Solar Energy 80 (2006) 715–722
www.elsevier.com/locate/solener
Thin film deposition of Cu2O and application for solar cells
K. Akimoto *, S. Ishizuka, M. Yanagita, Y. Nawa, Goutam K. Paul, T. Sakurai
Institute of Applied Physics, University of Tsukuba, CREST, Japan Science and Technology Corporation, 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8573, Japan
Received 16 June 2005; received in revised form 9 September 2005; accepted 18 October 2005
Available online 5 January 2006
Communicated by: Associate Editor Arturo Morales-Acevedo
Abstract
Deposition conditions of cuprous oxide (Cu2O) thin films on glass substrates and nitrogen doping into Cu2O were studied by using reactive radio-frequency magnetron sputtering method. The effects of defect passivation by crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, were also
studied. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu2O
at around 680 nm was enhanced, and the hole density was increased from 1016 to 1017 cm3. Finally, polycrystalline pCu2O/n-ZnO heterojunctions were grown for use in solar cells. Two deposition sequences were studied, ZnO deposited
on Cu2O and Cu2O deposited on ZnO. It was found that the crystallographic orientation and current–voltage characteristics of the heterojunction were significantly influenced by the deposition sequence, both being far superior for the heterojunction with structure Cu2O on ZnO than for the inverse structure. We successfully obtained a photoresponse for the first
time in the deposited thin film of Cu2O/ZnO.
2005 Elsevier Ltd. All rights reserved.
Keywords: Cuprous oxide; Zinc oxide; Solar cell; Cyanide treatment; Defect passivation; Photoresponse
1. Introduction
Cuprous oxide (Cu2O), a direct-gap semiconductor with a band-gap energy of 2.0 eV, has been
regarded as one of the most promising materials
for application to photovoltaic cells (Pollack and
Trivich, 1975). The attractiveness of Cu2O as a photovoltaic material lies in the fact that the constituent
materials are nontoxic and abundantly available on
the earth, and that the Cu2O has a high absorption
*
Corresponding author. Fax: +81 298 53 5205.
E-mail address: akimoto@esys.tsukuba.ac.jp (K. Akimoto).
coefficient in visible regions and low-cost producibility. There are several reports regarding the deposition method of Cu2O thin films (Siripala et al.,
1996; Matsumura et al., 1996; Kawaguchi et al.,
1994; Padiyath et al., 1994; Drobny and Pulfrey,
1979) and the dopant impurities (Musa et al.,
1998). We have systematically investigated the deposition conditions and dopant impurities, and found
that nitrogen acts as a p-type dopant in Cu2O with
an activation energy of 140 meV (Ishizuka et al.,
2000, 2001). Based on these results, we have been
able to control the hole density in polycrystalline
Cu2O thin films from 1015 to 1017 cm3. We have also
0038-092X/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.solener.2005.10.012
716
K. Akimoto et al. / Solar Energy 80 (2006) 715–722
Nomenclature
I–V
XRD
PL
P
Na
Nd
Nv
Ea
current–voltage
X-ray diffraction
photoluminescence
hole density
acceptor density
donor density
effective density of state in the valence
band
acceptor activation energy
reported that the hydrogen treatment, which is generally used for the passivation of defects in polycrystalline Si and Si/SiO2 interface states, is effective in
improving electrical and optical properties of the
polycrystalline Cu2O thin films (Ishizuka et al.,
2002). The hydrogen treatment for polycrystalline
Cu2O was, however, effective only in the absence of
heat treatment above 150 C. Therefore, it is desired
to develop another passivation technique whose
effects are unchanged after relatively high-temperature heat treatment.
Crown-ether cyanide treatment, which was
recently developed for passivating interface states
at Si/SiO2 interfaces and trap states in polycrystalline Si (Kobayashi et al., 1998, 2000) was applied
to polycrystalline Cu2O, and its effects on electrical
and optical properties were compared with those of
the hydrogen treatment.
Since Cu2O is a unipolar p-type semiconductor,
probably due to Cu vacancies, a p-Cu2O/n-ZnO
heterostructure was grown and the current–voltage
(I–V) characteristics were studied. In this paper,
successful control of hole density by nitrogen doping, defect passivation by cyanide treatment, and
photoresponse of a Cu2O/ZnO/substrate thin film
structure are reported.
