The metal ion Bi3+ has been shown to be an efficient broadband sensitizer for Yb3+ ion in the Y2O3 host lattice. Upon ultraviolet-light excitation in the allowed 1S0→3P1 transition of Bi3+ ions, the characteristic near infrared luminescence of Yb3+ ions due to the 2F5/2→2F7/2 transition was observed as a result of energy transfer from Bi3+ to Yb3+ ions. It demonstrates that cooperative downconversion can be realized for Bi3+–Yb3+ coupling. Yb3+-concentration-dependent quantum efficiency was evaluated and the maximum efficiency reached 173.8%. These phosphors are promising for the enhancement of energy efficiency of crystalline Si solar cells via spectrum shifting.