PVICOKEST · INTERNATIONAL COOPERATIVE PROGRAMME FOR PHOTOVOLTAIC KESTERITE BASED TECHNOLOGIES

INTERNATIONAL COOPERATIVE PROGRAMME FOR PHOTOVOLTAIC KESTERITE BASED TECHNOLOGIES

Throughout the exchange program PVICOKEST, the individual expertise available at the partners from European Union and Easter Countries (Table A) has been combined to study kesterites on films and crystals that are especially appropriate for their use as materials for high-efficient, ecologically lovely and low-cost photovoltaic devices. Besides the intensification of the cooperation between partners, a spectacular step forward in the knowledge of the kesterites has been attained. More specifically the main achievements are summarized below: • Homogeneous polycrystalline samples as well as single crystals of different kind of ternaries, quaternaries and pseudo quaternaries compounds Cu2ZnSnxGe1-xS4 and Cu2ZnSnxGe1-xSe4, Cu2ZnSiTe4, as well as stoichiometric and off-stoichiometric Cu2ZnSn(SxSe1-x)4 polycrystalline powders were grown by directional crystallization of the melt (polycrystalline bulk crystals), iodine chemical vapour transport (CVT) and solid state reaction to study Spectroscopic Ellipsomety, Raman Scattering, Photoluminiscence and crystal structure. • Thin films samples belonging to the studied solid solution in this project have been developed by different methods. Cu2ZnSnxGe1-xS4 by flash evaporation and subsequent sulfurization in Ar atmosphere, Cu2ZnSnS4 by sulfurization of metallic precursor, Cu2ZnSnSe4 by ion beam sputtering and Cu2ZnSnS4 by spray pyrolysis • Solar cell device prototypes were also made with these layers in order to investigate the impact of their chemico-physical and structural properties on their performance as absorber layers in solar cells. • Using electron and optical microscopy, Raman scattering, X-ray and neutron diffraction the phases present in the several bulk, powder and thin film samples have been determined. Structure refinements of the diffractograms are now under evaluation for the pure quaternary compounds Cu2ZnSnS4 and Cu2ZnSnSe4 and for the pseudo quaternary Cu2Zn(Ge,Sn)(S,Se)4 single crystal samples. The full structural analysis is in progress. Up to now, the grown crystals crystallize in a tetragonal structure either I-4 or I-42m. • An intensive and continuous research has been made about Raman analysis in bulk samples. The main kesterite vibrational modes, as well as many other peaks, have been identified for Cu2ZnSnS4, Cu2ZnSnSe4, Cu2ZnGeS4, Cu2ZnGeSe4, Cu2ZnSiS4, and Cu2ZnSiSe4 and Cu2ZnSiTe4 single crystals and related to the presence of ordered and disordered phases. The process started with the thin film samples of Cu2ZnSnS4 or Cu2ZnSn1-xGexS4, and it has been determined the relation between the presence of secondary phases and the growth conditions. • The analysis has been extended to the characterization of the main secondary phases presented in the solar cells as ZnS in Cu2ZnSnS4 –developing a methodology for the assessment of the crystal size in ZnS nanocrystals – and Mo(S,Se)2 phases that appear at the interfacial region between the kesterite absorber and the Mo back contact layer • The room temperature spectroscopic ellipsometry study of Cu2ZnSnSe4 and Cu2ZnSn(SexS1-x)4 (x=0.4;0.2). polycrystals were performed. The spectral dependence of the complex pseudodielectric functions was derived. The observed structures in the optical spectra, the fundamental band gap energy E0 and transition energies E1A, E1B were determined from the complex pseudodielectric functions in the frame of the Adachi’s model. In addition, the refractive index, absorption coefficient, and normal-incidence reflectivity were evaluated. • Cu2ZnSn1-xGexS4 single crystals were investigated by spectroscopic ellipsometry at room temperature. The fundamental band gap energy E0 and transition energies E1A, E1B were determined from the complex pseudodielectric functions. Band-gap engineering is possible via the partial substitution of Sn with Ge: E0 increases when the Ge content is higher. This is a very interesting point for the development of a future tandem device. All these results have been summarized in 42 articles in international journals (39 indexed) and in 23 works presented in more than 12 different international conferences listed at the end of document. Among the published articles more than 10 have been cited 10 times and 4 articles have been cited more than 20 times, showing the high impact of the project results in the scientific community. All this work has been done mainly during the 66 secondments of researchers involved in the project. Consequently we can say that throughout this exchange program, a great success has been achieved in the study of Kesterites crystals and films, and an important impact in the development of photovoltaic devices based in these compunds is expected. In summary: • The key objective of intensify cooperation between different groups has been achieved • All the scientific specific objectives have been covered • Very big success in both scientific and cooperation objectives

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