Spin-orbit coupling in synthetic semiconductor-protein complexes for solar energy harvesting

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%). Recently, there has been much academic and industrial interest in developing new singlet fission sensitizers, but to-date no material has proved ideal [1].

The biggest hurdle for using singlet fission sensitizers for solar harvesting is that of transferring the triplet excitons to the photovoltaic. Previous work suggests direct energy- or charge-transfer of the triplets into silicon is extremely inefficient [1]. Instead radiative transfer (i.e. photon emission then reabsorption into silicon) has been suggested [1]. Unfortunately, triplet excitons are non-emissive! They could transfer to an efficient emitter as suggested in Ref. [2], but we suggest an alternative approach: protein-induced triplet emission, i.e. phosphorescence.

We have recently created a new range of synthetic ‘maquette’ proteins that bind organic semiconductors (non biological pigments) in a specific dimer configuration. You will use these proteins to induce triplet emission (phosphorescence) in the organic semiconductors.

Room temperature phosphorescence has been known to occur in proteins since the 1970s [4,5]. We aim to design proteins that exploit this behaviour, hindering non-radiative deactivation and enabling pigment phosphorescence. To increase the phosphorescence efficiency further, we will use external spin-orbit coupling, as demonstrated in molecular crystals [6]. Compared with molecular crystals, our synthetic proteins offer a much more fine-tuned approach to achieving phosphorescence: we can place heavy elements at specific locations within the protein core. Achievement of this would constitute a major breakthrough.

In this project, you will work closely with biologists to develop and characterize semiconductor-proteins capable of efficient triplet emission. To achieve this aim, you will use time-resolved optical spectroscopy to track the singlet fission process in absorption and emission as a function of protein structure at the recently opened ‘Lord Porter Laser Facility’ in Sheffield.


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