2D materials combine, polarize and give rise to a photovoltaic effect

For the first time, researchers have discovered a way to obtain the polarity and photovoltaic behavior of certain non-photovoltaic, atomically flat (2D) materials. The key lies in the particular way the materials are arranged. The resulting effect is different and potentially superior to the photovoltaic effect commonly found in solar cells.

Solar energy is seen as a key technology to move away from fossil fuels. Researchers are continually innovating more efficient ways to generate solar energy. And many of these innovations come from the world of materials research. Research associate Toshiya Ideue from the Department of Applied Physics at the University of Tokyo and his team are interested in the photovoltaic properties of 2D materials and their interfaces where these materials meet.

“Quite often the interfaces of multiple 2D materials exhibit different properties for individual crystals alone,” Ideue said. “We have found that two specific materials that generally show no photovoltaic effect do so when stacked in a very particular way.”

The two materials are tungsten selenide (WSe₂) and black phosphorus (BP), both of which have different crystal structures. Originally, both materials are nonpolar (have no preferred conduction direction) and do not generate a photocurrent under light. However, Ideue and his team found that by stacking sheets of WSe₂ and BP together in the right way, the sample exhibited polarization, and when light was projected onto the material, it generated a current. The effect occurs even if the illumination area is far from the electrodes at each end of the sample; this is different from the operation of the ordinary photovoltaic effect.

The key to this behavior is how the WSe₂ and BP are aligned. The crystal structure of BP has reflective, or mirror, symmetry in a plane, while WSe₂ has three lines of mirror symmetry. When the lines of symmetry of the materials align, the sample gains in polarity. This kind of layering is delicate work, but it also reveals to researchers new properties and functions that could not be predicted by just looking at the ordinary shape of the materials.

“The biggest challenge for us will be to find a good combination of 2D materials with higher power generation efficiency and also to study the effect of changing battery angles,” Ideue said. “But it’s so gratifying to discover new emerging properties of materials. Hopefully one day this research can improve solar panels. We would like to explore more properties and functionality unheard of in nanomaterials.”

The study is published in Science.