Mahmoud Chakaroun

Optimal design of a microcavity organic laser device under electrical pumping

The quality factor of microcavity organic lasers, designed for operation under electric pumping, has been numerically investigated. The microcavity structure consists of an organic light emitting diode set in between multilayer dielectric mirrors centered for an emission at 620 nm. In order to optimize the quality factor, different parameters have been studied: the impact of high and low index materials used for the multilayer mirrors, the role of a spacer inserted in between the mirrors to obtain an extended cavity, and the effect of an absorbing electrode made of metallic or transparent conductive oxide layer. The results of our different optimizations have shown a quality factor (Q) as high as 15,000.

Experimental optimization of the optical and electrical properties of a half-wavelength-thick organic hetero-structure in a Micro-cavity

In the context of progress towards the organic laser diode, we experimentally investigate the optical and electrical optimization of an OLED in a vertical λ/2 microcavity. The microcavity consists of a quarter-wavelength TiO₂/SiO₂ multilayer mirror, a half-wavelength-thick OLED and a semitransparent aluminum cathode. The Alq3/DCM2 guest-host system is used as the emitting layer. This study focuses on the design and the fabrication of a half-wavelength thick organic hetero-structure exhibiting a high current density despite both the thickness increase and the cathode thickness reduction. The emission wavelength, the line-width narrowing and the current-density are studied as a function of two key parameters: the hetero-structure optical thickness and the aluminum cathode thickness. The experimental results show that a 125 nm thick cavity OLED ended by a 20 nm thick aluminum cathode exhibits a resonance at 606 nm with a full width at half maximum of 11 nm, and with current-densities exceeding 0.5 A/cm². We show that even without a top high-quality-mirror the incomplete microcavity λ/2 OLED hetero-structure exhibits a clear modification of the spontaneous emission at normal incidence.

Self-organized nanoparticle photolithography for two-dimensional patterning of organic light emitting diodes

We report a new simple and inexpensive sub-micrometer two dimensional patterning technique. This technique combines a use of a photomask featured with self-organized particles in the micro- to nano-meter size range and a photoresist-covered substrate. The photomask was prepared by depositing monodispersed silicon dioxide (SiO(2))- or polystyrene- spheres on a quartz substrate to form a close-packed pattern. The patterning technique can be realized in two configurations: a hard-contact mode or a soft-contact mode. In the first configuration, each sphere acts as a micro ball-lens that focuses light and exposes the photoresist underneath the sphere. The developed pattern therefore reproduces exactly the same spatial arrangement as the close-packed spheres but with a feature size of developed hole smaller than the diameter of the sphere. In the soft-contact mode, an air gap of few micrometers thick is introduced between the 2D array of self-organized spheres and the photoresist-covered substrate. In this case, a phase mask behavior is obtained which results in an exposure area with a lattice period being half of the sphere diameter. A 2D lattice structure with period and feature size of a developed hole as small as 750 nm and 420 nm, respectively, was realized in this configuration. We further applied this technique to host the deposition of organic films into the 2D nanostructure and demonstrated the realization of green and red nano-structured OLEDs.

Electrical and Optical Impulse Response of High-Speed Micro-OLEDs Under UltraShort Pulse Excitation

The electric and optical impulse response of two types of high-speed OLED (HSOLED) driven by ultrashort electrical pulses is investigated. The two HSOLED were designed and manufactured to be characterized in the presence of electrical pulses ranging from 10 to 100 ns in duration and a repetition rate of 10 Hz. The impact of the OLED geometry and the fabrication process on the time response is investigated. This is the first time that an optimized HSOLED exhibits an electrical time response as low as 2.1±0.6 ns and also shorter than the device optical decay time (9.8± 0.2 ns). Moreover, the HSOLED measured current density reaches 3.0 kA/cm2, the highest value reported in the literature, with state-of-the-art electroluminescence of 12 W/cm2.

Study of the Light Coupling Efficiency of OLEDs Using a Nanostructured Glass Substrate

We study theoretically the enhancement of the light extraction from an OLED (Organic Light-Emitting Diode) with nanoair-bubbles embedded inside a glass substrate. Due to such a nanostructure inside the substrate, the critical angle which limits the light extraction outside the substrate from the OLED is increased. The theoretical results show that the nanoair bubbles near by the substrate surface can improve the efficiency of the light extraction by 7%. Such a substrate may also be suitable for photovoltaic cells or display screens.

Experimental and theoretical study of the optical and electrical properties optimization of an OLED in a microcavity

In this work, we experimentally and theoretically investigate half-wavelength-thick Organic Light Emitting Diode (OLED) in a vertical microcavity. The latter is based on a quarter-wavelength multilayer mirror on one side and a thin aluminum semi-transparent layer on the other side. Two key parameters are studied for an optimal design of a cavity- OLED: the organic layer and the metallic cathode thicknesses. The experimental study shows that a 627 nm peak emission is obtained for a 127 nm-thick OLED hetero-structure. To achieve both desired optical transmission and effective electron injection, we investigate the influence of the Al cathode thickness on the performance of the microcavity devices. The experimental results are compared to those obtained by simulations of the emission spectra using the transfer matrix method and taking into account the organic emitter position inside the cavity.

Plasmonically enhanced blue OLED subject to exciplex emission

Blue OLEDs are of a particular interest as they are needed in the development of full color display and white lighting. However, blue OlEDs have a very poor efficiency in comparison to green and red OLEDs [1]. In addition, the color of the emitted light is generally not stable due to the apparition of Green Emission Bands (GEB) allowing a broadened electroluminescent spectrum of the OLED S. Liu, R. Wu, J. Huang, and J. Yu, “Color-tunable and high-efficiency organic light-emitting diode by adjusting exciton bilateral migration zone,” Appl. Phys. Lett. 103,133307 (2013).

Localized surface plasmon for electroluminescence enhancement of organic light sources

In this paper, we study the effect of the localized surface plasmon resonance (LSPR) of two types of metallic nanoparticles (NPs) on the organic light-emitting diodes (OLED) performances. Numerical and experimental results will be presented.

Organic Light-emitting Diodes

The earliest work on electroluminescence of organic materials is often attributed to Tang and Van Slyke, from the Kodak laboratories in Rochester, New York. However, in 1953, the Frenchman Andre Bernanose from the Faculty of Pharmacy in Nancy presented work on the electroluminescence of acridine, and Pope, in 1963, reported electroluminescence in a crystal of anthracene. However, this pioneering work produced very little in the way of results, because the electrical devices used were capacitors, rather than diodes. Indeed, in a structure made up of an organic monolayer sandwiched between two electrodes, the difference in mobility between the holes and electrons gives rise to recombination, not in the heart of the material electroluminescent, but mainly in a region near to the electrode injecting the minority charge carriers, i.e. near to the electrode. The efficiency of electric current-to-light conversion, therefore, is very poor.

Organic Plasmonics: Toward Organic Nanolasers

The LSP (Localized Surface Plasmon) is one of the more interesting optical properties of metal nanostructures. It results from the collective oscillation of the electron cloud at the surface of a metal nanoparticle (NP). Thus, the electromagnetic (EM) field produced in the immediate vicinity of the NP may surpass the excitation field by several orders of magnitude. This property is used for various applications – in particular, for Metal-Enhanced Fluorescence (MEF).