Measuring the device-level EQE of multi-junction photonic power converters

Multi-junction photonic power converters (PPCs) are photovoltaic cells used in photonic power transmission systems that convert monochromatic light to electricity at enhanced output voltages. The junctions of a multi-junction PPC have overlapping spectral responsivity, which poses a unique challenge for spectrally resolved external quantum efficiency (EQE) measurements. In this work, we present a novel EQE measurement technique based on a wavelength-tunable laser system and characterize the differential multi-junction device-level EQE (dEQE_MJ) as a function of the monochromatic irradiance over seven orders of magnitude. The irradiance-dependent measurements reveal three distinct irradiance regimes with different dEQE_MJ. For the experimentally studied 2-junction GaAs-based device, at medium irradiance with photocurrent densities between 0.3 and 90 mA/cm², dEQE_MJ is independent of irradiance and follows the expected EQE of the current-limiting subcell across all wavelengths. At higher irradiance, nonlinear device response is observed and attributed to luminescent coupling between the subcells. At lower irradiances, namely, in the range of conventional EQE measurement systems, nonlinear effects appear, which mimic luminescent coupling behavior but are instead attributed to finite shunt resistance artifacts that artificially inflate dEQE_MJ. The results demonstrate the importance of measuring the device-level dEQE_MJ in the relevant irradiance regime. We propose that device-level measurements in the finite shunt artifact regime at low monochromatic irradiance should be avoided.