Abstract
We study the effects of strain on the properties and dynamics of Wannier excitons in monolayer (phosphorene) and few-layer black phosphorus (BP), a promising two-dimensional material for optoelectronic applications due to its high mobility, mechanical strength, and strain-tunable direct band gap. We compare the results to the case of molybdenum disulphide () monolayers. We find that the so-called funnel effect, i.e., the possibility of controlling exciton motion by means of inhomogeneous strains, is much stronger in few-layer BP than in monolayers and, crucially, is of opposite sign. Instead of excitons accumulating isotropically around regions of high tensile strain like in , excitons in BP are pushed away from said regions. This inverse funnel effect is moreover highly anisotropic, with much larger funnel distances along the armchair crystallographic direction, leading to a directional focusing of exciton flow. A strong inverse funnel effect could enable simpler designs of funnel solar cells and offer new possibilities for the manipulation and harvesting of light.
- Received 7 April 2016
DOI:https://doi.org/10.1103/PhysRevX.6.031046
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Advances in solar-cell technologies are rapidly changing the landscape of the global energy supply. Even so, conversion efficiencies are ultimately limited to a maximum of roughly 40% in conventional silicon-based solar-cell designs. New materials, particularly atomically thin semiconductors such as black phosphorus, have recently heralded promise as being able to break this limit, in particular, by engineering their optoelectronic properties via applied strain. Here, we demonstrate how strain-engineered, nonuniform band gaps might allow black-phosphorus solar cells to funnel harvested energy toward collector regions for greatly enhanced efficiencies.
Conventional solar cells rely on a process in which a photon with an energy larger than the material’s band gap knocks out an electron from an atom and sends it into an outer circuit to do useful work. Often, however, this newly freed electron ends up falling back onto the atom from whence it came. As a result, the energy is absorbed into a new photon or is lost as useless heat. By theoretically applying specific strain profiles to black phosphorus—a remarkably pliable material that can withstand large strains and whose band gap can be tuned according to strain—we are able to accelerate photoexcited electrons away from the originating atom and into the outer circuit, thereby increasing the efficiency of the solar cell. This mechanism, dubbed the inverse funnel effect, enables efficient control of energy flow in black phosphorus and opens up a whole new mode of operation of strain-engineered solar cells.
We expect that our findings will pave the way for black phosphorus becoming a valuable platform for the development of cost-effective renewable energy sources.