Current Flow and Efficiency of Ge p-n Junctions in Triple-Junction GaInP/Ga(In)As/Ge Solar Cells for Space Applications

Photovoltaic converters based on n-GaInP/n-p Ge heterostructures grown by OMVPE at different conditions of p-n junction formation have been investigated. The heterostructures are intended for narrow bandgap subcells in triple-junction GaInP/GaInAs/Ge solar cells. It has been demonstrated that along with diffusion current component there is tunneling one in the Ge p-n junctions. Therefore, the two-diode electrical equivalent circuit of Ge p-n junction was used. The diode parameter values for both current components were determined by analyzing both dark and light I-V dependences. It has been shown that elimination of the tunneling current component allows increasing the Ge photovoltaic converter efficiency by ~1 % at non-concentrated solar radiation. Due to using the concentrated sunlight, the effect of tunneling current on the Ge based photovoltaic device efficiency can come to naught at the photogenerated current density of ~ 1.5 A/cm.