EQE measurement technique for multi-junction photovoltaics with overlapping subcell absorptance

The standard method to measure subcell external quantum efficiency (EQE) for multi-junction photovoltaics (MJPV) uses light biasing to bring each subcell into current limitation. This method is suitable when each subcell absorbs in a different wavelength range. However, isolating individual subcells via light biasing is difficult for semitransparent subcells with overlapping absorptance, as in MJPV designed for monochromatic irradiance in power-by-light systems. For these cells, the standard measurement approach falls short. Here, we present an alternative technique that incorporates a negative bias voltage to overcome this limitation. We demonstrate subcell EQE measurements in MJPV devices with up to six GaAs subcells.