(a) 100 µm - Helmholtz-Zentrum Berlin

30 State of the art
Figure 2.17: Si concentration profile around growing grains. The Si concentration within
the grains is cg. Right in front of the grain it is ci. Far away from the grains is the
nucleation concentration cn. Si is depleted in the concentration profiles on a distance d.
(a) If neighboring grains are further then 2 d apart nucleation is still possible. (b) Once
the grains are closer than 2 d nucleation is suppressed. [52].
Si layer is formed by the ALILE process. (c) On the top of the poly-Si film an Al
layer with silicon inclusion is formed which has to be removed. (d) The poly-
Si layer is subsequently thickened by low-temperature epitaxy forming a p-doped
absorber. Finally, a low temperature, n + -emitter is deposited. This can either be
done epitaxially by changing the dopant or by deposition of an amorphous silicon
emitter. In the latter case a transparent conducting oxide (TCO) layer is needed to
enhance the lateral conductivity of the emitter. A mesa structure has to be etched
in order to define the solar cell area and contact the absorber layer.
Historically, the University of New South Wales (UNSW) in Sydney and the Hahn-
Meitner-Institut Berlin (HMI) were both involved in ALILE research due to O. Nast
preparing his dissertation in both facilities. In order to produce a smooth, epi-
ready surface on the ALILE layer a wet chemical lift-off process for the Si islands