Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se)2solar cell
M. Rusua, W. Eiselea, R. Wu ¨rza, A. Ennaouia,*, M.Ch. Lux-Steinera,
T.P. Niesenb, F. Kargb
aHahn-Meitner Institut, Bereich Solarenergieforschung SE2, Glienicker Strasse 100, Berlin D-14109, Germany
bShell Solar GmbH, D-81739 Mu ¨nchen, Germany
Temperature-dependent current-voltage measurements are used to determine the dominant recombination mechanism in
thin-film heterojunction solar cells based on Cu(In,Ga)(S,Se)2absorbers with chemical bath deposited ZnS buffer layer. The
measurements are carried out in the dark and under illumination in the temperature range 200–320 K. The activation energy of
the recombination under illumination follows the absorber band gap energy Eg¼ 1:07 eV of bulk Cu(In,Ga)(S,Se)2. The
thermal dependence of the diode ideality factor is described by classical Shockley–Read–Hall (SRH) recombination via an
exponential distribution of trap states in the bulk of the absorber. In the dark, the current flow is dominated by tunnelling
enhanced bulk recombination with a tunnelling energy E00¼ 18 meV. Two activation energies higher than Eg; namely 1.21and
1.40 eV, have been found. These results may be explained by dominant recombination in a region close to the surface of the
Cu(In,Ga)(S,Se)2absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se)2solar
cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy
E00influenced by illumination.
q 2003 Elsevier Ltd. All rights reserved.
Thin-film ternary Cu-chalcopyrite semiconductors
CuInSe2, CuGaSe2, CuInS2and their alloys are among the
leading absorber materials for low-cost and most efficient
terrestrial solar cells. Photovoltaic conversion efficiencies
up to 18.9% on a laboratory scale and 12% for large area
monolithically integrated submodules  have been
achieved by using Cu(In,Ga)Se2absorbers and CdS buffer
layers at the absorber–window interface. The usage of the
Cu(In,Ga)(S,Se)2(CIGSSe) alloys as absorber with a wider
band gap is in progress to rise the open circuit voltage of the
solar cells and, finally, their efficiency. Strong research
effort is forwarded towards replacement of the CdS by a less
toxic alternative buffer material. Solar cells based on
CIGSSe have already achieved an efficiency of 15% using
such buffer materials as Zn(X,OH) or In(X,OH) with X ¼ S,
Se prepared by chemical bath deposition (CBD) [2–7].
However, the knowledge about current transport and
recombination mechanisms in Cd-free CIGSSe-based
devices is still limited.
The aim ofthis contribution is toevaluate the dominating
recombination mechanism which limits the device perform-
ance. In order to determine the transport mechanism in
ZnO/ZnS/Cu(In,Ga)(S,Se)2 structures, the temperature
dependence of dark JV curves and short-circuit current
ðJscÞ versus open circuit voltage ðVocÞ plots were analysed.
Solar cells based on CIGSSe absorbers were processed
by the following procedure. A chemical bath was used for
the deposition of Zn(S,OH) buffer layer on the CIGSSe/-
Mo/glass substrates provided by Shell Solar GmbH
(Mu ¨nchen). Details concerning Zn(S,OH) preparation can
be found elsewhere . A double ZnO (100 nm intrinsic
and 400 nm Ga-doped) window layer was subsequently
sputtered. Ni–Al grids were deposited by e-beam evapor-
ation through a shadow mask as front contacts. Solar cell
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
Journal of Physics and Chemistry of Solids 64 (2003) 2037–2040
* Corresponding author. Tel.: þ49-30-8062-3038; fax: þ49-30-
E-mail address: firstname.lastname@example.org (A. Ennaoui).
parameters were determined under AM1.5 conditions in a
sun-simulator and device performance is quoted as total area
to include grid losses. Measurements of the current-voltage
characteristics as a function of temperature ðJV–TÞ and
illumination were performed in an evacuated N2cooled
cryostat using a source measure unit in four-point
configuration. An ELH-type halogen lamp was used for
illuminating the sample and a set of neutral density filters
served for adjusting the light intensity. For the determi-
nation of the absorber band gap energy, we used the
measurements of the spectral dependence of the internal
quantum efficiency (IQE). The external quantum efficiency
(EQE) was measured using a Xe and a halogen lamp, a
Specs 1680 monochromator and a Si reference diode.
