Chloro-sulfide glass with low phonon energy, GeS2–Ga2S3–CsCl, is co-doped with Er and Yb. This active glass is a potential downconversion material for modifying the solar spectrum to improve the efficiency of solar cells. Two downconversion processes from visible to near infrared are observed. In the first process, an energy transfer between Er3+ ions and Yb3+ ions occurs. In that case, one photon is absorbed by the 4I15/2→2H11/2 (Er3+) transition and then two photons are emitted by 2F5/2 →2F7/2 (Yb3+) and 4I13/2→4I15/2 (Er3+), respectively. In the second process, downconversion takes place from the charge-transfer state of Yb3+–S2- to the 4f states of Yb3+ ions, which leads to an intense excitation band between 400 nm and 600 nm, and an emission at 1000 nm. Quantum yields for downconversion are measured. The highest quantum yields of emission below 1200 nm and 1650 nm are equal to 51% and 76%, respectively.