Direct growth of III-V nanowire-based top cell for tandem on Silicon

We grow and characterize solar cells based on Ga-catalyzed GaAs NWs radial p-i-n homojunctions grown by Molecular Beam Epitaxy onto inactive p-type Si(111) substrates. The solar cells feature a relatively low efficiency of 2.09% mainly due to poor Voc=0.39V, FF=0.4. To assess the intrinsic quality of the GaAs NWs, we use a hyperspectral imager to determine the absolute luminescence of GaAs homojunction devices. Between 5 suns and 130 suns illumination, we find an ideality factor of 2.2 and 1.4 from J-V curves and J
_sc
-PL curves, respectively. Moreover, the quasi-Fermi level splitting Δ
_EF
, which represents the maximum achievable Voc, is tentatively extracted from the absolute luminescence and is estimated to be above 0.8 V at 1 sun. It confirms the NWs high crystalline quality and indicates a considerable scope for improvement in device processing.