All in NaY(WO4)₂:Er³⁺/Yb³⁺: Quantum cutting, upconversion, and temperature sensing

Introducing light conversion materials into silicon-based photovoltaic devices is one of the effective ways to improve the photoelectric conversion efficiency. Light conversion materials include quantum cutting materials and upconversion materials. The purpose of introducing quantum cutting materials is to divide a short wavelength photon into two or more photons that can join the photoelectric conversion in silicon-based photovoltaic devices. The purpose of introducing upconversion materials is to combine two or more infrared photons into one photon that can also be used for photoelectric conversion in silicon-based photovoltaic devices. The introduction of light conversion materials can improve the photoelectric conversion efficiency without changing the performance of silicon-based solar cells themselves. This way can greatly reduce the technical difficulty of improving the efficiency of silicon-based photovoltaic systems. In addition, silicon-based photovoltaic devices are exposed to sunlight, so their temperature must be managed. In order to manage the temperature, it should be measured in advance. It can be imagined that if three materials that can individually achieve quantum cutting, upconversion, and temperature sensing are simultaneously introduced into silicon-based solar cells, it will lead to difficulties in solar cell structure design and unnecessary increase in the product costs. Therefore, finding and developing high-performance materials that combine the above three functions will be a great challenge.

In a new paper published in Light Science & Application, a bunch of authors from the School of Science, Dalian Maritime University presented that highly efficient photo split, nearly pure infrared upconversion emission, and suitable temperature sensing for thermal management in silicon-based solar cells were achieved by adjusting the doping concentrations of Er³⁺ and Yb³⁺ in NaY(WO₄)₂ phosphor. It was revealed that this all-in-one material is an excellent candidate for application in silicon-based solar cells for improving their photoelectric conversion efficiency and enhancing their heat management.

An in-depth understanding to the quantum cutting mechanism is significant for designing and assessing the quantum cutting materials. However, in many cases, quantum cutting processes are complicated and hard to know in detail. In this work, the authors decrypted subtly the photo-splitting steps in Er³⁺/Yb³⁺ co-doped NaY(WO₄)₂ in assistance of the doping-concentration-dependent spectroscopy and fluorescence dynamics. The authors concluded that “Based on the optical spectroscopic analyses, the quantum cutting mechanism was discovered, and the photon splitting process includes two-step energy transfer processes, namely, ⁴S_3/2+²F_7/2 ®⁴I_11/2 +²F_5/2 and ⁴I_11/2 + ²F_7/2® ⁴I_15/2 + ²F_5/2”.

The quantum cutting efficiency can be confirmed experimentally and theoretically. In the ideal case, the measured quantum cutting efficiency is also defined as the internal quantum efficiency, but it is different from the traditional definition of internal quantum efficiency. The measuring technique for the quantum efficiencies is still not satisfactory since the measuring results are interfered by too many uncontrollable factors. Therefore, the theoretical internal quantum cutting efficiency becomes significant. The authors claimed that “The quantum cutting mechanism was discovered by the optical spectroscopic analyses, and the quantum cutting efficiencies were calculated in assistance of Judd-Ofelt theory, Föster-Dexter theory, energy gap law”. The authors estimated the internal quantum cutting efficiencies for NaY(WO₄)₂: Er³⁺/Yb³⁺ by taking radiative transitions, non-radiative transitions, and energy transfers into account, and an efficiency as high as 173% was achieved.

Another selling point of this work is that nearly pure near-infrared emission of Yb³⁺ was acquired. The authors addressed that  “These upconversion mechanisms tell us that both Er³⁺ and Er³⁺/Yb³⁺ doped NaY(WO₄)₂ phosphors exhibit strong near-infrared emissions from ⁴I_11/2®⁴I_15/2 of Er³⁺ and ²F_5/2®²F_7/2 of Yb³⁺ that indicates the studied phosphors are good light conversion candidate for silicon-based solar cell applications”.

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