Understanding the spin-entangled triplet-pair state in organic semiconductors

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In collaboration with theoreticians at the University of Oxford, this project involves studying the fundamental electronic behaviour of excited states in organic semiconductors (specifically 'triplet-pair' states) for applications in photovoltaics, displays and polaritonics. You will study model systems (either thin films, photonic devices or protein-bound semiconductors) using ultrafast spectroscopy at Sheffield's ultrafast laser facility. [[Background]]Singlet fission is a process whereby one photon creates two excited states. This two-for-one mechanism could dramatically increase solar cell efficiency (from 33% to >40%). There has therefore been significant academic and industrial interest in developing new singlet fission sensitizers for photovoltaic or optoelectronic applications recently. Unfortunately, to-date no material has proved ideal. This is in part because of a fundamental lack of understanding of the singlet fission process and what molecular parameters control it. What is known is that singlet fission proceeds through an intermediate excited state, known as (TT): a correlated pair of triplets (spin S=1 excitations). The spins of the individual triplets may remain entangled over microseconds at room temperature, even as the triplets move apart, making it an exciting area of research with possible applications well beyond photovoltaics. [[Project details]]In this project you will study the magnetic-field dependent spectroscopy of organic semiconductors using ultrafast (femtosecond) pulses of light to map the electronic landscape of the singlet fission process in Sheffield's ultrafast laser facility. The emphasis will be on developing an understanding of triplet-pair states (TT): their lifetime and energetics; how long they remain spin-coherent; how far and fast they move. The aim is to understand which molecular and film parameters govern these properties and how they can be optimized for use in a variety of optoelectronic and photonic devices.