Evidence for triplet interchain polaron pairs and their transformations in polyphenylenevinylene

Photoluminescence of films of poly-(p-phenylenevinylene) (PPV), 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: (i) a narrow (1.7 mT width at the half height), (ii) a broad (140 mT) enhancement signal at g=2, and (iii) the signal at g=4. 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 g=4 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.