Phosphorene could lead to ultrathin solar cells

How to make it using simple sticky tape; peeling off layers changes its properties
July 22, 2015

Australian National University | Sticky tape the key to ultrathin solar cells

Scientists at Australian National University (ANU) have used simple transparent sticky (aka “Scotch”) tape to create single-atom-thick layers of phosphorene from “black phosphorus,” a black crystalline form of phosphorus similar to graphite (which is used to create graphene).

Unlike graphene, phosphorene is a natural semiconductor that can be switched on and off, like silicon, as KurzweilAI has reported. “Because phosphorene is so thin and light, it creates possibilities for making lots of interesting devices, such as LEDs or solar cells,” said lead researcher Yuerui (Larry) Lu, PhD, from ANU College of Engineering and Computer Science.

Properties that vary with layer thickness

Phosphorene is a thinner and lighter semiconductor than silicon, and it has unusual light emission properties that vary widely with the thickness of the layers, which enables more flexibility for manufacturing. “This property has never been reported before in any other material,” said Lu.

Schematic of the “puckered honeycomb” crystal structure of black phosphorus (credit: Vahid Tayari/McGill University)

“By changing the number of layers [peeled off] we can tightly control the band gap, which determines the material’s properties, such as the color of LED it would make.* “You can see quite clearly under the microscope the different colors of the sample, which tells you how many layers are there,” said Dr Lu.

The study was recently described in an open-access paper in the Nature journal Light: Science and Applications.

* Lu’s team found the optical gap for monolayer (single-layer) phosphorene was 1.75 electron volts, corresponding to red light of a wavelength of 700 nanometers. As more layers were added, the optical gap decreased. For instance, for five layers, the optical gap value was 0.8 electron volts, a infrared wavelength of 1550 nanometres. For very thick layers, the value was around 0.3 electron volts, a mid-infrared wavelength of around 3.5 microns.

Abstract of Optical tuning of exciton and trion emissions in monolayer phosphorene

Monolayer phosphorene provides a unique two-dimensional (2D) platform to investigate the fundamental dynamics of excitons and trions (charged excitons) in reduced dimensions. However, owing to its high instability, unambiguous identification of monolayer phosphorene has been elusive. Consequently, many important fundamental properties, such as exciton dynamics, remain underexplored. We report a rapid, noninvasive, and highly accurate approach based on optical interferometry to determine the layer number of phosphorene, and confirm the results with reliable photoluminescence measurements. Furthermore, we successfully probed the dynamics of excitons and trions in monolayer phosphorene by controlling the photo-carrier injection in a relatively low excitation power range. Based on our measured optical gap and the previously measured electronic energy gap, we determined the exciton binding energy to be ~0.3 eV for the monolayer phosphorene on SiO2/Si substrate, which agrees well with theoretical predictions. A huge trion binding energy of ~100 meV was first observed in monolayer phosphorene, which is around five times higher than that in transition metal dichalcogenide (TMD) monolayer semiconductor, such as MoS2. The carrier lifetime of exciton emission in monolayer phosphorene was measured to be ~220 ps, which is comparable to those in other 2D TMD semiconductors. Our results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using monolayer phosphorene.