DOI: 10.1126/science.1137014
, 798 (2007); 315Science
et al.Nathan S. Lewis,
Toward Cost-Effective Solar Energy Use
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believe that a hydrogen economy that used
electrolysis to generate H
2
and O
2
from water,
and a fuel cell to convert H
2
and O
2
back to
water and electrons, could make a substantial
contribution to global energy without a much-
improved oxygen electrode. The identification
of this problem is not in any sense new: The
redox chemistry of oxygen has been a subject of
active interest (but limited success) for decades.
We simply need new ideas.
Another reason to work on these big prob-
lems is that they will attract the most talented
young people. Over the past 30 years, the Na-
tional Institutes of Health has used stable and
generous support to recruit and build a very
effective community of biomedical scientists.
Solving the problems of energy and global
stewardship will require the same patient,
flexible, and broadly based investment, if
society believes that the problems in these areas
are sufficiently important to provide a life’s
work for its most talented young people.
References and Notes
1. President’s Council of Advisors on Science and
Technology (PCAST), The Energy Imperative. Technology
and the Role of Emerging Companies (2006), www.ostp.
gov/PCAST/pcast.html.
2. World Energy Outlook 2004 (International Energy
Agency, Paris, 2004), www.worldenergyoutlook.org/.
3. Basic Research Needs to Assure a Secure Energy Future ,
J. Stringer, L. Horton, Chairs [workshop report, U.S.
Department of Energy (DOE) Office of Basic Energy Sciences,
2003], www.sc.doe.gov/bes/reports/abstracts.h tml#SEC.
4. Basic Research Needs for Solar Energy Utilization,
N. S. Lewis, G. W. Crabtree, Chairs (workshop report,
DOE Office of Basic Energy Sciences, 2005), www.sc.doe.
gov/bes/reports/abstracts.html#SEU.
5. Systems and life-cycle energy Technology Analyses
(National Renewable Energy Laboratory), www.nrel.gov/
analysis/tech_analysis.html.
6. See discussions of global climate science from the
National Center for Atmospheric Research, www.ucar.edu/
research/climate.
7. J. M. Deutch, R. K. Lester, Making Technology Work:
Applications in Energy and the Environment (Cambridge
Univ. Press, New York, 2004).
8. N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. U.S.A.
103, 15729 (2006).
9. M. S. Dresselhaus, I. L. Thomas, Nature 414, 332
(2001).
10.1126/science.1140362
PERSPECTIVE
Toward Cost-Effective
Solar Energy Use
Nathan S. Lewis
At present, solar energy conversion technologies face cost and scalability hurdles in the
technologies required for a complete energy system. To provide a truly widespread primary energy
source, solar energy must be captured, converted, and stored in a cost-effective fashion. New
developments in nanotechnology, biotechnology, and the materials and physical sciences may
enable step-change approaches to cost-effective, globally scalable systems for solar energy use.
M
ore energy from sunlight strikes Earth
in 1 hour than all of the energy con-
sumed by humans in an entire year. In
fact, the solar energy resource dwarfs all other
renewable and fossil-based energy resources
combined (1). With increasing attention to-
ward carbon-neutral energy production, solar
electricity—or photovoltaic (PV) technology—is
receiving heightened attention as a potentially
widespread approach to sustainable energy pro-
duction. The global solar electricity market is
currently more than $10 billion/year, and the in-
dustry is growing at more than 30% per annum
(2). However , low-cost, base-loadable, fossil-
based electricity has always served as a for-
midable cost competitor for electrical power
generation. To provide a truly widespread primary
energy source, solar energy must be captured,
converted, and stored in a cost-effective fashion.
Even a solar electricity device that operated at
near the theoretical limit of 70% efficiency would
not provide the needed technology if it were
expensive and if there were no cost-effective
mechanism to store and dispatch the converted
solar energy upon demand (3). Hence, a com-
plete solar-based energy system will not only
require cost reduction in existing PV manufac-
turing methods, but will also require science and
technology breakthroughs to enable, in a conve-
nient, scalably manufacturable form, the ultralow-
cost capture, conversion, and storage of sunlight.
One key step is the capture and conversion
of the energy contained in solar photons.
