On the device-level and subcell resolved quantum efficiency of multijunction photonic power converters

Multijunction (MJ) photonic power converters (PPC) are becoming the standard for power-by-light applications (optical wireless power transfer/OWPT, power over fiber/PoF), because of benefits from higher output voltage, lower current and thus increased power before series resistance limitations, and operation at more suitable load resistance. The spectral response or external quantum efficiency (EQE) of PPC devices is crucial for developers. However, for MJ PPCs, experimenters face overlapping subcell responses. Unlike MJ solar cells, MJ PPCs consist of multiple stacked subcells made from the same absorber material, responding in the same wavelength range. We emphasize the importance of distinguishing between two metrics: device-level EQE and subcell-resolved EQE. The former is relevant for the performance in operation, while the latter provides insights into structural details and allows for quantitative comparison with modeling. In this work, we present two novel experimental routines to measure both quantities and we highlight limitations of commonly applied “traditional” techniques that often lead to false or misleading data interpretations.