Researchers at the Los Alamos National Laboratory in the US have developed the first ultrafast photodetector made from quantum dots that is capable of directly observing the extra electrons in a process called “carrier multiplication”. This process has the potential to boost the efficiency of solar cells and understanding how it occurs could lead to the development of new types of light and radiation detectors.
When a conventional solar cell or photodetector absorbs a photon, a single electron–hole pair called an exciton is created within the device’s active semiconductor layer. In nanometre-sized pieces of semiconductor called quantum dots, electrons can interact more strongly with each other after they have absorbed light, and this results in multiple electrons being unleashed by a single photon. This effect is known as carrier multiplication, and it could help to make cheaper and more efficient solar cells as well as new types of detectors.
Rapid changes
Until now, it has been difficult to observe and quantify this multiplication process as it happens in real time in working devices. To address this problem, researchers at the Center for Advanced Solar Photophysics at the Los Alamos National Laboratory led by Victor Klimov have created a specially engineered photodetector that reveals carrier multiplication by monitoring the rapid changes in electrical current that occur when light is absorbed by the device. Indeed, the device can distinguish between events occurring just 50 ps apart.
“Previous research in this field mainly relied on optical spectroscopy for detecting carrier multiplication and quantifying its efficiency,” Klimov explains. “However, it remained unclear whether results from these spectroscopic measurements could be reproduced in photocurrent measurements in real-life devices. Our new study has allowed us to address this important question.”
At the heart of the new photodetector are quantum dots made from lead selenide, which form the active photoconductive layer of the device. As team member Jianbo Gao explains, “Using an appropriate photodetector design combined with ultrafast electronics, we have been able to resolve very short photocurrent spikes coming from multiexcitons produced in a carrier multiplication process.”
Avoiding Augers
One of the main difficulties when studying carrier multiplication is being able to quickly extract charge carriers (electrons and holes) before they recombine, adds team member Andrew Fidler. “In the case of multiexcitons, the recombination process is governed by ‘Auger decay’, which occurs on extremely short, picosecond time scales. We have shown that by treating the outer layers of the quantum dots with 1,2-ethanedithiol and hydrazine, we can indeed extract the charges from the quantum dot before they recombine,” he says.
The research is described Nature Communications.
- This article first appeared on nanotechweb.org
