Subcell-Resolved EQE Method Using Reverse Voltage Biasing for Multijunction Photovoltaics With Overlapping Subcell Absorptance

External quantum efficiency (EQE) measurements of individual subcells in multijunction photovoltaic devices are essential to evaluate current matching and to iterate the design process. The standard light biasing technique used to measure subcell EQE falls short when multiple subcells absorb within the same spectral region. In this work, we demonstrate a three-step reverse voltage biasing EQE method, which measures any number of subcells with overlapping absorptance: (1) A light bias is applied to generate current mismatch between the subcells. (2) Current–voltage (I–V) characteristics are measured into reverse bias, where the limiting subcell enters reverse-bias breakdown and the device current climbs to a plateau at the photocurrent of the next limiting subcell, producing a staircase I–V curve. (3) Each subcell EQE curve is measured using a voltage bias within its current plateau. We demonstrate this approach for a two-junction GaAs-based photonic power converter, comparing to the standard light biasing method and revealing better than 0.8% absolute agreement when the top junction is preferentially biased in the reverse voltage biasing method. We demonstrate the viability of the method by measuring the EQE of all subcells in a six-junction GaAs-based photonic power converter.