High-Performance Copper Nanowire/Graphene Hybrid Transparent Conducting Electrodes for Emerging Optoelectronic Devices
Transparent conducting electrodes (TCEs) based on indium tin oxide (ITO) have been widely used as an essential element of various optoelectronic devices, including liquid crystal displays, organic light-emitting diodes, touch screen panels, and solar cells. Vacuum-deposited ITO possesses good physical properties such as high optical transmittance and low sheet resistance as a TCE. However, it has several drawbacks such as brittleness, low optical transmittance, high refractive index, and high processing temperature. Furthermore, the price of ITO has been highly volatile recently, due to the scarcity of indium resources and the increased consumption of the material. Therefore, cheap, flexible, and solution-processed TCEs have been required for emerging optoelectronic devices such as flexible solar cells and displays. Recently, silver nanowire (AgNW) TCEs showed optical and electrical performance superior to that of ITO. However, the mass production of AgNWs is limited by its price and scarcity. Copper is 1000 times more abundant and 100 times less expensive than silver. Moreover, the electrical resistivity of copper is as low as that of silver, which has the lowest electrical resistivity. Therefore, a copper nanowire (CuNW) TCE has attracted considerable interest as a potential alternative to ITO and AgNW TCEs. More recently, researchers have shown that CuNW TCEs can possess remarkable physical properties such as excellent electrical conductivity, optical transparency, and mechanical flexibility. However, there is still an issue regarding long-term stability, which makes it difficult for practical use. Thus, it is necessary to suppress the oxidation of the CuNW in order to enhance the long-term stability of CuNW TCEs. Recently, many efforts have been made to develop novel protection layers for CuNW TCEs. However, these protection layers tend to cause a rough surface morphology, inefficient electrical connection, and diminished optoelectronic properties of CuNW TCEs. Therefore, the challenge still remains to develop a new protection layer which is cheap, gas-impermeable, and electrically conductive while maintaining optoelectronic properties and low cost. Here, I will present high performance CuNW/graphene hybrid transparent conducting electrodes based on copper nanowire@graphene core@shell (CuNW@G) nanostructures. The CuNW@G core@shell nanostructures were successfully prepared by using a low temperature plasma enhanced chemical vapor deposition process at temperatures as low as 400oC for the first time. The CuNW/graphene hybrid TCEs exhibited excellent optical and electrical properties comparable to those of conventional ITO. In addition, they showed remarkable thermal oxidation and chemical stability due to the tight encapsulation of the CuNW with gas-impermeable graphene shells. The potential suitability of the CuNW/graphene hybrid TCEs was demonstrated by fabricating bulk heterojunction polymer solar cells.