The incorporation of materials that undergo fission of singlet excitons into triplets in solar cells is known to augment the efficiency of photovoltaic devices by doubling the number of excitons created by high energy photons [Hanna and Nozik, 2006]. Although internal power conversion efficiencies as high as 160% have been reported in devices made of pentacene/poly(3hexylthiophene) [Baldo, 2013], efficiencies of most singlet fission devices are sub-optimal and can be improved with better design of materials. A principal factor determining efficiency is the rate of singlet fission, which is dependent not only on the energy gap but also on the coupling between the singlet exciton and the multiexcitonic state. The effect of this coupling is best demonstrated using covalently linked chromophore dimers made of tetracene [Bardeen, 2007], diphenylisobenzofuran [Michl, 2013], and TIPS-pentacene [Guldi, 2016]. Here we present a comprehensive set of excited state properties of ortho-, meta- and para-ethynyltetracene dimers in solution and neat film phases that lets us compare and contrast the effects of inter- and intra-dimer coupling on singlet fission. In solution, only the para-dimer forms distinct triplets while the meta-dimer predominantly undergoes radiative decay and only the multiexcitonic state is observed in the ortho-dimer. In neat films, inter-dimer chromophore interactions leads to singlet fission in all three dimers although at different rates as observed using broadband femtosecond transient absorption spectroscopy. These results highlight the effect of ground state conjugation as well and inter- and intra- dimer coupling on singlet fission that we hope will drive the design of materials for photovoltaic applications with better efficiency.

Linker dependent singlet fission in tetracene dimers

256th National Meeting (2018)

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