Titre de la dissertation:“Using Computational Methods to Describe Heat Transport Properties in Organic Materials: A Promising Route Towards Thermoelectric Applications”.

Promoteur: Monsieur Jérôme CORNIL

Résumé de la dissertation:Faced with the challenge of climate change and dwindling fossil fuels, the need for developing technologies that rely on sustainable and renewable energy sources has rapidly emerged as one of the most important goals Humanity ever had to achieve. In this ingoing quest for finding new energy recovery systems, the field of thermoelectric applications has triggered a growing interest in the scientific community during the last decades.Thermoelectricity relies on the ability of a material to convert heat into electricity or on the reverse process and offers a great potential for applications in power generation as well as solid-state cooling and heating. The energy conversion efficiency is usually quantified by the dimensionless figure of merit zT = σS²T/(κ_l+κ_e) where σ, S, κ_l and κ_e stand for the electrical conductivity [S cm^-1], the Seebeck coefficient [V K^-1], and the lattice and electronic thermal conductivities [W m^-1 K^-1], respectively. Nowadays, commercial modules are built with materials such as bismuth telluride (Bi₂Te₃) that can act both as p-type and n-type materials in the thermocouple as a function of the nature of the atomic species in excess.However, the fact that they display a high cost of production, a limited range of applications due to their brittleness together with dramatic impact on environment has recently motivated the use of doped organic semiconductors. Unlike their inorganic counterparts, conducting polymers and small  molecule organic materials combine a high mechanical flexibility with an eco-friendly and cost-effective processability, which render them particularly attractive for the development of state-of-the-art thermoelectric applications over large areas. To this aim, thermoelectric properties of various polymer semiconductors have been measured, with the highest zT values reported at 0.42 (p-type) for PEDOT:PSS and 0.3 (n-type) for poly (Ni-ethylenetetrathiolate) at room temperature, thus approaching those of inorganic materials (zT of Bi₂Te₃ is around unity).Whereas the charge transport processes in organic semiconductors have been thoroughly investigated over the past decade, the description of their thermal properties remains a domain scarcely explored up to now, both experimentally and theoretically. This is a critical point because it must be borne in mind that the operating efficiency of any thermoelectric device is strongly influenced by thermal effects; as proof, active layers with a high thermal conductivity would detrimentally affect the overall figure of merit zT.In the light of these considerations, it is clear that the design of novel materials for organic thermoelectric applications is still cruelly limited by a lack of data on thermal transport. To fill this gap, the main objective of this thesis work is twofold: (i) to perform a critical comparison of different theoretical approaches giving access to thermal conductivity values; (ii) to contribute to the establishment of structure-property relationships (via a joint theoretical and experimental investigation) that will further guide the synthesis of high performing organic-based thermoelectrics.To achieve this goal, the general approach followed in this work consists in using and comparing the three most widely known atomistic molecular dynamics (MD) algorithms to estimate the thermal conductivity of an organic prototypical system. In the second part, the most suitable MD approach selected from the previous benchmarking has been used to rationalize experimental measurements of thermal properties of promising molecules for the fabrication of highly-efficient thermoelectric devices. In particular, two classes of compounds have captured our attention: dinaphto-thienothiophene derivatives (C_n-DNTT-C_n with n = 0,8) and benzothieno-benzothiophene derivatives (C_n-BTBT-C_n with n = 0,8) that recently emerge as top-class organic semiconductors.These joint theoretical and experimental studies weree made possible by a close collaboration with the group of Henning Sirringhaus (University of Cambridge, United Kingdom) and the group of Dominique Vuillaume (Institute for Electronics Microelectronics and Nanotechnology, France), which have now developed an expertise in measuring the thermal conductivity of organic thin films by means of the 3ω–Völklein method and scanning thermal microscopy, respectively. These systematic studies reveal that the in-plane thermal conductivity is significantly reduced by adding alkyl chains on the terminal aromatic rings of the fused conjugated core, by strongly localizing vibrational modes with respect to unsubstituted conjugated cores, thus pointing to strong anharmonicity effects.