In this work the degradation effects of the Ga0.TIn0.3As (1.0 eV) and Ga0.42In0.58As (0.7 eV) sub-cells for IMM4J solar cells are investigated after 1-McV electron irradiation by using spectral response and photoluminescence (PL) signal amplitude analysis,as well as electrical property measurements.The results show that,compared with the electrical properties of traditional single junction (S J) GaAs (1.41 eV) solar cell,the electrical properties (such as Isc,Voc,and Pmax) of the newly sub-cells degrade similarly as a function of logφ,where φ represents the electron fluence.It is found that the degradation of Voc is much more than that of Isc in the irradiated Ga0.42In0.58As (0.7 eV) cells due to the additional intrinsic layer,leading to more serious damage to the space charge region.However,of the three types of SJ cells with the gap widths of 0.7,1.0,and 1.4 eV,the electric properties of the Ga0.7In0.3As (1.0 eV) cell decrease largest under each irradiation fluence.Analysis on the spectral response indicates that the Jsc of the Ga0.7In0.3As (1.0 eV) cell also shows the most severe damage.The PL amplitude measurements qualitatively confirm that the degradation of the effective minority carrier life-time (τeff) in the SJ Ga0.7In0.3As cells is more drastic than that of SJ GaAs cells during the irradiation.Thus,the output current of Ga0.7In0.3As sub-cell should be controlled in the irradiated IMM4J cells.