*2.2. Beam-Area Expansion Method*

This examination determines the difference in solar cell degradation between the two beam-area expansion techniques, scanning and defocusing. Only proton was use in this examination because no electron accelerator capable of providing a defocused beam was

available. High (10 MeV) and low (50–100 keV) energy levels were selected for the proton beams. Tables 3 and 4 summarize these test conditions. In the case of the defocus beam of 10 MeV proton, a primary beam with a Gaussian distribution over a diameter of ~80 mm was uniformized using multipole magnets to form a rectangular exposure area up to 80 mm × 100 mm. This unique technique was developed at QST Takasaki [6–8].

**Table 3.** Beam conditions of high-energy proton experiment for difference in degradation between scanned and defocused beams.


**Table 4.** Beam conditions of low-energy proton experiment for difference in degradation between scanned and defocused beams.


For the 10 MeV proton experiment, proton irradiation and LIV measurements were alternately implemented in an irradiation chamber. Irradiation was interrupted for each fluence step, and LIV measurements were made using a single-source (xenon lamp) AM0 solar simulator. Schematic top-view of the chamber is posted in Figure 3. This unique setup enables us to collect solar cell's degradation characteristics from a single sample if the obtained data are properly corrected for sample temperature using the temperature coefficients of its output parameters.

coefficients of its output parameters.

**Figure 3.** Schematic top view of the 10 MeV-proton irradiation vacuum chamber. A test solar cell can be illuminated by simulated solar light (AM0) to measure output characteristics instantly. **Figure 3.** Schematic top view of the 10 MeV-proton irradiation vacuum chamber. A test solar cell can be illuminated by simulated solar light (AM0) to measure output characteristics instantly. Beam area 30 mm × 30 mm Beam-area expansion method Scan Defocus

beams. Tables 3 and 4 summarize these test conditions. In the case of the defocus beam of 10 MeV proton, a primary beam with a Gaussian distribution over a diameter of ~80 mm was uniformized using multipole magnets to form a rectangular exposure area up to 80

For the 10 MeV proton experiment, proton irradiation and LIV measurements were alternately implemented in an irradiation chamber. Irradiation was interrupted for each fluence step, and LIV measurements were made using a single-source (xenon lamp) AM0 solar simulator. Schematic top-view of the chamber is posted in Figure 3. This unique setup enables us to collect solar cell's degradation characteristics from a single sample if the obtained data are properly corrected for sample temperature using the temperature

mm × 100 mm. This unique technique was developed at QST Takasaki [6–8].

For both the experiments, beam profiles were observed by using an alumina fluorescent plate right before the irradiations. We confirmed that the adopted scan frequencies and the beam spot sizes provided uniform proton exposure without voids on the entire beam area. For both the experiments, beam profiles were observed by using an alumina fluorescent plate right before the irradiations. We confirmed that the adopted scan frequencies and the beam spot sizes provided uniform proton exposure without voids on the entire beam area. Scan frequency Horizontal: 89 Hz, Vertical: 502 Hz ─ Fluence rate 3.5 × 109 cm−2 s−1 1.0 × 1011 cm−2 s−<sup>1</sup> Beam spot size ~2 mm ϕ ─ Institute QST Takasaki WERC

#### **Table 3.** Beam conditions of high-energy proton experiment for difference in degradation between **3. Results and Discussion 3. Results and Discussion**

