Bandgap Optimization for Bifacial Tandem Solar Cells

Bifacial tandem solar cells can generate more power than their monofacial counterparts by exploiting albedo at their rear. Optimum bandgap (E_g) values for subcells in both two-terminal (2T) and four-terminal (4T) bifacial tandems under different albedos are discussed using radiative-recombination-limit calculations. Our investigation considers both AM1.5G and field spectra, incident on silicon-constrained and unconstrained tandems. We find that for an albedo in the 0.2-0.5 range for 2T tandems and ≥0.3 for 4T tandems, the optimal bottom E_g is ∼1.12 eV, indicating that silicon, besides practical purposes, can also theoretically be the best bottom cell. Furthermore, our work highlights that the 4T configuration may be more attractive than its 2T counterpart for bifacial deployment due to a higher theoretical energy yield with lower sensitivities to albedo variations. To maximize 2T-tandem power conversion efficiency, counterintuitively, rather than precise current matching, the subcell with the highest fill factor needs to be current-limiting.