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Spectroscopy of Polarons, Bipolarons, and Excitons in Semiconducting Polymers


Organic electronics are an exciting alternative to traditional inorganic electronics. Compared to inorganic materials, organic electronics are lightweight, flexible, solution-processable, and inexpensive, and are made with non-toxic earth-abundant elements. Semiconducting polymers, discovered in the 1970s, are promising materials for use in organic electronics; the materials are easily tunable through organic synthesis, and their promise allowed researchers to earn the Nobel Prize for their discovery in 2000. Semiconducting polymers have potential applications in LEDs, solar cells, electrochromics, and thermoelectrics. Like the case with inorganic semiconductors, doping of organic semiconductors produces a charge (usually a positive charge) on the polymer. If the charge is mobile, the material’s electrical conductivity is enhanced. The belief in the literature is that initially, at low dopant concentrations, trapped polarons (singly charged cation radicals) form, which can be Coulombically-bound to the dopant counterion. As the concentration increases, the Coulomb wells of the dopants overlap and create mobile free polarons. As the dopant concentration increases further, polarons become too crowded on the polymer chains and can pair up to form doubly-charged bipolarons instead. My thesis work investigates polarons and bipolarons in conjugated polymers to better understand the nature and formation of these charge carriers.

The work presented in this thesis is the first use of ultrafast spectroscopy to understand and identify the electronic structure of the doped species on conjugated polymers. The first chapter in my thesis introduces semiconducting polymers and their applications, as well as the techniques I used to study them. The second chapter in my thesis demonstrates that ultrafast transient absorption spectroscopy can be used to distinguish the roles of free carriers and carriers that are Coulomb bound to the dopant counterions. The work presented in Chapter 2 also refutes one model of polaron energy levels, the band-bending model, based upon the transitions observed in the ultrafast transient absorption experiments. The final chapter of my thesis applies the ultrafast spectroscopy techniques developed in Chapter 2 to rule out the formation of polarons in a novel doped polymer system and to show that in this novel polymer, doping produces solely bipolarons instead of single polarons. We argue based on quantum chemistry calculations that it is the particular chemical structure of the polymer that produces this unusual behavior. Overall, my thesis work has made great strides in identifying and understanding the charge carrier(s) present in doped polymer systems.

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