Figure 1 shows the fully amortized cost of elec-
tricity as a function of the efficiency and cost of
an installed PV module (2, 4). Because the total
energy provided by the Sun is fixed over the 30-
year lifetime of a PV module, once the energy
conversion efficiency of a PV module is estab-
lished, the total amount of “product” electricity
produced by the module at a representative mid-
latitude location is known for the lifetime of the
system. The theoretical efficiency limit for even
an optimal single–band gap solar conversion
device is 31%, because photons having energies
lower than the absorption threshold of the active
PV material are not absorbed, whereas photons
having energies much higher than the band gap
rapidly release heat to the lattice of the solid and
therefore ultimately contain only a useful in-
ternal energy equal to that of the band gap (2).
Small test cells have demonstrated efficiencies
of >20%, with the remaining losses almost en-
tirely due to small reflection losses, grid shading
losses, and other losses at the 5 to 10% level that
any practical system will have to some extent.
Shipped PV modules now have efficiencies of
15 to 20% in many cases. At such an efficiency,
if the cost of a module is ~$300/m
2
(2), and if we
take into account the accompanying fixed costs
in the so-called “balance of systems” (such as
the inverter , grid connection, etc., which add a
factor of ~2 to the total installed system cost),
then the sale price of grid-connected PV elec-
tricity must be $0.25 to $0.30 per kilowatt-hour
(kWh) to recover the initial capital investment
and cost of money over the lifetime of the PV
installation (2, 4). Currently, however, utility-
scale electrical power generation costs are much
less, with current and new installations costing
~$0.03 to $0.05 per kWh (1). Hence, for solar
electricity to be cost-competitive with fossil-
based electricity at utility scale, improvements
in eff iciency are helpful, but manufacturing costs
must be substantially reduced.
In current manufacturing schemes for Si-
based solar cells, the cost of the processed and
purified Si is only about 10% of the final cost of
the PV module. Some of the Si is lost in cutting
up boules into wafers, and other costs are
incurred in polishing the wafers, making the
diffused junction in the Si into a photovoltaic
device, fabricating the conducting transparent
glass, masking and making the electrical con-
tacts, sealing the cells, connecting the cells
together reliably into a module, and sealing the
module for shipment. Hence, in such systems,
the energy conversion efficiency is at a premium
so as to better amortize these other fixed costs
involved with making the final PV module.
Improvements in efficiency above the 31%
theoretical limit are possible if the constraints that
are incorporated into the so-called Shockley-
Queisser theoretical efficiency limit are relaxed
(2). For example, if photons having energies
greater than the band gap of the absorbing
material did not dissipate their excess energy as
heat, but instead produced more voltage or
Beckman Institute and Kavli Nanoscience Institute, 210
Noyes Laboratory, 127-72, California Institute of Technol-
ogy, Pasadena, CA 91125, USA. E-mail: nslewis@its.
caltech.edu
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generated multiple, low-energy, thermalized
electrons from the energy of a single absorbed
photon, theoretical efficiencies in excess of 60%
would, in principle, be attainable. Absorbers hav-
ing a highly quantized band structure, such as
quantum wells and quantum dots, can theoret-
ically produce the desired effects (Fig. 2). In fact,
recent observations on PbSe quantum dots have
demonstrated the production, with high quantum
yield, of multiple excitons from a single absorbed
photon, thereby establishing an existence proof
for the process of interest (5). At present, how-
ever , there is no method for efficiently extracting
the photogenerated carriers from the quantum dot
structure to produce electricity in an external
circuit. Materials with “mini-bands” or with “in-
termediate bands” also offer the possibility for
ultrahigh energy conversion efficiency (2, 6). In
this approach, different incident photon energies
would promote absorption from different isolated
energy levels and therefore allow for the pro-
duction of different voltages (Fig. 2). The phe-
nomenon has been described theoretically but
has yet to be demonstrated in a practical im-
plementation. In addition, these materials are
currently extremely costly , and methods of re-
taining the high performance with scalable,
inexpensive manufacturing methods would also
be required.
In the absence of marked increases in cell
efficiency, the value of new solar cell materials
rests primarily with their potential to enable an
entirely different manufacturing process, such
as roll-to-roll manufacturing, printing, painting,
or other ultralow-cost approaches to imple-
mentation of PV technology. This area is where
breakthroughs in the science and technology of
solar cell materials can have the greatest impact
on the cost and widespread implementation of
solar electricity.
