Downconversion for solar cells in YF3:Pr3+, Yb3+

Abstract

Energy losses in solar cells caused by the spectral mismatch can be reduced by adapting the solar spectrum using a downconversion material where one higher energy visible photon is 'cut' into two lower energy near-infrared photons that both can be absorbed by the solar cell. Downconversion with the (Pr3+, Yb3+) couple in YF3 is investigated. Based on analysis of luminescence and diffuse reflectance spectra it is evident that two-step energy transfer takes place from the 3P0 level of Pr3+ (around 490 nm) exciting two Yb3+ to the 2F5/2 level giving emission around 980 nm. The transfer efficiency increases with Yb3+ concentration and is 86% for YF3 doped with 0.5% Pr3+ and 30% Yb3+. Due to concentration quenching the intensity of emission from Yb3+ is strongly reduced and the 2F5/2 emission intensity reaches a maximum for the sample with 0.5% Pr3+ and 2-5% Yb3+ at 300 K. Temperature dependent measurements reveal the role of the Pr3+ 1G4 level in the energy transfer between Pr3+ and Yb3+. Back-transfer of excitation energy from the Yb3+ 2F5/2 level to the 1G4 level of Pr3+ occurs and quenches the Yb3+ emission. The quenching is shown to become more efficient between 4 and 50 K due to faster phonon-assisted energy transfer between the Yb3+ donors. Upon raising the temperature from 50 to 300 K, the luminescence life time of the Yb3+ emission increases again because the small energy difference between the Pr3+ (1G4) level and the Yb3+ (2F5/2) level (∼300 cm-1) which makes the 1G4 less efficient as a trap for the excitation energy. The present results give insight into factors involved in the concentration quenching in downconversion materials based on the (Pr3+, Yb3+) couple