Characterization of Electron-Induced Defects in Cu (In, Ga) Se 2 Thin-Film Solar Cells using Electroluminescence
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Characterization of Electron-Induced Defects in Cu (In, Ga) Se2 Thin-Film Solar Cells using Electroluminescence
Shirou Kawakita1, Mitsuru Imaizumi1, Shogo Ishizuka2, Hajime Shibata2, Shigeru Niki2, Shuichi Okuda3 and Hiroaki Kusawake1 1
Japan Aerospace Exploration Agency (JAXA), Tsukuba, Ibaraki, 305-8505 Japan
2
Insititute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8568
Japan 3
Osaka Prefecture University (OPU), Sakai, Osaka, 599-8570 Japan
ABSTRACT CIGS solar cells were irradiated with 250 keV electrons, which can create only Cu-related defects in the cell, to reveal the radiation defect. The EL image of CIGS solar cells before electron irradiation at 120 K described small grains, thought to be those of the CIGS. After 250 keV electron irradiation of the CIGS cell, the cell was uniformly illuminated compared to before the electron irradiation and the observed grains were unclear. In addition, the EL intensity rose with increasing electron fluence, meaning the change in EL efficiency may be attributable to the decreased likelihood of non-irradiative recombination in intrinsic defects due to electron-induced defects. Since the light soaking effect for CIGS solar cells is reported the same phenomena, the 250 keV electron radiation effects for CIGS solar cells might be equivalent to the effect. INTRODUCTION CIGS solar cells have high attractive solar cells for space applications, since the cells have the highest efficiency among all thin-film solar cells [1], are lightweight and flexible with film substrates [2, 3, 4], and have excellent radiation tolerance in a space environment [5]. In particular, their radiation tolerance has been proved; not only in ground-based radiation irradiation tests but also demonstrations with small satellites in space [6]. CIGS solar cells have excellent radiation tolerance, which means their electrical properties are
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not degraded by 1MeV electrons. Conversely, cell performance is impaired with exposure to proton irradiation, similar to other solar cell types. The radiation damage to the cells caused by proton irradiation gradually recovers when the irradiated cells are kept even at room temperature and the recovery rate is temperature-dependent [7]. The radiation defect in CIGS solar cells, which impair their performance, was reported as an In antisite defect [8]. However, it remains unclear whether the other types of defects, namely Cu, Ga and Se Frenkel-pairs in CIGS, which are simultaneously generated by radiation, degrade cell performance or not. Therefore, we investigated these defects in CIGS solar cells induced by low energy electrons, enabling the type of radiation defect in the solar cells to be selected. The electrical output performance of CIGS solar cells was not degraded by 250 keV electron irradiation, which can generate Cu-related defects in CIGS [9] and the roll–over behavior featured in the current-voltage characteristic under light illumination was reduced by irradiation. The increased carrier density produced by 250 keV ele
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