Photovoltaic devices (PVs) were fabricated by spray-coating an ink of copper indium diselenide (CIS) nanocrystals as the light-absorbing layer. Without high-temperature post-deposition annealing, PVs were made on glass and plastic substrates with power conversion efficiencies of up to 1.9% and 1.1%
Commercialization is at the forefront of the University of Texas’ plans as a partnership with Konarka Technologies was announced in January.
Deposition of a CdS layer on CIS and CIGS films strongly enhanced the photocurrent; for example, a nanocrystal-CIS film showed about 100 times higher photocurrent with a thin CdS layer. Capacitance measurements and Mott−Schottky plots were obtained to find the flat band potential. Incident photon to current conversion efficiencies (IPCE) and absorbed photon to current conversion efficiencies (APCE) obtained from PEC measurements were about 20% and 40−70%, respectively.
Brian Korgel, a nanomaterials chemist at the University of Texas at Austin, is making nanocrystals for paintable solar panels. They are focused on ‘CIGS’–copper, indium, gallium, selenide–and they make small particles of this inorganic material that they can disperse in a solvent, creating an ink or paint.
Korgel describes the tiny collection devices as a “solar sandwich.” “So these devices are ‘sandwiches,’ where you have the metal contact on the bottom and metal contact on the top to extract the charge out; and the middle part is the part that absorbs out the light,” explains Korgel.
This paint, made of the CIGS nanocrystals, can be sprayed on plastic, glass and even fabric to create a solar cell.
“So what we’re able to do is create radically new ways of depositing inorganic films to make solar cells, and so we’re trying to meet this challenge of much lower cost of manufacturing,” he says.
One way to create these cells on a very large scale would be to print them on thin, flexible sheets, the same way huge presses now print newspapers. “And the final product would ideally look something like today’s shingles,” says Vahid Akhavan, one of Korgel’s graduate research assistants. “You want to produce something that is very user friendly. So you could go to your local hardware store, buy them and install them on your roof.”
These shingles would do double duty, generating electricity while serving as roofing material. They would be also stand up better in bad weather, such as hail and windstorms, than some of today’s more fragile solar collectors.
A lot of challenges need to be conquered before solar energy becomes so commonplace. High on that list is improving the efficiency of these nanomaterial cells. “Right now, we have made devices that have an efficiency of 3 percent, and to be commercial, you really need to be at 10 percent,” says Korgel. “But I think we can get to 10 percent. Those are just engineering challenges; they are not necessarily easy, but they are not fundamental roadblocks.”
Another obstacle will be determining what raw materials can be used if this technology can be mass produced. The copper, indium, gallium, and selenide are not all cheap or readily available.
“Ultimately, thinking much further out, you want to go with a technology where you use elements that are earth-abundant,” says Korgel.
One possibility is silicon, which is made from sand, abundant across our planet. But extracting the silicon from the sand is now an incredibly energy-intensive process and the chemicals it takes to do that are pretty harsh on the environment.
A Different Approach at MIT
Revisiting a once-abandoned technique, engineers at the Massachusetts Institute of Technology (MIT) have successfully created a sophisticated, yet affordable, method to turn ordinary glass into a high-tech solar concentrator. The technology, which uses dye-coated glass to collect and channel photons otherwise lost from a solar panel’s surface, could eventually enable an office building to draw energy from its tinted windows as well as its roof.