Flexible, thin-film solar panels from a Silicon Valley company are allowing drone makers to keep their craft in the sky for hours longer than is possible with batteries alone. The panels, from Alta Devices in Sunnyvale, are produced on thin plastic sheets that can be stuck on the top frame of drones like the Bramor ppX, developed by Slovenia's C-Astral Aerospace. On Tuesday, the two companies showed off a version of the drone with six solar panels affixed to its top. The basic drone can stay aloft for 3.5 hours, but the addition of the solar panels has extended this by two hours, they said. The two plan to offer a solar version of the drone commercially.