This thesis discusses structural and ferroelectric properties of two well-known classes of materials, perovskite oxides and Hydrogen bonded ferroelectrics, using first-principles calculations. Certain aspects of first principles calculations are central to the problems presented in this thesis. Such as the ability to calculate polarization based on the modern theory of polarization and calculation of ferroelectric property under finite electric displacement field. Therefore, these fundamental theoretical approaches are discussed following an opening section on the basic methodology of density-functional theory. In addition to the discussion on theoretical methods, a brief review of different phenomena and techniques crucial to alter/enhance ferroelectric properties at the interfaces of perovskite materials has been presented along with examples. The first problem presented in this thesis proposes and validates an alternative quantitative measure of ferroelectric(FE) and antiferrodistortive(AFD) instabilities by means of calculating inverse capacitance and layer inverse capacitance of layered perovskites. The presented methodological approach is applied to BaTiO$_{3}$/CaTiO$_{3}$ and PbTiO$_{3}$/SrTiO$_{3}$ superlattices and it precisely estimates FE and AFD instabilities. Here we also present an approach to accurately predict the ferroelectric instabilities in large period superlattices from the statistical coefficients obtained from short period superlattices. In the second problem, we study ferroelectricity in an organic crystal(croconic acid) for which ferroelectric polarization is close to that of bulk BaTiO$_{3}$. We employ new meta-GGA functional named SCAN and revisit all structural and ferroelectric properties. Calculated X-ray absorption spectra(XAS) qualitatively and quantitatively agrees well with experimental O K-edge spectra. By discussing the origin of each XAS peak and their characteristic we demonstrate with a systematic approach the connection between ferroelectricity and XAS in croconic acid. Best to our knowledge such relation has not been realized in past. This study could prove XAS as a new way to measure ferroelectric instability in hydrogen-bonded organic ferroelectrics. / Physics

Dans cette thèse sont développées les synthèses et caractérisations de nouveaux polymères conjugués pour des applications dans l’électronique organique. Ces polymères ont été synthétisés via des réactions de polymérisation sans utilisation de métaux de transition. Des polyazomethines à base de carbazole ont ainsi été synthétisés par polycondensation entre des carbazole portant des fonctions amine et aldéhyde en positions 2,7 et 3,6. Leurs propriétés optiques et électroniques ont été étudiées en fonction de la position des fonctions imines ainsi formées. Un comonomère de type EDOT a ensuite été intégré dans le polymère et l’impact de ce comonomère sur les propriétés du copolymère ainsi formé a été étudié.Des polymères à base d’acide squarique et croconique ont ensuite été synthétisés. En faisant varierles conditions de synthèse, les propriétés optoélectroniques ont pu être contrôlées, permettant d’obtenir des composés présentant une émission blanche, qui ont ensuite été intégrés en tant que couche active dans des dispositifs de type OLED.Enfin, des polymères plus originaux ont été étudiés, utilisant des réactions de polymérisation originale, permettant par exemple la formation de benzobisthiazole in situ. D’autres polymères ont été synthétisés en intégrant dans leur chaine des monomères originaux, comme la tetrazine ou la divanilline. Les propriétés optoélectroniques de ces composés ont ensuite été étudiées en vue deleur éventuelle intégration dans des dispositifs. / In this work, synthesis and characterizations of new conjugated polymers are described.These polymers, developed for their integration into devices, have been synthesized via transitionmetalfree polymerizations. Carbazole based polyazomethines have been synthesized via polycondensation reactions between di-substituted carbazoles, bearing amino and formyl functionsin positions 3,6 or 2,7. Optical and electronical properties of such polymers have been studieddepending of the linkage position. A comonomer EDOT has then been integrated into the polymer chain, and impact of such insertion has been studied. Squaric and croconic acid base polymers have also been synthesized. By varying polymerization conditions, optoelectronic properties have been tuned, leading to the formation of polymers exhibiting a white emission. These polymers have then been integrated into OLED, as the active layer. Finally, more original polymers have been synthesized, using more original reactions or monomers such as by forming in situ benzobisthiazole. Other polymers integrating more originals monomers, such a tetrazine or divanillin, have been synthesized. Optoelectronic properties of such materials have been studied for the purpose of their integration into devices.