It's fun to see how solar panel manufacturing processes affect how they look. Mono-Si is produced in cylinders, so the cells are cut to octagons, a compromise between wasting too much material, and having too much empty space when tilling. Poly-Si can be made directly into squares without waste, and is also cheaper, but the panels are less efficient.
https://en.wikipedia.org/wiki/File:Comparison_solar_cell_poly-Si_vs_mono-Si.png