Optimization of BBr₃-Based Co-Diffusion Processes for Bifacial N-Type Solar Cells.

We report on the co-diffused bifacial N-type solar cells based on N-type Si wafers using the process of spin on doping (SOD, phosphorous source) and boron tribromide (BBr₃) diffusion by atmospheric pressure chemical vapor deposition (APCVD). For bifacial co-diffusion, a phosphorous layer was deposited by SOD on the rear side of N-type Si wafer and a BBr₃ as boron dopant source deposited by APCVD. Co-diffusion process was controlled by changing the flowrate of carrier N₂ gas and drive-in temperatures. It was found that the fabricated bifacial co-diffused N-type solar cell with 2% H₃PO₄ doping, the flowrate of N₂ carrier gas of 15 slm and drive-in temperature at 930°C exhibited the highest conversion efficiency of 15.8% with high open circuit voltage (V(oc)) of 593 mV. As compared to high H₃PO₄ concentrations (5% and 9%), the low H₃PO₄ concentration of SOD showed the higher sheet resistance and decreased in the thickness of N + emitter layer, resulting in the high V(oc), shunt resistance, fill factor and conversion efficiency of solar cells.