Measuring IV Curves and Subcell Photocurrents in the Presence of Luminescent Coupling

High-quality, direct-bandgap solar cells emit significant luminescence at their band edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected III-V multijunction solar cell. We describe a technique that uses a set of light-emitting diodes (LEDs) and a Xenon-lamp white-light source to accurately determine the subcell photocurrents under a reference spectrum, taking the luminescent coupling current into account. The technique quantifies the luminescent coupling efficiencies and compensates for any spectral overlap between the LEDs and the other junctions. Since quantum efficiency curves are used in the adjustment of the simulator spectrum, we also show how to correct those curves to remove the effects of luminescent coupling.