Diffusion of Ga and In in Sequentially Prepared Cu(In, Ga)S 2 Thin Films for Photovoltaic Applications
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Diffusion of Ga and In in sequentially prepared Cu(In,Ga)S2 thin films for photovoltaic applications Axel Neisser, Reiner Klenk, Martha Ch. Lux-Steiner Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, D - 14 109 Berlin, Germany
ABSTRACT This contribution investigates the diffusion of In and Ga in Cu(In1−x Gax )S2 thin film absorbers for photovoltaic applications. Sequentially prepared Cu-rich CuGaS2 /CuInS2 thin film diffusion couples have been annealed at temperatures in the range of 500◦ C to 600◦ C. Annealed samples have been examined by XRD and SNMS depth profiling. It was found that despite significant interdiffusion of Ga and In the bilayer retains its two phase structure of two layers of almost homogeneous composition, where the composition of these layers depends on the annealing temperature. Furthermore the interdiffusion process is less effective if the binary copper-sulfide phase is removed before annealing. On the basis of a tentative two dimensional diffusion model, which includes rapid diffusion along path ways of high diffusivity, the Ga-depth distribution could be modeled by numerical fits of calculated XRD-spectra to the experimental data. CuS is proposed as a possible pathway for rapide In-Ga interdiffusion. INTRODUCTION The possibility of engineering the band gap of a Cu(In1−x Gax )S2 layer by simply adjusting the Ga-concentration at the interface and in the bulk of the layer offers great opportunities to tailor the band gap to the specific requirements of an electronic device. The maximum attainable open circuit voltage as well as the maximum efficiency in CuInS2 based solar cells could be significantly increased by Ga incorporation in the CuInS2 absorber layer [1] . Therefore the precise control of the elemental composition, particularly of the [Ga]/([In] + [Ga]) ratio x of a Cu(In1−x Gax )S2 absorber layer is a prerequisite in order to fully utilize the potential benefits of Ga-alloying in Cu(In1−x Gax )S2 /CdS/ZnO heterostructures. As reported earlier the growth of Cu(In1−x Gax )S2 absorber layers in a two step process is characterized by sequential segregation of two ternary phases leading to a phase of high Ga-content at the bottom of the layer and a phase of high In-content at the top. In a subsequent recrystallization phase Ga diffuses from the bottom phase into the top phase and In diffuses from the top phase towards the bottom. The degree of alloying of the resulting quaternary Cu(In1−x Gax )S2 phases, i.e. the final profile of the inhomogeneous Ga-depth distribution, strongly depends on annealing temperature and time of the recrystallization phase [2]. Since the Ga-depth distribution has a crucial influence on the final device performance a better understanding of the diffusion mechanism and its dependence on processing parameters is highly desirable in order to optimize the conversion efficiency of such devices and/or in order to adjust the device to a certain application. Interdiffusion of In and Ga in Cu(In1−x Gax )Se2 thin films has been investigated by several au
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