Structural and morphological investigations on the formation of Cu(In,Ga)Se2 by solvothermal, sonochemistry and sputter deposition methods

Photovoltaics is a highly attractive field due to the expectation of delivering cheap, reusable and inexhaustible clean energy. Among the various solar cell materials, the Cu(In,Ga)Se2 family of chalcopyrites are of much interest because of their highly attractive material properties including band gap and absorption coefficient. Preparation of this material has been investigated by various techniques for the eventual fabrication of solar cells.;It is beneficial to have a good understanding of the Cu(In,Ga)Se 2 formation reactions that can form high quality absorber layer with reduced manufacturing cost. In this thesis the formation reactions of a series of synthetic techniques are studied by relating physical properties of the Cu-In-Ga-Se systems and combining them with the experimentally obtained data. The formation reactions during the synthesis of Cu(In,Ga)Se2 by solvothermal, sonochemistry and sputter deposition techniques are investigated.;Nanoparticles and/or bulk-like particles are formed by reacting elemental precursors in ethylenediamine and triethylenetetramine solvents under by varying synthesis temperature and duration. The results show that the formation reaction of CuInSe2 is relatively slower than the formation of CuIn 0.7Ga0.3Se2 and this is attributed to smaller ionic radii of Ga and lesser equilibrium phases of Ga-Se system compared to In-Se system. To achieve a better understanding of the reaction pathways sonochemistry technique is used to homogenize and react elemental precursors. Sonochemistry technique has the advantage for creating fine dispersions of particles. After the sonication the suspension can be dried of to study formation pathway of the Cu(In,Ga)Se2 by annealing.;For sonochemistry reactions, three preselected solvents were investigated by sonicating Cu and Se precursors in each solvent followed by the selection of the solvent that shows the fastest reaction. Results showed that rapid selenization reactions take place in two minutes with 2-cyanopyridine solvent due to the removal of surface oxide from the precursors, as confirmed by XPS analysis. For studying the CuInSe2, CuGaSe2 and Cu(In,Ga)Se 2 formation reactions, the precursors are sonicated, combined stoichiometrically, and finally annealed in Ar(95%) – H2(5%) atmosphere. The effective heat of formation (EHF) model is used to predict the binary phase formation sequences and the model is found to be in agreement with the experimentally obtained data for as-sonicated and post-synthesis annealed samples.;A crystallographic relation model is proposed with the purpose of determining the reduced activation energy and suppressed reactions for the formation of Cu(In,Ga)Se2. In order to identify this relation first, the predominant crystal facets of reactant phases are predicted by using Bravais-Niggli-Donna-Harker (BNDH) law. Second, crystallographic relations between the predominant crystal facets of the reactant phases are analyzed such as to find if they are suitable for epitaxy. It is proposed that when the reactant phases are found to be structurally similar and a lattice mismatch is <10%, the formation reaction is expected to have reduced activation energy otherwise the formation reaction is suppressed. The experimentally obtained results show that it is in complete agreement with crystallographic relation model. Based on this model, another reason for Ga phase separation in thin film reactions is addressed due to the suppressed reaction pathways and these findings are supported by the experimental results.;Finally, Cu-In-O thin films are prepared by sputter deposition technique under different substrate temperatures and using different sputtering targets to study the selenization reactions of the prepared oxide thin films. XRD results showed In2O3 phase is detected in all films but Cu phase is only detected higher substrate temperatures. In2O 3 phase is found to be in different crystal orientations depending on the substrate temperature (300°C and 500°C). This is due to a templated growth between Cu and In2O3 phases, creating texturing in In2O3 phase. Additionally the selenization reactions showed that textured films affected the diffusion of Se in the sample, as revealed by the XRD results.