A mixed-integer optimization for bifacial two-terminal perovskite-on-perovskite tandem solar cells

We present a mixed-integer optimization for the optics of two-terminal perovskite-on-perovskite tandem solar cells, which accounts for thickness and material variations in all thin-film layers. The model, which is validated with experiments for both monofacial and bifacial devices, optimizes the short-circuit current density under six albedo conditions, identifying the best cell architectures and alternative material combinations that yield optimal and close-to-optimal results. In contrast to monofacial devices and low albedos, tandem devices with top subcells that are not semi-transparent are found to provide the highest short-circuit current densities under high albedos, such as albedos for white sand and snow. According to a sensitivity analysis, the short-circuit current densities in the optimized devices are not significantly affected by small perturbations in layer thicknesses. The device performances are also discussed in the context of the best optical structure. A power conversion efficiency of 34.6% and a bifacial equivalent efficiency of over 50% are predicted for the optimized monofacial device and bifacial device under high albedo values, respectively.