AlGaAsP Distributed Bragg Reflectors for GaAsP/Si Solar Cells

We investigate (Al)GaAsP distributed Bragg reflectors (DBRs) on Si (001) to improve the quantum efficiency (QE) of 1.7 eV GaAsP solar cells in GaAsP/Si tandem devices. Samples were grown on Si (001) by molecular beam epitaxy and consisted of a 2.1 $\mu \mathrm{m}$ GaAsP/GaP buffer followed by a ∼2 $\mu \mathrm{m}$ DBR with 20 periods of GaAsP/Al_xGa_1-xAsP alternating layers. Two different DBR designs were studied with x = 0.4 and x = 0.8, both targeting a peak reflectance wavelength of 700 nm. The average threading dislocation density on the DBRs was 1.4 × 10⁷ cm⁻², suitable for high-performance GaAsP cells. The reflectance profiles matched well to simulations, and the GaAsP/Al_0.8Ga_0.2AsP DBR had a significantly higher peak reflectance and reflectance bandwidth than the GaAsP/Al_0.4Ga_0.6AsP DBR due to the higher refractive index contrast. QE simulations of GaAsP cells showed an improvement of ∼1 mA/cm² in short-circuit current density with a DBR, which should enable a ∼5% relative efficiency boost in the GaAsP cell and superior current matching to a Si bottom cell in tandem devices.