Near infrared emissions from both high efficient quantum cutting (173%) and nearly-pure-color upconversion in NaY(WO4)2:Er3+/Yb3+ with thermal management capability for silicon-based solar cells.

Abstract

Raising photoelectric conversion efficiency and enhancing heat management are two critical concerns for silicon-based solar cells. In this work, efficient Yb³⁺ infrared emissions from both quantum cutting and upconversion were demonstrated by adjusting Er³⁺ and Yb³⁺ concentrations, and thermo-manage-applicable temperature sensing based on the luminescence intensity ratio of two super-low thermal quenching levels was discovered in an Er³⁺/Yb³⁺ co-doped tungstate system. The quantum cutting mechanism was clearly decrypted as a two-step energy transfer process from Er³⁺ to Yb³⁺. The two-step energy transfer efficiencies, the radiative and nonradiative transition rates of all interested 4 f levels of Er³⁺ in NaY(WO₄)₂ were confirmed in the framework of Föster-Dexter theory, Judd-Ofelt theory, and energy gap law, and based on these obtained efficiencies and rates the quantum cutting efficiency was furthermore determined to be as high as 173% in NaY(WO₄)₂: 5 mol% Er³⁺/50 mol% Yb³⁺ sample. Strong and nearly pure infrared upconversion emission of Yb³⁺ under 1550 nm excitation was achieved in Er³⁺/Yb³⁺ co-doped NaY(WO₄)₂ by adjusting Yb³⁺ doping concentrations. The Yb³⁺ induced infrared upconversion emission enhancement was attributed to the efficient energy transfer ⁴I_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_15/2 (Er³⁺) + ²F_5/2 (Yb³⁺) and large nonradiative relaxation rate of ⁴I_9/2. Analysis on the temperature sensing indicated that the NaY(WO₄)₂:Er³⁺/Yb³⁺ serves well the solar cells as thermos-managing material. Moreover, it was confirmed that the fluorescence thermal quenching of ²H_11/2/⁴S_3/2 was caused by the nonradiative relaxation of ⁴S_3/2. All the obtained results suggest that NaY(WO₄)₂:Er³⁺/Yb³⁺ is an excellent material for silicon-based solar cells to improve photoelectric conversion efficiency and thermal management.