**1. Introduction**

Most spacecraft are powered by electricity generated by photovoltaic cells mounted on solar panels. In space, solar cells are exposed to radiation environment, and electrons and protons contained in the environment degrade them and reduce their power output. Electrons and protons are relatively light particles that easily penetrate a solar cell, which is generally made of single-crystal semiconductor material. These electrons and protons create crystal defects in the material that form minority-carrier recombination centers or majority-carrier traps. The effect of these two types of defect states is to reduce the output power of a solar cell. Thus, radiation resistance is of the utmost importance to space solar cells.

To determine a material's radiation resistance (i.e., the radiation degradation characteristics) irradiation tests need to be performed on the ground with electron and proton using accelerators. A ground test is mandatory because the obtained degradation characteristics are necessary to predict the degradation of generated power during a mission in space; therefore, it is essential for a test to reflect the actual degradation in space accurately.

Space solar cell irradiation tests are executed in various facilities by many organizations worldwide using their own test protocols. However, to ensure accurate degradation predictions, the test results must be identical regardless of the test procedure and facility. Additionally, investigating the radiation resistance of a new solar cell requires that test results are perfectly reproducible, so standardizing solar cell irradiation tests is crucial.

To this end, the Japan Aerospace Exploration Agency (JAXA), European Space Agency (ESA), National Aeronautics and Space Administration of the United States (NASA), and US Naval Research Laboratory (NRL) have been collaborating to standardize the irradiation test procedures for space solar cells [1]. As a result, an international standard (ISO) of

**Citation:** Imaizumi, M.; Ohshima, T.; Yuri, Y.; Suzuki, K.; Ito, Y. Effects of Beam Conditions in Ground Irradiation Tests on Degradation of Photovoltaic Characteristics of Space Solar Cells. *Quantum Beam Sci.* **2021**, *5*, 15. https://doi.org/10.3390/ qubs5020015

Academic Editor: Akihiro Iwase

Received: 30 March 2021 Accepted: 18 May 2021 Published: 20 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

irradiation test methods was published in 2005 and revised in 2015 [2]. However, no quantitative definition of irradiation test conditions is contained in the standard because the effects of conditions of electron/proton beam irradiation on testing had not clarified when the standard was published. Currently, each facility/organization performs solar cell irradiation tests under its own test conditions. standard (ISO) of irradiation test methods was published in 2005 and revised in 2015 [2]. However, no quantitative definition of irradiation test conditions is contained in the standard because the effects of conditions of electron/proton beam irradiation on testing had not clarified when the standard was published. Currently, each facility/organization performs solar cell irradiation tests under its own test conditions. Our paper aims to provide information for standardizing radiation specifications for

To this end, the Japan Aerospace Exploration Agency (JAXA), European Space Agency (ESA), National Aeronautics and Space Administration of the United States (NASA), and US Naval Research Laboratory (NRL) have been collaborating to standardize the irradiation test procedures for space solar cells [1]. As a result, an international

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 2 of 12

Our paper aims to provide information for standardizing radiation specifications for testing space solar cells. This study clarifies the effects of two typical irradiation beam conditions, fluence rate, and beam-area expansion technique, on a solar cell's degradation for use in space. testing space solar cells. This study clarifies the effects of two typical irradiation beam conditions, fluence rate, and beam-area expansion technique, on a solar cell's degradation for use in space.

#### **2. Experimental 2. Experimental**

Two typical irradiation beam conditions were selected in this study, dose rate and beam-area expansion methods, for the following reasons: First, in the ground irradiation test, the dose rate is generally selected to complete the test within an acceptable duration. However, the rate of actual radiation exposure in space is several orders of magnitude lower than the rate for ground irradiation tests. A lower dose might be gentler for degradation because recovery of radiation damage can be expected [3]. Therefore, the dose rate employed for a ground test is likely to over-estimate degradation. Second, to secure a beam area for an objective solar cell, scanning with a focused spot beam is the usual technique in a ground irradiation test. On the other hand, radiation particles fall into a solar cell uniformly in space. In the case of scanning, intense particles in the focused beam spot are irradiated momentarily, which may induce greater degradation than a uniform defocus beam. Therefore, clarifying the potential difference in output degradation due to the difference of beam condition is the primary purpose of this study. Two typical irradiation beam conditions were selected in this study, dose rate and beam-area expansion methods, for the following reasons: First, in the ground irradiation test, the dose rate is generally selected to complete the test within an acceptable duration. However, the rate of actual radiation exposure in space is several orders of magnitude lower than the rate for ground irradiation tests. A lower dose might be gentler for degradation because recovery of radiation damage can be expected [3]. Therefore, the dose rate employed for a ground test is likely to over-estimate degradation. Second, to secure a beam area for an objective solar cell, scanning with a focused spot beam is the usual technique in a ground irradiation test. On the other hand, radiation particles fall into a solar cell uniformly in space. In the case of scanning, intense particles in the focused beam spot are irradiated momentarily, which may induce greater degradation than a uniform defocus beam. Therefore, clarifying the potential difference in output degradation due to the difference of beam condition is the primary purpose of this study.

An InGaP/GaAs/Ge triple-junction (3J) space solar cell and a high-efficiency silicon (Si) space solar cell were used in this study, both made by SHARP Corporation. Figure 1 shows the schematic cross-section of the solar cells. The size of both cells was 2 cm × 2 cm. The typical efficiency of a 3J cell is 27%; for a Si cell, it is 17%. An InGaP/GaAs/Ge triple-junction (3J) space solar cell and a high-efficiency silicon (Si) space solar cell were used in this study, both made by SHARP Corporation. Figure 1 shows the schematic cross-section of the solar cells. The size of both cells was 2 cm × 2 cm. The typical efficiency of a 3J cell is 27%; for a Si cell, it is 17%.

