The growing difference between power supplies and power demand requires a new approach to displays in mobile devices.
Not long ago, mobile phone performance hinged on reception quality and talk time. Today’s standards are more complex, as handsets have transformed from primarily voice-driven devices to converged products that offer functions like video, Web browsing, gaming, and GPS. Engineers tasked with designing the next feature-rich device have numerous factors to consider, the most pressing of which is energy efficiency.
Advanced functions have enhanced consumers’ professional and personal mobile lives, but they come at a cost: dwindling battery life. Multimedia applications require significantly increased processing power and active screen time, putting a major strain on handset batteries. This dilemma is a recurring theme in consumer complaints about smart phones whose batteries die before the day ends—an issue that handset designers and engineers are hard-pressed to address.
The power gap between consumer demand and handset capabilities continues to widen as improvements in power-draining mobile functions greatly outpace advances in battery technology. We’re approaching an “energy crisis” in the handset sector, where the modest pace of battery advances cannot match the spike in demand for a multimedia-rich mobile lifestyle (Fig. 1).
This comparison is not meant to diminish the impact of progress in the battery sector, but rather to point out that the industry must supplement battery technology advances with other more immediate, effective energy solutions. The lithium-ion (Li-ion) battery is a central component in today’s mobile devices and will be with us into the foreseeable future. Yet like all technologies, it’s constrained by the laws of physics and held back by the pace of development. Consequently, it has its limitations.
With Li-ion technology on an incremental path, alternate solutions have emerged on the energy supply side. The most interesting include solar charging (requiring behavioral adaptation), fuel cells (requiring both behavioral change and system/infrastructure integration), and wireless charging (requiring system development/integration). While some of these may hold merit in the future, the power gap is an immediate problem that requires a solution to facilitate the current momentum in the handset sector and appease today’s mobile addicts.
Solar is widely recognized as a viable energy solution, whether deployed on a grand scale or in commercial or residential sectors. Unsurprisingly, it has also been suggested as a charging option for future handsets, which would constitute an exciting development for the mobile industry.
A transition to solar charging as a supplementary or primary source of power hinges on innovation on the hardware side as well as a behavioral change on the part of consumers, as dependence on solar energy would require mobile users to spend sufficient time outdoors on a daily basis. Behavioral considerations aside, the technology to support solar charging is still in development, and it does not present a viable charging solution to address the mobile energy gap in the immediate future.
To avoid constant recharging, fuel cells have been proposed as an alternate energy supply to today’s batteries. Fuel cells have a thermodynamically open system, meaning they consume reactant from an external source, which must then be replenished. Batteries in today’s handsets, on the other hand, have a thermodynamically closed system and store electrical energy chemically.
In layman’s terms, this means that a shift to fuel cells would require mobile users to carry extra fuel-cell batteries to swap out, which essentially is a return to what is conceived to be an outdated model. Consumers have adapted to the charging model, and requiring them to carry additional components could be inconvenient, detracting from—not increasing—mobility. Furthermore, adopting a model that relies on disposable batteries would greatly increase waste, contradicting the consumer electronics industry’s attempts to be more eco-friendly.
Out of all the proposed supply-side solutions, wireless charging alone requires no change in consumer behavior. The idea is that handsets would pick up the strongest TV station signals (or other suitable sources) to charge wirelessly, eliminating the need for power cords. While wireless charging presents a viable solution in the long term, the technology is not available today to address the growing mobile power gap.
As engineers continue to explore wireless charging, they will need to look for ways to mitigate the addition of hardware so it doesn’t result in heavier, bulkier phones. With designers and consumers alike pushing for sleeker, lighter devices, the transition to wireless charging is aesthetically unlikely until the technology has matured.
Without any near-term, feasible solutions on the supply side of the handset energy equation, the industry must reduce energy consumption on the demand side. If neither occurs, the widening power gap will have ramifications for the entire handset value chain. The gap will limit designers, handcuff product managers, reduce available revenue time (ART) for carriers, and ultimately turn consumers off. As a result, designers must explore solutions on the demand side that enable continued innovation in the mobile application space and don’t force consumers to limit their mobile behavior or remain chained to power cords.
