This presentation will cover some emerging ideas in our laboratories for designing organic semiconductors with properties that make them relevant for established optoelectronic devices and that may open up new opportunities for materials design. The topics to discuss include the following:
(a) The synthesis, design and processing of regioregular narrow bandgap conjugated polymers containing alternating cyclopentadithiophene (CPDT) and pyridyl[2,1,3]thiadiazole (PT) subunits in the backbone. When processed atop nanostructured substrates under conditions that allow slow evaporation of solvent, it is possible to obtain fibrous thin films with a very high degree of structural order from which one can fabricate p-type thin film transistors with mobilities in excess of 25 cm2/V.s. The synthetic methodology can be applied for the preparation of more complex polymer architectures, for example a backbone comprised of CPDT-PT-IDT-PT repeat units (IDT = indacenodithiophene) and strictly organized PT orientations, such that the pyridyl N-atoms point toward the CPDT fragment. This CPDT-PT-IDT-PT copolymer is noteworthy because the Voc value of bulk heterojunction solar cells is particularly high (~0.86 V) for a donor polymer with a band gap < 1.5 eV.
(b) A relatively new class of anionic narrow bandgap conjugated polyelectrolytes (NBGCPE) is interesting because they are easily doped in water and can be used to provide conductive thin films. The choice of pendant ionic group and the ionization potential of the internal repeat units within the backbone is critical for favoring the doped state in aqueous media. These NBGCPEs are excellent dispersants for single walled carbon nanotubes (SWCNT). One finds that it is possible to obtain predominantly either n- or p-transport in the NBGCPE/SWCNT blends by the choice of the charged group in the pendant group, while keeping the backbone structure the same. These blends can be therefore used to fabricate thermoelectric devices.
(c) We will also examine how to provide semiconductor materials that can be used to fabricate organic solar cells using environmentally benign solvents. These considerations are particularly relevant in view of emerging chemical sustainability regulations and the need to minimize the use of toxic solvents in mass fabrication protocols.