The key issue involves the trade-off between
material purity and device performance. In a
typical planar solar cell design, the charge
carriers are collected in the same direction as
light is absorbed. A minimum thickness of the
cell is set by the thickness of material required to
absorb >90% of the incident sunlight. However,
the required thickness of the material also
imposes a constraint on the required purity of
the material, because the photoexcited charge
carriers must live sufficiently long within the
absorbing material to arrive at the electrical
junction, where they can be separated to produce
an electrical current flow through the metallic
contacts to the cell. Impure absorber materials
with short charge carrier lifetimes can therefore
effectively absorb sunlight but cannot effec-
tively convert that absorbed energy into elec-
tricity. In turn, absorber materials with the
necessary purity are generally costly to produce
and manufacture. Cheaper materials, such as
organic polymers or inorganic particulate solids
with small grain sizes, generally have short
charge carrier lifetimes and/or induce recombi-
nation of charge carriers at the grain boundaries
of such materials. This cost-thickness-purity
constraint is largely why all current PV cells fall
in the green region, labeled zone I, in Fig. 1.
Approaches to circumventing
this cost/efficiency trade-off gen-
erally involve orthogonalization of
the directions of light absorption
and charge carrier collection. High–
aspect ratio nanorods, for example,
can provide a long dimension for
light absorption while requiring only
that ca rriers move radially, along
the short dimension of the nano-
rod, to be separated by the metal-
lurgical junction and collected as
electricity (Fig. 3) (2, 7). A con-
ceptually similar approach involves
the use of interpenetrat ing net-
works of inorganic absorbers, such
as CdTe “tetrapods” (8) and/or or-
ganic polymeric absorbers (9), such
as the organic conducting polymer
100
80
60
40
20
Thermodynamic limit
at 46,200 Suns
Shockley-
Queisser limit
Ultimate
thermodynamic
limit at 1 Sun
US$0.10/W US$0.20/W
min BOS
US$0.50/W
US$1.00/W
US$3.50/W
Efficiency (%)
0 100 200 300 400
Cost (US$/m
2
)
II
III
I
Fig. 1. Solar electricity costs as function of module efficiency and cost. The theoretical efficiencies are
shown for three cases: the Shockley-Queisser limit for a quantum conversion device with a single band
gap, in which carriers of lower energy are not absorbed and carriers of energy higher than the band gap
thermalize to the band gap; the second-law thermodynamic limit on Earth for 1 Sun of concentration;
and the second-law thermodynamic limit for any Earth-based solar conversion system. Current solar cell
modules lie in zone I. The dashed lines are equi-cost lines on a cost per peak watt (W
p
) basis. An estimate
for the minimum balance-of-systems cost given current manufacturing methods is also indicated. A
convenient conversion factor is that $1/W
p
amortizes out to ~$0.05/kWh over a 30-year lifetime of the
PV module in the field. [Adapted from (4)]
Conduction band
Valence band Valence band
Mini-band
Photon
Photon
Quantum
dots
Quantum
wells
Conduction band
To contacts
To contacts
E
V
E
F
E
C
A
E
V
E
C
E
2
E
1
E
2
E
1
B
Fig. 2. Possible methods of circumventing the 31% efficiency limit for thermalized carriers in a single–band
gap absorption threshold solar quantum conversion system. (A) Intermediate-band solar cell; (B) quantum-well
solar cell. [Adapted from (2 )]
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poly(phenylenevinylene). Such systems are un-
der widespread investigation at present, and the
key is not only to obtain intimate contact be-
tween the light-absorbing and charge-collecting
phases, but also to control the chemistry at the
interface between the two phases that make up
the device. Junction recombination is a delete-
rious loss pathway even in many planar solar
cell devices, and such junction recombination
generally b ecom es dominant in disordered
systems that, by definition, have a
large increase in their interfacial
contact area relative to their pro-
jected geometric area for light
absorption. Methods for controlling
the chemical properties of the sur-
faces and junctions of such sys-
tems, and thereby reducing their
natural tendency to promote dele-
terious charge carrier recombina-
tion, are therefore critical. Such
methods have been developed for
certain well-defined semiconduc-
tor surfaces (10) and will need to
be developed and implemented suc-
cessfully for the high–junction area
systems to obtain high (>5%) en-
ergyconversionefficiencies from
such devices.