2. Experimental
Cu2O thin films were deposited by means of reactive radio-frequency (rf) magnetron sputtering on
glass substrates (Corning 7059) at a substrate temperature of 400–500 C using a Cu target (99.99%
purity) and Ar sputtering gas. Nitrogen and oxygen
were introduced during the deposition through a
nozzle whose end was placed near the substrate. The
flow rate of oxygen used as the reactive gas was
k
T
mh
h
Voc
Jsc
FF
AFM
Boltzmann constant
temperature
effective hole mass
Planck’s constant
open circuit voltage
short circuit current
fill factor
atomic force microscope
fixed at 200 ml/min, which was found to be the
optimum condition to deposit a single phase of
Cu2O (Ishizuka et al., 2000). The flow rate of nitrogen used as the source of a p-type dopant was varied
from 0 to 20 ml/min. The film thickness was typically 2 lm, as determined by scanning electron
microscopy.
The structural properties were studied by X-ray
diffraction (XRD) in the h–2h mode using Cu-Ka
radiation. The resistivity, Hall mobility and carrier
density were measured at temperatures ranging
between 180 and 300 K by the Van der Pauw
method using a BIO-RAD HL5500PC system. The
specimen for the Hall measurement was square
shaped with a size of 7 · 7 mm2, and the electrodes
consisted of evaporated 1 mmB Au dots. Photoluminescence (PL) spectra of the Cu2O thin films were
measured at 77 K using a He–Cd laser with an excitation wavelength of 325 nm.
The crown-ether cyanide treatment was carried out by the following procedure: 0.2 mol of
18-crown-6 (C12H24O6) was dissolved in xylene
and the solution was added to a 0.1 M KCN aqueous solution with the same volume. After keeping
the solution still for 30 min, the part of the xylene
solution which was well separated from the aqueous
phase was taken out of the separatory funnel, and
the Cu2O film was immersed in the KCN solution
of xylene containing crown-ether for about 10 min,
followed by successive rinses in acetone, ethanol,
and de-ionized water. Hereafter, we call this treatment the ‘‘cyanide treatment’’. The 18-crown-6
molecule effectively captures a K+ ion, and consequently contamination by K+ ion is completely prevented. That is to say, K+ ions do not directly
contact Cu2O, only CN ions do. Details of the cyanide treatment are described elsewhere (Kobayashi
K. Akimoto et al. / Solar Energy 80 (2006) 715–722
717
et al., 1998, 2000). Hydrogen treatment was carried
out by exposing Cu2O thin films to hydrogen
plasma (Ishizuka et al., 2002).
In order to study the thermal stability of the passivation effects, the Cu2O thin films with the hydrogen
or cyanide treatment were annealed at temperatures
between 150 and 350 C for 3 min in a rapid thermal
annealing system under the vacuum of 1 · 104 Pa.
A polycrystalline p-Cu2O/n-ZnO heterostructure
was grown also by means of rf magnetron sputtering
on Corning 7059 glass substrates using a Cu target of
99.99% purity, a ZnO target and Ar sputtering gas.
At the interface between n-ZnO and p-Cu2O, an
insulating ZnO (i-ZnO) layer was deposited. The iZnO and n-ZnO were deposited using undoped
and 1 wt% Al2O3-doped ZnO targets, respectively.
Two types of heterostructures, n-ZnO/i-ZnO/
p-Cu2O/Au/glass substrate (structure-1) and pCu2O/i-ZnO/n-ZnO/glass substrate (structure-2),
were grown and the structural and electrical properties were studied. The deposition temperature of
Cu2O was varied between 400 and 500 C to study
the deposition temperature effect on the device performance. The carrier densities of n-ZnO and pCu2O were estimated to be about 5 · 1020 and
1 · 1015 cm3, respectively, by Hall-effect measurements on films of the individual materials.
3. Results and Discussion
Fig. 1. XRD profiles of copper oxides thin films for various
oxygen flow rates.
Fig. 2. Hole density (a) and Hall mobility, (b) for Cu2O thin films
deposited at various N2 flow rates.
3.1. Deposition of Cu2O and nitrogen doping
Fig. 1 shows the results of XRD for film deposited
at 400 C. Cu and Cu2O phases were observed for an
oxygen flow rate of 180 ml/min. Also, Cu2O and
CuO phases were observed for oxygen flow rates
higher than 225 ml/min. Instead, for oxygen flow
rates 210 ml/min and 225 ml/min, a single Cu2O
phase was obtained, showing a major preferred crystal orientation on the (2 0 0) axis with a small (1 1 1)
component.
