UC Santa Barbara

Organic photovoltaics (OPVs) based on conjugated polymer and small molecule semiconductors are a promising technology for solar energy capture. The compatibility of organic semiconductors with solution processing allows OPV devices to be prepared using high-throughput manufacturing processes on flexible substrates, opening up applications not available to most commercial technologies. The most efficient devices utilize a bulk heterojunction (BHJ) active layer, a two-component solid state mixture that provides efficient photocurrent generation, but also presents several performance-limiting issues. The consequences of structural and energetic disorder in these systems can be difficult to quantify using existing experimental techniques. This dissertation describes three studies presenting new approaches to measuring the morphological, energetic, and interfacial properties of BHJ solar cells. First, a study of static disorder in the density of states (DOS) is carried out in-situ in several BHJ blends. DOS tail states are characterized here by combining the charge carrier density dependence of Voc with disorder-dependent band bending behavior. A direct connection is found between disorder observed within donor and acceptor phases and the overall impact of disorder on open-circuit voltage. Disorder is shown to be both material- and morphology-dependent, significantly impacting device performance. Next, a new technique, polarization-dependent photoconductive atomic force microscopy, is used to measure local orientation-dependent photocurrent in BHJ active layers with resolution below 100 nanometers. A high-performing small molecule donor is found to exhibit in-plane, micron-scale orientational order not seen by typical diffraction or topography measurements. The observed liquid-crystalline networks provide a unique picture of morphology that goes beyond the typical binary of crystalline vs amorphous. Lastly, a technique is presented to measure the ion transport properties in conjugated polyelectrolytes (CPEs) used as interlayers in OPVs. Ion motion is characterized using Kelvin probe force microscopy to observe the relaxation over time of bias-stressed CPE device channels. Based on studies of systematically varied CPE structures, the number of ionic side chains per monomer shows a strong correlation with ionic conductivity. These results help inform the material design of CPEs, not just for interlayers in organic optoelectronic devices, but also for a number of emerging mixed ionic-electronic conductor applications.