Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Luminescence properties of Yb 3+ -Tb 3+ co-doped amorphous silicon oxycarbide thin films Loreleyn F. Flores a,b,∗ , Karem Y. Tucto a , Jorge A. Guerra a , Jan A. Töfflinger a , Erick S. Serquen a , Andres Osvet b , Miroslaw Batentschuk b , Albrecht Winnacker b , Rolf Grieseler a , Roland Weingärtner b a Departamento de Ciencias, Sección Física, Pontificia Universidad Católica Del Perú, Av. Universitaria 1801, Lima, 32, Perú b i-MEET, Department of Materials Science and Engineering, University of Erlangen Nürnberg, Martensstr. 7, Erlangen, 91058, Germany ARTICLEINFO Keywords: Silicon oxycarbide Photoluminescence lifetime Ytterbium Terbium rf magnetron sputtering ABSTRACT This work analyzes the photoluminescence emission of Yb 3+ and Tb 3+ ions in co-doped silicon oxycarbide thin films, their activation by thermal treatment, and reveals their luminescent properties regarding the energy transfer between them. Three samples of silicon oxycarbide were prepared by rf magnetron sputtering from SiC, Yb and Tb targets in an oxygen/argon atmosphere. The first one is the undoped silicon oxycarbide sample, the second one is Tb single doped, and the last one is the Yb-Tb co-doped sample. All three samples are identified as silicon oxycarbides with a low carbon content using energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The latter shows the presence of the vibrational modes of Si–C and Si–O bonds. Subsequent annealing treatments up to temperatures of 750 °C led to the rare earths optical activation in the samples. For each annealing step, we present the photoluminescence spectra using an above-bandgap excitation of325nm.TheTb 3+ and Yb 3+ -related luminescence lines were identified. The Yb 3+ ions luminescence for the co-doped sample shows concentration quenching above annealing temperatures of 500 °C whereas the Tb 3+ ions luminescence intensity remains almost constant. The analysis of the photoluminescence excitation spectra shows the direct excitation of the Tb 3+ ions in the Tb-doped samples. A high suppression of the direct excitation of the Tb 3+ ions in the Yb-Tb co-doped samples was observed. The energy transfer from Tb 3+ toYb 3+ ions in co-doped samples is evidenced, first, by the decrease of the Tb 3+ photoluminescence intensity in the co-doped compared to the Tb doped sample. Second, a change from nearly single exponential to nonexponential decay in the Tb 3+ photoluminescence decay curves and third, by the reduction of the Tb 3+ decay time from 1.2 ms in the Tb- doped sample to 0.5 ms in the Yb-Tb co-doped sample. 1. Introduction One of the main limiting factors regarding the conversion efficiency of solar energy into electricity in silicon solar cells is the spectral mis- match. Low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently due to thermalization [1,2].The application of rare earth ions (RE) offers an approach to improve the efficiency of crystalline silicon-based solar cells by using spectral con- verters, based on up- and downconversion (UC and DC). In the case of upconverters, two low energy photons which cannot be absorbed by the solar cell, are absorbed by the RE ions emitting one high energy photon which can be subsequently absorbed by the solar cell [3,4]. In the case of DC one high energy photon is absorbed by the RE ions to emit two lower energy photons which can be efficiently converted by the solar cell [5,6]. Among different RE systems for DC those including Yb 3+ ions seem to be the most appropriate because the Yb 3+ ion has a transition at ∼975nm (1.27eV) from the 2 F 5/2 to the 2 F 7/2 energy level just above the crystalline silicon bandgap of 1.1 eV. As a consequence, the losses by thermalization are diminished in the silicon solar cells. The couple Yb 3+ -Tb 3+ is suitable because the Tb 3+ ion has a transition from the excited state 5 D 4 to the ground state 7 F 6 at about 488nm (2.54eV) which has twice the photon energy of the transition of Yb 3+ ions. The energy transfer from Tb 3+ to Yb 3+ allows two-photon processes, in which the absorption of one high energy photon by Tb 3+ ion may lead to the transfer of energy to two neighboring Yb 3+ ions by cooperative energy transfer (CET) [7]. Another possible energy transfer path is the one-photon process, in which the energy transfer from one Tb 3+ to one https://doi.org/10.1016/j.optmat.2019.04.003 Received 30 November 2018; Received in revised form 12 March 2019; Accepted 1 April 2019 ∗ Corresponding author. Departamento de Ciencias, Sección Física, Pontificia Universidad Católica del Perú, Av. Universitaria 1801, Lima, 32, Perú. E-mail address: flores.lf@pucp.edu.pe (L.F. Flores). Optical Materials 92 (2019) 16–21 0925-3467/ © 2019 Published by Elsevier B.V. T