(522c) Self -Assembly of Conjugated Diblock Copolymers within Nanofibers for Solar Cell Applications

We report the fabrication and self-assembly characterization of nanofibers of oligothiophene-block-oligophenylenevinylene (OT-b-OPV), a new class of fully conjugated diblock copolymers, using a technique called electrospinning. Electrospinning is a fiber formation technique that uses strong electric field to accelerate and thin a polymer melt/solution jet forming nanoscale fibers with diameters in the range of 50-500 nm. The unique self assembly between electron conducting (OPV) and hole conducting (OT) blocks will minimize recombination of electrons and holes created on photo-excitation of the active layer in photovoltaics, which can potentially increase the efficiency of organic solar cells. The formation of nanofibers of these unique materials will facilitate the development of ultra-light weight breathable smart textiles with integrated devices.

Prior to electrospinning, solution-cast films of OT-b-OPV were fabricated and characterized to better understand the effects of electrospinning on block copolymer self assembly. Within the thin film, covalently bonded OT and OPV blocks micro-phase separate to form self-assembly structures with approximately 5 nm domain spacing. In addition, the oligothiophene (OT) blocks exist as highly ordered crystalline domains with spacing of less than 2 nm through intermolecular packing with their alkyl side chains aligned normal to the substrate and the thiophene rings aligned parallel to the substrate through π-π stacking, thereby resulting in hierarchically-ordered self assembly.

Oligomers are not electrospinnable by themselves due to the lack of chain entanglement in solution, which is a prerequisite for the electrospinning process. To overcome this limitation, in this study, nanofibers were fabricated by coaxially electrospinning OT-OPV (oligothiophene-co-oligophenylenevinylene) as the core with poly vinyl pyrrolidone (PVP) as the shell. PVP serves as a spinning aid and allows the formation of a continuous OT-OPV jet in the core. Pure OT-OPV nanofibers were successfully obtained by selective removal of PVP shell post-electrospinning. The external nanofiber morphology was characterized using scanning electron microscopy (SEM). The block copolymer undergoes micro phase separation and exhibits hierarchical self-assembly inside the fibers as seen via both transmission electron microscopy (TEM) and wide and small angle x-ray scattering (SAXS/WAXS). Prior to TEM, nanofibers were first microtomed into 60 nm thin sections to study the self assembly inside nanofibers both along and perpendicular to the fiber axis.

Owing to the fast solvent evaporation and strong extensional deformation during electrospinning, the OT-OPV exhibit a disordered, but percolating self assembly within nanofibers   WAXS data reveals the reduction in the p-p distance in nanofibers compared to films. Similar effect has been seen in nanofibers of pure polythiophene (P3HT) compared to films of the same materials. Annealing studies of both films and nanofibers show an enhancement in the materials crystallinity, which is known to enhance conductivity.