Electroluminescence and Laser Emission of Soluble Pure Red Fluorescent Molecular Glasses Based on Dithienylbenzothiadiazole

Soluble molecular red emitters 1a/1b are synthesized by Stille coupling from 2-(3,5-di(1-naphthyl)phenyl)thiophene precursors. The compounds show emission maxima at ca. 610 nm in CH2Cl2 solution and 620 nm in solid films. Replacing the n-hexyl substituent by 4-sec-butoxyphenyl produces a marked increase of glass transition temperature (Tg) from 82 °C to 137 °C and increases the solubility in toluene and p-xylene, thus improving the film-forming properties. Cyclic voltammetry shows that the compounds can be reversibly oxidized and reduced around +1.10 and −1.20 V, respectively. A two-layered electroluminescent device based on 1b produces a pure red light emission with CIE coordinates (0.646, 0.350) and a maximal luminous efficiency of 2.1 cd A−1. Furthermore, when used as a solution-processed red emitter in optically pumped laser devices, compound 1b successfully produces a lasing emission at ca. 650 nm

Tuneability of the ASE in thin organic films

Explains the tuneability of the ASE in thin organic films

Influence of surface-related states on the carrier dynamics in (Ga,In)N/GaN single quantum wells

We report on the influence of surface-related states on the relaxation of carriers within single (Ga,In)N/GaN quantum wells. Two identical samples that differ only in the thickness of the top GaN cap layer were studied. Photoluminescence and pump-probe measurements reveal significant variations in the quantum well integrated emission and the carrier relaxation decay times in the two samples, when probing both the ground and excited states of the wells. The variations are attributed to the presence of an efficient nonradiative relaxation channel associated with the proximity of the quantum well excitations to the surface-related states in the thin-cap sample

Hybrid solar cells from the Blend of Poly(3-hexylthiophene) and ligand-capped TiO2 nanorods.

International audienceHybrid bulk heterojunction solar cells based on nanocrystalline TiO2 (nc-TiO2) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3-hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active-layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer-metal oxide interaction at the nc-TiO2/ P3HT interface, resulting in a six-fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc-TiO2 particle surface is found to be crucial for charge separation. Ligand-exchange procedures applied on the TOPO-capped nc-TiO2 nanorods with an amphiphilic ruthenium-based dye are found to further improve the charge-separation yield at the polymer-nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor-acceptor bulk heterojunction systems, we conclude that charge transport in the nc-TiO2:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc-TiO2 particles

Synthesis and properties of monodisperse oligofluorene-functionalized truxenes: highly fluorescent star-shaped architectures

This paper describes the strategy toward novel monodisperse, well-defined, star-shaped oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-dimensional nanosized macromolecules T1-T4. The radius for the largest oligomer of ca. 3.9 nm represents one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible or quasi-reversible oxidation and reduction processes (E-ox = +0.74 to 0.80 V, E-red = -2.66 to 2.80 eV vs Fc/Fc(+)), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band gaps of the oligomers are quite high (E-g(CV) = 3.20-3.40 eV). Close band gaps of 3.05-3.29 eV have been estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good thermal stability (up to 400-420 degreesC) and improved glass transition temperatures with an increase in length of the oligofluorene arms (from T-g = 63 degreesC for T1 to 116 degreesC for T4) and show very efficient blue photoluminescence (lambda(PL) = 398-422 nm) in both solution (Phi(PL) = 70-86%) and solid state (PhiPL = 43-60%). Spectroelectrochernical experiments reveal that compounds T1-T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state (similar to340-400 nm) to colored (from red to purple color; similar to500-600 nm) in the oxidized state

Hybrid bulk heterojunction solar cells based on blends of TiO2 nanorods and P3HT.

International audienceOver the past decades, organic solar cells based on semiconducting polymers or small molecules have become a promising alternative to traditional inorganic photovoltaic devices. However, to address the intrinsic limitations of organic materials, such as charge separation yield, charge transport and durability, new strategies based on hybrid organic/inorganic materials have been explored. One such approach exploits mesoporous inorganic nanostructures as electron acceptors, which takes advantage of the potential to control the active layer structure and interface morphology through nanoparticle synthesis and processing. In this work, the potential of hybrid photovoltaics will be discussed and illustrated through a recent study of bulk heterojunction systems based on the blend of TiO2 nanorods with a conjugated polymer

Efficient energy transfer in organic thin films - implications for organic lasers

We show that efficient nonradiative (Förster) energy transfer between solid films of two highly luminescent perylene dyes blended in a solid film can be used to control the amplified spontaneous emission (ASE) emitted from the films under pulsed optical excitation. Perylene orange, which acts as the donor, and perylene red, which is the acceptor, are doped into a host matrix of poly(methylmethacrylate) (PMMA). We report the ASE behavior as a function of acceptor concentration, and observe a sudden change in the spectral position of the ASE at an acceptor:donor concentration of 1:9 by weight. Below this concentration, emission is at 590 nm, which is characteristic of ASE from undoped perylene orange:PMMA blends, whereas films with higher acceptor concentrations produced ASE spectra centered at 620 nm, which is characteristic of perylene red:PMMA blends. In order to understand this behavior, the rate constant for energy transfer between the dyes was measured and found to be 5.0±0.2×1011 s−1 (mol/dm3)−1. We used this to deduce an upper limit for the stimulated emission rate of 4.9±0.2×108 s−1