Recombination Parameters of Si Solar Cells with Back Surface Field Formed by ION Implantation

Solar cells of 1 Omegacm FZ p-Si with an n⁺-p-p⁺ structure were fabricated using combined thermal diffusion/ion implantation processing. The solar cell fabrication technology under investigation is based on the implantation of 30 keV B ions for back p ⁺ layer formation, P gas phase thermal diffusion for emitter formation, and defect annealing with simultaneous drive-in thermal treatment. The effect of the p⁺ layer formation on recombination in the solar cell base region was studied. Recombination effects due to back ion implantation were determined by analysis of back IQE, using a model of step-wise defect distribution in the base region. After implantation of 3.1middot10¹⁵-6.3middot10¹⁵ cm^-2 of B ions and subsequent annealing at 950 degC effective surface recombination, S_eff2 at the inner interface of the implanted p⁺ layer was in the range 25-60 cm/s. Formation of a defect layer of 0.5-0.6 mum adjoining the p⁺ layer was discovered, and S_eff1, at the inner interface of the defect layer is between 970-1080 cm/s. S_eff1 due to this defect layer is responsible for ~1/2 of a solar cell reverse saturation current calculated for a cell with open circuit voltage exceeding 630 mV. Non optimized combined thermal diffusion/ion implantation processing yields n⁺-p-p⁺ solar cells with efficiencies in the 19-20% range