N. T. Harrison

Current heating in polymer light emitting diodes

We present an investigation of current-induced heating in polymer light emitting diodes. Using short electrical pulse measurements, we were able to quantify the temperature rise in the active region. We consider that heating effects play a major role in limiting the maximum efficiency of devices and in initiating degradation mechanisms. Heating and heat sinking are also discussed in the context of electrically pumped polymer lasers.

Electronic excitations in luminescent conjugated polymers

Abstract 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.

Electronic processes of conjugated polymers in semiconductor device structures

Abstract 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 Lumens/W and peak brightnesses up to 90,000 cd/m 2 . 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.

Enhanced photostability of poly(1,3-phenylene diphenylvinylene)-derivatives by diphenyl-substitution

Abstract We report measurements of the UV-photodegradation of the poly( para -phenylene vinylene) derivatives poly-(4,4′-diphenylene diphenylvinylene) (PDPV) and poly-(1,3-phenylene diphenylvinylene) m -PPV-DP in the presence of oxygen. We find that degradation of both PDPV and m -PPV-DP is slower than that of PPV, and attribute this to the replacement of the vinylene hydrogens by phenyl rings. The photoluminescence decay and the photoluminescence excitation spectra were measured as a function of UV-irradiation time. Initially a reversible luminescence quenching process was observed for films of either PDPV or m -PPV-DP in moist air in the presence of oxygen and/or water. We propose that this arises from conformational changes of the polymer chain produced following optical excitation. At longer irradiation times, FTIR studies reveal that the degradation process is oxidative. This oxidation is rate limited by oxygen diffusion into the film. Models for the photoluminescence properties are presented.

Peak current density and brightness from poly(p-phenylenevinylene) based light-emitting diodes

Abstract We report measurements of peak current and brightness from polymer light-emitting diodes based on poly(p-phenylenevinylene). The devices were operated at low duty cycles and supported peak currents in excess of 700 A/cm2 and produced peak brightnesses of up to 5 Mcd/m2. The values are higher than was previously thought attainable and we discuss them with reference to application in passive matrix displays and polymer laser diodes.

The effects of phenyl-di-substitution of PPV on its photophysical and photostability properties

Abstract We present measurements of photoluminescence (PL) and optical absorption of the phenylated PPV (poly( p -phenylene vinylene)) derivative poly(1;3-phenylene diphenyl-vinylene) ( m -PPV-DP) and the model compound tetraphenylethylene (TPE). We also assess the effect of the di-phenylsubstitution in both m -PPV-DP and PDPV (poly-(4,4′-diphenylene diphenylvinylene)) on their photostability. Our work confirms that the meta-link in m -PPV-DP breaks the conjugation and leaves the polymer with a longest fully conjugated unit identical to the molecule TPE. This allows to transfer our quantum chemical modelling results for TPE to m -PPV-DP and to show that the photophysical properties of the polymer are controlled by ring-torsional relaxation of the excited states rather than by effects of chain-extended conjugation. The UV-photodegradation is shown to be slowed down significantly by the phenylation in comparison to PPV as is evident from the measurements of the PL decay. We show by means of FTIR studies that the degradation is an oxidation process.

Photoinduced absorption and excitation spectra studies of excited states in a cyano-derivative of poly (p-phenylenevinylene)

Abstract We have identified in a cyano-substituted derivative of poly ( p -phenylenevinylene) a broad photoinduced absorption signal at 1.3 eV from a photoexcited state with a lifetime of 0.2 ms at 20 K, which we assign to the triplet exciton. We present here excitation spectra in the range 2–5 eV of the triplet-triplet photoinduced absorption, and photoluminescence signals, which are both broadly flat above the onset of the linear absorption. From these results we conclude that quantum efficiencies of singlet and triplet exciton generation are constant over the excitation range.

High excitation density in light-emitting polymers

Properties and processes in poly (p-phenylenevinylene) are described in the context of lasing structures. In this communication we highlight the high-excitation-density regime, which seems to be the regime where new laser systems/technologies starts. We also illustrate, in the context of polarization memory, how device structure can be used to probe material properties.

Photoluminescence excitation spectra of pristine and photo-oxidised poly(p-phenylenevinylene)

Abstract We present photoluminescence excitation (PLE) spectra for pristine and photo-oxidised samples of poly( p -phenylenevinylene) (PPV). In pristine PPV we observe a constant photoluminescnce efficiency for excitation wavelengths in the range 500-250 nm. In contrast, in photo-oxidised PPV samples the PLE spectrum is very sensitive to the degree of oxidation. In particular the PL efficiency decreases at short excitation wavelengths in an oxidised sample.

Stimulated emission, lasing, and line narrowing in conjugated polymers

We present time-resolved and time-integrated measurements of optical gain and its effects in films and microcavities made from poly(p-phenylenevinylene) (PPV). We demonstrate that PPV films yield net gain due to stimulated emission with a lifetime of about 20 ps when excited with 200 fs pulses. The short duration of this gain is shown to result from exciton- exciton annihilation. We present results showing how this gain leads to spectral narrowing of the emission from PPV films and discuss why it is unlikely cooperative phenomena play a role in this phenomenon. We present evidence that it results from waveguiding of amplified spontaneous emission. Newly developed high-finesse microcavities are described, for which it is possible to obtain lasing thresholds below the onset of exciton-exciton annihilation. We present measurements of the far-field emission patterns from these structures and also demonstrate a measurement of the coherence time of their emission.