The Display: Problem And Solution
Given limitations in the energy supply department, solutions for the power gap will rely on making handsets more energy efficient on the demand side. Chipset technology is often targeted as a great way to more effectively manage the device’s power capacity, and significant strides have been made in this sector. However, designing more energy-efficient handsets naturally requires building them with more energy-efficient components, and the component most at fault for battery drain is the display.
As devices evolve into highly visual platforms for multimedia, the display is becoming increasingly central to the mobile experience as the window to the mobile world. But with consumers’ reliance on the display increasing alongside their demand for sophisticated mobile functions, the display has also become the chief offender with respect to reduced battery life.
So where does the display come into the equation? Today’s dominant player, the LCD, offers a bright, vibrant interface for mobile content. But it is also the primary culprit when it comes to energy consumption. Consumers enjoy a sophisticated mobile experience, though at the price of limiting their mobile usage or remaining tied to power cords and outlets.
First developed in 1968, LCD technology rapidly gained a foothold in the display market. Continuous improvements to the chemical mixtures and display-drive electronics, as well as optical films, have overcome the initial problems of the super-twisted nematic (STN) displays—namely, low contrast and low resolution. While scientists continue to work on reducing the power requirements of the STN and thin-film transistor (TFT) LCDs, limitations inherent in the technology make it difficult to achieve meaningful improvements.
Almost without exception, LCDs modulate polarized light to create images. The initial polarization of light incident on the LCD discards at least 50% of that light. Additional film layers within the LCD, such as the color filters, reduce light usage even further as a typical LCD will transmit only 6% of the light incident upon it. So, LCDs require power-sapping backlights to achieve the bright, crisp visual experience we’ve become accustomed to. Additionally, LCDs also suffer a significant decrease in readability when in direct sunlight.
The displays of the future need to address the widening power gap, but without compromising the display experience. As a result, we’re seeing the emergence of new energy-efficient display technologies seeking to address the deficiencies of LCDs and enable consumers’ desire to be truly mobile. The most promising of these can be placed in two categories: emissive and reflective.
The display sector has been historically slow in moving, but the past few years have seen the creation of several new and exciting display technologies. These include electrofluidic displays, Microsoft Research’s telescopic pixel, and Unipixel’s Time Multiplexed Optical Shutter (TMOS), all of which can be classified in the emissive display category.
Unveiled by the University of Cincinnati in April 2009, electrofluidic displays comprise two sheets of plastic: one sheet with pixels that each includes a hole filled with pigment and surrounded by air or oil, topped with another sheet containing a transparent electrode. When a voltage is applied, the pigment is drawn from the hole and revealed. The surrounding air or oil prevents it from mixing with pigments from surrounding pixels. While the creators promise superior brightness, color saturation, and video speed, this technology is still in its early stages and is years away from commercialization.
Microsoft Research has positioned its telescopic pixels display technology as a superior alternative to LCD, with faster response time and the ability to form a brighter display. Telescopic pixels release 36% of the incident light, compared to 6% with LCD. This addresses another deficiency of LCD by providing enhanced viewability in outdoor lighting environments. Like electrofluidic displays, telescopic pixels have not yet been tested in the commercial market.
The last of the emerging technologies in the emissive category is Unipixel’s TMOS display technology. Interestingly, TMOS was originally developed as a display solution in avionics applications. The display’s light source is essentially a total-internal-reflection light box driven by color-field-sequential LEDs. The pixel elements are flexible optical shutters switched to frustrate internal reflection and emit light from the display screen. Unipixel claims that its TMOS display technology provides a brighter, higher-contrast, more colorful image that is readable in sunlight, while also significantly reducing power.
