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HZB scientists printed and explored several compositions of cesium 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 affect electronic properties. The study provides a quick and easy method for evaluating novel compositions of perovskite materials in order to identify candidates for applications in thin-film solar cells and optoelectronic devices.
Halide hybrid perovskites (ABX3) have become highly efficient new materials for thin-film solar cells in just a few years. The A stands for a cation, an organic molecule or an alkali metal, the B is a metal, most often 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%. Also, most perovskite thin films can be easily processed from the solution at moderate processing temperatures, which is very economical.
World record efficiencies have been achieved by organic molecules such as methylammonium (MA) as cation A and Pb and iodine or bromide at the other sites. But those organic perovskites are still not very stable. Inorganic perovskites with cesium at site A promise higher stability, but simple compounds such as CsPbI3 or CsPbBr3 are not very stable or do not provide the electronic properties necessary for applications in solar cells or other optoelectronic devices.
Now, a team from HZB has explored the compositions of CsPb (BrXI1−X)3, which provide tunable optical band gaps between 1.73 and 2.37 eV. This makes these blends really attractive for multi-junction solar cell applications, especially tandem devices.
For the production they used a newly developed method of combinatorial printing of perovskite thin films to produce systematic variations of (CsPb (BrXI1−X)3 thin films on a substrate. To achieve this, two print heads were filled with CsPbBr2I or CsPbI3 and then programmed to print the required amount of liquid droplets onto the substrate to form a thin film of the desired composition. After annealing at 100 degrees Celsius to drive out the solvent and crystallize the sample, they obtained thin strips with different compositions.
With a special high-intensity X-ray source, the liquid metal jet in HZB’s LIMAX laboratory, the crystalline structure of the thin film was analyzed at different temperatures, ranging from room temperature up to 300 degrees Celsius. “We found that all of the compositions studied convert to a high-temperature cubic perovskite phase,” explains Hampus Näsström, PhD student and first author of the publication. After cooling, all samples pass to metastable tetragonal perovskite and distorted orthorhombic phases, making them suitable for solar cell devices. “This proved to be an ideal use case of XRD in situ with the laboratory-based high-brightness X-ray source,” adds Roland Mainz, head of the LIMAX laboratory.
Since the transition temperatures in the desired phases are found to decrease with increasing bromide content, this would allow for lower processing temperatures for perovskite inorganic solar cells.
“The interest in this new class of solar materials is enormous and the possible variations in composition almost infinite. This work demonstrates how to systematically produce and evaluate a wide range of compositions,” says Dr Eva Unger, who directs the Young Investigator Group Training and scaling of hybrid materials. Dr Thomas Unold, head of the Combinatorial Energy Materials Research group agrees and suggests that “this is a prime example of how high-throughput approaches in research could greatly accelerate the discovery and optimization of materials in future research.”
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Materials provided by Helmholtz Berlin Center for Materials and Energy. Note: The content can be changed by style and length.
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