  A temperature accelerated life test on Upright Metamorphic
Ga0.37In0.63P/Ga0.83In0.17As/Ge triple-junction solar cells has been carried
out. The acceleration has been accomplished by subjecting the solar cells to
temperatures (125, 145 and 165{\deg}C) significantly higher than the nominal
working temperature inside a concentrator (90{\deg}C), while the nominal
photo-current (500 suns) has been emulated by injecting current in darkness.
The failure distributions have been fitted to an Arrhenius-Weibull model
resulting in an activation energy of 1.39 eV. Accordingly, a 72 years warranty
time for those solar cells for a place like Tucson (AZ, USA), was determined.
After the ALT, an intense characterization campaign has been carried out in
order to determine the failure origin. We have detected that temperature soak
alone is enough to degrade the cell performance by increasing the leakage
currents, the series resistance, and the recombination currents. When solar
cells were also forward biased an increase of series resistance together with a
reduction of short circuit current is detected. The failure analysis shows
that: a) several metallization sub-products concentrate in several regions of
front metal grid where they poison the silver, resulting in a two times
reduction of the metal sheet resistance; b) the metal/cap layer interface is
greatly degraded and there is also a deterioration of the cap layer crystalline
quality producing a huge increase of the specific front contact resistance, c)
the decrease of short circuit current is mainly due to the GaInP top subcell
degradation.
