*2.1. Fluence Rate*

This investigation aims to clarify the effects of the dose rate by fluence (i.e., the fluence rate) on solar cell degradation. The adopted radiation particles are 10 MeV protons or 1 MeV electrons; both are regarded as the standard radiation particles for radiation resistance evaluation of a space solar cell. Since the particles' energies are sufficiently high to pass through each of the solar cells, irradiation of the particles forms uniform damage throughout the entire device structures. Table 1 lists the beam conditions. The fluence rate was varied over two orders of magnitude in both the 10 MeV proton and 1 MeV electron irradiation experiments: in five levels for the proton beam and in four levels for the electrons. The samples were placed in air for electron irradiation and in a vacuum for proton one.

**Table 1.** Beam conditions of high-energy proton and electron experiments for degradation dependence on fluence rate.


In addition, the effects of fluence rate with low-energy protons (50–150 keV) were examined for the 3J solar cell. A 3J cell has a stacked structure with an InGaP top cell, a GaAs middle cell, and a Ge bottom cell as shown in Figure 1b. Low energy protons stop in the cell, so radiation damage is localized around the Bragg peak of the protons. According to the TRIM simulation [3], 50, 100, and 150 keV protons stop in the InGaP top cell, around the interface between the InGaP top cell and GaAs middle cell, and in the GaAs middle-cell, respectively. Therefore, the effects of the damage may differ. Table 2 shows the beam conditions.

**Table 2.** Beam conditions of low-energy proton experiment for degradation dependence on fluence rate.

