Broadband-sensitized upconversion of ATiO<sub>3</sub>:Er,Ni (A = Mg, Ca, Sr, Ba)

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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 1650 nm 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 ∼4.8%, considering the present value of ∼25% in the optimized c-Si solar cell. We used octahedrally oxygen-coordinated Ni2+ ions to harvest 1060–1500 nm photons and transferred the absorbed energies to the Er3+ ions. Those photons along with the 1450–1650 nm 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 ATiO3 (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.

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