SPEAKER:
Ms Leah Weiss, Optoelectronics Group (Physics), University of Cambridge
SYNOPSIS:
Spin plays a key role in the dynamics of organic systems, with examples ranging from spin-dependent loss channels in organic photovoltaics and light-emitting diodes to biological mechanisms of magneto-sensing. One such spin-dependent process, singlet fission, involves the production of two triplet excitons each with spin S=1 following excitation of one singlet exciton (spin S=0). This pair production process has the potential to boost the theoretical efficiency of photovoltaics beyond the Shockley-Queissar limit and has been used to produce photovoltaics with >100% external quantum efficiency [1,2]. Recently we have used singlet fission as a means of optically generating excitonic spin pairs to study the fundamental spin interactions of triplet excitons [3,4]. Using a combination of electron spin resonance and magneto-optical spectroscopy in high magnetic fields (<68 T), we observe and measure triplet spin-exchange coupling directly. These investigations have provided evidence for strong spin-coupling and high spin (S=2) formation in solid-state organic semiconducting materials. We further observe microsecond timescales for loss of coherence in the triplet pair state at low temperatures. Spin interactions and coherence properties, such as those measured here in triplet exciton pairs, not only affect dynamic processes in electronic devices, but also present an opportunity for development of molecular spin-based technologies.
References:
[1] Dexter, D. L. Two ideas on energy transfer phenomena: Ion-pair effects involving the OH stretching mode, and sensitization of photovoltaic cells. Journal of Luminescence, doi:10.1016/0022-2313(79)90235-7 (1979).
[2] Congreve, D. N. et al. External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell. Science 340, 334-337, doi:10.1126/science.1232994 (2013).
[3] Weiss, L. R. et al. Strongly exchange-coupled triplet pairs in an organic semiconductor.
Nature Physics, doi:10.1038/nphys3908 (2016).
[4] Bayliss, S. L. et al. Spin signatures of exchange-coupled triplet pairs formed by singlet fission. Physical Review B 94, doi:10.1103/PhysRevB.94.045204 (2016).