Transfer of the Metal Wrap Through Solar Cell Concept to n-Type Silicon

Beschreibung

This thesis presents the transfer of the metal wrap through (MWT) solar cell concept from p-type to n-type crystalline Czochralski-grown silicon (Cz-Si) wafers with 156 mm edge length. An industrially-feasible MWT solar cell structure with screen-printed metallization is developed, which combines the advantages of n-doped silicon with those of a rear contact cell structure. The studies focus on reverse bias stability, emitter diffusion, and electrical contacting of highly boron-doped surfaces. The vias and the rear p-type contacts represent the most important MWT specific structures. Detailed investigations result in optimizations and process simplifications with respect to the rear contact configuration. New tube furnace diffusion processes are developed to form deep driven-in boron dopings with low surface concentration in a single process step. The emitters formed therewith feature low dark saturation current density of (30 ± 3) fA/cm² for passiva ted and textured surface with a sheet resistance of about 120 Ω/sq. Despite low maximum dopant concentration of only (1.8 ± 0.2)·1019 cm-3, low specific contact resistance of less than 4 mΩcm² is obtained for screen-printed and fired metal contacts. It is experimentally shown for the first time that the depth-dependent course of the dopant concentration contributes to a low-ohmic contact formation. This correlation is also demonstrated by means of an analytical model which is based on metal crystallites. Furthermore, it is shown that charge carrier recombination underneath the metal contacts correlates with both the metal crystallites and the junction depths of the boron dopings.

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Elmar Lohmüller

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MWT n-type silicon solar cells

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