Ryoga Murata was studying a strange crystal in a Tokyo lab when he noticed something impossible: a solar-like current flowing without any voltage, even though the material’s structure should have made it inert. The crystal, made of copper, chromium, and sulfur, was supposed to stop generating power at room temperature. Instead, it got stronger.
This wasn’t how solar cells were supposed to work. Most rely on carefully engineered junctions to pull apart light-generated charges, a method capped by a hard efficiency limit—about 33% for the best silicon cells. But here, in a layered material called copper chromium thiophosphate (CuCrP2S6), light was creating electric current without any such junction. Even more surprising, it kept working when the crystal’s average structure looked symmetrical—a condition that, according to textbook physics, should kill this effect.
The effect is called the bulk photovoltaic effect (BPVE), and it’s long been tied to materials with lopsided atomic arrangements. When light hits them, electrons get pushed in one direction, creating current without needing extra voltage. But in this case, the symmetry seemed to return at room temperature. Yet the current didn’t vanish. It surged—reaching about 1.5 times the strength seen in its low-temperature phase.
The secret lies in how fast electrons move compared to atoms. As the crystal warms up, copper ions start hopping between positions in trillionths of a second (picoseconds). Over time, their average location looks symmetrical. But electrons react in quadrillionths of a second (femtoseconds)—so fast they only see the brief moments when the copper ions are off-center. To them, the material still looks asymmetric. These fleeting, lopsided snapshots are enough to keep the current flowing.
"Consequently, dynamically generated polar clusters can contribute to the shift-current response even though the macroscopic symmetry appears centrosymmetric in diffraction measurements," says Takao Sasagawa, who led the team at Institute of Science Tokyo. In simpler terms: motion itself becomes a tool for making electricity. The jiggling atoms aren’t a flaw—they’re the engine.
This discovery opens a new path for solar materials. Instead of relying only on fixed, rigidly asymmetric crystals, scientists could design materials where atomic motion boosts energy conversion. It’s not about static structure anymore—it’s about controlled chaos. And because the BPVE isn’t bound by the same rules as traditional solar cells, efficiencies could one day surpass today’s hard limits.
The work, published in Advanced Functional Materials, was done entirely on lab-grown crystals at low light levels. But the principle could inspire a new class of solar devices—ones that don’t just capture sunlight, but use the dance of atoms inside materials to amplify it.