Crystal formation and epitaxy of thin film organic semiconductors

Abstract

As the Earth continues to warm due to human related emissions from burning fossil fuels, alternative and renewable energy sources are becoming increasingly important. This crucial decarbonization transition will require a mix of energy sources spanning solar, wind, hydroelectric, biomass, nuclear, and more to reach carbon neutrality. One of the most abundant sources of energy comes from the sun. While there are numerous methods of harnessing the sun's power, directly converting it into electricity, a highly useful form of energy, is one of the most desirable techniques. For this reason, solar cells have been lauded for their high potential in combating humanity's energy crisis. The vast majority of commercial photovoltaics are currently made using silicon. The resulting solar panels are heavy and rigid and suffer from high installation costs. A new generation of solar cell technology relying on organic semiconductors could mitigate these problems. Their low processing temperatures allow fabrication on light-weight, flexible substrates opening the door to roll-to-roll processing potentially driving cost down. However, a common issue with organic semiconductors is their poor charge transport properties compared to their inorganic counterparts, one reason being that these materials are almost always applied in a disordered, amorphous phase despite organic crystals having superior charge carrier mobilities and diffusion lengths. The focus of this thesis is to further our understanding of organic crystal growth and their integration into future devices. Beginning with crystal growth, we expand the number of known organic materials that are able to grow large, single crystal domains as a thin film. Compiling the thermal and structural properties of these materials we develop a better understanding of what controls crystal growth and morphology allowing for easier identification of new candidate materials. Next, we turn to investigating growth methods that will allow these thin film crystals to be incorporated into devices. Homoepitaxy, which permits tuning the thickness of a crystalline template, is found to result in remarkably smooth thin films but only for materials with low molecular aspect ratios. These molecules experience low energy barriers to diffusion on a crystalline surface. Finally, heteroepitaxy on crystalline templates is studied due to the prevalence of heterojunctions in organic devices. In this work we show one-to-one, commensurate growth between two lattice matched materials, a first for organics, and lay out criteria dictating heteroepitaxy with perfect registry.