A full-color, low-power, wearable display for mobile applications

Display technology has advanced rapidly with the increasing demands of consumer electronics. One of the future trends is to realize flexible displays, which are highly desired for mobile applications in a lightweight, non-breakable, and ultimately rollable form factor.^1–4 Figure 1 shows a four-phase com- mercial roadmap of flexible display development. First, low-power, ultra-thin, and unbreakable displays are built on a plastic or metal foil substrate. Second, displays become bendable and conformable, starting to exhibit true flexibility. Third, rollable displays are extremely flexible in one dimension so that they can be rolled around a cylinder. Finally, free-form displays can be made into any form, including rolled or folded, in the ultimate flexible phase.

Figure 1. Commercialization roadmap of flexible displays.

Among display technologies, organic light-emitting devices (OLEDs) are the most suitable for high-information-content full-color video applications. An OLED device structure is a series of thin organic films deposited between two electrodes (see Figure 2). By applying a voltage across the device, charges are injected and converted into photons to generate light. OLEDs provide exceptional image quality, offering very high contrast and excellent color reproduction, as well as fast video response rates. In particular, the simple device structure and thin-film nature of OLEDs make them the perfect candidate for flexible displays. In addition, phosphorescent OLED (PHOLED^TM) technology enables OLED displays to have low power consumption,^{5, 6} making them well-suited for portable electronics, where long battery life is a critical concern.

Figure 2. Illustration of an organic LED (OLED) device structure. ETL: Electron transport layer. EML: Emissive layer. HTL: Hole transport layer. HIL: Hole injection layer.

Fabricating an OLED display requires us to integrate key technologies. The individual pixels need to be addressable. The most commonly used addressing scheme for video applications is ‘active matrix’, which is realized using a thin-film transistor (TFT) ‘backplane’. An OLED display built using an active matrix backplane is referred to as an AMOLED display. On top of the backplane is the OLED ‘frontplane’. Finally, OLEDs degrade in the presence of oxygen or moisture, and so the final device needs to be encapsulated to ensure long operational lifetimes.

The standard process of AMOLED display fabrication can be transferred to flexible substrates (see Figure 3). However, flexible substrates present unique challenges to building TFT arrays. Plastic has the advantage of high flexibility and transparency,² but usually brings a low-temperature constraint that limits the backplane fabrication. Plastic is also a poor barrier that adds complexity and cost because of the additional encapsulation process. By contrast, thin metal foil has reasonable flexibility, excellent thermal stability and is an excellent barrier to moisture and oxygen.¹ For this reason, we used stainless steel foil as the substrate. However, thin metal foils can present challenges in mass production, and an additional planarization step is usually required to prevent electrical shorting.

Figure 3. The cross-section of a flexible active matrix OLED (AMOLED) display structure. BL: Blocking layer. TFT: Thin-film transistor.

For the backplane technology, amorphous silicon (a-Si) TFTs have the advantages of showing uniform electrical characteristics over large areas, a reasonable field-effect mobility (∼0.5cm²/V·s), a low-temperature process (<300^°C), and low-cost fabrication.⁴ Simple circuitry can be added to compensate for threshold-voltage instability shifts. Moreover, the low drive voltage of high-efficiency PHOLEDs further reduces these shifts. We have demonstrated a flexible AMOLED display using a-Si backplane technology.

To reduce power consumption to a minimum, we used phosphorescent emitters. During operation, charge is injected into the OLED device and converted into photons through the formation and subsequent recombination of excitons, the bound molecular excited state. Excitons come in two different forms: singlet and triplet, depending on the spins of the electrons and holes that came together to form the exciton. Phosphorescent emitters contain a heavy metal atom that facilitates the mixing of singlet and triplet states, which enables the triplet states to radiate and therefore to potentially achieve 100% internal quantum efficiency.^{5, 7} This is up to four times higher than that of fluorescent OLEDs where only singlet states emit light. This level of efficiency enables low power consumption, and significantly extends battery life.

Another challenge facing flexible AMOLED displays is the development of an encapsulation technology. OLEDs degrade when exposed to atmospheric oxygen and water. Rigid glass displays are typically sealed in an inert atmosphere using a glass or metal lid. For flexible displays, however, flexible, thin-film encapsulation is required. We have reported a low-stress, transparent, single-layer permeation barrier using single-chamber plasma-enhanced chemical vapor deposition.⁸ Under accelerated storage tests at 85^°C and 85% relative humidity, the active area of the encapsulated device remains 100% after 500 hours. This shows great promise for achieving a high-quality, simplified thin-film encapsulation for flexible displays.

We put all these technologies together to demonstrate a wrist unit prototype using a 4.3” HVGA (half-size video graphics array, 640×240 pixels in size) 134dpi full-color flexible PHOLED display. This prototype is designed to fit over the user's wrist, enabling the wearer to see real-time video and graphics information (see Figure 4). The displays use a-Si backplanes fabricated on stainless steel substrates, with a total thickness of less than 0.3mm. The power consumption in video mode is only 0.3W. Full specifications of the display are shown in Table 1. These wrist units combine high luminance, good uniformity, low power consumption, and a thin, lightweight housing system. We believe flexible AMOLEDs will keep improving in optical performance, lifetime, and flexibility in the near future, and then move into rollable and ultimately free-form phases. Looking forward, we expect in the near future that flexible AMOLEDs will be adopted in specialized applications where rugged displays are needed.

Figure 4. Demonstration of a soldier wearing the wrist unit with a navigation display on.

The authors thank their collaborators at LG Displays for providing TFT backplanes, and L3 Communications Display Systems for designing and fabricating the wrist units. The authors gratefully acknowledge financial support from the Communications-Electronics Research, Development and Engineering Center, the Air Force Research Laboratories, and the Army Research Laboratories.