Apr 7, 2016: Mobile personal electronics and wearables have become immensely popular around the globe, and this has given rise to a new dilemma. Screen content has grown, mainly from multimedia applications, resulting in increased display pixel count. The initial black and white character displays in early mobile phones have evolved into full-HD (FHD), full-color video displays and beyond. Even the screen diagonals have become larger: There is only a single smartphone product on the market featuring FHD active-matrix organic light-emitting diodes (AM-OLEDs) at <5 in. (Samsung Galaxy S4), but many at larger screen formats. Besides the technical issues of achieving high pixel density, most mobile display screen sizes are bigger than 5 in., and, depending on the geographic region, even greater than 7 in.
However, content will continue to increase, and will continue to do so, especially with upcoming virtual- and augmented-reality applications. Yet users generally prefer to keep their smartphones “mobile” and thus need to limit the size of the device, hence the display screen size. Fortunately, set manufacturers have managed to keep the device size very close to the display size, and do not require much extra space beyond it.
There are two general ways to address the balancing act of mobile screen content versus size: The first is to create a physically larger screen size at a reduced form-factor. This would be a move away from rigid planar screens; for example, they could be made rollable, to fit into smaller spaces.
Another option is to move from direct view toward projection, by generating a larger screen perception toward the user by either real or virtual optical magnification. The latter one is the opportunity for microdisplays in general since, due to their unprecedented pixel density, they are able to generate high-resolution virtual images with a large viewing angle at a very small device footprint.
All microdisplay technologies on the market comprise an image-creating pixel modulation, but only the emissive ones (for example, OLED and LED) feature the image and light source in a single device, and therefore do not require an external light source. This minimizes system size and power consumption, while providing exceptional contrast and color space. Organic light-emitting diodes (OLEDs) are made from ultra-thin organic semiconducting layers, which light up when they are connected to voltage (charge carriers become injected and luminance mainly is proportional to the forward current).
The major layers comprise several organic materials in sequence (for example, charge transport, blocking and emission layers — each with a thickness of several nanometers), which are inserted between an anode and a cathode. OLED microdisplays cover the major portion of emissive microdisplays on the market today as they are well-suited for extremely small form-factor and low-power consuming optical engines. They are the first choice for mobile.
Near-to-eye applications growing
The growing market for wearable devices requires a high number of small and lightweight displays for different applications in sports, medicine or at work. The global market for microdisplays expanded in the 1990s, mainly due to rising demand for projection applications, specifically front projectors and rear-projection TV (RPTV). Whereas RPTV significantly declined after 2005, other applications have arrived, mainly in near-to-eye (NTE) displays.
Due to the size, power, contrast and color-space advantages, NTE applications represent the largest opportunity for OLED microdisplays. This relates to both personal viewers (PV) and electronic viewfinders (EVF). As of today, there are three main markets for near-eye OLED micro-displays. The consumer market encompasses video and virtual-reality (VR) glasses and EVF; industrial covers augmented-reality (AR) smart glasses for logistics; defense market targets AR helmets for pilots.
PV are either see-through data glasses used for mixed- and augmented reality (AR) applications, or non-see-through/immersive video glasses used for entertainment or virtual reality (VR) applications in gaming, training or entertainment. Due to their emissive nature OLED microdisplays are specifically suited for see-through/AR smart glasses, since they prevent a virtual gray-shaded monitor-like perception inside the user’s field of view, which is caused by the insufficient backlight suppression of non-emissive microdisplays.
Although other products had been on the market for quite some time, Google garnered global attention with the launch of Google Glass in 2012, ostensibly aimed at the consumer market. Instead, it was shipped to individual customers/developers in limited quantities and has been used in a few niche professional applications, including fabrication logistics at Volkswagen in Wolfsburg, Germany.1
Today, suppliers of personal viewers include Vuzix, Google, Epson, Brother, Sony, Zeiss, NVIS Inc., eMagin and Lumus Ltd., with others including Meta, Atheer, Optinvent and Samsung Electronics. There are also personal viewers, mainly for fully immersive VR applications, that make use of midsize TFT displays (which are not microdisplays), Oculus Rift or Zeiss VR One.
