Photovoltaic solar cells made from lead halide perovskites (LHPs) have rapidly emerged over the last decade or so as viable materials for creating commercial thin film solar cells. They show high efficiency and have the potential for low manufacturing costs.
Despite the fact they show excellent optoelectronic performance, there is a lack of consensus about the local symmetry and dynamics of the materials. Understanding the local structure of LHPs is key to being able to control their performance and understand unexplained phenomenon like charge carrier lifetime and ion migration.
In this study, published in Joule, Dr Nicholas Weadock, and the rest of his team from the University of Colorado and several other institutions, built a comprehensive picture of the structure and dynamics of two LHPs by combining X-ray and neutron diffuse scattering with inelastic neutron scattering measurements on the Merlin instrument at ISIS.
The group investigated two prototypical LHPs, CH3NH3PbI3 and CH3NH3PbBr3, which exhibit surprising diffuse scattering features at LHP-device relevant temperatures.
They found that the materials contained network of local two-dimensional, circular pancake-like regions of dynamically tilting lead halide octahedra. These pancake regions then induce longer-range intermolecular correlations within the CH3NH3+ sublattice. The correlations on the CH3NH3+ sublattice could improve free charge carrier lifetimes by introducing transient ferroelectric or antiferroelectric domains.
This extended dynamic local order was neither observed nor predicted in previous studies of structural dynamics in these materials. Being able to relate this structure to the properties of an LHP that make it suitable for a commercial device is key when considering the design of new, improved materials.
In one example, the results from this work can help other researchers better understand ion migration in LHPs. Ion migration of both the halide anion and organic cation are detrimental to the performance of a potential device, especially under illumination. However, the origin of this ionic movement is not well understood, and the two-dimensional pancakes found in this work may provide a barrier to halide migration. Manipulating the size and lifetime of the pancakes could reduce halide migration in these materials overall.
Their approach of combining single-crystal X-ray and neutron diffuse scattering data with molecular dynamics simulations will provide unparalleled insights into the structure of hybrid materials such as these, and other materials with engineered disorder.
“I am very proud of this work and grateful to our collaborators and instrument scientists who all contributed to our understanding of these complex materials," says Dr Weadock. “This work would not have been possible without neutron diffuse scattering and neutron inelastic spectroscopy. The CH3NH3+ cations are invisible to high-energy X-rays, and the dynamic component of the correlations is only observed with the excellent energy resolution that neutrons provide."
The full paper can be found at DOI: 10.1016/j.joule.2023.03.017