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Title: Theoretical studies of charge transfer excitations, absorption, and polarisation in organic photovoltaic materials
Author: Few, Sheridan
ISNI:       0000 0004 5918 5118
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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To optimise organic photovoltaic devices, design rules relating chemical and physical structure to the ability of the active material to absorb light, separate charges and transport charges, are required. We assess the predictive power of computational modelling techniques, comparing results with experiment. In Chapter 4, we calculate the influence of chemical structure on conformation and light absorption in conjugated polymers, and explain the remarkably high optical absorption in poly-thieno[3,2b]-thiophene-diketo-pyrrolopyrrole-co-thiophene (PDPP-TT-T) in terms of its high persistence length. We calculate that diketopyrrolopyrrole does not act as an electron acceptor in PDPP-TT-T or other oligomers, and find no correlation between transition dipole moment and degree of charge transfer between units. Chapter 5 reviews recent developments in modelling charge pair generation, focussing on electronic structure calculations of interfacial states, electrostatic models, excited state dynamics, and remaining challenges in achieving a predictive approach. In Chapter 6, we study the influence of chemical structure and mutual position of oligomer:fullerene pairs on interfacial state properties using time-dependent density functional theory, reproducing energy trends measured by electroluminescence in corresponding polymer:fullerene systems. For several isoindigo and diketopyrrolopyrrole based cooligomers, we calculate a lowest excitation energy close in energy to that of pairs with fullerene. The natures and energies of these pair states are strongly dependent on fullerene position. We calculate the effect of thiophene substitution in PDPP-TT-T, and rationalise observed charge separation efficiency of corresponding experimental systems. In Chapter 7, we calculate the influence of chemical structure on electronic polarisabilities of functionalised fullerenes using the Hartree Fock method, and the influence of molecular polarisability, lattice structure, and direction of charge separation on dielectric constants in C60 and PCBM, using a polarisable dipole model. We calculate the influence of electronic polarisation on charge delocalisation, and its dependence upon chemical structure and packing, and discuss relevance to charge transport properties.
Supervisor: Nelson, Jenny Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available