A one-dimensional theoretical model of the Tandem-Junction Solar Cell (TJC) with high base resistivity and under low-level injection is derived. The model provides a theoretical basis for a previously published conceptual model and allows for the calculation of the spectral response and other performance parameters, namely, I
sc, V
oc, P
m,

, and η, under variation of one or more of the geometrical and material parameters and 1-MeV electron fluence. Sample calculation results of computer simulation of this model are presented.