We report on multijunction GaInP/GaAs photovoltaic cells with efficiencies of 29.5% at 1-sun concentration and air mass (AM) 1.5 global and 25.7% 1-sun, AM0. These values represent the highest efficiencies achieved by any solar cell under these illumination conditions. Three key areas in this technology are identified and discussed; the grid design, front surface passivation of the top cell, and bottom surface passivation of both cells. Aspects of cell design related to its operation under concentration are also discussed.
The refractive index change caused by changes in the absorption edge of GaAs is determined by analysis of the spontaneous emission spectrum of a buried heterostructure window laser. The spontaneous emission spectrum is converted to a gain spectrum from which changes in the imaginary part of the refractive index can be calculated as the laser is excited from low current up to threshold. The real change in refractive index is then determined by a Kramers-Kronig transformation. The change in refractive index n′ of the GaAs active layer is slightly sublinear with minority carrier density nc. At the laser line, including the calculated contribution of free carriers, Δn′ = −0.025±0.005 and dn′/dnc = −(1.8±0.4)×10−20 cm3 in lasers with carrier densities at threshold estimated as 1.02×1018 cm−3. Near threshold, the ratio of the change in the real index to the change in the imaginary index is about 6.2.
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