Don't think the 10 or 20 or 30 watt output from the photocell that directly drives the DC motor in the solar attic fans could cause much of if any fire. Maybe a little warmth if the motor freezes. I had a few installed by SOLARGUYS and they could probably be directly driven by 2x AA batteries but changing them would be a PITA.
Not sure if they are uL Approved. They are made by "SOLAR STAR".