Highlights from EuroDisplay 2013
by Ian Sage and Alasdair Campbell
For the fourth time since 1987, EuroDisplay – the Biennial International Display Research Conference (IDRC) – came to the UK. The September 2013 event, co-sponsored by the Society for Information Display and the Institute of Physics, had, as always, a distinct European flavor compared to other display-related meetings. The program this year provided a concentrated insight into the part that Europe and the UK in particular continue to play in the invention and development of cutting-edge display technologies. The venue this year was Imperial College, the prestigious, science-based university in the heart of London’s museum district and a stone’s throw from famous landmarks such as Hyde Park (Fig. 1).
Fig. 1: Tower Bridge by night was just one of many iconic views that EuroDisplay 2013 attendees could take in during the special event dinner cruise along the Thames.
Three plenary presentations started off events with a show of strength from the European display research and development community. Two of the technologies described originated in Cambridge University’s Cavendish Laboratory. Jeremy Burroughs (Cambridge Display Technology) provided an excellent summary of the latest developments in solution-processed organic semiconductor-based display technologies. Improved efficiencies have been achieved in light-emitting- polymer (LEP) based OLEDs through such factors as emissive dipole alignment, and triplet-triplet annihilation has been shown to boost the singlet yield of fluorescent polymer OLEDs from 25 to 40%. For lighting and backlights, 6-in. tiles with a slot-die-cast white-emitting LEP have achieved 42 lm/W. Polymer-based electrochemical-cell OLEDs currently have a long turn-on time, but could be used in low-cost flexible devices. Improved organic TFT performance for backplanes has also been achieved by lowering contact resistance through improved formulation design.
Henning Sirringhaus (University of Cambridge) took as his theme the rapid improvements in solution-processed organic semiconductor and oxide TFTs. Such TFTs can be used in both backplanes and driver circuitry for flexible displays that are shatterproof and robust and feature novel form factors and usage modes. AMOLED drive currents require TFTs with mobility values of about 2 cm2/V-sec, but TFT instability can require compensation circuits with values of 5–10 cm2/V-sec for video. Oxide TFTs can easily reach these values, but normally require processing at temperatures too high for plastic substrates. Sirringhuas has developed a sol-gel method for oxide TFTs with wet-atmosphere processing at temperatures of 250°C and has used dopants such as Ba (barium) and Sr (strontium) instead of Ga (gallium) to produce much higher stability. Donor-acceptor polymer organic field-effect transistors (OFETs) have also been achieved with values of 5–10 cm2/V-sec.
The quality of liquid-crystal displays (LCDs) is so routinely excellent that the underlying technology is largely lost to view. Michael Heckmeier (Merck KGaA) provided a broad overview of the electro-optic effects used in the latest devices and the special material properties required by each. Each leading type of display occupies its own place in the applications field, with different types of VA-mode devices dominating TV displays and in-plane switching (IPS) and fringe-field switching (FFS) displays used in computer and mobile devices. Not only the liquid-crystal fluids but their accompanying reactive monomer materials are crucial for PS-VA devices and optical films. Heckmeier continued by describing emerging device modes such as blue phase and uniformly lying helix displays, which offer the promise of still faster response speed and higher-quality images.
Beyond the Status Quo
A number of presentations gave views of displays which, if realized, will greatly extend the capabilities of present technologies. One of the highest profile initiatives is the drive toward ultra-high-resolution (8K) television technology, referred to as Super-Hi Vision and presented by a team of authors from NHK. The highly ambitious specifications for such a system include a screen resolution of around 7680 × 4320, combined with frame rate, color gamut, and bit depth all exceeding HDTV standards. Such a system will provide an immersive experience based on very-large-diagonal displays. No technologies currently satisfy all the requirements of the proposed new standards, while impressive demonstration devices that achieve the target resolution and approach other specifications have been made using plasma, LCD, and projection systems. This talk stimulated a lively audience discussion, centering on the practicality of such systems against their requirements for bandwidth, power, and footprint.
Adrian Travis (Microsoft Corp.) and Adrian Geisow (Hewlett-Packard Laboratories) each presented stimulating talks describing a re-think of LCD technology. The Hewlett-Packard work addressed the unsatisfactory performance of reflective color displays and described the company’s work in providing bright, saturated colors using a stacked subtractive-color dichroic LCD. In the past, attempts to exploit subtractive-color stacks have suffered from absorption spectra in the colorants, which are far from ideal and result in poorly saturated “muddy” colors. Geisow described how to radically improve color rendering by incorporating color-selective reflectors between the dichroic LCD layers; each reflector avoids the corresponding color of light having to traverse the layers deeper in the display stack, and the brightness and saturation are markedly improved. Reflectors can be based on vacuum-coated multilayers or on cholesteric polymer layers.
Travis described radical approaches to power savings in LCDs, based on providing steered backlight illumination that directs light specifically toward the users’ eyes by making use of head-tracking. Steering the output light to follow the user while maintaining a compact form factor presents a real challenge, which has been overcome using a hybrid system incorporating one small and one linear tracking mirror, allowing a compact grating to scatter light into the correct optical path in a slab waveguide. Such a system allows steering of light in the x-y plane, but this is not sufficient for a display backlight, which should also converge light toward the eye. A switchable grating in the waveguide synchronized with the optical scan angle allows this, and Travis offered his audience the prospect of future displays with a near-zero power budget.
