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.
Films of poly-(2-phenyl-1,4-phenylene-vinylene) (PPPV) and photodiodes with PPPV as an active layer were studied by optically (ODMR) and electrically (EDMR) detected electron-spin resonance (ESR). Two different signals were observed in ODMR: enhancement of the photoluminescence (PL) at g=2.01 due to recombination of the photogenerated polarons (s=1/2), and a half-field enhancement signal, attributed to the fusion of triplet excitons. Both processes lead to the formation of singlet excitons. The spectral dependence of the s=1/2 signal follows the low energy part of the PL spectrum, indicating that delayed recombination of distant polarons is influenced by ESR, whereas the cw PL contains both prompt and delayed contributions. The linewidth and the intensity of the ODMR signal depend on the PL excitation intensity. Both effects are due to a decrease of the recombination lifetime of the polaron pairs at higher intensities. The relative decrease of the short-circuit photocurrent ISC through a PPPV photodiode by ESR saturation is due to recombination of nonthermalized, nongeminate excess charge polarons in the active layer of the device. This effect is at least two orders of magnitude stronger than the enhancement of total PL at the same temperature. This feature is found to be common for conjugated polymers investigated so far, and reflects the fact that the total photogenerated ISC is spin dependent, whereas ODMR selects only the nongeminate portion of recombining species in the sample.
The new fullerite C60−PΦ4+ · (ClPΦ4)2 was investigated by EPR and static susceptibility. Charge transfer from PΦ4 to C60 was found to be incomplete. C60−PΦ4+ separates into a system of localized and delocalized spins at higher temperatures. At 40 K an Anderson-type localization occurs. The low-temperature susceptibility is shown to be a mixture of a Curie susceptibility and a contribution that resembles the behaviour of a spin singlet with an activated low-lying triplet state. The EPR linewidth below 40 K is only about 400 mG.
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