Origins of the short circuit current of a current mismatched multijunction photovoltaic cell considering subcell reverse breakdown

Abstract

In the photovoltaic community, short circuit current (I_sc) of a current mismatched multijunction photovoltaic (MJPV) cell was usually thought to be limited by the lowest subcell photocurrent (I_min). However, under certain conditions for multijunction solar cells, I_sc≠I_min was observed by researchers, while this effect has not been studied in multijunction laser power converters (MJLPCs). In this work, we provide an in-depth analysis of the formation mechanisms for the I_sc of the MJPV cell by measuring I-V curves of the GaAs and InGaAs LPCs with different number of subcells and simulating the I-V curves with the reverse breakdown of each subcell considered. It is found that I_sc of an N-junction PV cell can be theoretically equal to any current value within a range from a current lower than I_min to the maximum subcell photocurrent, which is up to the number of subcell current steps in the forward biased I-V curve. An MJPV cell with a constant I_min will demonstrate a higher I_sc if it has more subcells, smaller subcell reverse breakdown voltage and smaller series resistance. As a result, I_sc tends to be limited by the photocurrent of a subcell closer to the middle cell and is less sensitive to the optical wavelength than I_min. This should be another possible reason why the measured EQE of a multijunction LPC exhibits a wider spectrum width than the calculated I_min-based EQE, whereas this was usually attributed to the luminescent coupling effect merely.