Advanced concepts for high-efficiency germanium photovoltaic cells

Germanium is an important material for today's highest efficiency solar cells with three np-junctions based on GaInP, GaInAs and Ge. The Ge subcell in these structures consists of a 100–300 nm thin diffused n-type emitter passivated with GaAs or GaInP and a 150 μm thick base layer which is not passivated. Therefore, the current generation of the Ge subcell mainly results from the emitter and covers the direct absorption range of Ge up to ∼1600 nm. Most of the minority carriers which are generated in the Ge base layer are lost due to nonradiative recombination before reaching the np-junction. This paper describes a new approach for improving the performance of the Germanium bottom subcell by the application of a proper rear surface passivation combined with a low substrate doping level in the range of p=10¹⁶ cm⁻³. The new passivation layer sequence consists of amorphous SixC1-x layers, followed by SiO2 and Al. The thicknesses of these materials can be additionally adapted to achieve high infrared reflectivity of up to 80 % below the bandgap of Ge. This adds another design parameter to the solar cell structure and can lead to reduced heat generation in the solar cell device. Up to 10 % higher current generation was measured for Ge subcells with the new passivation layer. This has specifically advantages for metamorphic GaInP/GaInAs/ Ge solar cells where the Ge subcell is current limiting.