Nanoscale interfaces in hybrid materials for exciton fission and fusion (Conference Presentation)

Abstract

The ability to downconvert (1 photon to 2 photons) and upconvert (2 photons to 1 photon) energy can have applications in many fields, including solar energy. Singlet fission provides a way to convert one photon into a pair of triplet excitons. It occurs efficiently in organic semiconductors, but the question remains how to extract the triplet excitons in a useful form. In this talk, we will describe efforts to transform triplet excitons into other forms through energy transfer into inorganic semiconductors like silicon. Heterogeneous solid-liquid approaches to use spin-orbit coupling to enhance the triplet excitons’ oscillator strength so they can emit photons will also be described. The solid-solid and solid-liquid interface appears to be critical for these schemes to succeed. Upconversion occurs via the reverse process, where a pair of triplet excitons fuse into a high-energy singlet state. A new approach to triplet state sensitization involves absorption of low energy photons by the semiconductor nanocrystals followed by energy transfer to the molecular triplet states. These states can then undergo triplet-triplet annihilation to create high energy singlet states that emit upconverted light in the visible and ultraviolet regions. By using conjugated organic ligands to form an energy cascade, the upconversion can be enhanced by up to three orders of magnitude. The mechanism of the nanocrystal-to-triplet energy transfer is investigated using time-resolved spectroscopy. Again, the role of organic ligand-inorganic surface interactions is important for determining the ultimate efficiency.