Diffusion-based model of local Al back surface field formation for industrial passivated emitter and rear cell solar cells

In this work, the back surface field (BSF) formation of locally alloyed Al paste contacts employed in recent industrial passivated emitter and rear cell solar cell designs is discussed. A predictive model for resulting local BSF thickness and doping profile is proposed that is based on the time dependent Si distribution in the molten Al paste during the firing step. Diffusion of Si in liquid Al away from the contact points is identified as the main differentiator to a full area Al BSF; therefore, a diffusion based solution to the involved differential equation is pursued. Data on the Si distribution in the Al and the resulting BSF structures are experimentally obtained by firing samples with different metal contact geometries, peak temperature times and pastes as well as by investigating them by means of scanning electron microscopy and energy dispersive X ray spectroscopy. The Si diffusivity in the Al paste is then calculated from these results. It is found that the diffusivity is strongly dependent on the paste composition. Furthermore, the local BSF doping profiles and thicknesses resulting from different contact geometries and paste parameters are calculated from the Si concentration at the contact sites, the diffusivity and solubility data. These profiles are then used in a finite element device simulator to evaluate their performance on solar cell level. With this approach, a beneficial paste composition for any given rear contact geometry can be determined. Two line widths are investigated, and the effects of the different paste properties are discussed in the light of the solar cell results obtained by simulation.