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.

Supply-Side Solutions

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.