Studying the Density of States of buried interfaces in organic semiconductor thin films using electronic sum frequency generation

Abstract: New nanostructured semiconductor materials such as nanocrystals and organic semiconductors constitute an attractive platform for optoelectronics design due to the ease of their processability and highly tunable properties. Incorporating these new nanostructured materials into electrical circuits requires forming junctions between them and other layers in a device, yet the change in dielectric properties about these junctions can strongly perturb the electronic structure of the two layers. Specifically, the morphology of the interface between two materials greatly affect their ability to transfer charge and energy through the system, and the method through which this energy travels across a junction is poorly understood. To study these processes, an interfacial technique is required that measures the Density of States (Dos) at buried interfaces in working devices. In this thesis, we adapt an interface-selective optical technique, electronic sum frequency generation (ESFG), to study the dynamics of energy transfer across interfaces in these materials. We begin by developing “direct” detected ESFG to study the electronic states and morphology at the interface of thin films made from known organic semiconductor materials. Using direct ESFG, we examine the differences in the DoS at an interface in an organic thin film relative to its bulk. Through polarization optics, we study morphological changes in the film caused at the junction of the OSC and substrate. To account for interference from multiple ESFG active interfaces present in a thin film, we use a modeling system to separate contributions to the measured ESFG signal from the air exposed and buried interface of interest. We then adapt the direct detected ESFG to “heterodyne” detected ESFG (HD-ESFG), which significantly increases the detection ability of the ESFG spectrometer. Additionally, HD-ESFG allows us to measure the phase of the materials response, which direct ESFG cannot. This phase information can give a better understanding of the morphology at the interface and additional inputs for thin film interference modeling to better deconvolute the signal from the buried interface.