Broadband-sensitive upconversion emission of Er,Ni,Nb-codoped Gd3Ga5O12 garnet

We synthesized Gd3Ga5O12 garnet codoped with Er, Ni, and Nb to realize transparent ceramics of broadband-sensitive upconverters used for crystalline silicon solar cells. We investigated energy transfer between the Er3+ and Ni2+ dopants using time-resolved measurements of both Ni2+ and Er3+ emissions. Most of the Ni2+ dopants occupied the six-coordinated Ga sites in the synthesized powder samples. Under 1.1–1.45 µm excitation, energies absorbed by the six-coordinated Ni2+ sensitizers almost perfectly transferred to the Er3+ emitters, leading to upconversion (UC) emission of the Er3+ at 0.98 µm. Combined with the UC emission directly excited at 1.45–1.65 µm, a broad sensitivity range of 1.1–1.65 µm was realized. However, the internal quantum efficiency of the UC emission was lower than that of previously developed La(Ga0.5Sc0.5)O3:Er,Ni,Nb, most likely owing to energy dissipation to a small amount of the Ni2+ dopants locating at the four-coordinated Ga sites. To improve the UC performance, the four-coordinated Ni2+ should be eliminated.