Direct Semiconductor Bonding Technology (SBT) for high efficiency III-V multi-junction solar cells

In summary, we have demonstrated a high quality direct bond between GaAs and InP wafers. The wafers were polished to an excellent surface finish with RMS roughness of below 0.5nm, making them suitable for direct wafer bonding. A mechanically robust bonded interface with electrical resistance of as low as 0.3 Ωcm2 and optical absorption loss of less than 3% across the bonded interface is achieved by optimizing the bonding process parameters. This technique was applied to fabricate 4and 5-junction solar cells grown on GaAs and InP substrates and integrated through the bonding process. The multi-junction solar cells thus fabricated have exhibited greater than 83% fill factor and external quantum efficiencies exceeding 90% in the bottom subcells, attesting to the low electrical resistance and high optical transmittance of the bonded interface. The highest AMO efficiency of 33.5% is achieved for 4-junction cells.