#### scanned and defocused beams. *3.1. Fluence Rate 3.1. Fluence Rate*

**Particle Proton Acceleration Energy 10 MeV**  Fluence (Φ) 1.0 × 1011, 3.0 × 1011, 6.0 × 1011, 1.0 × 1012, 2.0 × 1012, 3.0 × 1012, 5.0 × 1012, 7.0 × 1012, 1.0 × 1013, 2.0 × 1013, 3.0 × 1013, 5.0 × 1013 cm−<sup>2</sup> Beam-area expansion method Scan Defocus Beam area 100 mm × 100 mm 80 mm × 100 mm Scan frequency Horizontal: 50 Hz, Vertical: 0.5 Hz ─ Figure 4 exhibits the LIV characteristics of the (a) Si and (b) 3J solar cells before and after 10 MeV proton irradiation with a fluence of 5.0 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> . The fluence rates for the blue and red curves are 6.6 <sup>×</sup> <sup>10</sup><sup>8</sup> and 6.6 <sup>×</sup> <sup>10</sup><sup>10</sup> cm−<sup>2</sup> s −1 . The Si cell shows noticeable degradation, but there is no difference in LIV after irradiation. On the other hand, the 3J cell does not degrade as greatly as the Si, but there is a slight difference in Isc after irradiation. Figure 5 indicates the remaining factors of Isc, Voc, and Pmax for the (a) Si and (b) 3J solar cells as a function of 10 MeV proton fluence rate [9]. Two samples were irradiated for each beam condition. From the results, no significant dependence in degradation on fluence rate can be observed within the range of the experiment for either the Si or 3J solar cell. This fact confirms that the difference in Isc observed in Figure 4b is not significant and comes from a fluctuation of the sample characteristics. Figure 4 exhibits the LIV characteristics of the (a) Si and (b) 3J solar cells before and after 10 MeV proton irradiation with a fluence of 5.0 × 1012 cm−2. The fluence rates for the blue and red curves are 6.6 × 108 and 6.6 × 1010 cm−2 s−1. The Si cell shows noticeable degradation, but there is no difference in LIV after irradiation. On the other hand, the 3J cell does not degrade as greatly as the Si, but there is a slight difference in Isc after irradiation. Figure 5 indicates the remaining factors of Isc, Voc, and Pmax for the (a) Si and (b) 3J solar cells as a function of 10 MeV proton fluence rate [9]. Two samples were irradiated for each beam condition. From the results, no significant dependence in degradation on fluence rate can be observed within the range of the experiment for either the Si or 3J solar cell. This fact confirms that the difference in Isc observed in Figure 4b is not significant and comes from a fluctuation of the sample characteristics.

Beam spot size ~10 mm ϕ ─

**Figure 4.** Light current–voltage characteristics of (**a**) high-efficiency silicon and (**b**) In-GaP/GaAs/Ge triple-junction solar cells before and after 10 MeV proton irradiation with a fluence of 5.0 × 1012 cm−2. The adopted fluence rates for blue and red curves are 6.6 × 108 (6.6E8) and 6.6 × 1010 (6.6E10) cm−2 s−1. **Figure 4.** Light current–voltage characteristics of (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells before and after 10 MeV proton irradiation with a fluence of 5.0 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> . The adopted fluence rates for blue and red curves are 6.6 <sup>×</sup> <sup>10</sup><sup>8</sup> (6.6E8) and 6.6 <sup>×</sup> <sup>10</sup><sup>10</sup> (6.6E10) cm−<sup>2</sup> s −1 .

**Figure 5.** Remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) for (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells as a function of 10 MeV proton fluence rate. The fluence is 5.0 × 1012 cm−2. **Figure 5.** Remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) for (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells as a function of 10 MeV proton fluence rate. The fluence is 5.0 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> . **Figure 5.** Remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) for (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells as a function of 10 MeV proton fluence rate. The fluence is 5.0 × 1012 cm−2.

