Highly Conductive Cu 2– x S Nanoparticle Films through Room-Temperature Processing and an Order of Magnitude Enhancement of Conductivity via Electrophoretic Deposition

© 2014 American Chemical Society. A facile room-temperature method for assembling colloidal copper sulfide (Cu2-xS) nanoparticles into highly electrically conducting films is presented. Ammonium sulfide is utilized for connecting the nanoparticles via ligand removal, which transforms the as-deposited insulating films into highly conducting films. Electronic properties of the treated films are characterized with a combination of Hall effect measurements, field-effect transistor measurements, temperature-dependent conductivity measurements, and capacitance-voltage measurements, revealing their highly doped p-type semiconducting nature. The spin-cast nanoparticle films have carrier concentration of ∼1019 cm-3, Hall mobilities of ∼3 to 4 cm2 V-1 s-1, and electrical conductivities of ∼5 to 6 S·cm-1. Our films have hole mobilities that are 1-4 orders of magnitude higher than hole mobilities previously reported for heat-treated nanoparticle films of HgTe, InSb, PbS, PbTe, and PbSe. We show that electrophoretic deposition (EPD) as a method for nanoparticle film assembly leads to an order of magnitude enhancement in film conductivity (∼75 S·cm-1) over conventional spin-casting, creating copper sulfide nanoparticle films with conductivities comparable to bulk films formed through physical deposition methods. The X-ray diffraction patterns of the Cu2-xS films, with and without ligand removal, match the Djurleite phase (Cu1.94S) of copper sulfide and show that the nanoparticles maintain finite size after the ammonium sulfide processing. The high conductivities reported are attributed to better interparticle coupling through the ammonium sulfide treatment. This approach presents a scalable room-temperature route for fabricating highly conducting nanoparticle assemblies for large-area electronic and optoelectronic applications.