Helmold Dipl Phys Kausche

Economical Patterning of Series Connected a-Silicon Modules

Different patterning methods are used for manufacturing series connected stripe like cells in a-Si modules whith thin film technologie. These line patterns cause a loss of active cell area, defined by the edge quality of the lines. Screen printing is favorized as a highly economic process. Two new screen printing methods for patterning SnO2 and metal are presented. The patterning sequence: TCO with screen printing/lift off technique, a-Si with mechanical milling or slotting and metal with screen printing/lift off technique is qualified as the most inexpensive patterning process for solar modules leading to patterning costs of about 1.-DM for modules of a 30 × 30 cm2 area.

A Comparative Study of the Electronic Stability of Hydrogenated Amorphous Silicon and Silicon-Germanium Alloy Material

The electronic stability of a-Si:H and a-Si1-xGex:H films of different preparations has been investigated by keV-electron irradiation. Employing an electron dose of about 60 J/cm2, a metastable defect density near its saturation value was created. It was found that a-Si:H films exhibiting a high stability, if bulk sensitive measurement techniques (CPM, conductivity measurements) are applied, still have quite a large number of stable surface defects (detected by PDS) whose concentration is raised by irradiation. a-Si1-xGex:H alloy films are obviously more stable than pure a-Si:H films, whereby the relative stability increases with the Ge content x. Alloy films containing a small amount of germanium (≈5%) could be of special significance for practical application. The electronic properties of this material are almost identical to those of a-Si:H but the electronic stability is increased.

Enhanced Stability of Amorphous Silicon Pin Solar Cells by Doping Profiles

pin cells with the light entering the n-layer (pinITO) or the p-layer (SnO 2 p(C)in) were prepared taking into account the ‘basic’ boron profile in the i-layer. Their efficiencies (up to 7.4% and 10.1%) show a light-induced degradation. This can be removed by doping the i-layer of pinITO cells with a decreasing boron profile. The initial efficiency is only slightly affected. Cells containing fluorine due to prior plasma-etching require higher amounts of boron for stability. For the cell type SnO 2 p(C)in, phosphorus profiles, however increasing towards the n-layer, lead to reduced cell degradation without affecting the efficiency. The results of profiling were interpreted in terms of a recombination model.

Stability of a-Si/Ge:H Device Material to Light and keV-Stress Exposure

The development of highly efficient and stable (a-Si:H/a-Ge:H) thin-film tandem solar cells presupposes a high quality and stable a-Ge:H material for the bottom cell. We have found conditions for manufacturing a high quality material which shows excellent stability to illumination with white light. The evaluation of the defect density with PDS did not reveal any increase in defects after illumination. Nor did the photo and dark conductivity change after 2500 hours AM1 light soaking. Furthermore, using keV electron irradiation, we demonstrated a significant increase of stability of a-Si/Ge:H alloys with rising germanium content. This behavior might be explained by the higher diffusion mobility of hydrogen in a-Ge:H compared to a-Si:H, so that a lower annealing temperature may result. In order to verify this assumption, the temperature-dependent dark conductivity sd of different doped and undoped a-Ge:H films was measured at different cooling rates. The results indicate a glass-like transition between a thermal equilibrium state above a certain freeze-in temperature T C * and a slowly relaxing state below that temperature. The values for T C * depend on the position of the Fermi level and agree well with the glass transition temperatures T G * defined by the diffusion coefficient DH of the hydrogen sub-lattice. For undoped a-Ge:H, T G * and T C * are about 50°C to 70°C lower than in a-Si:H. This will improve the annealing and stability behavior of a-Si/Ge tandem solar cells.

Integrated Series Connection of a-Si:H Solar Cells

We tested several methods of patterning amorphous-silicon (a-Si:H) solar cells to form stripe cells and to connect these stripe cells integrated with cell deposition process to a series-connected module. The patterning techniques used included: laser cut, screen print lift-off techniques, adhesive tapes with lift-off, and a special mechanical milling method. The three layers of a cell which must be patterned are 1. SnO 2 :F on glass, 2. a-Si (p(C)in), 3. metal-electrode. We developed a convenient patterning method for a laboratory size with a loss of area of 13 %: 1. laser cut, 2. milling, 3. adhesive tape lift-off. For a higher level of production we tested the following sequence with a loss of area of 4 %: 1. screenprint and lift-off, 2. milling/slotting, 3. screen print and lift-off. The last method has the advantage that without any laser treatment the costs are only third of those of the other methods with a laser cut; so we will eliminate the laser cut in future for manufacturing modules.