Gallium phosphide window layer for silicon solar cells

The integration of III-V compound semiconductors on a silicon bottom cell offers the opportunity to form two- and three-junction solar cells with a conversion efficiency exceeding 30%. This paper reports on the progress in the heteroepitaxial nucleation of gallium phosphide (GaP) on silicon, which allows the fabrication of a silicon bottom cell with front-surface passivation by a thin single-crystalline GaP window layer. GaP has a low lattice-mismatch to Si and an indirect bandgap energy of 2.26 eV, which leads to low absorption. At the same time, GaP can be doped with silicon to form an n-type contact layer. In this publication, we investigate n-Si/p-Si homojunction solar cells with a GaP window and contact layer. Metal-organic vapor phase epitaxy was used to deposit the 60-nm GaP window layer with a low density of antiphase boundaries at the heterointerface and without misfit dislocations. Open-circuit voltages of up to 634 mV have been obtained under 1-sun AM1.5g conditions for devices without antireflective coatings.