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

Abstract

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 ($V_{oc}$) 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 $V_{oc}$ benefit of using a dielectric/metal mirror compared to a bare metal mirror ($\Delta V_{oc}$) is small ($\le$10 mV) when the internal luminescent efficiency ${\eta }_{int}$ is lower than 0.95 for all mirror architectures investigated. Only in very-high-quality cells, $\Delta V_{oc}$ becomes significant, reaching $\sim$30 mV at ${\eta }_{int}$ $=$ 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 $V_{oc}$ benefits when ${\eta }_{int}$ is very close to unity. Furthermore, we find that for lossy mirrors, transition metal adhesion layers do not have a strong impact on $\Delta V_{oc}$, while for highly reflective mirrors like Ag, adhesion layers thicker than 1 nm are found to be detrimental to the already small $V_{oc}$ gains. Lastly, $\Delta V_{oc}$ 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.