Theoretical screening of dielectric/metal mirrors for enhanced photon recycling in GaAs solar cells

Dielectrics are often employed for high-reflectivity mirrors in semiconductor devices, since they leverage total internal reflection to reduce optical losses at semiconductor/metal interfaces. In this work, we investigate the impact of a range of dielectrics (ZnS, Si3N4, Al2O3, SiO2, MgF2, air) on mirror reflectivity, photon recycling probability and open-circuit voltage (Voc) in thin-film GaAs solar cells with Au- or Ag-based mirrors. The impact of transition metal adhesion layers is investigated, as well as the influence of the dielectric and active layer thickness. It is found that the Voc benefit of using a dielectric/metal mirror compared to a bare metal mirror (ΔVoc) is small (≤10 mV) when the internal luminescent efficiency ηint is lower than 0.95 for all mirror architectures investigated. Only in very-high-quality cells, ΔVoc becomes significant, reaching ∼30 mV at ηint = 1 when using a 250-nm air-gap to enhance the reflectivity of a lossy Au mirror. This shows that dielectric/metal rear mirrors only provide significant Voc benefits when ηint is very close to unity. Furthermore, we find that for lossy mirrors, transition metal adhesion layers do not have a strong impact on ΔVoc, while for highly reflective mirrors like Ag, adhesion layers thicker than 1 nm are found to be detrimental to the already small Voc gains. Lastly, ΔVoc is shown to be higher in cells with thinner active layers and in cells with planar compared to textured mirrors. In textured cells, however, the short-circuit current density and thereby the power conversion efficiency are affected more strongly by incorporating a dielectric into the rear mirror.