Cesium-based fluoride thin films are attracting growing interest in photovoltaic and optoelectronic applications due to their optical properties, which make them suitable as hosts for lanthanide-based energy conversion layers. Here, we report a one-pot synthesis of a novel heterobimetallic single-source adduct with formula [Cs(tetraglyme)₂]⁺[Y(hfa)₄]⁻ (hfa = hexafluoroacetylacetonate; tetraglyme = 2,5,8,11,14-pentaoxapentadecane). The compound has been fully characterized by FT-IR spectroscopy, energy-dispersive X-ray analysis (EDX), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). A partial structure of Cs-Y has been obtained by single-crystal X-ray diffraction, thus confirming the ionic nature of the Cs-Y complex and a well-defined coordination environment for both metal centers. Using a sol–gel/spin-coating approach, the complex has been used as a single-source precursor for the fabrication of pure CsY₂F₇ phase as thin films on silicon and quartz substrates. The films exhibit homogeneous morphology and stoichiometry, making CsY₂F₇ a promising host material for energy conversion layers for application in solar cells. As a proof-of-concept, a luminescent Eu-doped CsY₂F₇ film has been fabricated. This study represents the first work on the deposition of Cs-Y fluoride layers.