Effect of luminescent coupling on current mismatched multijunction photonic power converters

We demonstrate the impact of luminescent coupling on the performance of dual-junction InGaAs photonic power converters under current matched and current mismatched conditions through variations in the absorbing layer thicknesses. Comparison between simulation and experimental devices show good agreement when luminescent coupling is included in the model. Its inclusion is necessary to match the experimentally observed increase in spectral responsivity as a function of internal radiative efficiency. We show that on-substrate devices that are bottom limited benefit more from the effects of luminescent coupling than top limited devices. Also, we demonstrate that luminescent coupling provided an additional 16% absolute boost in efficiency for devices where the top subcell absorbed about 100 times more light than the bottom subcell compared to if the effect is excluded from the model. The difference in behaviour between top and bottom limited devices is due to losses through the rear side of the device. Losses can be mitigated by including a planar back reflector, which further improves performance. Increases in efficiency of over 5% absolute are possible for devices designed with a back reflector compared to the same design on-substrate. We show that including a back reflector also improves reabsorption across the device. These efficiency gains indicate that luminescent coupling is a significant process in these devices and its inclusion necessary for accurate device modelling.