When a conventional solar cell or photodetector absorbs a single photon, a single electron-hole pair (or exciton) is generated. However, in quantum dots (which are small pieces of semiconductor just several nanometres in size), electrons can efficiently interact with each other after they have absorbed light, generating multiple electrons from a single photon. This effect is known as carrier multiplication, and could help make cheaper and more efficient solar cells as well as new types of photodetectors.

Until now, it has been difficult to observe and quantify this multiplication process in working devices. Researchers at the Center for Advanced Solar Photophysics at Los Alamos led by Victor Klimov say they may now have a found a way around this problem and have directly resolved carrier multiplication by monitoring photocurrent transients in specially engineered photodetectors. These devices can distinguish between events occurring just 50 picoseconds apart.

Addressing an "important question"

“Previous research in this field mainly relied on optical spectroscopy for detecting carrier multiplication and quantifying its efficiency,” he tells nanotechweb.org. “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.”

We incorporate quantum dots made from lead selenide (PbSe) as part of the photodetector device itself, and the dots form its active photoconductive layer, explains team member Jianbo Gao. “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.”

"Overcoming" Auger decay

One of the main difficulties when studying carrier multiplication is being able to quickly extract photogenerated charge carriers (electrons and holes) before they recombine, adds team member Andrew Fidler. “In the case of multiexcitons, this 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.”

The research is detailed in Nature Communications doi: 10.1038/ncomms9185.