A solar cell is like a layered cake: thin films of different materials are stacked on top of each other, and when light enters, it knocks out an electric current. One promising material for such films is a compound of cesium, lead, and iodine. Until now, it was thought that there were no built-in electric fields inside it, meaning the current doesn't flow as freely as we'd like.
Scientists peered into such a film using an electron microscope that can resolve individual atoms. They found thin seams in the crystal — boundaries where rows of atoms tilt slightly and don't meet perfectly. At some seams, lead atoms shifted from their positions, creating a weak electric field around them, as if tiny batteries had appeared inside a neutral material. These seams occur every 30–50 nanometers. That's hundreds of times thinner than a human hair.
At other seams, there's no electric field, but the tilt of the atomic 'bricks'—octahedra of iodine around lead — changes. This is another hidden lever for controlling the material's properties. If we learn to arrange these seams in the right order, the current in a solar cell will flow more easily, and it will produce more energy.
So boundaries inside the crystal aren't defects; they're working tools. It's a case where a seam in fabric doesn't ruin the product but makes it stronger and more interesting.
The distance between the found seams—30–50 nanometers — is so small that about two thousand of them would fit across the width of a human hair.
In the series 'The Expanse,' solar panels on spaceships rely on materials that humanity is just beginning to understand, such hidden internal fields could be a step toward similar technologies.