The efficiency and time-dependence of luminescence from poly (p-phenylene vinylene) and derivatives (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.
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.
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.
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.
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.
Synthetic Metals, 2005
We report photoluminescence quantum efficiency (PLQE) measurements on thin films of a variety of in-house synthesized conjugated polymers such as PPV, MEH-PPV and CN-PPV with improved techniques using integrating sphere and synchronous detection. Our method allows use of low level of excitation avoiding problems due to degradation during measurement. Time correlated single photon counting (TCSPC) has been used to study PL decay to obtain radiative lifetime controlling efficiency in these materials. The measured PL efficiency in PPV synthesized by xanthate precursor route is measured to be 0.07 ± 0.01, which is lower than reported for similar films owing to presence of intrinsic defects. The measured PL quantum efficiency of CN-PPV and MEH-PPV are 0.24 ± 0.07 and 0.17 ± 0.01, respectively. These values are comparable to the ones reported by others [
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.
Synthetic Metals, 1996
Processible conjugated polymers can be used to fabricate a range of thin-film diodes which can be designed to show good characteristics both as electroluminescent diodes and as photoconductive diodes. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and, thus, bring about electron-hole capture to generate excitons at this interface. We present results on the photophysical properties of the cyanosubstituted poly(phenylenevinylene)s which provide electron transport layers if these heterostructure devices; we find evidence for strong interchain interactions (excimer formation) for the neutral excited states in these polymers, with strong red shifts of the luminescence, though preserving high photoluminescence efficiencies.