Two Diffusion Step Selective Emitter : Comparison of Mask Opening by Laser or Etching Paste

Two dimensional solar cell structures to form a selective emitter on the front side or back contact solar cells are often created with the aid of a selectively opened masking layer. In this paper we compare two methods of opening a PECVD SiNX layer. A nanosecond laser with a wavelength of 355 nm was used to ablate a PECVD SiNX layer. Alternatively, an etching paste was screen printed and subsequently heated to remove the SiNX. Both methods were successfully applied for the formation of a selective emitter structure on standard Cz monocrystalline solar cells. The process sequence includes two diffusion steps and uses one SiNX layer as a mask and also as an ARC. Since the front contact grid and the mask opening have the same shape, they have to be aligned accurately. The best cell efficiency of 18.1% was achieved on an industrial scale solar cell with a SiNX ARC opened by the etching paste. Scanning electron microscope pictures showed that the laser ablation method melts the silicon underneath the SiNX layer and thus a wet chemical damage etch must also be applied, whereas the screen printed etching paste removes the SiNX without damaging the silicon.