Among the emerging photovoltaic technologies perovskite solar cells have attracted a great deal of attention. They surprised with incredibly rapid improvement in efficiency, which increased from 2% in 2006 to more than 20% in 2015. In addition to the availability of raw materials, their cost is lower than conventional silicon solar cells. The active light-absorbing layer of these solar cells is a 3D hybrid organic-inorganic perovskite material with chemical formula ABX3. These materials have exceptional properties for photovoltaic applications such as adequate bandgap, high absorption coefficient, high charge mobility and simple film formation with low-temperature deposition procedures. Perovskite solar cells predominantly use methylammonium lead halide perovskite CH3NH3PbX3, X = Br, I. Despite many improvements, the technology of perovskite solar cells is still in the early stages of commercialization due to lead stability and toxicity concerns. A number of other organic-inorganic perovskite compositions are also being studied because of that reason. By changing cations and anions, the crystal structure and consequently optical and electronic properties of this material can easily be altered. Tolerance and octahedral factors offer guidelines for the selection of ions with appropriate radius sizes according to cubic symmetry, which provides optimum properties for use in photovoltaics.
In this master's thesis the synthesis process of CH3NH3PbI3 perovskite was researched based on the available literature data. The synthesis consists of two parts, the preliminary synthesis of the organic halide, followed by mixing the latter with the inorganic halide in the appropriate solvent. Annealing thus prepared precursor solutions results in crystallization of the desired product. The key parameters in the synthesis are the molar ratio of the reactants, the choice of solvent and the influence of moisture. CH3NH3PbI3 perovskite was formed in three different solvents - GBL, DMF in DMSO, with secondary phases only present in the later. The synthesis was successful both times, when using the stoichiometric ratio of the reactants as well as when using the excessive amounts of PbI2. There were no visible differences between XRD measurements. Syntheses were made only under atmospheric conditions.
Following the successful synthesis of CH3NH3PbI3, other 24 compounds were selected, which in theory can form a perovskite crystal structure based on their tolerance and octahedral factors. For the synthesis of these perovskites a precursor solution in GBL with 40 wt% reactants in stoichiometric ratio was used. Most of the starting compounds are soluble in the selected solvent with the exception of hydrazinium and imidazolium iodide and all inorganic bromides and chlorides. For solubility of PbI2 addition of organic halide was required. The synthesis of the desired perovskites failed in case of unsuccessful preparation of reactants. The use of the same synthesis process for perovskites with hydrazinium ion proved to be unsuccessful, as only amorphous and the degradation products were observed. Similar results were observed in perovskites containing strontium and calcium. In some samples synthesis of bromide and chloride perovskites was successful regardless of insolubility of the reactants.
|
