Broadband-sensitized upconversion of ATiO3: Er, Ni (A = Mg, Ca, Sr, Ba)

Upconverters that utilize two or more low energy photons to generate a single high energy photon are promising materials for solar energy conversion. Herein, we present a broadband-sensitive upconverter to utilize broad solar spectrum ranging from 1060 to 1650nm which is not utilized by present crystalline Si (c-Si) solar cells. Our calculation shows that the broadband-sensitive upconverters designed can increase the efficiency of c-Si solar cell by similar to 4.8%, considering the present value of similar to 25% in the optimized c-Si solar cell. We used octahedrally oxygen-coordinated Ni2+ ions to harvest 1060-1500nm photons and transferred the absorbed energies to the Er3+ ions. Those photons along with the 1450-1650nm photons absorbed by the Er3+ ions themselves are upconverted to 980 nm, which is efficiently utilized by c-Si solar cells. We optimized the efficiency of the broadband-sensitive upconverters by monitoring host cations and active-ions (Ni2+ and Er3+) concentrations. Absorption and Stokes emission band positions of Ni2+ changed remarkably depending on the A-site cations in the ATiO(3) (A = Mg, Ca, Sr, Ba) hosts making difference in the Ni2+ to Er3+ energy transfer efficiencies and hence the overall upconversion (UC) emission intensities. Further, absorption and emission intensities of the Ni2+ and Er3+ ions largely pronounced in the CaTiO3 host compared to the CaZrO3 due to more distorted nature of the CaTiO3 lattice. Intense Ni2+ emission with larger Stokes shift favored efficient Ni-to-Er energy transfer in the forward direction with minimum-energy back transfer making more intense Er3+ UC emission in the CaTiO3: Er3+, Ni2+ upconverter. Thus, to realize efficient broadband-sensitive UC, it is essential to design a host material with low symmetry lattice to confirm higher emission efficiency of Er3+ and controlled Ni2+ absorption and emission bands to suppress the energy back transfer while maintaining efficient energy transfer in the forward direction. (C) 2017 The Ceramic Society of Japan. All rights reserved.