Super Broadband-Sensitive Upconversion in Tm and Ni Codoped Perovskites

We investigated broadband-sensitive upconversion (UC) processes in a series of Tm- and Ni-sensitized ABO(3) (A = Ca/Sr/Ba and B = Ti/Zr/Hf) perovskites. We have designed combinations of the sensitizers and host cations such that super broad solar radiation ranging from 900 nm to nearly 2000 nm can be efficiently upconverted to 800 nm and shorter wavelengths. The Ni2+ ions located at the center of O2- octahedra absorbed photons in the 900-1500 nm range and transferred those energies to the nearby Tm3+ ions. The Tm3+ ions upconverted those energies at 800 nm, along with the energies absorbed by themselves in the 1100-1250 and 1550-2000 nm ranges, exhibiting super broadband sensitivity. Among the ABO(3):Tm, Ni (A = Ca/Sr/Ba and B = Ti/Zr/Hf) upconverters, CaTiO3:Tm, Ni exhibited the best performance due to its most distorted crystal structure, which intensified the emission and absorption extents by increasing the optical transition probabilities of Tm3+ and Ni2+ ions. Introduction of alkali ions at the Ca2+ sites and Nb5+ ions at the Ti4+ sites intensified the UC emission by many folds, mainly due to a charge balance mechanism. At the same time, bigger and smaller codoped alkali ions created an asymmetric crystal field around the active ions and further enhanced the UC emission. Importantly, the upconverted photons are within the absorption edges of GaAs, Cu2ZnSnS4, and dye-sensitized solar cells making wider applications of these upconverters besides crystalline Si solar cells.