University of St Andrews

Multiscale modeling refers to a combination of multiple models at different scales used simultaneously to describe a system. These models focus on different scales of resolution, atomistic through mesoscopic to continuum (macroscale). Combining the models allows us to understand how microscopic processes governing charge and energy transport influence behaviour, e.g. charge mobilities, at a macroscopic length scale. I will describe our use of random walk Monte Carlo models at the mesoscopic level and link the length scales. I will show how we can predict the performance of novel optoelectronic devices such as perovskite and organic solar cells and light emitting diodes. In perovskite solar cells, I will describe how our drift diffusion model of ionic-electronic conduction has shown how mobile ion vacancies cause hysteresis. I will also demonstrate predictions from a mesoscopic model in which the charge carriers are treated as polarons and that shows the conditions under which hot polarons can beat the Shockley–Queisser limit. Charge carriers in organic semiconductors in organic solar cells and in charge transport layers are also polarons. Here, polaron transport depends sensitively on molecular packing arrangements. I will describe simulation of charge and energy transport in small molecule and polymer semiconductors with a recently developed fast electrostatics solver and with morphologies obtained with the code Simulation of Atomistic Molecular Structures using an Elastic Network (SAMSEN) developed by my group [1]. Large systems, e.g. 100 polymers each 10 monomers long, can be simulated with SAMSEN in around a day with a desktop computer.

[1]        A R Smith, I R Thompson, A B Walker J. Chem. Phys. 150, 164115 (2019)

The Nonlinear Ghost Imaging is a novel imaging concept at the core of the Emergent Photonics Lab (EPic) ERC project “TIMING” proposed to overcome inherent limitations in the state of the art of terahertz imagers, in terms of resolution and imaging speed. Space-time coupling affects near-field time-domain imaging leading to fundamental systematic reconstruction issues.  The seminar will present the methodological and technological route towards high-fidelity subwavelength imaging, establishing a rigorous framework to reconstruct hyperspectral images of complex samples inaccessible through standard fixed-time methods. It will also introduce the effort of EPic Lab in the investigation quasi-2D structure, enabling element in this research, based on exotic nonlinear field-matter interaction processes at dielectric surfaces.