In theory, the standard AM 1.5 spectrum [6] power conversion efficiency of a single-junction solar cell can be increased up to 38.6% by the application of an ideal down-converter. UV photons (280–
400 nm) account for a 6.2% of the total absorbed in crystalline Si (Eg = 1.12 eV) solar cells (280–1100 nm), and many of these UV photons can be converted theoretically into pairs by highly efficient down-conversion mechanisms and collected by the solar cell. However, the trade-off between DC on one side, and increased front surface reflection losses, concentration quenching, and isotropic luminescence not collected by the solar cell on the other side, should be considered for achieving a net increase in the PV device efficiency [7,8]. To date, rare earths [9] and dyes [5] are the two main active species to be considered as down-converters for Si-based solar cells. In this work, we will focus on rare earths. The first down-converters incorporating lanthanide ions in solar cells were CaF2:Eu(III) single crystals [10] and organically modified silicate composite phosphor films doped with europium(III) [11]. However, poor mechanical properties of the materials and high temperature thermal treatments during the production of solar cells made these approaches unviable [12]. Subsequent experimental results reported a 9.5% increase in efficiency by introducing Eu(III)-doped silica layers formed by sol-gel processes and placed on top of the Si-based solar cells [8], and a 5% increase using KMgF3:Sm obtained by the vertical Bridgman method [13]. Though, these time and energy consuming processes are hardly transferred to the industry.