Figure 6 presents the LIV characteristics of the (a) Si and (b) 3J solar cells before and after 1 MeV electron irradiation with a fluence of 1.0 × 1015 cm−2. The fluence rates for blue and red curves are 1.7 × 1011 and 1.7 × 1013 cm−2 s−1. Again, the Si cell shows considerable degradation. Furthermore, there is a clear difference between the blue and red curves. The 3J cell exhibits far less degradation and has no significant difference between its blue and red curves. Figure 7 depicts the remaining factors of Isc, Voc, and Pmax of the (a) Si and (b) 3J solar cells as a function of 1 MeV electron fluence rate [9]. Two samples were used for each beam condition as well. No dependence in degradation on fluence rate is conformed for the 3J cell. However, the degradation in the Si cell becomes greater as the fluence rate increases. We attributed this to the sample temperature rising during the electron irradiation; therefore, this tendency is not pertinent to this study. The sample temperatures were 30 and 90 °C at the fluence rates of 1.7 × 1011 and 1.7 × 1013 cm−2 s−1. Details of this temperature effect have been described elsewhere [9]. However, this fact demonstrates that controlling the sample temperature is important especially for electron irradiation tests on Si solar cells. Figure 6 presents the LIV characteristics of the (a) Si and (b) 3J solar cells before and after 1 MeV electron irradiation with a fluence of 1.0 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> . The fluence rates for blue and red curves are 1.7 <sup>×</sup> <sup>10</sup><sup>11</sup> and 1.7 <sup>×</sup> <sup>10</sup><sup>13</sup> cm−<sup>2</sup> s −1 . Again, the Si cell shows considerable degradation. Furthermore, there is a clear difference between the blue and red curves. The 3J cell exhibits far less degradation and has no significant difference between its blue and red curves. Figure 7 depicts the remaining factors of Isc, Voc, and Pmax of the (a) Si and (b) 3J solar cells as a function of 1 MeV electron fluence rate [9]. Two samples were used for each beam condition as well. No dependence in degradation on fluence rate is conformed for the 3J cell. However, the degradation in the Si cell becomes greater as the fluence rate increases. We attributed this to the sample temperature rising during the electron irradiation; therefore, this tendency is not pertinent to this study. The sample temperatures were 30 and 90 ◦C at the fluence rates of 1.7 <sup>×</sup> <sup>10</sup><sup>11</sup> and 1.7 <sup>×</sup> <sup>10</sup><sup>13</sup> cm−<sup>2</sup> s −1 . Details of this temperature effect have been described elsewhere [9]. However, this fact demonstrates that controlling the sample temperature is important especially for electron irradiation tests on Si solar cells. Figure 6 presents the LIV characteristics of the (a) Si and (b) 3J solar cells before and after 1 MeV electron irradiation with a fluence of 1.0 × 1015 cm−2. The fluence rates for blue and red curves are 1.7 × 1011 and 1.7 × 1013 cm−2 s−1. Again, the Si cell shows considerable degradation. Furthermore, there is a clear difference between the blue and red curves. The 3J cell exhibits far less degradation and has no significant difference between its blue and red curves. Figure 7 depicts the remaining factors of Isc, Voc, and Pmax of the (a) Si and (b) 3J solar cells as a function of 1 MeV electron fluence rate [9]. Two samples were used for each beam condition as well. No dependence in degradation on fluence rate is conformed for the 3J cell. However, the degradation in the Si cell becomes greater as the fluence rate increases. We attributed this to the sample temperature rising during the electron irradiation; therefore, this tendency is not pertinent to this study. The sample temperatures were 30 and 90 °C at the fluence rates of 1.7 × 1011 and 1.7 × 1013 cm−2 s−1. Details of this temperature effect have been described elsewhere [9]. However, this fact demonstrates that controlling the sample temperature is important especially for electron irradiation tests on Si solar cells.

**Figure 6.** Light current-voltage characteristics of (**a**) high efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells before and after 1 MeV electron irradiation with a fluence of 1.0 × 1015 cm<sup>−</sup>2. The adopted fluence rates for blue and red curves are 1.7 × 1011 (1.7E11) and 1.7 × 1013 **Figure 6.** Light current-voltage characteristics of (**a**) high efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells before and after 1 MeV electron irradiation with a fluence of 1.0 × 1015 cm<sup>−</sup>2. The adopted fluence rates for blue and red curves are 1.7 × 1011 (1.7E11) and 1.7 × 1013 (1.7E13) cm<sup>−</sup>2 s−1. **Figure 6.** Light current-voltage characteristics of (**a**) high efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells before and after 1 MeV electron irradiation with a fluence of 1.0 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> . The adopted fluence rates for blue and red curves are 1.7 <sup>×</sup> <sup>10</sup><sup>11</sup> (1.7E11) and 1.7 <sup>×</sup> <sup>10</sup><sup>13</sup> (1.7E13) cm−<sup>2</sup> s −1 .

