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Minding the Gap Makes for More Efficient Solar Cells Print
Thursday, 19 December 2013 11:01

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.


 

Angle-resolved photoemission data on (a) single layer, (b) bilayer, (c) trilayer, and (d) 8 layer samples of MoSe2, with theoretical calculations for single layer and 8 layer systems. Experimental data show clear indications of a direct band gap for the single layer and an indirect band gap for the samples thicker than bilayer.

 

Crystal structure of single layer MoSe2 overlaid with angle-resolved photoemission data from the single layer sample. Experimental data show a clear indication of a direct band gap.

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)

 


 

Work performed on ALS Beamline 10.0.1

 

Citation: Y. Zhang, T.-R. Chang, B. Zhou, Y.-T. Cui, H. Yan, Z. Liu, F. Schmitt, J. Lee, R. G. Moore, Y. L. Chen, H. Lin, H.-T. Jeng, S.-K. Mo, Z. Hussain, A. Bansil and Z.-X. Shen, “Direct observation of the transition from indirect to direct band gap in atomically-thin epitaxial MoSe2,” Nature Nanotechnology (2013),DOI: 10.1038/nnano.2013.277