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A Study of the Morphology Dependence of Singlet Fission in Organic Materials Using Magnetic Field Effects and Time Resolved Photoluminescence

  • Author(s): Piland, Geoffrey
  • Advisor(s): Bardeen, Christopher J
  • et al.

To further enhance photovoltaic efficiencies, singlet fission (SF) has been studied as a possible avenue to exceed the Shockley-Quiesser limit which prohibits single junction solar cells from going beyond 34% efficiency. SF occurs in organic materials when an excited singlet exciton within the organic material splits into two triplet excitons. This process, being spin allowed, occurs rapidly and the resultant triplets have the benefits of longer lifetimes (~ms) and longer diffusion lengths than their singlet counterparts. In order to better utilize this process for applications such as photovoltaics, there must be a better understanding of what systems give rise to high yields of triplets and how to harvest these triplets to do work. This work uses time-resolved fluorescence techniques along with magnetic fields to study how morphology plays a role in the singlet fission process. In order to do this, studies were done on the molecular systems of tetracene, rubrene, and diphenylhexatriene. Expanded versions of the Merrifield kinetic model were created to model the dynamics of these systems and better understand how spin and triplet-triplet annihilation affect the dynamics of the system. We find that amorphous rubrene has a slower singlet fission lifetime than tetracene at 2 ns. Using our expanded model, diphenylhexatriene was found to have a fission lifetime of 290 ps in the monoclinic form while having a 435 ps lifetime in the orthorombic form. Also, differences in fluorescence behavior between thin polycrystalline films and single crystals of tetracene are explored to reduce discrepancies in the literature as to tetracene's singlet fission rate and whether or not its activated by temperature. We find here that single crystal's of tetracene have an average singlet fission lifetime of 170 ps while the polycrystalline films have a lifetime of 70 ps, which we attribute to differences in the number of defects due to grain boundaries and sample preparation. This study also studies the integration of tetracene films with n-type silicon in order to harvest the generated triplets for photovoltaic applications, but evidence was only seen for singlet transfer to the silicon.

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