New material design space discovered for light-based applications

Materials scientists at Duke University claim to have computationally predicted the electrical and optical properties of semiconductors made from extended organic molecules sandwiched by inorganic structures. 

These types of so-called layered "hybrid organic-inorganic perovskites" (HOIPs) are popular targets for light-based devices such as solar cells and LEDs. The ability to build accurate models of these materials atom-by-atom will allow researchers to explore new material designs for next-generation devices.

"Ideally, we would like to be able to manipulate the organic and inorganic components of these types of materials independently and create semiconductors with new, predictable properties," said Professor David Mitzi of Duke. "This study shows that we are able to match and explain the experimental properties of these materials through complex supercomputer simulations.”

HOIPs are a promising class of materials according to the scientists, due to their combined strengths of their constituent organic and inorganic pieces. Organic materials have more desirable optical properties and may be bendable, but can be ineffective at transporting electrical charge. Inorganic structures, on the other hand, are typically good at conducting electricity and offer more robust mechanical strength.

Combining the two can affect their individual properties while creating hybrid materials with the best of both worlds, the researchers reveal. But, understanding the electronic and atomic-scale consequences of their interaction is challenging at best, since the resulting crystals or films can be structurally complex. However, as these particular HOIPs have their organic and inorganic components in well-ordered layers, their structures are somewhat easier to model, and researchers are now beginning to have success at computationally predicting their behaviours on an atomic level.

"The computational approach we used has rarely been applied to structures of this size," said Associate Professor Volker Blum of Duke. "We couldn't have done it even just 10 years ago. Even today, this work would not have been possible without access to one of the fastest supercomputers in the world."

That supercomputer - dubbed Theta - is currently the 21st fastest in the world and resides at Argonne National Laboratory. The group was able to gain time on the behemoth through Blum securing one of only a dozen Theta Early Science Projects, aimed at paving the way for other applications to run on the system first launched in late 2017. They are now co-investigators on one of Department of Energy's "Innovative and Novel Computational Impact on Theory and Experiment" (INCITE) awards, enabling them to continue their work.

In their study, the team used Theta's computational power to model the electronic states within a layered HOIP first synthesised by Mitzi more than ten years ago. While the electrical and optical properties of the material are well-known, the physics behind how they emerge have been much debated.

The team believe to now have settled this debate.

In a series of computational models, the team calculates the electronic states and localises the valence band and conduction band of the HOIP's constituent materials, the organic bis(aminoethyl)-quaterthiophene (AE4T) and the inorganic lead bromide (PbBr4). These properties dictate how electrons travel through and between the two materials, which determines the wavelengths and energies of light it absorbs and emits, among other important properties such as electrical conduction.

The results showed that the team's computations and experimental observations match, proving that the computations can accurately model the behaviours of the material.

The team furthered their study by tweaking the materials – varying the length of the organic molecular chain and substituting chlorine or iodine for the bromine in the inorganic structure – and running additional computations.

The team are also working on synthesising these variations to further verify their theoretical models.

The work is part of a larger initiative called the HybriD3 project aimed at discovering and fine-tuning new functional semiconductor materials. The collaborative effort features a total of six teams of researchers. Joining those researchers located at Duke University and the University of North Carolina at Chapel Hill, Professors Kenan Gundogdu and Franky So at North Carolina State University are working to further characterise the materials made in the project, as well as exploring prototype LEDs.

"By using the same type of computation, we can now try to predict the properties of similar materials that do not yet exist," said Prof. Mitzi. "We can fill in the components and, assuming that the structure doesn't change radically, provide promising targets for materials scientists to pursue."

The idea is that this will allow scientists to more easily search for better materials for a wide range of applications. For this particular class of materials, that includes lighting and water purification.

Inorganic light sources are typically surrounded by diffusers to scatter and soften their intense, concentrated light, which leads to inefficiencies. This class of layered HOIPs could make films that achieve this more naturally while wasting less of the light, according to the scientists. For water purification, the material could be tailored for efficient high-energy emissions in the ultraviolet range, which can be used to kill bacteria.

"The broader aim of the project is to figure out the material space in this class of materials in general, well beyond the organic thiophene seen in this study," said Assoc Prof. Blum. "The key point is that we've demonstrated we can do these calculations through this proof of concept. Now we have to work on expanding it."