Photovoltaics, Essay

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364 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 3, JULY 2012 Compositional Gradients in Cu(In,Ga)Se2 Thin Films for Solar Cells and Their Effects on Structural Defects Jens Dietrich, Daniel Abou-Ras, Thorsten Rissom, Thomas Unold, Hans-Werner Schock, and Christian Boit, Member, IEEE Abstract—Cu(In,Ga)Se2 (CIGSe) absorber layers used in thinfilm solar cells exhibit, when grown in a multistage process, compositional gradients of Ga and In. In this study, the correlations between the Ga gradient and the microstructure are studied by means of transmission electron microscopy (TEM) imaging combined with energy-dispersive X-ray spectroscopy (EDX), allowing the determination of structural defects and elemental distributions at identical sample positions. The occurrence of linear defects (dislocations) and planar defects (stacking faults and microtwins) of CIGSe layers was studied by means of TEM images. The Ga distributions obtained from EDX elemental distribution maps and structural parameters from the literature were used to calculate the lattice parameters c and a and the gradient dc/dx perpendicular to the substrate. We found a correlation between the magnitude of dc/dx and the occurrence of dislocations within individual large grains. From the presented results, a threshold value of the Ga gradient of 12–13at.%/μm can be estimated for the formation of misfit dislocations. Index Terms—Chalcopyrite-type, Cu(In,Ga)Se2 (CIGSe), dislocation, Ga gradient, microstructure. I. INTRODUCTION HIN-FILM solar cells with polycrystalline Cu(In,Ga)Se2 (CIGSe) absorber layers and embedded in a ZnO/CdS/CIGSe/Mo/glass stack have reached powerconversion efficiencies of more than 20% [1], which is already comparable with those of multicrystalline silicon solar cells [2]. CIGSe thin films grown in a multistage coevaporation process exhibit compositional gradients

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