The market is expected to grow significantly in PV applications, with major expected growth for see-through data glasses. The market analysis firm Tractica estimates that smart augmented-reality glass shipments will surpass 12 million units between 2015 and 2020, while more than 200 million virtual reality head-mounted displays will be sold by 20202. Consumer and enterprise segments will both drive sector growth, with additional sizable markets for industrial and sports applications.
Electronic viewfinders in cameras
In the past few years several leading camera makers including Sony, Panasonic, Olympus and Fujifilm released cameras — mostly high-end models that adopt OLED microdisplays for the EVF. In 2012 microdisplay unit shipments for electronic viewfinders were 4 million, with 7 million in 2013, according to a report from Insight Media in 20133. The market should continue to grow significantly, with an expected shift of EVFs from primarily high-end cameras toward midrange consumer devices. There is also a market in EVF for digital video cameras and camcorders; however, this is expected to decline driven by a shrinking camcorder market and the use of direct view panels in high-end viewfinders.
Micro-projection in sight
Projection is still the major application of microdisplays. Due to limited luminance and luminous flux capabilities of OLED, the use of OLED microdisplays for front-projection has been limited. However, in 2010 the European Commission-funded HYPOLED project demonstrated OLED microdisplays in micro-projection, though full-color projection had only been feasible using a three-panel approach comprising three monochrome subpanels (R, G, B) and optical color-combining. Red and green monochrome micro-projectors worked quite well at limited luminous flux, but displays had to be driven at around 10000 nits (a nit is a unit of luminance equivalent to one candela per square meter). This significantly affected OLED lifetime.
Alternative OLED microdisplay projection applications, mainly in combination with Fraunhofer FEP’s bidirectional OLED microdisplay approach, have been presented by Fraunhofer IOF. A bidirectional OLED microdisplay refers to a device capable of image display and image sensor/acquisition by the same chip and in parallel.4
The prominent emissive microdisplay technology on the market is OLED-on-silicon. A single-crystalline silicon CMOS chip provides the active-matrix circuitry to address and drive the millions of individual pixels. Since the silicon substrate itself is intransparent in the visible spectrum, a top-emission OLED setup is required. Today’s relevant commercial players in the OLED microdisplay market are Microoled in France, eMagin in the U.S., Sony in Japan and Olightek in China.
It’s worth noting that emissive microdisplay technology might be most suitable for so-called bidirectional microdisplay techniques, which combine both image display and image acquisition in a single chip. That’s mainly due to the fact that there is no intrinsic saturation of photodetectors embedded inside the microdisplay backplane caused by the external illumination of “modulating” displays, in contrast to the top-emission, though optical cross-talk inside the emissive microdisplay device should be factored in.
The term “microLED” refers to a variety of approaches that combine III/V-based inorganic LEDs with a silicon backplane, either monolithically or hybrid. Examples come from several startups and universities, including Ostendo, InfiniLED, LuxVue, Lumiode or the University of Strathclyde/the University of Edinburgh5.
Due to recent promising activities of III/V LED or RF/power component manufacturers to transfer III/V processes onto silicon-compatible 8-in. equipment for cost reasons, monolithic silicon-III/V hetero-integration — GaN-on-Silicon, for example — could become a competitive technology for emissive microdisplays in about 10 years. Cost may be comparable to OLED-on-silicon over the long run, but some characteristics such as lifetime, high-temperature performance, spectral width and switching speed, could prove superior.
Future forward: OLED-on-silicon
There is a trend toward high-brightness, and improved current and power efficiency, as can be seen in the development of stacked OLED devices. This will be accompanied by a monochrome subpixel micropatterning approach, which will extend color space and battery life for mobile applications. That’s always an important driving force, which may lead to new low-power microdisplay architectures. Specifically the bidirectional OLED microdisplay feature provides great opportunity for OLED microdisplay applications well beyond near-to-eye, for example, in optical inspection and optoelectronic sensors.