Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se)2 solar cell

Article (PDF Available)inJournal of Physics and Chemistry of Solids 64(9):pp. 2037-2040 · September 2003with98 Reads
DOI: 10.1016/S0022-3697(03)00175-6
Abstract
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)2 absorbers 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 Eg=1.07eV 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 E00=18meV. Two activation energies higher than Eg, namely 1.21 and 1.40eV, 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)2 absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se)2 solar cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy E00 influenced by illumination.
Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se)
2
solar cell
M. Rusu
a
, W. Eisele
a
,R.Wu
¨
rz
a
, A. Ennaoui
a,
*
, M.Ch. Lux-Steiner
a
,
T.P. Niesen
b
, F. Karg
b
a
Hahn-Meitner Institut, Bereich Solarenergieforschung SE2, Glienicker Strasse 100, Berlin D-14109, Germany
b
Shell Solar GmbH, D-81739 Mu
¨
nchen, Germany
Abstract
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)
2
absorbers with chemical bath deposited ZnS buffer layer. The
measurements are carried out in the dark and under illumination in the temperature range 200320 K. The activation energy of
the recombination under illumination follows the absorber band gap energy E
g
¼ 1:07 eV of bulk Cu(In,Ga)(S,Se)
2
. The
thermal dependence of the diode ideality factor is described by classical Shockley ReadHall (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 E
00
¼ 18 meV. Two activation energies higher than E
g
; namely 1.21 and
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)
2
absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se)
2
solar
cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy
E
00
influenced by illumination.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
Thin-film ternary Cu-chalcopyrite semiconductors
CuInSe
2
, CuGaSe
2
, CuInS
2
and 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 [1] have been
achieved by using Cu(In,Ga)Se
2
absorbers 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) [27].
However, the knowledge about current transport and
recombination mechanisms in Cd-free CIGSSe-based
devices is still limited.
The aim of this contribution is to evaluate 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
ðJ
sc
Þ versus open circuit voltage ð V
oc
Þ plots were analysed.
2. Experimental
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 [7]. A double ZnO (100 nm intrinsic
and 400 nm Ga-doped) window layer was subsequently
sputtered. NiAl 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.
doi:10.1016/S0022-3697(03)00175-6
Journal of Physics and Chemistry of Solids 64 (2003) 2037–2040
www.elsevier.com/locate/jpcs
*
Corresponding author. Tel.: þ49-30-8062-3038; fax: þ 49-30-
8062-3199.
E-mail address: ennaoui@hmi.de (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 N
2
cooled
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)
2
cells
using a Cary 500 spectrometer (Varian) in combination with
an integrating sphere in order to collect the scattered light as
well.
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 [810]. The forward current density
J of the heterojunction is described by
J ¼ J
0
exp
qV
AkT

¼ J
00
exp
2E
a
AkT

exp
qV
AkT

ð1Þ
where A and J
0
are the ideality factor and the saturation
current density of the diode, respectively. kT=q is the
thermal voltage, J
00
is a weakly temperature-dependent
prefactor, and E
a
is the activation energy of the recombina-
tion. According to Eq. (1) the open-circuit voltage V
oc
is
V
oc
<
AkT
q
ln
J
sc
J
0

¼
E
a
q
2
AkT
q
ln
J
00
J
sc

ð2Þ
If A; J
sc
; and J
00
are independent of the temperature T; a plot
of V
oc
versus T should yield a straight line and the
extrapolation to T ¼ 0 K gives the activation energy E
a
:
However, when tunnelling processes becomes important,
the ideality factor becomes temperature dependent. In this
caseamorerenedevaluationofE
a
is possible by
reorganising Eq. (1) to the relationship
A lnðJ
0
Þ¼2
E
a
kT
þ A lnðJ
00
Þð3Þ
Thus, a plot of the corrected saturation current density A
lnðJ
0
Þ versus the inverse temperature 1=T should yield a
straight line with a slope corresponding to the activation
energy E
a
: The value of E
a
indicates either bulk
recombination when E
a
¼ E
g
(E
g
is the band gap energy),
or interface recombination for E
a
, E
g
:
The thermal dependence of the diode ideality factor is
described depending on the recombination mechanism. For
tunnelling enhanced interface recombination A is given by
A ¼
1
a
E
00
kT
coth
E
00
kT

