Efficient blue light-emitting diodes from a soluble poly (para-phenylene) internal field emission measurement of the energy gap in semiconducting polymers (original) (raw)
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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.
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.
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.
Synthetic Metals, 2003
The dependence of the linear optical properties of some soluble poly(terphenylenevinylene)s on the relative amount of cis-and trans-vinylene units has been investigated. A regular blue-shift of the absorption spectra in solution has been observed upon the addition of cis double bonds. Luminescence spectra show a vibronic structure which accounts for the reduced interring torsion in the excited state with respect to the ground state. Moreover, an increased electron-phonon coupling for cis-rich polymers has been observed.
Chemical Physics, 1996
We present an experimental investigation of the spectral, field and concentration dependence as well as the ultrafast transient behavior of electric field-induced photoluminescence quenching in tris(stilbene)amine (TSA) doped into polycarbonate (PC) and different polystyrene (PS) derivatives. The results are interpreted in terms of field-assisted dissociation of neutral excitations with a binding energy of E b ~-0.4 eV. Comparison with corresponding experimental data obtained previously on the conjugated polymer poly(phenyl-p-phenylene vinylene) (PPPV) supports the notion that PPPV can be treated as an ensemble of subunits with 'molecular' electronic properties. In view of the striking similarity between the behavior found in TSA and PPPV a band type model for PPV type conjugated polymers seems inappropriate.
Journal of Applied Polymer Science, 2009
Two classes of light emitting Poly(p-phenylenevinylene) (PPV) based conjugated-nonconjugated copolymers (CNCPs) have been synthesized. The conjugated chromophores containing 2-methoxy-5-(2 0-ethylhexyloxy)-1,4-phenylenevinylene (MEHPV) and 2,5-dimethyl-1,4-phenylenevinylene (DMPV) moieties are rigid segments and nonconjugated portion containing hexyl units are flexible in nature. All copolymers were synthesized by wellknown Wittig reaction between the appropriate bisphosphonium salts and the dialdehyde monomers. The resulting polymers were found to be readily soluble in common organic solvents like chloroform, THF and chlorobenzene. The effect of chromophore substituents on the optical and redox properties of the copolymers has been investigated. Color tuning was carried out by varying the molar percentage of the comonomers. The UV-Vis absorption and PL emission of the copolymers were in the range 314-395 nm and 494-536 nm respectively. All the polymers show good thermal stability. Polymer light-emitting diodes (PLEDs) were fabricated in ITO/PEDOT:PSS/emitting polymer/ cathode configurations of selected polymers using doublelayer, LiF/Al cathode structure. The emission maxima of the polymers were around 499-536 nm, which is a bluegreen part of the color spectrum. The threshold voltages of the EL polymers were in the range of 5.4-6.2 V. V
Site-selective fluorescence studies of poly(p-phenylene vinylene) and its derivatives
Physical Review B, 1996
We report fluorescence measurements on poly(p-phenylene vinylene͒, PPV, and four derivatives of this polymer, all of which show strong luminescence and can be used as emissive materials in electroluminescent diodes. We measure the variation of the emission spectrum with excitation energy at low temperature, and find a threshold energy above which emission is independent of excitation energy and below which the emission energy tracks with the excitation energy. This information makes it possible to separate out the effects of spectral diffusion by exciton migration from other forms of excited-state relaxation. We find that PPV and two derivatives with asymmetric, branches side chains show little or no excited-state relaxation. In contrast, the other two derivatives ͑one with bromine and dodecyloxy attachments at the two and five positions on the phenylene, the other with hexyloxy attachments at these sites, and cyano groups at the vinylic carbons͒ show further relaxation by about 0.25 eV. We consider that emission in these two polymers is from an interchain excimer excited state. Supporting evidence for the cyano-PPV is seen in the differences between the dilute solution and solid-state fluorescence spectra.
Excitation transfer processes in a phosphor-doped poly(p-phenylene vinylene) light-emitting diode
Physical Review B
We present experimental measurements and theoretical calculations of the electrical and optical properties of phosphor-doped poly ͑p-phenylene vinylene͒ light-emitting diodes to determine the excitation processes that lead to radiative recombination from the phosphor molecule. Three possible phosphor excitation processes are considered: ͑1͒ sequential electron and hole capture by the phosphor, ͑2͒ energy transfer from the polymer triplet exciton ͑Dexter transfer͒, and ͑3͒ energy transfer from the polymer singlet exciton ͑Förster transfer͒. The properties of the doped polymer are investigated for doping levels up to about 20 wt %. At the highest doping density, all radiative recombination occurs in the phosphor molecule and the observed electroluminescence decay time increases significantly compared to the undoped polymer. Built-in potential and current-voltage measurements indicate that the electron and hole energy levels of the phosphor are outside the energy gap of the polymer, and that the phosphor molecule does not capture either individual electrons or holes. Measurements of triplet optical absorption show that the triplet population in the polymer is not affected by the presence of the phosphor, indicating that Dexter transfer processes are weak. Calculations of the triplet opticalabsorption cross section combined with the measurements of the triplet optical absorption determine the triplet exciton density in the device. In an analogous chemically substituted polymer, no significant excitation transfer occurs when there is no overlap between the emission spectrum of the polymer and the absorption spectrum of the phosphor. These results demonstrate that the dominant excitation transfer path from the polymer to the phosphor is dipole-dipole ͑Förster͒ coupling. Calculations of the charged and neutral electronic excitation energies of the polymer and phosphor are performed using hybrid and time-dependent, density-functional theory. The results of these calculations show why Förster transfer is strong in this system, and why the other two transfer processes do not take place.