Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 252

surface of 0.5 m 2 and efficiencies ranging from 8 to 10 % have been commis-
sioned.

Incident light Incident light

Metal rear contact Metal rear contact

CdTe CdTe

ZnO ZnO

CdS CdS

CdS CdS

ITO ITO

Cu(In,Ga)Se 2 Cu(In,Ga)Se 2

Metal rear contact Metal rear contact

Glass superstrate Glass superstrate

Glass superstrate Glass superstrate

Incident light Incident light

Fig. 6.11 Layer order of a CdS/CdTe (left) and a CdS/Cu(In,Ga)Se 2 (right) hetero-
structure solar cell (ITO indium stannous oxide)

With efficiencies above 18 %, CdS/Cu(In,Ga)Se 2 hetero-structure solar cells
achieve the highest electrical efficiencies of all thin film solar cells. Fig. 6.11,
right, illustrates the layer sequence of such hetero-structure solar cells. Manufac-
turing of this solar cell type starts with depositing a molybdenum rear contact,
followed by the deposition of the photovoltaically active Cu(In,Ga)Se 2 layer of a
thickness below 2 µm. The following two deposition methods are suitable for this
purpose on an industrial scale.
− Firstly, physical co-evaporation of all elements (i.e. Cu, In, Ga and Se) onto the
heated substrate, so that the Cu(In,Ga)Se 2 compound is already built up during
vapor deposition.
− Secondly, deposition of all elements onto the unheated substrate (e.g. by sput-
tering). Subsequently, a second heating step, called selenization, is performed
to obtain the Cu(In,Ga)Se 2 compound.
Subsequent to absorber manufacturing by one of the two described methods, a
CdS layer of an approximate thickness 0.05 µm is deposited from the chemical
bath prior to depositing the ZnO front electrode by sputtering.
Pilot projects and small scale manufacturing plants have been installed and are
operated for Cu(In,Ga)Se 2 solar modules. Modules with efficiencies of up to 12 %
are commercially available. Both technologies (i.e. CdTe and Cu(In,Ga)Se 2 ) will
have to prove within the next years whether they can sustain themselves in the
marketplace. Generally speaking, for crystalline silicon solar cells the manufactur-
ing process is quite reasonable and well understood based on the experience from
the semiconductor industry. But the material costs are high due to the thick