ð4Þ
where E
00
is 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
1
A
¼
1
2
1 2
E
2
00
3ðkTÞ
2
þ
T
T
p
!
ð5Þ
kT
p
is the characteristic energy of the exponential
distribution of recombination centers. In the limit T
p
!1;
Eq. (5) describes the tunnelling enhanced recombination via
midgap states, whereas in the limit E
00
! 0; from Eq. (5)
results A
21
¼ð1 þ T=T
p
Þ=2 for the description of classical
ShockleyReadHall (SRH) recombination via an expo-
nential distribution of trap states [11].
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/cm
2
) the
following parameters: V
oc
¼ 577 mV, J
sc
¼ 35.9 mA/cm
2
,
a fill-factor of FF ¼ 70% and an efficiency of 14.6%
certified at NREL [12].
Typical dark JV T curves from a CIGSSe-based solar
cell at different temperatures together with the J
sc
versus V
oc
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 J
sc
versus
V
oc
plots exhibits an exponential dependence over more
than two orders of magnitude in the temperature range
200320 K. The dark and illuminated curves were fitted by
using models described in Section 3.1.
Fig. 1. Temperature-dependent JV curves and J
sc
versus V
oc
plots
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 V
oc
to 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 E
a
¼ 1.11 eV and
matches the absorber band gap energy of E
g
¼ 1:07 eV
determined from IQE measurements in this work and also
the data published by Probst et al. [13] 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 absorber.
The temperature dependence of the diode quality factors
A is given in Fig. 3. The ideality factor values calculated by
using J
sc
versus V
oc
plots 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 A
21
¼ð1 þ T=T
p
Þ=2; a tempera-
ture T
p
¼ 577 K was found corresponding to an energetic
width for the distribution of recombination centers of
kT
p
¼ 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
E
00
¼ 18 meV with the same characteristic energy of the
distribution kT
p
extracted 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 E
00
influenced by illumination.
The modified Arrhenius plots of A lnðJ
0
Þ versus 1=T are
shown in Fig. 4, where the saturation currents are obtained
from the analysis of the dark JV characteristics and J
sc
versus V
oc
plots. The slope of the Arrhenius plot with
extracted A and J
0
data from J
sc
versus V
oc
curves yield an
activation energy of 1.10 eV which is similar to the
extrapolated value of V
oc
at T ¼ 0K(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
the temperature regions 270320 K (1=T ¼ 0:0031
0:0037 K
21
) and 200 230 K (1=T ¼0:00430:0050 K
21
),
respectively. The temperature dependence of the diode
quality factor indicated tunnelling enhanced bulk recombi-
nation. The activation energies of 1.21.4 eV are higher
than the previously found bulk E
g
of 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)
2
layer 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
Fig. 2. Temperature dependence of the photovoltage V
oc
at different
illumination conditions (W ¼ 100 mW/cm
2
).
Fig. 3. Temperature dependence of the diode quality factors as
extracted from dark JV curves and J
sc
versus V
oc
plots. The dashed
lines 1 and 2 show the fits to Eq. (5).
Fig. 4. Arrhenius plot of the saturation current density J
0
corrected
by diode quality factors A:
M. Rusu et al. / Journal of Physics and Chemistry of Solids 64 (2003) 2037–2040 2039
also on CuInSe
2
thin films [17]. Such enlargement of the
band gap energy towards the film surface is an important and
operative element in all Cu-poor Cu(In,Ga)(S,Se)
2
films
[18]. 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.
4. Conclusion
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 a characteristic energy
kT
p
¼ 50 meV and a tunnelling energy of E
00
¼ 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 E
g
of 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 kT
p
¼ 50 meV and an exponential
distribution of trap states in the CIGSSe bulk.
The investigated ZnO/ZnS/Cu(In,Ga)(S,Se)
2
devices
have shown similar photoelectrical properties as
ZnO/CdS/Cu(In,Ga)(S,Se)
2
solar cells [18]. This fact
confirms that ZnS may be used as an efficient buffer
layer.
References
[1] M.A. Green, K. Emery, D.L. King, S. Igari, W. Warta, Prog.
Photovolt: Res. Appl. 10 (2002) 355 360.
[2] A. Ennaoui, C.D. Lochande, M. Weber, R. Scheer, H.J.
Lewerence, Proceedings of the 14th European Photovoltaic
Solar Energy Conference, Barcelona, Spain, H.S. Stephens
and Associates, UK, 1997, p. 1220.
[3] F. Cooray, K. Kushiya, A. Fujimaki, I. Sugiyama, T. Miura, D.
Okumura, M. Sto, M. Ooshita, O. Yamase, Sol. Energy Mater.
Sol. Cells 49 (1997) 291 297.
[4] T. Nakada, K. Furumi, M. Mizutani, Y. Hagiwara, A. Kunioka,
11th International Photovoltaic Science and Engineering
Conference, Hokkaido, Japan, Technical Digest, 1999, p. 81.
[5] A. Ennaoui, M. Lux-Steiner, F. Karg, 11th International
