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

Abstract

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 GalnP/GalnAs/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.

Degradation of Ge subcells by thermal load during the growth of multijunction solar cells

“…However, for the solar cell from structure #E3, the curve is shifted to lower voltages, which implies that there is an increase in the reverse saturation current density of etched 3JSC solar cells regarding the as‐grown one of about 1 order of magnitude (from 1.4 × 10 −6 to 1.5 × 10 −5 A/cm 2 ). In addition, no shunt resistance was observed in the solar cells analyzed in this work, neither in the as‐grown nor in the etched 3JSC samples, which simplifies the EQE measurement of Ge subcells in 3JSCs …”

Artifact Interpretation of Spectral Response Measurements on Two‐Terminal Multijunction Solar Cells

“…This method was widely reported in literature. [5,6,9,[11][12][13][14][15]32,33] On one hand, it gives useful information of how the incident photons can be distributed and utilized in different subcells, thus an important means to obtain the feedback needed for optimizing the cell fabrication and achieving current matching between the subcells. On the other hand, the short-circuit points of the subcells do not correlate with the short-circuit point nor other relevant operation conditions of the whole device under solar irradiation.…”

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

“…3) to determine the suitable voltage biasing range to perform an EQE measurement. The width of Region III mainly depends on the values of V br and V oc of the BC (measured under such light bias) [4]. At the same time, the V br depends on 1/N A , being N A the doping level of the base layer of the BC [8].…”

“…This artifact consists of a lower than expected EQE of the Ge subcell under test together with the simultaneous measurement of some response in wavelengths corresponding to another subcell. The measurement artifact has been related to the presence of a low shunt resistance in the Ge junction [2], luminescence coupling (LC) [3] or a low breakdown voltage (V br ) in conjunction with an incorrect voltage bias (Vb MS ) [4].…”