Laura Hrostea

Etude des propriétés de transport optique et électronique de certains films minces (basés sur polymères conjugués) pour des applications de cellules solaires organiques: Compte tenu de la crise énergétique mondiale et de la volonté de protéger la planète en réduisant les émissions de dioxyde de carbone et l'effet de serre, le remplacement des sources d'énergie conventionnelles par des sources non-conventionnelles prime, tant dans le domaine technologique, ainsi que dans celui de la recherche scientifique. Axée principalement sur l'énergie solaire, la thèse présente des notions approfondies sur les processus physiques et les phénomènes sous-jacents à la conversion de l'énergie solaire en électricité, par exemple,l'effet photovoltaïque ou les phénomènes de transport de charges électriques dans un matériau organique semi-conducteur. L'étude expérimentale est basée sur l'investigation de couches minces de polymères conjugués (P3HT et PBDBT-SF) déposé par centrifugation sur des substrats en verre ou en oxyde d’indium dopé à l'étain (ITO), sous diverses formes et structures: monocouche, mélangé avec des fullerènes (PCBM) et des non-fullerènes (IT-4F), ou chimiquement sensibilisé par chlorure de fer. Les structures ont été caractérisées optiquement (spectrophotométrie et ellipsométrie), structurellement (diffractométrie aux rayons X, spectroscopie aux rayons X), morphologique (microscopie à force atomique) et électrique (extraction des charges électriques par augmentation linéaire de la tension - CELIV). Les objectifs de cette étude sont de mettre en évidence les propriétés du polymère PBDB-T-SF par rapport au P3HT et de comprendre les relations entre les propriétés physiques possédées dans différentes structures, d'identifier les limites et les possibilités de son amélioration afin d'augmenter l'efficacité d'une cellule solaire.

The ellipsometric analyses reported in this paper present a novelty by bringing an in-depth optical investigation of some ternary organic blends. This study focuses on the tunability and control of the relative permittivity of active layers by varying the weight ratio of blended materials spin-coated as thin films. To investigate this, an extensive approach based on spectroscopic ellipsometry was conducted on ternary blend (D:A₁:A₂) thin films, involving a donor [D = chlorinated conjugated polymer (PBDB-T-2Cl)] and two acceptor materials [A₁ = a non-fullerene (ITIC-F) and A₂ = a fullerene (PCBM)]. The refractive index constitutes a key parameter that exposes insights into the feasibility of photovoltaic cells by predicting the trajectory of light as it transits the device. In this term, higher obtained refractive indexes support higher absorption coefficients. Notably, the dielectric constant can be rigorously tuned and finely calibrated by modest variations in the quantity of the third element, resulting in considerable modifications. Moreover, the inclusion of fullerene in blends, as the third element, results in a smooth topographical profile, further refining the surface of the film. From an electrical point of view, the ternary blends outperform the polymer thin films. The synergistic interaction of constituents emphasizes their potential to enhance solar conversion devices.

This study examines the optimal parameters for obtaining fluorine-doped SnO 2 (FTO) films with promising potential for photovoltaic applications. Due to its properties, tin oxide is used in a wide range of technologies, among which the manufacture of solar cells is one of the most important. Being doped with fluorine, tin dioxide becomes a good transparent and conductive electrode, suitable for solar cell applications. The chemical stability and low cost of the doped SnO 2 makes it an advantageous alternative to tin-doped indium oxide (ITO). Among the most important characteristics of FTO thin films are high photoconductivity under sunlight irradiation and strong UV absorption. The SnO 2 compound, doped with fluorine, exhibits a considerable chemical and physical stability, good electrical conductivity and high transmission (over 85%) in the visible range. The spray pyrolysis technique is the most preferable and efficient deposition method of fluorine-doped SnO2 thin films. This work aims to identify the optimal parameters for the spray pyrolysis of SnO 2:F films and to analyze the morphology, transparency and strength of as obtained films in relation to the doping amount in the precursor solution, spraying distance and film thickness.

Highly transparent thin films with the chemical formula BaSrMgWO₆ were deposited by spin coating using a solution of nitrates of Ba, Sr, and Mg and ammonium paratungstate in dimethylformamide with a Ba:Sr:Mg:W ratio = 1:1:1:1. XRD, SEM, EDX, and XPS investigations evidenced that annealing at 800 °C for 1 h results in an amorphous structure having a precipitate on its surface, and that supplementary annealing at 850 °C for 45 min forms a nanocrystalline structure and dissolves a portion of the precipitates. A textured double perovskite cubic structure (61.9%) was found, decorated with tetragonal and cubic impurity phases (12.7%), such as BaO₂, SrO₂, and MgO, and an under-stoichiometric phase (24.4%) with the chemical formula Ba_2−(x+y) Sr_xMg_yWO₅. From transmittance measurements, the values of the optical band gap were estimated for the amorphous (E_gdir = 5.21 eV, E_gind = 3.85 eV) and nanocrystalline (E_gdir = 4.69 eV, E_gind = 3.77 eV) phases. The presence of a lattice disorder was indicated by the high Urbach energy values and weak absorption tail energies. A decrease in their values was observed and attributed to the crystallization process, lattice strain diminution, and cation redistribution.

Highly transparent thin films with the chemical formula BaSrMgWO6 were deposited by spin coating using a solution of nitrates of Ba, Sr and Mg and ammonium paratungstate in dimethylformamide with a ratio Ba:Sr:Mg;W=1:1:1:1. XRD, SEM, EDX and XPS investigations evidenced that an annealing at 800 ºC for 1h leaves the structure amorphous with a precipitate on the surface and that a supplementary annealing at 850 ºC for 45 min forms a nanocrystalline structure and dissolves a part of precipitates. It was evidenced a textured double perovskite cubic structure (61.9%) decorated with tetragonal and cubic impurity phases (12.7 %) like BaO2, SrO2 and MgO and an under-stoichiometric phase (24.4%) with the chemical formula Ba2-(x+y) SrxMgyWO5. From transmittance measurements the values of optical band gap were estimated for the amorphous (Eg= 5.15) and nanocrystalline (Eg=4.58 eV) phases. The presence of a lattice disorder was indicated by the high values of Urbach and weak absorption tail energies A decrease in their values was observed and attributed to the crystallization process and cation redistribution.

In this paper, the physical properties of a new series of multilayer structures of oxide/metal/oxide type deposited on glass and plastic substrates were studied in the context of their use as transparent conductive layers for solar cells. The optical properties of TiO₂/Ag/TiO₂, TiO₂:Nb/Ag/TiO₂:Nb and NiO/Ag/NiO tri-layers were investigated by spectrophotometry and ellipsometry. Optimized ellipsometric modeling was employed in order to correlate the optical and electrical properties with the ones obtained by direct measurements. The wetting surface properties of single layers (TiO₂, TiO₂:Nb and NiO) and tri-layers (TiO₂/Ag/TiO₂ TiO₂:Nb/Ag/TiO₂:Nb and NiO/Ag/NiO) were also studied and good correlations were obtained with their morphological properties.

The electrical and optical properties stability of poly(3-hexylthiophene) (P3HT) thin films sensitized with nitromethane ferric chloride (FeCl₃) solution was investigated. The optical properties modifications were studied by spectrophotometry and ellipsometry. For electrical characterizations, electrical resistivity measurements were performed. In agreement with the observations of other authors, an important decrease in the electrical resistivity by six orders of magnitude was noticed. In addition, the repeatability and stability of this phenomenon were investigated over a few weeks after sensitization and during different cycles of heating and cooling, both in the dark and under illumination.