Silicon Solar Cell Emitter Profile Tailoring Using the DOSS Diffusion Technique

Abstract

A tight control of the dopant surface concentration is important during phosphorus emitter diffusion due to the opposite requirements of a lowly doped emitter for good blue response and a sufficiently high surface concentration for a good ohmic contact. The Doped Oxide Solid Source (DOSS) diffusion technique is well suited for fine-tuning of the surface concentration. The sources are made in the laboratory using the standard POCl₃ diffusion technique and the source doping level can be controlled by changing the partial pressure of POCl₃. DOSS diffusions were carried out in the temperature range 850-1050°C using sources with different doping levels obtained by varying the POCl₃ partial pressure from 0.004% to 4.28%.  The electrical profiles were measured using the Stripping Hall profiling technique. For the entire range of source doping concentrations and temperatures investigated, the obtained emitters were of the finite source emitter type and the profiles showed a surface concentration below the electrically active limit, thus implying the absence of a dead layer. The residual diffusion oxide was characterized using ellipsometry and XPS profiling. The oxide thickness increased with both temperature and source doping level within the range 7.5-30 nm.  XPS profiling indicated that the composition of the residual glass was a mixture of P₂O₅ and SiO₂. The phosphorus was not homogeneously distributed across the oxide but exhibited a pile-up in the near oxide surface region and also at the oxide/Si interface.