Speaker: Lin X. Chen
Affiliation: Chemistry Department, Northwestern University

Electronic Processes, Morphologies and Structural-functional Correlations in Conjugated Oligomers and Polymers for OPV and Photocatalysis

Conjugated polymers with charge transfer characters in a large extend are responsible for the recent advancement in organic photovoltaic (OPV) applications in bulk heterojunction (BHJ) devices. In the DOEBES supported Energy Frontier Research Center (EFRC) Argonne-Northwestern-Solar-Energy-Research (ANSER) Center, we have carried out collaborative studies on electronic processes and in-situ morphological development of these low bandgap polymers and small molecules using ultrafast optical spectroscopy, static and in-situ grazing incident X-ray scattering (GIXS). Conventional organic photovoltaic models, in which donor molecules are treated as anonymous electron sources and charge carrier diffusion channels, are challenged by near-infrared transient absorption results of low bandgap polymers indicating strong correlations between intramolecular donor dynamics in < 100 fs and corresponding device power conversion efficiencies. The other conventional model being challenged is the driving force for exciton splitting in the bulk heterojunction environment which has been described by the LUMO-LUMO energy off-set between conjugated polymer electron donor and fullerene derivative electron acceptor. Our study suggests the intramolecular charge transfer characters must be combined with local and global conformations of conjugated polymer chains to achieve the low band gap. Moreover, the morphology of the BHJ films is also investigated by in-situ GIWAS/GISAX methods including the effects of additives which suggest the interplays of the additives and the polymers in solution. The morphology of the heterojunction films has been correlated directly with the yield of the charge separation on time scales from femtosecond to microsecond. In addition, new photophysical studies are also carried out on a series of metal chelating conjugated polymers showing the capability and potential in photocatalytic hydrogen generation.

About the Speaker

Lin X. Chen is a Chemistry Professor at Northwestern University and a Senior Chemist in Division of Chemical Sciences and Engineering, Argonne National Laboratory. She received her B.Sc. from Peking University in chemistry, and Ph.D. from the University of Chicago in physical chemistry. After her postdoctoral research at the University of California, Berkeley, she joined Argonne National Laboratory as a staff scientist. Starting 2007, she joined the Chemistry Department in Northwestern University where she has four current research focuses: 1) making molecular snapshots and movies of the excited state molecule in photochemical process-es using ultrafast optical and X-ray spectroscopy and X-ray scattering at synchrotron sources and X-ray free electron laser source; 2) fundamental electronic processes in organic materials for photovoltaic device applications using ultrafast laser spectroscopy, theoretical modeling and grazing incident X-ray diffraction; 3) understanding roles of ultrafast and coherent electronic and atomic motions in transition metal complexes and nanoparticles in photochemical reactions, and 4) transient structures of metalloproteins on multiple spatial and temporal scales during their functions.