Published on March 1st, 2012 | by Zachary Shahan3
New World Record for Tandem Polymer Solar Cell Efficiency
“Scientists boosted the significance of tandem polymer solar cells by successfully testing cells with low-bandgap polymers that achieved certified conversion efficiencies of 8.62 ± 0.3% with respect to standard terrestrial reporting conditions,” the National Renewable Energy Laboratory (NREL) announced today.
“Further, after the researchers incorporated a new infrared-absorbing polymer material provided by Sumitomo Chemical of Japan into the device, the device’s architecture proved to be widely applicable and the power-conversion efficiency jumped to 10.6 percent — a new record,” UCLA adds.
This is the highest independently measured efficiency for a polymer solar cell.
NREL and UCLA researchers achieved the new record efficiency and have authored a report, “Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer,” published in the journal Nature Photonics on February 12.
“We have been doing research in tandem solar cells for a much shorter length of time than in the single-junction devices,” said Gang Li, a member of the research faculty at UCLA Engineering and a co-author of the paper. “For us to achieve such success in improving the efficiency in this short time period truly demonstrates the great potential of tandem solar cell technology.”
“Everything is done by a very low-cost wet-coating process,” Yang said. “As this process is compatible with current manufacturing, I anticipate this technology will become commercially viable in the near future.”
Yang is hoping for 15% efficiency in the next few years.
The record efficiency was achieved in a test at NREL’s Spectrolab X-25 solar simulator (aka the One-Sun Solar Simulator), a solar simulator with wide current and voltage ranges.
“Accurately measuring tandem cells is difficult. The NREL simulator provide unparalleled accuracy by precisely adjusting the spectrum, and did so in a fraction of the time that other simulators could do the job,” said NREL Principal Engineer Keith Emery. Each device junction must behave the same under the simulator spectrum as it would under the reference spectrum. It requires significant adjustment of the simulator spectrum, normally a very tedious process.
NREL’s One-Sun Solar Simulator was able to turn an ordeal that typically takes all day into a five-minute task. “We think it’s also more accurate because we can better adjust the spectrum,” Emery said.
Tandem Solar Cells & Polymer Solar Cells
Tandem solar cells are also known as multi-junction solar cells, a type of technology we’ve written about a number of times.
“Envision a double-decker bus,” said Yang Yang, a professor of materials science and engineering at UCLA Engineering and principal investigator on the research. “The bus can carry a certain number of passengers on one deck, but if you were to add a second deck, you could hold many more people for the same amount of space. That’s what we’ve done here with the tandem polymer solar cell.”
Tandem or multi-juntion solar cells have been advancing for years, but advancements in polymer solar cells have lagged a bit due to one specific handicap.
“Tandem solar cells by their design can harvest a broader spectrum of the sun’s rays than single solar cells,” NREL notes. “But polymer solar cells have lagged because it’s been difficult finding a suitable low-bandgap polymer.”
Achieving the Record Efficiency
I’ll be honest, the technicalities here are beyond my expertise, as I’m sure they are for 99.99% of the population and most of our readers, but if you like reading about such scientific technicalities, here are more details (summarized, albeit) from NREL:
In sophisticated tests, the researchers were able to demonstrate highly efficient single and tandem polymer solar cells featuring a low-bandgap conjugated polymer (PBDTT-DPP: bandgap, 1.44 eV). When they tested a single-layer device with the polymer it converted the sun’s rays into electricity at an efficiency of about 6%. When the polymer was applied to tandem solar cells, the power conversion efficiency reached 8.62%.
The UCLA group recently improved on this result by incorporating a new infrared-absorbing polymer from Sumitomo Chemical in Japan. NREL measured the power conversion efficiency at 10.6+/-0.3% under standard terrestrial reporting conditions.
Stacking layers of different materials in a solar cell means multiple bandgaps, each of which captures a different part of the solar spectrum. The challenge is to achieve a high current by efficiently using the low-energy portion of the solar spectrum, and achieving a small energy bandgap – less than 1.5 eV.
Support for the study came from the National Science Foundation, the U.S Air Force Office of Scientific Research, the U.S. Office of Naval Research, and the U.S. Department of Energy.