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Impact of nanometer air gaps on photon recycling in mechanically stacked multi-junction solar cells.

We investigate photon recycling at the top subcell in mechanically stacked multi-junction solar cells with nanometer air gaps between the subcells. We determine the incident-angle-dependence of the reflectivity from the rear surface of the top subcell. The results show that more than 30% of the luminescence at the top subcell is reflected at the air gap even for an air gap thickness of 10 nm. In addition, we demonstrate enhanced luminescence extraction in GaAs//InGaAsP dual-junction devices with nanometer air gaps compared to a device with no gap between the subcells. Our findings indicate that an efficient photon recycling can be realized even for air gaps of a few tens of nanometers.