Distinct Design for Two Terminal Crystalline Silicon Bottom-Based III–V Tandem Devices via Direct Electrode Interconnection and Enlarged Bottom Area

An innovative two-terminal III–V/c-Si multijunction device is constructed using direct electrode connections and extending the bottom area to improve carrier transport and current matching between the top III–V and bottom c-Si devices. A commercial triple junction GaInP₂/InGaAs/Ge top cell (1 cm²) with a complete cover electrode at the rear is connected directly to a silicon heterojunction (SHJ) bottom device using silver paste. The area of the SHJ bottom device is enlarged from 0.69 cm² to 15 cm² to match the current density of the top. With a small active area of 0.69 cm², the bottom device has a current density of 13.6 mA/cm², comparable to the top device’s (13.5 mA/cm²). By optimizing the SHJ bottom, the multijunction device achieves an impressive efficiency of 39.5% with a short circuit current density of 13.5 mA/cm², an open circuit voltage of 3.45 V, and an 85% fill factor. This initial design demonstrates the potential to significantly improve the efficiency of a two-terminal III–V/c-Si tandem device by increasing the matching current levels of both top and bottom cells.