What Is Special About the Low Bandgap Polymers: From Local Molecular Properties to OPV Device Performance?
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 ~100 fs intramolecular donor dynamics and corresponding device power conversion efficiencies. These 100 fs dynamics correspond to the tendency of polymers to dissociate their own excitons into intramolecular charge-transfer or charge-separated populations in the absence of an external acceptor molecule.
These correlations can be explained if molecular dynamics are included in the traditional models, allowing for intramolecular exciton dissociation cost defrayment prior to donor-acceptor charge transfer, and therefore more facile exciton dissociation. The results imply that charge-transfer polymers, which are already achieving record-breaking efficiencies, can be predictably altered to enhance corresponding device efficiency by optimizing the electron-withdrawing or -pushing interaction of neighboring backbone building blocks to facilitate exciton dissociation.