ARTICLE DOI: 10.1002/zaac.201200279 Crystal Structures of Photovoltaic Chalcogenides, an Intricate Puzzle to Solve: the Cases of CIGSe and CZTS Materials Alain Lafond,* [a] Léo Choubrac, [a] Catherine Guillot-Deudon, [a] Philippe Deniard, [a] and Stephane Jobic [a] Keywords: Photovoltaics; Chalcogenides; Crystallochemistry; CZTS; CIGSe Abstract. Light harvesting chalcogenide materials have strong poten- tial applications for photovoltaic due, in part, to the ability of their structures to accommodate shift from the ideal stoichiometry. This study is devoted to the chemical and structural investigations of two specific series of materials, Cu(In,Ga)Se 2 (CIGSe) and Cu 2 ZnSnS 4 (CZTS). Both of them receive currently a strong incentive in the do- main of thin film solar cells. On the basis of accurate chemical analy- Introduction Since the sixties, numerous researches have been dedicated to chalcogenides. Part of this interest lies in their strong pro- pensity to crystallize in strong anisotropic structures that leads to fascinating chemical (e.g. ability to intercalate lithium and to exfoliate in polar solvents), and physical (e.g. electronic in- stabilities as charge and spin density waves) properties. [1–5] Unfortunately, apart several groups throughout the world, this specific activity strongly declined in the nineties, mainly in relation with their low chemical stability towards air and moisture that often delay or hinder their launching on the mar- ket. Nevertheless, there are real interests for chalcogenides with strong potential applications in the industry. Let us men- tion for instance the fabrication of chalcogenide based optics for infrared cameras, [6] the emergence of CdS/CdSe colloidal quantum dots for imaging, labeling and sensing, [7,8] the benefit of photo-induced structural phase-change in telluride for op- tical recording (e.g. DVD-RAM), [9,10] the potentialities of promising electric field induced resistive switches for new class of RRAM memories, [11] chalcogenide for thermoelectric- ity, [12–14] etc. Among all these applications, one appears in the seventies and continuously receives strong incentives. It con- cerns photovoltaics strongly dominated so far by silicon based (multi)junction devices (namely crystalline, multicrystalline, or * Prof. Dr. A. Lafond Fax: +33-240-37.39.95 E-Mail: Alain.Lafond@cnrs-imn.fr [a] Institut des Matériaux Jean Rouxel Université de Nantes CNRS 2 rue de la Houssinière BP 32229, 44322 Nantes cedex 3, France Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/zaac.201200279 or from the au- thor. Z. Anorg. Allg. Chem. 2012, 638, (), 1–8 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 ses, conventional powder and single-crystal X-ray diffraction, resonant X-ray scattering with synchrotron radiation the capacity of the chalco- pyrite and kesterite structures of CIGSe and CZTS to accommodate deviations from the stoichiometry is discussed. Formally, the former is found to be more flexible than the latter even if this one can self adapt to copper-poor and copper-rich compositions without any struc- tural change except in terms of the cation distributions. amorphous silicon cell) but which compete now with CuInSe 2 (and its gallium derivatives) based devices. [15,16] In that framework, we have initiated a few years ago in Nantes, basic researches in solid state chemistry on chalcogen- ides used as absorbers in thin film based solar cells. While most investigations on photovoltaic chalcogenides are devoted to the deposition processes and their impact on the photovol- taic performances, only a few studies have been conducted on the relationship between the absorber, i.e. its chemical compo- sition and its crystal structure, and the capacity of the cell to convert sunlight into electrical power. The determination of such a causal relationship is of prime importance because a slight deviation from stoichiometry may perturb the conversion efficiency of a solar cell and because layer deposition tech- niques go naturally along with local composition changes that request a strong flexibility of the host structure to hold defects. Hereafter, our work is organized as follows. First, we will focus on compounds in the Cu 2 Se-In 2 Se 3 -Ga 2 Se 3 system (i.e. CuIn 1–x Ga x Se 2 compounds), hereafter named CIGSe. Second, we will describe recent investigations carried out on Cu 2 ZnSnS 4 and its congeners (hereafter labeled CZTS), a very promising series of substitutes for CIGSe materials, where costly indium is replaced by zinc and tin, two elements much more abundant, deemed to open the door in the next future to low-cost solar cells. In both cases, we will put the emphasis on the ability of the host lattice to accommodate vacancies and/or substitutions that opens up the door to control of the physical properties and self-curing. Results and Discussion Before going further into the discussions, let us remind that the CIGSe and CZTS series are known to present shifts from