Electrical, Structural and Compositional Characterization of Interlayer Material and New Active Layers in Organic Solar Cells

Abstract

This dissertation focuses on the electrical, structural, and compositional characterization of atomic layer deposited ZnO, and the structural characterization of modified titanyl (TiOPc 6 and TiOPc 9) and copper phthalocyanines (Pc 5). All materials studied have application in organic photovoltaic devices, either as an interlayer (ALD ZnO) or as part of the active layer (TiOPcs and Pc 5). The goals of this research are to advance the understanding of defect chemistry and electronic properties of ALD ZnO after exposure to oxygen plasma, Ar+ sputtering, and aryl phosphonic acid modifications, and to understand the relationship between the chemical structure of modified phthalocyanines and their molecular organization.Based on X-ray photoelectron spectroscopy and photoluminescence, it was determined that the predominant defects in as-received ALD ZnO are zinc vacancies mostly located in the top layer of the ZnO film. Oxygen plasma treatment of as-received ALD ZnO changed the predominant defects to oxygen interstitials, which migrated out of the sample when left exposed in air. Phosphonic acid modification of oxygen plasma treated ALD ZnO was found to suppress the migration of oxygen interstitials from the ALD ZnO sample.Ultraviolet photoelectron spectroscopy was used to study the electronic properties of ALD ZnO. It was determined that surface chemistry strongly influences the work function of ALD ZnO. Oxygen plasma treated ALD ZnO showed the highest work function and as-received ALD ZnO the lowest work function. The phosphonic acid modification of ALD ZnO decreased the work function and surface free energy when compared to oxygen plasma treated ALD ZnO. Near edge X-ray absorption fine structure spectroscopy results showed planes of benzyl rings in aryl phosphonic acid modifiers tilted at approximately 30 - 38° with respect to the surface normal.X-ray diffraction studies on modified phthalocyanines powders and thin films were performed to correlate their chemical composition with their crystal structure. It was determined that strong interactions between molecules lead to higher-order lattices (monoclinic or triclinic). However, upon annealing at higher temperatures, higher-order lattices were transformed to columnar phases because of the side chains incorporated into the modified phthalocyanines.