Kyocera’s solar-powered smartphone: A thin solar panel sits below the display.

Some promising new technologies for smartphone charging are among the important, if relatively low-profile, exhibits at a mobile device trade show now taking place in Barcelona.

These technologies, on display at this year’s Mobile World Congress here, seem to represent key new trends in the ways mobile devices are charged.

Japan’s Kyocera is showing off a rugged smartphone with a solar panel built into its display.

Kyocera’s solar-powered smartphone prototype is a much-improved version of the one it showcased at last year’s MWC.

“We have come much closer to launching products based on this technology,” said Yoshihiro Tokushige of Kyocera’s department responsible for telecommunications equipment management strategy, who was at the company’s booth at the show.

An ultra-thin solar panel sits between the liquid crystal display and its touchscreen layer.

Smartphone users tend to watch the display much longer than those of conventional cell phones. Kyocera’s solar-powered smartphone takes advantage of the long display-watching time to charge its battery.

The Kyoto-based electronics maker has greatly improved the efficiency of the solar panel, developed jointly with Sunpartner Technologies, a French company specializing in solar technology.

With the new, enhanced prototype, three minutes of being exposed to sunlight give the user one minute of talk time, whereas the previous model exhibited at the show last year required two hours of light rays for five minutes of voice.

Last year, few people paid serious attention to the product, according to Tokushige. But the improved performance has led to a wave of interest in the technology among visitors.

Unlike ordinary solar panels, the one used in Kyocera’s solar-powered smartphone is almost transparent, allowing 85% of light to pass through it. This quality makes it suitable for use in smartphone displays.

In addition to the panel’s own better performance, improvement in the electric circuit design has contributed to higher efficiency, according to the Kyocera representative.

The company is now seriously considering mass production and an early market rollout of smartphones using the solar technology.

Besides selling its own solar-powered smartphones, Kyocera may also sell such panels to other manufacturers.

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