of 8
820 mV open-circuit voltages from Cu
2
O/CH
3
CN junctions
Chengxiang Xiang, Gregory M. Kimball, Ronald L. Grimm, Bruce S. Brunschwig, Harry A. Atwater
*
and Nathan S. Lewis
*
Received 12th October 2010, Accepted 25th November 2010
DOI: 10.1039/c0ee00554a
P-Type cuprous oxide (Cu
2
O) photoelectrodes prepared by the thermal oxidation of Cu foils exhibited
open-circuit voltages in excess of 800 mV in nonaqueous regenerative photoelectrochemical cells. In
contact with the decamethylcobaltocene
+/0
(Me
10
CoCp
2
+/0
) redox couple, cuprous oxide yielded open-
circuit voltage,
V
oc
, values of 820 mV and short-circuit current density,
J
sc
, values of 3.1 mA cm

2
under
simulated air mass 1.5 illumination. The energy-conversion efficiency of 1.5% was limited by solution
absorption and optical reflection losses that reduced the short-circuit photocurrent density. Spectral
response measurements demonstrated that the internal quantum yield approached unity in the 400–500
nm spectral range, but poor red response, attributable to bulk recombination, lowered the overall
efficiency of the cell. X-Ray photoelectron spectroscopy and Auger electron spectroscopy indicated
that the photoelectrodes had a high-quality cuprous oxide surface, and revealed no observable
photocorrosion during operation in the nonaqueous electrolyte. The semiconductor/liquid junctions
thus provide a noninvasive method to investigate the energy-conversion properties of cuprous oxide
without the confounding factors of deleterious surface reactions.
I. Introduction
Cuprous oxide (Cu
2
O) is an attractive material for water
photoelectrolysis and for photovoltaics because of the low cost,
high availability,
1
and straightforward processing of Cu
2
O.
2–4
Cu
2
O is a native p-type semiconductor, with a 2.0 eV band gap
and a relatively high absorption coefficient in the visible
region.
2,3,5,6
Cu
2
O synthesized by high temperature thermal
oxidation of Cu has been shown to have high hole mobilities and
long minority-carrier diffusion lengths.
2,7,8
The efficiency of Cu
2
O solar cells is however limited by the
difficulty of preparing high-quality n-type Cu
2
O and by the lack
of a suitable n-type heterojunction partner. Interfacial chemical
reactions at Cu
2
O/metal Schottky junctions result in the
precipitation of deleterious Cu metal that lowers the barrier
height of the resulting Schottky junctions and limits the open-
circuit voltage to <350 mV.
2,9
Interfacial Cu formation also
degrades the performance of p-Cu
2
O heterojunctions
10
formed
from metal–insulator–Cu
2
O contacts
11
and transparent con-
ducting oxide (TCO)/Cu
2
O heterojunctions that incorporate
Beckman Institute and Kavli Nanoscience Institute, Division of Chemistry
and Chemical Engineering, 210 Noyes Laboratory, MC 127-72, California
Institute of Technology, Pasadena, California, 9125. E-mail: haa@caltech.
edu; nslewis@caltech.edu
† Electronic supplementary information (ESI) available: Refractive index
and absorption coefficient of Cu
2
O substrates (Fig. 1S) and the
absorbance spectra for CH
3
CN–Me
10
CoCp
2
+/0
solution (Fig. 2S) in the
spectra range of interest. See DOI: 10.1039/c0ee00554a
Broadercontext
Low cost, earth-abundant light absorbers are of great interest for use in terrestrial photovoltaics and in artificial photosynthesis.
After early research work in the 1980s, cuprous oxide (Cu
2
O) has recently received renewed interest for such applications. With
a band gap of 2.0 eV, the theoretical efficiency of a Cu
2
O-based solar conversion device under AM 1.5 illumination is on the order of
20%. However, the highest efficiency reported to date is 2%, in a p–n heterojunction structure. The major challenge for formation of
a highly efficient Cu
2
O solid-state cell is the lack of emitter materials that possess suitable band-edge alignment and that do not
participate in deleterious interfacial reactions with the cuprous oxide. Semiconductor/liquid junctions offer the opportunity to
explore the limits on the open-circuit voltage,
V
oc
, that can be obtained by the use of redox species that have very negative Nernstian
redox potentials. In this work, high-quality Cu
2
O substrates were produced by high-temperature oxidation of copper foils. P-Type
Cu
2
O photoelectrodes in contact with the decamethylcobaltocene
+/0
redox couple displayed
V
oc
values in excess of 800 mV, and near-
unity internal quantum yields in the 400–500 nm spectral range.
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ZnO, In
2
O
3
, SnO
2
, or CdO.
12–16
Devices with higher barrier
heights have been fabricated by minimizing interfacial Cu
formation. For example, a Cu
2
O/ZnO heterojunction solar cell
has been reported to display an open-circuit voltage of 595 mV
and an energy-conversion efficiency of 2%.
13
Semiconductor/liquid junctions present an alternative to
conventional solid-state photovoltaic devices. Semiconductor/
liquid junctions offer the opportunity to tune the electrical and
chemical properties of the interface to produce either highly
rectifying or ohmic contacts to a semiconductor of interest.
17–20
In aqueous solutions, cuprous oxide is only thermodynamically
stable in a limited range of pH and electrochemical potential.
21
However, in acetonitrile
22
and other nonaqueous electrolytes,
23–25
cuprous oxide exhibits minimal photocorrosion.
The very low energy (3.2 eV
vs.
vacuum)
26
of the conduction-
band edge of Cu
2
O poses special challenges in forming high
barrier-height contacts to this semiconductor. Typically only
very reactive metals would be expected to yield high barrier
heights in p-Cu
2
O/metal Schottky barriers, and these metals
would only be useful if thermodynamically allowed interfacial
chemical reactions, that produce species such as Cu metal, can be
avoided during contact formation. Few heterojunction contacts
have the needed low electron affinity to provide suitable band-
edge offsets for junctions with Cu
2
O. For example, the built-in
voltage in Cu
2
O/ZnO heterojunctions is only 0.75–0.87 V, due to
the band-edge misalignment.
27
High quality p–n Cu
2
O homo-
junctions have not been reported due to the difficulty of
preparing n-type Cu
2
O. For semiconductor/liquid contacts,
redox species that have very negative Nernstian redox potentials
should be required theoretically to produce p-Cu
2
O/liquid
junctions that provide high open-circuit photovoltages and thus
high solar energy-conversion efficiencies. The potentials of such
species are far more negative than the reduction potential of
water to produce H
2
, and thus such redox species should be short-
lived in aqueous solution. However, the use of one-electron, outer-
sphere redox couples that have very negative reduction potentials,
such as cobaltocenium/cobaltocene (CoCp
2
+/0
)ordecamethy-
lcobaltocenium/decamethylcobaltocene (Me
10
CoCp
2
+/0
)ininert,
nonaqueous solvents offers an oppo
rtunity to explore the limits on
open-circuit voltage that can be obtained by use of very high
barrier height contacts to Cu
2
O photocathodes. The perfor-
mance of such systems, investigated herein, can be used to
establish the photovoltage and energy-conversion properties of
Cu
2
O contacts that do not suffer from interfacial chemical
reactions or interdiffusion processes that are typically present in
Cu
2
O-based solid-state devices.
II. Experimental
A. Preparation of Cu
2
O substrates
Large-grain polycrystalline Cu
2
O substrates,

