Overcoming symmetry limits in photovoltaics through surface engineering

Conventional solar cells rely on carefully engineered interfaces, such as p–n junctions, to turn light into electricity. A more exotic mechanism—the bulk photovoltaic effect—can generate electrical current directly in a material without such junctions, but only if its crystal structure lacks inversion symmetry. This strict requirement has long restricted the search for practical materials. In this new study, a group of researchers demonstrates that this limitation can be overcome: even perfectly symmetric materials can produce sizeable photocurrents thanks to the special electronic states that naturally form at their surfaces.

Using first-principles calculations, we show that the surfaces of metals and semiconductors with strong relativistic spin–orbit interaction can host electronic states that behave very differently from those in the bulk. These surface states break inversion symmetry locally and respond nonlinearly to light, giving rise to robust charge currents and, remarkably, pure spin-polarized currents flowing along the surface. After benchmarking the mechanism on the well-known Au(111) surface, we identified Tl/Si(111) as an ideal material platform, predicting photocurrents comparable to those of leading ferroelectrics along with clear experimental signatures for detection.

The findings reveal a new strategy for light-to-electricity conversion: rather than searching for complex non-centrosymmetric crystals, scientists can “engineer” photocurrents by tailoring the surface electronic structure of otherwise symmetric materials. Beyond energy harvesting, the ability to generate and control spin currents with light—without magnets or applied voltages—opens promising opportunities for ultrafast, low-power spintronic devices.