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978-3-8439-1878-7, Reihe Ingenieurwissenschaften
Metallization Losses in Industrial Silicon Solar Cells
190 Seiten, Dissertation Universität Stuttgart (2014), Softcover, A5
This thesis deals with fundamental understanding of the efficiency losses associated with the metallization of the industrial silicon solar cells. The optical, electrical and electronic losses associated with the front metallization limit the efficiency of the industrial silicon solar cells.
This work introduces a novel front grid optimization method which takes into account the optical, electrical and electronic losses. By optimizing the number and width of the conventionally screen printed fingers, this method predicts 0.2 % efficiency gain for high ohmic emitter cells with or without selective emitter structure. The experimental results verify the expected efficiency gain, thus yielding 18.4 % efficiency on large area silicon solar cells.
Furthermore, this thesis deals with understanding the contact formation and the current transport mechanism at the contact between the silver front metallization and the phosphorus doped emitter. This thesis reveals the growth mechanism of the silver crystallites and analyzes the role of phosphorus doping atoms for formation of the silver crystallites. Not the electrically active, but the inactive phosphorus atoms support the growth of the silver crystallites within the emitter. The presented results show that the current transport mechanism depends on the emitter type. The current is transported via silver crystallites for the furnace diffused emitters, and via silver colloids for the laser doped emitters. The presented model indicates that low contact resistance requires: high amount of silver crystallites, but for the case of less crystallites grown within emitter, there is a need of a lot of silver colloids in the glass layer or high active phosphorus concentration in the emitter.
Although the front grid optimization allows an efficiency gain, still the main efficiency loss factor for the industrial silicon solar cells with front metallization is the front metallization shadowing. Therefore, the last part of this thesis deals with back contacted back junction silicon solar cells. The main goal is optimizing the geometry, the doping, as well as the metallization structure of the back contacted silicon solar cells by using numerical and analytical simulations. The model predicts a maximal 22.2 % efficiency for a large area silicon cell with laser doped boron emitter, laser doped phosphorus back surface field, and rear side metallization with six bus bars.