Electrical and optical properties of hybrid transparent electrodes that use metal grids and graphene films
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Wenjun Liu School of Electrical and Electronic Engineering (EEE), Nanyang Technological University, Singapore 639798
Ju Nie Tey, Chek Kweng Cheng, Eugene Kok, and Jun Weia) Singapore Institute of Manufacturing Technology, Singapore 638075 (Received 6 September 2012; accepted 13 November 2012)
There have been efforts to develop alternative transparent conductors to replace indium tin oxide (ITO). A hybrid transparent conductor that integrates a metallic Cu grid and graphene film promises to be a suitable candidate. Flexibility, sheet resistance, and transmittance comparable to ITO have been demonstrated. Here, we show that the optical and electrical properties of the hybrid transparent conductor can be easily tuned by clever design of the metal grid. The outcome of our study provides unprecedented guidelines for future design of metal grids integrated in transparent conductors. We also find that the graphene film forms an effective barrier to retard the degradation of the copper grid when the hybrid transparent conductor is heated in air up to high temperatures for an extended period of time. Hence, a superior hybrid transparent conductor, which can be carefully engineered to display desirable properties, has been demonstrated. I. INTRODUCTION
In recent years, there has been an increasing demand for transparent conductors. The growth in demand is anticipated to persist as optoelectronic devices and components, which utilize transparent conductors, become ubiquitous in our lives. Some of these devices include panel displays, touch panels used in tablet personal computers and smart phones, light-emitting diodes, and photovoltaic cells. Films made from doped metal oxides, especially indium tin oxide (ITO),1,2 are commonly used as the transparent conductors. Besides good electrical conductivity and optical transparency, the production of ITO films can be scaled up. However, ITO has several drawbacks. ITO is very brittle and can crack and fracture at relatively low strains,3 which limits its integration in flexible devices, an emerging area of electronic and optoelectronic systems. A component material of ITO, indium, is very expensive due to its scarcity. Production of ITO films via costly vacuum deposition technique also drives the cost of ITO films up. Hence, many research groups have explored various alternatives to ITO. Alternative metal oxides4–6 were considered but they also suffered the similar drawback of brittleness. Nanomaterials such as carbon nanotubes (CNTs),7–12 graphene,13–17 and metal nanowires18–21 were also examined. However, it a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.399 620
J. Mater. Res., Vol. 28, No. 4, Feb 28, 2013
http://journals.cambridge.org
Downloaded: 20 Jan 2015
remains challenging for any of these materials to approach sheet resistance of ;10 X/u and transmittance of ;90%, which is achievable for ITO. Films of CNTs and metal nanowires are not able to attain very low sheet resistance while maintaining high optical transmittance be
