Tag: Cathy Wong

Spectroscopic Study of Squaraine Molecule Aggregate Formation for use in Solar Cells

Presenter: Laura Leibfried – Chemistry, Physics

Faculty Mentor(s): Cathy Wong

Session: (In-Person) Poster Presentation

Using organic photovoltaic (OPV) devices to harvest solar energy is uniquely enticing as they allow for mass manufacture, greater accessibility, and extraordinary chemical tunability. This study aimed to investigate a class of organic dyes called squaraines (SQs) which are potential donor molecules in OPVs and form molecular aggregates, affecting their electronic structure and energy transfer dynamics. Spatially encoded transient absorption was used to study restructuring SQ films during thermal annealing to reveal how the extent of aggregation affects exciton dynamics. Rapid and verging on total energy transfer from the targeted excitation of monomer molecules to aggregates is observed and dynamics are replicated by a kinetic model that evolves as a function of annealing temperature and the consequent extent of aggregation. Results indicate potential exciton trapping as a consequence of rapid energy transfer to optically darker states, which could imply less effective exciton diffusion in OPVs with only partially aggregated SQ donor domains.

Optimization of Deposition Techniques for Thin Film Production and Analysis

Presenter(s): Madelyn Scott − Chemistry

Faculty Mentor(s): Cathy Wong, Kelly Wilson

Poster 15

Research Area: Physical Chemistry

Funding: Community for Minorities in STEM (CMiS) Travel Award Scholarship; Phil and Penny Knight Campus for Accelerating Scientific Impact

Organic semiconductors offer a green alternative to conventional conductive materials because they can be solution- processed on an industrial scale for use in solar cells and OLEDs. The electronic transitions in organic semiconducting materials determine their charge-carrying efficiency for use in such devices. Transient absorption spectroscopy can be used to track the population of mobile electron-hole pair combinations at controlled delay times after photogeneration by a laser pulse. This technique is typically used to study equilibrated systems, like static solutions or films, but not materials as they evolve. For in situ studies of non-equilibrated systems, the Wong Lab has developed a single-shot transient absorption (ssTA) spectrometer to measure the excited state dynamics of thin films during deposition by a capillary or slot die coater. The solution capillary is two microscope slides spaced by aluminum shims and housed in an aluminum frame. The slot die coater is an apparatus designed to mimic solution-processed films that are manufactured roll-to-roll on an industrial scale. In both deposition techniques, a mechanical slide pusher is attached to the deposition device and positioned over an aluminum
stage to produce films on microscope slides. Experimental parameters considered during optimization of each deposition method included the following: slide pusher velocity, cleaning methods of the deposition slides, temperature of the depositing solution, and materials constructing the slide pusher apparatus. It was determined that the slot die coater enables more control over film quality than the solution capillary, producing films with more homogenous solution coverage. As a result, the slot die coater will be incorporated into the spectroscopy apparatus for the first in situ ssTA measurements of non- equilibrated material systems.

Correction of evolving background signals in single-shot transient absorption measurements

Presenter(s): Madelyn Scott

Faculty Mentor(s): Cathy Wong & Kelly Wilson

Oral Session 3 S

The electronic properties of organic molecules can be tuned to attain target electronic functionality. This feature of organic molecules enables their use in technologies like solar cells and light-emitting diodes (LEDs), in replacement of conventional silicon materials. The electronic properties of organic systems can change depending on way individual molecules pack together to form larger aggregate structures. Understanding how the behavior of organic molecules changes while molecular aggregation occurs enhances our insight into how target electronic functionality can be obtained by altering the environment of the molecular system. Conventional methods of studying the electronic properties of molecular systems are not equipped to measure evolving materials. To examine the changing electronic properties of materials systems, we have developed a single-shot transient absorption (SSTA) spectrometer capable of measuring structurally non-equilibrated samples, like molecules in a solution stacking into a final aggregate structure. However, evolving samples have changing background signals which can hinder SSTA measurements of the electronic properties of a sample. In this work, we demonstrate a shot-to- shot correction of dynamic background signals for SSTA measurements. Our correction scheme improves the robustness of SSTA for measurement of materials systems during molecular aggregation. Characterizing the electronic properties of organic semiconducting molecules during molecular aggregation will ultimately facilitate the achievement of target electronic properties for use in technological devices, like solar cells and LEDs, which are becoming increasingly prevalent in our contemporary society.

Aggregate Packing Structure and Photophysical Properties of Pseudoisocyanine Thin Films

Presenter(s): Rima Pandit

Faculty Mentor(s): Kelly Wilson & Cathy Wong

Poster 12

Session: Sciences

Electronic coupling between organic molecules in an aggregate gives rise to distinct features in the measured linear absorption spectra. Electronic coupling is determined by the physical arrangement of the molecules within the aggregate packing structure, and this results in specific photophysical properties of the aggregate. In dropcasted thin films of pseudoisocyanine (PIC), in situ absorption spectra reveal a distinct intermediate aggregation stage with potentially useful photophysical properties. Single-shot transient absorption (SSTA) spectroscopy can measure the exciton dynamics of the intermediate aggregation stage and of the entire aggregation process. This work describes improvements to a novel SSTA spectrometer that can concurrently measure exciton dynamics, absorption, and fluorescence during the PIC aggregation process. These measured photophysical properties are correlated with aggregate packing structure and composition inferred from fitting in situ absorption spectra with a Holstein-Hamiltonian. This strategy provides insight into the evolving composition and properties of aggregates during the process of aggregation, and can inform initiatives to tune aggregate packing structure to yield aggregates with desired electronic properties for photovoltaics and semiconductors.

Quantifying the spatial morphology of organic films through polarization- dependent imaging

Presenter(s): Madelyn Scott—Chemistry, Physics

Faculty Mentor(s): Kelly Wilson, Cathy Wong

Session 2: Cells R Us

Organic semiconducting materials are appealing, green alternatives to conventional semiconductors because they can be solution-processed into flexible films . However, solution-processing fabrication methods can be prone to morphological disorder, meaning that crystalline structures in the
film exhibit a variety of sizes and shapes . A large degree of morphological disorder inhibits the electronic functionality of a film for use in technological devices . Examining how film morphology varies with different deposition conditions allows us to connect the physical properties of organic semiconducting films to macroscopic perturbations in their formation environments . In this work, we used a homebuilt microscope to image the polarization-dependent absorption of organic films, and developed an image analysis software package to characterize their spatial morphology . A series of pictures are collected of the sample, rotating the polarizer between each image . For every pixel in the image, the absorption signal as a function of polarization angle is fit to a sinusoidal curve . These fits are employed to assign pixels in the image to discrete aggregate domains within the film . Quantitative domain metrics are computed to describe the morphology of the film . Several organic films are produced under different deposition conditions and their resulting morphologies are compared . By better understanding the relationship between deposition conditions and film formation, existing solution-processing techniques can be further controlled and refined to achieve target physical properties in organic semiconducting materials .