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 perovskite 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].

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.

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 thin film 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. 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.

XRD patterns have found almost parallel orientation for the SCN-rich and a high degree of preference of the Cu rich to orient the c-axis of β-CuSCN perpendicular to the substrate.

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 in the J-V curves, the onset voltage determined that to observe 5 mA/cm² 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 the electrodeposition. Successful operation of organic light emitting diode was also achieved by employing the electrodeposited CuSCN as a hole-injection layer. The device employing the electrodeposited CuSCN achieved a smaller turn-on voltage than that with spin-coated CuSCN in the I-V-L measurements.

[1] Vinod E. Madhavan et al., ACS Energy Lett. 2016, 1, 1112−1117.

[2] Lina Sun et al., Physics Procedia, 2011, 14, 12-24.