350
m
m in thick-
ness, were prepared by the thermal oxidation of 250
m
m thick
copper sheet (99.9999% pure, Alfa Aesar) that was diced into
6mm

6 mm squares. The Cu samples were then suspended in
a quartz tube and loaded into a tube furnace. Oxidation to
Cu
2
O was initiated by heating the tube furnace to 800

C under
<5

10

6
Torr. The temperature was ramped to 980

C under
a rough vacuum of 1

10

3
Torr and the Cu foil was then
oxidized by exposure to oxygen (ultra-high purity, Air Liquide)
at 6 Torr for 2 h. Following oxidation, the tube furnace was
cooled stepwise to room temperature, under successively
decreasing oxygen pressures, to maintain the Cu
2
O phase
according to the Cu–O phase diagram.
28
The as-oxidized
Cu
2
O foils were lapped using diamond abrasive films, and
polished in a colloidal silica slurry (South Bay Technology), to
produce substrates that had a specular finish and a thickness of
100
m
m–150
m
m, as measured by digital calipers.
B. Characterization of Cu
2
O substrates
Scanning electron microscope (SEM) images as well as maps of
the crystallographic orientation by Electron Back Scattering
Diffraction (EBSD) were collected using a ZEISS 1550. X-Ray
diffraction (XRD) data were collected using a Philips XRD with
Cu K
a
radiation. Square Cu
2
O substrates with evaporated gold
contacts patterned at the four corners through a shadow mask
were used for room temperature Van der Pauw and Hall effect
measurements (MMR Technologies).
Spectroscopic ellipsometry was performed at an angle of
incidence of 50

,60

,or70

, for 300 nm <
l
< 850 nm, using a Xe
lamp as the light source. The dielectric property
j
(
u
),
D
(
u
)
data were converted to
n
(
u
),
k
(
u
) values assuming a bulk,
isotropic substrate. The solution absorption of diluted electrolyte
in sealed quartz cuvettes was measured using a UV-vis
spectrometer (Cary 50).
X-Ray photoelectron spectroscopy (XPS) and X-ray excited
Auger electron spectroscopy (XAES, or AES) data were
obtained at base pressures <1

