Nine samples of caesium-based halide perovskite with different compositions, from pure CsPbBr2I (left) to pure CsPbI3 (right). Image: H. Näsström/HZB.
Nine samples of caesium-based halide perovskite with different compositions, from pure CsPbBr2I (left) to pure CsPbI3 (right). Image: H. Näsström/HZB.

Scientists at Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany have printed and explored different compositions of caesium-based halide perovskites (CsPb(BrxI1−x)3). Over a temperature range from room temperature to 300°C, they observed structural phase transitions that influenced the electronic properties of the perovskite. This study showcases a quick and easy method for assessing new compositions of perovskite materials in order to identify candidates for applications in thin film solar cells and optoelectronic devices.

In only a few years, hybrid halide perovskites (ABX3) have proved to be highly efficient new materials for thin film solar cells. In these materials, the A stands for a cation, either an organic molecule or some alkali metal; the B is a metal, most often lead (Pb); and the X is a halide element such as bromine (Br) or iodine (I).

Some hybrid halide perovskites have achieved power conversion efficiencies above 25%. What is more, most perovskite thin films can easily be processed from solution at moderate processing temperatures, which is very economic.

The high conversion efficiencies have been achieved with organic molecules such as methylammonium (MA) as the A cation, lead as the metal, and iodine or bromine as the halide. But these organic perovskites are not very stable. Inorganic perovskites with caesium at the A-site promise higher stabilities, but simple compounds such as CsPbI3 or CsPbBr3 are either not very stable or do not provide the electronic properties needed for applications in solar cells or other optoelectronic devices.

Now, a team at HZB has combined CsPbI3 and CsPbBr3 to produce various compositions of CsPb(BrxI1−x)3, which provide tunable optical band gaps between 1.73eV and 2.37eV. This makes these mixtures really interesting for multi-junction solar cell applications, in particular for tandem devices.

The scientists utilized a newly developed method for printing combinatorial perovskite thin films to produce systematic variations of CsPb(BrxI1−x)3 thin films onto a substrate. To achieve this, two print heads were filled with either 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°C to drive out the solvent and crystallize the sample, the scientists obtained thin stripes with different compositions.

Using a special high intensity x-ray source, the liquid metal jet in the LIMAX lab at HZB, the scientists analyzed the crystalline structure of the thin films at different temperatures, ranging from room temperature up to 300°C. "We find that all investigated compositions convert to a cubic perovskite phase at high temperature," said Hampus Näsström, PhD student and first author of a paper on this work in the Journal of Materials Chemistry A.

On cooling down, all the samples transitioned to metastable tetragonal and orthorhombic distorted perovskite phases, making them suitable for solar cell devices. "This has proven to be an ideal use case of in-situ XRD with the lab-based high-brilliance X-ray source," said Roland Mainz, head of the LIMAX laboratory.

The study also revealed that the transition temperatures into the desired phases decreased with increasing bromide content, offering a way to lower processing temperatures for inorganic perovskite solar cells.

"The interest in this new class of solar materials is huge, and the possible compositional variations near to infinite," says Eva Unger, who heads the Young Investigator Group Hybrid Materials Formation and Scaling at HZB. "This work demonstrates how to produce and assess systematically a wide range of compositions."

This story is adapted from material from HZB, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.