Abstract
Ion-implanted poly-crystalline silicon (poly-Si), in combination with a tunnel oxide layer, is investigated as a carrier-selective passivating contact in c-Si solar cells based on an interdigitated back contact (IBC) architecture. The optimized poly-Si passivating contacts enable low interface recombination, resulting in implied VOC (iVOC) of about 720mV and 704mV for n-type and p-type, respectively, before any hydrogenation step. It is found that high-quality passivation can be obtained when confining the dopants within the poly-Si layers and realizing a shallow diffusion of dopants into the c-Si bulk, meaning a sharp decrease in doping concentration in the c-Si at the poly-Si/c-Si interface. The doping profile at the poly-Si/c-Si interface can be influenced by poly-Si layer thickness, poly-Si ion-implantation parameters, and post-implantation annealing conditions. The detailed discussion on the passivation properties of the poly-Si passivating contacts and their preparation conditions are presented in this paper. In addition, IBC solar cells with/without front surface field (FSF) are fabricated, with the optimized poly-Si passivating contacts as back surface field, BSF (n-type poly-Si), and emitter (p-type poly-Si). The best cell shows an efficiency of 21.2% (VOC=692mV, JSC=39.2mA/cm2, FF=78.3%, and pFF=83.5%).
Highlights
The conversion efficiency of crystalline silicon (c-Si) solar cells, continuously improved owing to both material and surface passivation innovation, is minimizing the gap with the theoretical efficiency limit
In Section III.A we report on the nature of our poly-crystalline silicon (poly-Si)/c-Si interface; in Section III.B we analyse and discuss the passivating/conductive behaviour of our poly-Si layers; and in Section III.C we present our interdigitated back contact (IBC) solar cells and give an outlook on how to further enhance the conversion efficiency
The Raman measurement was done with a green laser λ( laser 1⁄4 514 nm) on a 500-nm thick poly-Si layer deposited on c-Si wafer which was pre-covered with 200 nm thick thermal SiO2
Summary
The conversion efficiency of crystalline silicon (c-Si) solar cells, continuously improved owing to both material and surface passivation innovation, is minimizing the gap with the theoretical efficiency limit. By using such a carrier-selective contact, very good passivation of c-Si surface was obtained with an VOC of 720 mV for the c-Si solar cell test structure nþ -SIPOS/thin SiO2/p-type c-Si/thin SiO2/n þ -SIPOS [10]. Materials and similar test structures, research is currently ongoing and high-quality passivation is obtained with n-type doped poly-Si/tunnelling SiO2/c-Si structure [7,8,9], which is referred as the TOPCon structure With such structure on the back side of a 2-cm wide front/rear contacted cell, AlOx/SiNxpassivated front emitter and selectively heavily-doped p þ þ front emitter under the metallic contact area, a record efficiency of 25.1% was achieved [11]. The effect of the front side structure and passivation on the IBC solar cell performance is discussed
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