A new study conducted by researchers from EHU, the Materials Physics Center, nanoGUNE and DIPC presents a new approach in the fields of solar energy conversion and spintronics. The study addresses the "need for a non-centrosymmetric crystal" issue, a long-standing constraint in the bulk photovoltaic effect, by demonstrating that even materials with perfect symmetry can generate significant photocurrent through engineered surface electronic states.
Researchers from EHU, Material Physics Center, nanoGUNE and DIPC have conducted a new study that presents a novel approach in solar energy conversion and spintronics. The study addresses the "non-centrosymmetric crystal requirement," a long-standing limitation in bulk photovoltaic effect, by demonstrating that even materials with perfect symmetry can generate significant photocurrent through specially engineered surface electronic states.
This discovery opens new pathways for designing efficient light-to-electricity conversion systems and ultrafast spintronic devices. Conventional solar cells rely on carefully designed interfaces such as p-n junctions to convert light into electricity. A more exotic mechanism, the bulk photovoltaic effect, can generate electric current directly within a material without such junctions; however, this is only possible if the material's crystal structure lacks inversion symmetry. This strict requirement has long constrained the search for practical materials. In this new study, a group of researchers demonstrates that this limitation can be overcome: materials with perfect symmetry can still generate considerable photocurrents thanks to special electronic states that naturally occur at their surfaces.
Using first-principles calculations, the researchers show that surfaces of metals and semiconductors with strong relativistic spin-orbit coupling can host electronic states that behave very differently from those in the bulk. These surface states locally break inversion symmetry and respond nonlinearly to light, leading to strong charge currents and, notably, pure spin-polarized currents flowing across the surface. After testing the mechanism on the well-known Au(111) surface, they identified Tl/Si(111) as an ideal material platform, predicting photocurrents comparable to those of leading ferroelectrics and clear experimental signatures for detection. The findings present a new strategy for light-to-electricity conversion: instead of seeking complex, non-centrosymmetric crystals, scientists can engineer photocurrents by tailoring the surface electronic structure of normally symmetric materials. Beyond energy harvesting, the ability to generate and control spin currents with light without magnets or applied voltages offers promising opportunities for ultrafast and low-power-consuming spintronic devices.
Source
Publication details
Javier Sivianes et al, Surface-State Engineering for Generation of Nonlinear Charge and Spin Photocurrents, Physical Review Letters (2025). DOI: 10.1103/h8rp-rtn8. On arXiv: DOI: 10.48550/arxiv.2503.14360
Journal information: Physical Review Letters, arXiv
https://phys.org/news/2026-01-symmetry-limits-photovoltaics-surface.html
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