10

9
Torr using an M-Probe
spectrometer. A monochromatic Al K
a
source generated X-rays
incident on the sample with a 300
m
m spot. High-resolution
spectra were acquired with a 100 mm hemispherical electron
analyzer set to a 20 eV pass energy and controlled using the
ESCA 2000 E Capture software (Service Physics). All XP and
XAE spectra were energy corrected assuming an adventitious
C(1s) photoelectron binding energy peak of 284.6 eV, and fit
using a Shirley baseline. The Cu
2
O samples were freshly prepared
and immediately loaded into the spectrometer. In addition to
freshly prepared Cu
2
O samples, XP and Auger spectra were also
acquired on Cu
2
O substrates that had been used as electrodes in
photoelectrochemical cells. All Auger and XP spectra were fit
using a software package that was written in-house that
minimizes the
c
2
fitting error with the Levenberg–Marquardt
algorithm. In all cases, spectra were fit with Gaussian–Lor-
entzian lineshapes above a Shirley background with no asym-
metric profiles. The Cu(2p
3/2
) XP spectra were well fit with two
peaks, with no
a priori
constraints placed on the peak locations,
areas, or widths. Conversely, fits to the Cu LMM Auger spectra
required up to four peaks. This somewhat more flat optimization
space required successive iterations in which the full width at half
maximum (FWHM) of the peaks was constrained to 3.0 eV while
the peak position was optimized, and the resulting peak positions
then were fixed while the peak width was optimized.
C. Photoelectrochemistry and spectral response properties
Cuprous oxide semiconductor electrodes were prepared accord-
ing to established techniques.
17,29,30
Ohmic contacts were made
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by evaporation of 100 nm of gold onto the unpolished side of the
Cu
2
O substrate. The Cu
2
O substrates were then attached to
copper wire by use of conductive silver paste between the wire
and the gold film on the back of the Cu
2
O. Exposed copper,
silver, and gold surfaces were sealed to a supporting glass tube
using epoxy (Hysol 1C), such that only the polished Cu
2
O face
was exposed to the electrolyte solution. The front surface of the
photoelectrode was etched in 8 M HNO
3
(aq) for 1–2 s, followed
by copious rinsing with 18 M
U
resistivity H
2
O (Nanopure
Infinity, Barnstead), followed by drying in a stream of N
2
(g).
12,13
For
all nonaqueous photoelectrochemical
experiments,
CH
3
CN (anhydrous, 99.8%, Sigma Aldrich) was distilled under
N
2
(g) (ultra-high purity, Air Liquide) from CaH
2
(
$
97%,
Fluka), LiClO
4
(battery grade, 99.99%, Sigma Aldrich) was dried
by fusion at 350

C under <1

10

3
Torr and was stored in
a VAC Omni-Lab glovebox that had <0.2 ppm of O
2
(g).
Bis(cyclopentadienyl) cobalt(
II
) (cobaltocene, CoCp
2
0
) and
bis(pentamethylcyclopentadienyl)
cobalt(
II
)
(decamethyl-
cobaltocene, Me
10
CoCp
2
0
) were purchased from Sigma Aldrich
and were purified by sublimation. Bis(cyclopentadienyl)
cobalt(
III
) hexafluorophosphate (cobaltocenium, CoCp
2
+
$
PF
6

)
and bis(pentamethylcyclopentadienyl) cobalt(
III
) hexafluoro-
phosphate (decamethylcobaltocenium, Me
10
CoCp
2
+
$
PF
6

)
were purchased from Sigma Aldrich and recrystallized
before use.
Current density
vs.
potential (
J
E
) data were collected at
50 mV s

1
using a Princeton Applied Research (PAR 273)
potentiostat. White light from a Sylvania ELH-type halogen
bulb was passed through a quartz diffuser to provide the
equivalent of air mass 1.5 illumination calibrated using a Si
photodiode electrode inside the cell. Various illumination
intensities were obtained by use of a combination of quartz
neutral density filters. Conventional three-electrode photo-
electrochemical cells were assembled in an inert atmosphere
glovebox, and included a Pt gauze counter electrode and a Pt
wire reference electrode poised at the solution potential. An
additional Pt disk working electrode was used to measure the
uncompensated ohmic resistance of the cell, typically

50
U
. The
stir bar was placed directly next to the Cu
2
O photoelectrode and
was rotated at

3000 rpm to minimize mass-transport effects.
The electrochemical data were analyzed and reported as-
collected, without correction for any solution resistance or
concentration overpotential losses.
31
Previous work has shown
that such three-electrode potentiostatic measurements yield
photoelectrode performance characteristics that are very close to,
and essentially identical to, those obtained in optimized, unstir-
red,
two-electrode,
thin-layer,
regenerative,
full
photo-
electrochemical solar cell devices.
30
Aqueous photoelectrochemical experiments were performed in

20 mL of 0.5 M K
2
SO
4
–0.050 M methyl viologen dichloride
(MV
2+
, 98%, Aldrich) that was buffered at pH
¼
3.6 by potas-
sium hydrogen phthalate. The Nernstian potential of the solu-
tion was driven to

