Speaker: Dr. Tak W. Kee, Department of Chemistry, Adelaide University, South Australia, Australia.
Title: Ultrafast Singlet and Triplet Exciton Dissociation in Organic Photo-Catalysts for Hydrogen Generation
Abstract: Hydrogen gas (H2) is an important energy source that is attracting significant attention due to its zero-carbon emission. The current methods of producing H2, however, which are powered by fossil fuels, release a high level of carbon dioxide. A cost-effective and green alternative to current methods is photocatalytic H2 evolution using nanoparticles of organic semiconductors. Typically, this process uses light to generate singlet excitons, which then undergo exciton dissociation at interfaces between electron donor and acceptor materials. The generated charges upon exciton dissociation can migrate to the photocatalyst’s surface to facilitate proton reduction to form H2. (left figure). We use broadband ultrafast transient absorption spectroscopy to study singlet exciton dissociation to form charge carriers with a time resolution of ~10 femtoseconds. We find that exciton dissociation occurs with time constants ranging from 28 femtoseconds to 149 femtoseconds. Ultrafast exciton dissociation correlates with the morphology of the photocatalyst. The presence of mixed H/J aggregations increases energetic disorder to promote ultrafast exciton dissociation. Our results indicate that exciton dissociation occurs at sufficiently high rates to outcompete most other photophysical processes in the photocatalyst.
I will also discuss a novel approach of using triplet energy transfer (right diagram) to generate triplet excitons of an organic semiconductor to increase the efficiency of H2 evolution. Triplet excitons are longer-lived species than singlet excitons due to their spin-forbidden relaxation to the ground state. As a result, the longer triplet exciton lifetime may allow a higher probability for charge separation than that using singlet excitons. The use of triplets could pave the way for singlet fission, a process that converts a singlet into two triplets, to be explored for maximising the efficiency of H2 evolution in organic semiconductors.