Solar ♦ news digest
In a solar cell, the ideal material would absorb a lot of light and efficiently convert that energy into electric charges that can be easily extracted as a current.
To study the details of this process, the scientists used quantum dots composed of a light-absorbing CdSe core encased in a protective ZnS shell and surrounded by a conductive polymer. They tested the ability of the polymer to extract electric charges generated when the quantum dots absorbed light, and conducted experiments using quantum dots with cores of different sizes.
“We knew from theoretical predictions that particle size should have an effect on the charge transfer with the polymer, but no one had done this as an experiment until now, and in particular at the single-particle level,” Cotlet says.
When they varied the size of the quantum dot’s core, the scientists found that the smaller the diameter, the more efficient and more consistent the charge transfer process.
“By using a smaller core, we increased the efficiency of the charge transfer process and narrowed the distribution of the charge-transfer rate so it was closer to the ideal with less variability,” Zang comments.
The scientists were exploring a particular type of charge transfer created by the movement of holes. In electronic devices, holes can be channelled just like electrons to create electric current. And in this case extracting holes had an added benefit - it increased the time that quantum dots, which turn on and off in a blinking pattern, remained in the ‘on’ condition.
“Hole transfer inhibits blinking,” Cotlet explains. “It keeps the quantum dot optically active longer, which is better for the photovoltaic process, because charges can only be extracted when the quantum dot is on.”
“It would be impossible to see this effect with bulk samples because you can’t see the ‘on’ and ‘off’ states. When lots of quantum dots are mixed together, the signals average out. You can only see it by looking at the single nanoparticles.”
(center), and Prahlad Kumar Routh, a graduate student in the Materials Science Department at Stony Brook University, hope their research on quantum dots for solar cells will brighten our energy future. The scientists are wearing laser safety goggles required for their experiments
Cotlet’s group had previously conducted a similar study pairing quantum dots with carbon-rich buckyballs. In that study, they found the opposite effect: Buckyballs decreased the dots “on” time while enhancing the transfer of electrons.
In other applications combing dots and polymers, such as LEDs or biosensors, scientists are looking for ways to suppress charge transfer as this process becomes detrimental.
“Knowing these fundamentals and how to control these processes at the nanoscale should help us optimise the use of quantum dots for a wide range of applications,” Cotlet concludes.
More detail of this work has been published in the paper, “Core size dependent hole transfer from a photoexcited CdSe/ZnS quantum dot to a conductive polymer,” by Huidong Zang et al in Chem. Commun., 2014. DOI: 10.1039/C3CC47975G.
This research was funded by the DOE Office of Science and by the Air Force Office of Scientific Research.
Ascent Solar expands in Japan
The CIGS manufacturer is concentrating on marketing its charging products in Japan
Ascent Solar has announced distribution partnerships with Kyodo International Corp. and Nara Co Ltd. for the company’s EnerPlex line of consumer products in Japan.
Kyodo International based in Osaka, specialises in supporting the entry of consumer electronic goods into the Japanese marketplace. Tokyo based Nara has a history of bringing both large electronics as well as consumer oriented electronics, to market in Japan.
CFN’s Mircea Cotlet (standing), post-doc Huidong Zang
John Maslanik, Ascent’s Manager of Business Development says, “Japan represents a strong consumer market for our EnerPlex brand, a highly tech-savvy and affluent customer base which is also highly engaged in the renewable energy marketplace. In addition these partnerships open new opportunities for Ascent’s revolutionary solar technology to be integrated into larger
January / February 2014
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