Wafer-Bonded AlGaAs///Si Dual-Junction Solar Cells

Abstract

Monolithic two-terminal III-V on Si dual-junction (2J) solar cells were fabricated by means of Surface-Activated direct wafer Bonding (SAB). Al0.2Ga0.8As single-junction cells are grown on GaAs substrate by Metal-Organic Vapor Phase Epitaxy (MOVPE) and bonded at room temperature to independently fabricated Si solar cells. The n + -GaAs//n + -Si bonding interface is characterized by Transmission Electron Microscopy (TEM) revealing a 2-3 nm thick amorphous interlayer. The performance of the 1 cm² tandem cells, designed for low concentration applications, was studied by External Quantum Efficiency (EQE)and J-V measurements showing a power conversion efficiency of 17% under one-sun AM1.5G spectrum. To our knowledge, this is the highest efficiency ever reported for a wafer-bonded 2J III-V on Si solar cell. Limitations to performance have been identified and therefore higher efficiencies are expected.

Characterization of dual‐junction III‐V on Si tandem solar cells with 23.7% efficiency under low concentration

“…The main differences between the cells presented in this work and the previous realizations, reported in Veinberg‐Vidal et al and Vauche et al , are the III‐V top subcell design (AlInP window and GaInP emitter instead of AlGaAs), the reduction of the bonding layer thickness by 100 nm and the ARC (dual‐layer instead of single‐layer).…”

“…For the bottom subcell, the J L is 16.2 mA/cm 2 (to be compared with 12.7 mA/cm 2 from previous experiments). This improvement is mostly attributed to the use of a SiN x /SiO 2 double layer ARC that highly reduces reflection in the bottom subcell region with values around 5%.…”

“…EQE measurements of the top and bottom subcells of the tandem cell are presented in Figure , along with the AlGaAs and Si top and bottom single‐junction component cells. The top subcell presents a much better carrier collection efficiency compared with precedent realizations, especially for short wavelengths from 350 to 600 nm, which results in a J L of 16.5 mA/cm 2 (versus the 13.5 mA/cm 2 obtained in previous works). This is the result of using a higher bandgap AlInP (2.3 eV) window and an Al‐free GaInP emitter.…”

“…The Si bottom cells were independently manufactured by thermal diffusion processes on p‐type 4″ (100) 525‐μm thick single‐side polished Czochralski (CZ) c‐Si wafers, with resistivity in the range of 14–22 Ωcm, resulting in a simple design with a P‐doped n + emitter and a B‐doped p + back surface field (BSF). Stand‐alone Si single‐junction cells manufactured in a similar way presented an efficiency of 13.8% under one sun III‐V and Si materials were combined at room temperature by SAB through an oxide‐free, permanent, electrically conductive, and optically transparent n + ‐GaAs//n + ‐Si interface, using EVG580 ComBond equipment as previously reported in Vauche et al .…”

“…For this reason, instead of using a real n‐on‐n isotype, a bottom pseudo‐isotype was manufactured using a Si single‐junction cell with the same design as the bottom subcell, together with a 720‐nm long‐pass filter (LPF720) placed on top so that the absorption of the top subcell is reproduced. An n‐on‐p Al 0.2 Ga 0.8 As single‐junction solar cell, grown on a GaAs substrate in upright configuration as reported in Veinberg‐Vidal et al , was used as the top pseudo‐isotype. The differences with the top subcell of the tandem cell are the material used for the emitter and the growth direction, which is inverted because no wafer bonding was performed in this case.…”