A new study shows that perfectly symmetric materials can generate strong photocurrents when their surfaces are designed to accommodate special electronic states, overcoming a long-standing limitation in photovoltaic design. The work points to new strategies for both solar energy conversion and ultrafast spintronic technologies by exploiting the physics of surfaces rather than relying solely on the properties of bulk crystals.
Conventional solar cells use interfaces such as p-n junctions to separate photo-generated charges and produce usable power. Another mechanism, known as the bulk photovoltaic effect, can create direct current in a homogeneous material without such bonds, but traditionally requires a non-centrosymmetric crystal structure, severely narrowing the pool of candidate compounds for practical devices.
The research team from EHU, the Materials Physics Center, nanoGUNE and DIPC investigated how this symmetry requirement can be relaxed if the electronic structure of the surface is taken into account. Using basic calculations, they investigated metals and semiconductors with strong relativistic spin-orbit interactions and found that their surfaces can support electronic states significantly different from those in the bulk interior.
These surface states locally break the inversion symmetry, even though the underlying crystal remains perfectly symmetrical. As a result, when light shines on the surface, the electronic response becomes nonlinear and robust photocurrents are generated along the surface. The calculations predict not only charging currents, but also pure spin-polarized currents confined to the surface region.
After pinpointing the mechanism on the well-studied Au(111) surface, the researchers identified Tl/Si(111) as a particularly promising platform to realize this effect. According to their results, Tl/Si(111) should exhibit photocurrents similar in magnitude to those observed in leading ferroelectric materials that rely on bulk non-centrosymmetry to produce large photovoltaic responses.
The work suggests a new direction for the conversion of light to electricity, with device designers focusing on tuning the electronic properties of the surface rather than just looking for complex, non-centrosymmetric crystals. By designing suitable surface states on otherwise symmetrical substrates, it may become possible to build efficient photovoltaic systems from a much wider range of materials.
In addition to energy harvesting, the ability to generate and control spin currents with light at a surface has important implications for spintronics. The proposed mechanism does not require magnetic materials or applied voltages to create spin-polarized currents, opening a path to ultrafast, low-power spin-based devices that operate purely via optical excitation.
The study appears in the journal Physical Review Letters, where the authors outline both the theoretical framework and specific experimental signatures that could confirm their predictions. These features include characteristic flow directions and surface spin textures, which experimentalists can investigate using established spectroscopic and transport techniques.
Research report: Surface-State Engineering for the Generation of Nonlinear Charge and Spin Photocurrents
