20%-efficient epitaxial GaAsP/Si tandem solar cells

We present epitaxial 1.7 eV/1.1 eV GaAs_0.75P_0.25/Si tandem cells with an NREL-certified efficiency of 20.0%, enabled by a thermally stable tunnel junction interconnect along with a hydrogenated amorphous Si (a-Si:H) carrier-selective contact for the Si bottom cell. Building on these promising tandem results, we also demonstrate a 16.5%-efficient GaAs_0.75P_0.25 single-junction top cell on Si and a 7.78%-efficient GaAs_0.75P_0.25-filtered Si bottom cell (both NREL-certified) with improved short-circuit current densities. The quantum efficiency of the GaAs_0.75P_0.25 single-junction top cell on Si is boosted across the whole wavelength range due to the use of a higher growth temperature, indicating an improved minority-carrier diffusion length. The implementation of random pyramid texturing at the Si back surface enables improved quantum efficiency at wavelengths of 900–1200 nm, corresponding to an increase of 1.42 mA/cm² in short-circuit current density. The improved short-circuit current densities of the sub-cells together show a pathway to >23% efficiency in a two-terminal tandem configuration.