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 La2−2xTm2xBaZnO5 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 La1.936Tm0.064BaZnO5 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.