(1.7E13) cm<sup>−</sup>2 s−1.

**Figure 7.** The remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) for (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells as a function of 1 MeV electron fluence rate. The fluence is 1.0 × 1015 cm−2. **Figure 7.** The remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) for (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells as a function of 1 MeV electron fluence rate. The fluence is 1.0 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> . **Figure 7.** The remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) for (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells as a function of 1 MeV electron fluence rate. The fluence is 1.0 × 1015 cm−2.

The remaining factors of Isc, Voc, and Pmax of the 3J solar cells as a function of the fluence rate of protons with the energies of (a) 50, (b) 100, and (c) 150 keV are exhibited in Figure 8. Although there is some scattering in the remaining factor of Pmax of the (a) 50 and (b) 100 keV results, the degradation is likely to be independent of the fluence rate in the examined range. The remaining factors of Isc, Voc, and Pmax of the 3J solar cells as a function of the fluence rate of protons with the energies of (a) 50, (b) 100, and (c) 150 keV are exhibited in Figure 8. Although there is some scattering in the remaining factor of Pmax of the (a) 50 and (b) 100 keV results, the degradation is likely to be independent of the fluence rate in the examined range. The remaining factors of Isc, Voc, and Pmax of the 3J solar cells as a function of the fluence rate of protons with the energies of (a) 50, (b) 100, and (c) 150 keV are exhibited in Figure 8. Although there is some scattering in the remaining factor of Pmax of the (a) 50 and (b) 100 keV results, the degradation is likely to be independent of the fluence rate in the examined range.

The adopted fluence rates in this investigation are reasonable values for radiation tests of solar cells. The results confirmed that degradation of Si and 3J solar cells due to electrons and protons is insensitive to the fluence rate that is generally used in actual radiation tests. Furthermore, this independence of degradation on fluence rate is established for both uniform and localized damage. The adopted fluence rates in this investigation are reasonable values for radiation tests of solar cells. The results confirmed that degradation of Si and 3J solar cells due to electrons and protons is insensitive to the fluence rate that is generally used in actual radiation tests. Furthermore, this independence of degradation on fluence rate is established for both uniform and localized damage. The adopted fluence rates in this investigation are reasonable values for radiation tests of solar cells. The results confirmed that degradation of Si and 3J solar cells due to electrons and protons is insensitive to the fluence rate that is generally used in actual radiation tests. Furthermore, this independence of degradation on fluence rate is established for both uniform and localized damage.

**Figure 8.** *Cont.*

**Figure 8. The** remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) of InGaP/GaAs/Ge triple-junction solar cell as a function of fluence rate of protons with energies of (**a**) 50, (**b**) 100 and (**c**) 150 keV. **Figure 8.** The remaining factors of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) of InGaP/GaAs/Ge triple-junction solar cell as a function of fluence rate of protons with energies of (**a**) 50, (**b**) 100 and (**c**) 150 keV.

#### *3.2. Beam-Area Expansion Method 3.2. Beam-Area Expansion Method*

Figure 9 shows a typical set of LIV data collected using the 10 MeV proton irradiation chamber equipped with a solar simulator of the previous section. This is the case of the Si solar cell and the scanned beam, but temperature correction is not applied. Fluence was varied from 0 (initial) to 5.0 × 1013 cm−2 with twelve steps. This figure displays representative deterioration in output as LIV characteristics of a solar cell due to radiation damage. Figure 9 shows a typical set of LIV data collected using the 10 MeV proton irradiation chamber equipped with a solar simulator of the previous section. This is the case of the Si solar cell and the scanned beam, but temperature correction is not applied. Fluence was varied from 0 (initial) to 5.0 <sup>×</sup> <sup>10</sup><sup>13</sup> cm−<sup>2</sup> with twelve steps. This figure displays representative deterioration in output as LIV characteristics of a solar cell due to radiation damage.

plied.

