Carmen Coya

Novel erbium(III) fluorinated β-diketonate complexes with N,N-donors for optoelectronics: from synthesis to solution-processed devices

Three novel ternary Er3+ complexes emitting in the C band transmission window for fiber optic communications have been synthesised and their structures have been elucidated by single crystal X-ray diffraction. The fluorinated β-diketonate ligand, 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione, combines a good absorption cross-section in the ultraviolet region with reduction of non-radiative quenching of the Er3+ emission, while the rigidity and bulkiness of the three different N,N-donors (2,2′-bipyridine, bathophenanthroline and 5-nitro-1,10-phenanthroline) have a pronounced impact on the emission intensity of luminescence. Furthermore, the choice of the ancillary ligand also determines the efficiency of the antenna effect, leading to complete quenching of the ligand-associated visible emission for the optimized complex with 5-nitro-1,10-phenanthroline. Solution processed 1.54 μm organic light-emitting diodes have been manufactured and characterized for this complex, confirming the aforementioned complete resonant energy transfer from the ligands to the Er3+ ion. The features of the reported device fabrication show a simple way to obtain large area NIR-OLEDs.

Full-solution-processed blue organic light emitting device based on a fluorescent 1,3,5-tristyrylbenzene stilbenoid small molecule

A full-solution-processed blue organic light emitting diode display based on a small molecule built on a fluorescent stilbenoid core has been fabricated and characterized. The structure of the blue device is ITO/PEDOT:PSS/active layer/Al. The optical characterization of the 1,3,5-tristyrylbenzene endowed with alkoxy (OC6H13) chain active layer reveals a change in the excited molecular level configuration due to the molecular packing in the thin film that gives as a result a large Stokes shift in the solid state. The electroluminescence in the deep-blue region of the single layer device corresponds well with the photoluminescence emission of the thin films (438nm) and is independent of the applied voltage. Raman analysis confirms the stability of the compound through the fabrication process. From the I-V study, an effective hole mobility of 9.3×10−6cm2∕Vs for the active layer has been obtained.

Influence of the 1,2,4-linking hyperbranched poly(arylenevinylene) structure on organic light emitting diode performance as compared to conventional 1,3,5-linking one

The influence of a novel 1,2,4-linking hyperbranched poly(arylenevinylene) (1,2,4-hb-PAV) material, designed to feature intramolecular energy-funneling, on the transport and emission properties of organic light emitting diodes (OLEDs) has been studied. A comparison to conventional hyperbranched 1,3,5-linking polymers (1,3,5-hb-PAV), which do not exhibit this effect, has been made. For this purpose, single-layer organic light emitting diodes with a glass/indium–tin oxide/poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate)/active layer/Ca/Al structure and different active layer thicknesses have been fabricated and characterized, using either 1,2,4-hb-PAV or 1,3,5-hb-PAV as active layers. The current–voltage response has been interpreted in terms of a numerical model that includes a field-dependent carrier mobility, which allows us to estimate carrier mobility in the diodes. Stable green emission with Commission Internationale de L’Eclairage coordinates at (0.41,0.56) and a high luminous efficiency of ...

Influence of the substrate on the bulk properties of hybrid lead halide perovskite films

In addition to the known effect of the substrate on the interfacial properties of perovskite films, here we show that the bulk properties of hybrid lead halide perovskite films depend on the type of substrate used for film growth. Despite the relative large film thickness, ∼600 nm, the roughness and nature of the substrate layer (glass, FTO, TiO2 and PEDOT:PSS) affect not just the degree of preferential orientation and crystal grain size but also the lattice parameters of CH3NH3PbI3 films synthesized from the PbCl2 precursor. The obtained changes in lattice parameters indicate that the Pb–Pb distance varies by around 0.7%. We suggest that the substrate roughness and chemical nature determine the concentration of defects mainly by varying the chlorine content and probably by the incorporation of oxygen and iodine vacancies during film nucleation and growth. These differences also have consequences in the observed light induced transformations. Upon laser illumination, the formation of additional defects, most probably related to oxygen, is revealed by 110 and 165 cm−1 Raman peaks. With increasing laser power the chemical transformation into PbOx is clearly identified by the 140 and 275 cm−1 Raman peaks. The irreversible photoluminescence enhancement observed at low power with illumination time, also dependent on the substrate nature, is proposed to be due to the localization of the electron–hole excitons created in the vicinity of the light generated defects. These results shed light on the performance of the perovskite layer and help to understand how bulk processes, where ion migration is a conspicuous example, are severely affected by interfacial properties such as those imposed by the substrate.

