Jahresbericht 2005 - IPHT Jena

92
The varying shades (spectrum from white to
greenish-blue in a colour picture) indicate the different
level of filling and positioning accuracy. In
order to improve the homogeneity a further optimisation
of this technique is required.
Thin film solar cells
For the first time layered laser crystallised (LLC)
thin film solar cells were prepared on borofloat
glass substrates covered with a tantalum pentoxide
layer.
Our standard cell structure is sketched in Fig. E7.
Immediately on top of the borofloat glass sub-
Fig. E7: Design of a standard LLC cell.
strate a multicrystalline silicon layer system with
grains exceeding 100 µm in size is deposited.
The layer system consists of a p + -doped highly
conductive seed layer, a p-doped absorber layer
and an n-doped emitter layer. A metal electrode
acts as a light reflector. The seed layer is prepared
by cw laser crystallisation of amorphous
silicon. The absorber and emitter are prepared by
continuously depositing amorphous silicon on top
of the seed combined with repeated excimer
laser irradiation in order to guarantee epitaxial
growth. In various papers it has been demonstrated
that intermediate layers between the
glass and silicon may increase the cell efficiency
by reducing the reflection loss. Moreover, the barrier
may act as a light trapping structure due to
surface scattering and as a diffusion barrier
against foreign atoms of the glass. In the case of
LLC cells the intermediate layer has to withstand
the laser crystallisation process. Standard layers
INNOVATIONSPROJEKT / INNOVATION PROJECT
usually applied in solar cells such as SnO 2, ZnO
or a-SiN x:H do not meet this requirement.
Because of its suitable optical properties and
high melting point of above 1800 °C we expect
tantalum pentoxide layers to be an interesting
alternative. Therefore, we tested the laser stability
of this material. We demonstrated that on
Ta 2O 5 layers LLC solar cells can be prepared successfully.
However, it turned out that Ta 2O 5 layers
require an excimer laser pretreatment. Without
the pretreatment layer damage was observed
after laser crystallisation of amorphous silicon
due to the formation of gas bubbles. Moreover,
due to the laser pretreatment the Ta 2O 5 layer surface
roughens so that increased light scattering
is observed which is beneficial for the solar cell.
In Fig. E8 a current-voltage diagram of a 2 µm
thick cell with a tantalum pentoxide intermediate
layer is shown, as compared to a standard cell. In
both cases no metal reflector for light trapping
was present. Even though the Ta 2O 5 cell has inferior
properties as compared to the standard cell it
is still a positive result. It was demonstrated that
the intermediate layer has the desired properties.
In order to improve the cell parameters the
excimer laser pretreatment has to be optimised
so that the crystal quality of the silicon is
increased to that of the standard cells.
Fig. E8: I–V-diagram of a LLC cell with tantalum
pentoxide intermediate layer (full line) as compared
to a standard LLC cell (broken line).