Intellectual Merit - Conjugated polymers (CPs) are a promising class of materials for use in the conversion of solar energy to electricity. For optimal performance in bulk heterojunction CPs, the morphology of the donor and acceptor materials must form percolating interpenetrating networks maximizing interfacial contacts w/ length scale of ~10 nm. Currently, we lack the fundamental understanding to guide the formation of bulk heterojunctions to these targeted nanoscale morphologies. In this collaborative proposal, an understanding of the fundamental driving forces that govern the nanoscale self-assembly and interfaces in conjugated block copolymer (BCP) thin films will be developed in order to enable the rational design and fabrication of the targeted bicontinuous nanoscale morphologies. This will be realized by completing an interdisciplinary research program that will detail the thermodynamic driving forces that control the formation of a bicontinuous interconnected percolated morphology in a thin film of conjugated BCP with controlled rigidity on a surface that is patterned incommensurately to the periodicity of the diblock copolymer as well as the synthesis and thin film structure of conjugated diblock polymers that exhibit traditional diblock morphologies. Therefore upon completion, we will attain an understanding of the thermodynamics that control the assembly of these systems in thin films; enabling the reproducible creation of the desired bicontinuous interconnected morphologies with this structure, providing a transformative method to rationally design, tailor and fabricate nanoscale morphologies with exquisite control of size and thickness for CP systems. The successful completion of these experiments will broaden the range of nanoscale thin film morphologies that can be targeted and rationally tuned in conjugated polymer thin films, and thus allow a systematic study of CP morphology on organic photovoltaics, a critical area in their optimization, yet a parameter that is not currently controllable experimentally.
Broader Impact - This project is an integrated collaborative effort between Chemistry, Chemical Engineering, and Materials Science research groups at the University of Tennessee. The broader impacts of the proposed program are embodied in this interdisciplinary collaboration, as well as the educational experiences to which it will lead. In the course of this project, the PIs will continue their outreach programs and use their research to provide training experiences for undergraduate and high school students and K-12 teachers, as well as provide input to their own teaching and exciting areas for discussion at K-12 visits. The execution of this collaborative project will also develop an interdisciplinary system of instruction, via classroom and laboratory, for training graduate and undergraduate students in chemistry, materials science, and mathematics who will be equipped to tackle modern science and engineering challenges. This project will also further develop the sustainable research infrastructure in Tennessee, an EPSCOR state, and will be implemented to ensure the participation of underrepresented groups in this research.
Organic photovoltaics (OPV) are promising materials to convert sunlight to electricity, as they are easy to fabricate, flexible, and lightweight, where the most promising OPV active layers consist of a mixture of conjugated polymers and fullerenes. However, in order for these materials to become economically competitive with silicon based solar cells, their efficiency must be improved. The research completed in this program develop a novel processing procedure, called solvent vapor annealing, that provides a method to tune the structure of polymer:fullerene mixtures in the OPV active layer with more precision and control than previously used techniques, such as thermal annealing or the addition of solvent additives. Moreover, the precise structure of the conjugated polymer:fullerene mixture is determined with neutron scattering and correlated to the photovoltaic performance of the mixture. In this way, we provide scientists who are interested in optimizing the structure and function of organic photovoltaics a new and readily controllable method to improve their structure and performance. More broadly, the work completed in this research program provides a unique method to exquisitely control and optimize the morphology of the polymer mixture thin films, and will be generally applicable to direct the morphology of a broad range of promising polymer nanocomposites as next-generation functional materials. At the same time, these novel processes have the potential to be used in a wide range of industries that will allow the processing of thin films at low temperature, thus lowering their environmental impact. This research program has also trained post-doctoral fellows (2), graduate students (2), and undergraduates (1), providing them with skills that are required to become an asset to the future workforce in the polymer, organic photovoltaic, and nanotechnology industries. Prof. Dadmun has also spent visited local primary schools demonstrating the concepts of temperature, recyclability, and novel materials to multi-grade advanced learning classes, introducing these young minds to problems that can be addressed with science.