0.6 V
vs.
SCE using the large carbon cloth
electrode as a working electrode and the frit-separated Pt mesh as
a counter electrode. During experiments with the Cu
2
O working
electrodes, the carbon cloth electrode served as the counter
electrode, a Pt wire that was poised at the solution potential
served as the reference electrode. The cell was purged with Ar gas
at all times, to minimize reaction between O
2
(g) and MV
+
c
.
Spectral response measurements were performed on a photo-
electrochemical cell that contained CH
3
CN–1.0 M LiClO
4
0.0020 M Me
10
CoCp
2
+
–0.0002 M Me
10
CoCp
2
0
. Relative to
the solution used to collect
J
E
data, the solution used for
spectral response measurements was diluted with CH
3
CN to
minimize solution absorption. A 50 W xenon arc lamp (Oriel)
was chopped at 13 Hz and passed through a quarter-wave
monochromator onto the working electrode in the photo-
electrochemical cell. The light intensity was determined using
a calibrated Si photodiode that was placed at the same position
as the working electrode. A Gamry Reference 600 potentiostat
was used to measure the photocurrent at short circuit. Lock-in
amplifiers (EG&G Princeton Applied Research) collected
signals from both the reference Si photodiode channel and from
the potentiostat output channel. The signal from both lock-in
amplifiers was fed into a computer that was controlled by
a LabVIEW program.
Fig. 1
(a) Scanning electron microscope image showing the poly-
crystalline grain structure of the Cu
2
O foil before lapping and polishing
(scale bar, 1 mm). (b) Electron backscatter diffraction image showing the
orientation map corresponding to the grain structure of the Cu
2
O
substrate. (c) X-Ray diffraction measurement showing phase-pure Cu
2
O.
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III. Results
A. Crystal structure and electrical properties of Cu
2
O
electrodes
The average grain size of an as-grown Cu
2
O substrate was 275

120
m
m in diameter, as shown by SEM images (Fig. 1a). Across
large sample areas, no strong preferential crystallographic
orientation was observed in the substrate, as shown by the cor-
responding false-color representation of the crystal planes
acquired by electron backscatter diffraction (EBSD) (Fig. 1b).
The X-ray diffraction pattern of the Cu
2
O substrate exhibited
reflection peaks that could all be indexed to the Cu
2
O cubic
structure (Fig. 1c). The absence of CuO and Cu peaks in the XRD
pattern indicated that the as-grown substrate was phase-pure
cuprous oxide. SEM and XRD results thus indicated that the
substrates synthesized by thermal oxidation were large-grain
polycrystalline Cu
2
O. Hall measurements indicated that the as-
grown multicrystalline Cu
2
O was p-type, with a hole concentra-
tion of 3.7

10
13
cm

3
and a hole mobility of 65 cm
2
V

1
s

1
at
room temperature, which are comparable to values reported in the
literature for Cu
2
O films prepared by an analogous method.
8,13
B. Photoelectrochemical behavior under simulated AM 1.5
illumination
Fig. 2a displays the
J
E
behavior of Cu
2
O photocathodes in
contact with either CH
3
CN–1.0 M LiClO
4
–0.020 M CoCp
2
+
0.002 M CoCp
2
0
(red trace) or CH
3
CN–1.0 M LiClO
4
–0.020 M
Me
10
CoCp
2
+
–0.002 M Me
10
CoCp
2
0
(blue trace). In contact with
the CoCp
2
+/0
redox couple, Cu
2
O photocathodes typically
exhibited
V
oc
values between 500 and 550 mV. In contrast, in
contact with the Me
10
CoCp
2
+/0
redox couple, Cu
2
O photocath-
odes exhibited
V
oc
values between 780 and 820 mV. In both
cases, the Cu
2
O photocathodes exhibited
J
sc
values between 3
and 4 mA cm

2
and fill factors of 0.5–0.6.
In the dark and under forward bias, the stabilized electrodes
passed only anodic current, as expected for a reversible photo-
electrochemical system whose absolute anodic current was
limited by the low concentration of the reduced species, either
CoCp
2
0
or Me
10
CoCp
2
0
, respectively. In contrast to stabilized
electrodes, freshly etched Cu
2
O electrodes displayed
V
oc
values
>1 V under 1 sun of illumination, and
V
oc
¼
0.3–0.4 V in the
dark. The potential offset however was only observed prior to the
first scan of the electrode. After passing a few mC cm

2
of
cathodic charge, the dark open-circuit voltage stabilized at 0.0 V,
and the open-circuit voltage under 1 sun of illumination stabi-
lized near 800 mV for Cu
2
O/CH
3
CN–Me
10
CoCp
2
+/0
contacts
and near 520 mV for Cu
2
O/CH
3
CN–CoCp
2
+/0
contacts during
several hour time period over which the experimental data were
collected.
C. Spectral response in photoelectrochemical cells
Fig. 2b shows the external quantum yield (
F
ext
or EQE, light
blue triangles) and the estimated internal quantum yield (
F
int
or
IQE, dark blue squares) of Cu
2
O photocathodes in contact with
CH
3
CN–Me
10
CoCp
2
+/0
. The external quantum yield peaked at
0.8 near 500 nm, with significant losses in both the blue and red
spectral regions. Reflection losses were estimated using the
Fresnel equations with the refractive indices of glass, CH
3
CN
and Cu
2
O in the spectral range of interest. The refractive index
of the Cu
2
O substrates was measured by spectroscopic ellips-
ometry (see ESI, Fig. S1†). The optical reflection in the cell
contributed

