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Optical properties

In this section we will deal with the optical aspects of the materials used in solar cells. Generally, solar cells are built from different elements, and beside their electric importance most of them also have to fulfill important optical requirements.

(Encapsulation)

Antireflection coating

Front contact

n-window layer

p-absorber layer

back contact

The figure shows a typical stack of the active layers in a solar cell. The incident light first strikes the encapsulation (not shown), ususally consisting of a glass plate and some orgainc glue. The first active functional layer of a solar cell is the antireflection coating. Its job is the minimisation of reflection losses by means of optical interference.

The next element is the front contact. This layer must combine high optical transparency with high electric conductivity, two properties which normally exclude each other. Electric conduction is usually observed in metals but they are not transparent. However, highly doped semiconductors are transparent for light with energy less than the bandgap and they can transport certain amounts of current. For transmission of visible light we need either bandgaps of more than 3 eV or low absorption coefficients like those of indirect semiconductors, e.g. silicon. For heterojunction devices oxides like ZnO and SnO2 are used due to their high bandgaps and easy dopability.

The next layer on the way into the solar cell is the n-layer, a vital part of the pn-junction. Usually it is intended to avoid absorption in the n-layer, thus, also this layer should consist of a wide bandgap semiconductor or one with low absorption.

Further on the light enters the p-layer where the absorption takes place. Direct bandgaps are a good choice because they have high absorption coefficients and allow for thin absorbers and a low material input. Indirect bandgap abosorbers like silicon require thick absorbers or light trapping techniques. The absorbed photons create electron hole pairs, the electric field of the pn-junction separates them and directs them to the electric contacts. It is particularly important to ensure a reasonable transport of the photo generated minority charge carriers because they dominate the electric behaviour of the pn-junction. In most semiconductors the mobility is higher for electrons than for holes, thus, the absorber should be p-type.

Finally, if light has not been absorbed and reaches the back contact, it should be reflected back into the absorber. Thus, the back contact must be a good electric contact and a good optical reflector. Recently, in thin film solar cells transparent back contacts have attracted much attention, they allow for bifacial illumunation or the application of separate, highly reflecting mirrors.


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