Fig. 2(a) and (b) shows the hole density and Hall
mobility of Cu2O thin films as a function of N2 flow
rate. As it can be seen, the hole density increases
nearly linearly with N2 flow rate, with unitary slope,
and varied from 1 · 1015 cm3 to approximately
1017 cm3 in controlling the electrical properties of
this p-type semiconductor. The nitrogen was found
K. Akimoto et al. / Solar Energy 80 (2006) 715–722
to have the effect of increasing hole density and to
act as an acceptor. It is expected that the nitrogen
in Cu2O can be incorporated in the oxygen lattice
site, and this may be the reason for the nitrogen acting as an acceptor. The linear relationship between
the hole density and nitrogen flow rate suggests that
the incorporation rate of nitrogen is constant and
no defects induced by nitrogen doping were created.
The Hall mobility decreased with increasing
nitrogen flow rate, as shown in Fig. 2(b). This result
is reasonable since the carrier scattering is enhanced
by ionized acceptors. These results indicate that the
nitrogen is very effective in controlling the electrical
properties in p-type conductivity.
3.2. Defect passivation in Cu2O by cyanide treatment
Fig. 3 shows PL spectra of the nitrogen-doped,
polycrystalline Cu2O thin films measured at 77 K
before and after cyanide treatment. Although no
luminescence peak was detected before the cyanide
treatment, a peak due to near-band-edge emission
was observed at 680 nm (1.82 eV) after the cyanide
treatment. This peak is most likely attributable to
acceptor-related luminescence, taking account of
the band-gap energy of Cu2O (2.0 eV) and the nitrogen acceptor level (0.14 eV above the valence band
maximum). These results suggest that CN ions passivate nonradiative recombination centers of Cu2O
grains. As we reported previously (Ishizuka et al.,
2002), the hydrogen treatment has an effect of
increasing a luminescence intensity of polycrystalline
Cu2O. The effect of the defect passivation from the
cyanide treatment is similar to that of the hydrogen
treatment since the luminescence intensities were
almost the same after both treatments.
Fig. 4(a) and (b) shows the hole density and Hall
mobility of the Cu2O thin films, respectively, as a
function of the nitrogen flow rate during the film
deposition. After the cyanide treatment, the hole
density increased and the Hall mobility decreased,
irrespective of the nitrogen flow rate. The increase
of the hole density and the decrease of the mobility
may indicate that some kinds of hole traps are passivated by the cyanide treatment. It is generally
believed that oxygen vacancies, that is, dangling
bonds of Cu, act as donors that also act as hole
traps (Philips, 1973). From these considerations, it
10
Hole density (cm-3)
718
17
10
16
10
15
0
(a)
10
20
N2 flow rate (ml/min)
H- treatment
He-Cd laser 77K
40
After
CN- treatment
As received
Mobility (cm2/Vs)
PL Intensity (a.u.)
Before
20
0
0
(b)
600
700
10
20
N2 flow rate (ml/min)
800
Wavelength (nm)
Fig. 3. PL spectra of the nitrogen-doped, polycrystalline Cu2O
thin films before and after cyanide treatment.
Fig. 4. Hole density (a) and Hall mobility (b) before (circle) and
after hydrogen treatment (triangle) and after cyanide treatment
(square), obtained from Hall-effect measurements for the Cu2O
thin films deposited at various N2 flow rates.
K. Akimoto et al. / Solar Energy 80 (2006) 715–722
P ¼ fðN a N d Þ=2N d gN v expðEa =kT Þ;
where Nv is the effective density of states in the valence band equal to 2ð2pmh kT =hÞ3=2 , and Na, Nd, Ea,
k, T, mh , and h are the acceptor density, donor density, activation energy of the acceptors, Boltzmann
constant, temperature, effective hole mass, and
Planck’s constant, respectively. The value of the
effective hole mass was taken to be 0.58m0 (where
m0 is the mass of the free electron) (Hodby et al.,
1976). From the slopes of the straight lines determined by the least squares method, as shown in
Fig. 5, the values of Ea were estimated to be 0.14
and 0.12 eV for the nitrogen-doped Cu2O thin films
before and after cyanide treatment, respectively.
The change in Ea may be due to the formation of
an impurity band. The compensation ratios Nd/Na
were determined to be 0.56 and 0.18 for the samples
before and after cyanide treatment, respectively.
