27-Apr-2022: Talk by Dr. L. A. Muscarella
Strain-induced stabilization of phase segregation in mixed-halide perovskites.
The demand for energy grows globally due to a rapid growth of the world’s wealth. While fossil fuels are still the primary energy source used to meet the demand for energy, there is an urgent need to exploit the full potential of climate neutral alternatives like solar energy to reduce greenhouse gas emissions and fight climate change. Lead-halide perovskites emerge as an excellent candidate for highly efficient solar cells. However, these perovskites are unstable under continuous illumination where a process called ion migration occurs, hampering the long-term stability of the corresponding devices and preventing commercialization. Thus, understanding the origin and manipulating the ion migration in these materials is crucial to exploit their full potential. Ion migration arises in part because of the soft nature of these semiconductors, but at the same time the softness leads to large strain under various conditions, which could also provide a solution for stabilizing these perovskites. Yet, a complete picture of the role of strain on the ion migration process remains elusive. In this context, unveiling the connection between the dynamically disordered perovskite lattice and the optoelectronic properties can provide concrete guidelines for compositional engineering toward a rational design of mixed-halide devices, where targeted strain engineering strategies can be used as fabrication routes to obtain stable and bandgap-tunable materials.
In this talk, I will first introduce the phase segregation process and the detrimental consequences associated with ion migration in halide-perovskite. The bulk modulus, a figure of merit used for describing the sensitivity of a material to external and internal strain, will be introduced for several mixed-halide compositions and put into context to highlight the soft nature of perovskites in comparison with other conventional semiconductors. I will then present a unique pressure-dependent transient absorption spectroscopy setup used to study phase segregation as a function of the applied strain and show that phase segregation is substantially suppressed at external pressures of 3000 bar. Similarly, stability against halide segregation can also be improved at ambient conditions by chemical compression of the perovskite, via partial replacement of the cations with smaller ones, e.g. cesium. I will continue by proposing the reason behind the enhanced stability at high pressure which combine kinetics and thermodynamics effects. Finally, I will give a perspective on the long-term stability of mixed-halide perovskites, which would be essential for any application of perovskites requiring tunable and stable bandgap energies.