Theoretical approaches which allow to calculate spectral dependences of external quantum yield of multijunction tandem solar cells have been considered; spectral characteristics of single-junction Ge, double-junction GalnP/GaAs and triple-junction GalnP/GaAs/Ge solar cells were calculated. The possibility of application of these approaches to investigation of minority carrier diffusion lengths in active regions of the solar cells through the comparison of experimental and calculated spectral characteristics was shown. Found out the values of minority carrier diffusion lengths for the layers of GalnP/GaAs/Ge solar cells, which provide almost full collection.