A high efficiency infrared quantum cutting effect in a Tb3+–Yb3+ codoped silicon oxynitride system is demonstrated. The thin films are deposited on Si substrates by reactive magnetron co-sputtering of a Si target topped with Tb4O7 and Yb2O3 chips under pure nitrogen plasma. The photoluminescence dynamics are investigated, revealing a quantum efficiency of this system at 980 nm up to 197% for the higher Yb3+ concentration. Thus, via a cooperative transfer mechanism between Tb3+ and Yb3+, an absorbed UV–visible photon gives rise to almost two emitted IR photons. Such a down-conversion effect is demonstrated upon indirect excitation of energy donors, via defect states in the host matrix. These down-converter films could be directly and easily integrated on top of the Si-based solar cell to improve the photoelectric conversion efficiency at a lower cost. An evaluation of the additional external quantum efficiency is deduced from this optical system and found to be almost 2%.