Solution-processed graphene and its display applications

Abstract

Graphene has been studied vigorously since its discovery in 2004. Intriguing properties of graphene such as high electrical and thermal conductivity, high specific surface area, excellent mechanical strength, and good gas barrier characteristics have attracted numerous researchers in the field of condensed-matter physics and materials science. Graphene can be produced mainly in three ways: mechanical cleavage, chemical vapor deposition (CVD), and solution processes. Mechanical cleavage, also known as the ‘Scotch tape method’, exfoliates graphene from graphite and was used in the early stage of graphene research. However, this method has inherent problems relating to reproducibility and scalability. The CVD method is well-established and commonly used to obtain high-quality graphene films. The CVD process, however, essentially has cost and scalability problems because it is a vacuum process. Meanwhile, solution processes usually obtain graphene from graphite through a series of chemical reactions. In the process of chemical reactions, we can also obtain graphene-like materials such as graphene oxide (GO), reduced graphene oxide (RGO), and many other functionalized graphene. These chemically derived graphene-like materials shows sometimes very similar but occasionally very different characteristics from graphene. Prior to graphene, there was another material made up of carbon which had been substantially investigated thanks to its excellent properties. Carbon nanotubes (CNTs), however, did not continue its prosperity to the actual device applications. It is thought that the gap between ideal and actually synthesized CNTs is inevitable and therefore limited the successful application of CNTs to many practical devices. Thus, we can easily expect that graphene and other two-dimensional materials including transition metal dichalcogenides are going to have similar development steps as CNT had in the past. Actually, the application of graphene to practical devices falls relatively short of expectations considering the vigorous studies so far. Although the theoretical progresses are very important, the actual development of graphene for practical applications is also significant for further advances of science and engineering. Accordingly, we have to find feasible ways to utilize graphene, GO, and RGO in many practical applications. Graphene, GO, and RGO have many applications including displays, photovoltaic devices, and sensors. Here, we focused on the display applications of GO and RGO. At first, we tried to fabricate conductive patterns composed by RGO using advanced inkjet printing technology referred to as reactive inkjet printing (RIP). Solution-processed conductive RGO patterns have usually been fabricated by printing GO first and subsequent reduction process, thus far. RIP technique, however, can fabricate conductive RGO patterns within just one step by printing multiple reactive inks with a same pattern. Inks, which are printed with identical patterns, then react with each other on a substrate in a suitable thermal and ambient environment. In this study, we printed a GO ink and reducing agent inks using RIP technique and obtained conductive RGO patterns. The performance of printed patterns were characterized and demonstrated by a simple LED circuit. We also utilized GO and RGO to enhance the photoluminescence (PL) of ZnO. ZnO usually emits visible lights from its band-edges and defect levels. Previously, there have been many reports on the enhanced PL from ZnO by adopting surface plasmons of noble metals. Recently, many researchers have investigated the plasmonic behavior of graphene and its applications. One of the applications is the enhancement of PL emission of light emitters like ZnO and a few groups reported the result very recently. The mechanism of the enhancement, however, is not thoroughly analyzed yet and previous reports used hardly-reproducible graphene production methods such as a mechanical cleavage. In this study, we exploited solution-processed graphene for the enhancement of PL from ZnO and investigated the fundamental mechanism underneath the PL enhancement. We applied GO and RGO to the organic light-emitting diodes (OLEDs) as well. OLEDs already dominate small-sized display market and are considered to substitute for liquid crystal displays and plasma display panels in large-sized display applications in near future. To achieve this development, low-cost fabrication methods and reasonable device reliability have to be investigated. Low-cost fabrication can be implemented by developing solution processes and materials. However, the widely-used and solution-processable hole-injection layer (HIL) material, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is known for its corrosive nature and exciton quenching, therefore degrading the performance and reliability of devices. In this study, we used GO and RGO as novel hole-injecting materials and investigated the enhancement of device performances with regard to the efficiencies. Varied reduction levels of RGO altered the efficiency of OLEDs and we could obtain OLEDs with optimized performance. We studied viable applications of GO and RGO in various devices as above. GO and RGO can be utilized as an electrode, as a light emitter, and as an OLED material. It is meaningful that we not only found the novel applications of GO and RGO, but also improved some device performances by adopting GO and RGO. We are certain that these results will contribute and accelerate the technology transfer from the research field to the practical applications of graphene, thereby making an advance in the science and engineering of graphene.