**Figure 9.** Typical set of LIV data for high-efficiency silicon solar cell collected using the 10 MeV proton irradiation chamber equipped with a solar simulator. No temperature correction was applied. **Figure 9.** Typical set of LIV data for high-efficiency silicon solar cell collected using the 10 MeV proton irradiation chamber equipped with a solar simulator. No temperature correction was applied. **Figure 9.** Typical set of LIV data for high-efficiency silicon solar cell collected using the 10 MeV proton irradiation chamber equipped with a solar simulator. No temperature correction was ap-

Figure 10 compares the radiation degradation trends of Isc, Voc, and Pmax with scanned (solid symbols) and the defocused (open symbols) beams on the (a) Si and (b) 3J solar cells due to 10 MeV protons. All the degradations in the remaining factors are nearly the same except for Isc of the 3J solar cell. However, the degradation of Pmax, the product of current and voltage, of the 3J cell does not differ between the scan and defocus results. We conclude that the discrepancy in the Isc degradation is insignificant. The Isc of Si solar cell shows an anomalous increase at the last fluence point of 5.0 × 1013 cm−2. This phenomenon can be seen right before a catastrophic degradation of a solar cell [10–12]. Thus, it is Figure 10 compares the radiation degradation trends of Isc, Voc, and Pmax with scanned (solid symbols) and the defocused (open symbols) beams on the (a) Si and (b) 3J solar cells due to 10 MeV protons. All the degradations in the remaining factors are nearly the same except for Isc of the 3J solar cell. However, the degradation of Pmax, the product of current and voltage, of the 3J cell does not differ between the scan and defocus results. We conclude that the discrepancy in the Isc degradation is insignificant. The Isc of Si solar cell shows an anomalous increase at the last fluence point of 5.0 <sup>×</sup> <sup>10</sup><sup>13</sup> cm−<sup>2</sup> . This phenomenon can be seen right before a catastrophic degradation of a solar cell [10–12]. Thus, it is an indication of the life of the Si solar cell in terms of radiation degradation. Figure 10 compares the radiation degradation trends of Isc, Voc, and Pmax with scanned (solid symbols) and the defocused (open symbols) beams on the (a) Si and (b) 3J solar cells due to 10 MeV protons. All the degradations in the remaining factors are nearly the same except for Isc of the 3J solar cell. However, the degradation of Pmax, the product of current and voltage, of the 3J cell does not differ between the scan and defocus results. We conclude that the discrepancy in the Isc degradation is insignificant. The Isc of Si solar cell shows an anomalous increase at the last fluence point of 5.0 × 1013 cm−2. This phenomenon can be seen right before a catastrophic degradation of a solar cell [10–12]. Thus, it is an indication of the life of the Si solar cell in terms of radiation degradation.

an indication of the life of the Si solar cell in terms of radiation degradation.

**Figure 10.** Comparison of degradation characteristics of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) of (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells by irradiations with scanned and defocused 10 MeV proton beams. **Figure 10.** Comparison of degradation characteristics of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) of (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells by irradiations with scanned and defocused 10 MeV proton beams. **Figure 10.** Comparison of degradation characteristics of short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax) of (**a**) high-efficiency silicon and (**b**) InGaP/GaAs/Ge triple-junction solar cells by irradiations with scanned and defocused 10 MeV proton beams.

Figures 11 and 12 present the degradation of LIV characteristics due to (a) 50 and (b) 100 keV protons with scanned and defocused beams for the Si and 3J solar cells, respectively. The fluence was 1.0 × 1012 cm−2 for both energies. Two samples were used for each condition. In Figures 10a and 11a,b, one result indicates different degradations, but the other three results show almost the same degradation. Therefore, the different degradations of the three solar cells are not thought to be caused by the difference in the beam-Figures 11 and 12 present the degradation of LIV characteristics due to (a) 50 and (b) 100 keV protons with scanned and defocused beams for the Si and 3J solar cells, respectively. The fluence was 1.0 × 1012 cm−2 for both energies. Two samples were used for each condition. In Figures 10a and 11a,b, one result indicates different degradations, but the other three results show almost the same degradation. Therefore, the different degradations of the three solar cells are not thought to be caused by the difference in the beam-Figures 11 and 12 present the degradation of LIV characteristics due to (a) 50 and (b) 100 keV protons with scanned and defocused beams for the Si and 3J solar cells, respectively. The fluence was 1.0 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> for both energies. Two samples were used for each condition. In Figure 10a and a,b, one result indicates different degradations, but the other three results show almost the same degradation. Therefore, the different degradations of the three solar cells are not thought to be caused by the difference in the beam-area

