(Invited) Up-Conversion Luminescence of Lanthanide Activated Phosphors

Abstract

The major problem that limits solar cells’ efficiency is their insensitivity to the whole solar spectrum which is the so-called spectral mismatch. Therefore, several mechanisms have been explored based on photoluminescence (PL) to convert the solar cell spectrum where the spectral response of the solar cell is low to regions where the spectral response of the solar cell is high. Downconversion, up-conversion (UC) and downshifting are some of the mechanisms that may be applied to improve the spectral response. Upconversion nanoparticles (UCNPs) have shown some promising possibilities to be considered in this respect, however, low UC efficiency of UCNPs is still the most severe limitation of their applications. This study reports on the PL and cathodoluminescence (CL) behaviour of different phosphors. The vastly studied lanthanide pairs in UC are Er3+, Ho3+ and Tm3+ with Yb3+ as a sensitizer. Whereas, very few UC studies have been carried out for Yb3+/Tb3+ and Yb3+/Eu3+ pairs. The reason behind these two categories of lanthanides is the way they transfer their energies. Er3+, Ho3+ and Tm3+ co-doped with Yb3+ can be excited by ground state absorption (GSA), excited-state absorption (ESA) and energy transfer up-conversion (ETU) mechanisms whereas the energy transfer from Yb3+ to Tb3+/Eu3+ is due to co-operative energy transfer (CET). Since, Tb3+ and Eu3+ do not have energy levels which can absorb the near infra-red (NIR) light directly, Yb3+ is the best choice in order to obtain the UC luminescence in Tb3+/ Eu3+ doped system. Yb3+ has a single electronic transition within the 4f subshell. The transition from the lower level through ground state absorption can be easily achieved by using NIR radiation, and resulting energy from two or more excited Yb3+ ions can be utilized in the excitation of a single Tb3+ or Eu3+ ion, for gaining improved UC emission. Doping different host materials with these lanthanide pairs were investigated and tested for possible increase in solar cell efficiency. Power tuneable visible UC and infrared emissions were detected upon excitation with a 980 nm diode laser.