15 to 20% to the incident photon loss, across the
entire spectral range. The internal quantum yield, corrected for
solution optical absorption (see ESI, Fig. S2†) and reflection
losses, showed near-unity carrier collection in the 400 to 500 nm
spectral range. The effective collection length,
L
eff
, was deter-
mined to be 490 nm by fitting the spectral response data to
eqn (1) (Fig. 2b inset):
32
Fig. 2
(a) Current density–potential characteristics of Cu
2
O photocathodes in the dark and under ELH-simulated 1 sun illumination in contact with
CH
3
CN–1.0 M LiClO
4
containing 0.020 M Me
10
CoCp
2
+
–0.002 M Me
10
CoCp
2
(blue) or 0.020 M CoCp
2
+
–0.002 M CoCp
2
(red), respectively. (b)
Spectral response of Cu
2
O photocathodes in CH
3
CN–1.0 M LiClO
4
–0.0020 M Me
10
CoCp
2
+
–0.0002 M Me
10
CoCp
2
, where the internal quantum yield is
calculated from the external quantum yield using estimates of the solution optical absorption data and reflection losses.
1314 |
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F
ð
l
Þ
int
¼
Ð
L
eff
0
q
G
0
a
ð
l
Þ
e

a
ð
l
Þ
x
d
x
q
G
0
¼
1

e

a
ð
l
Þ
L
eff
(1)
where
q
G
0
a
(
l
)e

a
(
l
)
x
is the electron–hole generation rate,
G
0
is the
incident photon flux per unit area, and
a
(
l
) is the absorption
coefficient of Cu
2
O substrates at each wavelength (see ESI,
Fig. S1†).
D. Light intensity dependence
Fig. 3a shows the effects of illumination intensity on the current
density–voltage characteristics of Cu
2
O/CH
3
CN–Me
10
CoCp
2
+/0
contacts. The maximum illumination intensity from the
W–halogen ELH-type illumination corresponded to 100 mW
cm

2
,
i.e.
1 sun, whereas a series of neutral density filters was used
to attenuate the light intensity to

2.3 mW cm

2
. Fig. 3b shows
the integrated external quantum yield as a function of the applied
potential, where the integrated external quantum yield is the
fraction of the observed photocurrent density relative to the
estimated total photon flux at all energies above the Cu
2
O band
gap. The discrepancy between the absolute value of the inte-
grated external quantum yield and the spectral response data
shown in Fig. 2b is attributable to solution absorption. The
integrated external quantum yield at
J
sc
increased as the light
intensity decreased. The increase in
F
ext
is attributable to the
decreased concentration of the highly absorbing Me
10
CoCp
2
0
species that is generated near the electrode surface at low
cathodic current densities.
The
V
oc
of a semiconductor/liquid junction is given by eqn (2),
where
n
is the diode quality factor,
k
B
is the Boltzmann constant,
T
is the temperature (in Kelvin),
q
is the (unsigned) charge on an
electron,
J
ph
is the photocurrent density, and
J
0
is the exchange-
current density:
V
oc
¼
nk
B
T
q
ln

J
ph
J
0

(2)
For a photoconductor, when the series resistance of the pho-
toelectrode is sufficiently large, the slope of the
J
V
behavior
near
V
oc
is expected to be inversely proportional to the majority-
carrier (hole) concentration (
N
A
) in the photoelectrode.
32
The
hole concentration (
N
A
) for Cu
2
O photoelectrodes as a function
of light intensity was thus computed from the cell series resis-
tance that was estimated by fitting the slope of the
J
E
data in the
region of
V
oc
32
(Fig. 3a inset). The deduced hole concentration
showed a linear relationship on light intensity, consistent with
other reports of photoconductivity in Cu
2
O.
33,34
Consistently, at
illumination intensities near 1 sun, a diode ideality factor of
approximately 2.1 (Fig. 3b, inset) was obtained from the slope of
V
oc
vs. J
sc
(eqn (2)). The error bars in the inset represent the
standard deviation from four samples that were analyzed under
a series of illumination intensities.
E. Stability of Cu
2
O photoelectrodes
Fig. 4a displays
J
E
data for freshly etched Cu
2
O electrodes in
contact with the aqueous MV
2+/+
redox couple (purple), as well as
the
J
E
behavior of Cu
2
O/Me
10
CoCp
2
+/0
contacts before (blue)
and after (orange) photoelectrochemical measurements of Cu
2
O/
MV
2+/+
(aq) contacts. The
J
E
data of the Cu
2
O/MV
2+/+
(aq)
contacts exhibited
V
oc
< 200 mV. Moreover, after the
Fig. 3
(a) Current density–potential characteristics and (b) external
quantum yield as a function of ELH illumination intensity for Cu
2
O
photocathodes in contact with CH
3
CN–1.0 M LiClO
4

