Structure-Property-Performance Relationships for Organic Photovoltaics and the Utilization of Photoconducting Atomic Force Microscopy for Characterizing Organic Thin Films

Progress in the development of organic photovoltaics (OPVs) depends on a continually growing understanding of the effects of chemical composition or processing method on the optoelectronic and structural properties and, in turn, how those properties influence device performance. Unfortunately, no single characterization method can provide all of the necessary information to develop these structure-property-performance relationships. This thesis details examples of structure-property-performance studies in which multiple characterization methods are used to identify the root cause of limited device performance for a particular photovoltaic system. As a large part of this work, the refinement and utilization of a nanoscale characterization technique, namely photoconducting atomic force microscopy (pc-AFM) is presented, not as an alternative to other characterization methods, but as a unique approach to characterizing the nanoscale morphology and local optoelectronic properties of an organic thin film simultaneously. The goal of this work was to make pc-AFM as robust and reliable a characterization tool and as close an analog to bulk OPV performance testing as possible.;The first sections of this thesis focus on the development of pc-AFM for the characterization of OPVs. The capabilities of pc-AFM at the time this research commenced are illustrated in a study of a multilayered p/i/n architecture OPV system comprising a thermoset small molecule donor, tetrabenzoporphyrin (BP) and one of two structurally similar soluble fullerene derivative acceptors. By investigating the nanoscale topography, dark current, and photocurrent in each layer of these multilayer devices, the differences in bulk device performance can be rationalized and the composition of specific morphological features can be identified. At the same time, this study demonstrates how interpretation of pc-AFM measurements was not straightforward and required knowledge of the bulk performance. This issue prompted efforts to improve the technique to increase reliability and applicability to OPVs. Subsequently, sensitivity studies of the experimental parameters such as contact force, illumination intensity and spot size, and film processing conditions were performed, further elucidating factors that must be taken into consideration when designing and interpreting a pc-AFM experiment. In order to better control and monitor the environmental conditions during a pc-AFM experiment, a custom glovebox was designed to house and interface with the pc-AFM. Realization of this glovebox allowed further improvements in reliability of pc-AFM measurements and enabled the use of low workfunction AFM probes that better mimic the low workfunction top electrode typically employed in bulk OPVs.;The initial use of BP as a donor material in pc-AFM measurements led to the utilization of BP in other OPV systems due to its rare properties as a solution-processable, thermoset, small molecule electron donor. The latter sections of this thesis describe two structure-property-performance relationship studies utilizing BP as a donor material. In the first study, the use of BP enables the facile testing of solution processable bilayer OPV devices with perylene-based acceptors as alternatives to solubilized fullerene derivatives. The second study compares the optoelectronic and performance characteristics of BP to a Cu-metalized derivative, CuBP, which, despite the promise of this material due to its high charge carrier mobility measured in FETs, performs poorly in OPV devices.;Altogether, this work demonstrates the importance of developing structure-property-performance relationships, provides insight to the continued improvement of OPV performance, and sets the groundwork for the reliable utilization of pc-AFM as a technique for characterizing organic thin films.