Electric field-induced photoluminescence quenching in molecularly doped polymer light-emitting diodes (original) (raw)
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Electronic excitations in luminescent conjugated polymers
Solid State Communications, 1997
We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W−1 and peak brightnesses up to 90 000 cd m−2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.
On the luminescence efficiency of polymer light-emitting diodes: a quantum-chemical investigation
Journal of Photochemistry and Photobiology A: Chemistry, 2001
Correlated quantum-chemical techniques are applied to the description of electronic excitations in luminescent conjugated polymers. We first address the role of intermolecular interactions on the emission properties of organic conjugated materials. The nature of the lowest excited states in molecular aggregates is discussed and a special emphasis is devoted to the chain-length dependence of the exciton coupling. By applying a molecular orbital perturbation approach, we then calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer processes. Application of our approach to a model system for poly (paraphenylenevinylene) shows that the ratio between the electroluminescence and photoluminescence quantum yields generally exceeds the 25% spin-degeneracy statistical limit.
Voltage-Dependent Luminescence Properties of Molecularly Doped Polymer System
Journal of Solid State Chemistry, 2001
Single-layer light-emitting diodes (LEDs) are fabricated using a mixture of a blue-emitting polymer and green-emitting 9, 10bis(phenylethynyl)anthracene as emitting layer. The blend device with these two components in the emitting layer exhibits voltageinduced evolution of the electroluminescence. But when polystyrene is also blended into the emitting layer, the EL spectra show emission bands from both ether}PPV and BPEA in proportion to concentrations of the two materials, and the spectra exhibit no change with applied voltage. This implies that doping inert polymer is helpful in suppressing voltage-induced evolution of electroluminescence in LED blends.
Journal of Photochemistry and Photobiology A: Chemistry, 2008
This paper describes the optical properties of an electroluminescent poly(phenylene vinylene-cofluorenylene vinylene) (BPPPV-PF)-based -conjugated polymer using absorption, photoluminescence (PL), time-resolved photoluminescence (TRPL), continuous wave (CW) and transient-photoinduced absorption (PA) spectroscopic techniques. The TRPL decay spectra of BPPPV-PF in chloroform and film exhibit single exponential decay with PL lifetimes of 0.8 ns and 0.2 ns, respectively. The CW-PA spectrum exhibits a single well-defined band centred at 1.5 eV, which is assigned to the T-T* transition of the triplet excitons. The monomolecular lifetime ( ) of the triplet excitons was estimated from the intensity dependence of CW-PA and found to be ∼1.2 ms at 80 K. The temperature dependence of the CW-PA signal at 1.5 eV was studied for a temperature range from 80 K to 298 K. It was observed that the PA signal at 1.5 eV was persistent to relatively high temperatures, which may be due to the bulky side chain of the polymer and morphology of the film. The relaxation process of this triplet excitation was studied by measuring the transient decay of the PA signal for various temperatures. The transient-PA signal shows monomolecular recombination process at all recorded temperatures and the temperature dependence of monomolecular lifetime ( ) was studied. It was found that the monomolecular lifetime ( ) is dependent on temperature but independent of the laser excitation intensity. The obtained results are analyzed based on the molecular structure of the PPV-PF copolymer.
The efficiency and time-dependence of luminescence from poly (p-phenylene vinylene) and derivatives
Chemical Physics Letters, 1993
We report measurements of the quantum efficiency and time decay of photoluminescence in the conjugated polymers poly(g phenylenevinylene) (PPV) and poly(Z-methoxy, 5-(2'ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV). MEH-PPV is soluble and we measure values for the quantum yield for luminescence of order 35% for dilute solutions in toluene and chloroform. By comparison of luminescence decay rates in solution and in solid films we estimate luminescence efticiencies in solid films, which can be as high as 50% in partially conjugated PPV. Decay time distribution analysis of the luminescence reveals a broad distribution of decay rates, and this is consistent with the distribution of conjugation lengths known to be present in these materials. Exciton migration in better conjugated material results in narrower distributions of emitting chromophores.
Luminescence efficiency and time dependence in a high electron affinity conjugated polymer
Synthetic Metals, 1996
We report measurements of the efficiency and time dependence of photoluminescence in a high electron affinity cyano-substituted derivative of poly (p-phenylenevinylene). In solution the photoluminescence quantum yield is 0.52 ± 0.05, and the luminescence lifetime is 0.9 ± 0.1 ns. In solid films the luminescence quantum yield is 0.35±0.03 and the luminescence lifetime, of 5.6 ± 0.2 ns, is much longer. These results strongly suggest that the emission from the film is from an inter-chain excitation, and that inter-molecular interactions are an important factor to consider in the design of highly luminescent conjugated polymers.
Photophysical study of a conjugated–non-conjugated PPV-type electroluminescent copolymer
Polymer, 2005
This paper reports the photo-and electroluminescence studies of emitting species of one of the first reported blue emitters, the conjugatednon-conjugated multi-block copolymer, poly[1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene-1,2-ethenylene-3,5-dimethoxy-1,4-phenylene]. Because the conjugation length of the emissive center is very well defined (two and half phenylene-vinylene units) the differences found between the fluorescence profile of the fluorophore in solution, at several concentrations and that in the solid state allowed us to conclude that in solid the emission comes from associated forms, such as ground-state dimers and/or excimers. Time-resolved fluorescence in a nanosecond time scale recorded at 6 em Z24,096 cm K1 showed a monoexponential decay of 1.5 ns, which is compatible with rigid forms of stilbene derivatives. q
Synthetic Metals, 1996
Polymer light·emitting diodes with poly(2·decyloxy·l.4-phenylene), DO·PPP, as the luminescent polymer emit blue light with external quantum efficieny of "" 3%. Using internal field emission (Fowler-Nordheim tunneling) in single carrier devices, the top of the 'IT band and the bottom of the 'IT* band have been determined for DO·PPP, and for poly(2·methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV. For DO-PPP, the single particle 'IT-1T* energy gap is 3.4-3.6 eV; for MEH-PPV, the 'IT-'lT* energy gap is 2.2-2.3 eV. Comparison with optical absorption (the optical gap) implies that the exciton binding energy is small; for both polymers, Ecxc ~ 0.2 e V. Such small residual values are probably not significant in the presence of disorder broadening which is also 0.1-0.2 eV and in the presence oflattice relaxation with self-localization energies of approximately 0.2 eV.
The Journal of Physical Chemistry B, 2010
The effects of electric field on absorption and photoluminescence (PL) of films of sulfide-substituted PPV derivative S3-PPV, poly[2-(phenyl)-3-(4′-(3,7-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene-co-2-(11′-decyl sulfanylundecanyloxy)-5-methoxy-1,4-phenylene vinylene], were investigated. Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of S3-PPV show the Stark shifts, indicating a significant alternation in the molecular polarizability (∆R j) associated with the optical transitions. Field-induced enhancement or quenching is also observed for PL of S3-PPV, depending on the photoexcitation energy, whereas the shape of the PL spectra is independent of the excitation wavelength. The field effects on the decay profiles of PL indicate that the quenching results from a diminished population of the emitting states on excitation at 300 nm, whereas the PL is enhanced on excitation at 471 nm because the emitting state has an increased lifetime. The efficiency of field-assisted generation of electron-hole pairs produced through excitons monotonically increases with increasing excitation energy, and the nonradiative decay rate in the emitting state is diminished by electric fields in S3-PPV. The photoirradiation of S3-PPV in ambient air resulted in rapid degradation of the polymer film.