Monocrystalline solar cells are made out of silicon ingots, which are cylindrical in shape. To optimize performance and lower costs of a single monocrystalline solar cell, four sides are cut out of the cylindrical ingots to make silicon wafers, which is what gives monocrystalline solar panels their characteristic look.
A good way to separate mono- and polycrystalline solar panels is that polycrystalline solar cells look perfectly rectangular with no rounded edges.
Very advanced standard silicon module: German module manufacturer SolarWorld offers glass-glass panels with bifacial cells and 5 busbars, based on monocrystalline PERC technology. The photo shows the module from the front and back.