Optical reflection measurements were performed at 300 K
on the top side of the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se)2cells
using a Cary 500 spectrometer (Varian) in combination with
an integrating sphere in order to collect the scattered light as
3. Results and discussion
3.1. The transport mechanisms evaluation theory
The dark and illuminated JV–T curves could be fitted
and analytically described by the models proposed for the
chalcopyrite solar cells [8–10]. The forward current density
J of the heterojunction is described by
J ¼ J0exp
where A and J0are the ideality factor and the saturation
current density of the diode, respectively. kT=q is the
thermal voltage, J00is a weakly temperature-dependent
prefactor, and Eais the activation energy of the recombina-
tion. According to Eq. (1) the open-circuit voltage Vocis
If A; Jsc; and J00are independent of the temperature T; a plot
of Voc versus T should yield a straight line and the
extrapolation to T ¼ 0 K gives the activation energy Ea:
However, when tunnelling processes becomes important,
the ideality factor becomes temperature dependent. In this
case a more refined evaluation of Ea is possible by
reorganising Eq. (1) to the relationship
A lnðJ0Þ ¼ 2Ea
kTþ A lnðJ00Þð3Þ
Thus, a plot of the corrected saturation current density A
lnðJ0Þ versus the inverse temperature 1=T should yield a
straight line with a slope corresponding to the activation
energy Ea: The value of Ea indicates either bulk
recombination when Ea¼ Eg(Egis the band gap energy),
or interface recombination for Ea, Eg:
The thermal dependence of the diode ideality factor is
described depending on the recombination mechanism. For
tunnelling enhanced interface recombination A is given by
where E00is the characteristic tunnelling energy and a is a
constant for the given heterojunction. For the case of
tunnelling enhanced bulk recombination the ideality factor
A is described by the equation
kTpis the characteristic energy of the exponential
distribution of recombination centers. In the limit Tp! 1;
Eq. (5) describes the tunnelling enhanced recombination via
midgap states, whereas in the limit E00! 0; from Eq. (5)
results A21¼ ð1 þ T=TpÞ=2 for the description of classical
Shockley–Read–Hall (SRH) recombination via an expo-
nential distribution of trap states .
3.2. The evaluated transport mechanisms and discussion
Investigations of the current transport mechanism in the
ZnO/ZnS/CIGSSe devices were carried out on solar cells
which show under AM1.5 conditions (100 mW/cm2) the
following parameters: Voc¼ 577 mV, Jsc¼ 35.9 mA/cm2,
a fill-factor of FF ¼ 70% and an efficiency of 14.6%
certified at NREL .
Typical dark JV–T curves from a CIGSSe-based solar
cell at different temperatures together with the Jscversus Voc
plots are shown in Fig. 1. The forward dark and illuminated
JV–T curves show similar behaviour in the region of lower
voltages and some differences at higher ones. The Jscversus
Vocplots exhibits an exponential dependence over more
than two orders of magnitude in the temperature range
200–320 K. The dark and illuminated curves were fitted by
using models described in Section 3.1.
Fig. 1. Temperature-dependent JV curves and Jscversus Vocplots
measured with the temperature step of 10 K.
