Ultrasonically Synthesized Self-Sintering Field’s Metal Semiconductive Inks towards water processable Organic Solar Cell Electrodes

Abstract

Field’s metal (FM) is a eutectic alloy of bismuth, indium, and tin that offers high electrical conductivity and a low melting point, which opens its potential application in flexible electronics. In view of preparing water processable organic solar cells, Field’s metal microparticles were synthesized via ultrasound in an ethanol–water medium, using polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA) as stabilizers. By XRD, the In4Bi2 crystalline phase was found, for both samples. DSC measurements showed a melting point at ~61 °C with low-temperature crystallization peaks at 22 °C, consistent with super-cooled core-shell particles with a liquid metal nucleus, coated with oxide and polymer layers. SEM analysis revealed that spherical particles are formed with both stabilizers, but the PVP-passivated samples exhibited a larger size distribution (4.3 m vs 0.84 m) and rougher surfaces, suggesting a stronger adhesion of PVP to the FM surface. This results in a longer stability of these FM-polymer particles suspension; in fact, PVA-coated particles flocculate after 10 minutes, while those in the PVP suspension are maintained up to 1 hour. Inks were thus prepared from PVP-coated particles with cellulose as base. HCl was also added to the reaction mixture to remove the oxide layer, which was corroborated with the appearance of the FM bulk peak in the thermogram, after HCl treatment. This process likely promotes particle coalescence as the dynamic light scattering (DLS) sizes increase in the inks. The removal of the insulating oxides and the particles coalescence allow to obtain semiconductive inks with resistance in the k order, which can be improved with the ink formulation optimization using conjugated polymers as co-passivants, work in progress.

Keywords: Field’s metal; semiconductive inks; flexible electrodes.