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For the normal solar cell geometry, there is shown to be an upper limit to the cell conversion efficiency for each value of the silicon substrate resistivity. This limit cannot be exceeded regardless of possible improvements in material lifetime properties. It peaks for a value of substrate resistivity of about 0.1 Ω.cm for p-type substrates corresponding to an acceptor concentration of 7 × 1023m-3. The limit can be exceeded if the cell structure is modified. A high-low junction incorporated near the ohmic back contact to a suitably designed device not only improves the current collecting properties for a given cell thickness, as has been shown previously, but also increases the ultimate conversion efficiency.