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 DOE-BES 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 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 &lt; 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.

Electronic processes, morphologies and structural-functional correlations in low bandgap oligomers and polymers for OPV

253rd National Meeting (2017)

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