The significant reduction of the compensation ratio
indicates the passivation of donor-like defects by the
cyanide treatment. These results are consistent with
the aforementioned passivation model that CN
ions terminate the Cu dangling bonds, resulting in
the formation of Cu–CN bonds.
10
19
10 18
Hole density (cm-3)
is suggested that the effects of the cyanide treatment
result from termination of Cu dangling bonds by
CN ions. A similar hole density was also obtained
from the hydrogen treatment.
Fig. 5 shows the temperature dependence of the
hole density for the nitrogen-doped Cu2O thin films
before and after cyanide treatment. The hole density
can be expressed as
719
10 17
10 16
10 15
0
100
200
300
400
Temperature (˚C)
Fig. 6. Hole density of the Cu2O thin films with hydrogen
treatment (circle), cyanide treatment (rectangular), and before
treatment (triangle) measured as a function of the annealing
temperature.
The annealing effects on the hole density of three
kinds of Cu2O films; cyanide treated films, hydrogen
treated films and as received films, were examined,
and the results are shown in Fig. 6. The data on
the axis of 0 C means the samples without annealing. In the case of hydrogen treatment, the hole density began to decrease at around 200 C. In the case
of cyanide treatment, on the other hand, the hole
density remained almost constant up to 350 C.
Therefore, it can be concluded that cyanide treatment has much better thermal stability than the
hydrogen treatment.
3.3. Photoresponse from polycrystalline Cu2O/ZnO
XRD profiles of the heterostructures are shown in
Fig. 7. The curves (a) and (b) show the XRD profiles
H-treatment
As received
Cu2O (110)
Cu2O (220)
Au (111)
10
Intensity (a.u.)
Hole density (cm-3)
CN-treatment
1018
17
10
1016
(a)
Structure-I
ZnO (0001)
Cu2O (111)
(b)
1015
3
4
1000/T
Cu2O (200)
5
Structure-II
(K-1)
Fig. 5. Temperature dependence of the hole density for the
nitrogen-doped Cu2O thin films before and after cyanide treatment. That of for hydrogen treatment was also plotted for
reference.
20
30
40
50
2θ (degree)
60
70
Fig. 7. XRD profiles of structure-I (a) and structure-II (b).
K. Akimoto et al. / Solar Energy 80 (2006) 715–722
from the n-ZnO/i-ZnO/p-Cu2O/Au/glass (structureI) and p-Cu2O/i-ZnO/n-ZnO/glass structures (structure-II), respectively. In the curve (a), three peaks
corresponding to reflections from Cu2O(1 1 0),
Cu2O(2 2 0) and Au(1 1 1) are observed. In the curve
(b), two intense and sharp peaks corresponding to
Cu2O(1 1 1) and ZnO(0 0 0 1) and a minor peak of
Cu2O(2 0 0) are observed. These results show that
in structure-I, Cu2O has a (1 1 0) orientation on
Au(1 1 1), though the crystallographic orientation
of ZnO is unknown. In structure-II on the other
hand, ZnO is highly (0 0 0 1) oriented and the orientation of Cu2O is dominated by (1 1 1). Hence
Cu2O is likely to have the (1 1 1) orientation
selectively on ZnO(0 0 0 1). The differences in these
structures may be due to the crystal structure; ZnO
is hexagonal (wurtzite) and Cu2O is cubic (cuprite),
and the hexagonal (0 0 0 1) and cubic (1 1 1)
orientations have similar atomic arrangement. A
schematic illustration of the atomic arrangements
of Cu2O(1 1 1) and ZnO(0 0 0 1) is shown in Fig. 8.
The lattice mismatch between Cu2O(1 1 1) and
ZnO(0 0 0 1) is calculated to be 7.1%. The similarity
in atomic arrangement would seem to be responsible for the crystallographic orientation in the
heterostructure.
The current–voltage (I–V) characteristics of samples with structure-I and structure-II were mea-
sured. The n-ZnO is used as the window layer and
also n-electrode since its resistivity was as low as
2 · 104 X cm for both structures. The electrode
for p-side was Au. Some of the samples with structure-I showed electrical rectification, however,
reproducibility was not good and the inverse current
density was relatively high. On the other hand, all
the samples fabricated with a type-II structure
showed electrical rectification, and the inverse current density was clearly lower than that of struc0.5
Current density (mA/cm2)
720
0.0
-0.5
-1.0
-1.5
-2.0
dark
dar k
light
li ght
-2.5
-3.0
0.0
0.1
Voltage (V)
0.2
0.3
Fig. 9. I–V characteristics of a sample of structure-II. The dark
current and the photocurrent are indicated by open squares and
open circles, respectively.