**Figure 1.** *Cont.*

**Figure 1.** Cross-section of (**a**) a high-efficiency silicon solar cell and (**b**) an InGaP/GaAs/Ge triplejunction solar cell. Note that the figures are not to scale. **Figure 1.** Cross-section of (**a**) a high-efficiency silicon solar cell and (**b**) an InGaP/GaAs/Ge triplejunction solar cell. Note that the figures are not to scale. **Figure 1.** Cross-section of (**a**) a high-efficiency silicon solar cell and (**b**) an InGaP/GaAs/Ge triplejunction solar cell. Note that the figures are not to scale.

The irradiation experiments were carried out at the National Institute for Quantum and Radiological Science and Technology (QST) [4], Takasaki Lab, and the Wakasa Wan Energy Research Center (WERC) [5]. High-energy (10 MeV) proton irradiation was executed using the cyclotron accelerator at QST Takasaki. Low-energy (50–150 keV) proton irradiation was performed using the ion implanters at QST Takasaki and WERC. Electron irradiation was carried out using the Cockcroft–Walton accelerator at QST Takasaki. Current–voltage output characteristics of the solar cells under light illumination (LIV) before and after irradiation were measured at the Japan Aerospace Exploration Agency (JAXA), Tsukuba Space Center. The LIV measurement was made at 25 °C using a dual-source (xenon and halogen lamps) solar simulator with an Air Mass 0 (AM0) spectrum and light intensity equivalent to the Sun in space around the globe. Figure 2 explains the currentvoltage characteristics of a solar cell, which is fundamentally a p–n junction diode and has the rectifying characteristic shown as "Dark I-V" in Figure 2. Once the solar cell is illuminated, a photocurrent is generated and the I-V curve shifts downward ("Light I-V") since the direction of the photocurrent is the opposite to that of the injected current. Three typical output parameters, the short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax), are indicated in the figure. The irradiation experiments were carried out at the National Institute for Quantum and Radiological Science and Technology (QST) [4], Takasaki Lab, and the Wakasa Wan Energy Research Center (WERC) [5]. High-energy (10 MeV) proton irradiation was executed using the cyclotron accelerator at QST Takasaki. Low-energy (50–150 keV) proton irradiation was performed using the ion implanters at QST Takasaki and WERC. Electron irradiation was carried out using the Cockcroft–Walton accelerator at QST Takasaki. Current–voltage output characteristics of the solar cells under light illumination (LIV) before and after irradiation were measured at the Japan Aerospace Exploration Agency (JAXA), Tsukuba Space Center. The LIV measurement was made at 25 ◦C using a dual-source (xenon and halogen lamps) solar simulator with an Air Mass 0 (AM0) spectrum and light intensity equivalent to the Sun in space around the globe. Figure 2 explains the current-voltage characteristics of a solar cell, which is fundamentally a p–n junction diode and has the rectifying characteristic shown as "Dark I-V" in Figure 2. Once the solar cell is illuminated, a photocurrent is generated and the I-V curve shifts downward ("Light I-V") since the direction of the photocurrent is the opposite to that of the injected current. Three typical output parameters, the short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax), are indicated in the figure. The irradiation experiments were carried out at the National Institute for Quantum and Radiological Science and Technology (QST) [4], Takasaki Lab, and the Wakasa Wan Energy Research Center (WERC) [5]. High-energy (10 MeV) proton irradiation was executed using the cyclotron accelerator at QST Takasaki. Low-energy (50–150 keV) proton irradiation was performed using the ion implanters at QST Takasaki and WERC. Electron irradiation was carried out using the Cockcroft–Walton accelerator at QST Takasaki. Current–voltage output characteristics of the solar cells under light illumination (LIV) before and after irradiation were measured at the Japan Aerospace Exploration Agency (JAXA), Tsukuba Space Center. The LIV measurement was made at 25 °C using a dual-source (xenon and halogen lamps) solar simulator with an Air Mass 0 (AM0) spectrum and light intensity equivalent to the Sun in space around the globe. Figure 2 explains the currentvoltage characteristics of a solar cell, which is fundamentally a p–n junction diode and has the rectifying characteristic shown as "Dark I-V" in Figure 2. Once the solar cell is illuminated, a photocurrent is generated and the I-V curve shifts downward ("Light I-V") since the direction of the photocurrent is the opposite to that of the injected current. Three typical output parameters, the short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax), are indicated in the figure.

**Figure 2.** A typical solar cell's current-voltage characteristics in the dark ("Dark I-V") and under illumination ("Light I-V"). Three typical output parameters are indicated: the short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax). **Figure 2.** A typical solar cell's current-voltage characteristics in the dark ("Dark I-V") and under illumination ("Light I-V"). Three typical output parameters are indicated: the short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax). **Figure 2.** A typical solar cell's current-voltage characteristics in the dark ("Dark I-V") and under illumination ("Light I-V"). Three typical output parameters are indicated: the short-circuit current (Isc), open-circuit voltage (Voc), and maximum power (Pmax).

We carried out two types of radiation experiments. First, we irradiated solar cells with electrons and protons to a specific fluence at different fluence rates. Second, we irradiated solar cells using defocused or scanned proton beams with various proton energies. The sample's output performance was characterized before and after the irradiation tests, and degradation of the output was compared. Details of the experimental conditions are described in the following sections.
