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On the way towards a sustainable future, the gradual abandonment of non-renewable fossil fuels such as coal, petroleum, and natural gas, in favor of alternative energy sources is required. One of the important alternative solutions is solar energy technology, in which sunlight is utilised to produce electricity. Whereas a growing role of this technology in global energy production is predicted, a step forward must be taken in terms of solar cells’ efficiency while simultaneously ensuring their low cost and small carbon footprint. In this regard, solar cells based on organic materials seem promising owing to a low production cost, abundance of organic materials, and their non-toxicity.  

- However, the widespread proliferation of organic solar cell technology in the global energy market is impeded due to a poor transfer of the excitation energy in organic materials, resulting in a low efficiency of conversion of sunlight into electricity, says Doctoral Researcher Ilia Sokolovskiifrom Ģ��ֱ��. 

Polaritons were meant to fly 

To address this challenge, placing molecules in optical cavities has been proposed. An optical cavity, which in the simplest case is just two parallel metallic mirrors, serves to confine light in a small volume, which makes it possible for strong interaction between electromagnetic field in the cavity, i.e. light, and molecular excitations, or excitons, if the energies of these two parts are at resonance. In the case of this so-called strong coupling regime, the cavity and the molecules begin to exchange energy of light intensively, so that it becomes “smeared out” between the two constituents. This results in the formation of hybrid light-matter quasiparticles, which are conventionally called exciton-polaritons, or just polaritons for shortness. 

- Perhaps the most remarkable thing about polaritons is that, as hybrid light-matter quasiparticles, they inherit the properties of both the molecular excitons and light in the cavity, which opens up plenty of potential applications, such as improving efficiencies of optoelectronic devices (solar cells, light-emitting diodes, lasers), developing logical elements for classical and quantum computers, and even controlling photochemistry, explains Sokolovskii. 

Enabling the next generation of organic solar cells 

As a part of the Computational Chemistry group of Professor Gerrit Groenhof, M.Sc. Ilia Sokolovskiiwas involved in the investigation of the dynamics and transport of exciton-polaritons, as well as of the effect of strong coupling on photochemical reactivity, by means of atomistic molecular dynamics simulations of over a thousand organic molecules in a cavity, which was performed on the high performance computing facilities of the Finnish national supercomputing center CSC. 

- In my dissertation, I present a simulation model developed by our groupand its application to the problem of polariton transport, says Sokolovskii. 

Considering different molecular species and various cavity structures allowed to propose a general mechanism for polariton transport in terms of extremely high velocities of polaritons and large densities of molecules in the cavity.  

- Not only do these simulations qualitatively agree with recent experiments, but they also allow us to deepen our understanding of excitation energy transfer in optical cavities, which might eventually pave the way towards rational design of next-generation organic solar cells, tells Sokolovskii. 

M.Sc. Ilia Sokolovskii defends his doctoral dissertation "Multiscale Molecular Dynamics Simulations of Enhanced Excitation Energy Transport in Organic Microcavities” on 11.11.2024 at 12:00 at Ylistönrinne Campus in lecture hall KEM4.  Opponent is Assistant Professor Milan E. Delor (Columbia University, USA) and custos is Professor Gerrit Groenhof (Ģ��ֱ��). The language of the dissertation is English. 

The dissertation "Multiscale Molecular Dynamics Simulations of Enhanced Excitation Energy Transport in Organic Microcavities” can be read on the JYX publication archive: .