letters to nature
..............................................................
A photovoltaic device structure
based on internal electron emission
Eric W. McFarland* & Jing Tang†
* Department of Chemical Engineering, † Materials Department, University of
California, Santa Barbara, California 93106-5080, USA
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There has been an active search for cost-effective photovoltaic
devices since the development of the first solar cells in the 1950s
(refs 1–3). In conventional solid-state solar cells, electron–hole
pairs are created by light absorption in a semiconductor, with
charge separation and collection accomplished under the influence of electric fields within the semiconductor. Here we report a
multilayer photovoltaic device structure in which photon absorption instead occurs in photoreceptors deposited on the surface of
an ultrathin metal–semiconductor junction Schottky diode.
Photoexcited electrons are transferred to the metal and travel
ballistically to—and over—the Schottky barrier, so providing the
photocurrent output. Low-energy (,1 eV) electrons have surprisingly long ballistic path lengths in noble metals4,5, allowing a
large fraction of the electrons to be collected. Unlike conventional
cells, the semiconductor in this device serves only for majority
charge transport and separation. Devices fabricated using a
fluorescein photoreceptor on an Au/TiO2/Ti multilayer structure
had typical open-circuit photovoltages of 600–800 mV and shortcircuit photocurrents of 10–18 mA cm22 under 100 mW cm22
visible band illumination: the internal quantum efficiency (electrons measured per photon absorbed) was 10 per cent. This
alternative approach to photovoltaic energy conversion might
provide the basis for durable low-cost solar cells using a variety of
materials.
The device structure is a solid-state multilayer; it consists of a
photoreceptor layer deposited on a ,10–50-nm Au film, which caps
200 nm of TiO2 on an ohmic metal back contact (Fig. 1). Au and
Figure 1 Electron transfer in the operating photovoltaic device. Process A, photon
absorption and electron excitation from the chromophore ground state, S, to the excited
state, S*; B, energetic electron transfer from S* into and (ballistically) through the
conducting surface layer and over the potential energy barrier into the semiconductor;
C, electron conduction as a majority carrier within the semiconductor to the ohmic back
contact and through the load; and D, reduction of the oxidized chromophore, Sþ, by a
thermal electron from the conductor surface. Shown schematically are the relative
energies of the electron levels within the device structures, the Schottky barrier, f, the
Fermi energy, E f, and the semiconductor bandgap, E g.
616
TiO2 are chosen and prepared so that a Schottky barrier, f, of
approximately 0.9 V is formed at the metal–semiconductor interface, and the dye, merbromin (2,7-dibromo-5-(hydroxymercurio)fluorescein), is selected such that the photoexcited donor level is
energetically above the barrier. The photon-to-electron conversion
in this device occurs in four steps. First, light absorption occurs in
the surface-absorbed photoreceptors, giving rise to energetic electrons. Second, electrons from the photoreceptor excited state are
injected into conduction levels of the adjacent conductor, where
they travel ballistically through the metal at an energy, 1e, above the
Fermi energy, Ef. Third, provided that 1e is greater than the Schottky
barrier height, f, and the carrier mean-free path is long compared to
the metal thickness, the electrons will traverse the metal and enter
conduction levels of the semiconductor (internal electron emission). The absorbed photon energy is preserved in the remaining
excess electron free energy when it is collected at the back ohmic
contact, giving rise to the photovoltage, V. The photo-oxidized dye
is reduced by transfer of thermalized electrons from states near Ef in
the adjacent metal. Like electrochemical dye-sensitized solar cells6,7,
this structure physically separates the photon absorption process
from charge separation and transport; but it does not have the
disadvantage of needing a reducing agent in an electrolyte for
intermolecular charge transport.
In addition to the desired production of ballistic electrons in the
metal, there are other processes available for energy dissipation by
the electron in the excited photoreceptor. The efficiency of the
device will depend upon favouring specific energy transfer pathways
from the major competing processes: (1) radiative intramolecular
de-excitation with photon emission (luminescence), (2) non-radiative intramolecular or intermolecular de-excitation with direct
coupling to phonons, (3) non-radiative intermolecular de-excitation with coupling to the metal conduction electrons, and (4)
electron transfer of the energetic electron from the photoexcited
state into the unoccupied conduction states of the metal. Both
pathways (3) and (4) may produce energetic electrons in metal
conduction levels above the Fermi energy (hot electrons) that also
have sufficient energy and momentum to traverse the Schottky
barrier; it is these electrons that give rise to the primary photocurrent in the device. We believe that the dominant pathway is (4) in
our device, but we cannot rule out contributions to the measured
photocurrent from pathway (3). Pathway (2) as well as nontransmitted electrons from (3) and (4) constitute the principal
competing ‘quenching’ processes whereby the electronic excitation
energy is dissipated as heat, and neither appears as light nor electron
free energy in the device current.