A conceptually related system is
the dye-sensitized solar cell, in
which a random, disordered net-
work of inexpensive TiO
2
particles
is used to collect the charge carriers.
The light absorption is performed
by an adsorbed dye molecule, and
the interfacial contact distance is kept small by
use of a liquid or conductive polymer to
penetrate the pore structure of the solid and
collect the other charge carrier type to complete
the circuit in the cell (Fig. 4) (11). Small
“champion” dye-sensitized solar cells have
shown efficiencies as high as 10 to 11%,
although at present large-area devices typically
have efficiencies of <5%. Improvements in the
efficiency of such systems will require improved
dyes, better electrolytes, and better control over
the recombination at the interfacial contact area
that currently limits the voltage produced by
such systems to about 50 to 60% of its theo-
retical value. The stability of such systems will
also need to be demonstrated under operational
conditions for extended periods (>10 years) to
allow them to be implemented in the market-
place. Clearly, advances in basic science are
needed to enable all such nanostructured sys-
tems to truly offer a practical, ultralow-cost
option for solar electricity production (2).
Although there is tremendous potential for
growth for PV in electricity generation, solar
electricity can never be a material contributor to
primary energy generation without cost-effective
methods for storing and distributing massive
quantities of electricity (2, 3, 12). Put simply, the
Sun goes out locally every night, and the inter-
mittency imposed by the diurnal cycle must be
dealt with to provide a full, base-loadable pri-
mary energy system from the Sun. The lack of
cost-effective large-scale electrical storage ca-
pacity on Earth underlies the call for develop-
ment of space-based solar power systems. On
Earth, the cheapest method for massive electric-
ity storage is pumped-water storage, which can
be relatively efficient, but even that process does
not scale well if every reservoir would have to be
filled up each day and emptied each night; ad-
ditionally, a staggering amount of water would
be needed to compensate for the diurnal cycle if
one were to provide a material contribution to
the primary U.S. or global energy generation
through this approach. Batteries are a natural ap-
proach to electricity storage, but for battery
storage to be cost-effective over the 30-year
amortized lifetime of a PV system, enormous
quantities of batteries would have to be hooked
up to the grid, and they would have to cost as
little as lead-acid batteries while providing the
cycle life of lithium-ion batteries. Innovative ap-
proaches to massive, low-cost energy storage—
including potentially a superconducting global
transmission grid, supercapacitors, flywheels,
etc.,aspromotedbySmalley(12)—will be im-
portant enablers of a full solar capture, conver-
sion, and storage ener gy system.
Perhaps the most attractive method for cost-
effective massive energy storage is in the form
of chemical bonds (i.e., chemical fuel). After all,
this approach is central to photosynthesis and is
the basis for much of the recent attention
devoted to development of biofuels. Photosyn-
thesis, however, saturates at about one-tenth of
the intensity of normal sunlight, and conse-
quently the yearly averaged energy storage ef-
ficiency of even the fastest-growing plants is less
than 1%, and typically less than 0.3 to 0.5% (2),
as compared to the >15% efficiency values
displayed by current PV devices (2). Hence, to
first order, land-related constraints dominate the
ultimate commercial potential of biofuels as
200 nm
Top electrode
Polymer
Light
Nucleation layer
TCO
Nanofibers
Fig. 3. Arrays of nanorods, illustrating an approach to orthogonalization of the directions of light
absorption (down the length of the rods) and charge carrier collection (radially outward to the surface
of the rods). [Adapted from (2)]
Fig. 4. Dye-sensitized solar cell, in which a nanoparticulate
network provides collection of charge carriers injected into it as
a result of absorption of sunlight by the adsorbed dye molecule.
The oppositely charged carrier moves through the contacting
liquid or polymeric phase to the counterelectrode, completing
the electrical circuit in the solar cell. [Adapted from (2)]
9 FEBRUARY 2007 VOL 315 SCIENCE www.sciencemag.org
800
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material contributors to primary energy supply,
whereas cost-related constraints dominate the
ultimate commercial potential of PV-derived
solar energy conversion and storage systems.
One approach to storing electrical energy in
chemical bonds is through electrolysis, in which
water is split into H
2
and O
2
in an electrolyzer.
