Advances in solar cell technologies are rapidly changing the landscape of global energy supply. Conversion efficiencies are however ultimately limited to a maximum ~40% in conventional silicon-based solar cell designs. New materials, particularly atomically thin semiconductors such as black phosphorus, have recently brought to the table radically new possibilities that could allow us to break this limit, in particular by engineering their optoelectronic properties through applied strain. In this work we demonstrate how strain-engineered non-uniform bandgaps could allow black phosphorus solar cells to funnel harvested energy towards collector regions for greatly enhanced efficiencies.
Conventional solar cells operation relies on a key process, whereby a photon of energy larger than the bandgap knocks out an electron from an atom in the material, and sends it into an outer circuit to do useful work. Often, however, the electron ends up falling back onto the atom whence it came, loosing the absorbed energy into a new photon or useless heat. By applying specific strain profiles to black phosphorus, photoexcited electrons are accelerated away from the originating atom and into the outer circuit, increasing the solar cell efficiency.
This mechanism, that we dubbed inverse funnel effect, allows for an efficient control of energy flow in black phosphorus, and enables a whole new mode of operation of strain-engineered solar cells. Coupled to its high-quality electrical properties and low fabrications costs, these advances could make black phosphorus a valuable platform for the development of cost-effective renewable energy sources. [Full article]