In this artist's representation, electrons race towards the junction between two atomic layer materials. As they impact the interface between layers, they give off energy that can free additional electrons and increase the overall current in the device.” Credits: QMO Lab and Max Grossnickle)
The photodetector used in the experiment is shown on top of a dime for scale. Just microns across and only nine atoms thick, this nanoscale detector could easily fit into tiny packages. (Credit: QMO Lab and Max Grossnickle)With research into atomic layer materials on the increase, scientists at the University of California, Riverside, have developed a prototype photodetector that improves the efficiency of its light-to-electricity conversion, and which could lead to significant developments in how solar energy is collected.
As well as being in solar cells, tiny photodetectors – measuring only a few microns – are already found in everyday devices such as mobile phones, cameras and remote controls. They convert light into electrons whose movement then generates an electronic signal. This new photodetector, which invokes quantum mechanical processes, was made by combining two different inorganic materials, with its efficiency being controlled by the fact that light energy is either converted into waste heat or useful electronic power.
In their study, published in Nature Nanotechnology [Barati et al. Nat. Nanotechnol. (2017) DOI: 10.1038/nnano.2017.203], the team stacked two atomic layers of tungsten diselenide (WSe2) on a single atomic layer of molybdenum diselenide (MoSe2), which works to provide properties very different from that of the parent layers, and bringing customizable electronic engineering.
“Understanding such processes, together with improved designs that push beyond the theoretical efficiency limits, will have a broad significance with regard to designing new ultra-efficient photovoltaic devices”Nathaniel Gabor
At the atomic level, electrons exist in states that determine their energy level, so when electron moves between states they either acquire or lose energy. Above a certain energy level, electrons are able to move freely, while an electron moving into a lower energy state can transfer sufficient energy to loosen another electron. Usually, one photon can at most generate a single electron, but here they showed one photon generating two electrons or more through a process known as electron multiplication.
Electron multiplication in standard silicon photocell devices usually needs voltages of between 10 and 100 volts, but to observe the doubling of electrons here they used only 1.2 volts. As most commercial semiconductor products have to operate at the one-volt level, this device is a substantial improvement for low power, high efficiency operation. As team leader Nathaniel Gabor said, “Understanding such processes, together with improved designs that push beyond the theoretical efficiency limits, will have a broad significance with regard to designing new ultra-efficient photovoltaic devices”.
In such tiny materials, electrons act like waves, and the process of generating two electrons from one photon is then realizable at very small length scales. The materials provide the ability to custom-build electronic devices at the atomic scale, and could lead to atom-thick transistors, while their flexibility could find applications in wearable photovoltaics. The team now hope to explore further the quantum optoelectronic properties of the devices, and assess whether energy storage could be made more efficient with devices that behave quantum mechanically.