Understanding the Roles of Intra- and Interchain Order on Spin-Dependent Electronic Processes in Self-Assembled Conjugated Polymer Aggregates

University Of New Mexico, Albuquerque NM

Investigators

Abstract

With this award, the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division is supporting Professor John Grey of the University of New Mexico to study spin-dependent processes in self-assembled polymeric systems. Conjugated polymers are large chains consisting of a basic molecular repeat unit that absorb light and conduct charges making them excellent candidates for next-generation optoelectronic applications. For example, these so-called 'plastic' semi-conductors are used in solar cells and light emitting displays, which may eventually replace existing technologies owing to their ever increasing efficiencies over the last 10 years and relatively low cost. The ways in which the polymer chains fold and pack in thin films has a significant impact on the overall performance of organic solar cells and light-emitting devices (OLEDs). The proposed research will use new strategies to fabricate ordered polymer structures and study how electrical charges are formed and how electrical current can be generated more efficiently in the resulting films. In addition, the PI and his NSF-sponsored graduate students actively participate in ongoing educational and research infrastructure development programs at the University of New Mexico. These activities include coordinating with the local Center for Integrated Nanotechnologies (CINT) to increase the participation of undergraduate and graduate researchers through collaboration with CINT staff scientists. Students take part by writing user proposals that undergo peer review thus providing valuable experience in designing and executing an independent research project. Lastly, student participants gain valuable and diverse skill sets in academic fields essential for national interests. The interactions between and inter-conversion of electrically neutral (exciton) and charged (polaron) states of different spin in conjugated polymer materials has significant bearing on their overall performance in cutting-edge optoelectronic devices, such as solar cells. Although the outcomes of these processes are strongly dependent on polymer structure, detailed molecular level structure-function relationships have proven difficult to obtain from conventional polymer functional forms (i.e., bulk thin films). This difficulty originates from morphological heterogeneity due to intrinsic molecular weight poly-dispersity of polymers. The proposed research seeks to overcome heterogeneity effects to understand how molecular structure regulates interactions and inter-conversion of excitonic and polaronic spin states. Self-assembly approaches are used to fabricate well-defined polymer aggregate supramolecular nanostructures that can be exploited to selectively control conformational and packing (intra- and interchain) order and electronic coupling. The Grey group at the University of New Mexico has pioneered this approach to direct exciton coupling and polaron interactions in polymer aggregates that are leveraged in the proposed research to address longstanding fundamental questions involving spin state exchange interactions and crossover in polymeric semiconductors. This research also takes new directions for interrogating the fates and interactions of different spin states in individual polymer nanostructures by introducing sensitive and selective electric- and magnetic-field dependent molecular spectroscopic and imaging techniques. The research aims to establish design rules for reliably tuning subtle polymer chain packing and order within supramolecular assemblies to understand and control exchange interactions and spin state inter-conversion for improved material performance.