Quantum tripling in Tm³⁺ doped La₂BaZnO₅ phosphors for efficiency enhancement of small band gap solar cells

The emission of three infrared photons at 1700 nm by Tm³⁺ for each absorbed photon at 465 nm is reported as a function of the Tm³⁺ concentration in La_2−2xTm_2xBaZnO₅ phosphors. It was observed that the intensity of the Tm³⁺³F₄→³H₆ emission with respect to emissions from the ¹G₄ state after ¹G₄ excitation at 465 nm increases when the Tm³⁺ concentration is raised from x=0.001 to x=0.032. This increase is the accumulated effect of four different possibilities for cross-relaxation that can take place between neighboring Tm³⁺ ions, which explains the efficiency of the quantum tripling process. An optimum in the cross-relaxation efficiency was found for the La_1.936Tm_0.064BaZnO₅ phosphor. At higher Tm³⁺ concentrations the ³F₄→³H₆ emission decreases as a result of concentration quenching. Quantum tripling can be used in high power IR phosphor converted LEDs or to increase the efficiency of small band gap solar cells like germanium. - Highlights: • Tm³⁺ doped La₂BaZnO₅ phosphors are prepared. • 1700 nm emission is enhanced with increasing Tm concentration. • The enhanced IR emission is due to increased 4f–4f cross-relaxations. • Cross-relaxations lead to emission of 3 IR photons after 1 blue photon is absorbed. • Quantum tripling can be used to increase efficiency of small band gap solar cells