The group thin silicon deals with Si thin-film solar cells and thin silicon wafer solar cells. This includes material characterization as well as device development.
Different approaches are applied to lower the cost of a silicon solar cell module. At the University of Konstanz we aim at lowering the costs for the silicon absorber; on the one hand the Si wafer can be extremely thin (well below 100 µm) and on the other hand Si is directly deposited from the gas phase on a large area. The advantage of the latter versus the conventional production of Si wafers is that the purified Si gas is directly used for the fabrication of the absorber instead of first crystallize it for feedstock generation and then subsequently melt it again and crystallize the Si ingots.
In the field of thin wafers the main topics are surface passivation, contactless material processing and metallization techniques.
An excellent surface passivation is needed to fully exploit their efficiency potential. Usually thermal oxidation, SiO2, or a SiNx layer is deployed. Unfortunately, the process temperature is rather high for the thermal oxidation and the SiNx layer is not suitable for passivating contacted p-doped Si wafer surfaces adequately. Alternative layers are of amorphous silicon, SiCX as well as Al2O3.
Figure 1: The machine is operated in the range of atomic layer deposition. 1.2 Å/cycle of Al2O3 are deposited.
Light trapping schemes
Since the absorption of light in silicon leads to a loss in short circuit current of about 5% for a reduction of wafer thickness from 160 µm to 80 µm despite of a front surface texture, light trapping schemes are essential for these thin solar cells. Textures on both sides of the cell, a Lambertian diffuser or a back reflector are approaches here.
Si layer growth and hetero solar cells
The deposition of Si solar cells from the gas phase is a new topic at the photovoltaics division. First of all we aim at establishing measurement methods to characterize amorphous and nanocrystalline Si layers.