Can solar panels be laid flat?

Can solar panels be laid flat?

Best not to lay regular solar panels completely flat.  You can use tilt frames.  Panels tilted past around 10 degrees tend to be self cleaning.
When panels lie flat, water will pool on top of them and stay there after it stops raining. This water can collect dust and windborne debris, such as leaves, which will stay behind when the water evaporates. 
Tropical locations will have a lower tilt angle