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.