M. Rusu et al. / Journal of Physics and Chemistry of Solids 64 (2003) 2037–20402038
The temperature dependence of the open circuit voltage
under different illuminations are presented in Fig. 2. Almost
linear curves were observed in a wide range of the light
intensity. The extrapolation of Vocto T ¼ 0 K for the curve
measured under conditions approximated by AM1.5 (see
curve 1.0 W, Fig. 2) results in the value of Ea¼ 1.11 eV and
matches the absorber band gap energy of Eg¼ 1:07 eV
determined from IQE measurements in this work and also
the data published by Probst et al.  on same CIGSSe
layers with the S/(S þ Se) ratio of up to 18%. Hence, the
dominating recombination process takes place in the bulk of
The temperature dependence of the diode quality factors
A is given in Fig. 3. The ideality factor values calculated by
using Jscversus Vocplots show a weak dependence on
temperature which indicates recombination of charge
carriers in the bulk of the absorber via an exponential
distribution in energy of trap states in the space charge
region [11,14]. Fitting with A21¼ ð1 þ T=TpÞ=2; a tempera-
ture Tp¼ 577 K was found corresponding to an energetic
width for the distribution of recombination centers of
kTp¼ 50 meV. This indicates that under illumination the
dominant recombination mechanism is described by classi-
cal SRH model with an exponential distribution of
recombination levels in the bulk of the absorber. However,
the A values extracted from dark JV curves were fitted
satisfactorily by Eq. (5). The fits yield a tunnelling energy
E00¼ 18 meV with the same characteristic energy of the
distribution kTpextracted from illuminated curves. This
shows that current path in ZnO/Zn(S,OH)/CIGSSe structure
is described by tunnelling enhanced bulk recombination
with a tunnelling energy E00influenced by illumination.
The modified Arrhenius plots of A lnðJ0Þ versus 1=T are
shown in Fig. 4, where the saturation currents are obtained
from the analysis of the dark JV characteristics and Jsc
versus Vocplots. The slope of the Arrhenius plot with
extracted A and J0data from Jscversus Voccurves yield an
activation energy of 1.10 eV which is similar to the
extrapolated value of Vocat T ¼ 0 K (Fig. 2). The analysis
of the dark measurement results gives two activation
energies 1.21 and 1.40 eV for the fitted linear segments in
–0:0037 K21) and 200–230 K (1=T¼0:0043–0:0050 K21),
respectively. The temperature dependence of the diode
quality factor indicated tunnelling enhanced bulk recombi-
nation. The activation energies of 1.2–1.4 eV are higher
than the previously found bulk Egof 1.07 eV. Therefore, we
conclude that the band gap of the absorber is enlarged in a
region of the absorber close to the ZnS/CIGSSe interface.
The surface region of the Cu(In,Ga)(S,Se)2layer with larger
band gap might be formed either by a Cu-poor composition
with respect to the bulk material [15,16] or by increasing of
the sulphur content during the processing of the absorber
and/or during the Zn(S,OH) buffer layer deposition. An
unusually large surface band gap of 1.4 eV was observed
270–320 K(1=T ¼ 0:0031
Fig. 2. Temperature dependence of the photovoltage Vocat different
illumination conditions (W ¼ 100 mW/cm2).
Fig. 3. Temperature dependence of the diode quality factors as
extracted from dark JV curves and Jscversus Vocplots. The dashed
lines 1 and 2 show the fits to Eq. (5).
Fig. 4. Arrhenius plot of the saturation current density J0corrected
by diode quality factors A:
M. Rusu et al. / Journal of Physics and Chemistry of Solids 64 (2003) 2037–20402039
also on CuInSe2thin films . Such enlargement of the
band gapenergy towardsthe film surface is animportant and
operative element in all Cu-poor Cu(In,Ga)(S,Se)2 films
. The band gap widening towards the surface is
accommodated by a bulk/surface valence band offset
which sufficiently increases the potential barrier for inter-
face recombination and recombination losses at the hetero-
interface are minimised [19,20]. Thus, in the dark, the
dominant recombination takes place in the region of the
enlarged band gap near the surface of the absorber and
results in tunnelling enhanced bulk recombination.
The transport mechanism in the ZnO/ZnS/Cu(In,Ga)
(S,Se)2 solar cells with an efficiency of 14.6% was
investigated. The type and place of the dominant recombi-
nation mechanism depends on illumination. Thus, in the
dark, the current loss is dominated by tunnelling enhanced
bulk recombination with
kTp¼ 50 meV and a tunnelling energy of E00¼ 18 meV.
The recombination occurs in a region of the absorber close
to the ZnS/CIGSSe interface. The band gap of this region is
larger compared to the bulk Egof 1.07 eV and ranges
between 1.2 and 1.4 eV. Under illumination, the transport
mechanism is described by classical SRH recombination
model with the same kTp¼ 50 meV and an exponential
distribution of trap states in the CIGSSe bulk.
The investigated ZnO/ZnS/Cu(In,Ga)(S,Se)2 devices
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