0.020 M
Me
10
CoCp
2
+
–0.002 M Me
10
CoCp
2
. The external quantum yield was
calculated using the theoretical maximum current density for the ELH
illumination intensity. (b, inset) Linear fits to
V
oc
as a function of ELH
illumination intensity using the ideal diode equation gave estimates of the
diode ideality factor for the junction.
Fig. 4
(a) Current density–potential characteristics under ELH-simu-
lated 1 sun illumination of freshly etched Cu
2
O photocathodes in contact
with CH
3
CN–1.0 M LiClO
4
–0.020 M Me
10
CoCp
2
+
–0.002 M Me
10
CoCp
2
(blue); etched, exposed to H
2
O–0.5 M K
2
SO
4
–0.050 M MV
2+
(aq) and
then in contact with CH
3
CN–1.0 M LiClO
4
–0.020 M Me
10
CoCp
2
+
–0.002
MMe
10
CoCp
2
(orange); etched Cu
2
O photocathodes in contact with
H
2
O–0.5 M K
2
SO
4
–0.050 M MV
2+
(aq) (purple). (b) Cu LMM Auger
spectra (left) and Cu(2p
3/2
) XP spectra (right) for chemically treated
Cu
2
O: (i) etched by aqueous HNO
3
; (ii) etched then exposed to CH
3
CN–
Me
10
CoCp
2
+/0
; (iii) etched then exposed to H
2
O–0.5 M K
2
SO
4
–0.050 M
MV
2+
(aq).
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measurements, visible Cu precipitation was observed at the
electrode surface, consistent with severe photocorrosion. Freshly
etched Cu
2
O electrodes demonstrated
V
oc
z
800 mV in contact
with CH
3
CN–Me
10
CoCp
2
+/0
electrolytes, but electrodes that had
first been exposed to MV
2+/+
(aq) electrochemical scans under
illumination exhibited
V
oc
< 300 mV in contact with
Me
10
CoCp
2
+/0
. The degradation of the photoelectrochemical
performance that was caused by photocorrosion of Cu
2
Oin
aqueous methyl viologen experiments was, however, reversed by
re-etching the Cu
2
O electrodes in HNO
3
(aq).
The photoelectron spectra in Fig. 4b further elucidate the
chemical transformations at the surface of Cu
2
O electrodes. In
Cu(2p
3/2
) photoelectron spectra, metallic Cu
0
exhibits a peak
centered at 932.6 eV, whereas Cu
2
O shows a peak at 932.4 eV,
and CuO shows a primary photoelectron peak at 933.6 eV as
well as secondary shake-up peaks at 940–945 eV.
35
Hence, the
Cu(2p
3/2
) spectral region unambiguously identifies the presence
of CuO surface species, but cannot readily resolve the 0.2 eV
difference between Cu
2
O and metallic Cu
0
surface species. In
contrast, metallic Cu
0
produces a peak centered at 918.5 eV in the
Cu LMM Auger electron region, whereas Cu
2
O exhibits a signal
at 916.8 eV, and CuO shows a signal at 917.8 eV.
35–38
The peak
shapes observed in the Cu LMM Auger region can thus
discriminate between Cu
2
O and metallic Cu
0
surface species
Fig. 4b depicts the spectra from the Cu LMM Auger electron
region (left) as well as the Cu(2p
3/2
) photoelectron region (right)
observed for Cu
2
O substrates that were either freshly etched (i),
used as photoelectrodes under illumination in contact with
CH
3
CN–Me
10
CoCp
2
+/0
(ii), or used as photoelectrodes under
illumination in contact with MV
2+/+
(aq) (iii). In all cases, spectra
were fit with Gaussian–Lorentzian peaks above a Shirley back-
ground, with the red traces denoting individual fitted peaks, the
blue traces denoting the sum of the individual peaks, and the
black traces denoting the baseline of the fitted spectra. While all
of the XP spectra were well fit with two peaks, additional peaks
at both high and low-binding energy were needed to effectively fit
the Auger spectra. These additional peaks have been observed
previously in the Auger LMM spectra of copper oxides.
34
As
a guide between spectra, and for comparison with literature
results, a dashed line has been included in the Auger spectra at
916.8 eV, indicative of the Cu
2
O species, and another dashed line
has been included at 918.5 eV, indicative of metallic Cu
0
.A
dashed line is shown in the XP spectra at 934.4 eV, indicative of
CuO species, and another dashed line has been included at 932.1
eV, indicative of either Cu
2
O or metallic Cu
0
.
As shown in part (i) of Fig. 4b, the Cu(2p
3/2
) spectral region of
the freshly etched Cu
2
O surface showed a large peak at 932.1 eV
and a smaller feature centered at 934.3 eV, indicating that little
CuO or surface hydroxide was present on the surface of the
sample. Further, the Cu LMM Auger peak at 917.1 eV, and
the absence of any significant feature at 918.5 eV, indicate that
the thermal oxidation of Cu foil to Cu
2
O, and subsequent etching
with HNO
3
(aq), produced a Cu
2
O surface that was free of
metallic Cu
0
. The Auger spectrum in part (i) and all further
Auger spectra show features attributed to an additional peak
towards the lower binding energy at

