Reviewed by Lexie CornerOct 14 2024
Researchers from North Carolina State University have developed a method that allows for precise control over the conversion of electrical charge into light at the atomic level within layered hybrid perovskites (LHPs). This breakthrough, published in Matter, could lead to the engineering of materials for photovoltaic devices and next-generation printed LEDs and lasers.
Perovskites, known for their advantageous optical, electronic, and quantum properties, have a crystalline structure. LHPs consist of ultra-thin perovskite semiconductor sheets separated by organic "spacer" layers. These LHPs can be deposited as thin films with multiple sheets, making them suitable for various applications.
The ability of these materials to efficiently convert electrical charge into light makes them highly promising for use in photonic integrated circuits, lasers, and next-generation LEDs. However, despite long-standing interest in LHPs, there has been limited understanding of how to engineer these materials to control their performance characteristics effectively.
To address this, the researchers focused on quantum wells—semiconductor sheets placed between spacer layers—providing a foundation for understanding how to manipulate and optimize these materials for enhanced performance.
We knew quantum wells were forming in LHPs–they are the layers.
Aram Amassian, Study Corresponding Author and Professor, Materials Science and Engineering, North Carolina State University
Furthermore, since energy moves from high-energy structures to low-energy structures at the molecular level, understanding the size distribution of quantum wells is critical.
A quantum well that is two atoms thick has higher energy than a quantum well that is five atoms thick. And in order to get energy to flow efficiently, you want to have quantum wells that are three and four atoms thick between the quantum wells that are two and five atoms thick. You basically want to have a gradual slope that the energy can cascade down.
Kenan Gundogdu, Study Co-Author and Professor, North Carolina State University
Amassian added, “But people studying LHPs kept running into an anomaly: the size distribution of quantum wells in an LHP sample that could be detected via X-ray diffraction would be different than the size distribution of quantum wells that could be detected using optical spectroscopy.”
“For example, diffraction might tell you that your quantum wells are two atoms thick, as well as there being a three-dimensional bulk crystal. But spectroscopy might tell you that you have quantum wells that are two atoms, three atoms, and four atoms thick, as well as the 3D bulk phase. So, the first question we had was: why are we seeing this fundamental disconnect between X-ray diffraction and optical spectroscopy? And our second question was: how can we control the size and distribution of quantum wells in LHPs?” added Amassian.
After conducting several experiments, the researchers found that nanoplatelets were crucial in answering both questions.
“Nanoplatelets are individual sheets of the perovskite material that form on the surface of the solution we use to create LHPs. We found that these nanoplatelets essentially serve as templates for layered materials that form under them. So, if the nanoplatelet is two atoms thick, the LHP beneath it forms as a series of two-atom-thick quantum wells,” Amassian noted.
“However, the nanoplatelets themselves aren’t stable, like the rest of the LHP material. Instead, the thickness of nanoplatelets keeps growing, adding new layers of atoms over time. So, when the nanoplatelet is three atoms thick, it forms three-atom quantum wells, and so on. And, eventually, the nanoplatelet grows so thick that it becomes a three-dimensional crystal,” Amassian added.
This discovery also solved the long-standing problem of why X-ray diffraction and optical spectroscopy yield different results: optical spectroscopy detects isolated sheets, while diffraction detects the stacking of sheets, which means it does not detect nanoplatelets.
Amassian stated, “What is exciting is that we found we can essentially stop the growth of nanoplatelets in a controlled way, essentially tuning the size and distribution of quantum wells in LHP films. And by controlling the size and arrangement of the quantum wells, we can achieve excellent energy cascades – which means the material is highly efficient and fast at funneling charges and energy for the purposes of laser and LED applications.”
After discovering their importance in the formation of perovskite layers in LHPs, the researchers decided to investigate whether nanoplatelets could be used to engineer the structure and properties of other perovskite materials, such as those used to convert light into electricity in solar cells and other photovoltaic technologies.
We found that the nanoplatelets play a similar role in other perovskite materials and can be used to engineer those materials to enhance the desired structure, improving their photovoltaic performance and stability.
Milad Abolhasani, Study Co-Author and ALCOA Professor of Chemical and Biomolecular Engineering, North Carolina State University
Kasra Darabi, Fazel Bateni, Tonghui Wang, Laine Taussig and Nathan Woodward, who are all Ph.D. graduates of NC State; Mihirsinh Chauhan, Boyu Guo, Jiantao Wang, Dovletgeldi Seyitliyev, Masoud Ghasemi and Xiangbin Han, who are all postdoctoral researchers at NC State; Evgeny Danilov, director of the Imaging and Kinetics Spectroscopy Laboratory at NC State; Xiaotong Li, an assistant professor of chemistry at NC State; and Ruipeng Li of Brookhaven National Laboratory were the study co-authors.
National Science Foundation under grant 1936527, and the Office of Naval Research under grant N00014-20-1-2573 supported the study.
Journal Reference:
Darabi, K. et. al. (2024) Cationic ligation guides quantum-well formation in layered hybrid perovskites. Matter. doi.org/10.1016/j.matt.2024.09.010