The energy transfer from Tm2+ to Tm3+, which has not yet been reported before, has now been observed for the first time. It was found that orthorhombic BaCl2:Tm2+,Tm3+ with the PbCl2 cotunnite structure shows the luminescence properties to enable Tm2+→Tm3+ energy transfer. Evaluation of the luminescence properties of BaCl2:Tm2+,Tm3+ in detail and other Tm2+/Tm3+-activated phosphors in general makes clear that the conditions for Tm2+→Tm3+ energy transfer are a strong overlap of the Tm2+ spin-allowed 4f125d1→4f13 emission with Tm3+ 3H6→3F3 or 3H6→3H4 (4f12→4f12) excitations or overlap of the Tm2+ spin-forbidden 4f125d1→4f13 emission with Tm3+ 3H6→3H4 (4f12→4f12) excitation, resulting in both cases in interconfigurational transitions, while the Tm2+ spin-allowed 4f125d1→4f13 emission should not overlap with the Tm2+ spin-forbidden 4f13→4f125d1 excitation. In addition, the Tm2+-Tm3+ distance has to be small, preferably for a high Tm2+ concentration to increase the absorption of excitation radiation in combination with a low Tm3+ concentration in order to avoid concentration quenching of the luminescence. Finally, implications of Tm2+→Tm3+ energy transfer for applications such as luminescent solar concentrators are discussed.

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.