912.5 eV and a further peak
towards the higher binding energy at

921.0 eV. These peaks are
attributed to other LMM Auger transitions not principally
diagnostic of copper oxidation.
34–37
By comparison with part (i),
part (ii) of Fig. 4b demonstrates that the surface composition was
essentially unchanged after photoelectrochemical experiments in
contact with CH
3
CN–Me
10
CoCp
2
+/0
. This result is consistent
with the expected stability of Cu
2
O in nonaqueous photo-
electrochemical cells.
In contrast, use of the Cu
2
O in contact with MV
2+/+
(aq) yielded
significant changes in the XP and Auger spectra, as shown in
Fig. 4b, part (iii). In particular, the strongest feature in the Cu
LMM Auger spectrum shifted from 917.1 eV to 916.4 eV, and an
additional feature appeared at 918.5 eV, denoted by the bold,
green fitted trace, which is ascribable to metallic Cu
0
. Although
not strictly quantitative due to the paucity of sensitivity factors
and other calibrations for Auger spectra of copper oxides, the
ratio of the area of the green fitted peak at 918.5 eV to the area of
the peak centered at 916.4 eV was 0.56. Assuming similar
probabilities for Auger emission and capture of secondary elec-
trons from Cu
2
O and Cu
0
, Fig. 4b part (iii) thus clearly indicates
the presence of significant quantities of interfacial metallic Cu
0
.
Thus, the Auger spectrum of Fig. 4b part (iii) corroborates the
results described above, in which interfacial metallic Cu
0
formed
by aqueous photocorrosion processes significantly degraded the
photoelectrochemical performance of the Cu
2
O/CH
3
CN–
Me
10
CoCp
2
+/0
contact.
IV. Discussion
A. Open-circuit voltage of p-Cu
2
O/CH
3
CN contacts
The behavior of the semiconductor/liquid contacts investigated
herein, in which the redox system in the liquid was not photo-
excited but only served to transport the charge across the inter-
face and to determine the equilibrium barrier-height of the solid/
liquid contact, arose from the photogeneration, transport, and
recombination processes in the p-Cu
2
O, along with the space-
charge field in the p-Cu
2
O produced by charge equilibration
between the Fermi levels of the Cu
2
O and the redox species in the
electrolyte. The resulting energy-conversion properties of such
systems are thus directly analogous to those displayed by
p-Cu
2
O-based heterojunctions or homojunctions. The 820 mV
open-circuit voltage of Cu
2
O/CH
3
CN–Me
10
CoCp
2
+/0
contacts is
significantly higher than the reported
V
oc
values of 595 mV
13
and
300 mV
14,15
for Cu
2
O/ZnO, 400 mV for Cu
2
O/CdO
12,16
and
Cu
2
O/Al-doped ZnO,
39
270 mV for Cu
2
O/ITO,
40
240 mV for
Cu
2
O
/
Cu
x
S,
41
or <430 mV for p–n-Cu
2
O junctions,
42,43
and
<400 mV for previous Cu
2
O/liquid junctions.
22,44
The open-
circuit voltages of photoelectrodes measured in a potentiostatic,
three-electrode configuration are identical to those of full,
regenerative, two-electrode, photoelectrochemical cells measured
for the same solid/liquid contacts under the same injection
levels.
30,45
These
open-circuit
voltages,
representing
the
maximum free energy produced and available to be driven
through an external load at steady state under the specified
illumination conditions, and also are directly related to the open-
circuit voltage, and thus maximum free energy produced, of
a solid-state photovoltaic energy conversion device under the
same illumination conditions.
30,45
The 3.2 eV electron affinity of
cuprous oxide constrains the choices of redox couples appro-
priate for semiconductor/liquid junctions, because a very nega-
tive reduction potential is required to minimize the discontinuity
1316 |
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, 2011,
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, 1311–1318
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between the Cu
2
O conduction-band edge and the solution
reduction potential. For instance, the CoCp
2
+/0
formal reduction
potential of

