PORTLAND, Ore. -- Scientists have demonstrated a doubling of the number of electrons produced by carbon-based photovoltaic polymer potentially doubling the efficiency of any solar cell. The process called "singlet fission" produces "identical twin" electrons from a single photon, instead of the normal one, dramatically boosting the theoretical maximum output of solar cells. Instead of loosing energy to heat, an extra electron is produced by the process of applying a polymer solution to an existing solar cell.
"One of the challenges in improving the efficiency of solar cells is that a portion of the absorbed light energy is lost as heat," lead scientist at Brookhaven National Labs, Matt Sfeir, told EE Times. "In singlet fission, one absorbed unit of light results in two units of electricity via a multiplication process rather than resulting in one unit of electricity and heat as would occur in a conventional cell."
To boot, the carbon-based polymer (BaTi2Sb2O and BaTi2As2O) can be liquified for mass production using cheap manufacturing processes that essentially "print" onto conventional solar cells.
Template of design for doubling the number of electronics produced by solar cell using a coating that induces singlet fission.
(Source: Brookhaven National Labs)
"Our materials would be used as a 'sensitizer' on conventional photovoltaic (organic or inorganic)," Sfeir told us. "Unlike previously reported fission materials, these polymers work efficiently while dissolved in liquids, potentially allowing for industrial scale manufacturing."
Other researchers have produced singlet fission materials, the most well known of which is called pentacene. But Sfeir claims their material is almost as good, and more importantly, easier to apply onto existing solar cells.
"Pentacene works with nearly 200% efficiency in films. However, ours is the only material that works well in solution, where fission is generated on a single polymer chain. Our material is 170% efficient in solution," Sfeir told us. "We also have a general framework that we hope can be used to generate a wide variety of singlet fission capable molecules and polymers. We have been exploring some of these concepts to make materials that are 200% efficient."
Sfeir's team is also working to go beyond conventional bulk type solar cells to a third-generation concept based on other inorganic (non-carbon) nanomaterials.
Postdoctoral fellow Erik Busby and Matt Sfeir with optical equipment they used to study charge carrier production in organic photovoltaic polymers at Brookhaven Lab's Center for Functional Nanomaterials.
(Source: Brookhaven National Labs)
"The dream is to build hot-carrier solar cells that could be fully assembled using solution processing of our organic singlet fission materials," Sfeir told us.
Sfeir claims that the material can also be tuned for specific applications -- specific types of solar cells, that, for instance, absorb different parts of the available spectrum of light.
"We have demonstrated a general design principle for a class of materials whose properties can be tuned for a specific application. It is our hope that this will lead to a much more diverse and abundant set of materials that can be optimized for particular types of solar cells," according to Sfeir.
The Center for Functional Nanomaterials [CFN] at Brookhaven Natl. Labs used time-resolved optical spectroscopy to induce and quantify singlet fission in Sfeir's material, in which two triplet type excitons are produced from a single laser photon. A technique called “transient absorption” was used in analogy to a camera with a very fast shutter, according to Sfeir.
"We impulsively put light energy into the material with a laser pulse and then watch what happens to that energy using a series of weaker light pulses," according to Sfeir. "Surprisingly, we identified the multiplication process (singlet fission) as the dominant decay process. In addition, the CFN computational cluster was used to model these materials and understand the design requirements for singlet fission."
The Laser-Electron Accelerator Facility (LEAF) at Brookhaven National Labs was used to compare the behavior of individual triplet excitons generated via pulse radiolysis to the behavior of triplet pairs generated via direct photon absorption (at CFN), according to Sfeir. "The differences observed between these two experiments allow us to unambiguously identify fission as the primary decay process."
Next, the scientists aim to produce a large catalog of materials that will work using the singlet-fission process, then begin to optimize the organic carbon-based materials for solar cell applications. Beyond demonstrating producing more electrons per photon, they want to push the efficiency to build a working device that harnesses the extra excitation even more efficiently. They also want to produce a third-generation inorganic solar cell that takes advantage of what they've learned by optimizing the organic solar cell.
"Our dream is to build a solar cell that could be fully assembled using solution processing based on inorganic nanoparticles decorated with our organic fission materials," Sfeir told us.
Columbia University researchers Jianlong Xia, Jonathan Low, Rui Song, and professsor Luis Campos produced the materials along with professor Xiaoyong Zhu. The experiments on the materials to identify and measure the singlet fission process were conducted at BNL by researchers Erik Busby (a postdoctoral researcher at CFN), Qin Wu (also at CFN), John Miller, and Sfeir at CFN.
Funding was provided by the Center for Re-Defining Photovoltaic Efficiency Through Molecular-Scale Control, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences; the National Science Foundation; the Center for Functional Nanomaterials and Brookhaven National Laboratory.
— R. Colin Johnson, Advanced Technology Editor, EE Times