Epitaxial Electrodeposition of Hole Transport CuSCN Nanorods on Au(111) at the Wafer Scale and Lift-off to Produce Flexible and Transparent Foils

Abstract

The wide bandgap p-type metal pseudohalide semiconductor copper(I) thiocyanate (CuSCN) can serve as a transparent hole transport layer in various opto-electronic applications such as perovsksite and organic solar cells and light-emitting diodes. The material deposits as one-dimensional CuSCN nanorod arrays, which are advantageous due to their high surface area and good charge transport properties. However, the growth of high-quality epitaxial CuSCN nanorods has remained a challenge. Here, we introduce a low cost and highly scalable room temperature procedure for producing epitaxial CuSCN nanorods on Au(111) by an electrochemical method. Epitaxial CuSCN grows on Au(111) with a high degree of in-plane as well as out-of-plane order with +0.22% coincidence site lattice mismatch. The phase of CuSCN that deposits is a function of the Cu 2+ /SCN − ratio in the deposition bath. A pure rhombohedral material deposits at higher SCN − concentrations, whereas a mixture of rhombohedral and hexagonal phases deposits at lower SCN − concentrations. A Au/epitaxial CuSCN/Ag diode has a diode quality factor of 1.4, whereas a diode produced with polycrystalline CuSCN has a diode quality factor of 2.1. A highly ordered foil of CuSCN was produced by epitaxial lift-off following a triiodide etch of the thin Au substrate. The 400 nm-thick CuSCN foil had an average 94% transmittance in the visible range and a 3.85 eV direct bandgap.

Epitaxial Electrodeposition of Ordered Inorganic Materials

“…Epitaxial lift-off can also be used to produce flexible foils of semiconductors without the metal substrate. We have shown that flexible foils of the transparent hole conductor CuSCN can be produced using the scheme that is shown in Figure A . A buffer layer of single-crystal-like Au(111) is deposited onto Si(111) followed by the electrodeposition of CuSCN(001) using the method outlined above.…”

“…This is shown in the scheme in Figure 2A for the electrodeposition of the transparent hole conductors CuI and CuSCN. 4,39 If the Cu(II) is complexed with lactate or tartrate ions and reduced in an alkaline solution, the product will be insoluble Cu 2 O, a p-type metal oxide semiconductor. 25,42,16,26,27 Another example of a material produced by this method is the magnetic material magnetite, Fe 3 O 4 .…”

“…Namely, a Cu(II) EDTA complex is electrochemically reduced in the presence of SCN − ions. 39 For the substrate in this case we used a 28 nm thick film of [111]-oriented Au on Si(111) as an inexpensive proxy for single-crystal Au(111). 50 An X-ray pattern of the CuSCN on Au(111)//Si(111) is shown in Figure 4F.…”

“…We have shown that flexible foils of the transparent hole conductor CuSCN can be produced using the scheme that is shown in Figure 6A. 39 A buffer layer of single-crystal-like Au(111) is deposited onto Si(111) followed by the electrodeposition of CuSCN(001) using the method outlined above. Although Au is normally considered to be inert, it can be etched using a triiodide etch, and the CuSCN(001) foil can be removed by epitaxial lift-off.…”

Epitaxial Single-Domain Cu-BTC Metal–Organic Framework Thin Films and Foils by Electrochemical Conversion of Cuprous Oxide

“…Electrochemical epitaxy including electrodeposition and electro-conversion is a low-cost and scalable technique for growing highly ordered thin films. − The conventional electrochemical method using Cu metal as the precursor can only produce polycrystalline Cu-BTC films by anodic dissolution–reprecipitation . Textured Cu-BTC films with only out-of-plane order can be produced by layer-by-layer liquid phase epitaxy using a surface-functionalized Si or Au substrate by functional groups (such as −COOH or −OH).…”