2655
• DOI: 10.1016/0379-6787(89)90028-8

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-dependent, normalized conversion efficiency temperature coefficients with those measured experimentally for state-of-the-art, single-junction 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 Ga 0.47In 0.53As layers lattice matched to InP substrates have been fabricated. The results of performance testing the Ga 0.47In 0.53As cells under mild concentration ratios suggest that a practical efficiency of at least 35% is possible for a GaAs/Ga 0.47In 0.53As mechanically stacked, two-junction tandem cell which is independently connected and operated under a concentration ratio of 500 suns (ASTM E891-87 direct spectrum, 25°C).