Multijunction solar cells with subcell materials highly lattice-mismatched to germanium

The performance of a series of metamorphic GalnP and GalnAs solar cells grown on Ge with lattice-mismatch ranging from 0% to 2.4% is reported, with emphasis on device structures with 0.5% and 1.6% mismatch. Dualjunction cells with moderately lattice-mismatched (0.2% and 0.5%) structures have already reached electrical performance comparable to lattice-matched devices, at about 26% efficiency under AM0, 1-sun condition. Development efforts to date on highly lattice-mismatched (16% mismatch) structures have resulted in 22.6% efficiency dual-junction cells, with many improvements still possible. Spectral response measurements reveal excellent quantum efficiency (QE) for metamorphic GalnP and GalnAs materials, with a measured internal QE of over 90%. The offsets between the bandgap voltage (Eg/q) and the open-circuit voltage (V^oc) of GalnP and GalnAs metamorphic cells were kept below 550 mV and 450 mV, respectively. Experimental results indicate that lattice-mismatched GalnP/GalnAs dual-junction cells can achieve higher energy conversion efficiency than latticematched GalnP/GalnAs dual-junction solar cells.