The molecular origin of electrical conductivity in hybrid inkjet-printable nanoinks
Hybrid nanoparticles with a metallic core and an organic (semi-)conducting ligand shell are the basis of our sintering-free inks for conductive inkjet printing. Gold nanoparticles with spherical or rod-like shape and dimensions below 100 nm were chemically synthesized and conductive polythiophene ligands such as poly(2-(3-thienyl)-ethoxy-4-butylsulfonate) (PTEBS) were attached to the gold surface in a ligand exchange reaction (see figure 1 a). The polymer shell facilitates inter-particle electron-transport, resulting in highly conductive structures upon drying of the deposited ink. [Reiser et al.] We used the hybrid nanoparticles to formulate inkjet-printable inks that are stable in different polar solvents for at least one year. We inkjet-printed structures with dimensions of 100 µm to 1 mm on various substrates including flexible materials such as paper and polymer foils (see figure 1 b). The printed structures had specific conductivities on the order of 1.0 x 106 S/m immediately after printing. Printed structures on flexible substrates stayed conductive even upon mechanical deformation.
The molecular arrangement of conductive polymers influences carrier mobility and carrier transport, a well-known fact in organic photovoltaics. [Pandey et al.] We studied the interaction of the metallic core and the soft polymer shell and their effect on conductivity. Carrier transport within semiconductor polymers is dominated by inter-chain and intra-chain hopping processes. [Qu et al.] Efficient charge transport in polythiophenes is therefore strongly affected by the molecular arrangement of the conjugated units. We characterized both spherical and rod-like hybrid particles using Raman spectroscopy and X-ray photoelectron spectroscopy to determine the structure of the polymer shell. We discuss which polymer motif binds to gold and relate it to the molecular orientation of the polymer chains in the ligand shell. These differences correlated with the electrical properties of hybrid particle films.