Technical: The goal of this proposal is to study how local nanoscale heterogeneity affects the performance of organic semiconductors using electrical scanning-probe microscopy. By bringing novel experimental capabilities to directly measure nanoscale variations in the electronic properties of model organic semiconductor films and study at the microscopic level how heterogeneity arising from variations in processing parameters leads to variations in performance. This project will address unique new challenges including: 1) Studying the evolution of local current distributions as a function of processing-induced disorder in charge transport in organic semiconductors; 2) Obtaining local information about vertical film structure to construct a picture of 3D morphology in photovoltaic blends using local scanning probes. 3) Using time-resolved electrostatic force microscopy to study trapping, de-trapping, and photochemical degradation kinetics on sub-100 microsecond, and sub-100 nm scales to probe correlations between nanoscale film structures and trapping in organic semiconductor films. These objectives are of fundamental scientific importance to the field of organic semiconductors and address basic description of transport processes in these systems: if 1D models are insufficient, what is the most physical yet efficient way for 3D models to include +the requisite spatial disorder? How are the disorder parameters in theory linked to experimental processing conditions and observables? While the morphology/performance relationship in organics is widely recognized as a central challenge that affects the field from theory to synthesis to manufacturing, most methods of characterizing morphology permit only a correlation of average performance with average local structure and thus overlook the important role of local heterogeneity. This proposal will help transform the understanding and use of organic semiconductors.
The project addresses basic research issues in a topical area of materials science with technological relevance, and is expected to provide unique opportunities for graduate and undergraduate training in an interdisciplinary field. This research project is also expected to have broader impacts through the training of scientists in this research field, through the wide dissemination of the findings of this research through publications. The proposed research program will take place at the intersection of chemistry, physics, and materials science, and has technological applications in the areas of photovoltaics, energy efficient displays, and lighting. This proposal will support established educational programs including ongoing recruitment of under-represented groups into scientific careers, public outreach to K- 12 students, and broader interactions with the local community. The project will create new undergraduate research projects, international collaborations, and new industrial educational experiences and partnerships.
Organic semiconductors are a technology for producing thin film electronic devices with applications in thin film transistors, displays, and solar cells – for instance many new smartphones use organic light emitting diode (OLED) displays for richer color and improved battery life. This project’s goal was to apply new microscopy methods to study how nanoscale variations in the texture of solution processed organic semiconductors can arise from different production methods and can ultimately affect the performance of the resulting organic semiconductor films. The project was extremely successful and produced the first direct nanoscale images of trap (defect) formation in model polymer semiconductor films, made critical contributions to understanding how scanning probe microscopes can be used to image small, fast processes, and produced the first nanoscale differential aging maps that revealed how materials composition and processing conditions can affect long-term materials performance. These results will ultimately help us better understand how to prolong the lifetime of devices made from organic semiconductors, which may lead to longer-lasting consumer electronics, and perhaps even help make low cost plastic photodectectors and solar cells a reality. In addition, the program was successful at providing hands-on research training for a number of undergraduate and Ph.D. students in interdisciplinary science at the ntersection of chemistry, physics, and materials science. The award also supported the participation of the investigator and his students in a large number of public science education events, with total attendance of over 20,000 people.
