The Race for Efficient Solar Cells Brightens Up
What's Hot in Physics 2011
by Simon Mitton
The hottest paper in physics for this period is now two years old, but still the citations are piling up, with upwards of 600 hits as of the spring of 2011, and it leaps from #5 last time to head the current leaderboard. Paper #1 reports the invention of a heterojunction solar cell with an internal power conversion efficiency approaching 100%. The principal investigators are Nobel laureate Alan J. Heeger (University of California, Santa Barbara) and Kwanghee Lee (Gwangju, South Korea), who have worked together for many years on devising efficient plastic solar cells. Heeger’s Nobel Prize in chemistry (2000) was for his discovery of conducting polymers, and his research on solar cells is an extension of that.
It’s clear from a careful examination of reports citing #1 that it is now firmly established as the groundbreaking report that set out a new approach to solar energy conversion. The quest for highly efficient polymer solar cells (PSCs) has become a major goal for photonics and polymer physics. Although solar cells based on silicon are still the best in functionality, they are rather expensive and cumbersome. Typically half the cost of a domestic rooftop system is gobbled up with installation expenses. Polymers offer a path to reduce the mass-to-energy ratio, and plastic is more flexible than silicon on a commercial production line.
A monocrystalline solar cell, from the
Wiki Commons.
Hot Paper #3 takes the field a step closer to realistic devices. Yang Yang of the University of California, Los Angeles first took an interest in PSCs nearly ten years ago (interviewed in May/June 2011). He and his team point out in #3 that PSCs are attracting so much attention because they provide the means to harvest the sun’s energy at much lower cost than silicon. It’s true that PSCs are not yet as efficient as silicon-based technology, but that’s what paper #3 is all about: enhanced efficiency. And meanwhile the plastic approach allows countless applications that simply do not work for silicon: solar cells on cars, umbrellas, clothing, gadgets that need recharging, or even solar-powered military tents so that infantry are not burdened with batteries.
Nevertheless, impressive progress with PSCs still falls short of the efficiency goal of ~10%, and to address this, a broad range of materials have been considered in order to make polymers with a small band-gap, good absorbance of sunlight, the right mix of energy levels, and good mobility for the charge carriers. PSC research is therefore a blend of photonics and electronics.
In #3 the critical factor receiving attention is how to maximize the open-circuit voltage. In this case, the researchers fabricated three polymers in which molecular design using different functional groups played a key part in enabling tuning of the energy levels. The nub of the problem is that a reduction in bandgap allows the polymer to harvest more sunlight, and lowering the highest occupied molecular orbital increases the open-circuit voltage, both of which increase efficiency. However the nightmare is that narrowing the bandgap leads to lower output voltage. Not good. But in #3 a step-by-step tweaking of energy levels resulted in a polymer solar cell with an efficiency of 6.77%.
That was two years ago. Since then the peak energy levels in PSCs have climbed. Solarmer Energy Inc., a start-up that has licensed from UCLA the technology developed by Yang Yang, has hit 8.13%. The killer application which developers have in sight are transparent windows that generate electricity, and similar photovoltaic concepts for the built environment. With that in prospect, it’s no wonder that papers #1 and #3 are indispensable references.
Hot Paper #5 is about an energy-efficiency development that could help consumers. Organic light-emitting diodes (OLEDs) are found in thin displays such as cellphones, televisions, digital cameras, and information displays. But you cannot find OLEDs as a fluorescent tube replacement in a lighting store. That’s because OLEDs emit too little light per unit area. Paper #5 reports a development that could make white-light OLEDs the light source of choice for the future. Karl Leo of the Technical University of Dresden, Germany, and his team report an improved OLED structure, using phosphors, that combines a novel concept for efficient photon generation with an improved approach to letting the light shine out. "The future of white OLEDs will be bright," they conclude.
Dr. Simon Mitton is a 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 Jan-Feb 11 |
Rank Last Period Nov-Dec 10 |
1 | 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 | 64 | 5 |
2 | 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 | 36 | 6 |
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 | 33 | 9 |
4 | P. Jenniskens, et al., "The impact and recovery of asteroid 2008 TC3," Nature, 458(7237): 485-8, 26 March 2009. [20 institutions worldwide] *423YJ | 22 | † |
5 | 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 | 21 | † |
6 | W.B. Atwood, et al., "The Large Area Telescope on the Fermi Gamma-Ray Space Telescope mission," Astrophys. J., 697(2): 1071-1102, 1 June 2009. [57 institutions worldwide] *446YT | 20 | 10 |
7 | A.A. Abdo, et al., "Fermi Large Area Telescope first source catalog," Astrophys. J. Suppl. Ser., 188(2): 405-36, June 2010. [67 institutions worldwide] *613HG | 18 | † |
8 | W.J. Borucki, et al., "Kepler planet-detection mission: introduction and first results," Science, 327(5968): 977-80, 19 February 2010. [34 institutions worldwide] *556XE | 18 | † |
9 | M. Hicken, et al., "Improved dark energy constraints from ~ 100 CfA supernova type Ia light curves," Astrophys. J., 700(2): 1097-1140, 1 August 2009. [7 institutions worldwide] *471ZU | 17 | † |
10 | 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 | 17 | † |
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend. |