Design Rules for Multi-Junction (InGaAsP) Photonic Power Converters

Photonic power converters (PPCs) attract more and more attention as they enable galvanically isolated optical power transmission. For power-over-fiber applications, wavelengths from within the optical communication bands between 1260 nm and 1675 nm are of interest. While power conversion efficiencies succeeding 50% were demonstrated for such devices, voltages are limited due to the low bandgap of the suitable absorber material. Monolithic series connection via the well-known multi-junction approach enables elevated output voltages due to the addition of individual subcell voltages. Current matching is achieved using adjusted subcell thicknesses with varying degree of semi-transparency for the upper subcells. In this work, we show how the output voltage of multi-junction PPCs is affected by the different thicknesses of thinner upper subcells and a potential rear side mirror. To this end we fabricated single- and dual-junction PPCs on substrate as well as on mirror and experimentally demonstrated the voltage effects in IV-measurements. We physically explain this effect and compare measurement results with theoretical predictions. Finally, we include the subcell dependent influence of series resistance in multi-junction PPCs and derive design rules for the ideal number of junctions in a PPC depending on the desired power regime accounting for these effects.