Herbert deSmet (CMST, Ghent) presented truly futuristic work directed toward the ultimate goal of incorporating a viable display into a contact lens. Even the generation of an image on the retina from such a close distance is difficult; deSmet described how projection from an array of LEDs equipped with pinhole apertures and microlenses could achieve this, albeit with limited resolution. A large range of materials-related problems have to be solved in order to provide air permeation to the cornea as well as barrier properties to protect electronic components from water and the eye from contact with any toxic materials. Power from organic photovoltaic or inductive pickup sources could provide enough energy to run a display as well as additional devices such as biosensors to monitor conditions like diabetes or glaucoma.
Holographic Futures
In consecutive talks, Tim Wilkinson (University of Cambridge) and Jamieson Christmas (Two Trees Photonics) gave contrasting and fascinating views of their approaches to holographic displays. Wilkinson began from the fundamentals of off-axis holography and succinctly described an efficient approach to generating holograms that can be projected to a screen, or alternatively viewed directly using the eye’s lens to provide the Fourier imaging element. A wide field of view is then available. This increase in image size requires higher-resolution pixels at the SLM than are available from standard liquid-crystal–on–silicon (LCOS) configurations. The problem arises from the LC pixel rather than limitations of the underlying silicon, and Wilkinson went on to describe approaches to ultra-high-resolution devices achieved through the use of nanotube arrays in the pixel to concentrate the electrical field into smaller volumes. This challenging approach has borne some fruit; a static hologram based on submicron-resolution nanotube arrays shows plasmon-enhanced diffraction efficiency and a diffraction angle up to 70°.
Jamieson Christmas was this year’s recipient of the Ben Sturgeon award, given annually by the UK & Ireland chapter of SID to mark a significant development in display science. In his award lecture, Christmas described for the first time his use of holographic principles to provide a light engine for 2-D head-up displays (HUDs). Improved algorithms for hologram design minimize computational load, and the use of a holographic light modulator leads to a substantial improvement in the overall optical and power efficiency of the system. The technology is understood to be targeted at road vehicle use and is under intense development for inclusion in high-end cars in the near future.
Organic Electronics Workshop and More
Several additional presentations covered OLEDs, other organics, and oxide TFTs – areas that are moving forward, particularly with regard to new device architectures, new materials, and flexible displays. The conference included a one-day workshop on organic electronics, which covered not only organic light-emitting diodes (OLEDs) and organic thin-film transistors (OTFTs) and their use in displays, but many other areas of the field. The workshop began with presentations from Peter Skabara (University of Strathclyde) on organic semiconducting materials, which explored the design rules for achieving properties such as a particular energy gap and from Anna Hayer (Merck KGaA), who focused on solution-processable OLEDs, how they work, and how to maximize performance.
Ian Underwood (University of Edinburgh) discussed designing and driving active-matrix OLED (AMOLED) displays, and Alastair Buckley (University of Sheffield) described the production of OLED microdisplays based on his industrial experience of a startup. Buckley covered all the challenges in going from “lab to fab.” Ifor Samuel (University of St. Andrews) finished off the morning session with a presentation on organic lasing and optical amplifiers, and how these devices can be used as sensitive explosive detectors.
The afternoon workshop session started with a presentation from Giles Horowitz (École Polytechnique Paris) on the use of OFETs in biomedical devices. This was then followed by a detailed presentation from Soeren Steudel (IMEC) on the fabrication of flexible large-area organic electronics circuitry using a batch process, while Gerhard Klink (Fraunhofer EMFT) followed with a talk on the roll-to-roll processing of organic electronics on foil. Following this was a presentation from Jonathan Halls (Solar Press) on the large-area roll-to-roll processing of organic solar cells and a presentation by Ilkka Kaisto (VTT) on the commercialization of large-area organic electronics.
Other OLED presentations at the main conference included one on major advances in lighting made possible through optical modeling and design to increase light outcoupling in the forward direction, reclaiming the 75% of light lost horizontally in the organic, ITO, and substrate layers. By using a Si microsphere stamp to pattern a hexagonal grating structure on the internal substrate surface of the OLED, and combining this with an external macro-lens and a high-index substrate, Franky So (University of Florida) was able to achieve an impressive 230 lm/W.
With regard to novel OLEDs under development, Russell Holmes (University of Minnesota) reported a high-efficiency OLED consisting of continuously graded electron-transport and hole-transport materials, and Matthew Fuchter (Imperial College London) reported a circularly polarized OLED based on a conventional LEP doped with a helically shaped chiral small molecule.
State of the Art of Displays
The above presentations represent only some of the ground-breaking research presented at EuroDisplay 2013. The conference was a unique opportunity to learn about the latest and future developments in display technologies – from OLEDs and LCDs to holograms and contact lens displays. Although speakers came from all over the world, the event offered a uniquely focused look at the work now being done at companies and universities in Europe. The next conference in the series to be held in 2015 is now being planned by the Mid-Europe chapter and again promises an event to look forward to.
SID Appoints Senior Members
SID’s Senior Membership Grade Committee has elevated Yasushi Motoyama of the Japan Chapter and Patrick Green of the Pacific Northwest Chapter to Senior Members, in recognition of their outstanding professional achievements and volunteerism. Motoyama has been a member of the Society since 2002. He currently conducts plasma-display research at NHK Science and Technology Research Laboratories. Green has been an SID member since the early 1980s. He is an engineer with Planar Systems. •