Influence Of Al-BSF Fabrication Process On The Microstructure And Electrical Properties Of Monocrystalline Silicon

At present, the production of crystalline silicon solar cells tends to use thinner silicon wafers. The purpose is to reduce the cost and recombination of non-equilibrium carriers in the diffusion process, so the treatment of back surface field (BSF) plays an extremely important role in the solar cell manufacturing. In this work, in order to improve the deficiencies of the conventional aluminum conductive paste, a new type of aluminum boron conductive paste used for solar cells BSF is prepared. Testing and comparison of solar cells performance confirms the superiority of the new aluminum boron conductive paste.Firstly, through four-point probe measurements, the IV curve test instrument and secondary ion mass spectrometry (SIMS), we measured the electrical properties of the back surface field, the doping concentration in the BSF, and the photovoltaic properties of solar cells. Compared with the back surface field prepared by the conventional aluminum conductive paste, the electrical properties of the back surface field prepared by the new aluminum boron conductive paste were improved. When the BSF was sintered at850℃, the alloyed aluminum boron conductive paste with the B content at1.0wt%in Al-B alloy powders, the B concentration reached about4.21×1019atoms/cm3within the BSF, while the highest Al concentration was about3.05x1018atoms/cm3. Meanwhile the BSF sheet resistance was11.6Ω/□for the paste without containing B, and it quickly dropped to about5.2Ω/□for B at1.0wt%. In the meantime, the open circuit voltage, short circuit current density and fill factor of monocrystalline silicon solar cell were improved. Along with the increase of boron content and sintering temperature, the sheet resistance of monocrystalline aluminum back surface field has showed a downward trend.Secondly, we used scanning electron microscopy (SEM) to characterize and analyze the surface and cross section morphologies of the back surface field. Through these SEM images, we found that the sputtering thickness of the aluminum must be greater than1μm to make sure that we had a dense conductive aluminum layer in the process of magnetron sputtering back surface field. The variation of the morphology of the back surface field along with the thickness of sputtering aluminum was studied.Finally, We used transmission electron microscopy (TEM) to observe and analyze the microstructures of back surface field and Al-Si alloy, and the particle structure within the aluminum electrode.