Optimizing phosphorus diffusion for photovoltaic applications: Peak doping, inactive phosphorus, gettering, and contact formation

Abstract

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl₃ source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860 °C for 60 min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density N[subscript peak] = 8.0 × 10¹⁸cm⁻³ and a junction depth dj= 0.4 μm, resulting in a sheet resistivity ρ[subscript sh] = 380 Ω/sq and a saturation current-density J₀below 10 fA/cm². With these properties, the POCl₃ process can compete with ion implantation or doped oxide approaches.