The Future Shines for
Efficient Polymer Solar Cells
by Simon Mitton
Polymer-based
solar cells with a high quantum efficiency
have entered the Physics Top Ten at #5. This is the first time a
paper on plastic electronics has featured in these physics
listings. The results reported in the paper are an important
step forward in the development of solar cells with a power
conversion rate that would allow a commercial return on the
conversion of solar energy to electrical energy.
In 1976,
Alan Heeger (University of California, Santa
Barbara) and the late Alan MacDiarmid discovered
conducting polymers and the techniques to
dope those polymers over the full range from insulator to
conductor. This opened up an exciting new field at the interface
between chemistry and condensed-matter physics, most importantly
the promise of a new generation of polymers with the electrical
properties of metals and semiconductors as well as the
processing advantages of plastics. That breakthrough won the
polymer scientists a share of the 2000 Nobel Prize in Chemistry.
The field of plastic optoelectronics took off in 1990 when
Richard Friend and his colleagues at the
Cavendish Laboratory (Cambridge, U.K.) discovered how to make
light-emitting diodes using polymers. In 2000, in his Nobel
lecture, Heeger described this result as a major stimulus for
the development of a wide variety of applications such as
lasers, photodiodes, photovoltaic cells, and integrated circuits
made of polymers alone. All these devices share a common
architecture: they are thin-films in which the active layers are
made from semiconductor or metallic polymers.
In 1992, Heeger and his colleagues made the next step toward the
solar cell described in #5 with their discovery of photoinduced
electron transfer in composites of polymers (the donors) and
C60 buckyballs (the acceptors). Years of research
followed, while they investigated the fundamental physics of the
transfer of photoelectrons. Which brings us to Hot Paper #5.
The paper explains that heterojunction solar cells are based on
composites comprised of an electron-donating polymer and an
electron-receiving fullerene. Such cells hold great promise for
manufacture on an industrial scale because the materials are
inexpensive, printable, portable, and flexible. Such
characteristics hold the promise that polymer solar cells could
become a consumer product (or gadget), following the trajectory of
LEDs and solid-state lasers.
But there’s a big problem to be solved before solar panels
become a product in the home-improvement section of a supermarket:
the conversion rate of solar photons to electrical energy is too
low.
Newcomer #5 is receiving attention because it describes in some
detail the device structure and energy level diagram of a
heterojunction that has an internal quantum efficiency approaching
100%. That is to say, essentially every absorbed photon results in
a separated pair of charge carriers (an electron and a hole). These
released charge carriers are collected at electrodes.
What’s new here is the device structure, which perhaps calls
to mind a club sandwich. The top layer, the electrode, is an Al
film. Sitting immediately below this is an optical spacer and hole
blocker made from the sub-oxide TiOx This layer
is the key to understanding the efficiency of the device: it
redistributes light inside the heterojunction by avoiding
destructive interference from internal reflections. Under the
TiOx optical spacer there is the layer with
C70 as the acceptor, and this in turn sits on the
co-polymer that produces the carriers in response to photons. The
outer layer is of course glass, to let the light in.
The conversion rate when illuminated with 532 nm monochromatic
light is 17%, and a standard test using a solar simulator gave an
overall efficiency of 6%. For Science Watch, Prof. Heeger
comments on the high citation rate. "The paper provides a
scientific basis for confidence that high efficiency will be
achieved with the Bulk HeteroJunction (BHJ) solar cell technology.
The demonstration of 17% power conversion efficiency for
monochromatic light within the absorption band shows that high
efficiency can be obtained."
Prof. Kwanghee Lee (Gwangju Institute of Science and Technology,
South Korea) adds: "We have set a world record of 6.1% conversion
efficiency for BHJ polymer solar cells. Our paper sets a new
direction in the pursuit of higher power efficiencies. The results
are path-breaking since they lay the foundation for further process
related innovations."
Dr. Simon Mitton was awarded a Ph.D. in physics (1972)
by the Cavendish Laboratory, University of Cambridge.
Physics Top 10
Papers
Rank
Paper
Citations
This Period
(Nov-Dec 09)
Rank
Last Period
(Sep-Oct 09)
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
132
1
2
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
55
2
3
J.K. Adelman-McCarthy, et al., "The
Sixth Data Release of the Sloan Digital Sky
Survey,"Astrophys. J. Suppl. Ser.,
175(2): 297-313, April 2008. [84 institutions
worldwide] *327WN
43
6
4
X.H. Chen, et al., "Superconductivity
at 43K in SmFeAsO1-xFx,"Nature, 453(7196): 761-2, 5 June 2008. [U.
Sci. & Tech., Hefei, China] *308UK
37
4
5
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
36
†
6
M. Kowalski, et al., "Improved
cosmological constraints from new, old, and combined
supernova data sets,"Astrophys. J.,
686(2): 749-78, 20 October 2008. [41 institutions
worldwide] *364YB
32
8
7
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
28
3
8
F.-C. Hsu, et al., "Superconductivity
in the PbO-type structure alpha-FeSe,"PNAS, 105(38): 14262-4, 23 September 2008.
[Acad. Sinica, Taipei, Taiwan; Natl. Tsing Hua U.,
Hsinchu, Taiwan; Duke U., Durham, NC] *353TY
25
10
9
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
25
†
10
Z.A. Ren, et al., "Superconductivity
at 55 K in iron-based F-doped layered quaternary
compound
Sm[O1-xFx]FeAs,"Chinese Phys. Lett., 25(6): 2215-6, June 2008.
[Chinese Acad. Sci, Beijing] *306MN