Lattice-matched 3-junction cell with 1.2-eV InGaAs/GaAsP superlattice middle cell for improved current matching
A strain-compensated InGaAs/GaAsP superlattice (SL) was successfully integrated into the GaAs middle cell of a lattice-matched InGaP/GaAs/Ge triple junction (3J) solar cell. Wafer shuttle technology was employed for proof of concept because the growth of the SL was optimized for a different metalorganic vapor phase epitaxy (MOVPE) reactor from the one for conventional 3J cells, but an entire structure can be grown in a single reactor in the future. The SL extended the absorption edge of the middle cell from 875 nm of GaAs to 1150 nm. Consequently, the photo current generated in the SL middle cells was enhanced by approximately 30% compared to a normal GaAs middle cell, improving current matching among subcells. The performance of SL cell, especially photo-current, was affected by the structure of the SL middle cell and the regrowth condition of the InGaP top cell. The conversion efficiency reached approximately 37% under 100-sun. The results encourage the implementation of InGaAs/GaAsP SL for high efficiency multi junction solar cells (MJSCs) with reduced total layer thickness.