Bill McMahon has conducted research at NREL for more than 25 years.  During this time, he has performed foundational research on a wide range of topics impacting the growth, fabrication, and performance of high-efficiency III‑V solar cells. As a postdoctoral researcher, he studied the atomic structure of MOCVD-prepared semiconductor surfaces used in III‑V growth on silicon and germanium surfaces.  His research into the atomic structure of dislocations explained how ordering of III-V alloys can increase the glide force on dislocations, impacting the growth of lattice-mismatched solar cells.  He has also led investigations into cell bonding, nano-imprinted optical coatings, distributed resistance modeling of 3-D current flow in multijunction solar cells, and energy-yield measurement and modeling.