Photovoltaic Science and Engineering Conference, Hokkaido,
Japan, Technical Digest, 1999, p. 79.
[6] A. Ennaoui, M. Weber, R. Scheer, H.J. Lewerenz, Sol. Energy
Mater. Sol. Cells 54 (1998) 277 286.
[7] A. Ennaoui, S. Siebentritt, M.-Ch. Lux-Steiner, W. Riedl, F.
Karg, Sol. Energy Mater. Sol. Cells 67 (2001) 31 40.
[8] U. Rau, Appl. Phys. Lett. 74 (1999) 111113.
[9] U. Rau, H.W. Schock, Appl. Phys. A 69 (1999) 131147.
[10] V. Nadenau, U. Rau, A. Jasenek, H.W. Schock, J. Appl. Phys.
87 (2000) 584593.
[11] T. Walter, R. Menner, Ch. Ko
¨
ble, H.W. Schock, in: R. Hill, W.
Palz, P. Helm (Eds.), Proceedings of the 12th European
Photovoltaic Energy Conference, Stephens, Bedford, UK,
1994, p. 1755.
[12] A. Ennaoui, W. Eisele, M. Lux-Steiner, T.P. Niesen, F. Karg,
Thin Solid Films, in press (2003).
[13] V. Probst, W. Stetter, W. Riedl, H. Vogt, M. Wendl, H.
Calwer, S. Zweigart, K.-D. Ufert, B. Freienstein, H. Cerva,
F.H. Karg, Thin Solid Films 387 (2001) 262 267.
[14] T. Walter, R. Herberholz, H.W. Schock, Solid State Phenom.
51 52 (1996) 309316.
[15] D. Schmid, M. Ruckh, F. Grunwald, H.W. Schock, J. Appl.
Phys. 73 (1993) 2902 2909.
[16] D. Schmid, M. Ruckh, H.W. Schock, Sol. Energy Mater. Sol.
Cells 41 (1996) 281294.
[17] M. Morkel, L. Weinhardt, B. Lohmu
¨
ller, C. Heske, E.
Umbach, W. Riedl, S. Zweigart, F. Karg, Appl. Phys. Lett.
79 (27) (2001) 44824484.
[18] M. Turcu, O. Pakma, U. Rau, Appl. Phys. Lett. 80 (14) (2002)
2598 2600.
[19] R. Klenk, Thin Solid Films 387 (2001) 135 140.
[20] T. Dullweber, G. Hanna, U. Rau, H.W. Schock, Sol. Energy
Mater. Sol. Cells 67 (2001) 145 150.
M. Rusu et al. / Journal of Physics and Chemistry of Solids 64 (2003) 2037–20402040
  • [Show abstract] [Hide abstract] ABSTRACT: Diffusion of Cu in ZnS thin films was investigated in the temperature range 80–400 °C using the energy-dispersive X-ray fluorescence (EDXRF) technique. ZnS thin films were deposited by spray pyrolysis on a glass substrate. The temperature dependence of the Cu diffusion coefficient is described by the equation D = 1.4 × 10−10 exp [–(0.18 eV)/kT]. It is shown that diffusion doping of n-type ZnS by Cu is accompanied by a significant increase in resistivity and a slight decrease in the band gap of the ZnS films. The observed results are attributed to the migration of Cu in polycrystalline ZnS films by means of diffusion both along grain boundaries and into grains. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Article · Oct 2004
  • [Show abstract] [Hide abstract] ABSTRACT: The cost-effective spray pyrolysis technique was applied to prepare ZnS thin films for large-scale applications. Aqueous solutions containing ZnCl2 and SC(NH2)2 were deposited onto glass and n-type Si(100) substrates at 430-600°C. The structure and phase composition were studied by XRD and FTIR, the surface morphology by SEM. The optical properties were determined from the transmittance and reflectance measurements in the UV-visible and near-IR regions. The phase composition, crystallinity, and surface morphology of the films could be controlled by the Zn : S molar ratio in the stock solution and the deposition temperature. Highly (001) oriented ZnS films with wurtzite structure were grown at temperatures above 500°C using the Zn:S molar ratio 1 :2 in the solution. The sprayed films had high optical transmittance in a wide spectral range and the refractive index of up to 1.9 at 632.8 nm. It was also found that the sprayed ZnS films are capable of being used as antireflection coatings in Si-based devices.
    Full-text · Article · Aug 2005
  • [Show abstract] [Hide abstract] ABSTRACT: ZnS thin films were prepared by spray pyrolysis technique using aqueous solutions of zinc chloride and thiourea at molar ratio of 1:1, 1:2 and 2:1. The films were characterized by XRD, FTIR, SEM and EDX. The phase composition, structure and morphology of sprayed films are controlled by both, the substrate temperature and the precursors molar ratio in the solution. The films deposited below 380 ºC are amorphous and contain thermal decomposition residues of Zn(tu)2Cl2. Chlorine content becomes negligible at 500 ºC, carbon and nitrogen in the form of zinc cyanamide are present in a trace amount up to 540 ºC. Highly (002) orientated ZnS films with wurtzite structure and closely stoichiometric composition could be grown at temperatures close to 500 ºC using Zn:S molar ratio of 1:2 in spray solution. The film consists of well-shaped hexagonal prisms with size of 100 nm. The spray of equimolar solution (Zn:S = 1:1) results in ZnS films with smaller grain size (∼25 nm) and slightly Zn-rich composition at 500 ºC. Higher temperature (530 ºC) is needed to grow the films with wurtzite structure. The use of Zn-rich (Zn/S > 1) solution results in the films consisting of ZnO and ZnS phases above 400 ºC. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Full-text · Article · Feb 2005
Show more