Downconversion for Solar Cells in YF₃:Pr³⁺, Yb³⁺

Abstract

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. KEYWORDS: downconversion/quantum cuttinglanthanidesluminescencephotovoltaics ACKNOWLEDGMENTS This work is part of the Joint Solar Programme (JSP) of the Stichting voor Fundamenteel Onderzoek der Materie FOM, which is supported financially by Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The JSP is co-financed by Gebied Chemische Wetenschappen of NWO and Stichting Shell Research. Financial support from the 'Energie Onderzoek Subsidie' (EOS) programme of SenterNovem, an agency of the 'Ministerie van Economische Zaken' of the Netherlands, is gratefully acknowledged. Notes The percentages in the bottom row give the (U (t) )2 strength for the 1G4 → 3H4 transitions relative to the sum of all (U (t) )2 values for a given value of t.