Enhancing IR to NIR upconversion emission in Er³⁺-sensitized phosphors by adding Yb³⁺ as a highly efficient NIR-emitting center for photovoltaic applications

Near-infrared (NIR) to visible upconversion (UC) luminescence in activator Er³⁺ and sensitizer Yb³⁺ codoped systems has been extensively studied. In this work, we report the results of Er³⁺-sensitized Yb³⁺ NIR UC emission at similar to 1000 nm originating from ²F_5/2 -> ²F_7/2 transition under infrared (IR) excitation at similar to 1500 nm responsible for Er^3+ 4I_15/2 -> ⁴I_13/2 transition absorption. The IR to NIR UC is a desirable strategy to increase solar light utilization by reducing the spectral mismatch between the solar spectrum and crystalline silicon (c-Si) solar cells. Er³⁺ singly doped phosphors are mostly used for realizing the IR to NIR UC emission derived from Er^3+ 4I_11/2 -> ⁴I_15/2 transition centered also at similar to 1000 nm. The efficiency of this transition, however, is limited by multiphonon relaxation and radiative transition to the ⁴I_13/2 state. Here, Yb³⁺ ions are codoped in Y₂O₃:Er³⁺, and NIR UC emission with the intensity enhanced 1.5 times is achieved due to efficient energy transfer from Er³⁺ to Yb³⁺ and highly efficient Yb³⁺ emission. The influence of the dopants' concentrations on UC dynamics is studied. The optimal concentration under IR excitation is 5% Er³⁺ and 1% Yb³⁺. Our results indicate that introducing Yb³⁺ ions in Er³⁺ singly doped phosphors may notably improve the IR to NIR UC performance.