The current–voltage characteristics of a representative device
measured in the dark and under 100 mW cm22 visible band
Figure 2 Current–voltage characteristics of the multilayer merbromin/Au/TiO2/Ti
photovoltaic device. The response is shown for dark conditions (curve A), and for
1,000 W m22 broadband visible illumination (B). For comparison, curves obtained from
the same device before adding merbromin are also shown for dark conditions (C), and
under the same illumination (D).
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letters to nature
illumination before and after applying merbromin are shown in Fig.
2. After deposition of merbromin, the surface-activated device had
an open-circuit photovoltage, Voc, of 685 mV, and a short-circuit
photocurrent, jsc, of 18.0 mA cm22. The fill factor was determined to
be 0.63 from the current–voltage characteristics under illumination.
Before deposition of merbromin on the Au surface, excitation of
defect levels in the thermally oxidized TiO2 is responsible for a weak
visible photoresponse (Voc ¼ 325 mV, jsc ¼ 0.9 mA cm22).
The photovoltage, V, is the difference, under illumination,
between the Fermi level of the semiconductor and the Fermi level
of the ultrathin metal film. For the surface-sensitized Schottky
diode, the expression for the maximum value under open circuit
conditions, Voc is identical to a conventional Schottky solar cell
where photon absorption occurs by bandgap excitation:
nkT
½lnðjg =ðT 2 A** ÞÞ þ efB =kTŠ
V oc ¼
ð1Þ
e
where n is the ideality factor (,1), k is Boltzmann’s constant, e is the
charge on the electron, A ** is the Richardson constant, and T is the
device temperature5. The photocurrent density produced by photon
absorption, j g, is balanced by the effective forward bias current from
Voc such that no net current is observed. The maximum photovoltages and photocurrents can be achieved by choice and preparation of the device materials to control and match the Schottky
barrier height, f, the semiconductor conduction band positions,
and the chromophore donor/acceptor levels.
The photocurrent produced from a monochromatic photon flux,
F(1 g), is determined largely by the photon capture efficiency and
Figure 3 The wavelength-dependent photoresponse and absorbance of the photovoltaic
device. a, The absorbance of merbromin adsorbed to the surface of the Au/TiO2/Ti device
(solid line). Shown for reference is the absorbance (in arbitrary units) of merbromin in
water (dotted line). b, The incident photon-to-electron conversion efficiency (IPCE) of the
dye-sensitized device under 0.4 mW cm22 illumination. c, The internal quantum
efficiency (IQE), determined by correcting the IPCE by the absorption efficiency of the
affixed dye, reflects the absorbed photon-to-electron conversion efficiency of the dye.
NATURE | VOL 421 | 6 FEBRUARY 2003 | www.nature.com/nature
the internal quantum efficiency, IQE, which is the number of hot
electrons injected into the semiconductor (and thus detected) per
absorbed photon:
jg ð1g Þ ¼ Fð1g Þhg ð1g ÞIQEð1g Þ
ð 1g
IQEð1g Þ ¼ hCM ð1g ; 1e ÞhMS ð1e ÞhS d1e
ð2Þ
ð3Þ
0
where h g(1g) is the efficiency for absorption of a photon of energy
1 g; h CM(1 g,1e) is the probability that absorption will result in the
injection of an excited electron into the metal at an energy, 1 e, above
the Fermi energy; h MS(1e) is the efficiency for charge transport
across the metal film and into the semiconductor; and hS is the
charge collection efficiency in the semiconductor. Experimentally,
the incident photon-to-electron conversion efficiency, IPCEð1g Þ ¼
jð1g Þ=Fð1g Þ; is typically determined by measuring the photocurrent
from a monochromatic photon source. The IQE is then calculated
by correcting the IPCE for photon absorption in the dye as
IQEð1g Þ ¼ IPCEð1g Þ=hg ð1g Þ:
What makes the IQE for this device structure acceptable is the
surprisingly high value of h MS(1 e). At energies of ,1 eV above the
Fermi level, the ballistic mean free path for electrons in Au and other
metals with low-lying or filled d bands has been measured to be
extremely long, ,20–150 nm (refs 4, 5). Thus, despite the competing processes of quasi-elastic scattering by phonons and inelastic
scattering from other electrons, provided the metal conducting film
is ultrathin, a significant fraction of the injected electrons will reach
the Schottky barrier.
