UV protection of polymer solar cells is significant to extend the duration of life and to improve their efficiency. Diatom shells doped with Eu3+/Tb3+ complexes (ETDSs) can be used as an efficient absorber and down-conversion agent for these purposes. In this work, the goal is effective UV protection and photovoltaic efficiency enhancement of polymer solar cells using DSs doped with Eu3+/Tb3+ complexes. These Eu3+/Tb3+ complexes in DSs are anchored at porous walls through a grafted coupling agent. As a strong UV absorber and efficiency enhancer, ETDSs were compounded into the cellulose acetate (CA) matrix to get an ETDS@CA flexible luminescent film. This luminescent film with high UV absorption in the range of 200-400 nm down converts UV photons into photons in visible, at 612 nm. We spin-coat the mixture solution of ETDS and CA onto the back surface of the transparent ITO/glass electrode to set up the ETDS@CA protected polymer solar cell. In this way, both UV protection and polymer solar cell efficiency enhancement are achieved. The experimental data indicated that ETDS@CA coated polymer solar cells can maintain 80% of the initial efficiency after UV-radiation for 1000 h, compared with 40% of unprotected polymer solar cells. Correspondingly, the efficiency of polymer solar cells has been enhanced by 4%. This method can be developed into a new and highly efficient way to UV protect polymer solar cells, as the original structure of polymer solar cells is not modified, with only an additional UV protection layer on the other side of the substrate.