High temperature accelerated life tests for GaInP/GaAs/Ge solar cells: forward versus forward-reverse bias

This paper presents the comparison of two temperature Accelerated Life Tests (ALTs) for space commercial solar cells. The tests are carried out in dark conditions to circumvent the inherent problems of illumination ALTs. In the first ALT the solar cell working conditions were emulated by means of forward bias (that emulates the electric performance under illumination) while in the second ALT, forward bias was used together with reverse bias (that emulates the shadowing at eclipse periods or any other shadowing event such as for example the shadows cast by antennas) in order to reproduce more adequately the real operation conditions of solar cells. Forward to reverse time ratio is 4:1. Furthermore, the high temperatures used in the second ALT (190, 210 and 230 degrees C) allow a considerable time reduction of the test with respect to the first ALT (150, 165 and 180 degrees C). The evaluation of the Arrhenius activation energy for an illumination power degradation of 10% was 0.88 eV in the forward bias test while 1.06 eV in the forward-reverse test. Reverse bias promotes similar degradation mode than forward one, namely parallel resistance reduction, but probably with a different degradation mechanism that degrades solar cell more intensely, thus reducing reliability which is more than 7 times shorter in the forward-reverse test than in forward one. Despite this reliability reduction, the time for a 90% reliability in the forward-reverse ALT is 32 years of continuous operation assuming a nominal temperature of 80 degrees C which is typical of many GEO missions. The solar cells used in both ALTs were commercial GaInP/Ga(In)As/Ge triple junction and perform very robustly and exhibiting a high reliability for many space applications. It should be noted that these reliability figures are related to the degradation caused exclusively by high temperatures and other stressors like radiation are not considered here.