Neutron interactions revealed that the orthorhombic structure of the hybrid perovskite is stabilized by the strong hydrogen bonds between the nitrogen substituent of the methylammonium cations and the bromides on the corner-linked PbBr6 octahedra. Image: ORNL/Jill Hemman.Neutron scattering has revealed, in real time, the fundamental mechanisms behind the conversion of sunlight into energy in hybrid perovskite materials. A better understanding of this behavior will allow manufacturers to design solar cells with increased efficiency.
The multi-institutional team of researchers from the US Department of Energy's Oak Ridge National Laboratory (ORNL), the University of Nebraska-Lincoln and Hunan University in China used photoluminescence measurements, along with neutron and x-ray scattering, to study the relationship between the material's microscopic structure and its optoelectronic properties. By examining the material under varying degrees of temperature, the researchers were able to track atomic structural changes and establish how hydrogen bonding plays a key role in the material's performance. They report their results in a paper in Advanced Materials.
Hybrid perovskites have the potential to be more efficient at converting light into energy than traditional solar cell materials. They are also easier to manufacture, as they can be spin cast from solution and do not require high-vacuum chambers for synthesis.
Unlike their singular silicon or germanium counterparts, hybrid perovskites are made of both organic and inorganic molecules. Their structure comprises inorganic lead and bromine molecules arranged in octahedral units that form cages around organic methylammonium cations (positively charged ions) consisting of carbon, nitrogen and hydrogen.
"The advantage of having both organic and inorganic molecules in a well-defined crystal structure means we can tailor the material by tuning either one group or the other to optimize the properties," said Kai Xiao, a researcher at ORNL's Center for Nanophase Materials Sciences (CNMS). "But even though researchers have been studying these materials for several years, we still don't fully understand on a fundamental level how the organic components are affecting the properties."
Finding the right combination and molecular orientation of the organic and inorganic components is the key to unlocking more functionality, but understanding those interactions requires the right tools.
"Neutrons are very good at this because they're sensitive to lighter elements like hydrogen," said ORNL instrument scientist Xiaoping Wang. "Because we're able to track each neutron, we get information about things like where the atoms are, what their temperature is, and how they're behaving."
Using the TOPAZ instrument at ORNL's Spallation Neutron Source, the team was able to observe hydrogen bonding interactions at the atomic scale. This experiment revealed that hybrid perovskites undergo significant structural changes between approximately 130K and 150K (roughly -190°F and -225°F). Cooling the material slowed the movement of the organic component into an ordered state, allowing the researchers to make precise in situ measurements of this component in real time. They were thus able to observe exactly how the organic molecules were binding to the lead-bromine component through hydrogen bonds.
"We saw the ordering is directly related to the hydrogen bonding in the structure, and how any changes can affect the energy gap of the material," said Wang. "That lets us know how well sunlight is being absorbed and what that could mean in terms of applications for photovoltaic materials."
Complementary photoluminescence and x-ray scattering measurements, along with crystal synthesis, were conducted at CNMS. Theoretical calculations were performed by scientists in ORNL's Materials Science and Technology Division.
"Hybrid perovskites are already a good material," said Xiao. "Now that we know how the orientation of the organic molecules impacts the crystal structure, and how we can tune them further to change the desired properties, this new fundamental understanding will enable us to design new materials with even greater potential."
This story is adapted from material from ORNL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.