by Jenna Bilbrey
Materials Research Society | Published: 28 March 2013
Schematic of quantum dot photovoltaic device
in action. Credit: Prashant Kamat, University of Notre Dame. Click
image to enlarge.
“Our worldwide energy demand is increasing close to 2% every year, and, if you put in the exponential growth model, it means in the next 35 years our energy demands will [almost] double,” Kamat says as he recalls BP’s recently-released Energy Outlook 2030. The report estimates rapid growth of renewable energy technologies, including solar energy, rising 8.2% per year from 2010 to 2030. But for renewable technologies to become cost competitive with fossil fuels, more efficient methods of harvesting renewables must be found. Quantum dot-based solar cells have gained enthusiasm in publications such as Science and the Nature family, winning the hearts of–and, more importantly, funding for–researchers worldwide.
In the early 1960s, Shockley and Queisser, two of the premier solar-cell researchers, calculated the efficiency limit for single-material, semiconductor solar cells to be around 32%. The ideal band gap for the material is estimated at 1.3–1.4 eV, which is close to that of silicon’s 1.1 eV band gap. Commercial solar cells have efficiencies between 15-21%–the “world record” of 24%, held by SunPower Corp., may be surpassed by Sun3D, Inc., who promised commercialization of 30%-efficient cells by the end of 2013. As of now, quantum dot solar cells have only reached a maximum efficiency of 10.9%.
Solar cells transform photons into electron-hole pairs, called excitons, and collect the pairs at opposite electrodes, generating current. When light of a certain wavelength hits a layer of quantum dots, electrons are excited into the conduction band. If there is nothing to whisk away the newly formed electron, it will recombine with its hole and release energy, typically in the form of heat. But if some material removes the electron–say, for example, TiO2–then, as the electron is carried toward one electrode and the hole toward the other, a current is generated.
Unique from both molecules and bulk material, the band gap of a quantum dot is controlled by its physical size, which is tunable by altering reaction conditions such as temperature and time. To create an exciton, the energy of the photon must be greater than the band gap; the band gap determines the usable solar spectrum. “In the case of quantum dots, you can control the optical and electronic properties just by changing the size,” says Kamat, “so you can get the entire region from infrared to ultraviolet just by changing the size.”
A large factor holding back the efficiency of quantum dot solar cells is the solar range the dots can absorb. Common materials, such as CdS and CdSe, only absorb in the visible region, while PbS picks up photons from the infrared region, which is a large part of the solar spectrum that reaches Earth. Ternary quantum dots, which use three types of elemental material, have an even greater potential to absorb a wide spectral range as properties can be tuned by both composition and size. Kamat notes it is possible to “tune the entire absorbance, within the visible range, by just changing the composition from sulfur to selenium.”
Forming layered cells is the leading method to expand the solar range. Ordered layers of material with different band gaps create a cascade effect, where high-energy electrons flow through the layers with minimal recombination. As the Shockley-Queisser limit only applies to single-layer solar cells, layered cells have the potentials to increase efficiency. One paper estimates that for quantum dot solar cells, with infinite layers perfectly matched to the solar spectrum, the conversion efficiency could increase to 66%.
Another way to increase the efficiency of cells is through multiple exciton generation, made possible by the unique size of quantum dots. Spearheaded by the National Renewable Energy Laboratory (NREL) in the mid-2000s, high-energy photons from ultraviolet light are absorbed to form multiple excitons from one photon. The produced “hot electrons” have short lifetimes, recombining with their holes to generate heat if not removed. “If you have two excitons in one quantum dot, then they will relax faster,” explains Matthew Beard, a senior scientist at NREL, “You need to get that exciton off in a few 10s of picoseconds, but that’s doable.” One trick to minimize recombination is to remove the “hot holes” by trapping with other materials, such as sodium biphenyl. Of quantum dot solar cells, Beard says, “It’s definitely an area of research that is exploding and has a lot of potential. More and more research efforts are going in that direction.”
Arthur Nozik, who holds a joint appoint at NREL and the University of Colorado, Boulder, has been studying the subject of photoelectrochemisty since its inception in the 1960s. He says quantum dot solar cells might surpass other methods, “but you have to underline might. The potential is there.” He emphasizes that further optimization is needed–“$1 per peak watt is the target,” which is about 6 cents per kilo-watt hour. The target, set by the former Secretary of the U.S. Department of Energy Steven Chu, can be reached by either achieving high efficiency cells or lowering the overall cost of materials; research is focused on both. Nozik illustrates that a cell with 50% efficiency and a total installed cost of $200 per square meter will lower the cost of power to 2 cents per kilowatt-hour, which is less than current price of coal.
Looking toward the future, “we have limited options,” says Kamat. “To make an impact or change, any technology that comes out should be transformative,” and to develop transformative materials, “we can make use of nanomaterials, in this case, quantum dots is one of the options. There is no one silver bullet, but [quantum dots] could be a major contribution.”
