Understanding and Engineering Molecular Order in Organic Semiconductors

Abstract

Organic semiconductors often exist in disordered material phases which have sub-optimal optical and electrical properties. Bringing some degree of order to these materials with crystals and even oriented amorphous phases has been shown to be fruitful for many applications, but is challenging to achieve. This is largely because of the variability between different materials and poorly understood dynamics in device-relevant thin films. This thesis describes progress towards understanding and tuning crystallization and ordering in organic thin films to realize enhancement in parameters relevant to organic optoelectronic devices. In particular, this thesis demonstrates that in thin-film crystallization processes, the crystal structure, crystal shape, and growth type can be controlled most effectively with film thickness, temperature, and strategic incorporation of secondary additives within the film. These variables change the rate at which crystals grow as well as the crystal shape during growth by altering the ability of molecules to attach and conform to the growing crystal front. When films are heated to bring about these processes through increased molecular mobility, secondary processes may occur to transform the microscopic film morphology through the addition, subtraction, and long-range motion of material. This motion can be connected to substrate-film interactions and the material phase of the starting film. Material interactions within the film bulk can kinetically trap molecular conformations, with the extent of this trapping depending on interaction type and deposition conditions. These properties can be further exploited to produce useful and spontaneous structures within a thin film. Ultimately, the desired result of ordering an organic semiconductor is to produce more efficient and stable structures for devices. This is demonstrated here through engineering the motion of excited states with crystallization, then applying such techniques to different organic solar cell geometries to study how different crystallization methods affect device properties. First, the mobility of excited states in boron subphthalocyanine chloride (SubPc), an archetypal organic solar cell molecule, is shown to increase upon crystallization with rigorous calculations explaining the origin. Such an increase provides motivation to study the effects of induced crystallization via homoepitaxial growth using the well-studied transistor material rubrene in a solar cell geometry. This serves as a platform by which to generalize the study to mixed epitaxial growth in a material which is not intrinsically crystalline, showing marked morphological and electronic changes with changes in mixture composition. This is ultimately applied to heteroepitaxial growth of SubPc, which does not crystallize when deposited onto a template at ambient temperature. These projects explore crystallization techniques as a solution to improve the performance of organic solar cells, resulting in an improved fundamental understanding of such processes and avenues for future progress.