Boron tube diffusion process parameters for high-efficiency n-TOPCon solar cells with selective boron emitters

Photovoltaic cell technology plays an important role in achieving carbon neutrality. However, a major challenge to further improving the conversion efficiency is the recombination and electrical contact of boron (B)-doped emitters in n-TOPCon solar cells. Boron-selective emitters (B-SEs) are ideal candidates for reducing the emitter recombination and contact resistivity of n-type silicon solar cells and for providing better blue response. In this study, the key process parameters of BCl3 tube furnace diffusion (drive-in temperature, oxidation temperature, and BCl3 gas flow, etc.) were identified, and their impact on the performance of the emitter dark saturation current density under the passivation layer (J0) and on the contact resistivity (ρc) was investigated. The results showed that the BCl3 gas flow rate (GBCl3) had much less impact than drive-in and oxidation conditions, which is mainly owing to the lower-temperature deposition with lower boron diffusivity. However, the O2 flow rate under high-temperature oxidation had the greatest impact on passivation and contact resistivity, which is mainly related to the thicker boron rich layer (BRL), resulting in high recombination. We introduced B-SEs formed by 3D printing mask technology and secondary diffusion with p++≈ 75 Ω/sq and p+ ≈ 230 Ω/sq, and the efficiency improved by 0.27% absolute to 24.05%, mainly because of the increased open-circuit voltage (Voc) of 10 mV. After the optimization of B-SEs and passivation processes, we manufactured industrial-grade TOPCon cells with efficiency (Eff), Voc, Jsc, and fill factor (FF) values as high as 24.2%, 721 mV, 40.88 mA/cm2, and 82.1%, respectively. This simplified B-selective emitter and the implementation of passivated contacts can thus be considered to further increase cell efficiency.