Highly Conductive Cu_2–xS Nanoparticle Films through Room-Temperature Processing and an Order of Magnitude Enhancement of Conductivity via Electrophoretic Deposition

Abstract

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 ∼ 10(19) cm(-3), Hall mobilities of ∼ 3 to 4 cm(2) 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 (Cu(1.94)S) 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.

Phase Transformations of Copper Sulfide Nanocrystals: Towards Highly Efficient Quantum‐Dot‐Sensitized Solar Cells

“…Electrical conductivity is an important parameter to characterize electronic transport properties . The conductivity of the Cu 2− x S NC films were measured by using a two‐point method based on a device structure of glass substrate/Cu 2− x S NC film/aluminum.…”

Prospects of Colloidal Copper Chalcogenide Nanocrystals

“…In fact, CdS/Cu 2 S heterojunction solar cells were among the earliest thin‐film solar cells to be investigated, but their further development was hindered by the low stability of the p/n junction, which deteriorated in air due formation of Cu vacancies and Cd–Cu interdiffusion . The possibility to make colloidal dispersions of monodisperse Cu 2− x S NCs has renewed interest in Cu 2− x S for PVs, as it allows the fabrication of solar cells by solution‐deposition techniques, such as ink‐jet printing by using nanocrystal inks. Nevertheless, ternary and quaternary copper chalcogenides have superior environmental stability and efficiency and are, therefore, preferred in the fabrication of PV devices (see Sections 3.3.2 and 4.3.1).…”

Cosolvent Approach for Solution-Processable Electronic Thin Films

“…For the Cu 2 S thin film, the conductivity decreases with decreasing temperature in the range 300 to 2 K (Figure 4b), showing a typical semiconductor behavior. 37 During the cooling process, the carrier mobility initially decreases and then increases below 100 K, which is also a common behavior of Cu 2 S. For the Cu 2 Se thin film, the overall conductivity does not change significantly with the temperature variation ( Figure 4e). The carrier mobility in Cu 2 Se shows similar behavior to that of Cu 2 S, which decreases first and then increases at low temperature.…”