Solution-processed organic devices developed by a novel cost-effective patterning technique based on electrical erosion

Application of arc erosion to the patterning of metallic contacts in organic devices is presented. A home-made systems and details of the working principles are described. Advantages and drawbacks of this novel technology are discussed.

Triindole based single crystals and thin films for OLED and OFET applications

The search of new organic molecules with improved properties is of fundamental relevance for hybrid organic-inorganic based devices (OLED, FET, PV, injection layers, flexible large area devices, lasers, etc.). Triindole based materials present extended aromatic cores with disk-like geometry that allow tailoring their electronic properties through chemical functionalization. In this work we present an optical and electronic study of new triindole based single crystals. Pistacking gives rise to highly ordered columnar structures yielding to high mobilities, around 0.4 cm2V-1s-1. External oxidation is found to increase orders of magnitude the conductivity. Slight modifications of this platform modify substantially the crystallization dynamics and therefore the quality of the thin films obtained by spin coating from solutions with different solvents. The morphology, stability and properties of the optimized films are found to be promising for device fabrication. A comparative study of the absorption and emission efficiency of solutions and thin films of the different derivatives is presented. The first tests for OLED and OFET devices are under way.

Development of electrical-erosion instrument for direct write micro-patterning on large area conductive thin films

We have developed a complete instrument to perform direct, dry, and cost-effective lithography on conductive materials, based on localized electrical discharges, which avoids using masks or chemicals typical of conventional photolithography. The technique is considered fully compatible with substrate transport based systems, like roll-to-roll technology. The prototype is based on two piezo nano-steppers coupled to three linear micro-stages to cover a large scale operation from micrometers to centimeters. The operation mode consists of a spring probe biased at low DC voltage with respect to a grounded conductive layer. The tip slides on the target layer keeping contact with the material in room conditions, allowing continuous electric monitoring of the process, and also real-time tilt correction via software. The sliding tip leaves an insulating path (limited by the tip diameter) along the material, enabling to draw electrically insulated tracks and pads. The physical principle of operation is based in the natural self-limitation of the discharge due to material removal or insulation. The so produced electrical discharges are very fast, in the range of μs, so features may be performed at speeds of few cm/s, enabling scalability to large areas. The instrument has been tested on different conducting materials as gold, indium tin oxide, and aluminum, allowing the fabrication of alphanumeric displays based on passive matrix of organic light emitting diodes without the use of masks or photoresists. We have verified that the highest potential is achieved on graphene, where no waste material is detected, producing excellent well defined edges. This allows manufacturing graphene micro-ribbons with a high aspect ratio up to 1200:1.

The use of electrical machining for patterning and oxidation of graphene in low cost manufacturing

Graphene patterning and local oxidation of graphene are performed at high speed over large areas by means of controlled electrical micro-discharges, using a home-made instrument. The optimum operating voltage is determined to obtain high quality straight lines of removed graphene in a dry environment. By properly adjusting the working voltage and humidity conditions, large spots of graphene oxide are obtained with high uniformity, which paves the way to achieve large single-layer graphene oxide substrates for different applications.

Synthesis, structural modelling and photoluminescence of Tb(III) polymeric complexes for solution-processed OLEDs

Current manufacturing technologies for OLEDs involve the use of expensive high vacuum techniques and call for thermal stability requirements which are not fulfilled by many materials. These problems disappear when the OLED films are deposited directly from solution. In this study, we have synthesized, modelled by semi-empirical quantum chemistry methods and characterized the optical properties of two novel Tb(III) complexes with carboxylic acids, which can be readily used as “complex-only” emissive layers in wet-processed OLEDs. Upon excitation in the UV region, very efficient energy transfer from the ligands to Tb3+ takes place, giving rise to intense green emission, both in powder and as a thin film. The good quantum efficiencies (31.3% and 24.7%, respectively) and adequate thin film forming properties make these materials promising chromophores for cost-effective OLEDs.

Direct-write patterning of metals and graphene oxide electrodes by arc erosion for organic device manufacturing

Electric arc erosion performed at low continuous voltages has been recently proven as a successful patterning technique for thin films of different conductive materials. In this work, we present an application of this procedure to materials typically aimed for device electrodes, such as indium tin oxide (ITO), gold (Au), graphene oxide (GO) or aluminum doped zinc oxide (AZO), as well as a more in depth study of the electrical discharge generation at submicron scale, which allows optimizing the procedure.