Computational study of light harvesting materials for applications in organic solar cells (Doctoral thesis)

Τράπαλης, Χαράλαμπος

The objective of this dissertation is to investigate morphological and structural characteristics of molecules used in the active material of organic photovoltaics using computational methods. In a rudimentary description organic photovoltaics consist of two electrodes and the active material which is placed between them. The active material contains a molecule as electron donor and a molecule as an electron acceptor. Incident light in the active material excites an electron of the donor from HOMO to LUMO which coupled with a hole they create an exciton. The exciton diffuses through the material and when it interacts with an acceptor molecule the electron and hole dissociate. The electron is directed through the acceptor towards one electrode and the hole through the donor towards the other. The efficiency of the process depends on the band gap between the donor and the acceptor, the diffusion length of the excitons, the morphology of the samples and the existence of additives or impurities. In order to improve the efficiency of the photovoltaic devices new molecular structures, and samples with varying concentration are tested to create improved morphologies in the active material. In addition, metallic nanoparticles are used to increase the efficiency through the phenomenon of localized surface plasmon resonance (LSPR). The mixtures of the active material are amorphous and it is challenging for the usual methods of microscopy to offer insight about the morphology at the atomistic level. This dissertation develops two methods for the production of parameters about (a) the dihedral angles for the oligomers (tetramer and octamer) of thiophene and 3-hexyl-thiphene (TH4, TH8, 3HT4, 3HT8) and (b) the interaction of organic molecules with the surface of metals. The FCC(111) surface of silver was chosen as the metal. A series of computational methods was developed for the mixing of molecules and the deposition to the surface. These methods were used to study 3 molecules (a) 3-hexyl-thiophene octamer (3HT8) as a model for poly (3-hexyl thiophene)(P3HT) which is an electron donor as well as (b) phenyl-C61-butyric acid methyl ester (PCBM) and (c) three isomers of indene C70 bis-adduct (ICBA) that are used as acceptors. The investigation of 3HT8 and PCBM on the surface of silver highlighted five important findings for the interaction of the molecules. The interaction of 3HT8 with the silver surface was stronger than that of PCBM. The interaction between 3HT8 molecules is weak. The interaction of each PCBM with the surface was strong and analogous to the interaction with other PCBM molecules. The interaction of each PCBM with the rest of the PCBM molecules was strong. The distribution of the distance of oxygens at the chain of PCBM from the surface indicated two favorable conformation around 4 Å and 12 Å from the surface. This result was independent of the deposition method. The oxygens at 4 Å correspond to conformations with the chain near the surface while the oxygens at 12 Å include conformation that have positioned the chain further away from the surface to accommodate more PCBM molecules near the surface. Finally, characterization of mixtures for ICBA isomers was performed. The morphology of the samples was determined as amorphous while the experimental differences cannot be associated with differences between the isomers.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Πολυτεχνική Σχολή. Τμήμα Μηχανικών Επιστήμης Υλικών
Subject classification: Φωτοβολταϊκά δυναμικά συστήματα
Keywords: Οργανικά φωτοβολταϊκά,Προσομοιώσεις,Μοριακή δυναμική,Θειοφαίνεια,Ολιγμοερή,Φουλερένια,Άργυρος,Δότες ηλεκτρονίων,Δέκτες ηλεκτρονίων,Organic photovoltaics,Simulations,Molecular dynamics,Thiophenes,Oligomers,Fullerenes,PCBM,ICBA,Silver,Electron donors,Electron acceptors
Appears in Collections:Διδακτορικές Διατριβές

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