The Quest Continues for High-Efficiency Polymer Solar Cells
What's Hot in Physics; March/April 2011
by Simon Mitton
The papers in the current Physics Top Ten can be grouped into three themes. Cosmology and survey astronomy still enthrall sky watchers, with three papers (#1, #4 and #5) in the top half of the rankings. Their total of 2,055 citations emphasizes the extent to which the cosmology "industry" now dominates astronomy, a field in which the number of possible universes, multiverses, and braneworlds has created a baffling, rich topological landscape cluttered with logical puzzles.
The second action item highlighted is graphene, in papers #6, #7, and #8. Graphene has the potential to displace silicon in spintronics. Much of the laboratory research is directed toward understanding how to manipulate the energy levels in graphene (#8).
Our third flavor, which I want to examine more widely, is the quest for highly efficient means of turning sunlight into electricity. Papers #2 and #3 consider polymer solar cells (PSCs) based on bulk heterojunction (BHJ) structures, which are composites of an electron-donating polymer and an electron-accepting fullerene.
The potential for useful devices based on BHJs is remarkable because of their relative ease of manufacture, compared to silicon. It is a simple recipe: blend polymers with fullerenes in organic solvents, and see the onset of self-organized networks that can be fabricated using spray coating. Suitable technologies include printing by inkjet, or gravure, or flexography. It is tempting to imagine a future in which the final coat of paint on an all-electric car is a PSC network. But that is a long way off because PSCs are still rather inefficient overall, and that’s the problem addressed in #2 and #3.
"There is still much to do before PSCs come to a store near you. A doubling of efficiency would help."
The effectiveness of a PSC is dependent both on the internal quantum efficiency, which measures the rate of incoming photons triggering pairs of charge carriers that can be harvested, and on the external efficiency, which is the conversion rate to useful energy. Paper #2, which has scored 336 citations in 18 months, reports on the performance of a heterojunction that marries a co-polymer to a fullerene derivative based on C70. The internal efficiency is close to 100%, which is as good as it gets. But then the next question to solve: how to boost the output efficiency? That’s where #3 is rather encouraging.
This paper deals with increasing the open-circuit voltage. There are many factors in play when converting sunlight into electricity. Two are obvious: the bandwidth of the absorption range needs to be maximized, and the photon-electron conversion rate (~ 100% in #2) must reach as high as possible. The big gain reported in #3 concerns reducing the bandgap in energy levels within the polymers. That’s important because low-bandgap polymers work at longer wavelengths, and therefore capture more sunlight. Much effort has already been expended in this field of research.
The kernel of #3 is the description of a PSC based on the structure of a low-bandgap polymer (known in the field as PBDTTT), which can be tuned little by little to produce an output voltage of 0.76 V. That does not produce any electric shocks, but it is sufficient to boost the power-conversion efficiency to 6.77% according to the U.S. National Renewable Energy Laboratory.
There is still much to do before PSCs come to a store near you. A doubling of efficiency would help. Then there’s the question of moving from the clean room of a lab to a consumer setting, where long device lifetimes are already looking rather challenging.
It would be unfortunate if electricity produced efficiently from sunlight then gets wasted in the reverse process of converting electricity into light. Paper #10 helps here with important progress on the development of organic light-emitting diodes (OLEDs). Although small LEDs emitting a pinpoint of light now shine in cheap consumer items, a highly efficient large-area light source is clearly preferable to a matrix of pinpoints or pixels.
In #10 Sebastian Reineke and colleagues describe an improved OLED layered structure, the key feature of which is the positioning of blue phosphor within the emitter layer and the use of high refractive index substrates. The result is emission that reaches the performance of a fluorescent tube. And that means white-light OLEDs with their soft area light and high color rendering could be the designer choices of future light sources.
Dr. Simon Mitton is a science writer and Fellow of St. Edmund’s College, University of Cambridge, U.K.
What's Hot in Physics | |||
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Rank | Paper |
Cites This Period Sep-Oct 10 |
Rank Last Period Jul-Aug 10 |
1 | E. Komatsu, et al., "Five-year Wilkinson Microwave Anisotropy Probe observations: Cosmological interpretation," Astrophys. J. Suppl. Ser., 180(2): 330-76, February 2009. [14 institutions worldwide] *406EI | 101 | 1 |
2 | S.H. Park, et al., "Bulk heterojunction solar cells with internal quantum efficiency approaching 100%," Nature Photonics, 3(5): 297-302, May 2009. [U. Calif., Santa Barbara; Gwangju Inst. Sci. & Tech., S. Korea; U. Laval, Quebec City, Canada] *447UY | 56 | 3 |
3 | H.Y. Chen, et al., "Polymer solar cells with enhanced open-circuit voltage and efficiency," Nature Photonics, 3(11): 649-53, November 2009. [Solarmer Energy, Inc., El Monte, CA; U. Calif., Los Angeles; U. Chicago, IL] *526PG | 45 | 8 |
4 | J. Dunkley, et al., "Five-year Wilkinson Microwave Anisotropy Probe observations: Likelihoods and parameters from the WMAP data," Astrophys. J. Suppl. Ser., 180(2): 306-29, February 2009. [14 U.S. and Canadian institutions] *406EI | 42 | 2 |
5 | K.N. Abazajian, et al., "The Seventh Data Release of the Sloan Digital Sky Survey," Astrophys. J. Suppl. Ser., 182(2): 543-58, June 2009. [110 institutions worldwide] *448UE | 41 | 4 |
6 | Y.M. Lin, et al., "100-GHz transistors from wafer-scale epitaxial graphene," Science, 327(5966): 662, 5 February 2010. [IBM T.J. Watson Res. Ctr., Yorktown Height, NY] *551ZD | 31 | † |
7 | K.V. Emtsev, et al., "Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide," Nature Materials, 8(3): 203-7, March 2009. [6 German and U.S. institutions] *410DK | 27 | † |
8 | Y.B. Zhang, et al., "Direct observation of a widely tunable bandgap in bilayer graphene," Nature, 459(7248): 820-3, 11 June 2009. [U. Calif., Berkeley; Lawrence Berkeley Natl. Lab., CA] *459EV | 24 | † |
9 | O. Adriani, et al., "An anomalous positron abundance in cosmic rays with energies 1.5-100 GeV," Nature, 458(7238): 607-9, 2 April 2009. [17 institutions worldwide] *427RK | 23 | 5 |
10 | S. Reineke, et al.," White organic light-emitting diodes with fluorescent tube efficiency," Nature, 459(7244): 234-8, 14 May 2009. [Inst. Angew. Photophys., Dresden, Germany] *445FR | 23 | † |
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend. |
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Scheme of the poly(3-dodecylthiophene)/ fullerene solar-cell. From the Wiki Commons.