Analysis of Proton Induced Defects in Cu(In,Ga)Se 2 Thin-Film Solar Cells
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Analysis of Proton Induced Defects in Cu(In,Ga)Se2 Thin-Film Solar Cells Shirou Kawakita1,2), Mitsuru Imaizumi1), Koichi Kibe1), Shinichi Yoda1,2) Takeshi Ohshima3), Hisayoshi Itoh3) and Masafumi Yamaguchi4) 1) Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan 2) Tokyo Institute of Technology, 4259 Nagatsuta-cho, Yokoyama, Kanagawa 226-8503, Japan 3) Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan 4) Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya, Aichi 468-8511, Japan e-mail:[email protected] ABSTRACT We investigated radiation-induced defects in CIGS solar cells with a solar-cell simulator to analyze the spectral response of the irradiated cells. The damage constant of the minority-carrier diffusion length of the cells irradiated with 1 MeV protons was determined to be 3.5 ×10-5. This analysis led to the relation between the defect introduction rate and proton energy, and was obtained using the same method, as was the defect annealing rate. This result agreed well with that estimated from an analysis of changes in short-circuit current degradation. INTRODUCTION A copper indium gallium di-selenide (CIGS) thin-film solar cell is one of several promising candidates for future thin-film space solar cells. CIGS thin-film cells have demonstrated conversion efficiencies exceeding 19%, which is significantly higher than other thin-film solar cells [1]. Radiation tolerance studies were performed on CIGS thin-film solar cells [2]. Of particular note is that the proton- and electron-irradiated cells gradually recover from radiation degradation. The recovery rate of the cells depends on temperature [3]. Since the cells thermally recover well from radiation damage, the recovery must be taken into account in predicting their on-orbit performance [2]. Meanwhile, the generation mechanisms of radiation defects in CIGS solar cells have not yet been clarified. To study the mechanisms, we analyzed the spectral response (SR) of irradiated CIGS solar cells using a PC1D solar-cell simulator [4]. We estimated the damage coefficient of the minority-carrier diffusion length (KL) from the analysis. In addition, we discuss the introduction rate and annealing rate of radiation defects.
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EXPERIMENTAL CIGS thin-film solar cell samples (effective area of 0.6 cm2) were separated from a 30cm×30cm integrated mini-module [5]. The solar cells have an average efficiency of 12.0%, open-circuit voltage of 600 mV, and short-circuit current density of 40 mA/cm2, all of which were measured under AM0, 1-sun condition. The SR was measured at 28˚C under white bias light. Before proton irradiation tests, the minority-carrier diffusion length of the CIGS solar cells was estimated by fitting the experimental SR data using PC1D [6]. The typical minority-carrier diffusion length before irradiation tests was 2.5 µm. This agrees well with the reported value [7]. The isochronical annealing was performed the proton irradiated CIGS solar cells in the air. T
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