Understanding the effects of biaxial stress on ion transport in perovskite caesium--lead--halides

• Applicant: Matthew J. Wolf, Ph.D.
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces ; Computer-Aided Design of Materials and Simulation of Materials Behaviour from Atomic to Microscopic Scale ; Physical Chemistry of Solids and Surfaces, Material Characterisation ; Theoretical Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546857271

The perovskite lead–halides (PLH's) are some of the most intensively studied materials of the past decade, primarily due to their excellent performance as active layers in inexpensive, solution processed photovoltaic cells. They been called the ‘poor man’s high-performance semiconductors’, and said to herald a ‘new era of solution-processed electronics’. Compared to conventional semiconductors, however, they exhibit a much greater degree of ion transport, and are much more susceptible to mechanical deformation due to their small elastic constants and large coefficients of thermal expansion. Both of these characteristics critically impact the performance of PLH’s, most notably their stability, which is the major impediment to commercial deployment. In light of the above, the main goal of this project is to develop a thorough understanding of the influence of biaxial stress—to which PLH’s are unavoidably subjected as a result of device fabrication and changes in ambient temperature under operational conditions—on ion transport in the PLH’s CsPbBr(3−x)Ix, where x=0,1,2,3. Density functional theory (DFT) will be used to determine the relationships between the biaxial stress-induced strain state, and activation free energies of vacancy-mediated cation and anion hopping. These quantities will be used within a kinetic model to calculate diffusivity tensors, providing a direct mapping between the atomic scale processes underlying ion transport and their macroscopic measurable consequences, as functions of strain. Additionally, the following scientific questions will be answered as a result of this project: How does stress affect the perovskite phase stability of the materials? How does stress affect formation energies, and thereby equilibrium concentrations, of ion vacancies? What, if any, are the structural predictors for ion transport, and how are they altered by stress? Thus, this project will establish the fundamental quantitative relationships between stress, structure, ion vacancy concentrations and transport in PLH’s, based on quantitative first-principles computational data. Such data are currently sorely lacking, and will help to enable this highly promising class of functional material to reach its full potential.

• DFG Programme: Research Grants