Preparation and Optimization of Thin Films of Rare-Earth Doped Carbon-Silica Systems for Luminescent Applications
The uses of rare-earth (RE) elements in luminescence materials led to important improvements in a variety of applications. In lighting, for example, YAG yellow phosphors (Y3Al5O12:Ce3+) are widely used for white LED devices. In telecommunication and internet systems, erbium (Er3+)-doped fiber amplifiers are used in the telecom wavelength region (1530-1550 nm). In laser, YAG:Nd3+ are used in solid-state lasers with an emission line at 1.06 μm. YAG can also be doped with Tm3+ (1.93-2.04 μm) or Er3+ (2.94 μm) and are mainly used in medical applications. In solar cells, spectral converter layers are designed to increase the efficiency of solar cells. The AM1.5G solar spectrum can be modified by processes called up-conversion, quantum cutting, and down-shifting. Thus, it would improve the EQE of solar cells. The central study of this work was the preparation and optimization of the luminescence of Tb3+ and Yb3+ ions in the relatively new a-SiOC:H matrix for luminescent applications as spectral downconverters. The method used for luminescence optimization is based on annealing treatments, matrix composition and co-doping with Tb and Yb elements. Experimental details are presented in Chapter 3. The first goal of this work focused on the preparation of a-SiOC:H layers that can be grown on the surface of a test solar cell (Si cell + antireflective layer). The literature reports previous works on SiOC. However, most of them are mainly based on preparation at temperatures close to 1000 °C, which would lead to damage in solar cell due to the high temperature. For this reason, an RF-sputtering system with substrate cooling was used. The matrix optical properties depend on the Si, C, and O composition. So, Chapter 4 focus on the study of (1) composition and structure analysis, (2) the bandgap and Urbach energy calculation, (3) the effect of the annealing treatment on the atomic network structure, and (4) the processes driving the matrix luminescence. The second goal of this work was the optimization of the annealing temperature and RE(Tb, Yb) concentration to improve the photoluminescence of the Tb3+ and Yb3+ ions in single doped a-SiOC:H samples. An increasing RE luminescence requires (1) to increase the number of active RE3+ ions, (2) to reduce the sources of energy loss, and (3) to increase the number of sensitizers. Chapter 5 examines those processes in detail. In the literature, the RE3+ ion emission is well reported. However, there is limited discussion of the mechanisms of non-radiative energy transfer from defect states to RE dopants in silicon-based amorphous materials. Furthermore, the RE3+ ion in such materials will not replace any of the ions in the matrix as it does in the case of RE doped crystalline materials. In amorphous materials (containing oxygen), the RE3+ ion will be located at a site locally surrounded by oxygen atoms. Hence, this work studies and proposes excitation mechanisms for Tb3+ and Yb3+ ions. These mechanisms can be applied to the a-SiOC:H matrix and be extended to Si and SiOx-based materials. Finally, previous studies of the effect of carbon showed an enhancement of the luminescence of RE3+ ions. Therefore, a systematic study of luminescence was carried out based on changing the carbon composition. This work seeks to study the advantages and disadvantages of carbon doping in activating RE luminescence. The results are also presented in Chapter 5. The third goal of this work was the optimization of the annealing temperature and RE(Tb, Yb) concentration to improve the Yb luminescence in co-doped a-SiOC:H samples. Among different RE3+ ions systems for near-infrared quantum cutting (QC) applications, those including Yb3+ ions seem to be the most appropriate because the Yb3+ ion has a transition at about 980 nm (~1.22 eV) just above the crystalline silicon bandgap of 1.1 eV. The Tb3+ ions (ion donor) are used to enhance the Yb emission. Thus, near-infrared downconverter (DC) layers have found a possible application in silicon solar cells. The spectral converter layer will be put on the top of the solar cell surface, incorporating the DC layer leads to the advantage of a layer that can be optimized independently of the cell. This work identifies and studies the mechanisms associated with the energy transfer from the host to the RE3+ ions. Photoluminescence quantum yield of the spectral converter layer was estimated. Also, the layers were prepared on the front surface of mc-silicon cells, and their effect on the external quantum efficiency was studied. In addition, the cooperative transfer mechanism between Tb3+ and Yb3+ ions were studied, which are used to explain the QC process. This cooperative mechanism has been attributed to QC processes in many crystalline and amorphous matrices. However, despite numerous documents that ascribe their results to the cooperative transfer mechanism, few actually prove it. Therefore, the role of the Tb3+ ions in the Yb luminescence is also discussed. The results are presented in Chapter 6.