Monocrystalline solar cells are made of silicon ingots, which are cylindrical. To optimize performance and lower the cost of a monocrystalline solar cell, four sides are cut from the cylindrical ingot to make silicon wafers, which gives typical monochristalin display solar panels.
A good way to separate the Solar Panel / Solar Panel mono- and polycrystalline panels is that the polycrystalline solar cells look very rectangular with no rounded edges.