Halide perovskite materials as chemical playground

Due to their excellent optoelectronic performance, and relatively easy processibility, halide perovskite semiconductors are used in highly efficient solar cells, LEDs, X-ray detectors, and more recently in photo(electro)chemistry. Eline M. Hutter aims to obtain stable, non-toxic perovskites for photoconversion applications.

What intrigues me most about halide perovskites is their compositional tunability,^1,2) allowing to manipulate the properties depending on the desired application. This compositional tunability originates from the ionic nature of halide perovskites, allowing to build in any cation or anion as long as geometric and charge constraints are obeyed. At present, methylammonium- or formamidinium lead iodide perovskites yield the highest efficiencies in single junction solar cells³⁾ due to their suitable bandgaps of ~1.6 eV. However, for multi-junction solar cells, LEDs, or photochemistry, larger bandgaps may be desired. For that, iodide is replaced with smaller halides like bromide or chloride to enlarge the bandgap, or even mix several halides to precisely tune the light absorption and emission in the entire visible region (i.e. between 2.9 and 1.6 eV).^1,2)

An ongoing challenge with these mixed-halide perovskites is to maintain phase-stability, as the halides tend to segregate under illumination conditions.⁴⁾ This segregation leads to inhomogeneous materials containing regions with different bandgaps. As the electrons tend to funnel to the lower bandgap regions, segregation leads to losses in open-circuit voltage (V_OC) in solar cells, and reduced colour purity of LEDs.⁵⁾ Although phase segregation is in itself not uncommon in materials science,⁶⁾ the reversibility of this phenomenon seems to be unique to halide perovskites, and is still poorly understood.

Phase segregation

Aiming to better understand the thermodynamics and kinetics of phase segregation, we have investigated this process under high external pressure with spectroscopic techniques.^7,8) By using ultrafast (sub-ns) transient absorption spectroscopy measurements it was possible to follow both the regions with high bandgap, enriched in bromide, and those with low bandgap, enriched in iodide. This led to a more complete picture than previous photoluminescence-based studies that could only probe the low-bandgap phase. On applying external pressure to the system, segregation decreased on compressing the perovskites, and even became completely absent at 0.4 GPa (i.e. 4000 bar).

Why is phase segregation suppressed at high pressure? We suspect that both kinetics and thermodynamics play a key role here. The kinetic role can be understood from the idea that segregation requires the anions to migrate, which is a thermally activated process with a certain activation energy.⁹⁾ At high pressure, the crystal structure is more closely packed, so that the energy barrier for ions to move is higher. As a consequence, ion migration is less likely and segregation slows down on compressing the perovskite.⁷⁾ The thermodynamics of phase segregation can be reasoned by considering the entropy and enthalpy of mixed halide perovskites. Entropically, mixing is favoured, whereas the enthalpy drives segregation because mixing anions leads to strain in the material. At high pressure, the strain is less, so that the entropy term overrules the enthalpy and mixed-halide perovskites are stable.⁸⁾

A similar stabilizing effect was achieved by compressing the perovskite chemically. Instead of pressurizing the perovskite from the outside, the lattice was contracted by replacing the organic cation with smaller caesium cations. Hence, we used a chemical approach to mimic physical pressure, showing reduced phase segregation at ambient conditions.

Is it now feasible to obtain mixed-halide perovskites that are fully stable at ambient conditions? This may likely require a combination of chemical tuning and strain release through substrate and surface engineering.

Double perovskites

Another fascinating class of halide perovskites are double perovskites, with alternating monovalent and trivalent metal cations at the B-site (elpasolite crystal structure). These materials recently gained new interest when researchers from the halide perovskite community proposed the semiconductor Cs₂AgBiBr₆ as an alternative material with reduced toxicity compared to widely investigated lead-based perovskites (see infobox).^10–12)

The presence of two metals instead of one offers a huge variety of compositions, and allows to design materials with novel features. For instance, in these double perovskites, not only the magnitude¹³⁾ but also the nature of the bandgap can be tuned from direct to indirect and in-between.^11,14) This allows rational design of direct-indirect semiconductors, that could be useful for applications requiring both strong absorption (direct bandgap) and slow recombination (indirect bandgap).¹⁵⁾ Furthermore, on incorporating magnetic metals in double perovskites, these may find applications in next-generation information technologies or optomagnetic sensing.¹¹⁾ Finally, the design of non-toxic halide perovskites with tunable properties will be important to exploit their full potential for photo-redox chemistry.

INFO: Toxicity

The toxicity of lead is considered a potential hurdle for commercializing perovskites, especially because the 2+ oxidation state may ease its uptake by humans. However, there have been very limited toxicity studies on lead halide perovskites, and predictions are mainly based on seminal toxicity studies using lead nitrate salts. In collaboration with plant scientists, we investigated the toxicity of lead halide perovskites and its precursors on the model plant Arabidopsis Thaliana.¹⁶⁾ On comparing the different precursors, we conclude that the iodide inhibited plant growth, before toxicity effects of lead appear. These results stress the importance of rigorous assessment on the potential harmfulness of halide perovskites, in parallel to optimizing its composition for the envisioned applications.

Three questions to the backer of the author: Markus Suta

What‘s the difference between research in Germany and the Netherlands?

In the Netherlands, research is much more interdisciplinary. In Germany, we often categorize chemical research to a higher extent (e.g. in organometallics, physical chemistry etc.) and use specifically targeted methods for that. In the Netherlands, researchers from different disciplines work on one particular research topic to contribute with different perspectives, the boundaries are smoother.

Why did you suggest Eline for an article?

Eline’s approach for the application of halidoperovskites is very unconventional. It is also impressive that both she and her husband, Ward van der Stam, are working in research and manage to balance that with the family life with two children.

What‘s your relationship with the Netherlands?

I did my postdoc at the University of Utrecht with Andries Meijerink – one of the primary experts in the field of inorganic luminescent materials. My family and I have truly enjoyed the life in the Netherlands and the scientifically enriching atmosphere there.

Markus Suta is a junior professor at the University of Düsseldorf. Author Eline Hutter, born 1990, became a Tenure Track assistant professor at Utrecht University in 2020 after a short postdoc at NWO Institute for Atomic and Molecular Physics (AMOLF). She received her PhD in chemical engineering at Delft University of Technology. In 2017, she was a Fulbright visiting scholar at Stanford University. Her research focusses on synthesis and spectroscopy of materials for photoconversion applications. Besides doing research, Eline Hutter sings in a chamber choir and has two children.

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• ¹⁶ E. M. Hutter, R. Sangster, C. Testerink, B. Ehrler, C. M. M. Gommers, iScience 2022, 25, 103583

Wissenschaft + ForschungSchlaglichtthema: Chemie in Europa