D. J. Pinner

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...

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.

Properties of light emitting organic materials within the context of future electrically pumped lasers

The quest for electrically pumped organic laser is now conducted in several places around the globe. In this paper we outline our approach to this long-term project and aim to describe both encouraging and challenging (discouraging?) results. We describe experimental results using polymers, polymer blends and triplet emitters in an LED configuration using either CW or pulsed mode of operation. We also describe results of modeling such structures with the aim of being able to design electrical and optical characteristics of future laser structures. Finally we describe an approach to modify material parameters so as to provide a range of building blocks for optoelectronic devices.

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.

Grazing emitted light from films of derivative polymer of polyfluorene

Abstract Emission patterns from the films of polyfluorene derivative on silica substrates are found to show light emission at grazing angles to the substrate. This light is found to be polarised and to have a narrowed emission, as well as enhanced intensity with respect to the light collected in the forward direction. Both the wavelengths of the peaks and their polarisation (s or p) are thickness-dependent. The peaks wavelength positions are used to characterise the spectrum of the refractive indices of the film both in and out of plane. The relative intensity of s- to p-polarised peaks serves as an independent indication of the dipole orientation. The deduced dipole in-plane alignment ratio (88%) and the magnitude of the birefringence (Δ n =0.2 at λ =600 nm) suggest that the main chains of this polymer lie preferentially in plane of the film. These findings seem to be consistent with the rod structure of the polymer studied.

Material and device related properties in the context of the possible making of electrically pumped polymer laser

Abstract It has been shown that conjugated polymers possess the two basic requirements for electrically pumped lasers: (1) they can support optically-pumped lasing, and (2) they can support current densities of kA/cm2. However, electrically pumped lasing has not been realised yet. The above-mentioned prerequisites are sufficient when traditional III–V lasers are concerned. However, conjugated polymers are in many ways different and hence require a slightly modified emphasis. We will discuss materials related issues such as charge-induced absorption, charge-mobility, exciton generation, as well as device-related properties such as architecture and driving scheme.

The use of electrical pulses to study the physics of bilayer organic light-emitting diodes

We present detailed experimental and theoretical analysis for both constant-wave and pulsed excitation for a variety of real bilayer organic light-emitting diodes (LEDs). We find from experiment and simulation that the recombination zone may be moved about the polymer-polymer heterojunction as a function of the applied voltage. We suggest a method for spectrally resolving the emission of both polymers of a bilayer LED as a function of time, and show that these results are in good agreement with time-dependent simulation results. The simulation model takes into account both the discontinuities in energy levels and mobilities across the polymer-polymer heterojunction.

Moving the recombination zone in two layer polymer LEDs using high voltage pulses

Abstract We have studied various two layer polymer light emitting diodes (LEDs) by applying a wide range of voltage pulses (100ns - 1 micosecond, 10–100V). We find that it is possible to move the recombination zone about the polymer/polymer heterojunction as a function of the applied voltage, and that the colour change depends on the charge injection, relative carrier mobilities and their field dependence. Preliminary simulation runs for these devices are also presented. Two current decay components (fast 100ns and slow 1ms) have been observed in the current response of a device after the application of microsecond pulses, which we attribute to the presence of mobile and immobile carriers. IVL and efficiency data presented show that LEDs perform better in pulsed than in d.c. operation. We also report peak brightnesses in LEDs in excess of 10 million Cd/m 2 at current densities of 500A/cm 2 under the application of 1MV/cm electric field.

Optoelectronic devices based on hybrid organic–inorganic structures

Conjugated polymers have become attractive for a range of device applications as light emitting diodes (display), photo-voltaic cells, transistors, and lasers. While there is true value in making all plastic devices most of the currently made ones involve the use of inorganic layers and or substrates. In this paper we outline some of the attractive properties of such devices and show that the hybridization on a nano-scale (as in blends) could be a powerful tool.

Time-resolved transport in conjugated polymers

Abstract We present a range of measurement techniques (time-resolved and CW) of various polymers in a variety of LED configurations. Using time-resolved pulsed electrical excitation, we are able to provide direct evidence for the motion of both types of carriers. We use pulses to probe the decay of carriers on time scales from nanosecond to millisecond. We address the issue of pulsed vs. CW properties and find that the functional form (power law) of the current–voltage curves is affected by the measurement technique; we also discuss a method to produce “intrinsic” curves. We introduce the method of using double electrical pulses to probe the dynamics of the stored charges after the application of a pulse.