Influence of Material Composition and Wafer Thickness on the Performances of Electron Irradiated Gallium-Doped Silicon Heterojunction Solar Cells

Past studies have underlined the importance of silicon material composition for optimum space solar cells performances. However, the maturity and performances of silicon cells have evolved over the last decades. Due to the increasing space photovoltaic power demand, it becomes crucial to assess modern silicon radiation hardness. Herein, the influence of material composition (resistivity and interstitial oxygen, gallium, and thermal donor concentrations) of modern gallium-doped silicon wafers on their electronic properties after electron irradiation is investigated. Results demonstrate stable majority carrier concentrations and mobilities within the doping ranges and fluences investigated. Regarding the post-irradiation carrier recombinations, the higher the resistivity the higher the carrier lifetime is at low injection level. Similarly, the electron diffusion length is six times higher for the 60 Ω.cm samples compared to the 0.9 Ω.cm ones. The Shockley–Read–Hall recombination signature of a vacancy-related defect (reported in boron-doped silicon) reproduces well this trend. Then, complete heterojunction solar cells are processed from these materials. While highest resistivity samples feature better carrier lifetimes after irradiation, the best conversion efficiencies are obtained for intermediate resistivity samples (15 Ω.cm). It is shown that it is essentially due to the positive effect of higher majority carrier concentration on the open-circuit voltage.