Estimation of the reliability figures of space GaInP/Ga(In)As/Ge triple junction solar cells from very high temperature accelerated life tests with forward and reverse biasing

Resumen

This paper presents a temperature Accelerated Life Test (ALT) for space solar cells. The test is carried in dark conditions to circumvent the inherent problems of illumination ALTs. This ALT is an evolution of our previous ALT in dark conditions where just forward bias was used. Now, solar cell working conditions are emulated by means of forward/reverse bias. Forward bias emulates electric performance under illumination while reverse bias emulates the shadowing at eclipse periods or any other shadowing event (for example antennas might cast also shadows). The forward to reverse time ratio is 4:1. Furthermore, the high temperatures used in the current ALT (190, 210 and 230 °C) allow a considerable time reduction of the test. The results obtained in this ALT onto commercial GaInP/Ga(In)As/Ge triple junction solar cells suggest that the degradation mode is related with the decrease of parallel resistance, namely an initial degradation by shunts occurs in the GaInP top subcell which is followed by a parallel resistance reduction in the Ga(In)As middle subcell. The activation energy in current ALT is higher (1.06 eV) than in the previous one (0.88 eV). Reverse biasing promotes similar degradation than forward one but more intensely, i.e. in a shorter time. Therefore, reverse biasing produces a significant lower reliability than in previous ALT without reverse bias. Despite this reliability reduction, the time for a reliability of 90% is 32 years of continuous operation at a nominal temperature of 80 °C (typical of many GEO missions). Therefore, these solar cells seem very robust and with a high reliability for many space applications. It should be noted that these figures are related to the degradation caused exclusively by high temperatures and other stressors like radiation are not considered here.