Long term effects of iPhone left in the sun?
If you cooled it down, then the problem is resolved for now and there is nothing more you can do. Try to avoid doing that again as this will slowly move from the short term effects to long term ones.
Short Term Effects of Overheating

Battery decreases- fast! (Hopefully temporarily, when you cool it down, it will return to 98% of what it was before)

Long Term Effects of Overheating
When the iPhone overheats, Apple also claims that you might experience these (more extreme) results:

device battery stops charging

display weakens

poor cellular signal.


To stop this from happening, try and turn your iPhone so that the screen is facing down. This will help it avoid contact with the sun. If you have a case, that will be better because the sun will be hitting the case instead of the phone directly.