Skip to main content

Atomic-level characterization of multi-component perovskite materials for optoelectronic applications

Periodic Reporting for period 1 - perovskites-NMR (Atomic-level characterization of multi-component perovskite materials for optoelectronic applications)

Reporting period: 2019-04-01 to 2021-03-31

About a decade ago, metal halide perovskites (MHPs) emerged as a new class of materials promising more efficient solar cells and light emission diodes (LEDs). Unlike silicone solar cells, they are easy to process in solution, allowing strategies such as solar cell printing. Perovskites can be represented by the general formula ABX3, where A is a small organic or inorganic component with a positive charge, B is a metal such as lead or tin, and X is a halogen, such as iodine, bromine or chlorine. In state-of-the-art solar cells, the “A”, “B” and “X” components are mixtures of chemical species and this strategy, referred to as compositional engineering, is what has led to solar cell efficiencies exceeding 25%. Generating more solar power and doing so more efficiently is essential to combat the ongoing climate crisis caused by burning fossil fuels. Halide perovskite solar cells and light-emitting devices are being intensely developed in academic institutions worldwide to diversify the available optoelectronic technologies and to constructively contribute to resolving the climate crisis.

However, the two major challenges associated with MHP photovoltaics are their poor long-term stability and the toxicity of lead. The first issue is being tackled by developing ever complex chemical compositions to improve humidity and light resistance of the perovskite light absorber. The toxicity of lead can be combatted by developing lead-free light absorbers using metals such as tin, silver and bismuth. These materials come with other problems – they are not as efficient as lead halide perovskites and tin halide perovskites are particularly sensitive to degradation in ambient air.

While new MHP compositions of arbitrary complexity can be readily prepared and tested, the main challenge is that our understanding of these new materials at the atomic level is insufficient. In order to understand the mechanisms of degradation as well as performance and stability improvements, it is necessary to look at the smallest building blocks of the structure. Until very recently, there were no methods that would allow studying MHPs at those small scales. Nuclear magnetic resonance (NMR) spectroscopy is a perfectly suited non-destructive method to address this challenge as it provides information on structure of materials at the atomic level. This project employs a combination of NMR and advanced optical spectroscopies to provide structure-property relationships in a broad range of materials developed as solar cell and LED materials today, thereby improving our understanding on how to rationally prepare more efficient optoelectronic materials.

In conclusion, the action has provided an unparalleled insight into the atomic-level structure of a highly technologically relevant class of materials for sustainable energy.
The work performed within this project included (a) solid-state mechanosynthesis of metal halide perovskite materials of current relevance to optoelectronics, (b) X-ray diffraction characterization of their long-range (>100 nm) structure, (c) solid-state NMR characterization of their short-range (<10 nm) structure, (d) characterization using optical spectroscopies, (e) establishing structure-property relationship based on the resulting data.
Mechanosynthesis is a highly atom-efficient way of preparing metal halide perovskites with yields approaching 100%. This allowed us to study a wide library of materials including low-dimensional lead halide perovskites, lead-free mixed-halide perovskites based on silver and bismuth, lead-free tin halide perovskites, and lead halide perovskites modulated with small organic molecules. We have successfully determined the atomic-level structure in these classes of materials using solid-state NMR.

Within the scope of understating lead-free materials based on silver and bismuth, we have successfully recorded NMR spectra, photoluminescence and charge carrier lifetimes, and correlated the results to obtain structure-property relationships. We have established the phase diagram of halide miscibility in Cs2AgBiX6 (X=Cl, Br, I) double halide perovskites. We then correlated the halide composition with the resulting charge carrier lifetimes and demonstrated the pernicious effect of iodide and bromide doping in this class of materials. The results have been published (Chem. Mater. 2020, 19, 8129–8138, doi: 10.1021/acs.chemmater.0c01255). We have also successfully synthesized and investigated the atomic-level structure and degradation pathways of a large library tin halide materials. These results have been published (J. Am. Chem. Soc. 2020, 17, 7813–7826, doi:10.1021/jacs.0c00647) as well as presented at international conferences.

In another study, we synthesized MHPs doped with a library of small organic molecules residing on the surface of the perovskite and revealed their atomic-level structure. These results were complemented by charge carrier lifetime measurements which have shown that one of the organic molecules leads to a substantial increase in charge carrier lifetimes and as such is a promising passivation agent for halide perovskites in optoelectronic devices. The study has also demonstrated that solid-state NMR is capable of determining the structure of dilute surface species on perovskite surfaces, which establishes it as a unique complementary tool to diffraction techniques in the field of metal halide perovskite materials research.
This project has capitalized on the rich information content achievable by solid-state NMR to develop comprehensive atomic-level understanding of multi-component halide perovskites used in solar cells and light emitting diodes. For the first time, we have established the atomic-level structure of tin halide perovskites by overcoming the challenges associated with tin-119 NMR data acquisition in this class of materials. The protocols established in this work lend themselves to investigating a variety of poorly understood phenomena in the field of tin halide perovskites including, for example, the microscopic mechanism of action of redox-active dopants which act as stabilizers. More generally, it is an important contribution to understanding how to stabilize tin-based light absorbers to use them as a more sustainable alternative to lead in photovoltaic and light emitting devices.
The driving force of halide perovskite optoelectronics today is the efficiency and stability improvement yielded by various passivation strategies. Here, we have shown for the first time that extremely dilute surface species on perovskite surfaces can be studied with high sensitivity using conventional NMR approaches to gain insight into the passivation mechanisms.
The improved understanding of these microscopic phenomena will allow researchers around the world to tackle the climate crisis by developing a new generation of more efficient and versatile solar cells and light-emitting devices. Taken together, this project has provided the research community with unparalleled level of detail on the atomic-level structure of halide perovskites and will contribute to the development of more efficient materials for optoelectronics, thereby supporting the diversification of sustainable energy technologies in the upcoming decade.
Solid-state NMR allows to study dilute dopants in metal halide perovskites.