How a Solar Cell Works

A solar cell is a thin silicon sandwich with two layers. The top is seeded with phosphorus, which carries spare electrons. The bottom is seeded with boron, which leaves electron-sized gaps.

When sunlight strikes the cell, the energy knocks electrons loose. A built-in electric field between the layers shoves those electrons in one direction, and that one-way flow is the electric current you use.

How Efficient Are They

A typical panel on a roof today turns about 20 to 23 percent of the sunlight hitting it into electricity. The rest becomes heat.

In the lab, exotic multi-junction cells under concentrated light have passed 45 percent, but they are far too costly for rooftops. The technology to watch is perovskite, cheap to make and thin enough to stack on top of ordinary silicon to squeeze out more power.