Downconversion for solar cells in NaYF4:Er, Yb

Downconversion is a promising avenue to boost the efficiency of solar cells by absorbing one higher energy visible photon and emitting two lower energy near-infrared (NIR) photons. Here the efficiency of downconversion for the (Er3+, Yb3+) couple is investigated in NaYF4, a well-known host lattice for efficient upconversion with (Er3+, Yb3+). Analysis of the excitation and emission spectra for NaYF4 doped with 1% Er3+ and codoped with 0%, 5%, 10%, or 30% Yb3+ show that visible to NIR downconversion is inefficient. Downconversion by the scheme based on the reverse of the upconversion process is hampered by fast multiphonon relaxation from the F-4(7/2) level (the starting level for downconversion) to the S-4(3/2) level. Energy transfer from the S-4(3/2) level of Er3+ to Yb3+ is shown to be inefficient. Efficient downconversion from the (4)G(11/2) of Er3+ level is observed, resulting in emission of two photons (one around 980 nm and one around 650 nm) after absorption of a single 380 nm photon. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3177257]