While electrofluidic displays, telescopic pixels, and TMOS are all exciting and innovative developments, their ability to address the widening power gap is questionable. As emissive displays, all three technologies consume significant power to emit light. While they mark an incremental improvement over LCD technology, they face the same challenges as their predecessor—high power consumption and significantly decreased readability in outdoor lighting environments.
Reflective Technology: Displays Of The Future?
So if emissive displays aren’t the answer to the mobile “energy crisis,” then what is? The second class of display technologies that has seen significant innovation in recent years is the reflective display category. Bistable reflective technologies are the most power savvy of the emerging display technologies. They also have proven their commercial viability through incorporation in today’s mobile devices. This category includes E Ink’s e-paper display and Qualcomm’s mirasol display.
The E Ink display is perhaps best known for its incorporation in Amazon’s Kindle and more generally for its role in defining the e-book market. It comprises a plastic film that has been laminated to a layer of circuitry, which forms a pattern of pixels controlled by a display driver. The display is designed to mimic ordinary ink on paper for maximum readability. It can hold a stable image without constantly needing to refresh, and it reflects ambient light rather than emitting light.
These attributes contribute to enhanced energy efficiency and visibility in direct sunlight—two of the primary deficiencies of the LCD. While black and white E Ink displays are well suited to e-reader applications, color e-paper displays have yet to be featured in devices. Current generations of e-paper displays are more suitable for relatively static displays that change less frequently and require little to no color. Furthermore, electronic paper has a relatively slow response time, rendering it unsuitable for video and similar functions.
Qualcomm MEMS Technologies’ Interferometric Modulation (“IMOD”) technology, however, makes a natural leap from bichrome to color and supports video-rate functions. The company’s engineers developed the mirasol display by studying and mimicking nature’s processes and structures. The display uses a combination of mirrors and thin-film layers common in LCD and semiconductor-like fabrication technologies to create a full spectrum of pure colors by reflecting light so specific wavelengths interfere with each other to select the emitted colors (Fig. 2).
This is the same phenomenon that makes a butterfly’s wings shimmer. When ambient light hits the display structure, it is reflected both off the top of the thin-film stack and off the reflective mirror membrane. Depending on the height of the optical cavity between the mirror membrane and the thin-film layers, light of certain wavelengths reflecting off the membrane will be out of phase with the light reflecting off the thin-film structure, and other wavelengths will be in phase. Based on the phase difference, some wavelengths will constructively interfere, while others will destructively interfere.
The mirasol display technology brings two essential value propositions to the mobile experience: longer battery life and direct-sunlight viewability. One of the key advantages is its bistable nature, which allows for near-zero power usage in situations where the display image is unchanged. This means that mirasol displays benefit from considerable power savings, especially compared to displays that continually refresh, such as LCDs.
The power currently used for battery-draining backlights can be reallocated to support functions and applications, improving response and refresh rate as well as lengthening battery life. Furthermore, the color remains bright, crisp, and saturated across a broad range of lighting conditions, including direct sunlight. While a relatively new technology, mirasol displays have already been selected for multiple products and will scale to greater sizes and enhanced color quality in the near future.
Bistable reflective technologies are the most power savvy of the emerging display technologies. They not only enhance the consumer experience, they also provide value throughout the mobile value chain. Designers can develop sleeker devices using smaller (and less expensive) batteries. Product managers don’t need to skimp on device features. And, carriers could see ART rise as consumers increase usage time on their handsets. These emerging display technologies are still maturing, but the core value proposition has the elements of a disruptive technology, making this a space to be watched.
1. Rosaleen Ortiz, “Ohio Engineers ‘Ink’ New E-Paper,” IEEE Spectrum, April 28, 2009, www.spectrum.ieee.org/semiconductors/materials/ohio-engineers-ink-new-electronic-paper-technology
2. Prachi Patel, “A New Competitor to LCD,” Technology Review, July 21, 2008, www.technologyreview.com/computing/21104/
3. Unipixel, “Technology,” www.unipixel.com/tech.htm