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 3.1middot10
¹⁵
-6.3middot10
¹⁵
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