Transition metal dichalcogenides (TMDCs) are highly tunable two-dimensional excitonic materials that allow rapid charge and energy transfer at their atomically thin and sharp interfaces. TMDCs paired with thin films of organic molecules result in unique heterostructures which offer new opportunities in developing solar energy functionalities by combining the best features of these two distinct classes of material. As mixed singlet and triplet excitonic states exist in TMDCs due to the material’s strong spin-orbit coupling, energy transfer from the TMDC to both the singlet and triplet states of the organic materials may occur. The two heterostructures which have been synthesized are tungsten disulfide with tetracene (WS2/Tc) and tungsten diselenide with rubrene (WSe2/Ru). In a joint theory-experiment study, we have examined the intra- and inter- layer exciton landscape in WS2/Tc via temperature-dependent (100-245K) photoluminescence (PL) and dynamics measurements. Our results confirm the predicted enhancement of PL intensity of interlayer excitons and the emergence of indirect phonon-assisted emission from interlayer exciton states. Preliminary results with selective excitation of WSe2 has revealed triplet-triplet energy transfer and subsequent photon up-conversion in WSe2/Ru. Transient absorption spectroscopy and microscopy are used to study the various kinetic processes associated with the upconverted emission, including triplet energy transfer, diffusion, and annihilation. The results of this study aim to provide not only fundamental insight into the energy and charge transfer of these heterostructures, but also guidelines when engineering new nanostructured photovoltaics.

Energy and charge transfer at two-dimensional organic-inorganic interfaces

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ACS Fall 2022

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