  The investigation of the degradation effects on triple-junction (TJ) solar
cells, operating in space environment, is of primary importance in view of
future space missions towards harsh radiation orbits (e.g. MEO with high
particle irradiation intensity) and for the new spacecraft based on electrical
propulsion. In the present work, we report the experimental results obtained
from the irradiation test campaign carried out both on (InGaP/GaAs/Ge) TJ solar
cells and on single junction (SJ) isotype sub-cells (i.e. InGaP top cells, GaAs
middle cells and Ge bottom cells). In particular, the electrical performances
degradation of the irradiated samples was analyzed by means of the Displacement
Damage Dose (DDD) method based on the NIEL (Non Ionizing Energy Loss) scaling
hypothesis, with DDD doses computed by the SR-NIEL approach. By plotting the
remaining factors as a function of the DDD, a single degradation curve for each
sample type and for each electrical parameter has been found. The collapse of
the remaining factors on a single curve has been achieved by selecting suitable
displacement threshold energies, $E_d$, for NIEL calculation. Moreover, DLTS
(Deep level Transient Spectroscopy) technique was used to better understand the
nature of the defects produced by irradiation. DLTS measurements reveal a good
correlation between defects introduced by irradiation inside the GaAs sub-cell
and the calculated Displacement Damage Doses.
