EXPLORATION OF SURFACE POTENTIAL IN ORGANIC SEMICONDUCTORS:\ud STATIC BEHAVIOR, KINETICS, AND MORPHOLOGY

Organic electronic devices offer cheaper solution processability than their inorganic counterparts and allow for the vast tailorability of synthetic chemistry to tune properties and efficiency. A critical fundamental challenge is to understand the dynamics and mechanisms of charge transport, particularly the role of defects and traps. This is motivated in part by research we performed in the origins and occurrence of negative differential resistance (NDR) in phthalocyanine systems, discussed below. In this dissertation, I have explored the surface potential energy distributions of organic, semiconducting thin films via Kelvin probe force microscopy and analyzed the effects of disorder in the samples. Thin films of commonly used materials in organic electronic devices were tested, both on short and long time scales, and throughout these experiments, a previously unnoted asymmetry in the energetic distribution was observed. To determine the cause of this asymmetry, the energetic distributions were compared to a dynamic Monte Carlo simulation, with experimental and theoretical results suggesting nanoscale charge heterogeneity providing the greatest cause. These results were followed with additional experiments, first testing the evolution of the potential energy distribution over time and then testing intentionally patterned dual-component films to witness whether the asymmetry persisted or not. Over long scan times, the energetic distributions equilibrate to a more Gaussian distribution and shift in value, first quickly then more slowly, indicative of two different regimes of energetic disorder: shallow and deep, respectively. The patterned films were created using multiple shapes at varying sizes, and they displayed no correlation between the degree of material patterning and the appearance of asymmetry. This indicates that while phase segregation may affect the potential energy distribution in organic semiconductors, it is not the main cause of asymmetry exhibited and explored here. It is important to more completely explore how disorder affects these materials, as they are commonly researched and utilized for organic electronic devices. With a greater understanding of disorder, more powerful and efficient devices can be created