  Motivated by the radiation damage of solar panels in space, firstly, the
results of Monte Carlo particle transport simulations are presented for proton
impact on triple-junction Ga$_{0.5}$In$_{0.5}$P/GaAs/Ge solar cells, showing
the proton projectile penetration in the cells as a function of energy. It is
followed by a systematic {\it ab initio} investigation of the electronic
stopping power for protons in different layers of the cell at the relevant
velocities via real-time time-dependent density functional theory (RT-TDDFT)
calculations. The electronic stopping power is found to depend significantly on
different channeling conditions, which should affect the low velocity damage
predictions, and which are understood in terms of impact parameter and electron
density along the path. Additionally, we explore the effect of the interface
between the layers of the multilayer structure on the energy loss of a proton,
along with the effect of strain in the lattice-matched solar cell. Both effects
are found to be small compared with the main bulk effect. The interface energy
loss has been found to increase with decreasing proton velocity, and in one
case, there is an effective interface energy gain.
