Optimierung der Vorderseitenmetallisierung und des Emitters für hocheffiziente industrielle Silizium Solarzellen

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Fineline printing options for high efficiencies and low Ag paste consumption

In this paper, we investigate and compare three different fine line printing techniques for the silver front side metallization of industrial-type silicon solar cells: single print, dual print and print-on-print. We produce solar cells using the same screen or stencil aperture of 40 μm and about 92 fingers and obtain finger widths below 60 μm for all three approaches. The print-on-print process achieves the highest finger heights of 20 μm after firing but with quite strong finger height variation. In contrast, the dual printed fingers have a very flat surface with a finger height of 14.5 μm which leads to the highest cross-section area of 530 μm2 of the three techniques. The single print shows the lowest cross-section area of 390 μm2 due to the lowest average finger height. The measured finger line resistance correlates with the finger cross-section area. The dual print allows us to use a non-firing through bus bar paste which increases the V oc by 2 mV and hence achieves the highest efficiency of 19.1% using full-area Al-BSF cells. Due to an optimized bus bar screen print in combination with only 30 μm finger aperture, the dual print has the lowest Ag paste consumption of only 75 mg/wafer, one of the lowest Ag paste consumption that has been reported so far. A first batch of PERC solar cells with dual-printed Ag front contacts shows efficiencies up to 19.6%.German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety/0325296SolarWorld Innovations GmbHRENA GmbHSINGULUS TECHNOLOGIES AGHeraeus Precious Metal

Record low ag paste consumption of 67.7 mg with dual print

We investigate and compare three different fine line printing techniques for the silver front side metallization of industrial-type silicon solar cells: single print, dual print and print-on-print. We obtain finger heights of 5.6 μm for single print, 9.5 μm for dual print and 15.1 μm for print-on-print as well as finger width between 46.2 μm and 61.3 μm. We process PERC solar cells with dual print and print-on-print. For the dual print, we test two different bus bar designs, a standard rectangular shaped bus bar and a segmented bus bar. The resulting PERC solar cells achieve conversion efficiencies of 19.8% for dual print and print-on-print. The dual print with segmented bus bar design reduces the Ag paste consumption to 67.7 mg, measured after printing prior to drying. To our knowledge, this is the lowest front side Ag paste consumption that has been reported so far. Additionally, we model optimum Ag finger width in dependence of electrical and geometrical parameters. We find that even when assuming very optimistic parameters, the optimum finger width of 26 μm is just a factor of two lower compared to the state of the art technology today.German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety/0325296SolarWorld Innovations GmbHRENA GmbHSINGULUS TECHNOLOGIES AGHeraeus Precious Metal

Optimized stencil print for low Ag paste consumption and high conversion efficiencies

We evaluate industrial-type PERC solar cells applying a dual printed front grid with stencil printed Ag fingers. We vary the Ag paste consumption for the finger print between 8.4 mg and 120.4 mg per 156 x 156 mm(2) wafer (weighted after printing before drying) by using polyurethane squeegees with different shore hardness as well as a metal squeegee and by varying the printing pressure to obtain different finger heights. The busbar consumes additional 19.5 mg Ag paste. We obtain average finger heights from 5.9 mu m up to 24.3 mu m for 55 mu m to 65 mu m wide fingers. The resulting PERC solar cells show an average efficiency of 20.2% for finger paste consumptions above 60 mg. In contrast, a strong reduction of the conversion efficiency with less than 60 mg finger paste consumption is observed since the increased series resistance reduces the FF. By analytical modelling, we compare the calculated series resistance to the experimental data and observe a good accordance for more than 40 mg finger paste consumption whereas the experimental series resistance slightly exceed the modelled values below 40 mg. In addition, we use numerical simulations to investigate the series resistance dependence on the finger height which shows higher experimental values for finger height below 10 mu m. The deviation of the measured series resistance and the two modelled cases is mostly due to inhomogeneous distribution of finger height profiles and finger interruptions on the solar cells with front finger paste consumption of less than 40 mg. For finger paste consumption below 60 mg, we find that also the specific contact resistance increases. A physical model of the root cause for this dependence still has to be found

Loss analysis and improvements of industrially fabricated Cz-Si solar cells by means of process and device simulations

We model currently fabricated industrial-type screen-printed boron-doped Cz silicon solar cells using a combination of process and device simulations. The model reproduces the experimental results precisely and allows us to predict both the efficiency gain after specific cell improvements and the associated thermal budgets. Separating the resistive losses (evaluated for various contributions) from the recombination losses (evaluated in different device regions) allows us to forecast the improvements of the emitter and the rear side necessary such that the recombination losses in the base dominate. We predict that to increase cell efficiency considerably beyond 19.7 %, the base material needs to be improved