A New Way to Improve Efficiency of Solar Cells by Overcoming Exciton 'Traps'

September 24, 2014
Phase diagram of exciton topological phases

Phase diagram of exciton topological phases: a,b, Diagrams for the upper and lower energy exciton Hamiltonians (v = L, U), respectively. Light and dark blue regions denote topologically non-trivial phases with Chern number equal to − 1 and 1, exhibiting edge states with anticlockwise and clockwise exciton currents, respectively. Switching the direction of the magnetic field to Bz < 0 inverts these chiralities. Black lines correspond to topologically trivial phases with and are located along the critical parameters θb = ± θa, π ± θa, where the two sublattices become identical and the gaps of the respective Hamiltonians vanish.. [Figure reprinted by permission from Macmillan Publishers Ltd: see Joel Yuen-Zhou, Semion K. Saikin, Norman Y. Yao, & Alán Aspuru-Guzik, "Topologically protected excitons in porphyrin thin films," Nature Materials (2014) | doi:10.1038/nmat4073.]

A major limitation in the performance of solar cells happens within the photovoltaic material itself: When photons strike the molecules of a solar cell, they transfer their energy, producing quasi-particles called excitons—an energized state of molecules. That energized state can hop from one molecule to the next until it's transferred to electrons in a wire, which can light up a bulb or turn a motor.

But as the excitons hop through the material, they are prone to getting stuck in minuscule defects, or traps—causing them to release their energy as wasted light.

Now a team of researchers at MIT and Harvard University [including Harvard Postdoc Norman Yao] has found a way of rendering excitons immune to these traps, possibly improving photovoltaic devices' efficiency. The work is described in a paper in the journal Nature Materials. (Read the rest of the Phys.org article...)