Evolving Morphology and Excited State Dynamics During Deposition and Thermal Annealing of Organic Thin Films

Abstract

Understanding the structure-function relationship for organic semiconducting materials is critical for developing more efficient organic photovoltaics (OPVs). Charge transfer takes place at the electron donor-acceptor interface and is highly dependent on film morphology. Since organic molecules have weak intermolecular forces, a variety of molecular orientations form when cast into films, which can impact OPV performance. Thermal annealing is a typical post-processing method used to improve sample crystallinity. This thesis first reports on thermal annealing of the prototypical donor-acceptor system, Poly(3-hexylthiophene-2,5-diyl) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) using in situ linear absorbance and single-shot transient absorption (SSTA). A sequential decay model revealed two different species in neat P3HT films with differing interchain ordering, where the species with less interchain ordering undergoes irreversible changes during thermal annealing. In P3HT:PCBM films, charge transfer increases significantly after passing through the glass transition temperature. This study shows the basis for studying how charge transfer evolves in a nonequilibrium system.

Squaraine (SQ) molecules have a donor-acceptor-donor (D-A-D) structure that leads to inhanced CT. In this study, an inert polymer was used to impede SQ aggregation, allowing for in situ spectroscopic studies of molecular aggregation during thermal annealing. In situ linear absorption and SSTA measurements during thermal annealing were used to reveal the changes in electronic structure and excited-state dynamics. A sequential decay kinetic model showed that energy transfer occurs between uncoupled and weakly-coupled monomers before annealing and energy transfer occurs between sub-populations of slip-stacked H-aggregates after annealing. We further use an ICT Hamiltonian model to predict the intermolecular spacing and intermolecular charge transfer probability as a function of aggregation. This model shows great promise in predicting both the species present and predicting excitonic behavior for applications in OPVs.

Copper phthalocyanine (CuPc), an electron donor molecule, forms crystallites in a pyramidal structure when cast as a thin film, but the mechanism of aggregation is not yet fully understood. In this work, in situ linear absorption and SSTA measurements revealed a novel intermediate stage during CuPc film deposition. Supersaturated solutions of CuPc showed that the steps of aggregation can be separated with H-aggregates forming in solution and J-aggregates forming in final step of solution evaporation. Films cast from solutions of supersaturated CuPc have a more crystalline structure and global fits to the TA spectra showed that exciton-exciton annihilation becomes more rapid compared to regular films. This study provides the framework for how in situ spectroscopies can be used to design new aggregate structures and control exciton dynamics.

Recent studies of high school / freshman chemistry laboratories show that online activities show some benefit, reducing the number of errors made in the lab and improving scores on post-lab questions. However, their impact on upper division chemistry laboratories have not been investigated. In this three-year study, we measured the impact of interacting with a computer simulation before an in-lab activity constructing a brightfield microscope in an upper-division physical chemistry laboratory. Students recorded their completion times for activities performed during the laboratory and took the Meaningful Learning in the Laboratory Instrument (MLLI) survey before and after building the microscope. The MLLI measured the students’ affective and cognitive experiences before and after the activity. Analysis of completion times showed that students performed the lab activities slightly faster with a narrower distribution of completion times when they had interacted with the simulation beforehand. Analysis of specific survey questions from the MLLI revealed that the students who completed the computer simulation before building the microscope were less worried about making mistakes and finishing on time. This study shows that while a simulation of in-lab activities may not provide any cognitive benefit to students, it may improve their confidence in the laboratory.This thesis includes previously published and unpublished coauthored material.