Organometallic vapor phase epitaxy growth of upright metamorphic multijunction solar cells

We demonstrate an integrated metamorphic AlGaInP/AlGaInAs/GaInAs/Ge 4 J solar cell on Ge substrate using organometallic vapor phase epitaxy (OMVPE). A step graded GaInAs buffer was grown right after the Ge subcell was formed to change the lattice constant from that of Ge to that of Ga _0.8In _0.2As lattice constant followed by a 1.14 eV Ga _0.8In _0.2As subcell, a 1.5 eV (AlGa) _0.8In _0.2As subcell, and a 1.85 eV Al _xGa _0.32-xIn _0.68P subcell. Transmission electron microscope (TEM) study shows the threading dislocation density (TDD) is about 6×10 ⁶ cm ^-2. The X-ray diffraction reciprocal space map (RSM) shows that the structure is 100% relaxed. Bandgap dependent (Al _xGa _1-x) _0.32In _0.68P subcell performance is systematically investigated. As the Al _xGa _0.32-xIn _0.68P cell bandgap goes up to 1.9 eV, the external quantum efficiency (EQE) goes down significantly. Theoretical simulation shows that the decrease of diffusion length causes the lower EQE, which indicates the material quality degrades with the increasing Al content. Integrated 4 J solar cells are fabricated and characterized with spectral response and tested under the AM1.5D terrestrial spectrum at both 1 sun and 2000 suns.