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Promoteur de la thèse: Monsieur David Beljonne

Résumé de la dissertation

This work reports on a computational study of the transport of electronic excitations in conjugated polymeric and molecular semiconductors. We have more specifically focused on the diffusion of singlet excitations in films of molecular donor-acceptor dyads, the motion of charges along and between rigid-rod conjugated polymer chains and the recombination of electron-hole pairs at polymer:non-fullerene acceptor heterojunctions, as detailed below.

Comprehensive Modelling Study of Singlet Exciton Diffusion in Donor-Acceptor Dyads: When Small Changes in Chemical Structure Matter. With this study we want to depict the multilevel computational protocol designed to provide a theoretical rationalization for two push-pull small molecule electronic and excitonic properties at the solid-state. Classical Force Field (FF) Molecular Dynamics (MD) simulations plus Time Dependent Density Functional Theory (TD-DFT) and Micro-Eletrostatic calculations were performed to evaluate conformational and the electrostatic component to the first singlet excited state energetic disorder. Then, we evaluated the internal and external reorganization energies as well as the excitonic couplings between neighboring molecules that were injected into the energy transfer rate expression taken from the MLJ. Based on these rates, Kinetic Monte Carlo (KMC) simulations were performed which yielded the diffusion coefficient and the exciton diffusion length.
Charge transport in TIFBT and IDTBT: Role of electronic connectivity. We have performed a theoretical study of charge transport in poly(indacenodithiophene-alt-benzothiadiazole): IDTBT and poly (dithiophene indenofluorene-alt-benzothiadiazole): TIFBT polymers, which show similar conformational resilience to torsions but differ in the functional sub-units. KMC simulations were employed on the atomistic input, ie, polymer morphologies built from atomistic molecular dynamic simulations and charge hopping parameters extracted from quantum-chemical calculations, to extract the electric filed dependent hole mobilities. TIFBT shows substantially larger mobilities, with respect to IDTBT, owing to not only the larger transfer integral values but also to the higher electronic connectivity, which enables the formation of percolation channels for charge transport.
Charge-Carrier Mobility along Conjugated Polymer Chains. We present the results of intra-chain charge transport simulations obtained by applying a robust surface hopping algorithm to a phenomenological Hamiltonian parameterized against first-principles simulations. Conformational effects are shown to provide a clear signature in the temperature dependent charge-carrier mobility that complies with recent experimental observations. Interestingly, by considering explicitly the contribution from the nonadiabatic couplings, both the band-like picture and the hopping-like picture can be reproduced. This highlights the crucial influence of nuclei dynamics on electron transport.
The role of charge recombination to spin-triplet excitons in non-fullerene acceptor organic solar cells. DFT and range-separated tuned TD-DFT calculations were performed on different polymer-non-fullerene acceptor complexes to evaluate singlet/triplet excited and CT (charge transfer) state properties. We have calculated the 1CT (singlet) and 3CT (triplet) energies at the equilibrium geometry. We find the energy ordering of the CT states is inverted from that expected when considering exchange interactions, with the 3CT higher than the 1CT by ~70 meV. To explore this further, we have run a rigid scan of the 1CT and 3CT excitation energies as a function of D/A separation. By analyzing the excited-state wavefunctions, we conclude that the inversion of 1CT and 3CT in PTB7-Th:IEICO-2F arises from CTE:LE hybridization. This inversion occurs because the NFA S1 is higher in energy than the 1CT and the NFA T1 is lower than the 3CT; hybridization of these states therefore stabilizes the 1CT and destabilizes the 3CT. We evaluated the internal reorganization energies as well as the excitonic couplings between donor-acceptor that were injected into the energy transfer rate expression derived from the Fermi Golden Rule. Calculations of kBCT (back charge transfer) as a function of intermolecular distance indicate that kBCT is reduced by an order of magnitude through hybridization effects. Implications for voltage losses in organic solar cells are discussed.

La défense publique de la thèse de Monsieur Rexiati Dilimulati aura lieu le 2 avril 2021 à 15h par vidéo-conférence, le lien actif se trouve en haut de cette page.