Optimization of four terminal rear heterojunction GaAs on Si interdigitated back contact tandem solar cells

High-efficiency, four-terminal tandem solar cells composed of thin GaAs films mechanically stacked onto interdigitated back contact silicon solar cells with a glass interlayer are demonstrated. The optimal thickness of the absorber layer of a rear heterojunction GaAs subcell for use in four terminal tandem solar cells was studied. GaAs top cells with absorber layer thicknesses of 1.5, 1.9, 2.3, 2.8, and 3.5 mu m were fabricated on glass and mechanically stacked onto interdigitated back-contact Si bottom cells. All tandem cells were found to have efficiencies above 30% under the AM1.5 G spectrum demonstrating a relatively weak sensitivity to thickness in the four-terminal configuration. We found the 2.8 mu m absorber layer cell to have the highest top cell and tandem cell efficiency at 26.38% and 32.57%, respectively. Optical modeling with transfer matrix method for the planar top cell and Lambertian light trapping in the textured Si subcell, along with drift-diffusion Hovel equations, were used to show photon recycling enhancement to the effective diffusion length and V-OC of the top cell as a result of the low-index glass interlayer.

Automated summary

This study demonstrated high-efficiency four-terminal tandem solar cells composed of thin GaAs films stacked onto silicon solar cells, with optimal thickness of the GaAs subcell absorber layer investigated. Tandem cells with different absorber layer thicknesses all showed efficiencies above 30%, with the 2.8 μm absorber layer cell exhibiting the highest efficiency. Optical modeling and equations were utilized to show the enhancement of photon recycling and cell performance due to a low-index glass interlayer.