Luciano Miozzo

Surface engineering for high performance organic electronic devices: the chemical approach

Self-assembled monolayers (SAMs) are rapidly becoming an essential part of organic electronics such as light emitting diodes (LEDs), organic field-effect transistors (OFETs), and complementary circuits, where they are employed to control the morphology and energetics of the interfaces. This review will focus on interface engineering and its influence on such devices. We will describe the growth of SAMs of organic molecules on various surfaces of interest for this purpose (mainly metals and oxides). For the growth of SAMs on metal electrodes, the most common approach makes use of thiols to modify the metal surface, but the preparation of SAMs on dielectrics requires other reactive groups such as carboxylic or phosphonic acids. We review also the control of the interfacial properties by appropriate molecular design of the SAMs and their effect on device performance. SAMs can modify the morphology of the organic semiconductor, the dielectric properties of the insulator, the electronic states at the dielectric interface and the level alignment at the electrode interfaces. All these factors can influence current transport mainly by modifying the charge mobility, the contact resistance and the trap density at the interfaces.

Structural characterisation of single crystals and thin films of α,ω-dihexylquaterthiophene

Single crystals and thin films of the p-type organic semiconductor α,ω-dihexylquaterthiophene have been studied using X-ray diffraction and TEM, respectively. The single crystal analysis reveals the presence of monomolecular layers of quaterthiophene cores with terminally bound hexyl chains. A herringbone molecular packing akin to that exhibited by the parent quaterthiophene molecule is observed. Electron diffraction of thin films on silica clearly shows the crystalline polymorph of the solution-grown crystals and the crystallites deposited by organic molecular beam deposition are identical. However, thin film crystallites are severely affected by multiple twinning.

Novel fluorinated amino-stilbenes and their solid-state photodimerization

We synthesized a series of polyfluoro-amino-stilbenes in satisfactory yields by Wittig reaction of 4-piperazinyl-benzalhehydes with pentafluoro-benzylidene-triphenyl-phosphorane and we analyzed the photochemical behavior of these stilbenes in the solid state; thanks to arene–perfluoroarene π–π interactions, some of them have shown a good propensity to give the corresponding cyclobutane photodimers in quantitative yields. The photocyclization is reversible both in solution and in polymeric matrix, affording the corresponding stilbenes with different cis–trans stereo-selectivity.

Electroluminescence from two fluorinated organic emitters embedded in polyvinylcarbazole

Electroluminescence and photoluminescence of two fluorinated dyes emitting in the blue and in the green, blended with polyvinylcarbazole and an oxadiazole compound, are reported. Organic light-emitting diodes realized with about 0.1wt% dye concentration show interesting performances. Excited state complexes reduce the effectiveness of energy transfer in the blue-emitting device while, for the green-emitting device, the main mechanism of exciton generation is based on charge trapping at the emissive dye.

Polarized absorption, spontaneous and stimulated blue light emission of J-type tetraphenylbutadiene monocrystals.

Blue amplified spontaneous emission at room temperature is demonstrated from the exposed face of the strongly emitting organic semiconductor 1,1,4,4-tetraphenyl-1,3-butadiene in single crystal form. The symmetry of the crystal and calculation of lattice sums indicate the J-type organization of the molecular transition moments. The minimum in the lowest exciton dispersion branch, from which emission takes place, is found at the edge of the Brillouin zone leading to a dominant vibronic emission since the zero-phonon line is forbidden. The observed gain narrowed line is attributed to the vibronic replica which becomes amplified with increased pumping. The reported emission is along the normal to the exposed crystal face, important for the development of vertical cavity geometry lasers based on organic single crystals. The threshold excitation fluence of 400 microJ cm(-2) is comparable to other organic crystalline systems, even if the amplification path is much reduced as a consequence of the vertical geometry. Considering these relevant aspects, the optical characterization of this material is provided. The polarized absorption spectra are reported and the properties of the lowest-energy excitonic state investigated. Calculation of the electronic transitions for the isolated molecule, lattice sums for the transition at lowest energy, and the symmetry of the crystal allow attributing the largest face of the samples and the observed optical bands in the spectra. Polarized time-resolved spectra are also reported allowing to identify the intrinsic excitonic emission.

Self-assembly of gold nanoparticles on functional organic molecular crystals

The utilization of metal nanoparticles (NPs) to fabricate metal electrodes under mild conditions is one of the most studied topic in recent years. In this work, colloidal Au NPs were deposited on two isostructural molecular crystals, namely 1,2,3,4-tetrafluoro-7-thiomethyl-acridine (MeSAcr) and 1,2,3,4-tetrafluoro-7-methoxy-acridine (MeOAcr), exposing S atoms and O atoms, respectively, at their largest crystal faces. The depositions were carried out mainly by drop casting under ambient conditions, increasing the contact time from 1 to 120 min, and the samples were then analyzed by atomic force microscopy (AFM) to evaluate the coverage. Thanks to the affinity between S and Au atoms, Au NPs are observed to adhere on the MeSAcr surface within 1-min contact time, whereas at least 1h is required to find NPs on the MeOAcr surface. NP adsorption is also affected by the substrate surface morphology; indeed, step edges represent preferential adsorption sites even in the absence of Au-S interaction. Experiments under different conditions were performed to maximize the coverage on MeSAcr, reaching values up to 13%. AFM equipped with fluid cell was also employed to simultaneously depositing and imaging NPs, achieving a better understanding of the adsorption mechanism.

Exciton self-trapping in tetrafluoro-dimethyl-aminoacridine single crystals

The UV-visible optical spectra of 1,2,3,4-tetrafluoro-7-(N,N)dimethyl-amino-acridine single crystals are reported. The results are discussed on the basis of the molecular transitions and crystal packing in the framework of the theory of molecular excitons under a fluctuating potential field due to dynamic disorder. A strong local geometry distortion is demonstrated by applying the Urbach rule to the absorption tails, which is the amplitude of the local potential fluctuation being larger than the intermolecular transfer energy. The lineshape and linewidth of the emission band and its temperature dependence give further evidence of exciton self-trapping.

Functional Π-conjugated polymers based on maleimide for photovoltaic applications

Abstract A series of new bis-(2-thienyl) maleimide monomers have been synthesized and characterized. The bis-(2- thienyl)maleimide unit has been copolymerized with different aromatic comonomers. Stille coupling polymerizations under various conditions have been utilized. The copolymers were then characterized by size-exclusion chromatography and their optical and electronic properties were investigated by UV-Vis absorption spectroscopy and cyclic voltammetry. All maleimide based copolymers shared similar LUMO energy levels, which are largely determined by the acceptor moiety, and are close to that of PC61BM to be effective for charge dissociation. These low band gap polymers have been tested for photovoltaic applications and have shown moderate photovoltaic performance. Interesting results were obtained by adding the polymer to the P3HT:PC61BM mixture, as a third component. The ternary blend BHJ solar cells showed power conversion efficiencies of 35% exceeding those of the corresponding binary blends.