High-Efficiency Cu(In,Ga)Se
2
Solar Cells with Indium Zinc Oxide (IZO) Window Layer
Richard Menner, Marco Cemernjak, Stefan Paetel, Wiltraud Wischmann, and Michael
Powalla
Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW),
Industriestrasse 6, D-70565, Germany
+49 711 7870 212, Fax +49 711 7870 230,
Cu(In,Ga)Se
2
(CIGS) based solar cells are amongst the most efficient thin-film solar cells
today and have reached efficiency levels exceeding 22 %. For the front contact usually a
transparent conductive oxide (TCO) based on doped ZnO is applied either by magnetron
sputtering or low-pressure chemical vapor deposition (LPCVD). At ZSW we typically
employ a bilayer window onto CdS-buffered CIGS substrates consisting of high-resistive
undoped ZnO and an Al-doped ZnO (AZO) by RF and DC magnetron sputtering,
respectively.
The sputtered Al-doped ZnO window layer has proven to work well with CIGS. However,
there is significant absorption in the visible and especially the near infrared (NIR) due to the
high free carrier concentrations necessary for high film conductivity. In order to reduce the
NIR absorption while maintaining the high conductivity, other TCO materials are studied,
which exhibit much higher carrier mobility. In this contribution we report upon experiments
with amorphous (In,Zn)O instead of the Al-doped ZnO layer. (In,Zn)O has been investigated
mainly for thin film transistor applications during the last decade. Besides the reported high
mobility, additional advantages are the possible deposition at room temperature and the
expected higher chemical stability against moisture, which can be detrimental for
insufficiently encapsulated solar modules.
(In,Zn)O window layers were deposited in an in-line system by magnetron sputtering from
linear In
2
O
3
:ZnO targets. Films were grown on bare glass substrates as well as on CIGS
substrates with CdS/i-ZnO buffer layers. RF or DC plasma excitation was applied and the
IZO films were optimized by parameter variations including discharge power, total pressure
as well as oxygen and hydrogen partial pressures. The films are usually deposited at room
temperature, but the influence of additional substrate heating was investigated, too. Targets of
two different In
2
O
3
:ZnO compositions were used.
The films deposited on glass were analyzed with respect to their optical, electrical, and
structural properties by spectral transmittance and reflectance, four-point-probe and Hall
measurements, and with X-ray diffraction and scanning electron microscopy. Highly
transparent and conductive films with mobilities above 50 cm
2
(Vs)
-1
, conductivities
exceeding 3000 S/cm, and weighted average absorption below 4 % between
400 and 1100 nm were achieved.
Solar cells and modules based on CIGS absorbers on glass were characterized by I-V under
simulated sunlight as well as spectral quantum efficiency. Comparable conversion
efficiencies to Al-doped ZnO coated reference cells were proven with up to 19.6 % for
champion cells. This is to our knowledge the highest reported efficiency for a CIGS solar cell
with IZO window layer so far. Enhanced stability against humidity is clearly demonstrated in
comparison to AZO in damp heat testing of mini-modules.
O 63
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