Organic molecules with low-lying spin-triplet states have received interest due to their potential to act as energy transducers within optoelectronic devices. For example, pairs of triplet excitations produced in these materials can fuse, yielding high energy spin-singlet states that can either radiate light or fuel energy-intensive catalytic transformations. By slightly altering their chemical structure, these materials can instead be optimized to undergo triplet fusion’s reverse reaction, singlet fission, which produces pairs of triplet excitons from high energy spin-singlet states. When incorporated into light harvesting systems, singlet fission materials can to boost the photocurrent they produce.  However, devices that use materials that undergo either of these processes to boost their performance necessitate forming functional semiconductor heterojunctions that readily transmit spin-triplet excitons. In this talk, I will present work describing my group’s efforts to develop functional interfaces for achieving molecule-to-semiconductor and semiconductor-to-molecule triplet energy transfer, with an emphasis on systems where the semiconductor in question is silicon. Achieving such junctions is key as silicon’s biocompatibility makes it attractive for photon upconversion imaging agents while its dominance in the solar energy marketplace makes it a key target for singlet fission-enhanced solar cells. First, I will present data showing silicon nanocrystals functionalized with organic molecules readily transmit triplet excitons from the nanocrystal to molecules at its surface following nanocrystal photoexcitation. Quantum confinement of charge carriers within silicon nanocrystals plays a key role in this process. Second, I will describe work whose aim is to extract triplet excitons produced by singlet fission in perylenediimide solids into bulk silicon. Using electronic sum frequency generation spectroscopy, we find strain within perylenediimide films at a semiconductor junction can lower their triplet energy at the junction, providing an energy sink that can attract triplets to this region. Importantly, we find evidence from transient reflectivity experiments for accelerated decay of triplet excitons at Si(111) surfaces, suggesting transfer of these species into bulk silicon.

Building functional organic/inorganic interfaces for triplet energy transfer

2019 Fall National Meeting

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