The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

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Graphene photonics and optoelectronics

Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Functionalized acenes and heteroacenes for organic electronics.

Electron and ambipolar transport in organic field-effect transistors.

We have fabricated organic thin-film transistor (OTFT)-driven active matrix liquid crystal displays on flexible polymeric substrates. These small displays have 16×16 pixel polymer-dispersed liquid crystal arrays addressed by pentacene active layer OTFTs. The displays were fabricated using a low-temperature process (<110 °C) on flexible polyethylene naphthalate film and are operated as reflective active matrix displays.

Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates

We have fabricated pentacene organic thin-film transistor (OTFT) driven active matrix organic light-emitting diode (OLED) displays on flexible polyethylene terephthalete substrates These displays have 48×48 bottom-emission OLED pixels with two pentacene OTFTs used per pixel Parylene is used to isolate the OTFTs and OLEDs with good OTFT yield and uniformity

All-organic active matrix flexible display

We report on parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films. The influence on the OTFT performance of the source and drain contact metal and the device design was investigated. Top contact (TC) and bottom contact (BC) gated transmission line model (gated-TLM) test structures were used to extract the combined parasitic contact resistance as a function of gate voltage swing and drain-source voltage for OTFTs with gold source and drain contacts. For comparison BC test structures with palladium contacts were studied. Differences in the bias dependence of the contact resistance for TC and BC OTFTs indicate that charge injection and device performance are strongly affected by the device design and processing. The results from this investigation show that TC and BC device performances may be contact limited for high mobility OTFTs with channel lengths less than 10μm.

An experimental study of contact effects in organic thin film transistors

We report on organic thin-film transistors fabricated using the small-molecule organic semiconductor naphthacene as the active layer material with device performance suitable for several large-area or low-cost electronics applications. We investigated naphthacene thin films deposited by thermal evaporation onto amorphous substrates held near room temperature. Using atomic-force microscopy and x-ray diffraction we find naphthacene films consist of a high density of submicron-sized grains with a surprisingly high degree of molecular order. Thin-film transistors fabricated using evaporated naphthacene films on thermally oxidized silicon substrates have field-effect mobility larger than 0.1 cm2/V s, current on/off ratio greater than 106, negative threshold voltage, and subthreshold slope of 1 V/decade.

Thin-film transistors based on well-ordered thermally evaporated naphthacene films

Pentacene organic thin-film transistors (TFTs)-driven active matrix organic light-emitting diode (OLED) displays has been investigated. This letter addresses several process issues unique to this type of display which are important in achieving bright and uniform displays. A bottom contact structure was used to fabricate the pentacene TFT backplane. Polyvinyl alcohol and parylene were used to isolate the pentacene active layer and passivate the backplane. The low processing temperature may allow the use of polymeric substrates and lower cost processing. Uniform TFT performance is achieved with reasonably good mobility and on/off ratio on the backplane. The initial OLED display performance is also presented.

Lisong Zhou

Papers

Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates

All-organic active matrix flexible display

An experimental study of contact effects in organic thin film transistors

Thin-film transistors based on well-ordered thermally evaporated naphthacene films

Pentacene TFT driven AM OLED displays