Fig. 8. Schematic illustration of atomic arrangements of Cu2O(1 1 1) and ZnO(0 0 0 1).
K. Akimoto et al. / Solar Energy 80 (2006) 715–722
721
Fig. 10. AFM images of samples of p-Cu2O/i-ZnO/n-ZnO/glass structures. The deposition temperatures of Cu2O are 500 C (a) and
400 C (b).
ture-I. Fig. 9 shows the I–V characteristics of Au/pCu2O (1.7 lm thickness)/i-ZnO (0.1 lm)/n-ZnO
(0.8 lm)/glass structure (structure-II). The Cu2O
layer of this sample was deposited at 400 C. As
shown in the figure, we successfully obtained a photoresponse from the structure-II samples. This is the
first reported demonstration of photoresponse from
a deposited thin film of p-Cu2O/n-ZnO heterostructure. The open circuit voltage (Voc), short circuit
current (Jsc) and fill factor (FF) were 0.26 V,
2.8 mA/cm2 and 0.55, respectively, under the condition of AM1.5. The conversion efficiency was calculated to be 0.4%. The value of Voc is smaller than
expected, which may be due to interface defects
and should be clarified in the future.
A photoresponse was barely obtained from samples with structure-I. Such a clear difference may be
due to the properties of the interface between ZnO
and Cu2O. The defect density can be expected to
be lower in structure-II due to the similarity of
the atomic arrangement, as discussed above. Therefore, it can be said that the continuous atomic
arrangement is important even in polycrystalline
heterostructures.
The effects of the deposition temperature of Cu2O
on the I–V characteristics and the photoresponse
were studied. The deposition temperature of Cu2O
of the samples shown in Fig. 9 was 400 C. For samples with Cu2O grown at a deposition temperature of
500 C, the ratio of the electrical rectification, determined as the ratio of the current densities at bias voltages +1 V and 1 V, was 1.4 times greater than that
for samples with Cu2O grown at 400 C, and the photocurrent was 1.2 times greater. The crystal quality
may also be improved at higher deposition temperatures, and grain size seems to play an important role
in improving device performance. Fig. 10 shows
atomic force microscope (AFM) images of structure-II samples with Cu2O deposited at (a) 500 C
and (b) 400 C. The average grain size increased from
about 0.2 to 0.4 lm as the deposition temperature
was increased. With increasing grain size, the density
of surface dangling bonds decreased. Therefore, the
improvement in device performance at higher deposition temperatures may be due to an increase in
grain size.
4. Conclusions
Cu2O thin films were deposited by the reactive rfmagnetron sputtering method on glass substrates.
Single Cu2O phase films can be obtained by controlling the oxygen flow rate. Nitrogen was found to
have the effect of increasing hole density by acting
as an acceptor in Cu2O and to generate an acceptor
level of about 0.14 eV.
The effects of cyanide treatment on the electrical
and optical properties of the nitrogen-doped, polycrystalline Cu2O thin films were studied. Luminescence properties were improved and the hole
density increased from 1016 to 1017 cm3 after the
cyanide treatment. These results indicate the passivation of nonradiative centers and hole traps, and it
is attributed to the termination of Cu-dangling
722
K. Akimoto et al. / Solar Energy 80 (2006) 715–722
bonds by the formation of Cu–CN bonds. The magnitudes of the improvements in the optical and electrical properties caused by the cyanide treatment are
comparable to those of the hydrogen treatment.
However, the cyanide treatment has an advantage
in that its passivation effect is more thermally stable
than that of the hydrogen treatment.
Polycrystalline p-Cu2O/n-ZnO heterojunctions
were grown by rf-magnetron sputtering. The photoresponse of the deposited thin film structure was
obtained for the first time. The I–V characteristics
and photoresponse properties were studied with
respect to the deposition sequence of ZnO and
Cu2O and the deposition temperature of Cu2O.
Crystallographic orientation, which relates to the
continuous atomic arrangement and surface defect
density (related to grain size), seem to be important
in getting high performance in electrical rectification
and photosensitivity.
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