The influence of interchain interactions on the photophysics of conjugated polymers

Abstract

This thesis presents findings on the photoluminescence properties of two conjugated polymers with special interest paid to interchain interactions and chemical degradation. The delayed luminescence of thin films of poly(3-methyl-4-octyl-thiophene) (PMOT) has been observed via time-resolved photoluminescence spectroscopy (TRS) in different temperature regimes. While at 15 K the emission consists of delayed fluorescence (DF) and phosphorescence (Ph) originating from radiatively decaying singlet and triplet excitons, room temperature emission is red-shifted with respect to the DF and originates from excimers. Supportingly, photo-induced absorption experiments detect very long-lived excitations allocated to the triplet counterparts of the emitting excimers. Furthermore, the influence of the photo-oxidisation of polyfluorenes on their luminescence is investigated. A series of polyfluorene/polyfluorenone co-polymers (PFO/PFl) with varying percentages of fluorenone repeat units was analysed via TRS as well as steady-state absorption, photoluminescence and photoexcitation spectroscopy in solid state and solution. Fluorenone repeat units (or keto defects) arising from polyfluorene photo-oxidation are the origin of the green luminescence band affecting the colour purity and quantum yield of polyfluorene devices. Fluorenone triplets play a major role and can be directly photo-excited. Moreover, they form upon polyfluorene degradation. Efficient energy transfer takes place from PFO singlets to fluorenone triplets entailing two types of triplet-triplet interaction processes, which lead to the formation of fluorenone excimers as well as polyfluorene-fluorenone exciplexes. Two types of the green emission can be assigned to emission from these states. This could be confirmed by the decay kinetics, thermal behaviour and keto level dependence of the emissions.