Although the relationship between Voc and j g in our devices is
identical to that of a conventional semiconductor Schottky solar
cell, equation (1), the bulk semiconductor is not utilized for photon
absorption; thus the bandgap and semiconductor thickness constraints are largely removed. The significance of the Schottky barrier
height, however, is increased. The absorption process occurs in the
photoreceptor, which is selected to balance high solar-spectrum
absorbance with maximum photovoltage as well as chemical stability. In this design, several different chromophores could be used
simultaneously to provide more efficient conversion (for example,
dyes, quantum structures); however, in an idealized cell using a
single dye, the optimal chromophore absorption maximum would
be the same as the ideal bandgap of a conventional Schottky diode
solar cell (,1.5 eV; refs 2, 8). Thus, the theoretical maximum power
output of the Schottky device utilizing internal electron emission
can be no greater than an idealized conventional cell for the solar
spectrum, ,25% (refs 2, 8).
The physical and electronic coupling of the chromophore to the
metal conduction levels are crucial to the performance of the device.
In aqueous solution, merbromin has an absorption maximum at
511 nm wavelength; but when merbromin is attached to the Au film,
the primary absorption peak appears to split into a blue-shifted
peak and a stronger red-shifted peak, giving rise to a broadening and
red-shift of the overall spectrum (Fig. 3a). The wavelength-dependent photoresponse of the device is shown in Fig. 3b, where we plot
the IPCE under short-circuit conditions, IPCE ¼ jð1g Þ=Fð1g Þ;
equation (2). The general features of the IPCE response spectrum,
and the broad maximum, are approximately the same as the surface-bound dye absorption; this is consistent with the mechanism of
action described in Fig. 1, with both the associated red- and blueshifted dye states coupling to the metal such that hot electrons are
produced. The IQE is determined by correcting the IPCE for the
fraction of incident photons absorbed in the dye (Fig. 3c). Although
the broad absorption edge of the TiO2 overlaps with the dye
adsorption between 400 nm and 450 nm, above 500 nm the TiO2
absorption is negligible and the IQE of approximately 10% reflects
the efficiency of the internal emission process.
The main considerations in choosing the materials for the device
are: (1) the relative energies of the donor/acceptor levels in the
© 2003 Nature Publishing Group
617
letters to nature
photoreceptor, the conductor Fermi level, the barrier height, and
the position of the semiconductor conduction band edges, (2) the
ballistic mean free path of electrons, and (3) the physical and
electronic coupling of the chromophore to the conductor for
high absorbance and high electron transfer efficiency. The devices
studied here represent only one of several different configurations of
photovoltaics taking advantage of ballistic electron transport and
internal electron emission in a photovoltaic device. For example,
modifications of the structure to utilize hot hole injection (rather
than hot electrons) in a p-type junction9 would allow use of hole
conducting polymers instead of inorganic semiconductors.
The IPCE and overall energy efficiency are limited by low dye
coverage (8 £ 1014 molecules cm22) and the resulting low photon
absorption. Significant increases are expected with improved optical design (reduced surface reflection), decreased metal thickness,
increased dye loading, and an engineered surface morphology with
significantly higher surface area structured such that multiple passes
through a dye-covered surface are possible for each photon.
Although the ultimate efficiency of an optimized device based on
the concept presented here is approximately the same as an ideal
conventional semiconductor cell, there appear to be practical and
economic advantages in terms of the wide choice of inexpensive,
durable, and readily synthesized device materials that may be
utilized.
A
Methods
Device fabrication
Devices were fabricated on titanium foil substrates (Alfa Aesar), which served as ohmic
back contacts. A 250-nm layer of titanium (99.9999%) was evaporated under vacuum
onto the foil following cleaning and polishing (using 10 mm grit). A 200-nm layer of TiO2
was grown on the substrate by thermal oxidation at 500 8C. The polycrystalline TiO2 is
predominately rutile phase, with oxygen vacancies giving rise to n-type doping. Au films
were electrodeposited onto the TiO2 from a solution containing 0.2 M KCN and 0.1 M
AuCN at pH 14. The TiO2 served as the working electrode, with a Pt wire counter
electrode. A 100-ms galvanostatic pulse at 2200 mA cm22 was used to nucleate Au
uniformly on the surface, followed by a periodic galvanostatic pulse train of 5 ms at
þ0.2 mA cm22 and 5 ms at 21.7 mA cm22 for 10 s to form a film ,10–50 nm thick.
