University of Illinois Urbana-Champaign

Impact of polymer solution aggregation and assembly on organic solar cell morphology and device properties

Khasbaatar, Azzaya

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https://hdl.handle.net/2142/125533

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Title
Impact of polymer solution aggregation and assembly on organic solar cell morphology and device properties
Author(s)
Khasbaatar, Azzaya
Issue Date
2024-07-09
Director of Research (if dissertation) or Advisor (if thesis)
Diao, Ying
Doctoral Committee Chair(s)
Diao, Ying
Committee Member(s)
  • Schroeder, Charles M
  • Sing, Charles E
  • Jackson, Nicholas E
Department of Study
Chemical & Biomolecular Engr
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
organic solar cells, polymer assembly, conjugated polymers
Abstract
Owing to various forms of intermolecular interactions, conjugated polymers self-assemble to form distinct types of aggregates in their solution-state, which play an important role on the film morphology and optoelectronic properties of organic electronic devices. Upon increasing the solution concentration, they can further assemble into more complex multistep secondary phases, which can be directly translated to the solid-state during evaporative solution processing of thin films. Numerous studies have shown that solution-state aggregation of conjugated polymers influence the film morphology of organic solar cells (OSCs); however, their detailed aggregate structures in the solution-state and their assembly pathways during solution processing remain unresolved. Therefore, the objective of my thesis is to elucidate the exact structure of solution-state aggregates and their assembly pathways and to further investigate how they influence the blend film morphology and device properties of organic solar cells. In my first study (Chapter 2), we investigated the impact of polymer backbone conformation on the solution aggregate structures of conjugated polymers. A combination of X-ray scattering and microscopic imaging techniques allowed us to observe the formation of semi-crystalline fiber aggregates and amorphous network aggregates depending on the polymer backbone conformation. We further reveal that amorphous, network-like aggregates are conducive to superior device performance in blade-coated OSCs owing to the formation of blend films with short π-π stacking distance, small domain spacing, and face-on preferred molecular orientation, whereas the fiber-like aggregates lead to large π-π stacking distance, large domain spacing, and isotropic molecular orientation in the blend film which deteriorate the device performance. It was also revealed that the fiber aggregates are highly temperature-sensitive compared to the polymer network aggregates, leading to more processing temperature sensitive performance. Next in Chapter 3, we demonstrated how polymer aggregate structures can be tuned by the solvent environment to give distinct aggregate structures that determine the blend film morphology and device performance of organic solar cells. We found that solvent affinity towards the polymer backbone and the alkyl side chains drastically tunes the polymer solution aggregate structures and the resultant blend film morphologies. Our findings reveal that a poor side chain solvent leads to semiflexible amorphous networks with strong side chain associations, whereas a poor backbone solvent gives a rise to semicrystalline fiber aggregates. Mutual solvents, on the other hand, balance the polymer backbone and side chain solubility, resulting in rigid amorphous networks with weak/no side chain interactions. Upon film deposition via blade coating, both semicrystalline fibers and flexible amorphous network aggregates yield highly crystalline films with large domains, whereas the rigid amorphous aggregates with weak or no side chain associations prevented excessive crystallization, yielding the most favorable blend film morphologies. Finally in Chapter 4, we explored the concentration-dependent lyotropic liquid crystalline phase mediated assembly of conjugated donor polymers and its influence on the blend morphology and device properties of organic solar cells. Using solution X-ray scattering and microscopic imaging tools, we discovered that D18 polymer assembles into both achiral and chiral lyotropic liquid crystalline phase upon increasing the solution concentration. We further show that the nature and extent of assembly during film deposition can be controlled by the coating regime and the processing solvent, which collectively determine the drying dynamics and kinetics of the meniscus-guided coating process. We demonstrate that coating in the evaporation regime drives the assembly towards the liquid crystalline phase formation, which not only enhances the film alignment and crystallinity but reduces unfavorable phase separation, resulting in improved OSC performance and stability for D18:Y6 blend system.
Graduation Semester
2024-08
Type of Resource
Thesis
Handle URL
https://hdl.handle.net/2142/125533
Copyright and License Information
Copyright 2024 Azzaya Khasbaatar

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Graduate Theses and Dissertations at Illinois