J. J. Eilers, D. Biner, J. T. van Wijngaarden, K. Krämer, H.-U. Güdel and A. Meijerink
Efficient visible to infrared quantum cutting through downconversion with the Er³⁺–Yb³⁺ couple in Cs₃Y₂Br₉
Appl. Phys. Lett. 96, 151106/1-3 (2010)                 

Abstract: Downconversion of one visible photon to two near-infrared photons may increase the efficiency of c-Si solar cells by 30%. The lanthanide ion couple Er³⁺–Yb³⁺ is well known for efficient upconversion but for the reverse process, downconversion, fast multiphonon relaxation from the ⁴F_7/2 level has been shown to compete with downconversion. Here we report efficient downconversion for the Er–Yb couple in Cs₃Y₂Br₉. The low phonon energy in this bromide host suppresses multiphonon relaxation and efficient two step energy transfer from the ⁴F_7/2 level of Er³⁺ is observed and results in strong 1000 nm emission from Yb³⁺. Based on emission spectra and luminescence life time measurements an intrinsic downconversion efficiency close to 200% is determined.