Elucidation of Chemistry-Structure-Function Relationships in Molecular Semiconductors for Organic Electronic Applications

Abstract

Molecular semiconductors are promising candidates for the active components of organic electronic devices as their optoelectronic properties can be tuned at the onset of synthesis, and they can be incorporated into lightweight, large-area, flexible devices at low costs. Yet, there are numerous challenges associated with molecular semiconductor systems and their commercial implementation, which largely stem from the structural heterogeneities present in polycrystalline thin films that strongly influence charge transport in devices. The microstructure of thin film active layers is dependent on both chemical structure and the processing conditions used. Thus, it is important to develop an understanding for the complex relationships that govern these systems.This thesis explores the development of chemistry-processing-structure-function relationships across multiple length scales in small molecule organic semiconductor systems. First, we explored the relationships between chemical structure and optoelectronic properties through targeted design of coronene derivatives for application in UV-absorbing, visibly transparent solar cells. We calculated the frontier orbital and excited state transition energies of over 350 candidate compounds and used the calculations to screen for promising molecules for synthesis and characterization. From our screening procedure, we selected and synthesized three coronene derivatives for use as donors in organic photovoltaic (OPV) active layers to produce visibly transparent OPVs, and in doing so demonstrated how integrated computational and experimental efforts can accelerate materials design. It is also important to understand how molecular semiconductors pack in the solid-state to elucidate the relationships between solid-state structure and device function. We explored the role of halogenated contorted hexabenzocoronene (cHBC) derivatives on the degradation of organic solar cells during stability testing and find that both fluorinated and chlorinated cHBCs, that start out amorphous as-deposited, crystallize during aging. The crystallization of cHBC derivatives produces gaps at the acceptor-buffer layer interface that hinder charge extraction, which results in the degradation of OPV device performance. We examined how atomistic substitution in the side group of triisopropylsilylethynylpentacene (TIPS-Pn) to produce triisopropylgermanylethynyl-pentacene (TIPGe-Pn), which maintains the size of the side group but increases its electron density, allows TIPGe-Pn to access a much broader structural phase space than TIPS-Pn. This work establishes that the solid-state packing of functionalized acenes depends on both the size of the side group and electron density, which may be tuned with simple atomistic substitutions. Finally, we explored the impact of grain boundaries on the kinetics of polymorphic transformations in a core-chlorinated naphthalene tetracarboxylic diimide. We determined that grain boundaries can lower the energy barrier by initiating polymorphic transformations. This work demonstrates the importance of grain boundaries, which are common in polycrystalline organic semiconductors, not only for their impact on charge transport but also in initiation of polymorphic transformations. Collectively, the work in this thesis highlights the importance of developing robust chemistry-processing-structure-function relationships that can guide material development. We demonstrate methodologies and illuminate concepts that will allow for further optimization and improvement to the performance and stability of organic electronics in the future.