Understanding The Morphology At Donor Acceptor Interfaces In Organic Semiconductors

Organic electronic devices, including organic photovoltaics (OPV), organic light-emitting diodes (OLEDs), and organic field-effect transistors (OFETs), have become increasingly important in consumer electronic applications due to the development of organic semiconductors, including organic small molecules, and conjugated polymers. They have advantages such as light weight, flexibility/stretchability, and the ability for roll-to-roll manufacturing. The structure and mechanism of organic devices are analogous to inorganic semiconductor devices, where donor materials (p-type) and acceptor materials (n-type) are used to create interfaces. To build high-performance organic electronic devices, it is essential to understand functionalities of organic heterojunction because they are building blocks of electronics. Organic heterojunction is interfaces created between donor and acceptor organic semiconductors. Exciting electronic action of devices occurs at organic interfaces. From a fundamental viewpoint, the role of interfaces must have optimal electronic and physical communication to yield highly efficient devices. From a technological viewpoint, one must understand, control, and have a rational design of the desired electronic and optical properties at organic interfaces for the development of different electronics and a host of potential new device concepts that have not yet been developed or realized. In this dissertation, we will use organic semiconductors for organic photovoltaics (OPV) as an example to investigat organic heterojunction interfaces, including interface fabrication, epitaxy, morphology control, and characterization, with the aim of building high-performance devices with good stability. We begin by discussing the growth of organic single-crystalline crystals with controlled orientations. Materials used are two small molecules: zinc phthalocyanine (ZnPc, p-type) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, n-type). In this study, a self-built vertical physical vapor transport (v-PVT) chamber is used for crystallization, and a graphene-coated substrate is used to control molecular packing. Although ZnPc and PTCDA have a planar molecular shape and face-on packing motif on graphene, we find that they have different growth modes. Such growth mechanism difference can be explained by competition between intermolecular and molecule-graphene interactions. We then continue the abovementioned study by building model heterojunctions on graphene substrates using ZnPc and PTCDA. We discover that thermodynamics and kinetics of the system affect P-N junction morphology. We find that ZnPc and PTCDA form the "line-on-line" organic weak epitaxy at heterojunction interfaces from X-ray studies and crystallography refinement. We also verify that P-N junctions can generate electron-hole pairs. This work will advance the knowledge and create enabling opportunities to fabricate single-crystalline-oriented nanostructures. Besides using organic small molecules, we also explore the structure-property-performance relationship of conjugated block copolymers (BCPs) for OPV applications. In this work, a new donor-acceptor BCP is synthesized and added into polymer blend solar cells. We find that adding BCP could potentially retain the relative degree of crystallinity of [pi]-[pi] stacking regions, and decrease the detrimental interaction between donor polymer and electrode under thermal stress, thus improving the solar device's thermal stability by 30%. Finally, we explore the possibility of using graphene engineering for epitaxial growth dynamics control of organic small molecules. We determine that two distinct, alternating morphologies of ZnPc crystals are simultaneously observed on a single epitaxial SiC-graphene substrate. We hypothesize that the different morphologies arise from electronic structure and surface energy differences of underlying SiC-graphene regions ZnPc is grown on. The result will enable selective patterning of organic semiconductors for use in advanced warfare device applications. We hope these studies throughout this work will advance knowledge on fundamental crystallization mechanisms, interface engineering, morphology control, and characterization in organic crystalline systems. This work could further produce various future architectures for different applications in organic electronics.