Sm2þ-doped SiAlO sputter deposited coatings for self-absorption and scatter-free luminescent solar concentrator applications

Combinatorial reactive co-sputtering using Al, Si and Sm targets in an Ar þ O2 atmosphere, resulted in Sm doped SiAlO thin films with a wide Sm concentration- and
Si:Al composition gradient. By combining position dependent EDX spectra and laser excited emission spectra, ternary phase diagrams were constructed that directly
show the relation between Sm emission intensity, index of refraction, thickness and composition. Using this approach, the Sm2þ and Sm3þ emission intensity ratio
was controlled towards films with predominantly Sm2þ emission, which is most favorable for luminescent solar concentrator (LSC) applications. The optimum Sm2þ
efficiency was reached when the Al content was about equal to the Sm content. When the Si:Al ratio decreases, the Sm2þ emission intensity strongly drops to almost
zero. However, sputtering without Al resulted in no Sm2þ emission intensity at all. The excitation and emission properties of Sm2þ in the optimized thin films,
especially the ratio between the 4f→4f and 5d→4f emission that is sensitively susceptible to the co-ordination polyhedron, closely resembles that of Sm2þ doped
crystalline powders with the same composition. This strongly suggest that the Sm2þ ions in our amorphous films are coordinated in the same way. A homogeneous
thin film on float glass clearly shows the light concentration effect of the red Sm2þ emission. Due to an unexplained low Sm2þ absorption of our films, even the
optimized thin films do not luminesce brightly.