Photoactive merbromin (2,7-dibromo-5-(hydroxymercurio)fluorescein disodium salt,
5 mM in water) was adsorbed onto the surface by immersion at room temperature for
10–12 h, followed by rinsing in water.
Characterization
Current –voltage (I–V) curves were measured using a voltage ramp rate of 0.05 V s21 in the
dark and under illumination from a 250-W tungsten lamp (Oriel, 6129), with intensity
measured using a radiometer (IL1700, International Light). The fill factor was calculated
at 1,000 Wm22 by dividing the maximum product of current and voltage from the
illuminated I–V curve by the product of open-circuit voltage and short-circuit current at
the same illumination. The spectral response was determined using a 150-W Xe lamp and
monochromator (Oriel 7240). IPCE was calculated from the current density under shortcircuit conditions and the photon flux as measured by the radiometer. The optical
absorbance (and absorption efficiency, h g(1 g)) of the dye on the device surface and dye
photon absorption was determined from the transmission and reflectance of a device
fabricated on a transparent substrate before and after application of the dye, using an
integrating sphere (LabSphere) and fibre-optic coupled monochromator (Ocean Optics).
Free-solution dye absorbance was measured with an optical spectrometer (UV-1610,
Shimadzu). Dye loading was determined by detaching the dye from the activated device
surface in 1 mM NaOH solution, and determining the amount removed from the
difference in optical absorbance at 511 nm of the NaOH solution.
Received 17 July; accepted 18 November 2002; doi:10.1038/nature01316.
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Acknowledgements We thank M. White, A. Tavakkoly, A. Kochhar, N. Shigeoka, G. Stucky and
W. Siripala for technical assistance and discussions. The project was supported by Adrena Inc.
Financial support for J.T. was provided by the NSF-MRSEC funded Materials Research
Laboratory (UCSB).
Competing interests statement The authors declare competing financial interests: details
accompany the paper on Nature’s website (ç http://www.nature.com/nature).
Correspondence and requests for materials should be addressed to E.W.M.
(e-mail: [email protected]).
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Hydrothermal recharge and
discharge across 50 km guided by
seamounts on a young ridge flank
A. T. Fisher*†, E. E. Davis‡, M. Hutnak*, V. Spiess§, L. Zühlsdorff§,
A. Cherkaoui*, L. Christiansenk, K. Edwards{, R. Macdonald‡,
H. Villinger§, M. J. Mottl#, C. G. Wheatq & K. Becker{
* Earth Sciences Department, † Institute for Geophysics and Planetary Physics,
University of California, Santa Cruz, California 95064, USA
‡ Pacific Geoscience Center, Geological Survey of Canada, Sydney, British
Columbia V8L 4B2, Canada
§ Earth Sciences Department, University of Bremen, Bremen D-28359, Germany
k Department of Earth and Planetary Sciences, Johns Hopkins University,
Baltimore, Maryland 21218, USA
{ Rosenstiel School of Marine and Atmospheric Science, University of Miami,
Miami, Florida 33149, USA
# School of Earth and Ocean Science and Technology, University of Hawaii,
Honolulu, Hawaii 96822, USA
q Global Undersea Research Unit, University of Alaska, Fairbanks, Alaska 99775,
USA
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Hydrothermal circulation within the sea floor, through lithosphere older than one million years (Myr), is responsible for 30%
of the energy released from plate cooling, and for 70% of the
global heat flow anomaly (the difference between observed
thermal output and that predicted by conductive cooling
models)1,2. Hydrothermal fluids remove significant amounts of
heat from the oceanic lithosphere for plates typically up to about
65 Myr old3,4. But in view of the relatively impermeable sediments that cover most ridge flanks5, it has been difficult to
explain how these fluids transport heat from the crust to the
ocean. Here we present results of swath mapping, heat flow,
geochemistry and seismic surveys from the young eastern flank
of the Juan de Fuca ridge, which show that isolated basement
outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than
50 km. Our analyses reveal distinct thermal patterns at the sea
floor adjacent to recharging and discharging outcrops. We find
that such a circulation through basement outcrops can be
sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling
studies.
Hydrothermal circulation on ridge flanks (crust older than
1 Myr) advects lithospheric heat from much of the sea floor,
contributing to enormous fluxes of fluid, energy and solutes1,6,7. It
is easy for fluid to enter and leave the crustal reservoir on most
young sea floor, where sediment cover is incomplete and permeable
basement rocks are widely exposed, but mechanisms by which fluids
penetrate through thick and more continuous sediments have
remained enigmatic5. The primary difficulty is that forces available
to drive hydrothermal circulation on ridge flanks are modest7–10,
being limited mainly to the difference in fluid pressures below
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