Metal is a good conductor of heat. Maybe I'll try an experiment when I do the laundry next time. I can take my clothes out of the dryer after they are dry and then feel the metal and the plastic in the dryer, to see if one feels hotter than the other. They should both be at the same temperature, so the one that feels hotter must be conducting heat better to my hand.

But something that conducts heat lets the heat go both in and out. So water in metal pipes might heat fast in the sun, but it would also cool fast when there is no sun.

So the simple answer to your question is that metals are a good conductor of heat for pipes in a solar panel.

According to thank you sun, the heating solar panels are usually built using Aluminum or Copper metals to receive and transmit heat.

Keep asking questions! Best wishes,