Tuning of Morphological, Crystallographic and Optoelectronic Properties in Electrodeposition of CuSCN for Device Applications

Abstract

Copper(I) thiocyanate (CuSCN) is known as a wide bandgap p-type semiconductor and recently demonstrated its high ability as a hole-transporting material in solar cells and light emitting diodes. Its thin film is usually fabricated by simple solution coating and drying [1]. In fact, little is known for physical properties of CuSCN, such as bandgap, band positions, optical transparency, carrier density and mobility.
We have established methods to electrodeposit well-crystallized CuSCN thin films in various forms. Although the electrochemistry is fairly simple as limited by diffusion of 1 : 1 complex between Cu²⁺ and SCN^- ions ([Cu(SCN)]⁺), the [Cu²⁺] : [SCN^-] ratio, its absolute concentration and solvent can significantly alter the morphology and crystal orientation of resulting CuSCN [2].
Hybridization with various cationic organic dyes has also been achieved to furnish nanostructures and even promote transition from rhombohedral β to orthorhombic α form [3]. These unique features of the electrodeposition technique let us anticipate possibilities to tailor-tune physical properties of CuSCN to match the demands for device applications. Moreover, electrodeposited CuSCN doesn’t hinder its use in flexible electronics unlike many other inorganic materials, since the process is done at room temperature.
In this study, we have carried out electrodeposition of CuSCN to vary its morphology and crystal orientation by tuning the bath composition and studied their band structure to explore the room for tuning its physical properties for diodes and hybrid electroluminescence devices.
Morphologies of CuSCN thin films electrodeposited from stoichiometric (REF), Cu-rich and SCN-rich baths are shown in Figs. 1 a-c. While the REF sample has an open structure made of relatively large bulky particles, the Cu-rich sample is dense, made of tiny grains to expose their hexagonally shaped top. XRD patterns have found almost random crystal orientation for the former and a high degree of preference of the latter to orient the c-axis of β-CuSCN perpendicular to the substrate. On the other hand, the one from the SCN-rich bath show rectangular sides of the grains (Fig. 1 a) and strongly oriented to lay down the c-axis in parallel with the substrate.
The Cu-rich film is highly transparent, whereas the others are opaque due to light scattering. Although all these films indicate nearly the same energy gap of about 3.6 eV estimated by Tauc plot, significant difference was found for their work function (WF) measured by photoelectron yield spectroscopy (PYS) (Fig. 2). The threshold energy moved downwards from 5.23 to 5.66 eV vs. VAC from Cu-rich to SCN-rich film. The result indicates a high level of p-type doping in the presence of excess SCN-, probably due to increased concentration of Cu2+ as stabilized by SCN- bound to it.
Simple devices as ITO/ electrodeposited CuSCN / Aluminium were fabricated to examine their diode behaviour. While all of the devices employing the electrodeposited CuSCN thin films showed good rectifications of the J-V curves, the onset voltage at 5 mA/cm² current moved from 1.21 to 2.57 V (ITO being positive) from Cu-rich to SCN-rich samples to confirm tunability of the device property by conditions of electrodeposition. Successful operation of organic light emitting diode was also achieved by employing the electrodeposited CuSCN as a hole-injection layer. Combination of photoelectron spectroscopies (XPS, UPS and IPES) is under way to examine the change of the band structure of CuSCN.