Researchers at the Australian National University (ANU) in Canberra, have successfully created an ultralight, ultrathin alternative to conventional silicon solar cells using an unconventional technique—sticky tape. With a thickness of just over 0.5 nanometers, this single-atom layer of black phosphorus called phosphorene could be the metamaterial that the world of solar power has been waiting for.
The ANU team adopted the “Scotch tape” technique used by Nobel Prize Winners Andre Geim and Konstantin Novoselov, who discovered graphene in 2004 by using sticky tape to remove single atom thick layers of carbon from pencil led. Inspired by the two Nobel laureates, researchers led by Dr. Yuerui (Larry) Lu, used sticky tape to carefully peel monolayers of phosphorene from black crystalline phosphorus.
Phosphorene is the 2D allotrope of phosphorus. An allotrope is an alternative form of a chemical element within the same physical state. In the same way that the allotropes of carbon are diamond, graphite and graphene, phosphorene is the monolayer form of crystalline black phosphorus.
While phosphorene is very similar to graphene in structure, the two materials have very different electrical properties. A monolayer of phosphorene has an optical band gap of 1.75 eV, and can emit red light at a wavelength of 700 nm. The property that has brought much excitement to the scientific community is the ability to use thickness of the layers to fine tune its optical gap and light emitting properties.
Stacking multiple layers of phosphorene has the unique effect of reducing the optical gap and increasing the wavelength of light emitted. Stacking 5 layers of phosphorene is enough to lower the optical gap to 0.8 eV and reach an infrared wavelength of 1550 nm. This property has the added bonus of allowing the scientist to tell how many layers of phosphorene are present based on the color of the sample. Dr. Lu says that “this property has never been reported before in any other material.” In addition to being lightweight, flexible and thin, phosphorene outperform silicon as a semiconductor.
Since phosphorene is a lightweight, ultrathin, p-type semiconductor it is a perfect candidate for thin-film solar cells. Conventional thin film solar cells are made by depositing one or more layers of photovoltaic material onto a plastic, glass or metal substrate. A typical sandwich may consist of an antireflection coating on top, an n-type “window” layer semiconductor, a p-type “absorber” layer semiconductor, an ohmic contact and a substrate.
Depending on the materials, the order may be a little different, as with the amorphous silicon cells where the p-type semiconductor is placed on top. The layer-number dependent optical band gap of phosphorene would allow you to tailor different semiconducting layers to different spectrums of light. The resulting multijunction cell would be able to cover a broader spectrum of light without the use of exotic materials. Furthermore, the ordered structure of phosphorene makes it a superior charge carrier to conventional thin film solar materials like silicon—despite its infancy as a new material, lab scale phosphorene trilayer solar cells have already been reported with efficiencies of 18%.
Thin film solar cells have the potential to revolutionize how solar power is gathered across the globe. Lightweight, flexible and semitransparent, they can be used in building photovoltaics, as a glazing material painted onto windows, or as rigid solar panels at photovoltaic power stations. Given enough time, phosphorene solar cells could one day supplant silicon as the semiconductor of choice for thin film solar cells.