We have developed a new broadband-sensitive photon upconversion (UC) material that can be used for transparent ceramic plates mounted on the rear faces of crystalline silicon solar cells. We selected the host material of a cubic crystal structure codoped with Er3+ and Ni2+ so that the Ni2+ dopants were fully activated to sensitize the Er3+ emitters. In garnet-type Ca3Ga2Ge3O12 with additional codopants of Nb5+ and Li+ for charge compensation, all the Ni2+ dopants occupied the six-coordinated Ga3+ sites, leading to highly efficient energy transfer from the Ni2+ to the Er3+. Formation of four-coordinated Ni2+ that quenches the UC emission of the Er3+ was prevented, because Ni2+ cannot substitute the four-coordinated Ge4+ much smaller than Ni2+. Consequently, energy dissipation from the Er3+ to the Ni2+ was well reduced compared with the previously developed Gd3Ga5O12:Er,Ni,Nb in which the Ni2+ dopants partially occupied the four-coordinated Ga3+ sites. Additional introduction of Y3+ and Li+ enhanced optical transitions and improved the UC performance, owing to more enhanced lattice distortion, along with eliminating different phases. The optimal composition (Ca0.6Er0.1Y0.1Li0.2)(3)(Ga0.98Ni0.01Nb0.01)(2)Ge3O12 exhibited a broadband sensitivity ranging from 1.1m (the absorption edge of silicon) to 1.6m for the UC emission at 0.98m.