Environmental Engineering Reference
In-Depth Information
Figure 5.6
Structure of a photovoltaic module.
The connections of the solar cells are linked to a module connection box. Individual
faulty cells or uneven shade can damage a PV module. Bypass diodes that compen-
sate for affected cells when a fault occurs are designed to prevent any damage. These
diodes are also usually integrated into the module junction boxes.
5.2.3 Thin Film Solar Cells
Crystalline solar cells require a comparatively large amount of costly semiconductor
material. Different production methods using thin fi lm cells are being tested to
reduce the amount of material needed. Whereas crystalline solar cells reach thick-
nesses in the order of tenths of millimetres, thin fi lm solar cells are thousandths of
a millimetre thick. The production principle is similar even when different materials
such as amorphous silicon (a-Si), cadmium telluride (CdTe) or copper indium dis-
elenide ( CIS ) are used.
The base of thin fi lm solar cells is a substrate that is usually made of glass. Plastic
can be used instead of glass for the substrate to produce modules that are fl exible
and bendable. A thin TCO (Transparent Conductive Oxide) layer is applied to the
substrate using a spraying technique. A laser or a micromiller then separates this
layer into strips. The individual strips constitute the single cells within the later solar
module. Like crystalline cells, these cells are also contacted in such a way that
they are connected in series to increase the electric voltage. The long strips make
it visually easy to distinguish thin fi lm modules from crystalline solar modules
(Figure 5.7 ).
The semiconductor and doping materials are then vaporized at high temperatures.
When silicon is vaporized as a semiconductor material, the crystalline structure of
the silicon is lost. This is then referred to as amorphous silicon. A screen-printing
procedure then applies materials like aluminium to the back side contact. A layer
of polymer seals the cell at the back to protect it from moisture.
The effi ciency of thin fi lm modules is currently still considerably less than that of
crystalline photovoltaic modules. This means that a larger surface is required for
