ORIGIN OF THE DEGRADATION OF TRIPLE JUNCTION SOLAR CELLS AT LOW TEMPERATURE

The degradation of solar cells under irradiation by high energy particles (electrons, protons) is the consequence of the introduction of defects trapping minority carriers, which are then not collected by the junction. However, at low temperature, defects located in the space charge region can also induce a tunneling current that results in an apparent decreases of the maximum power. The degradation produced by this tunneling current can depend on temperature, since the concentration of defects created by an irradiation is usually temperature dependent, and can be larger than the degradation associated with carrier recombination. For instance, as we shall see below, an irradiation with 1 MeV electrons at 120 K with a fluence of 3.0 x 10(15)/cm(2) induces a decrease of less than 10 % in the short-circuit current (I-SC) and open-circuit voltage (V-OC) of triple junction (TJ) cells, but a decrease of about 40 % in the maximum power (P-MAX), which implies that more than half of the total degradation of P-MAX should be assigned to another loss mechanism, tunneling in this case. In this work, we demonstrate that this additional degradation must indeed be ascribed to a tunneling process and we investigate the variation of the tunneling current versus fluence induced by electron irradiation in TJ cells, in order to tentatively ascribe the tunneling components to specific sub-cells.