High-Energy Electron and Proton Irradiation of Cu(In,Ga)Se 2 Heterojunction Solar Cells
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High-Energy Electron and Proton Irradiation of Cu(In,Ga)Se2 Heterojunction Solar Cells A. Jasenek1, A. Boden2, K. Weinert1, M. R. Balboul1, H. W. Schock1, and U. Rau1 1 2
Institute of Physical Electronics (ipe), University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany Hahn-Meitner-Institut (HMI), Glienicker Straße 100, 14109 Berlin, Germany
ABSTRACT We investigate radiation-induced defects in high-efficiency Cu(In,Ga)Se2/CdS/ZnO heterojunction solar cells after 1-MeV electron and 4-MeV proton irradiation. We use electron and proton fluences of more than 1018 cm-2 and up to 1014 cm-2, respectively. The irradiation experiments performed at three independent electron irradiation facilities consistently prove the superior radiation resistance of these Cu(In,Ga)Se2 devices compared to other types of solar cells. The reduction of the solar cell efficiency in all experiments is predominantly caused by a loss ∆VOC of the open circuit voltage VOC . An analytical model describes ∆VOC in terms of radiation-induced defects enhancing recombination in the Cu(In,Ga)Se2 absorber material. From our model we extract the defect introduction rates for recombination centers in Cu(In,Ga)Se2 for the respective particles and energies. Isochronal annealing steps fully recover VOC of the irradiated Cu(In,Ga)Se2 solar cells. Exposure to temperatures of approx. 400 K are sufficient to restore the initial VOC within less than 5 %, even after excessive irradiation. The annealing process displays an activation energy of E A = 1.1 eV. Admittance spectroscopy directly reveals the generation and the annealing of radiation-induced defects. INTRODUCTION Cu(In,Ga)Se2/CdS/ZnO (CIGS) solar cells represent a thin-film photovoltaic technology with very high energy conversion efficiencies. The growth of the CIGS absorber layer has successfully been demonstrated on light-weight metal [1] or polyimide foils [2]. Solar cells on metal foil have reached specific power-to-weight ratios as high as 1400 W/kg [1]. High efficiency, low weight, low cost, and high flexibility when using protective coating layers instead of stiff cover glasses, make CIGS very attractive for space applications. Starting in the 1980s, stability tests of CIGS-based solar cells under high-energy electron and proton irradiation have already been performed [3-5]. The first flight data of CIGS solar cells [6-8] further supported the very high radiation stability. This contribution investigates the stability of high-efficiency ZnO/CdS/CIGS heterojunction solar cells under high-energy electron and proton irradiation. We study the generation of recombination centers in the CIGS absorber layer by means of admittance spectroscopy. Our model quantitatively describes the loss ∆VOC of open circuit voltage VOC by increased space-charge recombination. Isochronal annealing steps in vacuum or in air lead to a reduction of recombination centers, observed with admittance spectroscopy, and to a full recovery of the initial VOC and conversion efficiency η of the CIGS cells. Cycles with differen
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