However, Pt-based electrolysis in acidic or
neutral media is expensive and unlikely to be
scalable to the levels that would be required
for this process to be m aterial in global
primary energy production. Ni-based electrol-
ysis in basic aqueous solutions is cheaper but
requires scrubbing the input stream to remove
the CO
2
(13); additionally, even the best fuel
cells are only 50 to 60% energy-efficient and the
best electrolysis units are 50 to 70% energy-
efficient (13), so the full-cycle energy storage/
discharge efficiency of such a system is cur-
rently only 25 to 30%. Clearly, better catalysts
for the multielectron transformations involved in
fuel formation are needed. Nature provides the
existence proof for such catalysts, with the
hydrogenase enzymes operating at the thermo-
dynamic potential for production of H
2
from
H
2
O, and with the oxygen-evolving complex of
photosystem II producing O
2
from H
2
Oinan
energy-ef ficient fashion. However, no human-
made catalyst systems, either molecular or
heterogeneous, have yet been identified that
show performance even close to that of the
natural enzymatic systems. Development of such
catalysts would provide a key enabling technol-
ogy for a full solar energy conversion and stor-
age system.
Whether the fuel-forming system is separate,
as in a PV-electrolysis combination, or inte-
grated, as in a fully artificial photosynthetic sys-
tem that uses the incipient charge-separated
electron-hole pairs to directly produce fuels with
no wires and with only water and sunlight as the
inputs, is an interesting point of discussion from
both cost and engineering perspectives. How-
ever, the key components needed to enable the
whole system remain the same in either case:
cost-effective and efficient capture, conversion,
and storage of sunlight. Each of these functions
has its own challenges, and integration of them
into a fully functioning, synergistic, globally scal-
able system will require further advanc es in both
basic science and engineering. Such advances,
together with advances in existing technologies,
will be required if the full potential of solar en-
ergy is to be realized.
References and Notes
1. World Energy Assessment Overview, 2004 Update,
J. Goldemberg, T. B. Johansson, Eds. (United Nations
Development Programme, New York, 2004) (www.undp.
org/energy/weaover2004.htm).
2. Basic Research Needs for Solar Energy Utilization
(U.S. Department of Energy, Washington, DC, 2005)
(www.er.doe.gov/bes/reports/abstracts.html#SEU).
3. M. I. Hoffert et al., Science 298, 981 (2002).
4. M. A. Green, Third Generation Photovoltaics: Advanced
Solar Energy Conversion (Springer-Verlag, Berlin, 2004).
5. R. D. Schaller, M. A. Petruska, V. I. Klimov, Appl. Phys.
Lett. 87, 253102 (2005).
6. M. A. Green, Prog. Photovolt. 9, 137 (2001).
7. B. M. Kayes, H. A. Atwater III, N. S. Lewis, J. Appl. Phys.
97, 114302 (2005).
8. D. J. Milliron, I. Gur, A. P. Alivisatos, MRS Bull. 30,41
(2005).
9. G. Yu et al., Science 270, 1789 (1995).
10. N. S. Lewis, Inorg. Chem. 44, 6900 (2005).
11. M. Gratzel, Nature 414, 338 (2001).
12. R. Smalley, Bull. Mater. Res. Soc. 30, 412 (2005).
13. J. Ivy, Summary of Electrolytic Hydrogen Prod uction
(National Renewable Energy Laboratory, Golden, CO,
2004) (www.nrel.gov/docs/fy04osti/36734.pdf).
14. Supported by the U.S. Department of Energy and NSF.
10.1126/science.1137014
PERSPECTIVES
Challenges in Engineering
Microbes for Biofuels Production
Gregory Stephanopoulos
Economic and geopolitical factors (high oil prices, environmental concerns, and supply instability)
have been prompting policy-makers to put added emphasis on renewable energy sources. For the
scientific community, recent advances, embodied in new insights into basic biology and technology
that can be applied to metabolic engineering, are generating considerable excitement. There is
justified optimism that the full potential of biofuel production from cellulosic biomass will be
obtainable in the next 10 to 15 years.
T
he idea of converting biomass-derived
sugars to transportation biofuels was first
proposed in the 1970s. Once again, the
idea is being seriously contemplated as a pos-
sible substitute for petroleum-based liquid fuels.
