When a high-end Swiss watchmaker introduces what appears to be a plain old dumb phone, odds are there’s something more going on below the surface. In the case of Tag Heuer’s Meridiist Infinite, the story is the surface: The phone’s 2.4-inch screen is also a solar panel. Engineers at Sunpartner Technologies, the French company that produces the screen, place microns-thin strips of photovoltaic material over the display. Inside the phone, micro lenses bend light so the LCD readout travels around the strips, making it invisible to the user. An hour of direct sun could charge the battery 10 to 30 percent, and even ambient indoor light could supply enough juice to keep the handset running in standby mode forever.