Solar cells with multiple junctions are more efficient than conventional photovoltaics, but they are way more expensive. One solution is to use tiny multi-junction cells and use lenses to focus light on them. This lets you get to higher power conversion efficiencies, but requires expensive trackers to follow the sun. A new set of designs obviates the need for trackers by sandwiching the cells between tiny lenses and mirrors.

A cutaway view of a concentrating photovoltaic system using tiny lenses