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In a MJ cell, the semiconductor with the highest band gap is used as the top layer. Band gaps are then reduced with each further layer. This design maximizes photon energy extraction as the top layer absorbs the highest energy photons allowing photons with less energy to transmit through. Each subsequent layer then extracts energy from photons closest to its band gap thereby minimizing thermalization losses. The bottom layer then absorbs all remaining photons above its band gap. As Figure 2 shows, cell efficiency is most sensitive to the band gap of the bottom subcell. The error bars indicate the range of subcell band gaps that would maintain a top theoretical efficiency within 1% of ideal. As a result, the material selection for the bottom layer is an extremely important consideration in the design of a MJ device. | In a MJ cell, the semiconductor with the highest band gap is used as the top layer. Band gaps are then reduced with each further layer. This design maximizes photon energy extraction as the top layer absorbs the highest energy photons allowing photons with less energy to transmit through. Each subsequent layer then extracts energy from photons closest to its band gap thereby minimizing thermalization losses. The bottom layer then absorbs all remaining photons above its band gap. As Figure 2 shows, cell efficiency is most sensitive to the band gap of the bottom subcell. The error bars indicate the range of subcell band gaps that would maintain a top theoretical efficiency within 1% of ideal. As a result, the material selection for the bottom layer is an extremely important consideration in the design of a MJ device. | ||
[[File:EffSensitivity.JPG|thumb|center|Figure 2: Efficiency sensitivities with band gaps (Marti and Araujo, 1996 | [[File:EffSensitivity.JPG|thumb|center|Figure 2: Efficiency sensitivities with band gaps (Marti and Araujo, 1996]] | ||
Figure 3 shows the detailed structure of a GaInP (1.8 eV) / GaInAs (1.4 eV) / Ge (0.67 eV) triple-junction solar cell. In this design a wide band gap (transparent) window layer is used to lower a cell's series resistance. It does so by enhancing the lateral flow of photogenerated electrons trying to reach an electrical contact or '''tunnel junction''' (see Tunnel Junctions). A buffer layer is also used between the bottom and middle layers to reduce lattice mismatch effects (see Lattice Constant Matching). | Figure 3 shows the detailed structure of a GaInP (1.8 eV) / GaInAs (1.4 eV) / Ge (0.67 eV) triple-junction solar cell. In this design a wide band gap (transparent) window layer is used to lower a cell's series resistance. It does so by enhancing the lateral flow of photogenerated electrons trying to reach an electrical contact or '''tunnel junction''' (see Tunnel Junctions). A buffer layer is also used between the bottom and middle layers to reduce lattice mismatch effects (see Lattice Constant Matching). |