North Carolina State University (NC State) researchers have developed and demonstrated a technique that allows them to engineer a class of materials called layered hybrid perovskites (LHPs) down to the atomic level, which dictates how the materials convert electrical charge into light. The technique opens the door to engineering materials tailored for use in next-generation printed LEDs and lasers – and holds promise for engineering other materials for use in photovoltaic devices. LHPs consist of thin sheets of perovskite semiconductor material that are separated from each other by thin organic “spacer” layers. LHPs can be laid down as thin films consisting of multiple sheets of perovskite and organic spacer layers. These materials are desirable because they can efficiently convert electrical charge into light, making them promising for use in next-generation LEDs, lasers, and photonic integrated circuits. The method required the understanding of how LHPs are formed. This started with quantum wells — sheets of semiconductor material sandwiched between spacer layers that make up LHPs. These are in turn made by nanoplatelets, which are perovskite materials sheets that form on top of the solution that is used to create LHPs. The researchers found a way to engineer LHPs to convert electrical charges into light more efficient by controlling the formation of nanoplatelets. The discovery could lead to the development of materials for use in next-generation printed LEDs and lasers as well as photovoltaic applications. Courtesy of North Carolina State University/Thomas Bormans. Measured in atoms, the researchers identified that the size distribution of quantum wells made the difference in the LHPs’ energy conversion efficiency. “A quantum well that is two atoms thick has higher energy than a quantum well that is five atoms thick,” said Kenan Gundogdu, professor of physics at NC State, “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.” This creates a gradual slope of energy flow. Nanoplatelets, in turn, dictate how many atoms thick a quantum well is, but it is not always exact. They act as templates for the material that will eventually form beneath them, said Aram Amassian, professor of materials science and engineering at NC State. This means if the nanoplatelet is one atom thick, then the quantum well will be one atom thick. But nanoplatelets have a tendency to keep on growing, meaning the corresponding quantum well will also. Discovery of the nanoplatelets’ function answered a question the researchers initially had about the inconsistencies they were facing in reading how thick each quantum well was. The researchers were using both X-ray diffraction and optical spectroscopy to measure the quantum wells, but each would give opposing answers. They found these results were due to the fact that diffraction detects the stacking of sheets and therefore does not detect nanoplatelets. Optical spectroscopy, on the other hand, only detects isolated sheets. The discovery also led to the understanding of how to stop the growth of the nanoplatelets in a controlled setting, allowing the researchers to engineer quantum wells to have a desired gradual slope of energy flow. “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,” said Amassian. Besides LHPs, the researchers tested the same method to engineer the structure and properties of other perovskite materials – such as the perovskites used to convert light into electricity in solar cells and other photovoltaic technologies. The test proved successful as well, showing improvements in photovoltaic performance and stability. The research was published in Matter (www.doi.org/10.1016/j.matt.2024.09.010).
