Reading between the lamellae: Elucidating block copolymer morphology using polarized resonant soft X-ray scattering

Open Access
Author:
Litofsky, Joshua Harrison
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 04, 2019
Committee Members:
  • Enrique Daniel Gomez, Dissertation Advisor
  • Enrique Daniel Gomez, Committee Chair
  • Michael John Janik, Committee Member
  • Scott Thomas Milner, Committee Member
  • Michael Anthony Hickner, Outside Member
Keywords:
  • block copolymers
  • organic electronics
  • x-ray scattering
  • conjugated polymers
  • organic semiconductors
Abstract:
In recent decades, organic electronics have penetrated into applications in our everyday lives. Stemming from their advantages of mechanical flexibility, chemical tunability, and solution processability, organic devices have the potential to work with inorganics to fully bring electronics into the polymer age. However, difficulties in understanding the true morphology and charge generation of these organic active layers hinders our ability to truly appreciate the true gains that this field of electronics can offer. From a perspective of organic solar cells, this dissertation, using fully conjugated donor-acceptor block copolymers, advances the nanoscale morphological understanding of charge generation layers of organic electronics. The first step in understanding nanoscale ordering in our polymers is through creating a computational model to determine the predicted morphology of polymer systems. Using a polarized Fast-Fourier Transform, we have developed a method of determining ideal X-ray scattering patterns through simulations to aid our understanding of experimental data. We find that using this simulated polarized scattering can provide valuable information about the domain spacing, semicrystalline orientation, and polymer chain tilt of homopolymer and block copolymer systems. From this computational data, we can begin to better understand our experimental scattering. Using polarized Resonant Soft X-ray Scattering and comparing it to our simulated polarized scattering, we can use the scattering anisotropy of the polymer to quantitatively understand the chain orientation and strength of nematic long-range order, as well as better determine the domain spacing in the polymers. This new method provides an understanding of nanoscale morphology of polymer films that was not seen before. The next step is to probe our ability to modulate the anisotropy of our polymers and block copolymers. Our simulations indicate that maximum anisotropy occurs when the sample is only 50% crystalline rather than fully crystalline as would be intuitive. Furthermore, our experimental scattering confirms this novel finding, as supported by polarized resonant soft X-ray scattering and differential scanning calorimetry. In these semicrystalline systems, both the chemical and orientational contrast lead to anisotropy; it follows that the most orientational contrast comes from a system divided evenly between amorphous and crystalline regions. Lastly, we use this enhanced morphological understanding to predict electronic device performance. From the lens or organic photovoltaics, we can use this quantitative scattering anisotropy to identify how the long-range order changes components of device performance. Coupling these photovoltaics with organic diodes to provide us with charge carrier mobilities, we have provided the first link between order and device performance of polymer solar cells using resonant soft X-ray scattering.