Aaron Le1 Sean Roberts1 Jon Bender1

1, University of Texas at Austin, Austin, Texas, United States

Singlet fission (SF) is a process that can occur in select organic molecules where a singlet exciton splits into two lower energy triplet excitons. While the intersystem crossing of singlets to triplets is typically slow in organic molecules, due to small spin orbit coupling singlet fission does not require a flip in spin and can be exceptionally fast compared to intersystem crossing. The SF process requires two chromophores whose triplet states should be approximately half of the singlet state’s energy to conserve energy. This process has applications in photovoltaics where thermalization of high energy excitations accounts for a major loss channel and limits the maximum performance of such devices. Utilizing materials that undergo efficient SF can potentially mitigate the losses these above band gap excitation experience --- high energy excitations can be down converted into lower energy excitations before thermalization can occur. Perylenediimides (PDIs) are a robust class of dye molecule employed as commercial pigments that contain the proper energetics for SF. These molecules form crystalline domains in the solid state that can be readily tuned by functionalization of the molecules, thereby allowing the structure-function relationship of SF and intermolecular geometry in the solid state to be explored. The intermolecular geometry of SF chromophores has been shown to have a strong connection to the SF rate chromophores that undergo SF. A collection of PDIs with different solid state packing structures were evaluated for potential use as SF chromophores in polycrystalline thin films. Employing a combination of ultrafast femtosecond and nanosecond spectroscopy we identify the formation of triplet state spectral features and identify the SF rates for these materials. Extracted experimental rates deviate from theoretical predictions by approximately three orders of magnitude which may be attributed to differences in energetic landscape of the polycrystalline films and the dimer employed in the calculation. A series of temperature dependent spectroscopic experiments were utilized to examine the energetic requirements of SF in these polycrystalline thin films.