Minding the Gap Makes for More Efficient Solar Cells

Using novel materials to develop thin, flexible, and more efficient photovoltaic cells is one of the hottest topics in current materials research. Transition-metal dichalcogenides,* such as MoS₂, MoSe₂, WS₂, WSe₂, have recently attracted a lot of attention in this effort. A growing number of studies suggest that the electronic properties of these materials go through a dramatic change that makes them ideal for solar energy applications. These materials can go from indirect band-gap semiconductors to direct band-gap semiconductors when their thickness reduces to a single-layer limit with the sizes of the band gap in the red to near-infrared spectrum, characteristics that are extremely advantageous for light-harvesting and light-detecting applications. Up until now however, researchers have not been able to make direct and detailed observation of how this transition occurs.

Using the molecular-beam epitaxial thin-film-growth capability at ALS Beamline 10.0.1, scientists have successfully grown high-quality thin film samples of MoSe₂ with layer-by-layer control of thickness down to a single layer. The films were characterized using in situ angle-resolved photoemission on Beamline 10.0.1, where for the first time, researchers were able to directly observe the indirect-to-direct band-gap transition. These photoemission results indicate a stronger tendency of single-layer MoSe₂ towards a direct band gap—with a larger gap size—than theoretically predicted. The ability to successfully produce uniform, large-size samples, and the observation of exact electronic structure and gap evolution with varying thickness, are huge steps on the design path to more efficient solar cells.

*dichalcogenide: Any chalcogenide containing two atoms of chalcogen (any of group 16 of the periodic table: oxygen, sulphur, selenium, tellurium, and polonium) per molecule or unit cell