Electron Radiation Damage in Cu(In,Ga)Se 2 analysed in-situ by Cathodoluminescence in a Transmission Electron Microscope

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Electron Radiation Damage in Cu(In,Ga)Se2 analysed in-situ by Cathodoluminescence in a Transmission Electron Microscope Hanne Scheela, Gerhard Franka, Niels Otta, Wolfram Witteb, Horst P. Strunka a Institute of Microcharacterization, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Cauerstr. 6, 91058 Erlangen, Germany b Center for Solar Energy and Hydrogen Research Baden-Württemberg, Industriestr.6, 70565 Stuttgart, Germany ABSTRACT We have equipped our transmission electron microscope (accelerating voltage up to 300 kV) with a cathodoluminescence (CL) system that covers a wavelength range of 180 – 1800 nm and temperatures from 10 K upwards. This contribution shows how this system can be utilized to study the initial damage process due to electron irradiation in Cu(In,Ga)Se2 thin solar films. This damage leads essentially to atomic defects that cannot structurally be imaged in the transmission electron microscope, but influence the luminescence spectra. We analyse in-situ the spectral evolutions with electron dose of Cu(In1-xGax)Se2 with [Ga]/([Ga]+[In]) ratio x ranging from x=0 to x=1 and interpret the defect formation kinetics with a first model. The obtained results indicate that the films with equal Ga and In concentration are the least radiation sensitive. The voltage dependence of the damage rate indicates that the damage arises essentially due to displacement by electron knock-on (in the voltage range 150 – 300 kV). INTRODUCTION Hardly any electronic device has to endure more severe operation conditions than solar cells in space, where high-energy electrons and protons can cause irradiation damage resulting in a degradation of solar cell efficiency. Several irradiation tests on solar cells with polycrystalline Cu(In,Ga)Se2 (CIGS) and CuInSe2 (CIS) absorber layers [1,2] as well as flight data [3] of these solar cells revealed an excellent radiation hardness superior to the other types of solar cell technology. Additional properties like high efficiency, low weight, low production costs as well as high flexibility make CIGS solar cells even more attractive for space applications. However, the microscopic origin of the remarkable stability of these solar cells is still unknown. This situation is probably due to the complexity of the material system but might also be a consequence of exclusively post-irradiation analyses so far. This contribution describes the first results of a completely new approach to study in-situ the defect generation during irradiation with high energy electrons (150 keV and 300 keV) in a transmission electron microscope (TEM) by means of cathodoluminescence (CL). An emerging CL signal indicates the generation of luminescent defects by irradiation. We can thus monitor the kinetics of the initial damage processes which occur on an atomic level. We investigate the radiation behaviour of polycrystalline Cu(In1-xGax)Se2 absorber layers with [Ga]/([Ga]+[In]) ratio x ranging from x=0 to x=1. We are particularly interested in the influence of the Ga conte

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