Gold Nanoparticle Assisted Self-Assembly and Enhancement of Charge Carrier Mobilities of a Conjugated Polymer

A composite of bithiazole-benzothiadiazole-based semiconducting conjugated copolymer and gold nano-particles (AuNP) was prepared in situ and characterized by transmission electron microscopy, therrnogravimetry, and UV-vis absorption spectroscopy. The polymer interacts with the nanoparticle surface through the nonbonding electrons of the nitrogen and sulfur atoms, which provides stability to the nanoparticles as well as planarity and rigidity to the polymer backbone. As a result, the effective conjugation length and delocalization of pi-electrons of the polymer improved as evident from 130 nm red-shift in the UV-vis absorption spectrum. The nanoparticle along with the chemisorbed layer of polymer acts as a template for the self-assembly of the remaining polymer which is dispersed in the solution through pi-pi-stacking and van der Waals interactions. The self-assembly process enhances the polymer packing as well as ordering as seen from the shorter d spacing and from the more than threefold increase in the intensity of X-ray diffraction of the composite film. The charge carrier mobilities in the short and long ranges were measured by flash-photolysis time-resolved microwave conductivity and space-charge-limited current methods, respectively, which showed enhancement for the composite material compared to the pristine polymer. A more significant increase was observed in the hole mobilities (more than 12-fold), and hence the p-type nature of the composite was further studied by preparing blend films with typical acceptors such as phenyl-C-61-butyric acid methyl ester (PCBM) and N,N'-bis(1-hexylheptyl)perylene-3,4-9,10-tetracarboxylbisimide (PBI). Due to its spherical geometry, PCBM was found to disturb the ordering of polymer chains in the composite, resulting in the lowering of photoconductivity signals. On the other hand planar PBI molecules coassemble with the composite leading to significant enhancement of photoconductivity. Thus, we demonstrated a versatile approach of controlling planarization, pi-stacking and ordering of a conjugated polymer leading to the improvement of optoelectronic properties using AuNPs as a template.