Evidence for triplet interchain polaron pairs and their transformations in polyphenylenevinylene

Abstract

Photoluminescence of films of poly-(p-phenylenevinylene͒ ͑PPV͒, and changes of its intensity under conditions of electron spin resonance as a function of temperature and light intensity were studied. The experimental technique was based on the modulation of the spin state of paramagnetic species by resonant microwave transitions between Zeeman sublevels of dynamically spin-polarized pairs. Three types of resonant signals were found in the magnetic resonance spectrum: ͑i͒ a narrow ͑1.7 mT width at the half height͒, ͑ii͒ a broad ͑140 mT͒ enhancement signal at gϭ2, and ͑iii͒ the signal at gϭ4. The results permitted one to conclude that Coulomb bound polaron pairs are produced in PPV with high yield under 488-nm photoexcitation. The narrow signal is assumed to appear due to microwave-induced resonant transitions in triplet polaron pairs. This implies that the resonant transitions change the rate of geminate recombination of the pairs that leads to the formation of triplet intrachain excitons. Those excitons annihilate in the second-order reaction and show themselves in the intensity of the photoluminescence. The annihilation rate was found to be influenced by resonant transitions in triplet exciton pairs as well and resulted in broad and gϭ4 signals. The lifetime of triplet intrachain excitons was estimated from microwave modulation frequency dependence of resonant signal intensities. The results showed that the energy level of the lowest polaron pair state situated below that of singlet intrachain exciton can act as a sink of the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence, and photoconductivity.

Morphology effects on spin-dependent transport and recombination in polyfluorene thin films

“…The zero-field splitting parameters of the triplet exciton are found to be very similar for the two phases, implying a similar degree of localization of the triplet. The values for D are also consistent with earlier measurements on similar polymers [54].…”

Ultrafast photogeneration mechanisms of triplet states inpara-hexaphenyl

“…Light absorption in such materials results in the generation of intrachain electron-hole pairs, called excitons. 7 Dissociation of excitons is the dominant mechanism of charge-carrier ͑polaron͒ generation following photoexcitation. This process, which can be enhanced by applying an external electric field, is relevant for photovoltaic applications.…”

Visualizing the radical-pair mechanism of molecular magnetic field effects by magnetic resonance induced electrofluorescence to electrophosphorescence interconversion

“…Analogous spin-pair processes can also arise in the solid state, and have been invoked to explain magnetic resonance phenomena and magnetic-field effects in the conductivity of amorphous silicon 11 and molecular organic semiconductor films [12][13][14][15][16][17][18] . Such measurements are, in principle, sensitive down to the single-charge level.…”