Economic and geopolitical factors (high oil
prices, environmental concerns, and supply
instability) have certainly played a role in re-
viving interest in renewable resources. However,
an additional impetus is now provided by sci-
entific and technological advances in biosci-
ences and bioengineering that support increased
optimism about realizing the full potential of
biomass in the liquid fuels area within the next
10 to 15 years. New approaches to biology are
being shaped by the genomics revolution; un-
precedented ability to transfer genes, modulate
gene expression, and engineer proteins; and a
new mind-set for studying biological systems in
a holistic manner [systems biology (1)]. We are
also seeing advances in metabolic engineering
(2–4), with the goal of overproducing useful
compounds by rationally and combinatorially
engineering cells and their metabolic pathways
(5). Combination of concepts and methods from
these fields will create a platform of technolo-
gies that are critical for overcoming remaining
obstacles in cost-efficient biofuel production
from cellulosic biomass.
Figure 1 shows the basic features of a
biomass-to-biofuels (B2B) process (6). After
harvest, biomass is reduced in size and then
treated to loosen up the lignin-cellulose fiber
entanglement in a step that can take from a few
minutes to many hours. Several methods have
been used for this purpose, such as biomass
treatment with saturated steam at 200°C, explo-
sion with ammonia, and cooking with warm
dilute acid (6). Dilute acid pretreatments are fast
(minutes), whereas steam-based treatments can
take up to a day. After pretreatment, the solid
suspension is exposed to cellulolytic enzymes
that digest the cellulosic and hemicellulosic bio-
mass components to release the hydrolysis pro-
ducts, primarily six- and five-carbon sugars,
respectively (along with acetic acid and lignin-
derived phenolic by-products). The type of
pretreatment defines the optimal enzyme mix-
ture to be used and the composition of the hy-
drolysis products. The latter are fermented by
ethanol-producing microorganisms such as ge-
netically engineered yeasts, Zymomonas mobilis
(Fig. 2), Escherichia coli,orPichia stipitis (Fig.
3). Presently, cellulose hydrolysis and fermenta-
tion are combined in a single unit, termed the
simultaneous saccharification fermentation
(SSF) stage. The rationale of combining sac-
charification (the bre aking up of complex
carbohydrates into monosaccharides) and fer-
mentation (the conversion of a carbohydrate to
carbon dioxide and alcohol) in a single unit was
to prevent inhibition of the hydrolytic enzymes
by the reaction products (7). The SSF step typ-
ically lasts 3 to 6 days, with cellulose hydrolysis
being the slow, limiting step. The product of SSF
is a rather dilute ethanol stream of 4 to 4.5% from
which ethanol is separated by distillation.
Department of Chemical Engineering, Massachusetts In-
stitute of Technology, Cambridge, MA 02139, USA. E-mail:
gregstep@mit.edu
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![Fig. 1. Solar electricity costs as function of module efficiency and cost. The theoretical efficiencies are shown for three cases: the Shockley-Queisser limit for a quantum conversion device with a single band gap, in which carriers of lower energy are not absorbed and carriers of energy higher than the band gap thermalize to the band gap; the second-law thermodynamic limit on Earth for 1 Sun of concentration; and the second-law thermodynamic limit for any Earth-based solar conversion system. Current solar cell modules lie in zone I. The dashed lines are equi-cost lines on a cost per peak watt (Wp) basis. An estimate for the minimum balance-of-systems cost given current manufacturing methods is also indicated. A convenient conversion factor is that $1/Wp amortizes out to ~$0.05/kWh over a 30-year lifetime of the PV module in the field. [Adapted from (4)]](/figures/fig-1-solar-electricity-costs-as-function-of-module-24ae2x3s.webp)
![Fig. 4. Dye-sensitized solar cell, in which a nanoparticulate network provides collection of charge carriers injected into it as a result of absorption of sunlight by the adsorbed dye molecule. The oppositely charged carrier moves through the contacting liquid or polymeric phase to the counterelectrode, completing the electrical circuit in the solar cell. [Adapted from (2)]](/figures/fig-4-dye-sensitized-solar-cell-in-which-a-nanoparticulate-3asa11zq.webp)
![Fig. 3. Arrays of nanorods, illustrating an approach to orthogonalization of the directions of light absorption (down the length of the rods) and charge carrier collection (radially outward to the surface of the rods). [Adapted from (2)]](/figures/fig-3-arrays-of-nanorods-illustrating-an-approach-to-12hp01wf.webp)