KEY ASPECTS FOR FABRICATION OF P-TYPE CZ-SI PERC SOLAR CELLS EXCEEDING 22 CONVERSION EFFICIENCY

摘要

This paper gives a close-up insight into recent and future developments that are performed with industrial focus at Fraunhofer ISE's PV-TEC pilot-line to increase the energy conversion efficiency of 6-inch p-type Czoch-ralski-grown silicon (Cz-Si) passivated emitter and rear cells (PERC) to 22% and above. First, the current status of PERC solar cell fabrication allowing for conversion efficiencies up to 21.5% is discussed. Then, we examine four key aspects in detail that need to be considered for optimizing the cells' front side to boost the cell efficiency to the 22% regime. We demonstrate selective emitter laser doping out of the phosphosilicate glass layer, which is formed by a gas phase phosphorus oxychloride diffusion process. After diffusion and wet-chemical emitter etch back, the field emitter features a very low saturation current density of only 31 fA/cm~2 (textured, SiN_x-passivated). Specific contact resistances of 1 mΩcm~2 confirm the low-resistance contacting of the laser-doped surfaces using a commercially available silver screen printing paste. Apart from developing an accurate alignment procedure to match laser-structured and screen-printed layouts, we have also optimized our single-step screen-printing process for finger widths of 38 μm at 16 μm height. Based on simulations we find that efficiencies up to 22.5% are possible when the optimized process routes are integrated into PERC solar cells.