even metallic periodic arrays [7-9] to aid the emitted light escape the bulk of the device. Phosphors are the materials most commonly used in solid-state lighting. [10] They usually consist of inorganic matrices doped with rare-earth (RE) cations. RE phosphors offer high thermal and chemical stability and are used as color-converting materials along with blue or UV light-emitting diodes (LEDs) to obtain emissions that cover different ranges of the visible spectrum. This approach has allowed the development of artificial illumination, featuring lower cost and more efficient devices, and rendering better quality lighting for the last two decades. Apart from illumination, the field of use of LED-based devices is wide, from sensors to wireless communication, passing through human-centric lighting or horticulture industry. [11] On this basis, directional and spectral control of the out-coupled light intensity are features of interest for many applications. Such

Fluoride-based compounds doped with rare-earth cations are the preferred choice of materials to achieve efficient upconversion, of interest for a plethora of applications ranging from bioimaging to energy harvesting. Herein, we demonstrate a simple route to fabricate bright upconverting films that are transparent, self-standing, flexible, and emit different colors. Starting from the solvothermal synthesis of uniform and colloidally stable yttrium fluoride nanoparticles doped with Yb 3+ and Er 3+ , Ho 3+ , or Tm 3+ , we find the experimental conditions to process the nanophosphors as optical quality films of controlled thickness between few hundreds of nanometers and several micrometers. A thorough analysis of both structural and photophysical properties of films annealed at different temperatures reveals a tradeoff between the oxidation of the matrix, which transitions through an oxyfluoride crystal phase, and the efficiency of the upconversion photoluminescence process. It represents a significant step forward in the understanding of the fundamental properties of upconverting materials and can be leveraged for the optimization of upconversion systems in general. We prove bright multicolor upconversion photoluminescence in oxyfluoride-based phosphor transparent films upon excitation with a 980 nm laser for both rigid and flexible versions of the layers, being possible to use the latter to coat surfaces of arbitrary shape. Our results pave the way toward the development of upconverting coatings that can be conveniently integrated in applications that demand a large degree of versatility.