External quantum efficiency measurements of Germanium bottom subcells: Measurement artifacts and correction procedures

The measurement procedures for the determination of the external quantum efficiency (EQE) of individual subcells in a monolithic, series connected stack of solar cells are known since the 1980s [1]. There, the importance of the choice of appropriate bias light spectrum and bias voltage was already addressed. However, especially when measuring the EQE of Germanium bottom subcells in monolithic III-V triple-junction cells artifacts in the measured EQE are frequently found. Such an artifact can be identified by concurrent observation of a significant parasitic EQE signal in the wavelength region of another subcell - in the majority of the cases in the response region of the middle subcell - AND a comparatively lowly determined EQE of the subcell intended to be measured - usually the Germanium bottom subcell. A procedure how to correct such an EQE measurement that shows an artifact is introduced in [2]. However, only a procedure correcting for the parasitic EQE signal in the response region of the subcell which is not under test, is given. In this paper we will present an additional correction procedure that also corrects for the too lowly measured EQE of the subcell under test.