expansion technique. Figure 10 shows that 50 keV protons inflicted greater damage on the Si solar cell than 100 keV protons because the 50 keV protons stop at shallower position in the Si solar cell and create dense defects there. The p–n junction where most photocarriers are generated is located below ~0.5 µm from the surface. Therefore, 50 keV protons induce greater degradation especially on the photocurrent. area expansion technique. Figure 10 shows that 50 keV protons inflicted greater damage on the Si solar cell than 100 keV protons because the 50 keV protons stop at shallower position in the Si solar cell and create dense defects there. The p–n junction where most photocarriers are generated is located below ~0.5 μm from the surface. Therefore, 50 keV protons induce greater degradation especially on the photocurrent. area expansion technique. Figure 10 shows that 50 keV protons inflicted greater damage on the Si solar cell than 100 keV protons because the 50 keV protons stop at shallower position in the Si solar cell and create dense defects there. The p–n junction where most photocarriers are generated is located below ~0.5 μm from the surface. Therefore, 50 keV protons induce greater degradation especially on the photocurrent.

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 11 of 12

**Figure 11.** Light current–voltage characteristics of high-efficiency silicon solar cell before and after (**a**) 50 and (**b**) 100 keV proton irradiation with the fluence of 1.0 × 1012 cm−2. The beam-area expansion techniques for red and blue curves are scanning and defocusing, respectively. **Figure 11.** Light current–voltage characteristics of high-efficiency silicon solar cell before and after (**a**) 50 and (**b**) 100 keV proton irradiation with the fluence of 1.0 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> . The beam-area expansion techniques for red and blue curves are scanning and defocusing, respectively. **Figure 11.** Light current–voltage characteristics of high-efficiency silicon solar cell before and after (**a**) 50 and (**b**) 100 keV proton irradiation with the fluence of 1.0 × 1012 cm−2. The beam-area expansion techniques for red and blue curves are scanning and defocusing, respectively.

**Figure 12.** Light current–voltage characteristics of InGaP/GaAs/Ge triple-junction solar cell before and after (**a**) 50 and (**b**) 100 keV proton irradiation with the fluence of 1.0 × 1012 cm−2. The beamarea expansion techniques for red and blue curves are scanning and defocusing, respectively. **Figure 12.** Light current–voltage characteristics of InGaP/GaAs/Ge triple-junction solar cell before and after (**a**) 50 and (**b**) 100 keV proton irradiation with the fluence of 1.0 × 1012 cm−2. The beamarea expansion techniques for red and blue curves are scanning and defocusing, respectively. **Figure 12.** Light current–voltage characteristics of InGaP/GaAs/Ge triple-junction solar cell before and after (**a**) 50 and (**b**) 100 keV proton irradiation with the fluence of 1.0 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> . The beam-area expansion techniques for red and blue curves are scanning and defocusing, respectively.

The results led us to conclude that there is little difference in solar cell performance

degradation between the two beam-area expansion techniques of scanning and defocusing, suggesting that the intense spot beam in the scanning method has no significant influence. We can use either technique for a radiation test of solar cells. The results led us to conclude that there is little difference in solar cell performance degradation between the two beam-area expansion techniques of scanning and defocusing, suggesting that the intense spot beam in the scanning method has no significant influence. We can use either technique for a radiation test of solar cells. The results led us to conclude that there is little difference in solar cell performance degradation between the two beam-area expansion techniques of scanning and defocusing, suggesting that the intense spot beam in the scanning method has no significant influence. We can use either technique for a radiation test of solar cells.
