Downconversion for solar cells in NaYF4:Er,Yb

Abstract

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 4F7/2 level the starting level for downconversion to the 4S3/2 level. Energy transfer from the 4S3/2 level of Er3+ to Yb3+ is shown to be inefficient. Efficient downconversion from the 4G11/2 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.