Photoluminescence quantum efficiency (PLQE) and PL decay characteristics of polymeric light emitting materials (original) (raw)
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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.
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.
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.
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.
Efficiency of radiative emission from thin films of a light-emitting conjugated polymer
Physical Review B, 2001
We examine the efficiency of radiative emission from thin layers of light-emitting conjugated polymers. We compare our experimental results for photoluminescence of the conjugated polymer poly͑2-methoxy, 5-͑2Ј-ethyl-hexyloxy͒ 1,4 phenylenevinylene͒ ͑MEH-PPV͒ with those of a theoretical model, finding good agreement between the two. The specially developed model takes into account several factors including absorption in the emissive layer, a spread of emitter sites within the layer, and the broad emission spectrum of the polymer. We find that the photoluminescence quantum efficiency for radiative emission of a bare MEH-PPV film on a glass substrate is ϳ25%. We then apply our model to study electroluminescent devices. We show that for these structures the efficiency of radiative emission is ϳ10%. There is thus potential for considerable improvement in efficiency for both systems through recovery of some of the wasted waveguided light. Finally we use our model to reexamine some controversial results that indicate the probability of singlet exciton formation to be 0.4Ϯ0.05, and thus greater than the 0.25 expected from spin statistics. Our reanalysis supports a probability Ͼ0.25. We conclude by discussing the limitations of present models, including our own, in predicting the performance of realistic light-emitting diodes.
Improvement of luminescence efficiency and photostability in polymer thin films
Thin Solid Films, 2000
The photodegradation mechanism and luminescence ef®ciency of a series of thin polymer ®lms prepared under a variety of conditions was studied. The conjugated polymer, poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV), is shown by infra-red spectroscopy to degrade via the chain scission of the carbon double bond along the polymer backbone. This causes a reduction in conjugation length and a blue shift in its absorption and photoluminescence (PL) spectra. To reduce photodegradation effects, ®lms were prepared using argon (Ar) gas and were investigated in air and an oxygen free environment. The initial PL intensity increased by over 70% for Ar treated ®lms. The PL decay in air was bi-exponential in nature, with a sharp initial decay linked to atmospheric oxygen, and a longer second decay linked to oxygen embedded in the ®lm. The increase in both PL ef®ciency and degradation lifetime, coupled with device encapsulation, should signi®cantly improve the performance of electroluminescent devices. q
The Journal of Physical Chemistry C, 2009
To investigate the local environment's effect on the lifetime and quantum yield of extended polymer chains in the absence of intra-and interchain aggregation, short, rodlike polymers of poly(2,5-di-n-octyloxy-1,4phenylenevinylene) (DO-PPV) were dissolved in chloroform and then embedded in a polystyrene matrix. The fluorescence lifetime was found to increase by 45% in moving from the solution to the matrix form. By using the absorption and emission spectra of the chloroform solution to estimate the radiative and nonradiative rate constants for the polymer in solution, along with calculations based on an exciton model, the corresponding decay rate constants for the polymer embedded in the matrix were obtained. The close agreement between the calculated and experimental values of fluorescent lifetime in the matrix proved the applicability of the exciton model used. On the basis of the model, the average quantum yield of isolated polymers in the matrix was calculated to be a factor of 2 higher than in solutionsan effect arising from a 59% decrease in the nonradiative rate constant and, to a smaller extent, from a 20% increase in the radiative decay rate due to the different dielectric constants of the environments. These results suggest that by extending and isolating single luminescent polymers, high quantum yield devices are possible.
Photoinduced photoluminescence intensity enhancement in poly"p-phenylene vinylene… films
We report measurements of photoluminescence ͑PL͒ intensity enhancement in poly͑p-phenylene vinylene͒ ͑PPV͒ films induced by light irradiation in the presence of air. This effect is dependent on laser intensity and the ratio between film thickness and excitation penetration depth. The results suggest that an efficient spectral diffusion of excited carriers to nondegraded PPV segments by Förster energy transfer is an important consideration in the PL efficiency of conjugated polymers light-irradiated in air.
Polydispersity of the photoluminescence quantum yield in single conjugated polymer chains
Chemical Physics, 2005
The conjugated polymer poly(2-methoxy-5-(2 0 -ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) was studied by a single-molecule imaging technique. A comparison of statistical distributions of fluorescence intensity with molecular weight distributions revealed that the distribution of the photoluminescence quantum yield of the single polymer chains under study is significantly asymmetric, with a polydispersity J 2. The result implies that there are molecules whose quantum yield is a few times higher than the ensemble quantum yield. This conclusion suggests a possibility of a great improvement of the photoluminescence quantum yield of MEH-PPV, which is known to be several times less than 1.