Enhanced broadband excited upconversion luminescence in Ho-doped glasses by codoping with bismuth

We report enhanced green and red upconversion (UC) luminescence in Ho3+-doped oxyfluoride germanate glass by introducing bismuth near-infrared active centers as sensitizers. The UC excitation bands at 750 and 970 nm show a full width at half-maximum of 20 and 45 nm, respectively. Energy transfer from sensitizers, the excited-state absorption, and phonon-coupled absorption of Ho3+ jointly contribute to the enhanced UC luminescence. Our approach provides an efficient methodology to broaden the excitation bandwidth of UC luminescent materials, which may have the potential for promising application in solar cells.