Solution processed perovskite solar cells

Today’s carbon-based economy will not be sustainable in the future. Not only will the known reserves of fossil fuels, like oil, natural gas or coal, be significantly reduced within the next 100 years, but the continued burning of fossil fuels also emits greenhouse gases, which have led to a global increase in temperature, called global warming. To preserve the environment for future generations and to prepare for the time when we will inevitably run out of fossil fuel, we have to change the way we produce our primary energy and focus research and investments on renewable energy sources. While energy from wind and water is already harvested with very high efficiencies, the utilization of solar energy still offers big room for improvements. Although conventional crystalline silicon cells achieve efficiencies around 25 %, their production is very energy intensive and relies on advanced production technologies, which makes them still rather expensive. To make photovoltaics a major part of our energy landscape, an easily prepared type of solar cell consisting of cheap and abundant materials is required. Novel organometal halide perovskite-type materials fulfill these requirements and have proven to be serious competitors for conventional photovoltaics. After only four years of research they already achieve power conversion efficiencies above 20 %. This thesis introduces a fast and easy way to prepare planar heterojunction solar cells based on methylammonium lead iodide (MAPbI3). The photoactive layer is deposited in a 2-step deposition approach, where a thin film of the lead precursor is converted into the final perovskite simply by immersing it into a solution of the other component. The resulting films consist of individual crystals sizes a few 100 nm and covering the whole substrate without significant gaps or holes. Solar cells prepared by this method achieve power conversion efficiencies of 15 %. Furthermore, by adjusting the temperature of the immersion bath, the orientation of the perovskite crystals can be controlled. The orientation, together with the resulting change in efficiency and resistance, gives interesting insights into the anisotropic charge transport properties of this class of materials. Additionally, the conventionally used hole blocking layer, titanium dioxide, was replaced by one made of fullerene molecules. The efficiencies achieved by solar cells employing this kind of electron selective contact reached almost 10 %, although the reproducibility was initially very low. This was attributed to a partial dissolution of the fullerene film during the subsequent preparation steps. To increase the stability of the layer, it was photo-polymerized using UV radiation. This not only reduces the solubility and therefore increases the fraction of solar cells achieving high efficiencies; it also changed the energy levels close to the bandgap. The bandgap energy of organic lead halide perovskite materials is strongly dependent on the composition. By exchanging some or all of the iodide in MAPbI3 with bromide, the difference between valence and conduction band can be changed from 1.5 eV (pure iodide) to 2.25 eV (pure bromide). This substitution can be performed gradually, so that phase pure materials with properties in between the two extremes are obtained. The pure bromide MAPbBr3 perovskite, however, does not perform efficiently in a planar heterojunction solar cell. Its close relative based on formamidinium FAPbBr3 has also been investigated for its suitability as active solar cell material. Although it is structurally very similar to MAPbBr3, with equivalent light absorption and emission properties, a 10 fold higher efficiency was observed for the FA-based compound. This striking difference is mainly attributed to an increased photoluminescence lifetime, resulting in an increased diffusion length of the free charge carriers. Apart from their application as light absorbing materials in solar cells, perovskites have also been investigated for their application as light emitters. Depending on the perovskite used, it was possible to demonstrate red light emission (MAPbI3) or green emission (MAPbBr3).

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Solution processed perovskite solar cells is an episode from Fakultät für Chemie und Pharmazie - Digitale Hochschulschriften der LMU - Teil 05/06 by Ludwig-Maximilians-Universität München.

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This episode was published on Oct 15, 2015.

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