N. Tessler

Transient electroluminescence of polymer light emitting diodes using electrical pulses

Detailed experimental and theoretical analysis of the pulsed excitation of polymer light emitting diodes is presented. We find a set of universal transient features for a variety of device configurations (different polymers/cathodes) which can be reproduced using our phenomenological numerical model. We find that the temporal evolution of the electroluminescence in response to a step voltage pulse is characterized by: (i) a delay followed by; (ii) a fast initial rise at turn-on followed by; (iii) a slow rise (slower by at least one order of magnitude). The large mobility mismatch between holes and electrons in conjugated polymers allows us to separately time resolve the motion of holes and electrons. We suggest a method for extracting mobility values that takes into account the possible field-induced broadening of carrier fronts, and which is found to be compatible with mobilities determined from constant wave measurements. By using appropriate device configurations it is possible to determine the mobilit...

Integrated, high-mobility polymer field-effect transistors driving polymer light-emitting diodes

Abstract An improved poly-hexylthiophene (P3HT) polymer field-effect transistor with field-effect mobility of 0.05-0.1 cm2/Vs and ON-OFF current ratio of 106–108 is demonstrated. The high ON-OFF ratio is obtained by careful device processing in N2 atmosphere and use of a reducing surface capping layer. The device has sufficient post-processing robustness that it can be integrated without degradation into a simple multilayer optoelectronic device in which the FET supplies the current to a polymer LED. For comparable size of FET and LED the FET supplies a current density of 10 mA/cm2 to the LED resulting in a brightness in excess of 100 Cd/m2. The FET-LED device is a step towards optoelectronic integrated circuits such as all-polymer active-matrix LED displays.

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.

Role of optical properties of metallic mirrors in microcavity structures

In thin metal films the phase change on reflection of incident light is dependent on the wavelength, the angle of incidence, the type of metal, and the metal thickness. These properties have been exploited to improve the performance of planar metal mirror microcavities. We model substantial alteration of peak emission wavelength and linewidth with mirror thickness. This allows the tuning of the cavity resonance wavelength by variation of metal mirror thickness. The dependence of the phase change on wavelength and angle of incidence can also be used to suppress the angular dependence of the cavity resonance wavelength. These effects are observed in silver-mirrored cavities containing the polymers poly(p-phenylene vinylene), (PPV), and a cyano-substituted derivative of PPV, MEH-CN-PPV.

Pulsed excitation of low-mobility light-emitting diodes: Implication for organic lasers

We present a theoretical and experimental study of electrical pulsed excitation in polymer light-emitting diodes (LEDs). We find that the low mobility results in a relatively high charge density within an electrically pumped structure. The broad spectrum of the charge-induced absorption and its overlap with the ground-state absorption pose a significant barrier for achieving net gain and lasing. We suggest an approach that might circumvent this problem using suitable emitters and an appropriate driving scheme. We also suggest methods for ultrafast modulation of LEDs as well as direct determination of the space charge within the recombination zone.

Lasing characteristics of PPV microcavities

Abstract We report lasing characteristics of poly (p-phenylenevinylene) (PPV) microcavity devices.

Suppressed angular color dispersion in planar microcavities

We report an improved microcavity design which allows the suppression of the viewing angle dependence of the color emitted by a planar device. This is demonstrated for luminescent conjugated polymer based cavities, for which the wavelength change is reduced from ∼60 to 10 nm at an angle of 60°. We introduce the concept of cavity optical length dispersion and suggest structures for which the wavelength change with viewing angle is reduced to 5 nm at a viewing angle of 60° irrespective of the emissive material.

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.

Analysis of the turn-off dynamics in polymer light-emitting diodes

We present experimental techniques to analyze the electroluminescence (EL) of polymer light-emitting diodes following the removal of an applied voltage pulse. We explain the fast modulation of the EL intensity at turn-off in terms of the sudden reduction of the Langevin recombination rate, and extract the time evolution the device’s internal electric field at the recombination zone during the application of a voltage pulse. The results are compared to, and found to be consistent with, those of simple numerical modeling. The subsequent long-lived EL tail is analyzed to give the time evolution of the carrier distributions at the recombination zone once the voltage pulse has been removed.

Optical response of conjugated polymers excited at high intensity

The realisation of optically pumped lasing devices using poly(p-phenylenevinylene) has intensified interest in the optical response of conjugated polymers on ultra-short time-scales and at high optical injection density. We describe an investigation of these properties using time-resolved and time-integrated techniques. We demonstrate the importance of exciton-exciton annihilation at high injection densities and present evidence that this process produces photo-induced absorption (PA) through the generation of charges. We present a model which accounts for PA from both photoexcited singlet excitons and the charge pairs generated by annihilation. This model describes the time-evolution of the PA well and indicates that, in the presence of exciton-exciton annihilation, dissimilar stimulated emission and PA dynamics are consistent with a high quantum yield of excitons as deduced from PL lifetime and efficiency studies.