Stephanie L. Fronk

Varying the ionic functionalities of conjugated polyelectrolytes leads to both p- and n-type carbon nanotube composites for flexible thermoelectrics

Single-walled carbon nanotubes can be selectively doped by conjugated polyelectrolytes (CPEs) to form either p- or n-type composites. The selectivity of charge-transfer doping is found to be dictated by the polarities of CPE pendant ionic functionalities. This finding leads to a fundamentally new approach to both p- and n-type solution-processable composites for high performance, flexible thermoelectric devices.

Influence of molecular structure on the performance of low Voc loss polymer solar cells

Two regioregular narrow bandgap conjugated polymers (PM1 and PM2) containing the repeat unit BDT-PT-CPDT-PT (BDT = benzodithiophene, PT = pyridyl[2,1,3]thiadiazole, CPDT = cyclopentadithiophene) and different solubilizing alkyl side chains were prepared with the goal of understanding how chemical structure impacts the performance of low Voc loss bulk heterojunction (BHJ) solar cells containing PC61BM as the acceptor semiconductor. Both polymers show nearly identical orbital energy levels, a face-on orientation relative to the surface normal, and can be processed to yield continuous fiber-like networks in the active layer. Due to the choice of repeat units within the backbone structure, PM1 and PM2 exhibit shorter π–π stacking distances, relative to the previously reported low Voc loss regioregular polymer PIPCP. Finally, PM1 achieves an average PCE of 6.2 ± 0.2% and PM2 achieves an average PCE of 7.2 ± 0.1%. Devices exhibit low Voc loss and high short circuit current Jsc, but, most significantly, display improved fill factors compared to previously reported PIPCP. A discussion is provided that seeks to identify structural features in conjugated polymers that lead to devices with low Voc loss and high external quantum efficiencies.

Real-time observation of conformational switching in single conjugated polymer chains

Single-molecule experiments in organic solvents provide new insight into the conformational dynamics of conjugated polymers. Conjugated polymers (CPs) are an important class of organic semiconductors that combine novel optoelectronic properties with simple processing from organic solvents. It is important to study CP conformation in solution to understand the physics of these materials and because it affects the properties of solution-processed films. Single-molecule techniques are unique in their ability to extract information on a chain-to-chain basis; however, in the context of CPs, technical challenges have limited their general application to host matrices or semiliquid environments that constrain the conformational dynamics of the polymer. We introduce a conceptually different methodology that enables measurements in organic solvents using the single-end anchoring of polymer chains to avoid diffusion while preserving polymer flexibility. We explore the effect of organic solvents and show that, in addition to chain-to-chain conformational heterogeneity, collapsed and extended polymer segments can coexist within the same chain. The technique enables real-time solvent-exchange measurements, which show that anchored CP chains respond to sudden changes in solvent conditions on a subsecond time scale. Our results give an unprecedented glimpse into the mechanism of solvent-induced reorganization of CPs and can be expected to lead to a new range of techniques to investigate and conformationally manipulate CPs.