Energy migration and charge formation in conjugated polymers
Abstract: The main part of the thesis is concerned with charge photogeneration in conjugated polymers. MeLPPP and POMeOPT films were investigated by transient absorption spectroscopy using different excitation photon energies and light intensities with and without an external electric field. We conclude that electric field-assisted charge pair photogeneration occurs from vibrationally relaxed singlet excitons during their entire lifetime without any intermediate states in MeLPPP. This charge pair formation takes place in this manner even in the absence of an external electric field in POMeOPT. High photon energy excitation opens additional channels of charge pair generation such as dissociation of higher excited states and hot exciton dissociation in MeLPPP, while in POMeOPT the photon energy has no effect at all. We have suggested two models of charge photogeneration in conjugated polymers: the random site model for MeLPPP type of charge photogeneration and the dissociation site model for POMeOPT. These two models or a combination of them may be useful to address photoinduced charge generation in other conjugated polymers. From optical studies of Stark shift dynamics we estimate the distance between electron and hole in MeLPPP. Charge pairs may be separated by 5 - 8 Å immediately after they are created from neutral excitations and then the average distance increases to ~35 Å during 600 ps, where they can be considered as separated charges. In addition, energy transfer was studied in conjugated polymer (PTOPT, POMeOPT) films by transient absorption anisotropy with ~20 fs time resolution. Computer simulation by an incoherent energy migration model reproduces the experimental kinetics in PTOPT for times longer than ~100 fs. Singlet-singlet exciton annihilation was also observed by transient absorption spectroscopy under high peak excitation intensities at low repetition rate, resulting in a typical nonexponential character of the exciton decay kinetics. Singlet-triplet exciton annihilation was studied by fluorescence spectroscopy at high pulse repetition rates in POMeOPT; the annihilation leads to shortening of the singlet exciton lifetime, while monoexponential fluorescence kinetics behaviour is maintained. Finally, the excited electronic states of a polyfluorene-based co-polymer DiO-PFDTBT have been characterized using optical spectroscopy and semiempirical quantum-chemical (ZINDO) calculations.
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