Efficient visible to infrared quantum cutting through downconversion with the Er3+-Yb3+ couple in Cs3Y2Br9

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 F-4(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 F-4(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.