Ion beam assisted deposition of Cu(In,Ga)Se 2 films for thin film solar cells
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Ion beam assisted deposition of Cu(In,Ga)Se2 films for thin film solar cells Gerd Lippold, Horst Neumann, Axel Schindler Institute for Surface Modification, Permoserstr. 15, 04318 Leipzig, Germany, [email protected]
ABSTRACT We report on a novel ion beam selenization process. The reactive chalcogen component Se and a significant part of the thermal energy needed for CIGS formation is delivered directly into the growing surface by a low energy Se ion beam from a broad beam ion source. This highly controllable technique with respect to ion energy, dose and uniformity and with scaleup capabilities can be used in two ways either for selenization of metallic Cu/(In,Ga) thin film stacks or in co-deposition. In the case of co-deposition the CIGS growth temperature can be reduced to < 400°C Besides the description of the method we present results of Se ion beam analysis and properties of CIGS thin films, produced by the novel selenization process. INTRODUCTION Common Cu(In,Ga)Se2 (CIGS) thin film deposition procedures utilize selenization by Se vapor, gaseous Se compounds or solid phase reactions, respectively. Problems arise from limited controllability of the selenization process or from the use of highly toxic gases. Typical temperatures for the deposition or selenization of Cu(Ga,In)Se2 (CIGS) absorber layers for high-efficiency CIGS solar cells are in the 500°C range. This results in a high thermal budget in the process of CIGS/glass deposition and makes the use of alternative substrate materials such as polymers challenging. Thus, their is a strong interest in alternative selenization methods which yield improvements of process controllability and/or reduce the necessary substrate temperature. They should be up-scalable and avoid the excessive use of toxic compounds. It has been shown, that the partial ionization of the metal and/or selenium beams is a successful approach to prepare crystalline films at low substrate temperatures and to get new degrees of freedom for the control of compositional and structural properties [1-4]. This was explained by an enhanced migration due to the kinetic energy transferred from the ionized beams and an increased sticking coefficient. The ionized cluster beam (ICB) deposition was utilized a typical ion acceleration voltage between 1 and 4 kV and ratios of ionized clusters to deposited atoms between 0.1% and 0.2% [5]. The electric power density delivered by the beam with ion current densities of 1-10 µAcm-2 to the substrate was in the range 220 mW/cm2 [5]. Gnidash et al [4] used partially ionized beams of In and Se with lower ion energy of 300 eV and a ion-to-atom ratio of about 0.025 for the Se beam with an ion current density of about 5 µA/cm2. Despite the fact, that only a very small fraction of the evaporated material was deposited as ions with increased kinetic energy, both methods could deposit polycrystalline CuInSe2 films with large grains at substrate temperatures between 330°C and 400°C. Based on these first encouraging results it was our objective to develop an ion
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