Morphology Control in Polymeric Materials: Structural Monitoring with Scattering

Wei Chen (Argonne National Laboratory)

Polymers have shown great potential in practical applications from our daily lives to advanced technologies, arising from the ability to self-assemble into various structures at different length scales. However, one of the challenges for optimizing performance and developing novel applications is to control their self-assemblies precisely. Consequently, further advances in the understanding of polymers, especially in the control of morphology, are required. Our studies have demonstrated a reversible photocontrol over the disorder-to-order transition in thin films of anthracene-functionalized deuterated polystyrene-block-poly(methyl methacrylate) (d8-PS-b-PMMA) block copolymers by taking advantage of photodimerization and thermal dissociation of anthracene. This is potentially used to create long-range laterally ordered nanostructures over macroscopic distances in a non-invasive manner by photocombing. On the other hand, using a suite of tools spearheaded by neutron scattering, we have determined that the superior performance of PTB7:fullerene bulk heterojunction (BHJ) solar cells, one of the best-performance organic photovoltaic (OPV) systems, is attributed to hierarchical nanomorphologies with optimum crystallinity and nanoscale intermixing of copolymers with fullerenes, which together promote exciton dissociation, and consequently, contribute to photocurrent. Fine-tuning morphologies from molecule ordering to nanophases enables us to understand the fundamental physics underlying the superior performance of PTB copolymers and, thereby, realize improvement in the performance of PTB:fullerene BHJ solar cells with power conversion efficiencies (PCEs) up to approximately 9%, very close to a commercially viable PCE of 10%.

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