Experimental coupling process efficiency and benefits of back surface reflectors in photovoltaic multi-junction photonic power converters

Abstract

Current matching is crucial to maximize the efficiency of two-terminal multi-junction photovoltaic devices. However, even in perfectly designed devices, deviation from the target operating temperature and consequent changes in the subcell absorptances causes current mismatch between the subcell currents even at constant spectral conditions. Fortunately, luminescence coupling from current-overproducing subcells to current limiting subcells mitigates this effect. In this work, the coupling process efficiency in three-junction photonic power converters based on GaAs/AlGaAs rear hetero-junction subcells is experimentally quantified. A coupling process efficiency of 32% ± 9% from top and middle subcells to the limiting bottom subcell is found. Under constant monochromatic illumination, the observed coupling reduces the current mismatch, induced by raising the temperature from current matched conditions at 25°C to 70°C, from 4.4% to 1.6%. Furthermore, in this work, three-junction photonic power converters with back surface reflectors are implemented. Those reflectors improve the device response at elevated temperatures by increasing the optical path length in the limiting subcell. It is shown experimentally how a back reflector effectively redirects photons that are emitted by the bottom subcell towards the upper subcells to reinforce luminescence coupling.