Tuning of Morphological, Crystallographic and Optoelectronic Properties of CuSCN By Electrodeposition

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 thin film devices such as dye-sensitized and perovskite solar cells [1]. However, in fact little is known for its intrinsic physical properties 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 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 the 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.Morphologies of CuSCN thin films electrodeposited from stoichiometric (REF), Cu-rich and SCN-rich baths are compared 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. 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 is made of elongated platelets (Fig. 1 b) and strongly oriented to lay down the c-axis in parallel with the substrate.As expected from the morphology, the Cu-rich film is highly transparent, whereas the other two are opaque due to light scattering (Table 1). Tauc plot for indirect transition indicated a systematic increase of bandgap from 3.58 to 3.64 eV on increasing the Cu2+ content. More significant difference was found for their work function (WF) measured by photoelectron yield spectroscopy (PYS). The threshold energy moved downwards from 5.31 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.Enhanced photoluminescence has also been found for the electrodeposited CuSCN thin film especially for the SCN-rich sample, as compared to commercial CuSCN powder. Thus, electrodeposition has turned out to allow fine-tuning of physical properties of CuSCN for device applications.[1] Vinod E. Madhavan et al., ACS Energy Lett. 2016, 1, 1112−1117.[2] Lina Sun et al., Physics Procedia, 2011, 14, 12-24.[3] Yuki Tsuda et al., Chem., 2017, 148, 845-854. Figure 1