The deposition of a thin layer of a quantum-cutter material on top of silicon-based solar cells seems to be a promising solution to reduce the thermalization losses. This mechanism has been reported in materials codoped with Pr3+−Yb3+, where Pr3+ can sensitize two Yb3+ ions for one absorbed blue photon. In the present Letter, we analyze precisely energy transfers between Pr3+ and Yb3+ in CaYAlO4, and we measure a quantum-cutting rate of 145%. We show that a very efficient back transfer from Yb3+ toward the G41 level of Pr3+ ion leads to a strong reduction of the quantum yield.