Inside cameras and solar panels, photodetectors absorb light and convert it to useful electronic signals. How well these devices perform depends on how efficiently the detectors can turn incident photons into electrons and corresponding positively charged species called holes. These electron-hole (e-h) pairs can then move through the material to generate electricity.
One way to improve this efficiency is to shrink the materials down to the nanoscale. Scientists have used nanocrystal quantum dots, carbon nanotubes, and graphene to achieve efficiencies beyond 100%, meaning a single photon produces more than one e-h pair, an effect known as e-h multiplication.
A team of researchers at the University of California, Riverside, led by Nathaniel M. Gabor, now reports greater than 300% efficiency with a class of ultrathin two-dimensional materials called transition-metal dichalcogenides. The photodetector, made of two atomic layers of WSe2 stacked on a single layer of MoSe2, is almost transparent and about the size of a camera pixel (Nat. Nanotechnol. 2017, DOI: 10.1038/nnano.2017.203).
The researchers propose that when a photon strikes the top WSe2 layer, it sets an electron in motion that can then hop to the MoSe2 layer to create another e-h pair. By applying a small voltage to the layers, the team was able to further enhance the device’s efficiency to 350%, creating between 3–4 e-h pairs per photon.
Work on the 2-D metal dichalcogenides is still in the early stages, Gabor says, but the materials’ efficiencies are “on par” with those of more mature devices made with nanocrystal quantum dots. He adds that one advantage of using dichalcogenides is that they tend to be crystalline, which could make them better at electron transport compared with quantum dots.
Pasqual Rivera, who studies 2-D materials at the University of Washington, says e-h multiplication is promising for raising the efficiency of photovoltaic devices. “With a library of over a thousand 2-D materials, the vast majority of which are yet to be characterized, the prospect of engineering layered nanoscale devices that leverage this understanding for even greater efficiency is highly probable.”