Resolving puzzles in conjugated polymer photophysics: nanoseconds to microseconds

Thesis (Ph. D.)--University of Rochester. Department of Materials Science, 2017.Conjugated polymers are active candidates of research for their potential applications in organic electronic devices. This thesis contains experimental results of the photophysical behaviour of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) in the nanoseconds timescale. It is generally accepted that quantum yield of films are less than their solution counterparts and has a non-exponential “temporal tail” in the decay dynamics. This thesis reports the spectroscopy, excitation wavelength dependence, temperature dependence and electric field quenching of the temporal tail of the photoluminescence in MEH-PPV on a nanosecond timescale. We try to understand if a correlation exists between the “temporal tail” and quantum yield in films. We conclude that the tail represents emission from H-like aggregated regions in the polymer. Using a simple model of the photophysics, we estimate the formation yield of the aggregates responsible for the tail emission and conclude that they cannot account for the large reduction in fluorescence observed in densely packed films relative to that in solution. If recombination of injected charges in an OLED follows quantum spin statistics and all recombination forms the lowest excited states in a spin manifold, then the ratio of formed singlet to triplet exciton would be 1:3 setting an efficiency limit of 25% to the EL emission. However, in conjugated polymers, various studies have reported the formation ratio of singlet to triplet excitons to be greater than 1:3. These results have ambiguity due to issues of magnetoresistance, TTA, charge trapping, outcoupling etc. We try to reexamine the singlet-triplet branching ratio in Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt- (benzo [2,1,3] thiadiazol-4,8-diyl) (F8BT) along with MEH-PPV with the hope of circumventing these problems. This approach involves simultaneous detection of singlet and triplet state luminescence and then estimate the recombination branching ratios directly from the change in singlet and triplet signals produced by applying magnetic fields. The role of PP in F8BT films are addressed based on studying the effect of external magnetic field on the DF and triplet signals of gold nanoparticle doped films. This work is unique and important because it includes a delay dependent magnetic field effect study of the DF suggesting that spin randomization of charge pairs is a fast process that occurs on the several hundred nanoseconds time scale. The magnetic field effect on the fluorescence persists to tens of microseconds and we argue that this apparent inconsistency implies the existence of an intermediate state in the recombination process that has implications for measurements and theoretical treatments of the singlet recombination fraction in electroluminescent conjugated polymers