Scientists at HZB have printed and examined various compositions of cesium-based halide perovskites (CsPb (BrxI1 – x) 3 (0 ≤ x ≤ 1)). In a temperature range between room temperature and 300 degrees Celsius, they observe structural phase transitions that influence the electronic properties. The study provides a quick and easy method for evaluating new compositions of perovskite materials in order to identify candidates for applications in thin film solar cells and optoelectronic devices.
Hybrid halide perovskites (ABX3) have developed into highly efficient new materials for thin-film solar cells in just a few years. The A stands for a cation, either an organic molecule or an alkali metal, the B is a metal, mostly lead (Pb) and the X is a halide element such as bromide or iodide. Currently, some compositions achieve power conversion efficiencies in excess of 25%. In addition, most perovskite thin films can be easily processed from solution at moderate processing temperatures, which is very economical.
World record efficiencies have been achieved by organic molecules such as methylammonium (MA) as the A cation and Pb and iodine or bromide in the other places. But these organic perovskites are not yet very stable. Inorganic perovskites with cesium at the A site promise higher stabilities, but simple compounds such as CsPbI3 or CsPbBr3 are either not very stable or do not offer the electronic properties required for applications in solar cells or other optoelectronic devices.
A team at the HZB has now investigated compositions of CsPb (BrxI1 – x) 3, which offer adjustable optical band gaps between 1.73 and 2.37 eV. This makes these mixtures really interesting for multi-junction solar cell applications, especially for tandem devices.
To produce them, they used a newly developed method of printing combinatorial perovskite thin films to produce systematic variations of (CsPb (BrxI1 – x) 3 thin films on a substrate. To achieve this, two printheads were filled with either CsPbBr2I or CsPbI3 and then programmed to print the required amount of liquid droplets on the substrate to form a thin film of the desired composition After annealing at 100 ° C to drive off the solvent and crystallize the sample, they received thin strips of different sizes Compositions.
With a special high-intensity X-ray source, the liquid metal beam in the LIMAX laboratory at HZB, the crystal structure of the thin film was analyzed at various temperatures from room temperature to 300 degrees Celsius. “We found that all of the compositions investigated transform into a cubic perovskite phase at high temperatures,” explains Hampus Näsström, doctoral student and first author of the publication. On cooling, all samples change into metastable tetragonal and orthorhombically distorted perovskite phases, making them suitable for solar cell devices. “This has proven to be an ideal application for in-situ X-rays with the laboratory-based X-ray source with high brilliance”, adds Roland Mainz, head of the LIMAX laboratory.
Since it is found that the transition temperatures into the desired phases decrease with increasing bromide content, this would enable the processing temperatures for inorganic perovskite solar cells to be reduced.
“There is great interest in this new class of solar materials and the possible variations in composition are almost infinite. This work shows how a wide range of compositions can be systematically produced and evaluated,” says Dr. Eva Unger, Head of the Young Investigator Group Education and Scaling of Hybrid Materials. Dr. Thomas Unold, head of the Combinatorial Energy Materials Research Group, agrees and suggests that “this is an excellent example of how high-throughput research approaches can significantly accelerate the discovery and optimization of materials in future research”.
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