1：Experimental Design and Method Selection

The study involved the synthesis of NaYF4 and NaYF4:Eu3+ nanocrystals via a solvothermal method, followed by their deposition on the non-conducting side of FTO glass to form a down-conversion layer. The perovskite solar cells were then fabricated with this layer to evaluate its impact on photovoltaic performance.

2：Sample Selection and Data Sources

Fluorine-doped tin oxide conducting glass (FTO) was used as the substrate. Methylammonium iodide (CH3NH3I), PbBr2, CsI, and PbI2 were used for the perovskite layer. Spiro-OMeTAD was used as the hole transport material.

3：List of Experimental Equipment and Materials

X-ray diffraction (XRD, SmartLab 3 kW, Rigaku, Japan), transmission electron microscope (TEM, Tecnai G2 F20), field emission scanning electron microscope (FESEM, SU8010, HITACHI), UV/VIS/NIR spectrophotometer (Lambda950, PE), Keithley 2400 source meter, AAA solar simulator (Newport-94043A), IPCE measurement system (Newport, USA), electrochemical workstation (CHI 660E), Thermo Scientific lumina fluorescence spectrometer, Zennium electrochemical workstation (IM6).

4：Experimental Procedures and Operational Workflow

The NaYF4:Eu3+ layer was spin-coated on the back of the FTO, followed by annealing. The TiO2 ETL was prepared by spin-coating a titanium diisopropoxide dis(acetylacetonate) solution on the FTO, followed by annealing. The perovskite layer was then spin-coated on the TiO2 layer, followed by the deposition of the spiro-OMeTAD layer and Au electrodes.

5：Data Analysis Methods

The photovoltaic performance was analyzed using J-V characteristic curves, IPCE spectra, and electrochemical impedance spectra (EIS). The crystal structure and morphology were analyzed using XRD and TEM, respectively.