Implications of low breakdown voltage of component subcells on external quantum efficiency measurements of multijunction solar cells

The electrical and optical coupling between subcells in a multijunction solar cell affects its external quantum efficiency (EQE) measurement. In this study, we show how a low breakdown voltage of a component subcell impacts the EQE determination of a multijunction solar cell and demands the use of a finely adjusted external voltage bias. The optimum voltage bias for the EQE measurement of a Ge subcell in two different GaInP/GaInAs/Ge triple-junction solar cells is determined both by sweeping the external voltage bias and by tracing the I–V curve under the same light bias conditions applied during the EQE measurement. It is shown that the I–V curve gives rapid and valuable information about the adequate light and voltage bias needed, and also helps to detect problems associated with non-ideal I–V curves that might affect the EQE measurement. The results also show that, if a non-optimum voltage bias is applied, a measurement artifact can result. Only when the problems associated with a non-ideal I–V curve and/or a low breakdown voltage have been discarded, the measurement artifacts, if any, can be attributed to other effects such as luminescent coupling between subcells.

Why can't I measure the external quantum efficiency of the Ge subcell of my multijunction solar cell?

The measurement of the external quantum efficiency (EQE) of low bandgap subcells in a multijunction solar cell can be sometimes problematic. In particular, this paper describes a set of cases where the EQE of a Ge subcell in a conventional GaInP/GaInAs/Ge triple-junction solar cell cannot be fully measured. We describe the way to identify each case by tracing the I-V curve under the same light-bias conditions applied for the EQE measurement, together with the strategies that could be implemented to attain the best possible measurement of the EQE of the Ge subcell.