Microstructural study of the CdS/CuGaSe
2
interfacial region in CuGaSe
2
thin film solar cells
V. Nadenau,
a)
D. Hariskos, and H.-W. Schock
Institut fu
¨
r Physikalische Elektronik, Universita
¨
t Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany
M. Krejci, F.-J. Haug, A. N. Tiwari, and H. Zogg
Gruppe fu
¨
rDu
¨
nnschichtphysik, Institut fu
¨
r Quantenelektronik, ETH Zu
¨
rich, ETH-Trakt Technopark,
Technoparkstr. 1, 8005 Zu
¨
rich, Switzerland
G. Kostorz
Institut fu
¨
r Angewandte Physik, ETH Zu
¨
rich, 8093 Zu
¨
rich, Switzerland
͑Received 13 July 1998; accepted for publication 23 September 1998͒
The microstructure of the CdS/CuGaSe
2
interface region in Cu-rich CuGaSe
2
-based polycrystalline
thin film solar cells with KCN-treated absorber layers are characterized. Two recipes for the
chemical bath deposition ͑CBD͒ of CdS with different bath temperatures ͑60 and 80°C͒ are
compared. Coherent Cu–Se precipitates are observed in both cases in the grains of the absorber
layer. This precipitation cannot be avoided and seems to be a principal limitation for the
performance of Cu-rich CuGaSe
2
-based thin film solar cells. There is a significant difference
between both recipes concerning the interaction with the absorber layer surface. For bath
temperatures of 80 °C the interaction is much stronger and Cu–S inclusions are found in the buffer
layer. These may be responsible for shunts across the pn junction. Owing to the reduced interaction
of the CdS deposited at 60 °C there are no Cu–S inclusions. For the 80 °C recipe the CdS/CuGaSe
2
interface region consists of a continuous transition zone with low defect density, whereas for the
60°C recipe the interface is sharper, but the CdS layer contains a high density of stacking faults. The
structure of the CdS layer depends also on the bath temperature and the growth orientation of the
CuGaSe
2
grains. CdS͑80°C͒ crystallizes predominantly in the zincblende structure and contains less
linear and planar defects than CdS͑60°C͒ which tends to incorporate hexagonal regions in the cubic
matrix. Strains due to lattice mismatch as well as mixture between wurtzite and zincblende
structures were revealed in high resolution transmission electron microscopy ͑HRTEM͒ images of
the CdS͑60 °C͒ layer. For CdS͑80 °C͒ the strain is relaxed by twinning and small-angle grain
boundaries which were imaged by HRTEM. A suitable CdS buffer layer for Cu-rich absorber layers
could not be obtained by CBD because of either the low crystal quality ͓CdS͑60°C͔͒ or the
formation of Cu–S inclusions ͓CdS͑80 °C͔͒. The enhanced interaction with the Ga-rich absorber
layer and improved quality of CdS͑80 °C͒ results in an improved device performance of Ga-rich
CuGaSe
2
-based solar cells. © 1999 American Institute of Physics. ͓S0021-8979͑99͒03101-1͔
I. INTRODUCTION
CuGaSe
2
with a band gap of 1.68 eV is a member of the
Cu chalcopyrite family which has been investigated as an
absorber layer for solar cells for more than 20 years.
1
The
development of CuInSe
2
-based solar cells started at about
the same time, but the progress in achieving high energy
conversion efficiencies was completely different than for
CuGaSe
2
-based solar cells. Repeated optimization of deposi-
tion processes of CuInSe
2
-based solar cells resulted in a
rapid improvement of cell performance, which led to high
efficiency devices (
Ϸ18%) based on Cu͑In,Ga͒Se
2
with
low Ga content ͑band gap 1.2 eV͒.
2,3
In comparison, the
development of CuGaSe
2
-based solar cells has been much
slower. Arndt et al.
4
presented first results for polycrystalline
thin film cells with an efficiency of 5% in 1985. The progress
made until 1991 was limited to a record efficiency of 6.2%
based on a solar cell with a Cu-rich KCN-treated absorber
layer on sodium-free glass.
5
It is well known that the Cu/͑InϩGaϩCu͒ ratio influ-
ences most of the properties of Cu-based chalcopyrites. Cu-
rich CuGaSe
2
contains free carrier concentrations which are
about three orders of magnitudes higher (10
18
cm
Ϫ3
͒ than in
Ga-rich material, and was therefore ͑in the case of Na-free
substrate glasses͒ more suitable for applications in solar
cells. On the other hand, Cu
x
Se is formed if the film compo-
sition deviates slightly towards Cu-rich compositions. These
binary selenides segregate mainly on the film surface or at
the grain boundaries as was shown by x-ray diffraction
͑XRD͒, scanning electron microscopy ͑SEM͒, x-ray photo-
electron spectroscopy ͑XPS͒, and energy dispersive x-ray
͑EDX͒ analysis.
6
This behavior is similar for CuInSe
2
,
CuInS
2
, and CuGaSe
2
7–11
and in agreement with the phase
diagrams which predict the formation of the copper selenides
for small deviations from the 1-1-2-stoichiometry or even for
a͒
Electronic mail: nadenau@ipe.uni-stuttgart.de
JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 1 1 JANUARY 1999
5340021-8979/99/85(1)/534/9/$15.00 © 1999 American Institute of Physics