Gold Nanoparticle Assisted
Self-Assembly and Enhancement
of Charge Carrier Mobilities of a Conjugated Polymer
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Abstract
A composite of bithiazole–benzothiadiazole-based
semiconducting
conjugated copolymer and gold nanoparticles (AuNPs) was prepared in
situ and characterized by transmission electron microscopy, thermogravimetry,
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 π-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 π–π-stacking
and van der Waals interactions. The self-assembly process enhances
the polymer packing as well as ordering as seen from the shorter <i>d</i> 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<sub>61</sub>-butyric
acid methyl ester (PCBM) and <i>N</i>,<i>N′</i>-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, π-stacking, and ordering of a conjugated polymer
leading to the improvement of optoelectronic properties using AuNPs
as a template