Co-Diffusion Processing of p⁺/n/n⁺ Structure for n-Type Silicon Solar Cells Using Boron Doped Paper Sheets

Abstract

In this work we designed, fabricated and assessed a p⁺/n/n⁺ structure which constitute the basis and the core part of the n-type silicon solar cells. The process of fabrication is based on the co-diffusion of pre-deposited phosphorus and boron. It consists of carrying out simultaneously in one single high temperature step the diffusion of both boron and phosphorus of the p⁺ emitter and pre-deposited n⁺- back surface field (BSF), respectively. The first step was the pre-formation of the n⁺-BSF in a POCl₃ furnace system. After depositing the silicon nitride film onto the wafers rear side as a diffusion barrier to protect them from being boron doped, the wafers underwent an alkaline bath to etch the phosphorus layer of the front side. Onto this latter we used a boron source paper sheets to create a p⁺ emitter (preform source) simultaneously with n⁺-BSF drive-in step. This co-diffusion process was carried out at a temperature of 930 °C in a quartz tube devoted to this purpose. The resulting structure has a sheet resistance of 49 Ω/□ and 39 Ω/□ for emitter and BSF, respectively, corresponding to junction depths of 0.40 μm and 0.55 μm. The dopants surface concentrations are of 1.14 E20 atoms. cm⁻³ and 6.20 E19 atoms. cm⁻³ for emitter and BSF, respectively. This p⁺/n/n⁺ structure was used to fabricate solar cells after passivating the emitter and screen printing the front and rear side metallic contacts. A short circuit current density of 31.60 mA/cm², an open circuit voltage of 555 mV and an efficiency of 10.70% was measured indicating that our non-optimized fabrication process and the resultant device is viable.