The characterization of a wide range of luminescent thin films can be a long and tedious endeavor. With reactive combinatorial sputtering of multiple metal targets, it possible to fabricate thin films with a gradient in composition simply by not rotating the substrate. In this work, combinatorically sputtered thin films of Cr³⁺ and Nd³⁺ doped in the Al₂O₃–Y₂O₃ system (YAlO) are studied for thin film based luminescent solar concentrators (TFLSCs) application. Contrary to mm's thick plate type LSC's, TFLSCs of just several 100 nm thick require much higher Cr³⁺ concentration to achieve 40% absorption which can enable several 10's of W/m² LSC power efficiencies. Our transmission measurements on a Cr₂O₃ film with a thickness gradient result in an absorption cross section at 460 nm of 1.3 ± 0.7 × 10⁻¹⁹ cm² showing that the TFLSC absorption requirement can be fulfilled provided that the Cr³⁺ concentration is in the order of 10²² ions/cm³. The Y:Al ratio of the YAlO host in our films ranged between 0.5 and 3.5, thereby including the monoclinic (Y₄Al₂O₉), perovskite (YAlO₃) and garnet (Y₃Al₅O₁₂) stoichiometry's on a single film. Position dependent XRD, EDX, excitation, emission and lifetime measurements of Cr³⁺ and Nd³⁺ show that the unique gradient film sputtering method is able to characterize thin films as a function of host composition and doping concentration. Energy transfer between Cr³⁺ and Nd³⁺ in co-doped YAlO films is concluded from Cr³⁺ excitation bands observed while monitoring Nd³⁺ emission and from the matching of the rise-time of Nd³⁺ 1340 nm emission (⁴F_3/2 -> ⁴I_11/2) and the decay time of Cr³⁺ 840 nm emission (⁴T₂ -> ⁴A₂). Nd³⁺ lifetime systematically decreases from 0.24 to 0.05 ms with increasing Cr³⁺ concentration in Y₃Al_5-xCr_xO₁₂:Nd (0.05 < x < 2). The constraints of heavily doped Cr³⁺ thin films for enabling adequate absorption and energy transfer to Nd³⁺ in TFLSC applications are the subjects of the discussion.

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The broad class of Cu(Al,Ga,In) (S,Se,Te)2 solar absorber materials when doped with Yb3+ are interesting for thin film based luminescent solar concentrator (LSC's) application. In this work the strong and broad absorption properties of co-sputtered CuGaS2 (CGS) thin films combined with the luminescent properties of Yb are reported. Energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction, transmission, excitation, and temperature dependent emission as well as radiative lifetime measurements are performed on thin films with varying Cu:Ga ratios and Yb3+ concentrations. It is found that Yb3+ emission can be broadly sensitized by the host in the range of 200–600 nm. A lower Cu:Ga ratio, crystallinity and post annealing in air provides a positive impact on the sensitization of Yb3+ emission. The temperature dependent time integrated decay curves show a clear thermal energy barrier of about 0.2 eV. Because the exponential tail, with a lifetime of 110 μs, is constant with temperature, we conclude that the barrier is connected to the thermal release of electrons trapped at the Yb2+ ground state. The low energy transfer efficiency from the host to the Yb dopant is attributed to efficient non-radiative electron-hole pair recombination. The prospects and design criteria of Cu(Al,Ga,In) (S,Se,Te)2 solar absorber materials for LSC applications is the further subject of the discussion.@en