Ultrathin Ta₂O₅ electron-selective contacts for high efficiency InP solar cells

Heterojunction solar cells with transition-metal-oxide-based carrier-selective contacts have been gaining considerable research interest owing to their amenability to low-cost fabrication methods and elimination of parasitic absorption and complex semiconductor doping process. In this work, we propose tantalum oxide (Ta₂O₅) as a novel electron-selective contact layer for photo-generated carrier separation in InP solar cells. We confirm the electron-selective properties of Ta₂O₅ by investigating band energetics at the InP-Ta₂O₅ interface using X-ray photoelectron spectroscopy. Time-resolved photoluminescence and power dependent photoluminescence reveal that the T₂O₅ inter-layer also mitigates parasitic recombination at the InP/transparent conducting oxide interface. With an 8 nm Ta₂O₅ layer deposited using an atomic layer deposition (ALD) system, we demonstrate a planar InP solar cell with an open circuit voltage, V_oc, of 822 mV, a short circuit current density, J_sc,of 30.1 mA cm^-2, and a fill factor of 0.77, resulting in an overall device efficiency of 19.1%. The V_oc is the highest reported value to date for an InP heterojunction solar cells with carrier-selective contacts. The proposed Ta₂O₅ material may be of interest not only for other solar cell architectures including perovskite cells and organic solar cells, but also across a wide range of optoelectronics applications including solid state emitting devices, photonic crystals, planar light wave circuits etc.