Visible-to-infrared quantum cutting by phonon-assisted energy transfer in YPO4:Tm3+, Yb3+ phosphors

Quantum cutting (QC) of one visible photon into two infrared ones has been reported for the lanthanide ion couple (Tm3+, Yb3+) in a variety of host lattices. The mechanism responsible for QC was assumed to be a cooperative energy transfer (ET) process from Tm3+ to two Yb3+ ions, however, no solid evidence was presented. Herein we report visible-to-infrared QC for (Tm3+, Yb3+) in YPO4 phosphors. The ET process from the excited (1)G(4) level of Tm3+ to Yb3+ was investigated in detail by means of optical spectroscopy. By monitoring the steady-state photoluminescence (PL) and PL decay of the intermediate F-3(4) level of Tm3+ as a function of the Yb3+ concentration, we demonstrated the QC of one incident blue photon into one near-infrared emitting photon at 1004 nm from Yb3+ and simultaneously into one mid-infrared emitting photon at 1791 nm from Tm3+, rather than two emitting photons from Yb3+. It was revealed that such visible-to-infrared QC was induced by phonon-assisted ET instead of cooperative ET as previously reported. This kind of QC phosphors may have potential as solar spectral converters to enhance the external quantum efficiency in multi-junction solar cells based on narrow band-gap semiconductors such as Ge, PbS or In1-xGaxN.