Singlet exciton fission in perylenediimide thin films

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2019-09-18

Authors

Le, Aaron Kristopher

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Abstract

Singlet exciton fission (SF) is a process where a singlet exciton splits its energy with a neighboring molecule to form two lower energy triplet excitons. This is a process unique to organic molecules, and while intersystem crossing is typically slow in organic molecules due to small spin orbit coupling SF does not require a flip in spin to form these lower energy excitons and can occur on timescales extending from 10s of femtoseconds to nanoseconds. SF becomes efficient when the triplet energy is half of the energy of the singlet energy and occurs on timescales that outcompete deactivation pathways of the singlet exciton such as fluorescence. Efficient SF, or unity singlet to triplet interconversion via SF, has potential applications in improving the efficiencies of photovoltaic devices. Thermalization of high energy excitations in semiconductor photovoltaics pose a major loss channel for device efficiencies. An SF sensitizer can be incorporated into the device to absorb high energy excitations and following down conversion via SF to lower energy excitations thermalization losses can be mitigated. In this work we investigate SF in the perylenediimides (PDIs) in the solid state using ultrafast spectroscopy. The PDIs are deposited as polycrystalline thin films and structurally characterized to determine how the PDIs arrange in the thin films. SF is highly dependent on the intermolecular geometry of the of the participating chromophores, and the PDIs investigated in this work adopt different crystal structures in the solid state. This provides a platform to study the structure-function relationship of intermolecular geometry and SF. SF rates and triplet yield maxima are extracted from time-resolved emission and transient absorption spectroscopy with the aid of a kinetic models. The linear absorption and emission spectra suggest the singlet energy of the polycrystalline PDI films is lower than the monomeric PDIs due to aggregation. The potential disruption to the energetic requirement that the triplet energy be half the singlet would imply SF is an activated process in PDI thin films. We use a combination of temperature dependent time resolved emission and transient absorption spectroscopy to determine SF is endoergic in the PDI studied.

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