Three-photon quantum cutting infrared emission based on two-step energy transfers in Er3+ doped bi-perovskite Ca2ScSbO6 phosphors: Mechanism and efficiency

Quantum cutting (QC) materials still attract significant attention due to their high quantum and energy efficiencies, which stem from the effective utilization of the excitation energy. In this study, a one-to-three QC 1544 nm emission was first developed in an Er3+ single-doped Ca2ScSbO6 phosphor system. The Ca2ScSbO6 host was selected since it possesses moderate maximum-phonon energy, which suppresses non-radiative relaxation while maintaining phonon-assisted energy transfers between Er3+ ions. Spectroscopic measurements elucidated the QC mechanisms that two-step energy transfers, including ET1: 4S3/2(2H11/2) + 4I15/2 → 4I9/2 + 4I13/2 and ET2: 4I9/2 + 4I15/2 → 4I13/2 + 4I13/2 processes, are responsible for the three-photon generation. The radiative transitions, non-radiative relaxations, and energy transfers of pertinent levels were taken into consideration when calculating the QC efficiencies for Er3+ doped Ca2ScSbO6 phosphors with varying Er3+ concentrations. The concerned radiative transition rates of Er3+ in Ca2ScSbO6 were calculated in the framework of Judd-Ofelt theory, while the non-radiative transition rates were derived based on the energy gap law. The maximum energy transfer efficiencies for ET1 and ET2 were determined to be 99% and 93%. Finally, the QC efficiencies for Er3+ doped Ca2ScSbO6 phosphors were calculated, and the maximum value was confirmed in the 20 mol. % Er3+ doped sample to be 232%. The primary reason for the deviation of the QC efficiencies from the theoretical maximum value of 300% was attributed to the fluorescence self-quenching of Er3+.