GaAs Photovoltaics on Polycrystalline Ge Substrates

High efficiency III-V multijunction solar cells deposited on metal foil or even polymer substrates can provide tremendous advantages in mass and stowage, particularly for planetary missions. As a first step towards that goal, polycrystalline p/i/n gallium arsenide (GaAs) solar cells are under development on bulk polycrystalline germanium (Ge) substrates. Single junction p/i/n GaAs photovoltaic devices, incorporating indium gallium phosphide (InGaP) front and back window layers, have been grown and processed. Device performance has shown a dependence upon the thickness of a GaAs buffer layer. An unintentional Ge subjunction was formed in the initial devices and limited the efficiency to ~13% (estimated with an antireflection coating) due to the current mismatch and lack of tunnel junction interconnect. Work is underway to control the Ge junction formation and extend the development to polycrystalline InGaP devices