Fundamental Understanding of a High Performance Polymer for Organic Photovoltaics and New Material Development by Rational Molecular Engineering

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  • March 19, 2019
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  • Li, Wentao
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Organic photovoltaics are a promising renewable energy technology. Development of novel materials and device architecture for further enhancing their efficiency requires fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Among many high performance polymers for organic photovoltaics, poly(benzodithiophene-alt-dithienyl difluorobenzotriazole) (PBnDT-FTAZ) is a very special one, due to its extremely efficient conversion of photons to observed current density. Although its absorption range is narrow with a band gap of ~ 2.0 eV, the power conversion efficiency of its bulk heterojunction solar cell based on phenyl-C61-butyric acid methyl ester (PC61BM) breaches 7%. In this dissertation, we conclude two fundamental reasons to account for the exceptional device performance of PBnDT-FTAZ by comprehensive investigation into morphology and device physics. On one hand, the molecular weight determines the morphology in the non-crystalline region. An appropriate molecular weight helps to achieve a small domain size, thus a shorter exciton diffusion path together with larger interfacial areas in the PBnDT-FTAZ:PC61BM bulk heterojunction, leading to improved short circuit current density. On the other hand, fluorination introduces better backbone stacking in the crystalline region, leading to significantly improved hole mobility, which reduces bimolecular recombination and directly accounts for the observed high fill factor in the OPV device. Overall, two important structure-property relationships regarding the molecular weight and the degree of fluorination of PBnDT-FTAZ are elucidated. In order to extend the absorption range and to further enhance the device performance of benzotriazole based polymers, we developed a general yet versatile synthetic approach towards a diverse set of triazole based conjugated molecules bearing various electron accepting abilities. The structural differences of as-synthesized three new triazole acceptors have a significant impact on the optoelectronic properties of conjugated polymers incorporating these triazoles. Bulk heterojunction solar cells based on one of these new polymers feature a high open circuit voltage of ~1 V and a notable efficiency of 8.4% with an active layer thickness around 300 nm.
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  • In Copyright
Advisor
  • Cahoon, James
  • Nicewicz, David
  • Brookhart, Maurice
  • You, Wei
  • Johnson, Jeffrey
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2015
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  • Chapel Hill, NC
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