0.91 V
vs.
SCE
46
and the Me
10
CoCp
2
+/0
formal
reduction potential of

1.47 V
vs.
SCE
46
are close to the potential
of the Cu
2
O conduction-band edge of

1.6 V
26
vs.
SCE. The
difference
between
the
V
oc
values
in
Cu
2
O/CH
3
CN–
Me
10
CoCp
2
+/0
contacts and Cu
2
O/CH
3
CN–CoCp
2
+/0
contacts is
thus related, as expected, to the different Nernstian potentials of
the two redox couples, with the solution having the more nega-
tive Nernstian redox potential producing a higher barrier height,
and thus a higher open-circuit voltage, than solutions having
more positive Nernstian potentials.
45
The observed open-circuit voltage is nonetheless lower than
the
V
oc
expected from the bulk recombination limit for Cu
2
O.
The maximum theoretical
V
oc
of a semiconductor junction is
determined by the bulk electronic properties of the semi-
conductor (eqn (3)):
29,32,47
V
oc
y
kT
q
ln

J
ph
L
n
N
A
qD
n
n
2
i

(3)
where
L
n
is the minority-carrier (electron) diffusion length,
N
A
is
the concentration of acceptors,
n
i
is the intrinsic carrier density,
D
n
is the minority-carrier diffusion coefficient (estimated by
using
D
n
¼ð
m
h
m
e
m
h
Þ
kT
q
, where
m
h
is the hole mobility, and
m
h
m
e
z
0
:
58
is the effective mass ratio between the hole and the electron).
48–50
Using
N
A
¼
3.7

10
13
cm

3
,
D
n
¼
2.2 cm
2
s

1
,
n
i
¼
2

10
2
cm

3
,
and estimating
L
n
to be 100 nm to 10
m
m, yields a value of
1.20–1.32 V as the maximum theoretical
V
oc
for Cu
2
O photo-
cathodes. Achieving the maximum theoretical
V
oc
will require
the use of the redox couple with a more negative potential and
the passivation of Cu
2
O surface states.
B. Short-circuit current density of Cu
2
O/CH
3
CN contacts
The low observed
J
sc
values for Cu
2
O photocathodes in contact
with CH
3
CN–Me
10
CoCp
2
+/0
solutions can be quantitatively
attributed to incomplete collection due to bulk recombination in
the semiconductor and to incomplete absorption due to solution
optical absorption and reflection losses. Table 1 shows the
estimated
J
sc
values expected for Cu
2
O photocathodes under
AM 1.5 illumination, under ELH-simulated 1.0 sun of illumi-
nation, and under attenuated ELH-simulated 0.09 sun of
illumination, given the various expected loss mechanisms.
J
sc
MAX
represents the theoretical maximum photocurrent density given
the photon flux of the incident light that is above the 2.0 eV band
gap of Cu
2
O, using standard spectral data.
J
sc
IQE
refers to the
expected photocurrent density, given the spectrum of the incident
light and the energy dependence of the internal quantum yield.
J
sc
PEC
refers to the expected photocurrent density given the
measured solution absorption and estimated reflection losses
inherent in the photoelectrochemical cell in addition to internal
collection losses. At ELH-simulated 1.0 sun illumination, the
value
J
sc
EXP
is expected to be even less than
J
sc
PEC
, due to addi-
tional solution absorption caused by the Me
10
CoCp
2
0
that is
generated at the electrode surface under short-circuit conditions.
However, at the attenuated ELH-simulated 0.09 sun illumina-
tion, the values of
J
sc
PEC
and
J
sc
EXP
are in agreement, because the
photogenerated Me
10
CoCp
2
0
is produced at a lower rate and can
be removed effectively by convection. Routes to improving the
photocurrent of the Cu
2
O photocathodes in contact with
a nonaqueous Me
10
CoCp
2
+/0
solution include producing material
with longer diffusion lengths and better red response, as well as
utilizing a thin-layer photoelectrochemical cell
30
with minimized
solution absorption.
V. Conclusions
Phase-pure, multicrystalline Cu
2
O substrates were produced by
thermal oxidation of Cu foils. P-type Cu
2
O photoelectrodes were
demonstrated to be stable and free of deleterious interfacial
reactions in nonaqueous regenerative photoelectrochemical cells.
Highly rectifying contacts were made to p-type Cu
2
O using redox
couples with very negative Nernstian potentials. In contact with
CH
3
CN–Me
10
CoCp
2
+/0
,Cu
2
O photoelectrodes exhibited 820
mV open-circuit voltage, and near-unity internal quantum yield
in the 400–500 nm spectral range. The observed short-circuit
current density agreed well with the value expected after
correction for solution optical absorption and reflection losses.
The observation of high open-circuit voltages in the p-Cu
2
O/
liquid junction demonstrates that high efficiency p-Cu
2
O solar
energy conversion devices can be obtained provided that suitable
emitter materials, that form high barrier-heights to Cu
2
O
substrates and that allow for rigorous control of the surface
chemistry and surface passivation of p-Cu
2
O absorbers, can be
identified.
Acknowledgements
This work was supported by the Office of Energy Efficiency and
Renewable Energy, US Department of Energy under Grant
DE-FG36-08GO18006, the Caltech Center for Sustainable
Energy Research (CCSER), and the Dow Chemical Company.
One of us (GMK) acknowledges support under an NDSEG
graduate fellowship.
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