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Cooperative down-conversion (DC) with emission of two near-infrared photons for each blue photon absorbed was achieved in Tb³⁺/Yb³⁺ co-doped yttrium lithium fluoride single crystals grown by an improved Bridgman method. The luminescent properties of the crystals were investigated through photoluminescence excitation, emission spectra, and decay curves. With the excitation of Tb³⁺ ion by a 486 nm light, emission between 980 and 1030 nm from the Yb³⁺:²F_5/2 → ²F_7/2 transition was observed, and this emission originated from the DC between Tb³⁺ and Yb³⁺ ions. The energy transfer processes are studied based on the Inokuti-Hirayama's model, and the interaction between Tb³⁺ and Yb³⁺ is confirmed to be electric dipole-dipole. The large quantum cutting efficiency approaches up to 166.7% for 0.32 mol% of Tb³⁺ and 7.98 mol% of Yb³⁺ co-doped LiYF₄, which is potentially used as a DC layer in silicon-based solar cells.