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Elucidation of Excitation Energy Dependent Correlated Triplet Pair Formation Pathways in an Endothermic Singlet Fission System

Accepted version
Peer-reviewed

Type

Article

Change log

Authors

Stern, hannah 
Cheminal, Alexandre  ORCID logo  https://orcid.org/0000-0001-9969-672X
Petty II, John 
Anthony, John 

Abstract

Singlet fission is the spin allowed conversion of a photo-generated singlet exciton into two triplet excitons in organic semiconductors, which could enable single-junction photovoltaic cells to break the Shockley-Queisser limit. The conversion of singlets to free triplets is mediated by an intermediate correlated triplet pair (TT) state, but an understanding of how the formation and dissociation of these states depend on energetics and morphology is lacking. In this study, we probe the dynamics of TT states in a model endothermic fission system, TIPS-Tc nanoparticles, which show a mixture of crystalline and disordered regions. We observe the formation of different TT states, with varying yield and different rates of singlet decay, depending on the excitation energy. An emissive TT state is observed to grow in over 1ns when exciting at 480nm, in contrast to excitation at lower energies where this emissive TT state is not observed. This suggests that the pathway for singlet fission in these nanoparticles is strongly influenced by the initial sub-100 fs relaxation of the photoexcited state away from the Frank-Condon point, with multiple possible TT states. On nanosecond timescales, the TT states are converted to free triplets, which suggests that TT states might diffuse into the disordered regions of the nanoparticles where their breakup to free triplets is favored. The free triplets then decay on µs timescales, despite the confined nature of the system. Our results provide important insights into the mechanism of endothermic singlet fission and the design of nanostructures to harness singlet fission.

Description

Keywords

0306 Physical Chemistry (incl. Structural), 1.1 Normal biological development and functioning

Journal Title

Journal of the American Chemical Society

Conference Name

Journal ISSN

0002-7863
1520-5126

Volume Title

140

Publisher

American Chemical Society
Sponsorship
Engineering and Physical Sciences Research Council (EP/M006360/1)
Engineering and Physical Sciences Research Council (EP/M024873/1)
Engineering and Physical Sciences Research Council (EP/P027741/1)
European Commission Horizon 2020 (H2020) Marie Sk?odowska-Curie actions (705113)
We acknowledge EPSRC and the Winton Programme for the Physics of Sustainability for funding. A.T. thanks the Dr. Manmohan Singh scholarship, St John’s College, University of Cambridge, for funding. M.J.Y.T. acknowledges receipt of a Marie Curie Individual Fellowship.