Keywords: flexible solar cells, plastic substrates, light trapping, embossing processes
Thin film silicon solar cells are classified into p-i-n and n-i-p configurations which refer to their deposition sequence; n-i-p processing starts with the n-layer which is normally grown on a metallic back contact. Historically this configuration is connected to flexible substrates because it was used on opaque substrates or poorly transparent substrates like steel foils or high temperature polymers. However, the configuration is not limited to this choice, it is in fact compatible with any kind of substrates, such as rigid or flexible, transparent or opaque. Nevertheless, flexible substrates have remained the main application of n-i-p cells because roll-to-roll processing makes them very interesting to reduce the production costs as well as the energy payback time, particularly when low cost substrates like poly-ethylene are used.
The activities of the n-i-p group combine general aspects of thin film silicon solar cells with special requirements that are imposed by the deposition sequence and the desired compatibility with low temperature substrates. Two main lines of work can be distinguished:
. Substrate texturing
. Light scattering and absorption enhancement
Towards a more fundamental understanding of absorption enhancement in solar cells, we fabricate cells on periodic gratings that permit the study of coupling into guided modes .
We obtained fully flexible solar cells on a low cost poly-ethylene substrate with a stabilized efficiency of 9.8% for 0.25cm2 laboratory cells .
Amorphous and microcrystalline silicon are poor absorbers, particularly for light with energies just above their respective band gaps. Some means of absorption enhancement is required which is commonly called light trapping.
A common approach for n-i-p cells are back contacts made from so called "hot silver", which is the texture that silver develops by partial recrystallization during growth on a heated substrate.
It can be achieved by texturing of the interfaces.
Unfortunately this is too hot for poly-ethylene, we have to devise other ways. We investigate the incorporation of texture into the substrate itself, during cell fabrication this texture is carried into the other interfaces because of conformal coverage.
Periodic substrate textures
We obtained promising results on low cost poly-ethylene substrates like the one shown below. The substrate texture has been manufactured by a commercial manufacturer.