A.J. Pal

Langmuir—Blodgett light-emitting diodes of poly(3-hexylthiophene) : electro-optical characteristics related to structure

Poly(3-hexylthiophene) Langmuir-Blodgett (LB) films have been used as the emitting layer in light-emitting diodes (LEDs). In order to develop further the device and increase the quantum efficiency by balancing the number of injected holes and electrons, two different approaches have been taken. Following the more conventional way, LB films of hole- and electron-transporting materials have been used in a separate layer and/or in a blend with the emitting material. Insulating polyaniline LB films at the electrode interfaces have also been used in order to control the operation. A simple model relating the electro-optical characteristics of the LED to its structure has been proposed.

Electric field redistribution and electroluminescence response time in polymeric light-emitting diodes

The electric field redistribution due to injected and trapped charge carriers in Langmuir–Blodgett (LB) films of poly(3-hexylthiophene) (P3HT) sandwiched between indium tin oxide and aluminum (Al) electrodes as function of applied voltage has been studied using charge collection measurements by the time-of-flight technique. For μτE<d (the drift distance shorter than the interelectrode distance) the amount of the collected photocharge is a function of electric field near the Al electrode and has been used to probe the time evolution of it. The response time for the field to redistribute inside the P3HT LB film was found to be of the order of 5–200 μs, in good agreement with the delay time observed in time-resolved electroluminescence measurements in light-emitting diodes (LED) of similar LB films. We suggest a model for the response times in organic LEDs based on these results.The electric field redistribution due to injected and trapped charge carriers in Langmuir–Blodgett (LB) films of poly(3-hexylthiophene) (P3HT) sandwiched between indium tin oxide and aluminum (Al) electrodes as function of applied voltage has been studied using charge collection measurements by the time-of-flight technique. For μτE<d (the drift distance shorter than the interelectrode distance) the amount of the collected photocharge is a function of electric field near the Al electrode and has been used to probe the time evolution of it. The response time for the field to redistribute inside the P3HT LB film was found to be of the order of 5–200 μs, in good agreement with the delay time observed in time-resolved electroluminescence measurements in light-emitting diodes (LED) of similar LB films. We suggest a model for the response times in organic LEDs based on these results.

High-frequency response of polymeric light-emitting diodes

The frequency dependence of alternating-current polymeric light-emitting diodes has been studied. Langmuir–Blodgett (LB) films of poly(3-hexylthiophene) have been used as the active emitting material sandwiched between LB films of emeraldine base polyaniline to form the device. We have shown that by reducing the thickness of the emitting layer using the LB deposition technique, one can increase the high-frequency operating limit of the device. From the −3 dB frequency, we have calculated the carrier mobility in the emitting polymer layer, and compared it with the Poole–Frenkel model. The electroluminescence and photoluminescence spectra have been studied.

Langmuir-Blodgett films of conjugated polymers: electroluminescence and charge transport mechanisms

This paper reviews and reports our work on the development of polymeric light-emitting diodes (LEDs) based on Langmuir-Blodgett (LB) films. We have used LB deposition technique as a tool to fabricate dc and ac LEDs with a precise thickness ranging from a few molecular layers to tens of layers. In de LEDs, we have shown that as few as three LB layers of active polymer can yield the same luminance as the thicker ones. With the advantage of LB deposition technique to fabricate heterostructure (multilayer) devices, we have used carrier transporting materials and carrier blocking LB films to control and balance the charge injections in dc LEDs. The frequency dependence of (multilayered) ac LEDs has been studied and moderately high-frequency (20 kHz) electroluminescence (EL) intensity has been obtained. From the transient EL measurements, the role of interfaces in polymeric LEDs has been emphasized and the operation mechanism of LEDs has been discussed.

Frequency response of molecularly thin alternating current light-emitting diodes

The frequency response of molecularly thin alternating-current polymeric light-emitting diodes has been studied. Langmuir–Blodgett (LB) films of poly(3-hexylthiophene) (PHT) were used as the active emitting material and the device was formed by sandwiching PHT films between LB films of emeraldine base polyaniline. As a step towards molecular electronic devices, we have shown that even two molecular layers of PHT (≈6 nm) are sufficient for light emission. The high frequency operation limit of the device has been discussed in terms of a charge accumulation process at the polymer–polymer interface. The electroluminescence (EL) spectra of different structures have been compared with corresponding photoluminescence spectra. A significant blueshift in EL has been observed in thinner structures and its origin has been discussed.

High frequency alternating current light-emitting diodes using Langmuir–Blodgett films

Abstract Alternating current (AC) light emitting diodes (LEDs) have been fabricated where Langmuir–Blodgett (LB) films of quinquethiophene (QT) or poly(3-hexylthiophene) (PHT) have been used as the active material sandwiched between insulating LB layers of poly(methylmethacrylate) (PMMA) or emeraldine base polyaniline (PANI), respectively. The frequency response of the devices has been studied, and as the frequency limit of operation we have used the −3 dB frequency. We have shown that high frequency AC LEDs can be fabricated with as few as 10–15 LB layers of the active material. Electroluminescence (EL) is observed almost equal in intensity in both biases for PMMA/QT/PMMA devices. The EL spectrum for these devices shows a broadening to the low-energy side as compared with the photoluminescence (PL) spectrum. The role of the interfaces for the frequency response is discussed.

Light-emitting diodes of poly(3-hexylthiophene) Langmuir-Blodgett films

Poly(3-hexylthiophene) Langmuir-Blodgett films have been used as emitting layers in light-emitting diodes. The effect of the film thickness and additional carrier-transport layers on current-voltage characteristics and quantum efficiency were studied, and the electroluminescence spectra measured. Hole transporting poly(9-vinylcarbazole) was used both as a separate layer in a heterostructure device and in a blend with emitting material. The electron-transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole was used in a blend with the emitting material. The connections between the diode structure and the electro-optical properties as well as operation mechanisms are discussed.

Light-emitting diodes using quinquethiophene Langmuir-Blodgett films: Effect of electron-transporting layers

Abstract Langmuir-Blodgett (LB) films of quinquethiophene have been used as active emitting layers to fabricate light-emitting diodes. Green electroluminescence was visible in a dark room. The effect of the thickness of the film on the electroluminescence efficiency has been investigated. Even 5 LB layers have been shown to yield the same luminance as thicker films. Additional LB films of electron-transporting material have been used to increase the quantum efficiency, which has also resulted in a lower “turn-on” current for the device. The electroluminescence spectrum showed a profile identical to the photoluminescence spectrum of quinquethiophene.

Light-emitting diodes from dye-insulating matrix Langmuir–Blodgett films

Light-emitting diodes (LEDs) utilizing a dye-insulating matrix blend have been fabricated with the Langmuir-Blodgett technique. The emitting dye material used was Rubrene, while poly(methylmethacrylate) (PMMA) was used as the insulating matrix material. The luminance-current density characteristics were studied as a function of Rubrene/PMMA molar ratio which ranged from 5:95 to 60:40 mol.% of Rubrene/PMMA. Devices with low concentrations of Rubrene were found to emit approximately the same amount of light as devices containing higher concentrations of the emitting material. The thickness dependence of luminance vs. applied voltage was studied for devices with optimal molar ratio between Rubrene and PMMA. The thickness of the LEDs studied ranged from three to 11 molecular layers. Photoluminescence and electroluminescence spectra of the LEDs were compared. The quantum efficiency for the LED configuration was calculated.