1pm, Wednesday
1 May 2024

PLT

This is a departmental event for all staff and students. Please come along! Refreshments will be provided on the Science Concourse from 12:45.

"Proton Migration and Halide Ordering in Lead Halide Perovskites from Solid State NMR", Dr Michael Hope

Hybrid lead halide perovskites are promising materials for photovoltaic applications due to their high power-conversion efficiencies, tunability, and inexpensive solution processing. However, ion migration under applied bias can impact the efficiency and stability of perovskite solar cell devices, limiting their application. Here, I will present two new examples where solid-state NMR can reveal key information on the structure and dynamics of hybrid perovskite systems, that cannot be accessed via other methods.

In the first example, I will show how ²⁰⁷Pb chemical shift anisotropy and relativistic DFT calculations can be used to distinguish the different [PbX₆] configurations in both 2D and 3D mixed-halide perovskites. The large negative ²⁰⁷Pb CSA observed for 2D perovskites shows that iodide preferentially occupies the axial site, which may explain the suppressed halide mobility.

In the second example, 2D ²H exchange spectroscopy is used to measure the rate of H migration in MAPbI3 and double-cation (MA,FA)PbI₃ perovskites. Using ¹⁵N-labelled methylammonium (MA) effectively splits the cations into two distinct populations, labelled by their ¹⁵N spin state, allowing the exchange rate between the two to be measured. Comparison with bulk H diffusivity suggests that H ions travel a significant distance through the material before associating with another cation.

"Ultrafast Chemical Dynamics: Towards Turning the Lights Off", Dr Raj Pandya

Femtoseconds (and faster) are the natural timescale for the elementary electronic, ionic, vibrational, etc processes that lie at the core of many functional materials and devices.

I will showcase some of the techniques we have developed over the last few years to spatially & temporally capture ultrafast dynamics in chemical systems, with particular applications in organic semiconductors (polymers), 2D/colloidal materials, batteries/catalysts and hybrid light-matter (polaritonic) systems.

I will then finish with a flavour of our upcoming work at Warwick on capturing stochastic ultrafast dynamics, working with entangled and twisted photon states and following non-light driven reactions.