Hydrothermal synthesis of materials for intermediate band solar cells

Abstract

Solar energy is an abundant energy source that may provide an environmentallyfriendly alternative to fossil fuels. The utilization of the solar energy have so farbeen limited by various energy losses in the photovoltaic devices. Intermediateband solar cells use a three photon absorption process to take advantage of alarger range of photon energies compared to conventional solar cells, increasingthe maximum eciency of the solar cell. Iron substituted copper gallium disulphide(CGFS) has been proposed as one of the most promising systems, basedon eciency calculations. Recently a CGFS with promising absorption propertieshas been synthesized, suggesting that an intermediate band may indeed beintroduced to the CuGaS2 band gap.In this thesis, hydrothermal synthesis is used to make a parameter studyof the unsubstituted CuGaS2 to optimize the phase purity. Iron substitutioninto the lattice has been attempted and a preliminary study of the reactionmechanisms has been made. Phase identication of the products has beenmade using x-ray diraction (XRD) and the diractograms have been analysedusing Rietveld and Pawley renements to determine phase composition, latticeparameters and crystallite sizes. The microstructure of the products has beencharacterized using scanning electron microscopy (SEM), and energy dispersivespectroscopy analysis (EDS) has been used to determine the composition of thedierent microstructures. Reflectance spectroscopy has been attempted to find theabsorption properties of the material.The major findings are that a parameter combination of 200-250 degrees reactiontemperature, 30 hour reaction time and a copper and gallium precursor concentrationof 0.15M or higher gives the most phase pure product of ca. 95 atomicpercent CuGaS2. Degree of filling of the autoclave was not found to have alarge effect on the product, though extreme fill factors were not tested. It issuggested that the CuGaS2 crystallites grow by a continuous reaction betweenthe precipitated secondary phases and the unreacted precursors in the solution.A reaction temperature above 160 degrees or a reaction time over 1 hour is neededfor nucleation of CuGaS2. Indications of iron substitution into the CuGaS2lattice by hydrothermal synthesis were found, which has not been previouslyreported. The obtained products were not phase pure enough to make accuratereffinements on lattice positions.