Solar Cells with Methylammonium Lead Halide Perovskites
Jäntti, Paavo Petteri
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Methylammonium lead halide perovskites are proving to be extremely efficient photoactive components of organic-inorganic solar cells. Though their suitability for photovoltaics was discovered only recently, the results acquired are improving more rapidly than with any other type of solar cell. In this work perovskite cells were studied by making them in the laboratory by starting with the recipe from Shi et al. (2014a). The first cells were not very efficient, but alterations, to accommodate for the conditions, were made and the second set was significantly better. The recipe was further modified according to other published works and the 3rd and 4th sets were even better. The cells made had a lot of variation and therefore it is hard to say, which improvements to the recipe actually caused the better performance. The improvements that were most likely to increase cell performance were the heating of the substrates before spin coating them with the lead halide solution and the introduction of lead chloride in small concentrations to the lead halide solution used for spin coating. As the research went on, other directions apart from just tweaking the recipe came along and were duly studied with hopes of discovering something new. A perovskite powder was manufactured in a solution as well as via a solid-state reaction. The powder was then compressed into a pellet to study, if it was feasible to fabricate perovskite cells by this way as well. The resulting tablets were too brittle and hard to handle and due to lack of time this direction was abandoned. Attempts were made to make perovskite films in dimethylformamide (DMF), by making a supersaturated solution of lead halides and methylammonium iodide (MAI), as the solution of both solutes is highly dependent on the temperature of the solution. However the attempts to achieve a supersaturated solution, in which, upon cooling, perovskite would form, proved futile. The solution made was used to make a set of cells with the one-step precursor deposition. When testing the solubility of different lead halides it was discovered that lead chloride is more soluble in cold DMF than hot. Another interesting fact is that the presence of MAI assists the solution of both lead halides significantly. They must form some kind of complexes with each other and DMF. MAI also undergoes a slow reaction to complexes in 2-propanol, as observed by the changing of its colour in 2-propanol.