title: A Theoretical and Experimental Study into the Kinetics of  Solution Phase Thin Film Deposition
Creators: name: Schaaf, Cyrus
Publisher: name: Western Washington University
Year: 2017

description: <p>The performance of electronic and optoelectronic devices based on solution-processed organic semiconductor layers is strongly influenced by their mesoscale polycrystalline structure, including domain size and spatial distributions. In solution-processed films prepared by spin casting, solvent-based printing, and related methods, morphology is governed by a combination of interrelated thermodynamic and kinetic factors. Classical models of crystal formation in bulk solution or on bare surfaces in vacuum-deposited films fail to adequately capture these effects; the current theoretical understanding of crystallization in solution-deposited films is generally unable to provide much insight, let alone predictive design guidance for tailoring films with specific structural characteristics for a given set of experimental conditions and chemical properties. In this thesis solution-phase thin film formation has been studied for the purpose of developing and new experimental techniques new models for understanding and predicting mesoscale film structure and crystal morphology. I will describe how nucleation can be modeled, and the predictions tested against experiment, by an approach that enables quantitative prediction of crystal coverage and intercrystalline spacing statistics as a function of processing conditions, using only a small number of experimentally-measureable parameters. To do this, a model is introduced that combines a mean-field rate equation treatment of monomer aggregation kinetics with classical nucleation theory and a supersaturation-dependent critical nucleus size to solve for the quasi-two-dimensional temporally- and spatially-varying monomer concentration and nucleation rate. Excellent agreement is observed with measured nucleation densities and inter-domain radial distribution functions in submonolayer tetracene films. The model leads to the first universal set of predictive design rules for solution-phase thin film growth capable of guiding the selection of experimental conditions for truly engineered morphological control. Accompanying this theoretical work a first of its kind experiment is also reported, in which monomer concentration has been spatially and temporally mapped in real time during the film formation process. Through the use of high resolution dark field fluorescence microscopy employing an internal fluorescent standard and multi-wavelength imaging optics the concentration dependence is visualized throughout all regimes of thin film formation. In situ measurements of local concentration contributes to the development of models which treat the role of variations in monomer concentration on mesoscale film morphology of polycrystalline thin films. This work opens the door to numerous studies enabling further development of models which allow for predictive control of polycrystalline thin films in solution-phase deposition techniques. In addition to nucleation, growth of crystalline films is modeled through a set of numerical and computational methods which provide insight into the main factors influencing crystal growth habit. It is shown that crystal capture rate correlation with physical properties displays a distinct lack of agreement between the spacing and initial sizes of crystals with their relative growth rates. This lack of correlation points to the need for more sophisticated models. Through the use of a mean field numerical calculation of the volumetric growth rate changes in crystal morphology can be attributed to a variable sticking probability which depends on the crystal face. Kinetic Monte Carlo simulations are used to directly probe the physics which explain the deviation from the typical single sticking coefficient capture model. The change in shape at long deposition times further suggests that crystal growth occurs in distinct regimes which dictate the final morphology of the crystals. This work provides an explanation to the change in shape of crystalline material at long deposition times which can be used to develop models to predict final crystal morphology. This thesis is comprised of several parts. In the first chapter the broader context of the work is discussed. In chapter 2, I discuss the scientific background laying the foundation for theoretical models into solution-phase deposition. In the third chapter, I describe the experimental system as well as results from various measurements of fundamental chemical and physical properties needed later. The fourth chapter describes a set of models which I have developed to predict mesoscale film structure to create a set of universal design rules in order to engineer thin films grown in the solution-phase. In chapter 5, I describe a state of the art experimental set up allowing for monomer concentration to be mapped in real time. Finally in the last chapter I describe a set of exploratory models to describe change in crystal morphology during the course of thin film formation. This thesis creates new understanding, which will allow for an increase in production of thin films for applications where strict control over domain size, shape, spacing, and crystallographic orientation.</p>