Optical transition properties, 1550-to-980 nm upconversion, and temperature sensing of NaEr(WO₄)₂:Yb³⁺ phosphors synthesized via the microwave-assisted hydrothermal method

Improving photoelectric conversion efficiency and enhancing heat management are two critical considerations for silicon-based solar cells. In this study, efficient Yb 3+ infrared emissions through the upconversion process were achieved by adjusting the concentrations of Yb 3+ in Er 3+ highly condensed NaEr(WO 4 ) 2 phosphor. Additionally, the temperature sensing based on the fluorescence intensity ratio (FIR) was also studied in this tungstate system. Moreover, the radiative transition rates for all relevant transitions of Er 3+ in NaEr(WO 4 ) 2 :Yb 3+ phosphors were calculated in the framework of Judd-Ofelt theory, and the optical transition properties of Yb 3+ were also revealed by taking Er 3+ as a reference. It was found that the radiative transition rate of Yb 3+ : 2 F 5/2 → 2 F 7/2 (2977.52s −1 ) is significantly higher than that of Er 3+ : 4 I 11/2 → 4 I 15/2 (303.50s −1 ), thus suggesting the feasibility for the strong emission at 980 nm of Yb 3+ in assistance of the energy transfer 4 I 11/2 + 2 F 7/2 → 4 I 15/2 + 2 F 5/2 . Finally, strong and nearly pure NIR emissions of Yb 3+ were experimentally observed under 1550 nm excitation, and possible upconversion mechanisms were proposed. The temperature sensing performance of the studied materials was also assessed. All the results imply that NaEr(WO 4 ) 2 :Yb 3+ constitutes an excellent material for enhancing both the photoelectric conversion efficiency and thermal management of silicon-based solar cells.