Two-dimensional analysis of the interdigited back-contact solar cell

Using two-dimensional computer analysis, the interdigited back contact silicon solar cell (IBC) was analyzed at high illumination levels and the results were compared with the conventional Front Junction cell. The change of effectiveness of Chockley-Read-Hall bulk and surface recombination centers at high currents as well as the induced internal electric field are argued to explain the improved efficiency predicted for IBC cells at high illumination levels. For a 100 μm cell thickness and lifetime τ_p0 = 10 μsec the efficiency is indicated to increase from 7.5% at 1 sun to 14.0% at 100 suns AMO, when a surface recombination velocity (s₀) equal to 1000 cm/sec is assumed. The substrate thickness to provide maximum efficiency was found to be approximately 50 μm. It is confirmed that the surface lifetime is a significant factor in determining the device conversion efficiency. Since surface recombination dominates the efficiency, a new IBC cell design with a front doping gradient has been introduced to suppress the surface recombination. The IBC cell with 10¹⁸/cm³ front surface n⁺-doping concentration is optimum for an impurity diffusion depth of 10 μm, s₀ = 1000 cm/sec, τ_p0 = 10 μsec, for which an efficiency of 12% is computed at 1 sum AMO. A useful efficiency of about 8% at 1 sun AMO, even with s₀ = 10⁵ cm/sec, is predicted with front doping.